Product Class 5: Organometallic Complexes of Rhodium

1.5 Product Class 5: 

Organometallic Complexes of Rhodium 

I. Ojima, A. T. Vu, and D. Bonafoux 

General Introduction 

Previous literature on the syntheses and properties of rhodium complexes can be found 

in four comprehensive reviews. [1±4] 

Rhodium is a relatively rare and expensive metal, yet its organometallic complexes 

have played an extremely important role in transition-metal catalysis and organic synthesis. 

The use of rhodium catalysts in synthetic organic chemistry has been one of the most 

active research areas of the last three decades and continues to be extensively investigated 

at the present time. Amongthe most important reactions are oligomerization, isomerization, 

hydrogenation, hydroformylation, hydroboration, and hydrosilation of multiple 

bonds, which are extremely useful in organic synthesis. In industry, a number of processes 

currently used for the syntheses of pharmaceuticals as well as fine chemicals are based 

on the catalytic activity of rhodium complexes. 

Organorhodium complexes are normally encountered in oxidation states +1 or +3, 

although complexes of other oxidation states between +4 and ±3 have also been synthesized. 

Complexes of rhodium(I) (d 8 ) often possess either tetra-coordinate square-planar or 

penta-coordinate trigonal-bipyramidal structures, while those of rhodium(III) (d 6 ) are usually 

octahedral. Ligand dissociation of penta- to tetra-coordinate complexes is often proposed 

as a means of generating open-coordination sites for binding of substrates in catalytic 

reactions. A key feature of organorhodium chemistry is the facile oxidative addition 

to tetra-coordinate rhodium(I) and the reductive elimination from octahedral rhodium(III). 

It is the reversibility of such reactions connectingthe rhodium(I) and rhodium(III) 

oxidation states that brings about the powerful catalytic activity of organorhodium complexes 

to catalyze a wide range of organic transformations. 

Most rhodium(III) and many rhodium(I) complexes are relatively air stable. However, 

in many instances, reactions involvingªair-stableº rhodium complexes give inconsistent 

results. Thus, it is highly recommended that all rhodium complexes to be used in catalytic 

reactions should be stored in an inert atmosphere and freshly recrystallized prior to use. 

The toxicity of rhodium complexes has not been thoroughly investigated. However, 

it appears that most organorhodium complexes are not particularly toxic, although some 

have been shown to exhibit antitumor activity and have been used in cancer chemotherapy. 

Numerous techniques have been developed for the analysis of rhodium complexes. 

Elemental analysis is a simple method for quantitative determination of rhodium content, 

which involves combustion of the sample at 8008C followed by weighing the rhodium(III) 

oxide residue. IR, NMR, and MS are powerful tools for the characterization of organometallic 

complexes of rhodium. Rhodium±carbonyl complexes can be characterized 

by their distinct í(CO) absorptions in the IR spectra. Mass spectrometry is an excellent analytical 

method for determination of rhodium clusters. Since most organorhodium complexes 

are diamagnetic, 103 Rh NMR spectroscopy, together with 1 H, 13 C, and 19 F NMR techniques, 

provides detailed qualitative analysis of rhodium complexes. 

531 

for references see p 612 

I. Ojima, A. T. Vu, and D. Bonafoux, Section 1.5, Science of Synthesis, 2001 Georg Thieme Verlag KG

532 Science of Synthesis 1.5 Organometallic Complexes of Rhodium 

b1.5.1 Product Subclass 1: 

Rhodium±Arene Complexes 

Transition-metal±arene complexes have been of interest to chemists for many years. The 

coordinated ç 6 -arene ligand plays many roles in transition-metal chemistry. It usually 

functions as a labile ligand which temporarily occupies the vacant coordination sites to 

stabilize the complex, but can easily be displaced by other ligands. It can also be a spectator 

ligand, staying bound to the metal center throughout the course of the reaction, or act 

as a substrate ligand for either catalytic or stoichiometric transformations. In general, as 

substitution on the ç 6 -arene ligand increases, the stability of the rhodium complex increases. 

Rhodium±arene complexes are commonly synthesized by ligand substitution reactions 

which involve either exchange of a diene or a weakly bound ligand for an arene, or 

by direct displacement of a chloride or acetylacetonate ligand with an arene. Arene complexes 

of rhodium can also be formed by alkyne cyclotrimerization reactions, but yields 

are too low to be synthetically useful. [5] The disproportionation reaction of coordinated 

cyclohexa-1,3-diene to benzene and cyclohexene ligands provides an indirect method 

for the synthesis of rhodium±arene complexes. [6] 

Synthesis of Product Subclass 1 

1.5.1.1 Method 1: 

Cationic Complexes by Ligand Substitution 

Cationic arene complexes of transition metals are of special interest owingto their high 

reactivity towards nucleophiles. [7] A large number of cationic ç 6 -arene complexes of rhodium 

are prepared by ligand substitution reactions, as shown in Scheme 1. Complexes 

such as 1 can often be isolated as yellow, air-stable, crystalline salts. Labile ligands such 

as acetone, acetonitrile, and trifluoroacetate can be displaced by an arene to give cationic 

rhodium±arene complexes. 

Scheme 1 Synthesis of Cationic Rhodium±Arene Complexes by Ligand Substitution [8] 

Rh 2Cl 2L 4 

[Rh(diene) 2] + 

Rh(acac)L 2 

Ag + , arene 

arene 

diene = L2 

Ph3C + , arene 

R 1 

R 3 

R 2 

R 

Rh+ 

L L 

1 

3 R2 1.5.1.1.1 Variation 1: 

From Arenes and Cationic Rhodium±Diene Complexes 

As mentioned above, diene ligands of rhodium complexes can readily be displaced by arenes. 

[8] Thus, bis(bicyclo[2.2.1]heptadiene)rhodium(I) tetrafluoroborate (bicyclo[2.2.1]heptadiene 

= norbornadiene = nbd) or bis(cycloocta-1,5-diene)rhodium(I) tetrafluoroborate 

(cycloocta-1,5-diene = cod) undergoes a rapid exchange reaction at room temperature on 

treatment with a dichloromethane solution of the arene (arene = hexamethylbenzene, 

1,3,5-trimethylbenzene, m-xylene, toluene, benzene, phenol, or anisole) to form the correspondingcrystalline 

cationic rhodium±arene complexes in high to excellent isolated 

yields (Scheme 2). Reaction of rhodium complexes with norbornadiene ligands proceeds 

faster than those with cycloocta-1,5-diene ligands. The more reactive 2,4-dimethyl- 

I. Ojima, A. T. Vu, and D. Bonafoux, Section 1.5, Science of Synthesis, 2001 Georg Thieme Verlag KG 

R 3

1.5.1 Rhodium±Arene Complexes 533 

bpentadiene ligand is rapidly displaced by an arene ligand to produce cationic rhodium± 

arene complexes in quantitative yields. [9] The reactivity of the arene increases as the number 

of electron-donatingsubstituents on the arene increases. The reactivity sequence 

C 6Me 6 > 1,3,5-Me 3C 6H 3 > 1,3-Me 2C 6H 4 > MeC 6H 5 >C 6H 6 was qualitatively observed (Table 1). 

Scheme 2 Substitution of Diene Ligands with Arenes [8,9] 

[Rh(diene) 2] + BF 4 − 

arene, CH2Cl2, rt 

83−95% 

[Rh(arene)(diene)] + BF 4 − 

diene = nbd, cod, butadiene 

arene = C6Me6, 1,3,5-Me3C6H3, 1,3-Me2C6H4, toluene, benzene, PhOH, PhOMe 

The formation of rhodium±arene complexes is reversible. Thus, treatment of the arene 

complexes with an excess of a diene (norbornadiene or cycloocta-1,5-diene) results in regeneration 

of the [Rh(diene) 2] + BF 4 ± complex. 

Table 1 Synthesis of Rhodium±Arene Complexes via Displacement of Diene Ligands [8] 

Diene Arene Time Yield (%) Ref Diene Arene Time Yield (%) Ref 

nbd C6Me6 10 min 89 [8] nbd C6H5OMe 30 h 86 [8] 

nbd 1,3,5-Me3C6H3 1 min 95 [8] cod C6Me6 1h 93 [8] 

nbd C6H5Me 10 h 86 [8] cod 1,3,5-Me3C6H3 3h 83 [8] 

nbd C6H6 16 h 84 [8] cod 1,3-Me2C6H4 2h 85 [8] 

nbd C6H5OH 30 min 88 [8] cod C6H5Me 24 h 89 [8] 

(Bicyclo[2.2.1]hepta-2,5-diene)(ç 6 -hexamethylbenzene)rhodium(I) Tetrafluoroborate 

(1,R 1 =R 2 =R 3 = Me; L±L = nbd); Typical Procedure: [8] 

An excess of hexamethylbenzene (0.30 g, 1.9 mmol) was added to a soln of [Rh(nbd) 2] + BF 4 ± 

(0.10 g, 0.27 mmol) in CH 2Cl 2 (10 mL). After 10 min at rt, Et 2O (60 mL) was added to the pale 

yellow soln and the precipitated solid was recrystallized (CH 2Cl 2/Et 2O) to give the title 

complex as white crystals; yield: 0.11 g(89%); mp >270 8C; 1 H NMR (CDCl 3): ä 7.73 (s, 18H, 

Ar-CH 3). 

1.5.1.1.2 Variation 2: 

From Arenes and Rhodium±Acetylacetonate Complexes 

The acetylacetonate (acac) ligand bound to rhodium can be substituted with an arene ligand 

as shown in Scheme 3. Treatment of (acetylacetonato)bis(ethene)rhodium(I) with triphenylmethyl 

tetrafluoroborate in the presence of an arene at room temperature gives 

the corresponding cationic rhodium±arene complex as yellow crystals in good to high 

yields. [8] 

Scheme 3 Substitution of Acetylacetonate Ligand with Arenes [8] 

Rh(acac)L 2 

L = H2C CH2 arene = C6Me6, 1,3-Me2C6H4, benzene 

arene, Tr + BF 

− 

4 , CH2Cl2, rt, 24 h 

72−89% 

[Rh(arene)L 2] + BF 4 − 




bBis(ethene)(ç 6 -hexamethylbenzene)rhodium(I) Tetrafluoroborate 

(1,R 1 =R 2 =R 3 = Me; L = CH 2=CH 2); Typical Procedure: [8] 

A soln of Tr + BF 4 ± (0.127 g, 0.38 mmol) in CH2Cl 2 (6 mL) was added dropwise with stirringto 

a soln of [Rh(acac)(H 2C=CH 2) 2] (0.10 g, 0.38 mmol) and hexamethylbenzene (0.35 g, 

2.0 mmol). After 24 h at rt, Et 2O was added to the yellow soln and the precipitated solid 

was recrystallized (CH 2Cl 2/Et 2O) to give the title product as cream-colored crystals; yield: 

0.14 g(89%); mp >200±2018C (dec). 

1.5.1.1.3 Variation 3: 

From Arenes and Rhodium(III) Chloride 

Substitution of a chloride ligand by an arene ligand can be achieved as shown in Scheme 

4. The chloride ligand is usually trapped with silver perchlorate or silver hexafluorophosphate 

to generate cationic rhodium complexes of the type [RhL 2] + , whose solutions are 

reasonably stable under an inert atmosphere. [4] Upon treatment of these complexes with 

arenes, air-stable cationic rhodium±arene complexes are formed in high yield. Trapping 

of chloride ligands can also be carried out in the presence of an arene solvent to afford 

cationic rhodium±arene complexes directly. [10] 

Scheme 4 Substitution of Chloride Ligands with Arenes [4,10,11] 

RhClL 2 

Ag + 

− AgCl 

[RhL 2] + 

arene 

L2 = nbd, cod, hexadienes, (H2C CH2) 2, 

[P(OPh) 3] 2, (CO) 2 

arene = benzene, toluene, alkyl-substituted benzene 

[Rh(arene)L 2] + 

(ç 6 -Toluene)bis(triphenyl phosphite)rhodium(I) Perchlorate 

[1,R 1 = Me; R 2 =R 3 = H; L = P(OPh) 3]; Typical Procedure: [11] 

To a soln of [Rh 2Cl 2{P(OPh) 3} 4] (0.104 g, 0.068 mmol) in toluene/CH 2Cl 2 (2:1) was added 

AgClO 4 (0.028 g, 0.136 mmol) in the same solvent. After 20 min stirring at rt under exclusion 

of light, the resulting mixture was filtered and the filtrate was concentrated. The residue 

was filtered and washed with hexane to give the title complex as yellow crystals; 

yield: 0.033 g(90%); 1 H NMR: ä 8.15 (3H, Ar-CH 3), 4.25 (5H, Ar). 

1.5.1.1.4 Variation 4: 

From Arenes and Rhodium±Acetate Complexes 

Arenes displace relatively weakly bound ligands such as acetate and trifluoroacetate in 

the presence of trifluoroacetic acid to give cationic rhodium±arene complexes, which 

are isolated as hexafluorophosphate salts (Scheme 5). These reactions proceed readily under 

mild conditions to give the products in good yields. No reaction occurs in the absence 

of the acid. Presumably the function of the acid is to increase the electrophilicity of the 

metal towards the arene by removingthe acetate anion and thus generatinga cationic 

rhodium intermediate. [12] The acetate salt is then converted into the hexafluorophosphate 

salt by treatment with a saturated aqueous solution of ammonium hexafluorophosphate. 

Scheme 5 Substitution of Acetate Ligands with Arenes [12,13] 

Rh 2(OCOCF 3) 2L 4 

arene, CF 3CO 2H 

− CF 3CO2H 

arene = 1,3-Me2C6H4, 1,3,5-Me3C6H3, C6Me6 

[Rh(arene)L 2]H(OCOCF 3) 2 

NH4PF6 

[Rh(arene)L 2]PF 6 


1.5.1 Rhodium±Arene Complexes 535 

bBis(ethene)(ç 6 -hexamethylbenzene)rhodium(I) Hexafluorophosphate 

(1,R 1 =R 2 =R 3 = Me; L = CH 2=CH 2); Typical Procedure: [13] 

A mixture of [Rh 2(OCOCF 3) 2(H 2C=CH 2) 4] (0.462 g, 0.85 mmol), TFA (5 mL), and hexamethylbenzene 

(0.380 g, 2.34 mmol) was heated at 508C for 3 h. The resultingsoln was evaporated 

to dryness and the residue was dissolved in the minimum amount of CH 2Cl 2. Careful 

addition of Et 2O gave [Rh(H 2C=CH 2) 2(C 6Me 6)]H(OCOCF 3) 2 as an insoluble precipitate, which 

was dissolved in the minimum amount of acetone. This soln was treated with a sat. aq 

soln of NH 4PF 6 (2.5 mL) to give cream-colored microcrystals of [Rh(H 2C=CH 2) 2(C 6Me 6)]PF 6; 

yield: 0.348 g(88%); mp 1808C (dec); 1 H NMR (acetone-d 6): ä 2.75±2.33 (bm, H 2C=CH 2), 2.35 

(s, CH 3). 

1.5.1.1.5 Variation 5: 

Via Displacement of Weakly Bound Ligands 

Arenes can directly displace weakly bound ligands such as acetone [14] and acetonitrile. [15] 

Complexes of the type [RhCp*(solvent) 3] 2+ (solvent = acetone, acetonitrile, etc.), which are 

generated by reaction of [Rh 2(Cp*) 2Cl 2] with AgPF 6, readily undergo solvent±ligand exchange 

with arenes at room temperature to furnish the desired arene complexes in high 

yields (Table 2). It was found most convenient to use the isolated tris(acetonitrile) complex 

as the startingmaterial since the correspondingtris(acetone) complex is less stable. 

This provides a mild and convenient method for the preparation of cationic rhodium±arene 

complexes which are not easily accessible by other routes. The use of one equivalent 

of the arene per metal gives the optimum yields. Displacement of solvent ligands with arenes 

is reversible in some instances. Thus, the benzene in a rhodium±benzene complex 

can be completely displaced by acetone within 30 minutes at 208C. 

Table 2 Displacement of Weakly Bound Ligands with Arenes [14,15] 

[RhCp ∗ (solvent) 3](PF 6) 2 

Solvent Arene Yield 

(%) 

arene 

[RhCp ∗ (arene)](PF6)2 

Ref Solvent Arene Yield 

(%) 

Me 2CO C 6H 6 61 [14] MeCN 4-H 2NC 6H 4Me 88 [15] 

Me 2CO C 6H 5Me 82 [14] MeCN 4-Me 2NC 6H 4Me 84 [15] 

MeCN PhNH 2 82 [15] MeCN Ph 2NH 83 [15] 

MeCN PhNMe 2 92 [15] MeCN PhOMe 82 [15] 

(ç 6 -N,N-Dimethyl-p-toluidine)(pentamethylcyclopentadienyl)rhodium(I) Hexafluorophosphate 

(1, arene = 4-Me 2NC 6H 4Me; L±L = C 5Me 5); Typical Procedure: [15] 

N,N-Dimethyl-p-toluidine (62.3 mg, 0.5 mmol) was added to a soln of [RhCp*(MeCN) 3]- 

(PF 6) 2 [14] (300 mg, 0.5 mmol) in acetone (20 mL). After stirring the mixture for 5 h at 208C, 

the resultingclear yellow soln was evaporated to 2 mL in vacuo. Et 2O was added to the 

residue, and the resultingcrystalline product was collected by filtration, washed with 

Et 2O, and dried in vacuo to give the title complex as yellow crystals; yield: 279 mg (84%); 

1 H NMR (acetone-d6): 6.94 (d, 2H, Ar), 6.79 (d, 2H, Ar), 2.59 (s, 3H, Ar-CH 3). 

Ref 





Rhodium±Cumulene Complexes 

Transition-metal complexes containingcumulene ligands have received considerable attention 

in organometallic chemistry. The ð-complex chemistry of allenes, the simplest 

cumulenes, has been extensively studied and advances in this area of research have 

been well documented. [16] The chemistry of rhodium±allene complexes will be discussed 

in Section 1.5.4 under rhodium±diene complexes. Transition-metal complexes of higher 

cumulenes are much less known, which is probably due to the lack of general methods 

for preparing higher cumulenes. Cumulapolyenes are also less stable, showing a high propensity 

toward dimerization, oligomerization, and reaction with oxygen, thus making 

their synthesis even more difficult. Rhodium±cumulapolyene complexes are synthesized 

by ligand substitution, often involving displacement of phosphine or carbon monoxide 

ligands with cumulapolyenes. 


1.5.2.1 Method 1: 

Cumulatriene Complexes by Ligand Substitution 

As shown in Scheme 6, rhodium±cumulatriene complexes 2±11 are synthesized by displacement 

of a triphenylphosphine ligand with a cumulatriene. Chlorotris(triphenylphosphine)rhodium 

(Wilkinson s catalyst) is often chosen as the startingmaterial because 

of its low catalytic activity toward cumulene oligomerization. This method is applied 

to a number of cumulatrienes includingthose containingalkyl, [17] quinone, [18] cyclopropyl, 

[19] and group 14 metal [20] substituents as well as cyclonona-1,2,3-triene [21] to afford 

rhodium±cumulatriene complexes in uniformly good yields. These complexes are all airstable, 

crystalline solids which are usually isolated by chromatography on silica gel. Single-crystal 

X-ray analyses reveal ç 2 -bindingof cumulatriene via the central double bond 

and that the cumulatriene and chlorine ligands are trans to each other. These complexes 

adopt a square-planar geometry around the rhodium center. 

Scheme 6 Synthesis of Rhodium±Cumulatriene Complexes by Ligand Substitution [17±21] 

R 1 

R 1 

R 2 

R 3 

7 R 1 = t-Bu 87% 

8 R 1 = iPr 75% 

RhCl(PPh 3) 3 

− Ph 3P 

O O 

R 1 

R 1 

R 1 

Ph3P Rh PPh3 Cl 

R 1 

R 1 

R 1 

• 

R 2 

R 3 


2 R 1 = R 2 = R 3 = Me 95% 

3 R 1 = R 2 = R 3 = Ph 70% 

4 R 1 = Me; R 2 = TES; R 3 = H 62% 

5 R 1 = Me; R 2 = GeEt3; R 3 = H 94% 

6 R 1 = Ph; R 2 = SnEt 3; R 3 = H 44% 

R 1 

R 1 


9 R 1 = Me 74% 

10 R 1 = Ph 72% 


11 57% 


1.5.3 Rhodium±Dienyl Complexes 537 

bChloro[ç 2 -1,1,2,2-tetramethyl-3-(3-methylbuta-1,2-dienylidene)cyclopropane]bis(triphenylphosphine)rhodium(I) 

(9; R 1 = Me); Typical Procedure: [19] 

To a soln of [RhCl(PPh 3) 3] (185 mg, 200 ìmol) in degassed benzene (25 mL) under argon 

was added a soln of 1,1,2,2-tetramethyl-3-(3-methylbuta-1,2-dienylidene)cyclopropane 

(34.1 mg, 210 ìmol; 5% excess) in benzene (5 mL) by syringe. The mixture was stirred for 

2 h at rt and the benzene was removed under vacuum. The resultingred-brown residue 

was purified by chromatography using first MeCl/CCl 4 (1:4) to remove triphenylphosphine 

and excess butatriene, then MeCl/CCl 4 (1:2), and finally MeCl/CCl 4 (1:1) to give a yellow-orange 

solid. This was recrystallized (CH 2Cl 2/pentane) at ±158C to give 9 as air-stable, 

yellow-orange square plates; yield: 121.8 mg (74%); mp 167±1698C (dec); 13 C NMR (CDCl 3): 

ä 135.1 (t, J = 6.1 Hz), 132.4 (t, J = 21.1 Hz), 132.3, 132, 129.9, 127.8 (t, J = 4.8 Hz), 121.2, 

116.4, 28.9, 24.9, 24.6, 23.1, 21.2, 21.1. 

1.5.2.2 Method 2: 

Cumulapentaene Complex by Ligand Substitution 

In a similar manner, a rhodium±cumulapentaene complex 12 can be prepared by displacement 

of a triphenylphosphine ligand in chlorotris(triphenylphosphine)rhodium 

with tetraphenylhexapentaene (Scheme 7). [22] The reaction requires higher temperature 

(refluxingbenzene) and a longer reaction time (21 h) compared to cumulatrienes. The 

rhodium±cumulapentaene complex is obtained as air-stable, red needles after purification 

by silica gel chromatography. Interestingly, rhodium forms a ç 2 -complex with the 

second double bond of tetraphenylhexapentaene, as inferred by 1 H and 13 C NMR studies 

and confirmed by X-ray crystal analysis. The coordination geometry around rhodium can 

be described as square planar with two triphenylphosphine ligands occupying trans positions. 

Scheme 7 Synthesis of a Rhodium±Cumulapentaene Complex by Ligand Substitution [22] 

Ph 

Ph 

Ph 

Ph 

RhCl(PPh3)3, benzene, reflux, 21 h 

− Ph3P 

Ph 

Ph 


12 

Chloro(ç 2 -tetraphenylhexapentaene)bis(triphenylphosphine)rhodium(I) (12): [22] 

A soln of [RhCl(PPh 3) 3] (185 mg, 0.200 mmol) and tetraphenylhexapentaene (76 mg, 

0.20 mmol) was refluxed in benzene (25 mL) for 21 h. The benzene was then removed 

and the residue obtained was chromatographed on silica gel using first hexanes to elute 

the most mobile unreacted cumulene, followed by CHCl 3. The CHCl 3 fraction was concentrated 

and red crystals, which formed upon slow addition of hexanes, were collected by 

filtration to give 12; yield: 180 mg(86%); mp 201±2028C (dec); 1 H NMR (CDCl 3): ä 8.02 (dd, 

2H, J = 7.5, 1.7 Hz), 7.55±7.46 (m, 16H), 7.4±7.2 (m, 15H), 7.16±7.07 (m, 12H), 7.06±6.96 (m, 

3H), 6.69 (dd, 2H, J = 8.1, 1.5 Hz). 

1.5.3 Product Subclass 3: 

Rhodium±Dienyl Complexes 

Cyclopentadienyl and pentamethylcyclopentadienyl ligands are ubiquitous in organotransition-metal 

chemistry. Complexes of these ligands are known for all transition and 

most of the f-block metals, [23] owingto the great stability of the ç 5 -cyclopentadienyl bindingmode. 

In most organometallic reactions of transition-metal complexes the ç 5 -cyclopentadienyl 

ligand plays the role of spectator, staying tightly bound to the metal center 

Ph 

Ph 




bthroughout the course of the reaction. Undoubtedly, rhodium±cyclopentadienyl complexes 

are the most common complexes amongthe rhodium±dienyl complexes. There 

are many methods for preparing ç 5 -cyclopentadienylrhodium complexes. Most of these 

methods rely on the acidity of cyclopentadiene (pK a = 9) and often involve reactions of cyclopentadienyl 

anions with rhodium halide complexes. The synthesis and chemistry of 

other less-common rhodium±dienyl complexes will also be discussed in this section. 


1.5.3.1 Method 1: 

Cyclopentadienylrhodium Complexes by Ligand Substitution 

The term ªhalf-sandwich complexesº refers to complexes that contain only one cyclopentadienyl 

group together with other ligands. These make up a large portion of all cyclopentadienyl 

complexes of rhodium. The most important complex of this series is dicarbonyl(cyclopentadienyl)rhodium, 

which has been used as catalyst in a number of organic 

reactions includinghydroformylation, hydrogenation, and cyclotrimerization of alkynes. 

[24] As mentioned, the primary method for synthesizingcyclopentadienylrhodium 

complexes involves nucleophilic displacement of rhodium halides with cyclopentadienyl 

anions. 

1.5.3.1.1 Variation 1: 

From Cyclopentadienyl Anions and Rhodium Halides 

The simplest method to prepare half-sandwich carbonyl(cyclopentadienyl)rhodium complexes 

13 is from dicarbonylchlororhodium(I) dimer [Rh 2Cl 2(CO) 4] and lithium cyclopentadienide 

(or sodium cyclopentadienide) as shown in Scheme 8. The use of thallium 

cyclopentadienide and cyclopentadienyltrimethylsilane provides a convenient method 

for the introduction of cyclopentadienyl-type ligands to rhodium complexes under mild 

conditions. A number of substituted analogues of 13 have been synthesized by a similar 

route. [25] 

Scheme 8 Cyclopentadienylrhodium Complexes by Ligand Substitution [25,26,28±31] 

Rh 2Cl 2(CO) 4 

R 1 C5H4 − 

R 1 = H, Me, NO 2, CHO, CO 2Me 

OC 

Rh CO 

13 R 1 = H 

Rh 2Cl 2L 4 

RhClL 3 

[RhL 4] + Cl − 

L = PX3, PR 1 3, P(OR 2 )3 

OC 

L 

L = PPh3 80% 

L = PBu3 53% 

L = P[(OCH2)3CMe] 34% 

L = CNCy 61% 

C 5H 4 − 

Rh CO 

13 

L 

Rh 

15 

L 

L 

R 1 

Rh CO 


14


bReplacement of one of the carbonyl ligands of 13 (R 1 = H) by Lewis base ligands (L = phosphines, 

phosphites, or isocyanides) occurs under thermal conditions to yield [RhCp(CO)L] 

14 (Scheme 8). [26] The rates of these reactions are slow because 13 is coordinatively saturated. 

These reaction rates can be enhanced by the presence of electron-withdrawingsubstituents 

on the cyclopentadienyl ring, which increase the stability of the ªring-slippedº 

intermediate. [27] However, a better method to prepare 14 (L = PPh 3) is to use the reaction of 

carbonylchlorobis(triphenylphosphine)rhodium(I) with sodium cyclopentadienide. [28] In 

a similar manner, cyclopentadienylbis(phosphine)rhodium complexes 15 can be synthesized 

more directly by treatment of [Rh 2Cl 2L 4], [RhClL 3], or [RhL 4] + Cl ± with sodium cyclopentadienide 

or thallium cyclopentadienide (Scheme 8). [29] 

Dicarbonyl(cyclopentadienyl)rhodium(I) (13,R 1 = H); Typical Procedure: [30] 

A mixture of [Rh 2Cl 2(CO) 4] (0.55 g, 1.4 mmol) and freshly sublimed TlCp (1.54 g, 

5.72 mmol) in hexane (40 mL) was gently refluxed under N 2 for 19 h. The reaction was 

monitored by IR for the disappearance of carbonyl absorptions attributable to the reactant 

[Rh 2Cl 2(CO) 4] (2033, 2088 cm ±1 ). Filtration under N 2 and removal of solvent yielded 

RhCp(CO) 2 as an orange-red oil; yield: 0.60 g (94%); 1 H NMR (CDCl 3): ä 5.46 (s, 5H, Cp); IR 

(cm ±1 ): 2051 (vs), 1987 (vs). 

Carbonyl(cyclopentadienyl)(triphenylphosphine)rhodium(I) (14, L = PPh 3); 

Typical Procedure: [26] 

A mixture containing[RhCp(CO) 2] (250 mg, 1.1 mmol) and Ph 3P (280 mg, 1.07 mmol) in 

hexane (40 mL) was refluxed for 12 h. After removal of the solvent (about 10 mL) and 

coolingto rt, red crystals precipitated from the soln. The crystals were collected by filtration, 

washed with hexane, and dried at rt under vacuum to give the product; yield: 400 mg 

(80%); mp 1538C. 

Cyclopentadienylbis(trifluorophosphine)rhodium(I) (15, L=PF 3); Typical Procedure: [31] 

A soln of [Rh 2Cl 2(PF 3) 4] [31] (0.72 g, 1.15 mmol) in Et 2O (20 mL) was stirred with TlCp (1.2 g; 

excess) at rt for 30 min. The solvent was then removed in a stream of N 2 and the residue 

was extracted with pentane (3 ” 20 mL). The extract was concentrated and cooled to ±808C 

to give impure crystals which melted on warming to rt. The product was purified by sublimation 

at 25 8C/10 ±3 Torr) onto a ±808C probe to give the product as an orange oil at rt; 

yield: 0.228 g(58%). 

1.5.3.1.2 Variation 2: 

Cationic Bis(cyclopentadienyl) Complexes by Ligand Substitution 

The first ªsandwichº bis(cyclopentadienyl)rhodium complex 16 was synthesized by reactingtris(acetylacetonato)rhodium(III) 

with cyclopentadienyl anion. [32] A later preparation 

usingrhodium(III) chloride in place of tris(acetylacetonato)rhodium(III) provides a cleaner 

synthesis of this rhodacinium cation [RhCp 2] + (16) in much higher yield (Scheme 9). [33] 

Unsymmetrical cationic rhodium sandwich complexes such as 17 have been prepared by 

treatment of [Rh 2(Cp*) 2Cl 4] with cyclopentadiene. [34] Substituted rhodacinium cations 

such as 18 can also be synthesized from anhydrous rhodium(III) chloride and 5,5-dimethylfulvene 

by treatment with isopropylmagnesium bromide, but the yield was too low to 

be synthetically useful. [35,225] 




bScheme 9 Cationic Bis(cyclopentadienyl)rhodium Complexes by 

Ligand Substitution [33±35,225] 

RhCl3 + CpMgBr Rh + 

1. benzene, rt, 18 h, then 70 oC, 1 h 

2. H2O, 0 oC 3. HBr/Br2 or NH4PF6 

Rh + 

PF6 − 

Rh + 

17 18 

Pr i 

Pr i 

PF 6 − 

X − 

16 X = Br3, PF6 

Bis(cyclopentadienyl)rhodium(I) Tribromide (16, X=Br 3); Typical Procedure: [33] 

In a 500-mL 3-necked round-bottom flask charged with Mg turnings (6.0 g, 246 mmol), fitted 

with a N 2 inlet, a droppingfunnel, and a reflux condenser, was added a soln of EtBr 

(22 mL, 294 mmol) in anhyd Et 2O (60 mL) under N 2. After the Mgwas consumed, freshly 

distilled cyclopentadiene (22 mL, 268 mmol) in benzene (60 mL) was added to the soln, 

and the mixture was stirred at rt for 1 h then refluxed for 4 h. After coolingto rt, anhyd 

RhCl 3 (8.0 g, 38.2 mmol) was added and the mixture was stirred at rt for 18 h, then refluxed 

for 1 h. Ice-cold water (150 mL) was added to quench the reaction and the mixture 

was filtered and the two phases separated. The aqueous layer was filtered through activated 

carbon (3 ”) and treated with a soln of KBr and Br 2 (1:1) to precipitate the product. The 

resultingyellow solid was washed several times with small amounts of water and dried 

under vacuum to afford the product; yield: 16.7 g(93%). 

Cyclopentadienyl(pentamethylcyclopentadienyl)rhodium(I) 

Hexafluorophosphate (17); Typical Procedure: [34] 

A mixture of the rhodium±acetone complex, prepared from [Rh 2(Cp*) 2Cl 4] (0.2 g, 

0.32 mmol) and AgPF 6 (0.33 g, 1.34 mmol) in acetone (10 mL), and freshly cracked cyclopentadiene 

(0.06 g, 0.90 mmol) was stirred for 30 min. Removal of the acetone under reduced 

pressure gave an off-white solid which was recrystallized (acetone/Et 2O) to give 17 

as a pure white solid; yield: 0.27 g(93%); 1 H NMR (CD 2Cl 2): ä 7.87 (s, 15H, C 5Me 5), 4.51 (d, 

J H,Rh = 1 Hz, 5H, C 5H 5). 

1.5.3.2 Method 2: 

From Pentamethylcyclopentadienyl Anions and Rhodium Halides 

The pentamethylcyclopentadienyl ligand is widely utilized in transition-metal complexes 

compared to its less-substituted analogues as a consequence of the greater binding stability 

of the pentamethylcyclopentadienyl ligand. This stability may be attributable to the 

electron-donatingeffect of the five methyl groups that help stabilize the cationic species. 

The steric bulk of the pentamethylcyclopentadienyl ligand presumably also adds some kinetic 

stability to the otherwise reactive rhodium center. Thus, while the cyclopentadienyl 

ligand is easily removed from the rhodium center under acidic conditions or in the presence 

of hydrogen, the corresponding pentamethylcyclopentadienyl ligand survives under 

both acidic and basic, as well as reductive and oxidative, conditions. [36] The (pentamethylcyclopentadienyl)rhodium 

complexes, particularly in the +3 oxidation state, are normally 

air stable. 



b1.5.3.2.1 Variation 1: 

Dimeric Pentamethylcyclopentadienyl Complexes 

The first complex of this series, [Rh 2(Cp* 2)Cl 4](20), which is the primary startingmaterial 

for numerous (pentamethylcyclopentadienyl)rhodium complexes, can be synthesized 

through the reaction of rhodium(III) chloride trihydrate with hexamethylbicyclo[2.2.0]hexa-2,5-diene 

(19, hexamethyl-substituted Dewar benzene) (Scheme 10). [37] The 

discovery of this synthetic route was somewhat serendipitous. Both pentamethylcyclopentadiene 

and 1-(1-chloroethyl)pentamethylcyclopentadiene can also be used directly 

in the synthesis of 20. Complex 20 has been used as a catalyst for the hydrogenation of 

arenes and alkenes, [36] as well as for the disproportionation of acetaldehyde. [38] Metathesis 

reactions of 20 with sodium halides/pseudohalides give [Rh 2(Cp*) 2X 4](21, X = Br, I, N 3, 

NCO, or SCN). [37,39] 

Scheme 10 Dimeric (Pentamethylcyclopentadienyl)rhodium Complexes by 

Ligand Substitution [37,39] 

RhCl 3 3H 2O + 

19 

MeOH, 65 o C, 15 h 

NaX 

− NaCl 

X = Br 90% 

X = I 81% 

X = N3 95% 

X = NCO 89% 

X = SCN 82% 

Cl Cl 

Rh Rh 

Cl Cl 

20 

X X 

Rh Rh 

X X 

Dichlorodi-ì-chlorobis(pentamethylcyclopentadienyl)dirhodium(III) (20): [37] 

A mixture of RhCl 3 ·3H 2O (1.00 g, 3.80 mmol) and bicyclohexadiene 19 (2.0 g, 12.3 mmol) 

in MeOH (30 mL) was stirred at 658C under N 2 for 15 h. After coolingto rt, the solvent was 

removed under vacuum. The residue was washed with Et 2O to remove excess hexamethylbenzene 

and the resultingoily red crystals were extracted with CHCl 3. The soln was 

dried (Na 2SO 4), concentrated, and the residue was recrystallized (CHCl 3/benzene) to give 

20 as dark red crystals; yield: 1.10 g(93%); mp >230 8C; IR (cm ±1 ): 456, 281, 243. 

Diazidodi-ì-azidobis(pentamethylcyclopentadienyl)dirhodium(III) (21,X=N 3); 


A suspension of [Rh 2(Cp*) 2Cl 4] (2.00 g, 3.11 mmol) and NaN 3 (1.2 g, 18.7 mmol) in acetone 

(150 mL) was stirred for 18 h, duringwhich time the color of the mixture changed from 

dark red to bright red. The solvent was then removed in vacuo and the product was redissolved 

in CH 2Cl 2. The soln was filtered to remove NaCl and unreacted NaN 3, and crystallization 

of 21 was initiated by the addition of iPr 2O, which provided wine-red crystals; 

yield: 2.00 g(95%); 1 H NMR (CDCl 3): ä 1.67 (s, 15H, C 5Me 5); 13 C NMR (CDCl 3): ä 8.0 (C 5Me 5), 

93.4 (d, J C,Rh= 7.7 Hz, C 5Me 5). 

21 





Monomeric Pentamethylcyclopentadienyl Complexes 

The dimeric framework of 21 (X = Cl, Br, I, NCO, SCN) can be cleaved by bridge-splitting 

reactions upon treatment with donor ligands [L 1 = PPh 3, P(OR) 3, py, RNC, Me 2NNH 2, 

ArNHNH 2] to give neutral monomeric Cp* rhodium complexes 22 in good yield (Scheme 

11). [39±41] The use of two equivalents of donor ligand results in further displacement of another 

halide (X) ligand, affording cationic complexes of the type [RhXCp*L 1 2] + . [41] Likewise, 

the reaction of 22 with another ligand L 2 leads to the formation of the mixed-ligand cationic 

complex [RhCp*XL 1 L 2 ] + (23,L 2 = PPh 3). [42] 

Scheme 11 Monomeric (Pentamethylcyclopentadienyl)rhodium Complexes by 

Ligand Substitution [39±42] 

X X 

Rh Rh 

X X 

21 

L 1 , acetone 

X 

X 

Rh 

22 

L 1 

L 2 

L2 X 

Rh 

L1 + 

Bis(isocyanato)(pentamethylcyclopentadienyl)(triphenylphosphine)rhodium(III) 

(22,L 1 = PPh 3; X = NCO); Typical Procedure: [39] 

A soln of [Rh 2(Cp*) 2(NCO) 4] (0.20 g, 0.31 mmol) and Ph 3P (0.16 g, 0.62 mmol) in acetone 

(30 mL) was stirred at rt for 1 h. The solvent was then removed in vacuo and the product 

was crystallized (CH 2Cl 2/iPr 2O) to afford 22 as orange microcrystals; yield: 0.31 g (88%); 

31 P NMR (CDCl3): ä 33.6 (d, J P,Rh = 132 Hz). 

Chlorobis(methylisocyanido)(pentamethylcyclopentadienyl)rhodium(III) Hexafluorophosphate 

(23, L 1 =L 2 = MeNC, X = Cl); Typical Procedure: [41] 

MeNC (0.26 g, 6.3 mmol) was added to a soln of [Rh 2(Cp*) 2Cl 4](20; 0.5 g, 1.6 mmol) in 

MeOH (25 mL) with stirring. After 5 min, NH 4PF 6 (0.3 g) in MeOH (5 mL) was added to the 

clear orange soln, forming a bright yellow precipitate. The precipitate was filtered, 

washed with H 2O, MeOH, and Et 2O, and dried in vacuo to give the product as bright yellow 

crystals which were recrystallized (CH 2Cl 2/iPr 2O); yield: 0.70 g(87%); IR (cm ±1 ): 2258 (s), 

2242 (s); 1 H NMR (CDCl 3): ä 3.90 (s, 6H, CNCH 3), 2.05 (s, 15H, Cp*CH 3). 

1.5.3.3 Method 3: 

Indenyl Complexes by Ligand Substitution 

Indenylrhodium complexes 24 are usually prepared by displacement of the chlorine ligand 

of rhodium(III) chloride complexes with indenyllithium, although indenylpotassium 

or indenyltrimethylstannane can also be used (Scheme 12). [43,44] The coordinated ethene 

molecule in the bis(ethene)indenylrhodium complex 24 (L 1 =H 2C=CH 2) is readily displaced 

by donor ligands. This air-stable, crystalline complex is particularly useful as a 


23


breadily available precursor for the preparation of other indenylrhodium complexes such 

as 25. Coordinated ethene can be substituted with carbon monoxide, phosphines, fluorophosphines, 

and other diene ligands. [43,44] It should be noted that, unlike their cyclopentadienyl 

analogues, indenylrhodium complexes show high reactivity in ligand substitution 

reactions. This increase in reactivity is attributed to the ease of the indenyl ligand to undergo 

ring slippage to a lower hapticity (pentahapto to trihapto), providing a vacant orbital 

for the incomingnucleophile. [43,44] 

Scheme 12 Indenylrhodium Complexes by Ligand Substitution [43,44] 

Rh 2Cl 2L 1 4 + C 9H 7 − 

L 

Rh 

L1 1 

L1 = 

L1 = CO 70% 

L1 H2C CH2 90% 

= PMe3 54% 24 

L 2 and/or L 2 −L 2 

L 1 = 

H 2C CH 2 Rh L 2 

L 2 

L 2 Yield of 25 (%) Ref L 2 -L 2 Yield of 25 (%) Ref 

PF 3 ~ 100 [43,44] dppe 79 [43,44] 

PF 2NMe 2 93 [43,44] dppp 70 [43,44] 

PF 2CCl 3 97 [43,44] cod 68 [43,44] 

PPh 3 85 [43,44] cot 85 [43,44] 

CO 75 [43,44] isoprene 93 [43,44] 

Bis(ethene)indenylrhodium(I) (24,L 1 =H 2C=CH 2); Typical Procedure: [43] 

A suspension of indenyllithium (0.753 g, 6.17 mmol), prepared by stirring a mixture of 

freshly distilled indene (0.8 mL) in Et 2O (5 mL) with 1.8 M BuLi in hexane (3.5 mL) for 

12 h, was added to a suspension of [Rh 2Cl 2(H 2C=CH 2) 4] (1.200 g, 3.8 mmol) in hexane 

(25 mL). The brown mixture was stirred overnight at rt. The LiCl that precipitated was filtered 

off and the filtrate was concentrated and cooled to give a crystalline product which 

was washed with hexane and dried in vacuo to afford the product as dark yellow, airstable 

crystals; yield: 1.438 g (85%); crystallization of the hexane washings gave an additional 

small amount of the product (0.084 g, 5%); mp 1118C (dec); 1 H NMR (acetone-d 6): ä 

7.96 (d, 8H, H 2C=CH 2), 4.76 (d, 2H), 3.8 (q, 1H), 2.6±2.9 (m, 4H). 

[1,2-Bis(diphenylphosphino)ethane]indenylrhodium(I) (25,L 2 ±L 2 = dppe); 


A mixture of [Rh(C 9H 7)(H 2C=CH 2) 2] (0.100 g, 0.36 mmol) and dppe (0.145 g, 0.36 mmol) in 

benzene (2 mL) was stirred at rt for 7 h. After removal of the solvent and washingwith 

hexane, the product was obtained as a mustard-colored solid; yield: 0.177 g(79%); key 

1 H NMR signals (benzene-d6): ä 4.25 (m, 2H), 3.64 (q, 1H), 2.35 (m, 2H), 2.68 (m, 2H). 

Indenyl(isoprene)rhodium(I) (25,L 2 ±L 2 = isoprene); Typical Procedure: [44] 

An excess of isoprene (3.0 g, 45 mmol) was condensed into a Carius tube (50 mL) containinga 

solution of [Rh(C 9H 7)(H 2C=CH 2) 2] (0.4 g, 1.4 mmol) in hexane (10 mL). After 1 week at 

rt, all volatiles were removed and the residue was extracted with hexane. Chromatogra- 

25 




bphy (alumina) followed by recrystallization (hexane) afforded the product as yellow crystals; 

yield: 0.39 g(93%). 

1.5.3.4 Method 4: 

Acyclic Pentadienyl Complexes by Dehydration 

Protonation of rhodium±ç 4 -dienol complexes 26 results in the formation of ç 5 -pentadienylrhodium 

cations 27 (Scheme 13). [45] 1 H NMR analysis indicates that the initial protonation 

occurs at the terminal carbon of the diene ligand adjacent to the alcohol group, followed 

by elimination of a water molecule. This method is also applicable to the synthesis 

of optically active ç 5 -pentadienyl complexes. Thus, treatment of a diastereomeric mixture 

of the ç 5 -cyclopentadienyl-ç 4 -dienol complex 28 with tetrafluoroboric acid affords the 

cationic pentadienyl complex 29 in high yield as a single diastereomer. [46] 

Scheme 13 Pentadienylrhodium Complexes by Dehydration [45,46] 

R 1 

Rh 

26 

28 

RhCp 

OH 

OH 

HPF6 

R1 = H 89% 

R1 = Me 74% 

R1 = 3-MeOC6H4 85% 

R1 = 4-MeOC6H4 53% 

R1 = 4-FC6H4 72% 

HBF 4 

Rh+ 

29 

27 

R 1 

+ RhCp 

[1¢,2¢,3,3¢,4-ç 5 -4-(But-1¢-enyl)-6,6-dimethylbicyclo[3.1.1]hept-3-enyl](cyclopentadienyl)rhodium(III) 

Tetrafluoroborate (29): [46] 

To a soln of 28 (0.12 g, 0.27 mmol), prepared from nopadiene and [Rh 2Cl 2(H 2C=CH 2) 4], [46] in 

propanoic anhydride at 0 8C was added dropwise a cold (0 8C) soln of HBF 4 (0.02 mL, 50%) in 

propanoic anhydride. The product was precipitated with Et 2O, then recrystallized 

(CH 2Cl 2/Et 2O) to give 29 as a yellow powder; yield: 0.10 g(87%). 

1.5.3.5 Method 5: 

ç 5 -Cyclohexadienyl Complexes 

ç 5 -Cyclohexadienylrhodium complexes have been extensively studied as synthetic intermediates 

and have found many applications in organic synthesis as well as in the total 

synthesis of a number of natural products. Cationic cyclohexadienylrhodium complexes 

are generally active toward a wide range of nucleophiles, including organolithium, -copper, 

and -zinc reagents, as well as metal enolates. The most common methods for the synthesis 

of ç 5 -cyclohexadienylrhodium complexes are ligand substitution, hydride abstraction 

from a cyclohexadiene ligand, and nucleophilic attack on a ç 6 -arene ligand coordinated 

to a cationic rhodium center. The readily available arenes and cyclohexadienes 

(which can also be derived from arenes by Birch reduction) provide versatile startingmaterials 

for the preparation of ç 5 -cyclohexadienylrhodium complexes. 


PF 6 − 

BF 4 −



Via Ligand Substitution 

ç 5 -Cyclohexadienylrhodium complexes can be synthesized by displacement of a chloride 

ligand with phenoxide anions (Scheme 14). [47] Both IR and X-ray analyses indicate substantial 

double-bond character for the C-O bond of the phenoxo moiety. Thus, the structure 

of the phenoxo ligand is best represented by a ç 5 -hexadienoyl ligand as shown in complex 

30. Alternatively, treatment of [RhR 3 L 3] with excess phenol affords 30 directly through 

elimination of R 3 H. [48] 

Scheme 14 Cyclohexadienylrhodium Complexes via Ligand Substitution [47,48] 

R 1 

R 2 OLi 

R 1 

R 1 

R 2 OH 

R 1 

R 3 = Me, Ph, N(TMS) 2 

Rh2Cl2L4 or RhClL3 

− LiCl 

R1 = t-Bu; R2 = Me; L = CO/PPh3 75% 

R1 = t-Bu; R2 = Me; L = PPh3 27% 

R1 = t-Bu; R2 = Me; L2 = cod 42% 

R1 = t-Bu; R2 = Me; L = H2C CH2 48% 

RhR3L3 − R3H R1 = H; R2 = H; L = PPh3 70% 

R1 = t-Bu; R2 = Me; L = PPh3 72% 

R 2 

R 1 

L 

Rh 

30 

R 

L 

1 

Carbonyl(ç 5 -2,6-di-tert-butyl-4-methylcyclohexadienoyl)(triphenylphosphine)rhodium(I) 

(30,R 1 = t-Bu; R 2 = Me; L = CO/PPh 3): [47] 

A soln of [RhCl(CO)(PPh 3) 2] (3.45 g, 5.0 mmol) in toluene (200 mL) was added to a soln of 

[2,6-t-Bu 2-4-MeC 6H 2O·Li(OEt) 2] 2 (2.25 g, 3.7 mmol) in toluene (300 mL) at rt with stirring. After 

the addition was complete, the mixture was heated at 808C for 3 h with stirringand 

then concentrated under vacuum to 100 mL. The mixture was filtered and the filtrate was 

concentrated to 5±10 mL and refiltered. Hexane was added dropwise to the filtrate with 

stirring to give a yellow-orange precipitate. The precipitate was washed with hexane and 

dried under vacuum to afford the product; yield: 2.3 g(75%); IR (cm ±1 ): 1576 (s), 1589 (m); 

31 P NMR (benzene-d6): ä 17.7. 

Phenoxobis(triphenylphosphine)rhodium(I) (30,R 1 =R 2 = H; L= PPh 3); 


A suspension of [RhPh(PPh 3) 3] (0.95 g, 0.98 mmol) and phenol (1.01 g, 10.7 mmol) in degassed 

toluene (10 mL) was warmed to 1008C for 1 h and the resultingred soln was filtered 

while hot. On addition of Et 2O (5 mL), a small amount of an orange flocculant precipitate 

was observed. The filtered soln was allowed to crystallize at rt to give red crystals 

which were washed with toluene (2 ” 1 mL) and dried in vacuo to give the product as the 

tris(phenol)toluene addition compound; yield: 0.75 g(70%); mp 83±858C. 

O 





Cationic Complexes via Hydride Abstraction 

Treatment of the ç 4 -cyclohexadienerhodium complex 31 with trityl ion results in hydride 

abstraction from a cyclohexadiene ligand, affording (ç 5 -cyclohexadienium)rhodium complex 

32 (Scheme 15). [49] This type of cationic cyclohexadienylrhodium complex is susceptible 

to stereoselective nucleophilic attack to form exo-substituted dienerhodium complex 

34. Complex 32 reacts further with a second trityl ion to afford dicationic arenerhodium 

complex 33. 

Scheme 15 Cationic Cyclohexadienylrhodium Complexes via Hydride Abstraction [49] 

Rh 

Tr + BF4 − 

Rh+ − 

BF4 Tr + BF4 − 

31 32 33 

H 

Y 

Rh 

34 Y = OMe, D 

MeO − or BD 4 − 

Rh 

2+ 

(ç 5 -Cyclohexadienium)(cyclopentadienyl)rhodium(III) Tetrafluoroborate (32): [49] 

To a stirred soln of (ç 4 -cyclohexa-1,3-diene)(cyclopentadienyl)rhodium (31; 320 mg, 

0.96 mmol) [49] in CH 2Cl 2 (4 mL) was added an equimolar amount of Tr + BF 4 ± (420 mg, 

1.27 mmol) in CH 2Cl 2 (4 mL). After 2 min, the reaction was quenched with Et 2O (6 mL) 

and the resultingyellow precipitate was filtered and recrystallized (CH 2Cl 2)at±788C to 

provide 32; yield: 345 mg(81%); 1 H NMR (SO 2): ä 7.37 (d, 1H), 7.06 (q, 1H), 5.68 (t, 2H), 

4.40 (t, 2H), 4.14 (s, 5H), 3.12 (t, 1H). 

1.5.3.5.3 Variation 3: 

Via Nucleophilic Addition to an Arene Ligand 

It is known that coordination of an arene to a metal increases the arene s activity towards 

nucleophiles. [12,50] Accordingly, the cationic rhodium±arene complex 35 is a useful startingmaterial 

for the preparation of cyclohexadienylrhodium complex 36 through the addition 

of a nucleophile to the coordinated arene ligand (Scheme 16). Nucleophiles add directly 

to the arene ring, producing exo-substituted cyclohexadienylrhodium complexes 

with complete stereoselectivity. There is no evidence of direct metal participation, which 

may give rise to endo products. No addition to the pentasubstituted cyclopentadienyl ring 

has been observed. 


2BF 4 −


bScheme 16 Nucleophilic Addition to a Coordinated Arene Ligand [12] 

R 1 

R 

35 

2 

R 1 

R 1 R 1 

Rh2+ 

R1 R1 2PF6 − 

Y − 

R1 = R2 = Me; Y = H 95% 

R1 = H; R2 = Et; Y = H 71% 

R1 = H; R2 = Et; Y = PBu3 99% 

R1 = H; R2 = Et; Y = PMe2Ph 93% 

R1 R1 R1 R1 Y 

Rh+ 

R1 R1 (exo-H-1,2,3,4,5,6-ç 5 -Hexamethylcyclohexadienyl)(pentamethylcyclopentadienyl)rhodium(III) 

Hexafluorophosphate (36, R 1 =R 2 = Me; Y = H); Typical Procedure: [12] 

[RhCp*(ç 6 -C 6Me 6)](PF 6) 2 (0.34 g, 0.85 mmol) in H 2O (50 mL) was treated with a soln of NaBH 4 

(0.08 g, 2.11 mmol) in H 2O (5 mL). The suspension dissolved with frothingand a yellowgreen 

precipitate formed which was filtered and the solid was washed with H 2O, then dissolved 

in acetone. Then, aq NH 4PF 6 was added to the acetone soln of the yellow-green precipitate. 

A yellow precipitate formed which was collected by filtration. Recrystallization 

(acetone/Et 2O) gave the title product; yield: 0.26 g (95%); mp 2588C; 1 H NMR (CDCl 3): ä 9.00 

(d, 6H), 8.93 (d, 3H), 8.29 (s, 3H), 8.10 (s, 6H), 7.83 (s, 15H, Cp*-Me), 7.17 (dq, 1H). 

1.5.3.6 Method 6: 

Norbornadienyl Complexes by Dehydration 

Treatment of (1-hydroxyethylnorbornadiene)rhodium complex 37 or 38 with concentrated 

sulfuric acid results in dehydration to form the correspondingcationic norbornadienylrhodium 

complex 39 or 40, which can be isolated as stable hexafluorophosphate salt 

(Scheme 17). [51] The dehydration proceeds with complete stereospecificity, affording 39 

or 40 with the methyl group in the syn or anti position dependingon the configuration 

of the hydroxyethyl complex (37 or 38). The result is explained well by adoptinga transition 

state in which the leavinggroup (a water molecule in this case) is located anti to the 

metal center. These reactions are found to be reversible. Under aqueous alkaline conditions, 

complexes 39 or 40 are converted back into their hydroxyethyl precursors 37 and 

38, with retention of configuration. This result clearly indicates the occurrence of exo attack 

of hydroxide anion on the ç 3 -allyl complex. 

Scheme 17 Norbornadienylrhodium Complexes by Dehydration [51] 

1. H + 

2. NH4PF6 Rh 

OH 

PF 

− 

6 − 

Rh 

R1 R2 H 

R4 R3 37 R 1 = Me; R 2 = OH 

38 R 1 = OH; R 2 = Me 

+ 

39 R 3 = Me; R 4 = H 93% 

40 R 3 = H; R 4 = Me 92% 

1¢,2,3-ç 3 -2-[(Cyclopentadienyl)-1-rhodaethyl]norbornadiene Hexafluorophosphate (40); 


Sulfuric acid (3±4 drops) was added to a soln of 38 (0.2 g, 0.065 mmol) in Et 2O (15 mL) in an 

inert gas atmosphere and the mixture was stirred for 15±20 min. The solvent was decanted 

from the dark yellow oil formed, and the oil was washed with Et 2O (2 ” 10 mL) and dissolved 

in distilled H 2O (2 mL). To the resultingbright yellow soln, a soln of NH 4PF 6 (0.12 g, 

R 

36 

2 

PF 6 − 




b0.074 mmol) in H 2O (0.5 mL) was added dropwise with stirringto give a yellow precipitate. 

The precipitate was collected by filtration, washed with H 2O, dried in air, dissolved in 

CH 2Cl 2 (1±2 mL), and reprecipitated from Et 2O (150 mL) to give 40 as an amorphous yellow 

powder; yield: 0.26 g(92%). 

1.5.3.7 Method 7: 

ç 5 -Cycloheptadienyl Complexes by Nucleophilic Addition 

Reaction of cycloheptatriene with dichlorodi-ì-chlorobis(pentamethylcyclopentadienyl)dirhodium(III) 

and silver(I) hexafluorophosphate in acetone directly leads to the formation 

of the (ç 6 -acetylmethylcycloheptadienyl)rhodium complex 42 (Y = CH 2COMe) with 

complete stereoselectivity (Scheme 18). [33] The reaction proceeds through the formation 

of dicationic (ç 6 -cycloheptatriene)rhodium complex 41, followed by exo nucleophilic addition 

of acetone to the cycloheptatriene ligand of 41 to give 42 (Y = CH 2COMe). In a similar 

manner, the reaction in methanol rapidly yields 42 (Y = OMe) as a bright yellow solid. 

Scheme 18 Nucleophilic Addition to a ç 6 -Cycloheptatriene Ligand [33] 

Rh2+ 

YH 

Y = CH2Ac 77% 

Y = OMe 63% 

Rh+ 

41 

(ç 6 -Acetylmethylcycloheptadienyl)(pentamethylcyclopentadienyl)rhodium(III) 

Hexafluorophosphate (42, Y=CH 2COMe); Typical Procedure: [33] 

A soln of [Rh 2(Cp*) 2Cl 4] (0.4 g, 0.64 mmol) and AgPF 6 (0.65 g, 2.58 mmol) in acetone (10 mL) 

was stirred to give the acetone-solvated dicationic rhodium complex. Cycloheptatriene 

(0.12 g, 1.3 mmol) was then added to the filtered soln. After stirring for 25 min, the solvent 

was removed under reduced pressure and the residue was washed with Et 2O to give the 

product as a yellow solid; yield: 0.49 g(77%); recrystallization (acetone/Et 2O) gave the 

pure complex; key signals in 1 H NMR (CDCl 3): ä 6.46 (t, 1H, J = 5Hz), 5.59 (t, 1H, J = 8), 5.21 

(dd, 1H, J = 5.5), 4.48 (m, 1H, J = 8.5), 4.41 (m, 1H), 2.05 (s, 15H). 

1.5.3.8 Method 8: 

ç 5 -Cyclooctadienyl Complexes 

Early syntheses of cyclooctadienylrhodium complexes involved hydride abstraction 

from (cycloocta-1,5-diene)rhodium complexes [52] [e.g., RhCp(C 8H 12) +Ph 3CBF 4 ® Rh(Cp- 

CPh 3(C 8H 11) + ] and protonation of cyclooctatriene complexes [e.g., RhCp(C 8H 10) + 

CF 3CO 2H]. [53] These methods are inefficient in that the products are obtained in low yield. 

Protonation approaches are also inefficient since the products formed are highly unstable 

and are prone to isomerization. A more practical synthesis of cyclooctadienyl complexes 

involves deprotonation of a cationic cyclooctadienerhodium complex (Scheme 19). [33] The 

reaction requires at least six equivalents of cyclooctadiene to completely displace two coordinated 

solvent molecules to give a cyclooctadienerhodium intermediate, which subsequently 

undergoes deprotonation of the cyclooctadiene ligand to give the ç 5 -cyclooctadienylrhodium 

complex 43. Cyclooctadienylrhodium complexes such as 43 readily undergo 

addition reactions with nucleophiles at room temperature to give the corresponding 

neutral rhodium complexes. 


Y 

42 

H

1.5.4 Rhodium±Diene Complexes 549 

bScheme 19 Deprotonation of Cyclooctadiene Ligands [33] 

cod 

[RhCp ∗ (OCMe2) 3] 2+ − 

2PF6 [RhCp ∗ (cod)(OCMe2)] 2+ 2PF 

− 

6 

− H + 

Rh + Cp ∗ 

(1±3:5,6-ç 5 -Cyclooctadienyl)(pentamethylcyclopentadienyl)rhodium(III) 

Hexafluorophosphate (43): [33] 

A soln of [Rh 2(Cp*) 2Cl 4] (0.2 g, 0.32 mmol) and AgPF 6 (0.33 g, 1.34 mmol) was stirred in acetone 

(10 mL). The exothermic reaction gave a white precipitate of AgCl and a yellow soln 

of the dicationic rhodium±tris(acetone) complex. The soln was filtered through a short 

column (cellulose, acetone 5 mL). Then cod (0.42 g, 3.9 mmol) was added to the combined 

filtrate and washings and the mixture stirred for 24 h at rt. The soln was filtered again and 

concentrated under reduced pressure. Crystallization of the residue (acetone/Et 2O) gave 

yellow-green crystals of 43; yield: 0.23 g(73%). 


Rhodium±Diene Complexes 

Transition-metal±diene complexes are generally categorized as conjugated or nonconjugated 

diene complexes. Nonconjugated dienes have similar binding modes to those of 

monoalkenes. Complexes of conjugated dienes, although less common than those of nonconjugated 

dienes, have gained considerable attention owing to their diversity of binding 

modes. Amongthe nonconjugated dienes, it can be said that cyclic diene complexes of 

the type [Rh 2(ì-Cl) 2L 4](L 2 = cod, nbd) are the most important. These rhodium complexes 

are used as startingmaterials for the syntheses of mononuclear cationic rhodium±diene 

complexes, a very important class of complex that is widely used as catalyst precursors 

for a large number of organic transformations. In general, nonconjugated diene complexes 

of rhodium are synthesized by two methods. The first method uses the reduction 

of rhodium(III) chloride hydrate by a diene in refluxingaqueous alcohol. This method is 

also used for the syntheses of simple rhodium±alkene complexes. The second method is 

based on the displacement of two labile ligands by a diene. The former method is more 

direct, while the latter is more versatile because of its mild reaction conditions. For conjugated 

dienes, neutral rhodium(I) complexes constitute most of this class of complex, although 

many cationic complexes are known. For organization purposes, rhodium complexes 

of allenes (or 1,2-dienes), although not usually ç 4 , are also covered in this product 

subclass. 


1.5.4.1 Method 1: 

Allene Complexes by Ligand Substitution 

The ð-complex chemistry of allenes (1,2-dienes), the simplest cumulenes, has been extensively 

studied and well documented in terms of synthesis, structure, [16] and reactivity. [54] 

Allenes exhibit a high tendency toward dimerization and oligomerization in the presence 

of rhodium(I) complexes. In general, rhodium±allene complexes are synthesized by ligand 

substitution reactions in which phosphine or ethene ligands are replaced by allenes. 

Allene replaces one triphenylphosphine molecule of halotris(triphenylphosphine)rhodi- 

43 

PF6 − 




bum(I) in benzene at room temperature to give allene(halo)bis(triphenylphosphine)rhodium(I) 

(44) as yellow-orange, air-stable crystals (Scheme 20). [55] Both ethene molecules of 

(acetylacetonato)bis(ethene)rhodium(I) can be displaced by two allene molecules to give 

complex 45. [56] 

Scheme 20 Rhodium±Allene Complexes [55±58] 

H 

H 

H 

H 

PPh3 Rh X 

PPh3 

R1 R1 R1 R1 Rh 

44 X = Cl, Br, I 45 R 1 = H, Me 46 R 1 = H, Me; R 2 = Me, CF3 

O 

R 

OC 

CO 

Rh Rh 

1 2C CR1 2 

O O O O O 

R 2 

R2 R2 Displacement of carbonyl ligands provides another approach to rhodium±allene complexes. 

Reaction of (acetylacetonato)dicarbonylrhodium(I) with allene or tetramethyallene 

affords the unusual dimeric complex 46 in which both ð units of the allene molecule 

coordinate to a rhodium center. [57] These ligand substitution reactions with allenes are reversible. 

Allene(chloro)bis(triphenylphosphine)rhodium(I) (44, X = Cl); Typical Procedure: [55] 

Gaseous allene was bubbled into a benzene soln (10 mL) of [RhCl(PPh 3) 3] (0.5 g, 0.54 mmol) 

under N 2 at rt. After 3 h, the solvent was partially removed in vacuo. Yellow crystals 

formed and were filtered off, washed with EtOH, and dried in vacuo. Recrystallization 

(benzene) gave the pure product; yield: not given; mp 140±1428C; 1 H NMR (for allene 

only) (CDCl 3): ä 4.35 (1H), 4.08 (1H), 0.53 (2H); IR (cm ±1 ): 1730 (C=C), 835 (=CH 2), 302 

(Rh-Cl). 

(Acetylacetonato)bis(tetramethylallene)rhodium(I) (45, R = Me); Typical Procedure: [58] 

Addition of tetramethylallene (0.25 g, 2.60 mmol) to a suspension of [Rh(acac)(H 2C=CH 2) 2] 

(0.4 g, 1.55 mmol) in MeOH (12 mL) at rt under N 2 led to dissolution of the complex with 

evolution of ethene gas. After 3 h the soln was cooled in a freezer and, within a few days, 

orange crystals developed which were collected by filtration under N 2; yield: not given; 

the complex decomposed very rapidly in air; 1 H NMR (4 Me groups of allene only) 

(CDCl 3): ä 2.44, 1.96, 1.49 [J Rh,Me » 1 Hz] , 0.83 [J Rh,Me = 2 Hz]. 

1.5.4.2 Method 2: 

Alka-1,3-diene Complexes 

Complexes of 1,3-dienes and related conjugated polyalkenes show considerable delocalization 

in their bondingto rhodium. The delocalization of the electrons of a conjugated 

diene is due to the interaction of its extended ð-systems with a larger number of metal 

orbitals. In general, complexes of conjugated dienes are synthesized by ligand substitution 

or by displacingcoordinated ligands with excess 1,3-dienes. Other approaches include 

the isomerization of coordinated 1,4-dienes by a hydrogen shift mechanism or the 

formation of 1,3-dienes by rhodium-mediated cyclodimerization of alkynes. Since thermal 

rearrangement of coordinated 1,4-dienes yields a number of isomeric rhodium±1,3diene 

complexes, this approach is not a practical method for the selective synthesis of 

specific rhodium±1,3-diene complexes. 


R 2




Reduction of rhodium(III) chloride trihydrate with dienes in an alcohol solvent provides a 

direct method for the synthesis of rhodium±1,3-diene complexes. The reaction with butadiene 

yields monomeric complex 47 (R 1 =R 2 =R 3 =R 4 = H), [59] while those with cyclic 1,3-dienes 

such as cyclohexa-1,3-diene and cycloheptadiene give dimeric, chlorine-bridged 

complexes 48 [R 1 ,R 6 = (CH 2) 2 or (CH 2) 3;R 2 =R 3 =R 4 =R 5 = H] (Scheme 21). [60] Complexes 47 

and 48 form yellow crystals which are sensitive to air, but stable under nitrogen at room 

temperature. 

Bis(ç 4 -buta-1,3-diene)chlororhodium(I) (47, R 1 =R 2 =R 3 =R 4 = H); Typical Procedure: [59] 

Butadiene (0.7 g, 12.9 mmol) and RhCl 3 ·3H 2O (0.1 g, 0.38 mmol) were dissolved in EtOH 

(99.5%, 6 mL) and the resultingsoln was kept at ±58C for 8±10 days to give the title complex 

as yellow crystals; yield: not given; mp 38±40 8C (dec); IR (cm ±1 ): 1479, 1375. 

Di-ì-chlorobis(1,2,3,4-ç 4 -cyclohexa-1,3-diene)dirhodium(I) (48,R 1 ±R 6 =CH 2CH 2; 

R 2 =R 3 =R 4 =R 5 = H); Typical Procedure: [60] 

A mixture of RhCl 3 ·3H 2O (340 mg, 1.29 mmol) and cyclohexa-1,3-diene (0.6 mL, 

6.29 mmol) in MeOH/H 2O (5:1, 6 mL) was stirred for 4 h at rt. The resultingsolid was recrystallized 

(petroleum ether/CH 2Cl 2 6:1) to give the product as yellow, long needle crystals; 

yield: 114 mg(37%); mp 115±1208C (dec). 

1.5.4.2.2 Variation 2: 


Replacement of weakly bound ligands such as simple alkenes by dienes provides a convenient 

method for the synthesis of rhodium±1,3-diene complexes. The alkene ligands of 

[Rh 2Cl 2L 4](L=H 2C=CH 2,C 8H 14) can be easily replaced with a diene by heatingthe complex 

under mild conditions in the presence of excess 1,3-diene (2±10 equivalents). [61] Dependingon 

the nature of the substituents on the 1,3-diene used, the product could be either a 

monomeric complex 47 or a dimeric complex 48. It is noteworthy that the formation of 

47 or 48 is independent of the stoichiometry of the reactants. The reason why a particular 

diene forms only one type of chloro complex is not well understood. Both types of complex 

are red-orange solids (Table 3) which are stable in the solid state under nitrogen at 

room temperature, but decompose rapidly in organic solvents even under an inert atmosphere. 

Similarly, the ethene ligands of (acetylacetonato)bis(ethene)rhodium(I) can be displaced 

by 1,3-dienes (2±12 equivalents) in diethyl ether at room temperature for 2 hours 

to give the complexes 49 as red-orange solids (Scheme 21). 




bScheme 21 Rhodium±1,3-Diene Complexes [59,60] 

R 3 

R 6 

R 1 

R 4 

R 5 

R 2 

R 2 

H 

H 

R 4 

Rh 

R 3 

49 

R 1 

R 

Rh 

1 

Cl 

R 2 

H 

H 

R 3 

R 4 

R 6 

R 1 

R 5 

R 2 

Rh Cl 

Cl Rh 

R4 R3 R3 R4 47 48 

O 

O 

Table 3 Rhodium Diene Complexes 47±49 via Displacement of Weakly Bound Ligands [61] 

Complex Type Diene Colormp (8C) Rh-Cl (cm ±1 ) Ref 

47 buta-1,3-diene yellow 38±40 ± [61] 

2-chlorobuta-1,3-diene orange 140±142 (dec) 233 [61] 

isoprene yellow 42 (dec) 228 [61] 

trans-penta-1,3-diene red 42±44 (dec) 225 [61] 

trans-2-methylpenta-1,3-diene orange 135±137 (dec) 235 [61] 

trans-3-methylpenta-1,3-diene orange 135±138 (dec) 244 [61] 

48 2,3-dichlorobuta-1,3-diene orange 170±173 (dec) 233 [61] 

cis-3-methylpenta-1,3-diene orange 120±124 (dec) 220 [61] 

trans,trans-hexa-2,4-diene red 132±135 (dec) 278, 247, 220 [61] 

cis,trans-hexa-2,4-diene red 40±44 (dec) 236 [61] 

2,3-dimethylbuta-1,3-diene red 140 (dec) 227 [61] 

2,4-dimethylpenta-1,3-diene orange 40 (dec) 215 [61] 

trans,trans-1,4-diphenylbuta-1,3-diene red 228±231 (dec) 270 [61] 

49 2-chlorobuta-1,3-diene red 99±102 ± [61] 

2,3-dichlorobuta-1,3-diene yellow 156±159 (dec) [61] 

trans-penta-1,3-diene red 100±103 (dec) ± [61] 

cis-penta-1,3-diene red oil ± [61] 

trans-2-methylpenta-1,3-diene orange 85±88 (dec) ± [61] 

trans-3-methylpenta-1,3-diene red 95±97 (dec) ± [61] 

cis-3-methylpenta-1,3-diene red 65 (dec) ± [61] 

trans,trans-hexa-2,4-diene orange 128±130 (dec) ± [61] 

cis,trans-hexa-2,4-diene orange 40 (dec) ± [61] 

2,3-dimethylbuta-1,3-diene orange ± ± [61] 

2,4-dimethylpenta-1,3-diene red ± ± [61] 

Rhodium±Diene Complexes 47 and 48; General Procedure: [61] 

An excess of the 1,3-diene (2±10 mmol) was added to a suspension of [Rh 2Cl 2(H 2C=CH 2) 4] 

(1 mmol) in Et 2O (30 mL) and the mixture was refluxed for 5 min, with evolution of ethene 

gas. The red filtrate was concentrated to 5±10 mL to induce precipitation of the solid prod- 


R 2 

R 5 

R 1 

R 6


buct. If no solid was formed at this stage, the solution was cooled to ±788C. If this still did 

not initiate precipitation, all the solvent was removed and the red oily residue was triturated 

with cold pentane. Recrystallization (pentane/Et 2O) gave complex 47 or 48 as redorange 

solids; yield: ³70%. 

1.5.4.2.3 Variation 3: 

Cationic Complexes via Ligand Substitution 

Treatment of a tetrahydrofuran solution of [Rh(nbd)(PPh 3) 2] + with molecular hydrogen at 

room temperature, followed by reaction with an excess amount of diene (buta-1,3-diene 

or cyclohexa-1,3-diene) gives the corresponding cationic rhodium±1,3-diene complex 50 

as a red-orange solid (Scheme 22). [62] This method provides a highly efficient route to such 

cationic complexes. The reaction proceeds through hydrogenation of the norbornadiene 

ligand with formation of [RhH 2(PPh 3) 2S 2] + (S = solvent). This type of dihydridorhodium intermediate 

has been found to serve as an active catalyst species in a large number of homogeneous 

hydrogenation reactions. 

Scheme 22 Cationic Rhodium Complexes via Ligand Substitution [62] 

Ph3P + 

Rh 

Ph3P ClO 4 − 

1. H2, THF, 1 atm, 0.5 h 

2. R1CH CH CH CHR2 R1 = R2 = H 90% 

R1 ,R2 = (CH2) 2 94% 

Ph3P + 

Rh 

Ph3P (ç 4 -Cyclohexa-1,3-diene)bis(triphenylphosphine)rhodium(I) Perchlorate 

[50,R 1 ,R 2 = (CH 2) 2]; Typical Procedure: [62] 

H 2 (760 Torr) was introduced to a degassed solution of [Rh(nbd)(PPh 3) 2] + ClO 4 ± (500 mg, 

0.61 mmol) in THF (10 mL) and the suspension stirred vigorously for 0.5 h to give a pale 

yellow soln. Upon addition of cyclohexa-1,3-diene (0.25 mL, 2.6 mmol) there was an immediate 

formation of a deep red colored solution followed by precipitation of a red-orange 

powder. The product was filtered and excess Et 2O was added to the filtrate followed 

by filtration to collect additional product. The two crops were combined, washed with 

Et 2O, and air dried to give the title complex; yield: 463 mg (94%); 1 H NMR (CDCl 3): ä 8.80 

(m, 4H, methylene), 5.99 (2H, alkene), 4.30 (2H, alkene). 

1.5.4.2.4 Variation 4: 

Cyclobutadiene Complexes via Alkyne Cyclodimerization 

Although several different approaches to the syntheses of rhodium±cyclobutadiene complexes 

have been reported, the most practical method involves cyclodimerization of two 

acetylene units. [63] As shown in Scheme 23, the reaction of di-ì-chlorotetrakis(trifluorophosphine)dirhodium(I) 

with excess diphenylacetylene proceeds smoothly to afford the 

dimeric, chlorine-bridged, rhodium±cyclobutadiene complex 51 in moderate yield as 

air-stable, red crystals. Phosphine ligands rather than carbon monoxide are used in the 

startingrhodium complex to avoid carbonyl insertion reactions, which form cyclopentadienone. 

The major byproduct in this reaction is hexaphenylbenzene, which is 

formed in a substantial amount. Unfortunately, this approach is limited to diphenylacetylene. 

Reactions with other alkynes of type R 1 CºCR 2 (R 1 =R 2 =CF 3, Me, CO 2Me; R 1 = Ph, 

R 2 = TMS; R 1 = Ph, R 2 =H;R 1 = Ph, R 2 = Me) give only substituted benzene derivatives. It appears 

that di-ì-chlorotetrakis(trifluorophosphine)dirhodium(I) acts primarily as a catalyst 

for the acetylene trimerization process. Complex 51 reacts with thallium acetylacetonate 

and cyclopentadienylthallium to afford rhodium±cyclobutadiene complexes 52 (red crystals) 

and 53 (yellow crystals), respectively, in good yields. 

50 

R 1 

R 2 

ClO 4 − 




bScheme 23 Synthesis of Rhodium±Cyclobutadiene Complexes [63] 

Rh 2Cl 2(PF 3) 4 

Ph Ph 

50% 

Rh Cl 

Cl Rh 

Ph Ph Ph Ph 

Ph Ph Ph Ph 

51 

hfacac = hexafluoroacetylacetonato 

Tl(acac) or 

Tl(hfacac) 

R1 = Me 87% 

R1 = CF3 63% 

TlCp 

80% 

Ph 

Ph 

Ph 

Ph Ph 

Ph 

Rh 

52 

Rh 

Ph Ph 

53 

Macrocyclic diynes react with dicarbonyl(cyclopentadienyl)rhodium in refluxingcyclooctane 

to afford tricyclic rhodium±cyclobutadiene complexes of type 54 as air-stable, pale 

yellow solids (Scheme 24). [64] Although the isolated yields of 54 are somewhat low owing 

to losses duringthe purification, the initial product yields appear to be much higher. Reactions 

of simple alkynes RCºCR (R = Et, Ph) under similar conditions failed to give any 

rhodium±cyclobutadiene complex. 

Scheme 24 Synthesis of Tricyclic Rhodium±Cyclobutadiene Complexes [64] 

( )m 

( )n 

RhCp(CO)2, cyclooctane, heat, 2−3 d 

M n Yield (%) mp (8C) 1 H NMR (ppm) Ref 

1 1 21 96±98 5.07 (5), 2.23 (8), 1.7 (8) [64] 

1 2 68 82±83 5.13 (5), 2.3 (4), 1.9 (10), 1.6 (4) [64] 

1 3 55 58±60 5.10 (5), 2.1 (8), 1.6 (12) [64] 

2 3 39 92±94 5.17 (5), 2.0 (8), 1.5 (14) [64] 

Di-ì-chlorobis(tetraphenylcyclobutadiene)dirhodium(I) (51): [63] 

A mixture of [Rh 2Cl 2(PF 3) 4] (0.257 g, 0.4 mmol) and PhCºCPh (0.356 g, 2 mmol) was refluxed 

in hexane (30 mL) for 8 h. The color of the soln changed from yellow to dark red. 

The brown-orange precipitate of hexaphenylbenzene was removed by filtration. The filtrate 

was evaporated to dryness and the solid residue was recrystallized (benzene/hexane) 

to afford 51 as red crystals, which are air stable in the solid state as well as in solution; 

yield: 0.195 g(50%); mp 2008C (dec); 1 H NMR (CDCl 3): ä 7.6 (2H), 7.2 (3H). 

(Cyclopentadienyl)(cyclobutadienyldicycloalkanyl)rhodium(I) (54); General Procedure: [64] 

A soln of [RhCp(CO) 2] (0.52 g, 2.33 mmol) and a cycloalkadiyne (4.7±5.2 mmol) in cyclooctane 

(25 mL) was refluxed for 2±3 days. The resultingmixture was chromatographed (alumina, 

hexane). Removal of the volatiles under reduced pressure, crystallization of the res- 


( ) m 

Rh 

54 

( ) n 

O 

O 

R 1 

R 1


bidue (hexane) at low temperature, and a final sublimation at 50±70 8C/0.1 Torr gave complexes 

54 as air-stable, white to pale yellow solids; yield: 21±68%. 

1.5.4.2.5 Variation 5: 

ç 4 -Cyclopentadiene Complexes via Nucleophilic Addition 

to ç 5 -Cyclopentadienyl Ligands 

The bis(cyclopentadienyl)rhodium cation 55 reacts with a variety of nucleophiles to give 

neutral (ç 4 -cyclopentadiene)rhodium complexes 56 (Scheme 25). Nucleophiles attack the 

cyclopentadienyl exclusively from the exo face. Reduction of 55 with sodium borohydride 

in ethanol proceeds smoothly to give cyclopentadiene(cyclopentadienyl)rhodium (56) as 

orange-yellow crystals after vacuum sublimation at 130 8C. [65] Complex 56 (R 1 = H) is stable 

indefinitely in an inert atmosphere, but decomposes slowly in air. Similarly, reaction of 

55 with other nucleophiles such as phenyllithium or cyclopentadienylsodium in diethyl 

ether at room temperature affords neutral cyclopentadienyl(1-exo-phenylcyclopentadiene)rhodium(I) 

(56, R 1 = Ph) and cyclopentadienyl(1-exo-cyclopentadienylcyclopentadiene)rhodium(I) 

(56, R 1 = Cp), respectively, as yellow solids. [35,225] These complexes 

are stable in air but decompose slightly in solution. The exo stereochemistry of 56 (R 1 = Ph) 

was confirmed by comparingits X-ray diffraction powder photographs and IR absorptions 

with those of cyclopentadienyl(1-exo-phenylcyclopentadiene)cobalt. [65] 

Scheme 25 Synthesis of Neutral ç 4 -Cyclopentadiene Complexes [35,65,225] 

Rh + 

X − 

R 1− 

Rh 

55 56 

X− = Br3, Cl 

R 1 

H 

R 1 Solvent Time Yield (%) Colormp (8C) Ref 

H EtOH 30 min 80 orange-yellow 121±122 [65] 

Ph Et2O 2 min 69 yellow 118.5±119.5 [35,225] 

Cp Et2O 30 min ± yellow 79±80 [35,225] 

Cyclopentadienyl(1-exo-phenylcyclopentadiene)rhodium(I) (56,R 1 = Ph); 

Typical Procedure: [35,225] 

A mixture of [RhCp 2]Br 3 (1 g, 2.12 mmol; see Section 1.5.3.1.2) and several Zn pellets in 

THF (10 mL) was stirred at rt with for 24 h. The gray precipitate was removed by filtration 

and washed with small amounts of THF. This precipitate was then suspended in Et 2O 

(50 mL) in a 3-necked, round-bottomed flask under N 2. To this suspension was added 

PhLi (0.335 g, 4 mmol) in Et 2O (4 mL) with vigorous stirring. After stirring for 2 min, several 

small pieces of dry ice and then H 2O (50 mL) were added. The yellow Et 2O layer was separated 

and concentrated. From the residue, biphenyl was sublimed at 65 8C, and then 56 

(R 1 = Ph) at 95±1008C under high vacuum; yield: 0.45 g (69%). The resulting yellow solid 

was chromatographed (alumina, cyclohexane) and again sublimed to give pure product; 

mp 118.5±119.58C. 





ç 4 -Cyclopentadienone Complexes 

Reaction of tetracarbonyldi-ì-chlorodirhodium(I) with excess alkyne (R 1 = Ph, Et, Me) in 

refluxingbenzene results in acetylene cyclodimerization±carbonylation and smoothly affords 

(ç 4 -cyclopentadienone)rhodium complexes 57 as shown in Scheme 26. [66] The byproducts 

in this reaction are hexasubstituted benzene, [Rh 2Cl 2(CO)(R 1 CºCR 1 ) 2], and occasionally 

tetrasubstituted benzo-1,4-quinone. Under a high pressure of carbon monoxide 

(76 ” 10 3 Torr) at 1508C, tetracarbonyldi-ì-chlorodirhodium(I) becomes a catalyst in the 

conversion of hexafluorobut-2-yne into tetrakis(trifluoromethyl)cyclopentadienone. [67] 

Complex 57 (R 1 = Ph) can also be synthesized in good yield (65%) directly by reacting tetraphenylcyclopentadienone 

and tetracarbonyldi-ì-chlorodirhodium(I) in refluxingbenzene. 

The dimeric, chlorine-bridged complexes 57 undergo several bridge-cleaving reactions 

to give monomeric cyclopentadienonerhodium complexes. [66] Reaction of 57 

(R 1 = Ph) with pyridine for 2 h in dichloromethane at 258C gives the complex chlorobis(pyridine)(tetraphenylcyclopentadienone)rhodium(I) 

in high yield (90%) after recrystallization 

from petroleum ether. Triphenylphosphine reacts with 57 (R 1 = Ph) in benzene 

for 30 minutes to give chloro(tetraphenylcyclopentadienone)(triphenylphosphine)rhodium(I) 

in good yield (83%) as a dark red solid after recrystallization from dichloromethane/ 

petroleum ether. Nucleophilic displacement of the chloride ligand of 57 (R 1 = Ph) using 

thallium(I) acetylacetonate in benzene for 12 hours affords (acetylacetonato)(tetraphenylcyclopentadienone)rhodium(I) 

(58,R 1 = Ph) in high yield (88%) as a red precipitate after recrystallization 

from dichloromethane/petroleum ether. 

Scheme 26 (ç 4 -Cyclopentadienone)rhodium Complexes via Acetylene Cyclodimerization± 

Carbonylation [66] 


R 1 = Ph, Et 

R1 R1 heat 

R1 = Ph 47% 

R1 = Et 40% 

R 1 

Rh Cl 

Cl Rh 

57 

58 

Tl(acac) 

The generally accepted mechanism for the metal-mediated acetylene cyclooligomerization, 

which is responsible for the formation of metal±cyclobutadiene and ±cyclopentadienone 

complexes as well as metal±benzene complexes, is illustrated in Scheme 27. [1,68] 

The initial displacement of two ligands on the metal by two alkyne molecules, forming 

metal±dialkyne intermediate 59, is followed by oxidative couplingto yield metallacyclopentadiene 

60. Evidence for the formation of 60 is obtained by isolation and characterization 

of this intermediate with a number of metals, includingrhodium. [1±3] Subsequent reductive 

elimination and complexation affords cyclobutadiene complex 61 (see Section 

1.5.4.2.4). Intermediate 60 can also undergo carbonyl insertion to give metallacyclohexadienone 

62, followed by reductive elimination and complexation to yield metal±cy- 


O 

R 1 

R 1 

R 1 

R 1 

O 

R 1 

R 1 

R 1 

Rh 

O 

O 

R 1 

R 1 

R 1 

O 

R 1 

+ 

R 1 

R 1 

R 1 

R 1 

R 1 

R 1


bclopentadienone complex 63, which gives free cyclopentadienone upon decomplexation. 

Alternatively, the coordination of another alkyne to 60, followed by alkyne insertion, 

gives metalacycloheptatriene 64 that rearranges to form 66. Also, a Diels±Alder-type cycloaddition 

to form 65, followed by reductive elimination and complexation gives the 

metal±arene complex 66, which yields the arene upon decomplexation or by ligand exchange 

with unreacted alkyne. 

Scheme 27 Mechanism of Metal-Mediated Acetylene Cyclooligomerization [1,68] 

2 R 1 R 1 

R 1 

R 1 

R 1 

R 1 

R1 R1 R 1 

MLn−2 

R 1 

R 1 

R 1 

R 1 

R 1 R 1 

61 

R 1 

ML n−2 

59 60 62 

R 1 

R 1 

ML n−2 

R 1 R 1 

R 1 

R 1 

O 

R 1 

MLn−2 

O 

MLn−2 R 

MLn−2 

1 

R1 R1 R1 R 

MLn−2 

1 

R1 R1 R1 R 

64 63 

1 

R1 R1 R1 R1 R1 R 1 

R1 66 

MLn 

R 

MLn−2 

1 M 

R1 R1 R1 Ln−2 R 1 

Di-ì-chlorobis(tetraphenylcyclopentadienone)dirhodium(I) (57,R 1 = Ph); 


A mixture of [Rh 2Cl 2(CO) 4] (0.5 g, 1.25 mmol) and PhCºCPh (1.35 g, 7.5 mmol) in benzene 

(20 mL) was refluxed for 24 h. After an insoluble dark brown byproduct (0.11 g, 12%) was 

removed by filtration, the filtrate was chromatographed (Florisil, benzene) to give hexaphenylbenzene 

(0.12 g, 9%), and a red solid which eluted with CH 2Cl 2/EtOH (9:1). Recrystallization 

(cyclohexane) gave the product; yield: 0.63 g (47%); mp 316±3188C; IR (cm ±1 , 

KBr): 1621 (C=O), 1647. 

R1 

R 1 

65 

R 1 

R 1 




b1.5.4.3 Method 3: 

Cycloocta-1,5-diene Complexes 

Rhodium complexes of nonconjugated dienes are synthesized either by ligand substitution 

or by displacement of weakly bound ligands. The first method is based on the direct 

reduction of rhodium(III) chloride trihydrate by a diene. This route is convenient and frequently 

used to synthesize complexes of simple alkenes. The second approach involves 

displacement of two alkenes or carbonyl ligands from a rhodium complex by a diene. 

This method is versatile and applicable to a wide variety of dienes. Replacement of two 

monoalkenes of a rhodium complex by a bidentate dialkene was shown to be strongly 

exothermic. [69] 

1.5.4.3.1 Variation 1: 


A mixture of rhodium(III) chloride trihydrate and an excess amount of cycloocta-1,5-diene 

in aqueous ethanol on heating for 18 hours gives the dimeric, chlorine-bridged (cycloocta- 

1,5-diene)rhodium complex 67 (X = Cl) in excellent yield as an air stable yellow-orange solid 

(Scheme 28). [70] Interestingly, a similar reaction using cycloocta-1,3-diene at 508C for 

24 hours also affords 67 (X = Cl) in fairly good yield (66%). [71] Except for the longer induction 

period, the rate of formation of 67 (X = Cl) usingcycloocta-1,3-diene is comparable to 

that usingcycloocta-1,5-diene. It should be noted that no isomerization of the 1,3-diene 

other than that required for conversion to 67 (X = Cl) occurs in this reaction, i.e. excess 

diene recovered from the reaction is solely cycloocta-1,3-diene. On the other hand, in 

the reaction usingcycloocta-1,5-diene, careful analysis shows that all excess cycloocta- 

1,5-diene isomerizes either to cycloocta-1,3-diene or to cycloocta-1,4-diene. [72] Recently, a 

microwave technique was developed for the rapid synthesis of 67 (X = Cl) as well as its 

norbornadiene analogue. [73] Usinga microwave oven operatingat a frequency of 

2450 MHz, 67 (X = Cl) can be conveniently synthesized from rhodium(III) chloride trihydrate 

and excess cycloocta-1,5-diene in ethanol/water (5:1) in a sealed Teflon container. 

The reaction is complete in less than 1 minute, giving the product in excellent yield 

(91%). Complex 67 (X = Cl) can be converted into the correspondingbromide complex 67 

(X = Br) or iodide complex 67 (X = I) by shakingan acetone solution with finely powdered 

lithium bromide or sodium iodide, respectively. [70] The iodide complex is rather unstable, 

especially in solution. 

Scheme 28 (Cycloocta-1,5-diene)rhodium Complexes [70±74] 

Rh 

X 

X 

Rh 

67 X = Cl, Br, I 

Di-ì-chlorobis(cycloocta-1,5-diene)dirhodium(I) (67, X = Cl); Typical Procedure: [74] 

Reprinted from (Giordano; Crabtree, Inorganic Syntheses), Copyright (1979), p 218, 

with permission from Inorganic Synthesis Inc. 

A 100-mL, two-necked, round-bottomed flask fitted with a reflux condenser was charged 

with RhCl 3 ·3H 2O (2.0 g, 7.6 mmol) and Na 2CO 3 ·10H 2O (2.2 g, 7.7 mmol). Under N 2, degassed 

EtOH/H 2O (20 mL, 5:1) and cod (3 mL, 24 mmol) were added and the mixture refluxed with 

stirring for 18 h to form the product as a yellow-orange precipitate. After cooling, the precipitate 

was collected by filtration, washed with pentane and then with MeOH/H 2O (1:5) 

until the washings no longer contained chloride ions. The product was dried in vacuo to 

give 67 (X = Cl); yield: 1.67 (94%); mp 2568C; 1 H NMR (CDCl 3): ä 4.3 (8H), 1.7±2.6 (16H); IR 

(cm ±1 , Nujol mull): 998, 964, 819. 


Di-ì-bromobis(cycloocta-1,5-diene)dirhodium (67, X = Br); Typical Procedure: [70] 

A suspension of [Rh 2Cl 2(cod) 2] (3 g, 6.1 mmol) and finely powdered LiBr (3 g, 35 mmol) in 

acetone (200 mL) was shaken until an orange opalescent soln was obtained. This mixture 

was filtered and the solvent removed in vacuo. The residue was extracted with CHCl 3, and 

the extract was evaporated to give the crude product; yield: 3 g (85%). This was redissolved 

in cold CHCl 3 and purified by treatingwith charcoal followed by reprecipitation with 

light petroleum to give pure 67 (X = Br); mp 207±2128C (dec). 

1.5.4.3.2 Variation 2: 


Displacement of two alkene or carbonyl ligands from a rhodium complex by a diene provides 

an alternative route to rhodium±diene complexes. Complex 67 (X = Cl) can be prepared 

by treatinga methanol solution of di-ì-chlorotetrakis(ethene)dirhodium(I) with cycloocta-1,5-diene. 

[75] Ethene ligands are completely displaced by the diene within 2 min at 

room temperature. Reaction of (acetylaconato)dicarbonylrhodium(I) with excess cycloocta-1,5-diene 

in petroleum ether affords dienerhodium complex 68 (R 1 =R 2 = Me; 

X = H). [76] In this reaction, carbon monoxide ligands are displaced by the diene after 30 

minutes at room temperature. In a similar manner, complexes 68 (R 1 =R 2 = Me, t-Bu, CF 3, 

Ph, MeC 6H 4, MeOC 6H 4, ClC 6H 4,O 2NC 6H 4; X = H, Cl) are prepared from (acetylacetonato)dicarbonylrhodium(I) 

or (acetylacetonato)bis(ethene)rhodium(I) (Table 4). [77,78] Complexes 

68 are usually air-stable solids and have colors ranging from yellow to orange. Alternatively, 

reaction of 67 with the anion of a â-diketone provides a more practical method for the 

syntheses of complexes 68 (X = H). [79] Reaction of cyclopentadienylbis(ethene)rhodium(I) 

(69) with excess cycloocta-1,5-diene in diphenyl ether results in replacement of the two 

ethene ligands by the diene to give cycloocta-1,5-diene(cyclopentadienyl)rhodium (70) 

(Scheme 29). [80] However, the most efficient route to 70 is the reaction of cyclopentadienyl 

anions with di-ì-chlorobis(cycloocta-1,5-diene)dirhodium (67, X = Cl). [70] 

Table 4 Rhodium±Cycloocta-1,5-diene Complexes via Displacement of Weakly Bound Ligands [76±78] 

X 

X 

Rh 

68 

O 

O 

R 2 

R 1 

R 1 R 2 X Yield 

(%) 


mp (8C) Ref R 1 R 2 X Yield 

(%) 

mp (8C) Ref 

Me Me H ± 125±128 (dec) [76] Ph Ph Cl 94 216±218 [78] 

t-Bu t-Bu H 95 185 [77] MeC 6H 4 MeC 6H 4 Cl 66 240±242 [78] 

CF 3 CF 3 H 74 124 [77] MeOC 6H 4 MeOC 6H 4 Cl 76 226±228 [78] 

Me Me Cl 94 133.5±134.5 [78] ClC 6H 4 ClC 6H 4 Cl 89 218±220 [78] 

Me Ph Cl 46 153±154 [78] O 2NC 6H 4 O 2NC 6H 4 Cl 94 >350 [78] 

Scheme 29 Synthesis of Cycloocta-1,5-diene(cyclopentadienyl)rhodium 

via Alkene Ligand Substitution [80] 

Rh 

cod (14 equiv), Ph2O, 136 o C, 1 h 

69 70 

Rh 




b(Acetylacetonato)(cycloocta-1,5-diene)rhodium(I) (68,R 1 =R 2 = Me; X = H); 


A mixture of [Rh(acac)(CO) 2] (0.1 g, 0.38 mmol) and cod (2.0 mL, 16.3 mmol) in petroleum 

ether was stirred at rt for 30 min, and then all the volatiles were evaporated. Extraction of 

the residue with petroleum ether, followed by concentration and crystallization, gave the 

product as yellow crystals; yield: not given; mp 125±1288C (dec); 1 H NMR (CCl 4): ä 5.21 (s, 

1H, acac-CH), 3.97 (br, 4H, C=CH), 2.73±1.63, (m, 8H, CH 2), 1.88 (s, 6H, CH 3). 

(Acetylacetonato)(1,6-dichlorocycloocta-1,5-diene)rhodium(I) (68,R 1 =R 2 = Me; X = Cl); 


To a suspension of [Rh(acac)(H 2C=CH 2) 2] (0.26 g, 1 mmol) in toluene (2 mL) was added 1,6dichlorocycloocta-1,5-diene 

(0.3 mL, 1.7 mmol). After the evolution of ethene had 

stopped, the mixture was filtered, the soln concentrated, and the resultingyellow crystals 

washed (light petroleum ether) to give the product; yield: 0.35 g (94%); mp 133.5±134.58C. 

1.5.4.3.3 Variation 3: 

Homoleptic Cationic Cycloocta-1,5-diene Complexes 

via Anionic Ligand Abstraction 

Homoleptic cationic dienerhodium complexes, [Rh(diene) 2] + (71, diene = cod, nbd), are 

important precursors for many organorhodium complexes, including chiral complexes 

which exhibit extremely high catalytic activities in a large number of organic transformations. 

In general, homoleptic, cationic rhodium±diene complexes are prepared by two 

routes as shown in Scheme 30. The first method includes (1) the removal of chloride 

ligand from [Rh 2Cl 2(diene) 2] with a silver(I) salt to give an intermediate complex, [Rh(diene)(solvent) 

n] + , that is reasonably stable in solution under an inert atmosphere, and (2) 

the reaction of [Rh(diene)(solvent) n] + with an equimolar amount of the same diene to 

give 71, which can be isolated as reasonably air-stable, red crystalline solids with anions 

such as hexafluorophosphate. [5] It should be noted that removal of the chloride ligand of 

[Rh 2Cl 2(diene) 2]byAg + can be carried out in the presence of the diene to afford 71 directly 

in excellent yield. [81] Alternatively, removal of the acetylacetonato ligand of [Rh(acac)(diene)] 

(diene = cod, nbd) by trityl tetrafluoroborate (1 equiv) in the presence of an excess 

amount of the same diene affords 71 as orange-red crystals. [82] Mixed diene complexes, 

[Rh(diene 1 )(diene 2 )] + , such as [Rh(cod)(nbd)] + (72), are also prepared usingthese methods. 

[5] 

Scheme 30 Synthesis of Homoleptic Cationic 

(Cycloocta-1,5-diene)rhodium Complexes [5,81,82] 

Rh 2Cl 2(diene) 2 

Rh(acac)(diene) 

diene = cod, nbd 

Ag + 

− AgCl 

Tr + X − 

diene, rt 

[Rh(diene)(solvent) n] + 

[Rh(diene) 2] + X − 

72 

71 

diene 

+ 

Rh X − 



bBis(cycloocta-1,5-diene)rhodium(I) Tetrafluoroborate (71, diene = cod; X = BF 4) 

via Chloride Ligand Abstraction; Typical Procedure: [81] 

To a soln of [Rh 2Cl 2(cod) 2] (1.47 g, 2.98 mmol; see Section 1.5.4.3.1) in CH 2Cl 2 (20 mL) under 

aN 2 atmosphere was added cod (1.1 mL, 8.97 mmol) followed by a soln of AgBF 4 (1.33 g, 

6.83 mmol) in acetone (10 mL). This resulted in the immediate formation of a deep red 

soln, containinga white precipitate, which was stirred for 20 min and then filtered 

through Celite. To the filtrate was added THF (20 mL) and the soln was concentrated to 

10 mL to form orange-red crystals which were filtered, washed with THF (2 ” 5 mL) and 

Et 2O, and then air dried to give the title compound; yield: 2.35 g (97%) (for characterization 

data, see below). 

Bis(cycloocta-1,5-diene)rhodium(I) Tetrafluoroborate (71, diene = cod; X = BF 4) 

via Acetylacetonato Ligand Abstraction; Typical Procedure: [82] 

To a yellow soln of [Rh(acac)(cod)] (2.8 g, 9.1 mmol) and cod (5 mL, excess) in CH 2Cl 2 

(20 mL), was added Tr + BF 4 ± (2.98 g, 9.1 mmol) in CH2Cl 2 (40 mL) dropwise at 258C with stirring. 

After the addition was complete, the resulting deep red soln was stirred for an additional 

5 min and then added to Et 2O (400 mL) to give the product as orange-red crystals; 

yield: 3.44 g(94%); mp 206±2088C; 1 H NMR (CDCl 3): ä 5.33 (s, 8H, CH=CH), 2.53 (s, br, 

16H, CH 2CH 2); IR (cm ±1 , Nujol mull): 1060 (BF 4), 1030 (BF 4). 

1.5.4.3.4 Variation 4: 

Monomeric Cycloocta-1,5-diene Complexes 

Reaction of di-ì-chlorobis(cycloocta-1,5-diene)dirhodium(I) (67, X = Cl) with a stoichiometric 

amount of a donor ligand in chloroform, dichloromethane, or benzene leads to 

chlorine-bridge cleavage and the formation of the corresponding monomeric complex, 

[RhCl(cod)L] (73, diene = cod) (Scheme 31). [70,83±86] Donor ligands (L) can be a phosphine, 

[70,83] phosphite, [83] amine, [70,84] or pyridine. [75,85,86] However, the presence of two or 

more equivalents of the donor ligand in a polar solvent results in total dissociation of 

the halide ion and production of tetracoordinate, cationic rhodium±diene complexes 74 

(diene = cod). [62] These complexes 74 can be readily isolated from the solution by introducinga 

suitable counterion such as hexafluorophosphate, perchlorate, tetrafluoroborate, or 

tetraphenylborate. Donor ligands such as a phosphine, amine, or nitrile have been employed. 

[62,87,88] The formation of 74 from 67 (X = Cl) may proceed through the monomeric 

complex 73. The mechanism of the chlorine ligand displacement by donor ligands at a 

rhodium center is most likely to induce the formation of a pentacoordinate complex, 

[RhX(diene)L 2], with subsequent loss of halide ion to form 74. [62] Another widely used 

method for the synthesis of 74 (diene = cod) is based on the reaction of [Rh(cod) 2] + (71, diene 

= cod) with an excess amount of a donor ligand (Scheme 31). [87,88] Upon mixing, one 

mole of cod is immediately replaced by two moles of the donor ligand to give 74. The displacement 

of the cod ligand in 71 requires a large excess of a monodentate ligand (10:1), 

but only a slight excess is needed for a bidentate ligand (1.2:1). 

Alternatively, removal of the acetylacetonato ligand of [Rh(acac)(diene)] (68, diene = 

cod; 77, diene = nbd) by reaction with an equimolar amount of trityl tetrafluoroborate [89] 

or perchloric acid [62] followed by addition of the donor ligand affords 74 in good to high 

yields (Scheme 31). This method is particularly useful for the synthesis of complexes such 

as [Rh(nbd)(AsPh 3) 2] + , [Rh(cod)(diphos)] + , and [Rh(nbd){P(OPh) 3} 2] + , which cannot be obtained 

in satisfactory yields by direct methods using 67, 71, 75, or76. 




bScheme 31 Synthesis of Monomeric Dienerhodium Complexes [62,70,83±90] 

1/2 

Rh Cl Rh 

Cl 

67 X = Cl; diene = cod 

75 diene = nbd 

L 

− L 

Rh 

73 

L 

Cl 

L 

− L 

Rh 

71 diene = cod 


Rh 

74 

2L (or L L) 

L 

L 

+ 

Tr + BF4 − or 

HClO4, 2L 



Chloro(cycloocta-1,5-diene)(triphenylphosphine)rhodium(I) (73, diene = cod; L = PPh 3); 


Ph 3P (1.07 g, 4.0 mmol) was added to a soln of [Rh 2Cl 2(cod) 2] (1 g, 2.0 mmol) in CH 2Cl 2, and 

the resultingsoln was concentrated to dryness under reduced pressure. The residue was 

washed with MeOH, filtered, and recrystallized (EtOH) to give the product; yield: not given; 

mp 1468C (dec). 

(Cycloocta-1,5-diene)bis(triphenylphosphine)rhodium(I) Hexafluorophosphate 

(74, diene = cod; L = PPh 3;X=PF 6); Typical Procedure: [62] 

To a mixture of [Rh 2Cl 2(cod) 2] (500 mg, 1.01 mmol) in CH 2Cl 2 (10 mL) and KPF 6 (500 mg) in 

H 2O (10 mL) was added Ph 3P (2.0 g, 7.6 mmol) with vigorous stirring. After 15 min, the 

CH 2Cl 2 layer was separated, washed with H 2O (3 ” 10 mL), and reduced to ca. 5 mL with a 

flow of N 2. To this soln was slowly added EtOH (5 mL), followed by slow addition of Et 2O 

(ca. 10 mL) to complete crystallization of the product. The resultingorange crystals were 

collected by filtration, washed with benzene and Et 2O, and air dried to give the product; 

yield: 1.65 g(90%); 1 H NMR (CDCl 3): ä 4.61 (4H, CH=CH), 2.42 (m, 8H, CH 2CH 2). 

Other complexes were synthesized in a similar manner: [Rh(cod)(PPh 3) 2] + ClO 4 ± ; yield: 

>90%; [Rh(cod)(PPh 3) 2] + BPh 4 ± ; yield: 95%; [Rh(cod)(PPh2Me) 2] + ClO 4 ± ; yield: 86%. 

(Cycloocta-1,5-diene)[1,2-bis(diphenylphosphino)benzene]rhodium(I) Trifluoromethanesulfonate 

[(74, diene = cod; L = 1,2-(Ph 2P) 2C 6H 4; X = OTf]; Typical Procedure: [90] 

To a soln of [Rh(cod) 2]OTf (144 mg, 0.307 mmol) in CH 2Cl 2 (5 mL) was added dropwise a 

soln of 1,2-bis(diphenylphosphino)benzene (146 mg, 0.326 mmol) in CH 2Cl 2 (4 mL) at rt. 

The mixture was stirred for 30 min and then concentrated (ca. 1 mL) under vacuum. Et 2O 

(15 mL) was added to the mixture, and the resultingprecipitate was washed with Et 2Oto 

afford the product as an orange solid; yield: 239 mg (97%); 1 H NMR (CDCl 3): ä 2.24±2.60 (m, 

8H), 5.08 (br s, 4H), 7.40±7.70 (m 24H); 31 P{ 1 H} NMR: ä 58.2 (d, J Rh,P = 149 Hz). 


Rh 

O 

O 

+ 

X − 

X −


b(Cycloocta-1,5-diene)bis(triphenylphosphine)rhodium(I) Tetrafluoroborate 

(74, diene = cod; L = PPh 3;X=BF 4) via Reaction with Trityl Tetrafluoroborate; 

General Procedure: [89] 

To a soln of [Rh(acac)(cod)] in CH 2Cl 2 was added an equimolar amount of Tr + BF 4 ± . The resultingdark 

soln was then diluted with Et 2O and Ph 3P was added. The orange precipitate 

which immediately formed was separated and recrystallized (CH 2Cl 2/Et 2O) to give the 

product; yield: not given; 1 H NMR (CDCl 3): ä 7.35 (30H, Ph-H), 4.66 (4H, CH=CH), 2.9±1.8 

(8H, CH 2). 

(Cycloocta-1,5-diene)bis(triphenylarsine)rhodium(I) Perchlorate (74, diene = cod; 

L = AsPh 3; X = ClO 4) via Reaction with Perchloric Acid; Typical Procedure: [62] 

To a soln of [Rh(acac)(cod)] (250 mg, 0.79 mmol) in THF (3 mL) was added HClO 4 (115 mg, 

0.8 mmol; 70%) in THF (1 mL). Addition of Ph 3As (418 mg, 1.36 mmol) to this soln yielded 

an orange soln which slowly deposited crystals. Et 2O was added as needed to complete the 

crystallization. The crystals were collected by filtration, washed with Et 2O, and air dried 

to give the product; yield: 630 mg (88%); 1 H NMR (CDCl 3): ä 4.76 (4H, CH=CH), 2.60 (4H, 

CH 2), 2.08 (4H, CH 2). 

Other complexes were synthesized in a similar manner: [Rh(cod){P(OPh) 3} 2] + ClO 4 ± ; 

yield: 74%; [Rh(cod)(diphos)] + ClO 4 ± ; yield: 76%; [Rh(cod)(arphos)] + ClO4 ± (employing1-diphenylarsino-2-diphenylphosphinoethane); 

yield: 80%. 

1.5.4.4 Method 4: 

Norbornadiene Complexes 

As mentioned above, rhodium complexes of norbornadiene as well as cycloocta-1,5-diene 

are the most important amongrhodium complexes of nonconjugated dienes because 

these species serve as versatile startingmaterials for the syntheses of a variety of other 

rhodium complexes. Mononuclear cationic rhodium complexes of norbornadiene and cycloocta-1,5-diene 

have attracted considerable interest because of their practical utility as 

catalyst precursors for homogeneous hydrogenation and a number of other organic transformations. 

Practically, most of the synthetic procedures used for the synthesis of rhodium±cycloocta-1,5-diene 

complexes are also applicable to the syntheses of rhodium±norbornadiene 

complexes, i.e. rhodium±norbornadiene complexes are synthesized either 

by direct reduction of rhodium(III) chloride with norbornadiene or by displacement of 

weakly bound ligands. 1 H NMR studies of a number of rhodium±diene complexes suggest 

that rhodium±norbornadiene complexes are thermodynamically the most stable but kinetically 

the most labile. [91] A study indicates that the thermodynamic stability of rhodium±diene 

complexes decreases in the followingorder: norbornadiene > cycloocta-1,5-diene 

> cycloocta-1,3,5,7-tetraene. [69] 

1.5.4.4.1 Variation 1: 


Norbornadiene and other dienes can reduce rhodium(III) chloride, but the reduction by 

norbornadiene requires milder reaction conditions as compared to cycloocta-1,5-diene 

(Section 1.5.4.3.1). Thus, shakingan ethanol solution of rhodium(III) chloride trihydrate 

and excess norbornadiene at room temperature for 2 days yields the dimeric rhodium± 

norbornadiene complex 78 (R 1 =R 2 = H; X = Cl) (Scheme 32). [92] This reaction can also be 

carried out at 508C, which shortens the reaction time to only 6 hours. [62] The correspondingbromo 

complex 78 (R 1 =R 2 = H; X = Br) is easily obtained by anion metathesis using 

lithium bromide. [93] 




bScheme 32 Dimeric Rhodium±Norbornadiene Complexes [62,92±95] 

R 2 

R 2 

R 2 

R 2 

R 1 

R 1 

Rh 

R 1 = R 2 = H; X = Cl, Br 

X 

X 

78 

Rh 

R 1 

R 1 

R 2 

R 2 

Bis(bicyclo[2.2.1]heptadiene)di-ì-chlorodirhodium(I) (78,R 1 =R 2 =H;X=Cl); 


A soln of RhCl 3 ·3H 2O (0.7 g, 3.3 mmol) and nbd (2 mL, 18.5 mmol) in aq EtOH (10 mL) was 

shaken for 2 d at rt. The resultingyellow precipitate was recrystallized (hot CHCl 3/light 

petroleum ether) to give the product as fine yellow crystals; yield: 0.62 g (81%); mp 

2408C (dec); 1 H NMR (CDCl 3): ä 3.92 (t, 8H, CH=CH), 3.85 (4H, CH), 1.17 (4H, CH 2); IR 

(cm ±1 ): 1449 (C=C). 

Bis(bicyclo[2.2.1]heptadiene)di-ì-bromodirhodium(I) (78,R 1 =R 2 = H; X = Br); 


An aq soln of RhCl 3 ·3H 2O (0.4 g, 1.9 mmol) and solid LiBr (0.5 g, 5.7 mmol) was shaken for 

30 min prior to addition of nbd (1.5 mL, 13.9 mmol) and reacted for 2 d. The solid that 

formed was collected by filtration, dried, and recrystallized (CHCl 3) to afford the product 

as a bright yellow powder; yield: 0.2 g (40%); mp 190±2008C (dec). 

1.5.4.4.2 Variation 2: 


An alternative synthesis of rhodium±norbornadiene complexes is based on the displacement 

of two weakly bound ligands such as an alkene or carbonyl by norbornadiene. Reaction 

of di-ì-chlorotetrakis(ethene)dirhodium(I) with excess 2,3-bis(methoxycarbonyl)norbornadiene 

in pentane leads to rapid evolution of ethene gas and the formation of 

the substituted rhodium±norbornadiene complex 78 (R 1 =CO 2Me; R 2 = H; X = Cl) in high 

yield. [94] Buta-1,3-diene can also be displaced by norbornadiene to afford rhodium±diene 

complexes in nearly quantitative yields. [91] Thus, reaction of 1,3-bis(butadiene)chlororhodium 

with an equivalent amount of norbornadiene or a substituted norbornadiene 

at room temperature in a nitrogen atmosphere gives very pure dimeric rhodium±diene 

complex 78 (R 1 =R 2 = H; X = Cl). Displacement of butadiene is facile and the reaction is 

driven by its volatility. Norbornadiene can also displace carbonyl ligands from carbonylrhodium 

complexes but more forcingreaction conditions are required. [95] Thus, reaction 

of tetracarbonyldi-ì-chlorodirhodium(I) with hexachloronorbornadiene in petroleum 

ether for 3 hours affords 78 (R 1 =R 2 = X = Cl) in good yield. The latter complex can also be 

synthesized by direct reduction of rhodium(III) chloride trihydrate with excess hexachloronorbornadiene 

in refluxingethanol for 24 hours. [94] The latter route is impractical 

owingto its low product yield, however. 

In a similar manner, acetylacetonato(norbornadiene)rhodium complexes 80 (R 1 =H, 

Cl; R 2 = Me, CF 3, t-Bu) are readily synthesized by replacingethene and carbonyl ligands of 

79 with norbornadiene (Scheme 33). [76,77] It should be noted that no evidence for displacement 

of a carbonyl ligand by dienes such as butadiene, 1,4-diphenylbutadiene, or 2,5-dimethylhexa-2,4-diene 

has been reported. A more common route to complexes 80 is based 

on the nucleophilic substitution reaction of 78 with the enolate of a â-diketone. [96] 


R 2 

R 2


bScheme 33 Acetylacetonoto(norbornadiene)rhodium Complexes 

via Displacement of Weakly Bound Ligands [76,77,95] 

L 

L 

Rh 

O 

O 

79 

L = H 2C CH 2, CO 

R 2 

R 2 

nbd 

− 2CO 

R 1 

R 1 

R 1 

Rh 

R1 80 

Di-ì-chlorobis{2,3-bis(methoxycarbonyl)bicyclo[2.2.1]heptadiene}dirhodium(I) 

(78,R 1 =CO 2Me; R 2 = H; X = Cl): [94] 

To [Rh 2Cl 2(H 2C=CH 2) 4] (1.36 g, 3.50 mmol) suspended in pentane was added 2,3-bis(methoxycarbonyl)norbornadiene 

(2.0 g, 8.5 mmol), and the mixture was stirred at rt for 

30 min. Evaporation of the solvent to dryness gave an oily residue which was recrystallized 

(CH 2Cl 2/pentane) to give the product as orange-yellow prisms; yield: 1.97 g (81%); 

mp 178±180 8C. 

Di-ì-chlorobis(hexachlorobicyclo[2.2.1]heptadiene)dirhodium(I) (78,R 1 =R 2 = X = Cl); 


A mixture of [Rh 2Cl 2(CO) 4] and hexachloronorbornadiene in petroleum ether was heated 

at 608C for 3 h. After removal of the solvent, the residual solid was recrystallized (CHCl 3/ 

petroleum ether 3:1) to give 78 (R 1 =R 2 = X = Cl) as air-stable, yellow needles, which were 

further purified by sublimation at 190±2108C/10 ±3 Torr; yield: 70%; mp 296±2988C (dec); 

1 H NMR (CDCl3): ä 4.7 (d, CH=CH); IR (cm ±1 , KBr): 1380 (C=C). 

Acetylacetonato(bicyclo[2.2.1]heptadiene)rhodium(I) (80,R 1 =H;R 2 = Me): [76] 

A mixture of [Rh(acac)(CO) 2] (0.4 g, 1.55 mmol) and nbd (5 mL, 46 mmol) in benzene 

(20 mL) was refluxed for 24 h. After removal of the gelatinous norbornadiene polymer 

formed in the reaction, the solvent was evaporated. Purification of the crude product by 

chromatography (alumina, CH 2Cl 2) afforded the product as yellow needles; yield: 0.4 g 

(88%); mp 175±1778C; 1 H NMR (CCl 4): ä 5.16 (1H, acac-CH), 3.80 (6H, norbornadiene-H), 

1.84 (6H, CH 3), 1.19 (2H, bridgehead-CH). 

1.5.4.4.3 Variation 3: 

Homoleptic, Cationic Norbornadiene Complexes 

via Anionic Ligand Abstraction 

As discussed in Section 1.5.4.3.3, homoleptic, cationic norbornadiene complexes 

[Rh(nbd) 2] + can be synthesized by removal of an anionic ligand such as chloride or acetylacetonate, 

followed by reaction with excess norbornadiene (see Scheme 30). Accordingly, 

reaction of [Rh 2Cl 2(nbd) 2] with silver(I) tetrafluorophosphate in tetrahydrofuran under argon 

forms a discrete monomeric ionic complex [Rh(nbd)(solvent) n] + PF 6 ± , with concomitant 

precipitation of silver(I) chloride. [5] Subsequent reaction of [Rh(nbd)(solvent) n] + PF 6 ± 

with norbornadiene gives [Rh(nbd) 2] + PF 6 ± (76, X=PF6) (Scheme 34) as red crystals. An efficient 

route to 76 combines the displacement of an ethene ligand by norbornadiene and 

chloride ligand abstraction by a silver(I) salt in one pot. [81] Thus, reaction of di-ì-chlorotetrakis(ethene)dirhodium(I) 

with silver(I) tetrafluoroborate in the presence of excess norbornadiene 

in dichloromethane affords [Rh(nbd) 2] + BF 4 ± (76, X=BF4) in quantitative yield. 

Alternatively, reaction of (acetylacetonato)(norbornadiene)rhodium(I) with an equimolar 

amount of trityl tetrafluoroborate in the presence of excess norbornadiene also affords 

76 (X = BF 4) in high yield. [82] 

R 1 

R 1 

O 

O 

R 2 

R 2 




bScheme 34 Rhodium±Norbornadiene Complexes [81,82] 

Rh 

+ 

76 X = PF 6, BF 4 

X − 

Bis(bicyclo[2.2.1]heptadiene)rhodium(I) Tetrafluoroborate (76,X=BF 4) 

via Chloride Ligand Abstraction and Ethene Ligand Substitution: [81] 

To an ice-cooled suspension of [Rh 2Cl 2(H 2C=CH 2) 4] (1.5 g, 3.86 mmol) in CH 2Cl 2 (45 mL) 

was added a soln of nbd (2.0 mL, 19.3 mmol) in CH 2Cl 2 (15 mL). When the gas evolution 

had ceased (~30 min), solid AgBF 4 (2.25 g, 11.6 mmol) was added in one portion, and the 

soln was stirred for 45 min at rt. The resultingred soln, containinga white precipitate, 

was filtered by cannula through Celite, and THF (10 mL) was added. The mixture was 

then concentrated to ~5 mL, giving red needles which were collected by filtration out, 

washed with THF (2 ” 5 mL) and Et 2O, and dried in vacuo to give the product; yield: 2.9 g 

(100%); for characterization, see below. 

Bis(bicyclo[2.2.1]heptadiene)rhodium(I) Tetrafluoroborate (76,X=BF 4) 

via Acetylacetonato Ligand Abstraction: [82] 

A soln of Tr + BF 4 ± (1.79 g, 5.4 mmol) in CH2Cl 2 (15 mL) was added dropwise to a soln of 

[Rh(acac)(nbd)] (1.6 g, 5.4 mmol) and nbd (5 mL, excess) in CH 2Cl 2 (10 mL), with stirring. After 

the addition was complete, the resultingdeep red soln was stirred for 5 min at rt and 

then added to Et 2O (200 mL), formingred crystals. The crystals were collected by filtration 

to give the product; yield: 1.8 g (89%); mp 157±1598C; 1 H NMR (CDCl 3): ä 5.53 (d, 8H, 

CH=CH), 4.19 (s, 4H, CH), 1.57 (s, 4H, CH 2); IR (cm ±1 , Nujol mull): 1053 (BF 4), 1038 (BF 4). 

1.5.4.4.4 Variation 4: 

Monomeric Norbornadiene Complexes 

The dimeric complex [Rh 2Cl 2(nbd) 2](75) undergoes smooth chloride-bridge cleavage with 

a stoichiometric amount of a donor ligand at room temperature to yield the monomeric 

complex [RhCl(nbd)L] (73, diene = nbd) as shown in Scheme 31 (see Section 1.5.4.3.4). Donor 

ligands can be phosphine, phosphite, [83,97] amine, [98] or pyridine. [86] The chloride ion of 

75 can be completely dissociated from the rhodium center by reacting 75 with two or 

more equivalents of the donor ligand in a polar solvent, forming tetracoordinate cationic 

complex 74 (diene = nbd). [62] Displacement of one norbornadiene ligand of [Rh(nbd) 2] + (76) 

by a donor ligand provides an efficient route to 74 (diene = nbd). [99] Reaction of 

[Rh(acac)(nbd)] (77,R 1 =H;R 2 = Me) with an equimolar amount of perchloric acid followed 

by addition of a donor ligand leads to the formation of complex 74 (diene = nbd) in good 

to high yield (Scheme 31). [62] The ligand (L) of 74 (diene = nbd) can be substituted with 

another ligand L 2 by mixingan acetone solution of 74 with three or more equivalents of 

L 2 . [62] 

(Bicyclo[2.2.1]heptadiene)chloro(triphenylphosphine)rhodium(I) 

(73, diene = nbd; L = PPh 3); Typical Procedure: [97] 

A mixture of [Rh 2Cl 2(nbd) 2] (0.3 g, 0.65 mmol) and Ph 3P (0.17 g, 0.65 mmol) in CH 2Cl 2 was 

shaken for 15 min and the solvent was removed under reduced pressure. The resulting 

fine yellow crystals were recrystallized [EtOH or benzene/light petroleum ether (bp 40± 

608C) 1:1] and vacuum dried to give the product; yield: not given; mp 163±1648C; IR 

(cm ±1 , Nujol mull): 446 (w), 434 (w), 424 (w), 269 (sh), 286 (m, br, Rh-Cl). 

Other Rh complexes of this type were prepared in a similar manner (no yields given): 

[RhCl(nbd)(PPh 2Me)], orange crystals, mp >150 8C (dec); [RhCl(nbd)(AsPh 3)], orange crystals, 



bmp 183 8C (dec); [RhCl(nbd)(SbPh 3)], red crystals, mp >100 8C (dec); [RhCl(nbd)(p-toluidine)], 

flaky yellow crystals, mp 187±1898C (dec). 

(Bicyclo[2.2.1]heptadiene)bis(triphenylphosphine)rhodium(I) Perchlorate 

(74, diene = nbd; L = PPh 3; X = ClO 4); Typical Procedure: [62] 

To a soln of [Rh 2Cl 2(nbd) 2] (750 mg, 1.62 mmol) and Ph 3P (3.0 g, 11.4 mmol) in benzene 

(25 mL) was added a soln of NaClO 4 (500 mg, 4.08 mmol) in THF (7.5 mL), followed shortly 

by the addition of Et 2O (10 mL). The precipitate was collected on a filter pad, washed with 

benzene and Et 2O, and air dried. The resultingyellow-orange powder was dissolved off 

the filter pad with the minimum amount of CH 2Cl 2 (ca. 5 mL) (NaCl remained on the filter) 

and recrystallized by addition of EtOH and Et 2O to give the product; yield: 2.55 g (96%); 

1 H NMR (CDCl3): ä 4.57 (4H, CH=CH), 4.14 (2H, CH), 1.50 (2H, CH 2); 31 P NMR (CH 2Cl 2): 

±28.5 ppm (d, J Rh,P = 155 Hz). 

Other Rh complexes of this type were synthesized in a similar manner: [Rh(nbd)- 

(PPh 3) 2] + PF 6 ± ; yield: 87%; [Rh(nbd)(PPh3) 2] + BPh 4 ± ; yield: 89%; [Rh(nbd)(PPh2Me) 2] + ClO 4 ± ; 

yield: 81%; [Rh(nbd)(PPhMe 2) 2] + ClO 4 ± ; yield: 91%. 

(Bicyclo[2.2.1]heptadiene)(phosphine)rhodium(I) Perchlorate [74, diene = nbd; 

L±L = 1,2-bis(diphenylphosphino)ethane, 1,2-bis(diphenylphosphino)propane, 

1,2-bis(diphenylphosphino)butane, 1,2-bis(dicyclohexylphosphino)ethane, 

or (PPh 2Me) 2; X = ClO 4]; General Procedure: [99] 

To a soln of [Rh(nbd) 2]ClO 4 [81] (0.2 g, 518 ìmol) in degassed CH2Cl 2 (2.0 mL) under argon 

was added a phosphine or diphosphine (~515 ìmol) in small portions over 30 s with stirring. 

After stirring for 5 min, the mixture was filtered under argon through a Celite pad 

and the pad washed with CH 2Cl 2 (3 ” 1 mL). The combined filtrates were concentrated to 

2 mL and dry, degassed Et 2O (1 mL) was added dropwise, followed by, if necessary, dry degassed 

THF (1 mL) to give large orange crystals upon standing. The crystals were collected, 

washed (Et 2O/CH 2Cl 2 10:1; Et 2O), and dried under an argon flow to give the product; yields: 

90 5%. 

(Bicyclo[2.2.1]heptadiene)(1-diphenylarsino-2-diphenylphosphinoethane)rhodium(I) 

Perchlorate (74, diene = nbd; L±L = Ph 2AsCH 2CH 2PPh 2; X = ClO 4); Typical Procedure: [62] 

To a soln of [Rh(acac)(nbd)] (250 mg, 0.85 mmol) in THF (3 mL) was added a soln of HClO 4 

(120 mg, 70%, 1 equiv) in THF (1 mL). Then 1-diphenylarsino-2-diphenylphosphinoethane 

(365 mg, 1 equiv) was added dropwise with stirring to give an orange solution with formation 

of crystals. Et 2O was added to the mixture as needed to complete the crystallization 

of the product and the crystals were collected by filtration, washed with MeOH and Et 2O, 

and dried to give the product; yield: 550 mg (91%); 1 H NMR (CDCl 3): ä 5.30 (4H, CH=CH), 

4.28 (2H, CH), 2.5 (m, 4H, PCH 2CH 2As), 1.74 (2H, CH 2). 

Other complexes of this type were synthesized in a similar manner: [Rh(nbd)- 

(AsPh 3) 2] + ClO 4 ± ; yield: 90%; [Rh(nbd){P(OPh)3} 2] + ClO 4 ± ; yield: 67%; [Rh(nbd)(PPh2OMe) 2] + - 

ClO 4 ± ; yield: 42%; [Rh(nbd)(dppe)] + ClO4 ± , employing1,2-bis(diphenylphosphino)ethane; 

yield: 85%. 

1.5.4.5 Method 5: 

Cycloocta-1,3,5,7-tetraene Complexes 

Metal complexes of cycloocta-1,3,5,7-tetraene (cot) are of particular interest owingto the 

various bindingmodes of cyclooctatetraene to metal centers. Since cycloocta-1,3,5,7tetraene 

serves often as a tetrahapto ligand, these rhodium complexes are covered in 

this product subclass. Reaction of rhodium(III) chloride trihydrate with cycloocta-1,3,5,7tetraene 

in ethanol provides a direct route to the dimeric rhodium±cyclooctatetraene 

complex 81 (Scheme 35). However, this procedure suffers from a longreaction time (one 




bmonth at room temperature) and poor yield. A more efficient route to 81 is based on the 

displacement of weakly bound ligands of a rhodium±diene complex with cycloocta- 

1,3,5,7-tetraene. Thus, reaction of [Rh 2Cl 2(diene) 2] (diene = butadiene [91] or two cyclooctenes 

[100] ) with cycloocta-1,3,5,7-tetraene in petroleum ether at room temperature affords 

81 in good to excellent yield as an air-stable, yellow solid. However, 81 is unstable 

in organic solvents, dissociating one molecule of cyclooctatetraene to form an orangebrown, 

insoluble precipitate which may have a polymeric structure 82. As shown in 

Scheme 35, the reaction pattern of 81 is similar to that of rhodium±cyclooctadiene and 

±norbornadiene complexes. The chlorine-bridged dimeric framework of 81 can be 

cleaved by reacting 81 with a slight excess of a phosphine or an amine donor ligand at 

room temperature to give a monomeric rhodium complex, [RhCl(cot)L] 83. [98,100] Complex 

81 reacts with acetylacetone in the presence of a base to give a crystalline complex, (acetylacetonato)(cycloocta-1,3,5,7-tetraene)rhodium(I) 

(84). [100] Displacement of two carbonyl 

ligands from dicarbonyl(cyclopentadienyl)rhodium(I) by cycloocta-1,3,5,7-tetraene provides 

a direct route to the cyclooctatetraene(cyclopentadienyl)rhodium complex 85 

(R 1 = H). [101] However, this procedure suffers from low yield. A more practical method for 

the synthesis of 85 (R 1 =H, CO 2Me) is based on the nucleophilic substitution of cyclopentadienyl 

anions (Scheme 35). [25] 

Scheme 35 Rhodium±Cyclooctatetraene Complexes [25,98,100] 

Cl 

Rh Rh 

Cl 

81 

CS 2 or CHCl 3 

L 

− C 8H8 

acacH, K 2CO 3 

TlC 5H 4R 1 

Cl 

Rh 

Rh L 

Cl 

83 L = P or N ligands 

Rh 

84 

Rh 

O 

O 

82 

R 1 

85 R 1 = H, CO2Me 

Rh Cl 

Di-ì-chlorobis(cycloocta-1,3,5,7-tetraene)dirhodium(I) (81); General Procedure: [100] 

To a suspension of [RhCl(cyclooctene) 2] [59] (0.5 g, 1.4 mmol) in petroleum ether (bp 60± 

808C) was added an excess of cot (5.0 mL, 44.4 mmol). After stirringthe mixture for 1 h 

at rt, the resultingyellow crystalline solid was collected by filtration, washed with EtOH 

and Et 2O, and dried in vacuo at 258C to give 81; yield: 0.58 g(85%); mp 140±1458C (dec); 

1 H NMR (CDCl3): ä 5.79 (8H, uncoordinated CH=CH), 4.26 (8H, coordinated CH=CH); IR 

(cm ±1 , Nujol mull): 1630 (w, uncoordinated C=C), 1410 (w, coordinated C=C), 1345 (s), 

797 (s). 


n


b(Acetylacetonato)(1,2,5,6-ç-cycloocta-1,3,5,7-tetraene)rhodium(I) (84): [100] 

To a suspension of [Rh 2Cl 2(cot) 2] (81; 0.24 g, 0.49 mmol) and anhyd K 2CO 3 (0.20 g, 

1.44 mmol) in dry petroleum ether (bp 60±80 8C) was added acacH (0.2 mL, 1.94 mmol). After 

stirringthe mixture at rt for 1 h, the resultingyellow soln was filtered and the solvent 

was removed from the filtrate at 258C/15 Torr to give the crude product which was crystallized 

(petroleum ether) to afford 84 as yellow crystals; yield: 0.12 g(80%); mp 1308C; 

1 H NMR (CCl4): ä 5.96 (4H, uncoordinated CH=CH), 4.22 (4H, J Rh,H = 2 Hz, coordinated 

CH=CH); IR (cm ±1 , Nujol mull): 1630 (w, uncoordinated C=C), 1350 (m), 799 (s). 

1.5.4.6 Method 6: 

Synthesis of Cationic Chiral Diene Complexes 

The great demand for enantiopure pharmaceuticals and agrochemicals has fueled tremendous 

efforts in the area of asymmetric synthesis in the past two decades. [102] In recent 

years, attention has been focused on asymmetric catalytic processes for the conversion of 

prochiral substrates into valuable optically active products. [103] Chiral rhodium complexes 

are amongthe most widely used catalyst precursors in asymmetric catalytic processes. Although 

many chiral rhodium complexes are known to exhibit good catalytic activity in 

promotingasymmetric reactions, cationic complexes such as 86 are far superior to neutral 

ones in terms of efficiency and enantioselectivity. These complexes have provided access 

to a plethora of extremely useful chiral organic building blocks with extremely high 

levels of chiral induction through a number of reactions such as hydrogenation, [104,108] hydrosilation, 

[104,108] alkene isomerization, [104,105] hydroacylation, [106] and cycloaddition. [107] 

Almost all chiral rhodium catalysts are generated in situ from precursor rhodium complexes 

in the presence of the chiral ligand (often by displacing weakly bound ligands 

such as alkene or carbonyl), and used immediately for the reactions. However, a number 

of studies have indicated that higher reactivity and asymmetric induction are observed 

when isolated chiral rhodium catalysts are used compared to those generated in situ. Furthermore, 

isolation of well-defined chiral rhodium complexes and the study of their 

structures provides important insight into the understanding of a particular catalyst system, 

which could lead to the design and development of new and more effective chiral 

catalysts. Scheme 36 illustrates two generic synthetic routes to well-defined, cationic chiral 

rhodium catalyst precursors 86. Usinga similar protocol to that used for the synthesis 

of nonchiral complexes 74 (L 2 = P±P), [99] complexes 86 can be prepared through simple replacement 

of a diene ligand (71, diene = cod; 76, diene = nbd) with an equimolar amount 

of a chiral diphosphine ligand, [108] or through dissociation of the chloride ion of 

[Rh 2Cl 2(diene) 2](67, diene = cod; 75, diene = nbd) usinga silver(I) salt in the presence of 

two equivalents of a chiral diphosphine ligand. [109] The latter method is more direct, while 

the former is more versatile and applicable to a wide variety of chiral ligands. Typical chiral 

diphosphine ligands that are commonly used in asymmetric hydrogenation are shown 

in Scheme 37. Most of these chiral ligands are commercially available. Details of other 

chiral diphosphine ligands are available. [104,105] 




bScheme 36 Synthesis of Cationic Chiral Diene Complexes [108,109] 

1/2 

Rh 



Rh Cl Rh 

Cl 

+ 

67 X = Cl; diene = cod 


X − 

∗ 

P P 

− diene 

1. Ag + , acetone 

∗ 

2. P P 

− AgCl 

Scheme 37 Commonly Used Chiral Diphosphine Ligands in 

Asymmetric Hydrogenation [110±117] 

O 

O 

PPh 2 

PPh2 

PPh 2 

PPh 2 

Rh P 

P 

Diop Chiraphos DIPAMP 

BINAP 

PPh2 

PPh2 

R 1 R 1 

P 

R 1 R 1 

P 

NORPHOS 

PPh2 

PPh 2 

Ph2P H 

H 

PPh2 86 

MeO Ph 

P 

P 

R 1 R 1 

∗ 

P 

+ 

X − 

R 1 R 1 

DuPHOS BICP PennPHOS 

BPE 

P 

Ph OMe 

Only one enantiomer is shown for each ligand. Diop: (R,R)-2,3-O-isopropylidene-2,3-dihydroxy-1,4-bis(diphenylphosphino)butane; 

[110] Chiraphos: (S,S)-2,3-bis(diphenylphosphino)butane; 

[111] DIPAMP (R,R)-1,2-bis[(2-methoxyphenyl)phenylphosphino]ethane; [112] BINAP: 

(R)-2,2¢-bis(diphenylphosphino)-1,1¢-binaphthyl; [113] NORPHOS: (R,R)-5,6-bis(diphenylphosphino)norborn-2-ene; 

[114] BPE (R = Me): 1,2-bis{(2S,5S)-2,5-dimethylphospholino}ethane; 

[115] DuPHOS (R = Me): 1,2-bis{(2S,5S)-2,5-dimethylphospholino}benzene; [108] BICP: 

(1R,1¢R,2R,2¢R)-2,2¢-bis(diphenylphosphino)-1,1¢-bicyclopentane; [116] PennPHOS (R = Me): 

P,P¢-1,2-phenylenebis{(1R,2S,4R,5S)-2,5-dimethyl-7-phosphabicyclo[2.2.1]heptane}. [117] 

(Cycloocta-1,5-diene)[1,2-bis{(2R,5R)-2,5-diethylphospholino}benzene]rhodium(I) 

Trifluoromethanesulfonate [86, diene = cod; P±P = (R,R)-Et-DuPHOS; X = OTf]; 


In a N 2-filled glovebox was added a soln of (R,R)-Et-DuPHOS [108] (0.10 g, 0.28 mmol) in THF 

(5 mL) to a soln of [Rh(cod) 2] + OTf ±[62] (0.13 g, 0.28 mmol) in THF (10 mL) at 258C. The yellow 

soln turned orange upon adding the phosphine. The mixture was stirred for 15 min, and 


R 1 

R 1 

R 1 

R 1


bthen Et 2O (30 mL) was slowly added to the soln to produce an orange precipitate, which 

was collected by filtration, washed (Et 2O), and dried under reduced pressure. The product 

was redissolved in CH 2Cl 2 (5 mL) and filtered (if necessary to remove the impurities), and 

Et 2O (30 mL) was added slowly to the orange filtrate to precipitate orange microcrystals of 

the product, which were collected by filtration; yield: 0.112 g(56%); 1 H NMR (CD 2Cl 2): ä 

7.70 (m, 4H, Ph), 5.60 (m, 2H, cod-CH), 4.90 (m, 2H, cod-CH), 2.20±2.70 (m, 14H), 2.20 (m, 

2H), 1.85 (m, 4H), 1.2±1.6 (m, 6H), 1.02 (t, J = 7.3 Hz, 6H, CH 3), 0.86 (t, J = 7.3 Hz, 6H, CH 3); 

31 P NMR (CD2Cl 2): ä 69.5 (d, J Rh,P = 148.3 Hz). 

Other complexesof this type were prepared in a similar manner: [Rh(cod){(R,R)-Et- 

BPE}] + OTf ± , [Rh(cod){(R,R)-iPr-BPE}] + OTf ± , [Rh(cod){(S,S)-Me-DuPHOS}] + OTf ± , [Rh(cod)- 

{(R,R)-Pr-DuPHOS}] + OTf ± , and [Rh(cod){(R,R)-iPr-DuPHOS}] + OTf ± from [Rh(cod) 2] + OTf ± ; 

[Rh(cod){(R,R)-Me-DuPHOS}] + PF 6 ± from [Rh(cod)2] + PF 6 ± ; [118] [Rh(nbd){(S)-BINAP}] + ClO4 ± and 

[Rh(nbd){(S,S)-Chiraphos}] + ClO 4 ± from [Rh(nbd)2] + ClO 4 ± . [119] 

(Cycloocta-1,5-diene)[(S)-2,2¢-bis(diphenylphosphino)-1,1¢-binaphthyl]rhodium(I) 

Perchlorate [86, diene = cod; P±P = (S)-BINAP; X = ClO 4]; Typical Procedure: [109] 

To a soln of [Rh 2Cl 2(cod) 2] (0.123 g, 0.25 mmol; see Section 1.5.4.3.1) in acetone (30 mL) was 

added AgClO 4 (0.104 g, 0.5 mmol) and the mixture was stirred for 1 h at rt under N 2 or argon. 

The colorless precipitate which formed was removed by filtration and washed with 

acetone. To the pale yellow filtrate and the washings was added solid (S)-BINAP [113] 

(0.311 g, 0.5 mmol) and the resulting orange-vermilion soln was stirred for 1 h at rt. On 

concentration of the mixture (ca. 2 mL) under reduced pressure, the product began to 

crystallize and Et 2O (5 mL) was slowly added to complete the precipitation. The resulting 

solid was carefully recrystallized (acetone/Et 2O) to give the analytically pure product as 

deep orange crystals; yield: 0.34±0.41 g (70±85%) for either (S)- or (R)-BINAP complex; mp 

1648C (dec); 1 H NMR (CD 2Cl 2): ä 2.00±2.62 (m, 8H, CH 2), 4.58 (br, 2H, =CH), 4.84 (br, 2H, 

=CH), 6.42±8.22 (m, 32H, Ar). Better crystals were obtained by recrystallization from 

THF/Et 2O as the THF solvated complex; mp 153 8C (dec). 

The complex [Rh(cod){(R,R)-Diop}] + ClO 4 ± (45% yield, deep orange crystals) was prepared 

in a similar manner. 

Applications of Product Subclass 4 in Organic Synthesis 

1.5.4.7 Method 7: 

Reactions Involving Allenes 

In general, there are fewer numbers of reactions involving metal allene intermediates 

compared to their alkene or alkyne counter parts. This may be due to the fact that allenes 

are prone to undergo dimerization and oligomerization in the presence of transition-metal 

complexes. Accordingly, the use of rhodium±allene complexes in organic synthesis is 

rather limited. Some useful reactions of allenes catalyzed by rhodium complexes include: 

carbonylative [4+1] cycloaddition of vinylallene, [4 +2] diene±allene cycloaddition, and 

[5+2] vinylcyclopropane±allene cycloaddition. These reactions involve rhodium±allene 

complexes as intermediates in their catalytic cycles. 

1.5.4.7.1 Variation 1: 

Carbonylative [4+1] Cycloaddition of Vinylallene 

Vinylallenes 87 undergoes facile carbonylative [4+1] cycloaddition in the presence of a 

rhodium complex to furnish five-membered cyclic ketones 88 in good to excellent yields 

(Scheme 38). [90] Only cationic rhodium catalyst precursors are effective for this reaction, 

and the use of [Rh(cod)(dppbe)]OTf [dppbe = 1,2-bis(diphenylphosphino)benzene] gives 

the best yield and selectivity. The reaction begins with the coordination of vinylallene 




b87 to the rhodium catalyst to give 89, followed by oxidative cyclization to give rhodacyclopent-3-ene 

90. Subsequent coordination of carbon monoxide and migratory insertion 

of carbonyl into the Rh-C bond gives 2-acylrhodacyclohex-4-ene 91, which undergoes reductive 

elimination to afford the product 88. 

Scheme 38 Rhodium-Catalyzed [4 + 1] Cycloaddition of Vinylallenes with 

Carbon Monoxide [90] 

87 

89 

R 1 

RhL n 

RhL n 

R 1 

R 2 

R 2 

5 mol% [Rh(cod)(dppbe)]OTf 

CO (10 atm), DME, 20−60 oC R1 = R2 = Me 85% 

R1 = R2 = Pr 90% 

R1 = R2 = Ph 93% 

R1 = CO2Et; R2 = Ph 94% 

R1 ,R2 = (CH2) 4 92% 

R1 ,R2 = (CH2) 5 95% 

Usinga chiral diphosphine ligand, the asymmetric version of this reaction is realized 

(Scheme 39). [118] The cationic chiral complex [Rh(cod){(R,R)-Me-DuPHOS}] + PF 6 ± , generated 

in situ from [Rh(cod) 2] + PF 6 ± and (R,R)-Me-DuPHOS, effectively catalyzes enantioselective 

carbonylative [4 +1] cycloaddition of vinylallenes 92 to give optically active 2-alkylidenecyclopent-3-enones 

93. Stereoselective reduction of 93 gives optically active cis-cyclopentenols 

94 in high yields and up to 95% ee. 

Rh 

Ln 90 

R 1 

R 2 

CO 

− RhLn 

O 

O 

88 

91 

R 1 

R 1 

RhLn 

Scheme 39 Rhodium-Catalyzed Asymmetric [4 +1] Cycloaddition of Vinylallenes [118] 

92 

CO 2R 1 

Ph 

5 mol% [Rh(cod)2]OTf , CO (5 atm) 

(R,R)-Me-DuPHOS 

DME, 10 oC, 24−90 h 

NaBH 4, CeCl3 

O 

93 

CO 2R 1 

Ph 

R 2 

R 2 

OH 

CO2R 1 

Ph 

94 R1 = Et 93%; 92% ee 

R1 = iBu 96%; 91.5% ee 

R1 = Bn 94%; 95% ee 

4,5-Dimethyl-2-isopropylidenecyclopent-3-enone (88, R 1 =R 2 = Me); Typical Procedure: [90] 

A mixture of [Rh(cod)(dppbe)]OTf (16.5 mg, 20.5 ìmol; see Section 1.5.4.3.4) and vinylallene 

87 (R 1 =R 2 = Me; 50.0 mg, 409 ìmol) in DME (2 mL) was stirred under 7600 Torr of 

CO in an autoclave in an oil bath at 608C for 15 h. After the mixture was cooled and the 

gas released, the solvent was removed in vacuo. The residue was subjected to preparative 

TLC (silica gel, Et 2O/hexane = 1:7) to afford the product; yield: 52.2 mg(85%). 




[4+2] Diene±Allene Cycloaddition 

Diels±Alder reaction of unactivated dienophiles generally requires high temperatures. 

However, in the presence of a transition-metal complex this cycloaddition proceeds 

smoothly under mild conditions. Facile [4+2] cycloaddition of a diene with an unactivated 

allene is realized in the presence of a rhodium catalyst. [120] As shown in Scheme 40, the 

[4+2] cycloaddition of 95 occurs exclusively on the internal double bond of the allene 

moiety, giving 96 in almost quantitative yield as a single stereoisomer. By varying the 

length of the tether between the diene and allene, and heteroatom substitution on the 

tether backbone, this protocol allows for the construction of bicyclic 6,5-, 6,6-, and 6,7fused 

carbocycles and heterocycles under mild reaction conditions. 

Scheme 40 Rhodium-Catalyzed [4 + 2] Diene±Allene Cycloaddition [120] 

95 

O 

O 

O 

O 

5 mol% Rh2Cl2(cod) 2 

tris(biphenyl-2-yl)phosphite (0.4 equiv) 

THF, 45 oC, 10 h 

98%; (cis/trans) 100:0 

3aâ-Methyl-4-methylene-1,3,3a,5,7aâ-hexahydroindene-2-spiro-5¢-(2¢,2¢-dimethyl- 

1¢,3¢-dioxolane-4¢,6¢-dione) (96): [120] 

A 250-mL Schlenk flask was charged with 95 (0.30 g, 1.08 mmol) and freshly distilled THF 

(100 mL) under N 2 before addition of tris(biphenyl-2-yl) phosphite (0.23 g, 0.432 mmol) 

and [Rh 2Cl 2(cod) 4] (26 mg, 0.054 mmol). The pale soln was stirred at 458C for 10.5 h, filtered 

through a pad of neutral alumina, and concentrated in vacuo. Flash chromatography 

of the residue on silica gel afforded 96; yield: 0.29 g(98%). 

1.5.4.7.3 Variation 3: 

[5+2] Vinylcyclopropane±Allene Cycloaddition 

Rhodium complexes also effect [5+2] cycloaddition of vinylcyclopropanes with allenes to 

give bicyclic seven-membered ring compounds, which are difficult to obtain by conventional 

synthetic methods. [121] As illustrated in Scheme 41, the reaction of the allene±vinylcyclopropane 

97 catalyzed by a rhodium(I) complex affords 98 in high to excellent yield 

as a single stereoisomer. Both chlorotris(triphenylphosphine)rhodium(I) and tetracarbonyldi-ì-chlorodirhodium(I) 

are comparably effective, although the use of silver(I) trifluoromethanesulfonate 

as additive in conjunction with chlorotris(triphenylphosphine)rhodium(I) 

is necessary for some systems. In this reaction, the cis ring-fused product predominates, 

and in many cases with complete selectivity. This protocol provides a general 

and efficient route to various bicyclo[5.3.0]decenes, which is a common core structure for 

a large number of natural products. Application of this cycloaddition methodology to the 

asymmetric total synthesis of (+)-dictamnol from 99 via 100 has been reported (Scheme 

41). [122] 

H 

O 

O 

96 

O 

O 




bScheme 41 Rhodium-Catalyzed [5 + 2] Cycloaddition of Allene-vinylcyclopropanes [121,122] 

X 

HO 

97 

99 

R 2 

R 1 

Rh(I), heat 

X 

2.5 mol% Rh 2Cl2(CO)4 

80 o C, 7 h 

76% 

R 1 

H 

H 

98 

HO 

X R 1 R 2 Yield (%) Ref 

R 2 

H 

H 

100 

C(CO 2Me) 2 t-Bu H 96 [121,122] 

C(CO 2Me) 2 Me Me 90 [121,122] 

HC(CO 2Me) t-Bu H 91 [121,122] 

NTs H H 90 [121,122] 

H 

HO 

H 

(+)-dictamnol 

Dimethyl 4-(2,2-Dimethylpropylidene)-3,3a,4,5,6,8a-hexahydro-1H-azulene- 

2,2-dicarboxylate [98, R 1 = t-Bu; R 2 = H; X = C(CO 2Me) 2]; Typical Procedure: [121] 

To a base-washed, oven-dried Schlenk flask under an argon atmosphere was added 

[RhCl(PPh 3) 3] (0.12 mg, 0.1 mol%) in one batch in freshly degassed distilled toluene (1 mL). 

The soln was stirred for 5 min at rt, and a soln of the ene±vinylcyclopropane 97 [R 1 = t-Bu; 

R 2 = H; X = C(CO 2Me) 2; 42.3 mg, 0.132 mmol] in toluene (0.3 mL) was added over 10 s and 

the mixture was heated at 1108C for 5 h. After cooling, the mixture was filtered through 

a plugof neutral alumina and concentrated. Flash chromatography of the residue (silica 

gel, 10% EtOAc/hexane) gave the product as a colorless oil; yield: 40 mg (96%). 

1.5.4.8 Method 8: 

[4+2] Cycloaddition Involving 1,3-Dienes 

The remarkable versatility of the Diels±Alder reaction for the stereospecific construction 

of six-membered ringcompounds has made this reaction one of the most powerful tools 

in organic synthesis. [123,124] Purely thermal [4+2] cycloadditions often require vigorous reaction 

conditions, so various modifications have been made to circumvent the severity of 

the reaction. [125] In the past decade, attention has been focused on the use of metal catalysts 

to promote Diels±Alder reactions, which are Lewis acids or metal templates for ðbonding. 

While Lewis acids remain the most effective catalysts, [126] their applications are 

limited primarily to activated dienophiles. The use of transition-metal complexes allows 

efficient [4+2] cycloaddition of unactivated trienes or dienynes to occur under mild conditions. 

[127±129] Rhodium complexes are amongthe most widely used metal catalysts that 

promote these reactions. [130,131] A plausible mechanism for a metal-catalyzed [4 +2] cycloaddition 

is shown in Scheme 42. [127,132] Coordination of a diene and a dienophile to the 

metal forms triple ð-complex 101. Oxidative coupling occurs through pathway A to give 

ð-allyl complex 102. Alternatively, oxidative cyclization of 101 via pathway B forms 

metallacyclopentene 103. Both 102 and 103 can lead to the formation metallacycloheptadiene 

104, which ultimately undergoes reductive elimination to afford the carbocycle 

product and regenerates the active catalyst species. 



bScheme 42 Mechanism of Metal-Catalyzed [4+2] Cycloaddition [127,132] 

+ 

M A 

M 

101 102 

M 

B 

A 

103 104 

The first example of the rhodium-catalyzed [4 +2] cycloaddition of an unactivated 1,3-diene 

and a terminal alkyne is shown in Scheme 43. [133] Usinga cationic rhodium complex, 

the intermolecular reaction occurs under mild conditions to give the corresponding cyclohexadienes 

in good yields and high regioselectivity. However, internal alkynes are 

not suitable for this reaction. 

Scheme 43 Intermolecular [4 +2] Cycloaddition [133] 

+ 

Ph 

1−5 mol% [Rh(cod)(dppb)]PF6 

CH2Cl2, rt to 45 oC, 144 h 

85% 

M 

M 

Ph Ph 

− M 

+ + other adducts 

The intramolecular variation of the [4+2] cycloaddition is significantly facilitated in the 

presence of a rhodium complex such as chlorotris(triphenylphosphine)rhodium(I) or 

chlorobis[tris(2,2,2,3,3,3-hexafluoroisopropoxy)phosphine]rhodium(I). [132] As Scheme 44 

illustrates, the reaction occurs under exceedingly mild reaction conditions to afford a variety 

of fused 5,6- and 6,6-ring systems in good to excellent yields, e.g. 106 and 108 from 

105 and 107, respectively. The intramolecular [4 +2] cycloaddition occurs usinginternal 

alkynes and, more importantly, unactivated alkenes. In addition, rhodium(I)-catalyzed 

[4+2] cycloadditions were found to proceed with excellent diastereoselectivity, providing 

cycloadducts exclusively as single diastereomers. The use of cationic rhodium catalysts 

greatly accelerates the intramolecular [4 +2] cycloaddition. [130,131] 

93:3:4 

Scheme 44 Rhodium-Catalyzed Intramolecular [4+2] Cycloaddition [132] 

X 

R 1 

10% Rh(I), CF3CH2OH 55 oC, 15−45 min 

87−96% 

H 

105 106 

X = O, C(CO2Me)2 R1 = H, CH2OTBS 

Rh(I) = RhCl(PPh3)3, RhCl{P[OCH(CF3)2]3}2 

X 

R 1 




10% RhCl{P[OCH(CF 3) 2] 3} 2, THF 

22−55 

bX 

oC, 18−60 h 

X = O; R1 = H 98% 

X = CH2; R1 = OTBDMS 61% 

R1 X 

R1 107 108 

Scheme 45 illustrates (for 109 ® 110) the enantioselective variant of this cycloaddition 

reaction, employinga neutral chiral rhodium catalyst generated in situ from chlorotris(triphenylphosphine)rhodium(I) 

or chlorobis[tris(2,2,2,3,3,3-hexafluoroisopropoxy)phosphine]rhodium(I) 

with chiral Diop ligands. [134] The enantioselectivity of this reaction 

is not as high as the one achieved with chiral Lewis acid catalysts and activated alkenes; 

[126] however, the use of cationic rhodium complexes in conjunction with a variety 

of chiral diphosphine ligands improves the reactivity and enantioselectivity of the reaction, 

e.g. 111 ® 112 and 105 ® 106 (Scheme 46). [107] Amongthe chiral ligands used, (S)- 

BINAP and (S,S)-Me-DuPHOS are found most effective for the cycloaddition of trienes and 

dienynes, respectively. Enantiopurity of 98% ee for 112 and up to 95% ee for 106 are 

achieved. 

Scheme 45 Rhodium-Catalyzed Enantioselective [4 + 2] Cycloaddition [134] 

X 

R 1 

R 

109 110 

2 

O 

PAr 

O 

1 2 

PAr1 2 

L1 ∗ Ar1 = 2-F3CC6H4; L2 ∗ Ar1 = 2-Tol; L3 ∗ Ar1 L 

∗ 

n = 

= Ph 

Rh(I) = RhCl(PPh3)3, RhCl{P[OCH(CF3)2]3}2 

10% Rh(I), L n ∗ , CF3CH 2OH 

X = O; R1 = H; R2 = Me; L 

∗ 

1 79%; 62% ee 

X = C(CO2Me) 2; R1 = H; R2 ∗ 

= Me; L2 69%; 67% ee 

X = O; R1 = Me; R2 = H; L 

∗ 

3 84%; 73% ee 

Scheme 46 Enantioselective [4+2] Cycloaddition Catalyzed by Cationic 

Rhodium Complexes [107] 

O 

6% [Rh(nbd)(P P)] + SbF6 − 

111 112 

P-P Conditions Yield (%) ee (%) Ref 

(S,S)-Chiraphos CH 2Cl 2,258C, 18 h 76 72 [107] 

(S,S)-Diop CH 2Cl 2,258C, 3 d 40 78 [107] 

(S)-BINAP EtOAc, 558C, 3 d 64 >98 [107] 


O 

H 

H 

H 

H 

X 

H 

H 

R 1 

R 2

1.5.5 Rhodium±Allyl Complexes 577 

[Rh(nbd){(S,S)-Me-DuPHOS}] 

bX 

+ SbF6 − 

R 

6 mol% 

X 

1 

H 

105 106 

X, R 1 Conditions Yield (%) ee (%) Ref 

X = O, R1 =H CH2Cl2/EtOAc (6:1), 558C, 4 h 85 95 [107] 

X = O, R1 =Et CH2Cl2,258C, 4 h 98 81 [107] 

X = C(CO2Et) 2,R1 =H CH2Cl2/EtOAc (6:1), 558C, 11 h 78 91 [107] 

5-Methyl-1,3,3aâ,4,5â,7aâ-hexahydroisobenzofuran (108,R 1 = H; X = O); 


A reaction vessel containing[Rh 2Cl 2(C 8H 14) 4] (17.9 mg, 0.025 mmol) was flushed with argon 

and charged with dry THF (4 mL). To this soln was added P[(CF 3) 2CHO] 3 (53.1 mg, 

0.100 mmol) followed by 107 (R 1 = H; X = O; 276 mg, 2.000 mmol), with stirring. The resultingpale 

yellow soln was stirred at 258C for 60 h (or alternatively, warmed to 558C for 

2.5 h). The mixture was filtered through an alumina pad using 30% EtOAc/C 6H 14 as eluant, 

and the solvents were evaporated. Flash chromatography of the residue (silica gel) gave 

the product; yield: 270 mg(98%). 

5-Methyl-1,3,5â,7aâ-tetrahydroisobenzofuran (106,R 1 = H; X =O); Typical Procedure: [107] 

In a Schlenk tube was placed [Rh(nbd)(diphosphine)] + SbF 6 ± (0.044 mmol) (prepared from 

[Rh 2Cl 2(nbd) 4], AgSbF 6, and a chiral diphosphine, see Section 1.5.4.6) and dissolved in 

freshly distilled, degassed solvent (6 mL). H 2 gas was bubbled through the soln for 2 min 

(soln color changed to dark red) followed by N 2 for another 2 min. Substrate 105 (R 1 =H; 

X = O; 0.735 mmol) was added to the catalyst soln alongwith solvent (1 mL). The Schlenk 

tube was then freeze±pump±thaw±degassed (3 cycles) and the mixture stirred under N 2 at 

rt. The progress of the reaction was monitored by TLC. After the reaction was deemed 

complete, the solvent was removed under reduced pressure. The product was purified 

by flash column chromatography (silica gel); yield: 85%. 


Rhodium±Allyl Complexes 

The rhodium±allyl complexes covered in this section refer to ç 3 -allylrhodium complexes 

formally havingone ó-bond and one ç 2 -alkene coordination to the rhodium center, with 

an overall resonance hybrid. ç 1 -Allyl complexes are not discussed here. A review on the 

chemistry of transition-metal±allyl complexes has been published. [135] Rhodium±allyl 

complexes are prepared by four generic methods, i.e. transmetalation, hydrometalation, 

metalation, and oxidative addition. These complexes are usually thermally stable but 

highly sensitive to air, especially in solution. 


1.5.5.1 Method 1: 

Monoallyl Complexes via Transmetalation 

Transmetalation reactions using allylic Grignard reagents or allyltin compounds provide 

the most direct and effective method for the synthesis of allylrhodium complexes. Reaction 

of chlorotris(triphenylphosphine)rhodium(I) with allylmagnesium chloride gives 

R 1 




b(ç 3 -allyl)bis(triphenylphosphine)rhodium (113) in good yield as a crystalline yellow solid 

(Scheme 47). [136] Similarly, (ç 3 -methallyl)bis(triphenylphosphine)rhodium and (ç 3 - 

crotyl)bis(triphenylphosphine)rhodium are prepared from the correspondingallylic 

Grignard reagents. Alternatively, allyltin compounds can be used in place of Grignard reagents. 

Thus, transmetalation of tetracarbonyldi-ì-chlorodirhodium(I) with allyltrimethylstannane 

rapidly takes place at room temperature to give (ç 3 -allyl)dicarbonylrhodium 

(114) in moderate yield. [137] The complex 114 can also be obtained, of course, through 

the reaction of allylmagnesium chloride with tetracarbonyldi-ì-chlorodirhodium(I). [138] 

Scheme 47 Synthesis of Monoallylrhodium Complexes via Transmetalation [136±138] 

RhCl(PPh 3) 3 

1/2 Rh 2Cl 2(CO) 4 

H2C CHCH2MgCl, Et2O, rt, 24 h 

− Ph3P, − MgCl2 

80% 

H2C CHCH2SnMe3, Et2O, rt, 0.5 h 

− Me3SnCl 70% 

PPh3 Rh 

PPh3 113 

CO 

Rh 

CO 

114 

Allyl(cyclooctadiene)rhodium complex 115 is synthesized through transmetalation of diì-chlorobis(cyclooctadiene)dirhodium(I) 

with allylmagnesium chloride (Scheme 48). [139] 

This complex 115 serves as a valuable precursor for a wide range of rhodium complexes 

containing ð-allyl ligands since the cyclooctadiene ligand can be easily displaced. By carefully 

controllingthe stoichiometry of the added ligand, two or three molecules of tertiary 

phosphine or phosphite ligands can be introduced to 115, giving 116 or 117 selectively 

(Scheme 48). [140] 

Scheme 48 Synthesis and Application of Allyl(cyclooctadiene)rhodium(I) Complexes [139,140] 

1/2 Rh 2Cl 2(cod) 4 

L = phosphine or phosphite 

H 2C CHCH 2MgCl 

THF, 0 o C 

Rh 

115 

2L 

3L 

Rh 

116 

L 

L 

L 

Rh L 

L 

117 

(ç 3 -Allyl)bis(triphenylphosphine)rhodium(I) (113): [136] 

To a suspension of [RhCl(PPh 3) 3] (5 g, 5.5 mmol) in dry Et 2O (30 mL) in a Schlenk tube under 

N 2 (or argon) was added a soln of C 3H 5MgCl (0.60 g, 6 mmol), with stirring, at rt. The suspension 

gradually turned from red to yellow and the reaction was complete in 24 h. The 

solid was collected by filtration under an inert atmosphere, and then dissolved in benzene 

(or toluene) and filtered again by gravity. The filtrate was concentrated (5±10 mL) and the 

volume was doubled by addition of Et 2O. Yellow crystals precipitated which were filtered, 

washed with small amounts of Et 2O and hexane, and dried in vacuo to give 113; yield: 3 g 

(80%); 1 H NMR (PhCl): ä 2.19 (2H, J = 12.5 Hz, CH 2), 2.74 (2H, J = 6.7 Hz, CH 2), 4.95 (1H, CH). 

(ç 3 -Allyl)(cycloocta-1,5-diene)rhodium(I) (115): [140] 

A 500-mL, 3-necked, round-bottomed flask was charged with [Rh 2Cl 2(cod) 2] (4.93 g, 

10.0 mmol; see Section 1.5.4.3.1) and THF (150 mL), and the suspension was stirred under 

an inert atmosphere. The soln was cooled to 08C and a soln of C 3H 5MgBr (4.8 g, 25 mmol) in 

THF (100 mL) was added dropwise over 45 min, followed by stirringfor an additional 2 h at 



b08C. The solvent was removed under vacuum to yield a yellow-brown solid which was extracted 

with pentane until the pentane layer became colorless. The pentane was then removed 

to yield a yellow solid. The solid was sublimed under vacuum at 208C to a cold finger 

at ±788C to give 115 as yellow crystals; yield: 4.2 g(83%); mp 30.5±31 8C; 1 H NMR 

(CDCl 3): ä 1.62 (2H, allyl-CH 2), 2.65 (2H, allyl-CH 2), 3.05 (8H, cod-CH 2), 3.96 (4H, cod-CH), 

5.17 (1H, allyl-CH). Complex 117 is thermally unstable and should be stored at ±308C. 

(ð-Allyl)bis(trimethyl phosphite)rhodium(I) [116, L = P(OMe) 3]; Typical Procedure: [140] 

(MeO) 3P (236 ìL, 2 mmol, 2 equiv) was added to a soln of [Rh(ç 3 -C 3H 5)(cod)] (252 mg, 

1 mmol) in pentane (5 mL) and the resultinglight yellow soln was stirred for several 

min. All the volatiles were removed under vacuum to leave an orange oil. To facilitate 

the removal of the cod formed, the oil was redissolved in pentane that was again removed 

under vacuum. This procedure was repeated one more time. The resultingoily crystals 

were dissolved in a minimum amount of pentane and cooled to ±308C to yield a crop of 

light yellow crystals with orange-red bands running through the mass; yield: 379 mg 

(97%). Two more recrystallizations from pentane yielded the product as air-sensitive, 

pale yellow crystals; yield: 379 mg(97%); mp 27.8±28.58C. 

Other complexes similarly obtained: [Rh(ç 3 -C 3H 5){P(O-iPr) 3} 2]; yield: not reported; 

[Rh(ç 3 -C 3H 5){P(OEt) 3} 2]; yield: not reported; [Rh(ç 3 -C 3H 5){P(Ot-Bu) 3} 2]; yield: 93%; 

[Rh(ç 3 -C 3H 5)(PMe 3) 2]; yield: not given; and [Rh(ç 3 -C 3H 5)(PPh 3) 2]; yield: 92%. 

(ç 3 -Allyl)tris(trimethyl phosphite)rhodium(I) [117, L = P(OMe) 3]; General Procedure: [140] 

Complex 117 was prepared and characterized in the same manner as that described for 

118 except that (MeO) 3P (354 ìL, 3 mmol, 3 equiv) was used. Purification was carried out 

by passinga toluene soln of the crude oil through a 25 ” 200 mm column of Bio-Beads S-X2 

or Bio-Beads S-X8 followed by removal of the toluene under vacuum and two cycles of dissolution 

in pentane and vacuum removal of the pentane; yield: not given. On a larger 

scale (³5 mmol), direct crystallization from pentane gave pure 117 as waxy crystals. Other 

complexes, [Rh(ç 3 -C 3H 5){P(O-iPr) 3} 3] and [Rh(ç 3 -C 3H 5)(PMe 3) 3], were prepared in a similar 

manner. 

1.5.5.2 Method 2: 

Allyl Complexes by Hydrometalation 

Insertion of 1,2- and 1,3-dienes into a Rh-H bond provides an alternative method for the 

preparation of monoallyl complexes. Thus, carbonyl(hydrido)bis(triphenylphosphine)rhodium(I) 

undergoes hydrometalation of allene and buta-1,3-diene to give allylrhodium 

complex 118 (R 1 = H) and crotylrhodium complex 118 (R 1 = Me), respectively, in high 

yields (Scheme 49). [141] Similarly, insertion of buta-1,3-diene into hydridotetrakis(triphenylphosphine)rhodium(I) 

gives the corresponding crotylrhodium complex as a mixture of 

syn (119, R 1 =H;R 2 = Me) and anti (119, R 1 = Me; R 2 = H) isomers in moderate yield. [136] Hydrometalation 

of cyclic dienes such as cyclohexa-1,3-diene and cycloocta-1,3-diene with 

hydridotetrakis(triphenylphosphine)rhodium(I) yields cyclic allylrhodium complexes 

120 (n = 1) and 120 (n = 3), respectively, in good to excellent yields. [136] 

Scheme 49 Allylrhodium Complexes Synthesized by Hydrometalation [136,141] 

H 

H 

R 1 

H 

CO 

Rh PPh3 H 

PPh 3 

H 

H 

R 2 

H 

R1 Rh 

PPh3 PPh3 ( ) n 

118 R 1 = H, Me 119 R 1 = R 2 = H, Me 120 n = 1, 3 

PPh3 Rh 

PPh3 for references see p 612 



bInsertion of isoprene into the Rh-H bond of hydridotetrakis(trifluorophosphine)rhodium(I) 

occurs only at the less-substituted double bond of isoprene to yield 121 as a 

single regioisomer but a 2:3 mixture of syn and anti isomers 121 (Scheme 50). [142] Similar 

regioselectivity is observed for the reaction of hydridotetrakis(trifluorophosphine)rhodium(I) 

with penta-1,3-diene, which gives a 1:1 mixture of syn,syn- and syn,anti-isomers 

122. The reaction of hydridotetrakis(trifluorophosphine)rhodium(I) with penta-1,4-diene 

also affords 122. The result strongly suggests that penta-1,4-diene undergoes rhodiumcatalyzed 

isomerization to penta-1,3-diene prior to hydrometalation. In the same manner, 

hydrometalation of hexa-1,3-diene proceeds at the less-substituted double bond to 

give syn,syn-123 predominantly, alongwith a tiny amount of the syn,anti-isomer 123. 

The allylrhodium complexes containingtrifluorophosphine are volatile, yellow liquids 

at room temperature. They are stable in solution in the absence of air, and may be stored 

for a longer period in the presence of a slight pressure of trifluorophosphine. It should be 

noted that anti-121 totally isomerizes to syn-121 in 1 hour, by heatingthe mixture of synand 

anti-isomers of 121 at 608C in solution. Similarly, isomerization of the syn,anti- and 

syn,syn-complexes 122 and 123 is also observed. These isomerizations also take place at 

08C over a period of weeks. 

Scheme 50 Synthesis of Allylrhodium Complexes via Hydrometalation [142] 

RhH(PF 3) 4 

Et 

or 

heat 

H 

H 

PF3 Rh PF3 R PF3 1 

R2 syn-121 R1 = H; R2 = Me 

anti-121 R1 = Me; R2 = H 

H 

PF3 Rh PF3 

R PF3 1 

R2 syn, syn-122 R1 = H; R2 = Me 

syn, anti-122 R1 = Me; R2 heat 

= H 

PF3 H 

Rh PF3 

R PF3 1 

R2 syn, syn-123 R1 = H; R2 = Et 

syn, anti-123 R1 = Et; R2 heat 

= H 

Rhodium hydride complex 126 can be generated by reaction of di-ì-chlorobis(cyclooctadiene)dirhodium(I) 

and isopropylmagnesium bromide, as shown in Scheme 51. [143] 

The initial transmetalation displaces the chlorine ligand to form alkylrhodium intermediate 

124. Subsequent â-hydride abstraction to give 125 is followed by exchange of the alkene 

ligands, leading to formation of the rhodium hydride complex 126. Insertion of the 

butadiene ligand into the Rh-H bond yields the cyclooctadiene(methallyl)rhodium complex 

128 via 127. The Rh-H bond can also be generated using butyllithium in place of the 

isopropyl Grignard reagent. [144] Thus, a number of allyl(cyclooctadiene)rhodium complexes 

129±138 have been synthesized through the reaction of di-ì-chlorobis(cyclooctadiene)dirhodium(I) 

with either isopropylmagnesium bromide or butyllithium in the 

presence of various dienes (Scheme 52). [143±145] As mentioned earlier in this section, hydro- 



bmetalation occurs exclusively at the less-substituted double bond of a diene, as exemplified 

by the exclusive formation of 129 and 130. A similar selectivity is observed for the 

formation of syn- and anti-complexes, i.e. syn-isomers are formed preferentially over antiisomers. 

Scheme 51 Mechanism of Rh-H Bond Formation [143] 

1/2 Rh 2Cl 2(cod) 4 

H 

Rh 

iPrMgBr 

H 

Rh 

126 127 

Rh 

124 125 

Rh Rh 

Scheme 52 Synthesis of Allyl(cycloocta-1,5-diene)rhodium Complexes 

via Hydrometalation [143±145] 

Rh 

Cl 

Cl 

Rh 

67 X = Cl 

iPrMgBr or BuLi 

Ph Ph 

H 

Et 

Ph 

Bn 

129 

Rh 

130 

131 

132 

Rh 

Rh 

Rh 

133 

128 

Rh 




biPrMgBr Cl 

or BuLi 

Rh Rh 

Cl 

67 X = Cl 

(ç 3 -Allyl)(carbonyl)bis(triphenylphosphine)rhodium(I) (118, R 1 = H) or Carbonyl(ç 3 -crotyl)bis(triphenylphosphine)rhodium(I) 

(118,R 1 = Me); General Procedure: [141] 

Allene or buta-1,3-diene was bubbled through a soln of [RhH(CO)(PPh 3) 2] (0.3 g, 0.32 mmol) 

in toluene (10 mL) for 5 min. The addition of EtOH (20 mL) with stirringunder N 2 at 08C 

lead to the precipitation of yellow crystals which were collected by filtration, washed 

(EtOH), and dried in vacuo to give the product. 

(ç 3 -Cyclohexenyl)bis(triphenylphosphine)rhodium(I) (120, n=1)or(ç 3 -Cyclooctenyl)bis(triphenylphosphine)rhodium(I) 

(120, n = 2); General Procedure: [136] 

A Schlenk tube was charged with [RhH(PPh 3) 4] (5 g, 4.34 mmol) and cyclohexa-1,3-diene or 

cycloocta-1,3-diene (20±30 mL, 163±244 mmol) under an inert atmosphere, and the mixture 

was stirred at rt until a clear soln resulted (0.5±1 h). The diene was then removed under 

reduced pressure and the residue treated with Et 2O (5 mL) to form a yellow precipitate, 

which was washed with a small amount of Et 2O and hexane and dried in vacuo to 

give the product as yellow crystals; yield: 2.5±3 g (80±95%). 

(ç 3 -1,3-Dimethylallyl)tris(trifluorophosphine)rhodium(I) (122); Typical Procedure: [142] 

A mixture of [RhH(PF 3) 4] [142] (0.152 g, 0.333 mmol) and penta-1,3-diene (0.026 g, 

0.381 mmol) in pentane (2 mL) was sealed in vacuo and kept at 0 8C for 7 d to afford a 

bright yellow soln. Fractional distillation of this solution yielded F 3P (0.029 g, 

0.330 mmol), pentane, and 122 as a volatile yellow liquid; yield: 0.128 g(88%); mp ±158C; 

IR (cm ±1 , vapor phase): 921 (vs), 895 (sh), 878 (vs), 855 (s), 842 (vs), 535 (m), 520 (sh), 510 (s), 

485 (s). 

In a similar manner, the reaction of [RhH(PF 3) 4] (0.242 g, 0.531 mmol) with penta-1,4diene 

(0.035 g, 0.514 mmol) for 3 d at 08C afforded 122; yield: 0.139 g(62%). Other complexes 

were also prepared usingthis method: [Rh(ç 3 -allyl)(PF 3) 3]; yield: 70%; [Rh(ç 3 -cyclo- 


or 

Et 

Rh 

134 

135 

Rh 

136 

Rh 

137 

138 

Rh 

Rh


bhexenyl)(PF 3) 3]; yield: 68%; [Rh(ç 3 -1-methylallyl)(PF 3) 3]; yield: 73%; [Rh(ç 3 -1,2-dimethylallyl)(PF 

3) 3]; yield: 81%; [Rh(ç 3 -1-ethyl-3-methylallyl)(PF 3) 3]; yield: 86%. 

(Cycloocta-1,5-diene)(ç 3 -1-methylallyl)rhodium(I) (128); Typical Procedure: [143] 

To a soln of [Rh 2Cl 2(cod) 2] (0.98 g, 2 mmol; see Section 1.5.4.3.1) in Et 2O (20 mL) at ±788C 

was added buta-1,3-diene (ca. 15 mL, condensed at ±788C). To this mixture was added a 

soln of iPrMgBr, prepared from iPrBr (1 mL) and Mg (0.19 g, 8 mmol) in Et 2O (30 mL), over 

a period of 10 min. The initial orange color changed to yellow, and the mixture was 

stirred for 2 h while warmingto rt. Excess diene was boiled off at rt, the solvent was removed 

under reduced pressure, and the residue was dried in vacuo. The residue was then 

washed with pentane (3 ” 50 mL) and filtered through a pad of Al 2O 3 (5% H 2O). Recrystallization 

of the yellow pentane filtrate at ±788C gave analytically pure 128 as light yellow 

crystals; yield: 0.709 g(67%); mp 28±30 8C; 1 H NMR (benzene-d 6): ä 1.4 (d, J = 6 Hz, 3H, 

Me), 1.75 (m, 4H, cod-CH 2), 2.1 (m, 4H, cod-CH 2), 2.0±2.8 (m, 2H, CH), 3.1 (dd, J = 7 Hz, 1H, 

CH), 3.7±5.5 (b, 4H, cod-CH), 4.9 (m, 1H, CH). 

Other complexes were prepared in a similar manner: [Rh(ç 3 -1,2-dimethylallyl)(cod)]; 

yield: 68%; [Rh(ç 3 -1,3-dimethylallyl)(cod)]; yield: 96%; [Rh(ç 3 -1,1,2-trimethylallyl)(cod)]; 

yield: 47%; [Rh(ç 3 -1-ethyl-3-methylallyl)(cod)]; yield: 55%; [Rh(ç 3 -1-benzyl-3-phenylallyl)(cod)]; 

yield: 26%. 

1.5.5.3 Method 3: 

Allyl Complexes by Allylation of Metal Salts 

Allylation of anionic rhodium complexes with allylic chlorides leads to the formation of 

allylrhodium complexes. [142] Reaction of potassium tetrakis(trifluorophosphine)rhodate(I) 

with various allylic chlorides 139 affords allylrhodium complexes 140±142 in good to excellent 

yields (Scheme 53). This protocol is comparable to the hydrometalation method 

described in Section 1.5.5.2 in terms of its efficiency and scope. 

Scheme 53 Synthesis of Allylrhodium Complexes via Allylation [142] 

K[Rh(PF 3) 4] 

+ 

R 1 

R 1 

R 2 

139 

Cl 

R 2 

R 1 

PF3 

Rh PF3 

R PF3 1 

140 R 1 = R 2 = H 76% 

141 R 1 = H; R 2 = Me 92% 

142 R 1 = Me; R 2 = H 73% 

(ç 3 -2-Methylallyl)tris(trifluorophosphine)rhodium(I) (141); Typical Procedure: [142] 

A mixture of K[Rh(PF 3) 4] [142] (0.200 g, 0.405 mmol) and 2-methylallyl chloride (0.035 g, 

0.386 mmol) in Et 2O (2 mL) was sealed in vacuo and heated at 608C for 15 h to afford a 

yellow soln over a white precipitate (KCl). Fractional distillation of the mixture gave 

F 3P (0.036 g, 0.409 mmol), Et 2O, and 141 as a volatile yellow liquid; yield: 0.158 g(92%); 

mp ±208C; IR (cm ±1 , vapor phase): 970 (w), 920 (vs), 890 (sh), 858 (vs), 848 (vs), 810 (w), 

795 (vw), 538 (m), 522 (s), 508 (s), 490 (w). 

1.5.5.4 Method 4: 

Allyl Complexes by Oxidative Addition 

Rhodium(III) complexes containingmonoallyl ligands are readily prepared by oxidative 

addition of rhodium(I) complexes [RhXL 3] to allylic halides 143, yieldingoctahedral, 

monoallylrhodium complexes 144 (Scheme 54). [146] The reaction proceeds smoothly at 




broom temperature within a few minutes under nitrogen. Addition of lithium bromide or 

chloride ion to 144 leads to instantaneous and complete exchange of the halide ligands. 

Complexes such as [RhCl 2(ç 3 -allyl)(PPh 3) 2], [RhCl 2(ç 3 -allyl)(AsPh 3) 2], [RhCl 2(ç 3 -allyl)(SbPh 3) 2], 

[RhBr 2(ç 3 -allyl)(SbPh 3) 2], [RhCl 2(ç 3 -2-methylallyl)(PPh 3) 2], [RhCl 2(ç 3 -2-methylallyl)(AsPh 3) 2], 

[RhCl 2(ç 3 -2-methylallyl){As(4-Me 2NC 6H 4) 3} 2], [RhCl 2(ç 3 -2-methylallyl)(SbPh 3) 2], and 

[RhBr 2(ç 3 -2-methylallyl)(PPh 3) 2] can be prepared usingthis method. 

Scheme 54 Synthesis of Allylrhodium Complexes via Oxidative Addition [146] 

RhXL 3 + 

R 1 

R 1 

R 2 

143 

R1 = R2 = H, Me 

L = PPh3, AsPh3, SbPh3, As(4-Me2NC6H4)3 

X = Cl, Br 

X 

R 2 

R 1 

Rh L 

R1 X 

X L 

Di-ì-chlorotetrakis(ethene)dirhodium(I) oxidatively adds to allylic chlorides with concomitant 

loss of ethene ligands to give the polymeric, chlorine-bridged, monoallylrhodium 

complexes 145 (Scheme 55). [147] Addition of donor ligands, such as phosphine 

or pyridine, to this complex provides an alternative method for the synthesis of 144 

(R 1 =R 2 = H; X = Cl; L = PMe 2Ph, py). [147] 

Scheme 55 Alternative Synthesis of Bridged Allylrhodium Complexes 

via Oxidative Addition [147] 

Rh 2Cl 2(H 2C CH 2) 4 

R 1 = H, Me 

H2C C(R 1 )CH2Cl 

R 1 

144 

Cl 

Rh 

Cl 

n 

145 

Oxidative addition of an allylic halide to a cationic phosphite complex of type 

[Rh{P(OR 1 ) 3} 5] + gives the corresponding cationic allylrhodium complex 146 or 147 

(Scheme 56). [148] Complex 146 is formed as the sole product when methanol is employed 

as the solvent, while 147 is formed exclusively when pure allyl halide is used without solvent. 

Scheme 56 Cationic Allylrhodium Complexes [148] 

R 1 

Rh L 

L 

2+ 

L 

L 

Rh L 

X 

+ 

L 

L 

R 147 

1 146 = H, Me 

X = Cl, Br 

L = P(OMe) 3 

L = P(OMe) 3, P(O-iPr)3 

(ç 3 -Allyl)chlorobis(triphenylphosphine)rhodium(III) Chloride 

(144,R 1 =R 2 =H;L=PPh 3; X = Cl); Typical Procedure: [146] 

[RhCl(PPh 3) 3] (0.5 g, 0.54 mmol) was slowly added to allyl chloride (100 mL, 93.9 g, 

1.22 mol) with vigorous stirring. After 15 min, the resulting precipitate was collected by 

filtration and the filtrate added to a new portion of allyl chloride (100 mL, 93.9 g, 

1.22 mol). The yellow precipitate was again collected, combined with the first batch, 

washed with allyl chloride, and dried in vacuo to give the product; yield: not reported; 

mp 144±146 8C; 1 H NMR (CDCl 3): ä 2.90 (4H, CH 2), 5.20 (1H, CH). 



b(ç 3 -Allyl)tetrakis(trimethyl phosphite)rhodium(III) Bis(tetraphenylborate) 

[146,R 1 = H; L = P(OMe) 3; Y = BPh 4]; General Procedure: [148] 

A suspension of [Rh{P(OMe) 3} 4]BPh 4 (0.35 g, 0.4 mmol) or [Rh{P(OMe) 3} 5]BPh 4 (0.4 g, 

0.4 mmol), allyl chloride (ca. 3 mL), and NaBPh 4 (0.3 g, 0.9 mmol) in MeOH (ca. 15 mL) was 

stirred at rt for 30 min. The MeOH and the excess allyl chloride were removed under reduced 

pressure and the white precipitate that formed was recrystallized (acetone) to give 

the product; yield: not reported. 

(ç 3 -Allyl)chlorotris(trimethyl phosphite)rhodium(III) Tetraphenylborate 

[147, L = P(OMe) 3; X = Cl; Y = BPh 4]; Typical Procedure: [148] 

A soln of [Rh{P(OMe) 3} 5]BPh 4 (1.0 g, 1.0 mmol) in allyl chloride (ca. 10 mL) was stirred at rt 

for 30 min. The white precipitate of [Rh(ð-C 3H 4){P(OMe) 3} 4][BPh 4] 2 (0.45 g) that separated 

from the soln was filtered off, excess allyl chloride was removed from the filtrate under 

reduced pressure, and the residue (0.1 g) recrystallized (CH 2Cl 2/MeOH) to give 147; yield: 

not reported. 

1.5.5.5 Method 5: 

Bis(allyl) Complexes 

There are a variety of methods available for the synthesis of bis-allylic complexes of rhodium. 

The two most widely used methods are shown in Scheme 57. The first provides a 

direct and convenient route to bis(allyl)rhodium complexes of the type [Rh 2Cl 2(allyl) 4] 

(148) and is based on the oxidative hydrolysis of tetracarbonyldi-ì-chlorodirhodium(I) in 

the presence of an allylic chloride in aqueous methanol. [147] The use of potassium hydroxide 

dramatically facilitates this reaction to give high yields (>90%), whereas reactions in 

water alone are much slower and usually give lower yields. The second method for the 

synthesis of 148 is based on the reaction of a solution of sodium hexachlororhodate(III) 

in water/methanol with an allylic mercury complex (Scheme 57). [149] The reactions proceed 

smoothly at ambient temperature to give the corresponding bis-allylic rhodium 

complexes in yields comparable to those of the first method. [150] 

Scheme 57 Synthesis of Bis-allylic Rhodium Complexes [147,149,150] 


[RhCl 6] 3− 

R 1 CH C(R 2 )CH 2Cl, OH − , H 2O 

80−95% 

R 1 CH C(R 2 )CH2HgCl, MeOH/H2O 

74−89% 

R 2 

R 2 

R 1 

R 1 

Rh X Rh 

X 

A possible mechanism for the formation of 148 via oxidative hydrolysis in the presence of 

hydroxide ions is proposed in Scheme 58. [147] Coordination of a water molecule to tetracarbonyldi-ì-chlorodirhodium(I) 

cleaves the chloride bridge to give 149, which leads to 

rapid formation of 150. Coordination of allyl chloride to 150, followed by migratory insertion 

of an OH group to the carbonyl ligand gives 151, which upon protonation breaks 

down to give 153, presumably via hydridorhodium intermediate 152. Carbon dioxide and 

propene evolved duringthe reaction can be detected by IR spectroscopy. Oxidative addition 

of a second allyl chloride to 153 gives 154. The formation of 148 from 154 goes 

through the same set of reactions described above, but involving 155±158. 

R 1 

148 

R 1 

R 2 

R 2 




bScheme 58 Mechanism of Basic Oxidative Hydrolysis [147] 


OC Cl 

Rh − 

O 

OH 

Cl 

151 

OC 

Rh 

Cl Cl 

154 

H 

O 

H2O 

O 

H + 

H2O 

− 

Rh 

Cl Cl 

Cl 

157 

OC Cl 

Rh 

OC OH2 149 

OC H 

Rh 

Cl 

O 

O 

H Cl 

152 

OC 

Rh 

H2O Cl Cl 

155 

H + 

− CO2 

− 2HCl 

− H + 

Rh 

158 

− CO 2 

− HCl 

− H + 

OC Cl 

Rh − 

OC OH 

150 

OC 

153 

C3H5Cl 

Cl C3H5Cl Rh 

− C3H6 OC 

− 

Rh 

HO Cl Cl 

156 

C 3H 5Cl 

Cl Rh Cl Rh 

Cl 

148 R 1 = R 2 = H; X = Cl 

The allylic rhodium complexes of type 148 (X = Cl) are readily converted into the correspondingbromo-, 

iodo-, or acetatorhodium complexes by anion metathesis usinglithium 

bromide, sodium iodide, or silver(I) acetate, respectively, in acetone or chloroform. [147] 

The dinuclear framework of 148 undergoes chloride-bridge cleavage with thallium(I) 

acetylacetonate to afford (acetylacetonate)bis(allyl)rhodium complex 159 or with donor 

ligands to give mononuclear complex 160 (Scheme 59). [147] 

Scheme 59 Monomeric Bis(allylic)rhodium Complexes [147] 

R 1 

R 1 

Rh 

159 

O 

O 

R 1 = H, Me 

L = PPh 3, AsPh3, PMe2Ph, py 

R 1 

R 1 

Rh Cl 

L 

Tetrakis(ç 3 -allyl)di-ì-chlorodirhodium(III) (148, R 1 =R 2 = H; X = Cl) via 

Oxidative Hydrolysis; Typical Procedure: [147] 

To a soln of [Rh 2Cl 2(CO) 4] (0.398 g, 1.02 mmol) and excess allyl chloride (9 mL, 110 mmol) 

in MeOH was added dropwise a 5 M aq soln of KOH (ca. 3 mol/Rh atom) until the pH of the 

soln reached 7. In this process, gases were vigorously evolved and NaCl and some of the 

product were precipitated. Removal of the excess allyl chloride from the mixture, followed 

by addition of an equal volume of water to the resultingsoln, gave a yellow precipitate 

which was collected and recrystallized (CH 2Cl 2/MeOH) to give the product as yellow 

prisms; yield: 0.38 g(95%); mp 180±185 (dec); 1 H NMR (CDCl 3): ä 1.86 (d, br, 1H), 2.55 (d, br, 


160


b1H), 4.10 (d, br, 1H), 4.9 (m, 1H), 5.13 (br, 1H); IR (cm ±1 ): 410 (m), 280 (m), 236 (s), 208 (s, br, 

Rh-Cl). 

Other complexes were prepared in a similar manner: [Rh 2(2-methylallyl) 4Cl 2]; yield: 

80%; mp 158±1668C (dec); [Rh 2(1-methylallyl) 4Cl 2]; yield: 58%; mp 144±1488C (dec). 

Tetrakis(ç 3 -allyl)di-ì-chlorodirhodium(III) (148, R 1 =R 2 = H; X = Cl) via 

Transmetalation using Allylmercury Chloride; Typical Procedure: [149] 

A soln of Na 3RhCl 6 (2.5 mM) in MeOH/H 2O (5:1) was added to a soln of allylmercury(II) halide 

[150] (5 mM) in MeOH (25 mL). The cherry-red mixture was stirred for 5 h at 258C and 

then stored for 10 h at this temperature. The resultingyellow soln was filtered to remove 

NaCl, the filtrate was diluted with water, and extracted with CH 2Cl 2. After dryingthe organic 

phase (MgSO 4) and removal of the solvent, the residue was recrystallized (benzene/ 

heptane) to give the product; yield: 0.5 g (89%); mp 181±1838C (dec). 

Other complexes were prepared in a similar manner: [Rh 2(2-methylallyl) 4Cl 2]; yield: 

78%; mp 162±1648C (dec); [Rh 2(1-methylallyl) 4Cl 2]; yield: 76%; mp 144±1488C (dec); 

[Rh 2(2-phenylallyl) 4Cl 2]; yield: 81%; mp 165±1708C (dec); [Rh 2(1-phenylallyl) 4Cl 2]; yield: 

74%; mp 192±1988C (dec). 

1.5.5.6 Method 6: 

Tris(allyl) Complexes 

A direct, yet not as efficient, route to the homoleptic complex tris(ç 3 -allyl)rhodium(III) 

(161) involves transmetalation of anhydrous rhodium(III) chloride with excess allylmagnesium 

chloride (Scheme 60). [151] Alternatively, the reaction of allylmagnesium chloride 

with di-ì-chlorobis(ç 4 -hexa-1,5-diene)dirhodium(I) [147] or di-ì-chlorobis(ç 4 -hexamethylbicyclo[2.2.0]hexa-2,5-diene)dirhodium(I) 

[152] also gives 161 as yellow crystals in considerably 

better yields. The most efficient and high yielding synthesis of 161 is based on the 

transmetalation of tetrakis(allyl)di-ì-chlorodirhodium(III) (148,R 1 =R 2 = H; X = Cl) with allylmagnesium 

chloride. [153] Notably, any attempt to prepare a substituted tris-allylic rhodium 

complex failed, possibly owingto steric crowding. 

Scheme 60 Synthesis of Tris(ç 3 -allyl)rhodium(III) Complexes [147,151±153] 

RhCl 3 

Rh2(μ-Cl)2(η 4 -hexa-1,5-diene)2 

Rh2(μ-Cl)2(η 4 -C6Me6)2 

Rh 2(η 3 -C 3H 5) 4(μ-Cl) 2 

148 R 1 = R 2 = H; X = Cl 

C 3H 5MgCl, Et 2O, rt, 5 min to 1 h 

Tris(ç 3 -allyl)rhodium(III) (161) from Allylmagnesium Chloride and 

Tetrakis(allyl)di-ì-chlorodirhodium(III): [147] 

[Rh 2Cl 2(allyl) 4] (0.340 g, 0.77 mmol) was added to a 0.4 M soln of C 3H 5MgCl (6 mmol) in Et 2O 

(15 mL) and the mixture was stirred under N 2 for 30 min, cooled to ca. ±40 8C, and 

quenched with H 2O (20 mL). The Et 2O layer was dried (MgSO 4) and evaporated to dryness 

under reduced pressure. The product was obtained from the solid residue by sublimation 

at 408C/10 ±2 Torr to give 161 as yellow prisms; yield: 0.395 g(85%); mp 80±85 8C. 

Rh 

161 




bTri(ç 3 -allyl)rhodium(III) (161) from Allylmagnesium Chloride and 

Di-ì-chlorobis(ç 4 -hexa-1,5-diene)dirhodium(I): [147] 

[Rh 2Cl 2(ç 4 -hexa-1,5-diene) 2] (0.090 g, 0.2 mmol) was added to a 0.3 M soln of C 3H 5MgCl 

(3 mmol) in Et 2O (10 mL) and the mixture was stirred under dry air for 5 min, cooled to 

ca. ±408C, and quenched with H 2O (15 mL). The Et 2O layer was dried (MgSO 4) and evaporated 

to dryness under reduced pressure. Sublimation of the residue gave 161; yield: 

0.058 g(63%). 


1.5.5.7 Method 6: 

Metallo-Ene Cyclization 

It is well known that ªmetallo-eneº cyclizations are promoted by palladium and nickel 

catalysts (vide supra), but recently rhodium(I) complexes have also been shown to promote 

the same type of reaction efficiently. [154] Typically, the reaction of octa-1,6-dienyl-8carbonate 

162 or its aza counterpart is carried out usinghydridotetrakis(triphenylphosphine)rhodium 

and tris(2,4,6-trimethoxyphenyl)phosphine as the catalyst in acetic acid 

at 808C for 1.5±7.5 hours to give the corresponding 1-exo-methylene-2-vinylcyclopentane 

or -pyrrolidine 163 in high yields (Scheme 61). Formation of six-membered rings is also 

possible, albeit in lower yields. Higher catalyst loadings and longer reaction times are required 

for these reactions. The reaction of dimethyl 1-(trans-4-acetoxycyclohexenyl)allylmalonate 

(164) affords cis-fused bicyclo[4.3.0]nonene 165 exclusively. 

Scheme 61 Rhodium-Catalyzed Metallo-Ene Cyclization [154] 

MeO 2CO 

AcO 

162 

X 

MeO 2C CO 2Me 

164 

2 mol% RhH(PPh3)4 

4 mol% [2,4,6-(MeO)3C6H2]3P 

AcOH, 80 oC, 1.5−7.5 h 

X = C(SO2Ph) 2 88% 

X = C(CO2Me) 2 75% 

X = NTs 80% 

X = NCOCF3 83% 

X = NCO2Bn 63% 

2 mol% RhH(PPh3)4 

4 mol% [2,4,6-(MeO) 3C6H2] 3P 

AcOH, 80 oC, 1 h 

60% 

X 

163 

MeO 2C 

165 

CO2Me 

3-Methylidene-1-(4-toluenesulfonyl)-4-vinyltetrahydropyrrole (163, X = NTs); 


A soln of 162 (X = NTs; 200 mg, 0.589 mmol), [RhH(PPh 3) 4] (13.5 mg, 2 mol%), and [2,4,6- 

(MeO) 3C 6H 2] 3P (12.5 mg, 4 mol%) in AcOH (5 mL) was heated at 808C for 1.5 h under argon. 

Evaporation of the volatiles and purification of the crude product by flash chromatography 

(hexane/EtOAc 9:1) afforded 163 (X = NTs) as an oil; yield: 124 mg(80%). 


1.5.6 Rhodium±Alkyne Complexes 589 


Rhodium±Alkyne Complexes 

The high reactivity of alkynes towards rhodium complexes often leads to the uncontrollable 

formation of an array of products. In many cases, alkynes react instantaneously to produce 

oligomers or organic products which may be free or coordinated to the metal center. 

Sections 1.5.4.2.4 and 1.5.4.2.6 discussed the cyclodimerization, cyclodimerization±carbonylation, 

and cyclotrimerization reactions of alkynes in the presence of rhodium complexes 

to afford cyclobutadienes, cyclopentadienones, and benzene derivatives, respectively. 

Alkynes are known to undergo dimerization in the presence of a rhodium complex 

to yield rhodacyclopentadienes of type 60 as illustrated in Scheme 27 (Section 1.5.4.2.6). 

Owingto their high reactivity, well-defined metal alkyne complexes are often difficult to 

prepare. Nevertheless, under milder conditions a number of rhodium complexes containingan 

alkyne moiety can be isolated and characterized. Rhodium±alkyne complexes are 

synthesized by simple ligand addition or displacement of a weakly bound ligand. 


1.5.6.1 Method 1: 

Via Simple Alkyne Addition 

Reaction of chlorotris(triphenylstibine)rhodium(I) with excess hexafluorobut-2-yne in dichloromethane 

or benzene at room temperature gives chloro(hexafluorobut-2-yne)tris(triphenylstibine)rhodium(I) 

(166) (Scheme 62). [155] The two trifluoromethyl groups 

are not equivalent, as indicated by 19 F NMR. In solution, this pentacoordinate adduct 

shows a tendency to dissociate, presumably via loss of a triphenylstibine ligand. When 

the reaction is carried out at 808C, rhodacyclopentadiene 167 is formed as an air-stable, 

yellow-orange complex of empirical formula [RhCl(C 8F 12)(SbPh 3) 3], probably through the 

initial formation of 166. Similarly, addition of hexafluorobut-2-yne or dimethyl acetylenedicarboxylate 

to the cationic complex [Rh{P(OMe) 3} 4] + affords pentacoordinate, cationic 

rhodium complex 168. [148] However, the reaction of [Rh{P(OMe) 3} 4] + with phenylacetylene 

does not give 168 but polymeric materials instead. 

Scheme 62 Rhodium±Alkyne Complexes via Simple Ligand Addition [148,155] 

RhCl(SbPh3) 3 + F3C CF3 

[Rh{P(OMe) 3} 4] + 

R 1 = CF3, CO2Me 

R 1 

R 1 

CH 2Cl 2, rt, 5 min 

CH2Cl2 

rt, 0.5 h 

benzene 

80 oC, 3 h 

(MeO) 3P 

(MeO)3P + 

Rh 

(MeO) 3P 

(MeO) 3P 

168 

Ph3Sb Ph3Sb Rh 

Cl 

Ph3Sb 166 

F3C Ph3Sb Cl Rh 

Ph3Sb 

F3C 

167 

R 1 

R 1 

X − 

CF 3 

CF 3 

CF 3 

CF 3 




bChloro(hexafluorobut-2-yne)tris(triphenylstibine)rhodium(I) (166): [155] 

A soln of [RhCl(SbPh 3) 3] (0.3 g, 0.25 mmol) and excess of hexafluorobut-2-yne in dry CH 2Cl 2 

(5 mL) was sealed in vacuo and shaken at rt for 30 min. The resultingorange powder was 

recrystallized (CH 2Cl 2/Et 2O) to give 166 as dark red crystals; yield: 0.2 g(80%); mp 173± 

1758C; 19 F NMR (CDCl 3): ä ±12.7 (3F), ±9.8 (3F); IR (cm ±1 , CHCl 3): 1824 (s, CºC), 1820 (s, 

CºC), 1770 (sh), 1330 (w), 1305 (w), 1266 (w), 1242 (vs), 1217 (vs), 1130 (vs), 890 (w), 848 

(w), 799 (m). 

(Dimethyl acetylenedicarboxylate)tetrakis(trimethyl phosphite)rhodium(I) 

Tetraphenylborate (168,R 1 =CO 2Me; X = BPh 4); Typical Procedure: [148] 

A soln of [Rh{P(OMe) 3} 4] + BPh 4 ± (0.800 g, 0.87 mmol) and DMAD (0.123 g, 0.87 mmol) in 

CH 2Cl 2 (10 mL) was stirred at rt for 5 min. The solvent was removed under reduced pressure 

and the resultant oily solid was washed with MeOH and recrystallized (CH 2Cl 2/MeOH) 

to give the product as a pale-yellow solid; yield: not given; mp 109±1108C; IR (cm ±1 , Nujol 

mull): 1806 (sb). 

In the same manner, [Rh(F 3CCºCCF 3){P(OMe) 3} 4] + BPh 4 ± (168, R 1 =CF3) was prepared 

as a white solid; yield: not given; mp 172±1748C. 

1.5.6.2 Method 2: 


Displacement of weakly bound ligands such as phosphines and alkenes by alkynes provides 

an alternative method for the synthesis of monomeric rhodium±alkyne complexes. 

Reaction of complex 169 with excess alkyne under mild conditions leads to the formation 

of a square-planar, monomeric rhodium±alkyne complex 170 as a stable crystalline product 

(Scheme 63). [156±158] Complex 170, bearinga phosphine ligand, shows a tendency to 

dissociate, as indicated by molecular weight measurements, while the arsine complex 

does not. Anion metathesis usingsodium halide in acetone or benzene readily converts 

complex 170 (X = F) into the correspondingchloro-, bromo-, and iodorhodium complexes 

170 (X = Cl, Br, I). [159] Thermal displacement of a triphenylphosphine from cyclopentadienylbis(triphenylphosphine)rhodium(I) 

by an alkyne gives an alkyne(cyclopentadienyl)rhodium 

complex of the type [RhCp(R 1 CºCR 1 )(PPh 3)] (R 1 = Ph, CO 2Me). [160] 

Scheme 63 Synthesis of Monomeric Rhodium±Alkyne Complexes [156±158] 

L 

X Rh L 

L 

L = PPh3, PCy3, AsPh3 R1 = H, CF3, Et, Ph 

X = F, Cl 

R1 R1 benzene or CH2Cl2, rt, 2−45 min 

− L 

L 

X Rh 

L 

169 170 

Displacement of alkene ligands by alkynes provides an alternative method for the preparation 

of rhodium±alkyne complexes. Reaction of (acetylacetonato)bis(ethene)rhodium(I) 

or (dipivaloylmethanato)bis(ethene)rhodium(I) with excess hexafluorobut-2-yne at ±788C 

for 2 hours gives (acetylacetonato)(ethene)(hexafluorobut-2-yne)rhodium(I) (171, R 1 = Me; 

L=H 2C=CH 2) and (dipivaloylmethanato)(ethene)(hexafluorobut-2-yne)rhodium(I) (171, 

R 1 = t-Bu; L = H 2C=CH 2), respectively, in high to excellent yields (Scheme 64). [161] These 

complexes are air-stable, yellow crystalline solids, which are soluble in diethyl ether but 

can be recrystallized from diethyl ether/methanol at low temperatures. Complexes 171 

sublime readily in vacuo at ca. 508C. Similarly, the reaction of (acetylacetonato)bis(cyclooctene)rhodium(I) 

with hexafluorobut-2-yne at ±788C affords (acetylacetonato)(cyclo- 


R 1 

R 1


boctene)(hexafluorobut-2-yne)rhodium(I) (171, R 1 = Me; L = C 8H 14). In this case, however, 

the cyclooctene ligand is not displaced as readily as ethene from the rhodium center, 

and this reaction requires 12 h to complete. No rhodium±alkyne complex is formed 

when the reaction is carried out at room temperature. In solution, the ethene ligand of 

(171, R 1 = t-Bu; L = H 2C=CH 2) is readily displaced from the rhodium by alkenes such as cyclooctene, 

cycloheptene, propene, and cis-but-2-ene. When compound (171, R 1 = t-Bu; 

L=H 2C=CH 2) is exposed to carbon monoxide, a rapid displacement of both alkene and alkyne 

takes place to give dicarbonyl(dipivaloylmethanato)rhodium(I). However, the reaction 

of [Rh(dpm)(alkene)(F 3CCºCCF 3)] (alkene = H 2C=CH 2,C 7H 12, C 8H 14; dpm = dipivaloylmethanato) 

with one equivalent of ligand results in the exclusive displacement of the alkene 

ligand to give tetracoordinate rhodium complexes of the type [Rh(dpm)(F 3CCºCCF 3)L] 

(L = PPh 3, ArPh 3, SbPh 3), or with two equivalents of ligand to give pentacoordinate, 18-electron 

complexes of the type [Rh(dpm)(F 3CCºCCF 3)L 2] [L = PPh 3, ArPh 3, SbPh 3, PMePh 2, 

PEtPh 2;L 2 = (Ph 2PCH 2) 2]. [162] 

Scheme 64 Rhodium±Alkyne Complexes via Displacement of Alkene Ligands [161] 

L 

L 

Rh 

O 

O 

R 1 

R 1 

F3C CF3 

Et2O, −78 oC, 2−12 h 

− L 

R1 = Me; L = 

R1 = t-Bu; L = 

R1 H2C CH2 85% 

H2C CH2 93% 

= Me; L = C8H14 91% 

Chloro(diphenylacetylene)bis(triphenylarsine)rhodium(I) (170, R 1 = Ph; L = AsPh 3; X = Cl); 


PhCºCPh (0.1 g, 0.57 mmol, 3 equiv) was added to a soln of [RhCl(AsPh 3) 3] (0.2 g, 

0.19 mmol) in CH 2Cl 2 (ca. 5 mL) under N 2. The dark brown soln rapidly lightened in color, 

formingan orange precipitate. The precipitate was collected, washed with Et 2O, and dried 

in vacuo to give the product as orange crystals; yield: 0.16 g (90%); mp 218±2208C (dec); IR 

(cm ±1 , CHCl 3): 1883 (s, CºC). 

Other complexes were prepared in a similar manner: [RhCl(PhCºCPh)(PPh 3) 3], orange 

crystals; yield: 60%; mp 157±1638C (dec); [RhCl(F 3CCºCCF 3)(PPh 3) 3], [157] orange crystals; 

yield: 50%; mp 1418C (dec). 

(Acetylacetonato)(ethene)(hexafluorobut-2-yne)rhodium (171, R 1 = Me; L = H 2C=CH 2); 


F 3CCºCCF 3 (1.0 mL) was condensed (±1968C) into a soln of [Rh(acac)(H 2C=CH 2) 2] (0.25 g, 

0.97 mmol) in Et 2O (20 mL) in a 50-mL, round-bottomed flask. The flask was warmed to 

ca. ±788C and the suspension was stirred at this temperature for 2 h to give a clear yellow 

soln. The solvent was removed in vacuo and the yellow residue was recrystallized (Et 2O/ 

MeOH) at ±788C to give the product as yellow crystals; yield: 0.323 g (85%); mp 57±59 8C; 

sublimed at


b1.5.6.3 Method 3: 

Alkyne-Bridged Dimeric Complexes via Ligand Displacement 

Alkynes react with octakis(trifluorophosphine)dirhodium, [Rh 2(PF 3) 8], displacingtwo molecules 

of trifluorophosphine to afford transversely bridged, dinuclear rhodium±alkyne 

complexes of the type 172 (Scheme 65). [163] The best reaction conditions for this synthesis 

are simply heatinga mixture of octakis(trifluorophosphine)dirhodium with excess alkyne 

in the absence of solvent. Yields are almost quantitative for the reactions with disubstituted 

alkynes, but those with terminal acetylenes yield polymeric materials and the desired 

complexes 172 are formed only in poor yields. When the reaction of octakis(trifluorophosphine)dirhodium 

with an equimolar amount of an alkyne was carried out in a solvent, 

polymeric material was mainly formed accompanied by a small amount of 172. 

Complexes 172 are fairly air stable in the solid state as well as in solution. Their structures 

resemble well-known complexes of the type [Co 2(CO) 6(alkyne)] in that the coordinated alkyne 

is positioned perpendicular to the metal-metal bond axis. However, the rhodium 

counterparts of [Co 2(CO) 6(alkyne)], i.e. [Rh 2(CO) 6(alkyne)], are unknown. 

Scheme 65 Transversely Bridged Rhodium±Alkyne Complexes via Ligand Displacement [163] 

R 1 R 2 

R2 , 40−90 o R C 

1 

Rh2(PF3) 8 

− 2PF3 F3P 

F3P 

Rh Rh 

PF3 PF3 

F3P PF3 

172 

R 1 R 2 Colormp (8C) Ref 

H H yellow 60±61 [163] 

Ph H dark brown oil [163] 

Bu H brown oil [163] 

t-Bu H orange-brown 18 [163] 

Me Me yellow 118 [163] 

PrMe yellow ± [163] 

Ph Me orange 68 [163] 

Ph Et red oil [163] 

Ph Ph red 143±145 [163] 

Ph CO 2Me orange oil [163] 

CF 3 CF 3 yellow 159 [163] 

Complexes 172 (alkyne = PhCºCPh, PhCºCMe, or 4-O 2NC 6H 4CºCCO 2Et) undergo ligand 

substitution reactions with tertiary phosphines and arsines in diethyl ether or pentane 

to give complexes 173 as red, crystalline solids (Scheme 66). [164] The reaction appears to 

be general in scope, but only complexes containing arylacetylenes are isolable. The substitution 

reaction of 168 occurs at room temperature, which is more facile than that of 

the analogous complex [Co 2(CO) 6(alkyne)]. Single-crystal X-ray analysis of 172 

(R 1 =R 2 = Ph) confirms the transversely bridged, dinuclear framework. [164] 



bScheme 66 Transversely Bridged Rhodium±Alkyne Complexes via Ligand Substitution [164] 

R 1 R 2 

F3P F3P Rh Rh 

PF3 PF3 

F3P PF3 172 

L2 

− 2PF 3 

R1 = Ph, 4-O2NC6H4 R2 = Ph, Me, CO2Et L = PPh3, AsPh3, PMePh2, AsMePh2 R 1 R 2 

L 

F3P Rh Rh 

L 

PF3 

F3P PF3 173 

Dicarbonyl(cyclopentadienyl)rhodium(I) (13) reacts with hexafluorobut-2-yne at 1008Cto 

afford predominantly the alkyne-bridged dimeric complex trans-175, together with a 

number of other organorhodium products. [165] A more effective synthesis of trans-175 

uses ì-carbonyldicarbonylbis(cyclopentadienyl)dirhodium(I) (174) as the rhodium precursor 

in place of dicarbonyl(cyclopentadienyl)rhodium(I) (Scheme 67). [166] Single-crystal Xray 

analysis of trans-175 reveals the ó-bridging alkyne±dirhodium arrangement in which 

the alkyne unit is parallel to the Rh-Rh bond and the trans geometry of the two carbonyl 

ligands. In solution, trans-175 undergoes rapid carbonyl scrambling at room temperature, 

as evidenced by 13 C NMR studies. Amongmany side products formed in the reaction, the 

less stable isomer cis-175 is isolated in very low yield. The complex cis-175 can also be prepared 

in low yield by the reaction of tetracarbonyldi-ì-chlorodirhodium(I) with hexafluorobut-2-yne, 

followed by treatment of the resultingintractable products with cyclopentadienylthallium 

at room temperature. In solution, cis-175 is unstable and isomerizes to 

trans-175 at room temperature. 

Scheme 67 Synthesis of Dimeric Alkyne-Bridged Rhodium Complexes [166] 

O 

Rh Rh CO 

OC 

174 

F3C CF3 hexane, 100 oC, 24 h 

− CO 

F 3C CF 3 

Rh Rh 

OC CO 

trans-175 

+ 

F 3C CF 3 

Rh Rh 

OC CO 

cis-175 

(ì-Alkyne)hexakis(trifluorophosphine)dirhodium (172); General Procedure: [163] 

An excess of the alkyne was condensed into solid [Rh 2(PF 3) 8] (0.20 g) in a conventional 

high-vacuum system. The mixture was heated to 40±90 8C (dependingon the alkyne 

used) until the evolution of gas subsided (F 3P, IR identification) and a dark red or yellow 

oil was formed. Unreacted alkyne and liberated F 3P were removed under vacuo at rt (or 

below rt if the complex was very volatile as in the case of hexafluorobut-2-yne). The residue 

was purified by sublimation or distillation at 0.005 Torr on to a cold finger. Complexes 

of disubstituted alkynes were isolated in almost quantitative yields. However, terminal 

alkynes formed appreciable amounts of involatile polymeric materials, and the 

yields of their complexes were 20±50%. 

Bis(methyldiphenylarsine)(ì-1-phenylpropyne)tetrakis(trifluorophosphine)dirhodium 

(173,R 1 = Ph; R 2 = Me; L = AsMePh 2); Typical Procedure: [164] 

MePh 2As (0.30 g, 1.23 mmol, excess) in pentane (10 mL) was added to a soln of 

[Rh 2(PhCºCMe)(PF 3) 6] (0.21 g, 0.45 mmol) in pentane (15 mL) under N 2. The soln rapidly 

turned dark red. A small amount of flocculent buff precipitate was removed by filtration, 

and the filtrate was kept at ±108C for 2 d. The resultingburgundy-colored crystals were 

collected, washed with pentane (3 ” 10 mL), and dried in vacuo to give the product; yield: 

0.18 g(63%); mp 136±1388C. 




btrans-Dicarbonylbis(cyclopentadienyl)(ì-hexafluorobut-2-yne)dirhodium(I) 

(trans-175): [166] 

A mixture of [Rh 2Cp 2(CO) 3] (0.1 g, 0.23 mmol) and F 3CCºCCF 3 (0.3 g, 1.85 mmol, 8 equiv) in 

hexane (5 mL) was heated at 1008C for 24 h. The mixture was dissolved in CHCl 3, filtered, 

and concentrated under reduced pressure. Preparative TLC of the residue usinghexane/ 

CHCl 3 (9:1) as eluant gave trans-175 as an orange-red solid; yield: 0.082 g (62%); mp 162± 

1638C; 1 H NMR: 5.53 (s, Cp); IR (cm ±1 ): CO absorption 1992 (s) (CHCl 3) or 2004 (s) (hexane). 


1.5.6.4 Method 4: 

[2+2+2] Cycloaddition 

Transition-metal-catalyzed cyclotrimerization of acetylenes has attracted much attention 

as a powerful tool for the construction of substituted benzene derivatives. [167±169] A generally 

accepted mechanism of the metal-catalyzed acetylene cyclotrimerization suggests 

the involvement of metallacycles as key intermediates in the catalytic cycle (see Scheme 

27). Amongvarious metals species, cobalt, rhodium, palladium, and nickel complexes 

have been shown to be effective catalysts for promotingthese transformations. To date, 

there are only a few successful reports on the synthetical usefulness of intermolecular cyclotrimerization 

reactions of alkynes. [167] The chemo- and regioselectivity problems of 

this reaction lead to complex mixtures of products, which severely limits its utility. On 

the other hand, the intramolecular counterpart of this reaction, in which all three alkynes 

are incorporated in the substrate, provides one of the most efficient methods for 

the synthesis of polycyclic aromatic systems. 

1.5.6.4.1 Variation 1: 

[2+2+2] Cyclotrimerization 

The partially intramolecular [2 +2+2] cycloaddition reaction of hepta-1,6-diyne 176 with 

a monoalkyne catalyzed by Wilkinson s catalyst, chlorotris(triphenylphosphine)rhodium(I), 

shows a good level of chemoselectivity. [170] The reaction occurs readily with a catalyst 

loadingof 0.5±2 mol% in a range of solvents (tert-butyl alcohol, tetrahydrofuran, ethanol, 

ethanol/chloroform) to give substituted benzene derivatives 177 in fair to excellent 

yields (Scheme 68). An excess of the monoalkyne is required to suppress the formation of 

side products. This reaction can be used to synthesize substituted indanes and their heterocyclic 

congeners. Two completely intramolecular variants of this reaction are shown 

in Scheme 69. Triyne 178 (n = 1) readily cyclizes at room temperature for 3 hours to give 

tricyclic benzene derivative 179 (n = 1), while 178 (n = 2) requires refluxingin ethanol for 

3 days to effect cyclization to 179 (n = 2). The intramolecular cyclotrimerization of triynes 

has been applied to efficient syntheses of two illudalane natural products, pterosin Z and 

calomelanolactone. [171] 



bScheme 68 Rhodium-Catalyzed [2 + 2+2] Alkyne Cyclotrimerization [170] 

X 

176 

R 1 

R 1 

R 2 

0.5−2 mol% RhCl(PPh 3) 3, 0−78 o C 

41−99% 

X = CO2Et, CH2, CAc2, SO, SO2, S, NAc, O 

R1 = H, Me; R2 = H, Pr, Bu, CH2OH, CH2OMe, CH2OAc, (CH2)2OH, Ph 

Scheme 69 Rhodium-Catalyzed Intramolecular Cyclotrimerization of Triynes [171] 

( ) n 

2 mol% RhCl(PPh3)3, EtOH 

n = 1: rt, 3h 

n = 2: heat, 3d 

O O 

n = 1 74% 

n = 2 38% 

O 

178 179 

There are very few reports on the successful rhodium-catalyzed [2 +2+2] cyclotrimerization 

between alkynes and alkenes. [167] The cyclotrimerization of hepta-1,6-diynes 180 

with monoalkenes promoted by Wilkinson s catalyst is shown in Scheme 70. [172] However, 

this reaction is limited to substrates containingthree-atom tethers connectingunhindered, 

disubstituted diynes. The intramolecular cyclization of enediyne 181 in the presence 

of 2 mol% chloro(triphenylphosphine)rhodium proceeds smoothly at room temperature 

to afford the unusual bicyclic triene 182 in fair yield (Scheme 70). [172] The product 

182 is unstable and decomposes in chloroform at room temperature. 

X 

R 1 

R 1 

177 

( )n 

O 

Scheme 70 Rhodium-Catalyzed Cyclotrimerization of Diynes and Alkenes [172] 

O + 

O 

180 

181 

O 

CN 

56% 

2 mol% RhCl(PPh3)3 

t-BuOH, 80 o C, 6 h 

59% 

2 mol% RhCl(PPh 3) 3, EtOH, rt 

Bicyclic Compounds 177; General Procedure: [170] 

A soln of the diyne 176 and the monoalkyne in absolute EtOH was degassed by bubbling 

N 2 through the soln for 10 min prior to addition of [RhCl(PPh 3)]. The resultingclear red 

soln was stirred at an appropriate temperature until all the diyne had been consumed 

(GLC, NMR, or TLC monitoringas needed). Solvent was then removed and the residue 

was filtered through a short column (alumina or silica gel, Et 2OorCH 2Cl 2) to remove the 

catalyst. The solvent was removed and the residue recrystallized or distilled to give 177; 

yield: 3±80%. 

O 

O 

182 

O 

R 2 

CN 





Silylcarbotricyclization of Triynes 

A silane-initiated formal intramolecular [2 +2+2] cyclotrimerization of triynes catalyzed 

by rhodium complexes has been reported. [173] The silylcarbotricyclization (SiCaT) reaction 

of dodeca-1,6,11-triynes 183 with a hydrosilane in the presence of a rhodium complex 

proceeds at ambient temperature to afford highly substituted, tetrahydro-as-indacenes 

184 and 185 as well as the heterocyclic analogues in good to excellent yields (Scheme 

71). Amongthe rhodium complexes used, the clusters dodecacarbonyltetrarhodium(0) 

and dodecacarbonyldicobaltdirhodium(0) are the most effective catalyst precursors. The 

reaction is applicable to a wide range of hydrosilanes, and is tolerant of a variety of functional 

groups including esters, tosylamines, benzylamines, and ethers. The SiCaT reaction 

is applicable for the construction of 5±5±6 and 6±6±6 ringsystems. The proposed 

mechanism of the SiCaT reaction includes initial insertion of an alkyne moiety of 183 

into the Si-[Rh] bond of the active catalyst species followed by three consecutive carbometalations. 

Scheme 71 Rhodium-Catalyzed Silylcarbotricyclization (SiCaT) of Triynes [173] 

X 

R 1 

183 

X 

0.5−1 mol% Rh(0), R2 3SiH 

CO (1 atm), toluene, rt 

75−96% 

Rh(0) = Rh4(CO)12, Co2Rh2(CO)12 

R1 = H, Me; R2 3 = PhMe2, (OEt)3, Et3, t-BuMe2, Ph2Me, Ph3 

X = C(CO2Et) 2, C(CH2OMe) 2, C(CH2OBn) 2, NTs, NBn, O 

R 2 3Si R 1 R 1 

X 

184 major 

X 

+ 

X 

185 minor 

4-[Dimethyl(phenyl)silyl]-2,2,7,7-tetrakis(ethoxycarbonyl)-1,3,6,8-tetrahydro-as-indacene 

[184,R 1 =H;R 2 3 = PhMe 2; X = C(CO 2Et) 2] and 2,2,7,7-tetrakis(ethoxycarbonyl)-1,3,6,8-tetrahydro-as-indacene 

[184, R 1 = H; X = C(CO 2Et) 2]; General Procedure: [173] 

A reaction vessel equipped with a stirrer bar and a CO inlet was charged with [Rh 4(CO) 12] 

(6.8 ” 10 ±3 mmol, 0.5 mol%). After purging the vessel with CO, toluene (6 mL) was introduced 

to dissolve the catalyst, Me 2PhSiH (2.70 mmol, 2 equiv) was added, and the soln 

was stirred at rt for a few min. 183 [R 1 = H; X = C(CO 2Et) 2] (1.35 mmol) in toluene (10 mL) 

was added and the mixture stirred under CO (760 Torr) at rt for 34 h. All volatiles were removed 

under reduced pressure and the residue was subjected to column chromatography 

(silica gel, hexanes/EtOAc 13:1) to give 184 (86%) and 185 (10%). 

1.5.6.5 Method 5: 

Rhodium-Catalyzed Pauson±Khand Reaction 

Transition-metal-promoted cocyclization of an alkyne, alkene, and carbon monoxide 

(Pauson±Khand reaction) provides a useful tool for selective organic synthesis and has 

found many applications in natural product syntheses. [174] This reaction is one of the 

most powerful and highly convergent methods for the synthesis of cyclopentenones 

from readily available startingmaterials through a formal [2+2+1]cycloaddition process. 

Since 1990, the synthetic significance of this reaction has greatly been increased by the 

findingof catalytic Pauson±Khand conditions, in which cobalt, titanium, or ruthenium 

complexes serve as catalysts. [175±177] Recently, two examples of rhodium-catalyzed Pauson±Khand 

reactions have appeared. [178,179] Thus, the reaction of 1-en-6-ynes with carbon 

monoxide in the presence of a dimeric rhodium complex affords bicyclo[3.3.0]octenones 

186 in good to excellent yields (Scheme 72). The use of a dimeric complex, tetracarbonyl- 


X


bdi-ì-chlorodirhodium(I) at 130 8C ortrans-dicarbonyldi-ì-chloro[bis(1,3-bis(diphenylphosphino)propane]dirhodium(I) 

at 1108C, without an additive proved to be the most effective. 

The reaction is also applicable to terminal and trimethylsilyl-substituted alkynes as 

well as 1,1-disubstituted alkenes, although the reaction of these substrates requires higher 

reaction temperatures or carbon monoxide pressure and gives lower product yields. 

Notable advantages of the use of rhodium complexes compared to other catalysts are 

that the reaction is highly efficient and requires only an atmospheric pressure of carbon 

monoxide. 

Scheme 72 Rhodium-Catalyzed Pauson±Khand Reaction [178,179] 

X 

R 1 

Rh(I), CO (1 atm), toluene or Bu2O 

110−130 oC, 9−40 h 

60−99% 

Rh(I) = 2.5 mol% trans-Rh2Cl2(CO) 2(dppp) 2, 1−2 mol% Rh2Cl2(CO) 4 

R1 = Me, Et, Ph 

X = C(CO2Et) 2, O, NTs 

6-Methyl-2-(4-toluenesulfonyl)-2,3,3a,4-tetrahydrocyclopenta[c]pyrrol-5(1H)-one 

(186,R 1 = Me; X = NTs); Typical Procedure: [179] 

In a 50-mL, round-bottomed flask was placed a soln of trans-[Rh 2Cl 2(CO) 2(dppp) 2] (21.9 mg, 

0.019 mmol) and N-allyl-N-but-2-ynyl-4-toluenesulfonamide (200 mg, 0.759 mmol) in toluene 

(5 mL). The flask was charged with CO under balloon pressure at 208C and then heated 

at 110 8C for 40 h in an oil bath. After coolingthe mixture, the solvent was removed in 

vacuo, and the resulting residue was subjected to column chromatography (silica gel, hexane/EtOAc 

3:1) to give the product as a white solid; yield: 184 mg (83%); mp 102.5±1238C. 

1.5.6.6 Method 6: 

[5+2] Vinylcyclopropane±Alkyne Cycloaddition 

Rhodium complexes have been shown to efficiently promote [5+2] cycloaddition of vinylcyclopropanes 

with alkynes, [180] alkenes, [181] and allenes [121] (see Section 1.5.4.7.3). This 

method provides an efficient and practical route to seven-membered rings. As illustrated 

in Scheme 73, the reaction proceeds through metalacyclopentene 188 arisingfrom 187. 

Driven by the strain of the attached cyclopropane ring, metallacycle 188 undergoes ring 

expansion to form a metallacyclooctadiene 190. Reductive elimination of 190 yields bicyclo[5.3.0]product 

191 and regenerates the active catalyst species. An alternative mechanism 

involvingmetallacycle 189 in place of 188 is also possible. [181] 

Scheme 73 Mechanistic Pathway of [5 +2]-Cycloaddition Reactions [180,181] 

X 

MLn 

187 188 

X 

MLn 

X 

or 

X 

X 

Ln M 

186 

189 

R 1 

Ln 

M 

O 

− ML n 

190 191 

X 





Intramolecular [5+2] Cycloaddition 

A solution of vinylcyclopropane 192 and tetracarbonyldi-ì-chlorodirhodium(I) was heated 

at 1108C for 20 minutes to give the bicyclic cycloadduct 193 in good yield (Scheme 

74). [180] The reaction also occurs in comparable yields at temperatures as low as 308C, although 

longer reaction times are required. The reaction tolerates ether and ester functional 

groups and worked with substituted alkynes and trisubstituted alkenes 194. Wilkinson 

s catalyst, chlorotris(triphenylphosphine)rhodium(I), can be used for this transformation 

either by itself or with silver triflate as additive. [182] Some shortcomings associated 

with the use of Wilkinson s catalyst are the formation of isomerized secondary products 

in the reaction of 194 and the formation of polymers at higher substrate concentrations 

(2 M). However, it is worth mentioningthat Wilkinson s catalyst can catalyze the reactions 

of substrates containing terminal alkynes to give cycloadducts in fair to high yields, 

while no [5+2]-cycloaddition product is obtained when tetracarbonyldi-ì-chlorodirhodium(I) 

is used. [180,182] 

Scheme 74 [5 + 2] Vinylcyclopropane±Alkyne Cycloaddition [180] 

X 

R 1 

5 mol% Rh 2Cl 2(CO) 4, toluene 

192 193 

X R 1 Temp (8C) Time Yield (%) Ref 

O Ph 65 15 min 78 [180] 

O Ph 30 14h 80 [180] 

O TMS 30 14 h 78 [180] 

C(CO2Me) 2 Me 110 20 min 82 [180] 

C(CO2Me) 2 Me 30 16 h 79 [180] 

X 

194 

R 1 

5 mol% Rh2Cl2(CO)4, toluene 

X R 1 Temp (8C) Time Yield (%) Ref 

O Ph 110 20 min 80 [180] 

O Me 110 20 min 78 [180] 

C(CO2Me) 2 TMS 30 2 d 81 [180] 

C(CO2Me) 2 Me 110 3 h 84 [180] 


X 

X 

R 1 

R 1


bDimethyl 8-Methyl-3,3a,6,7-tetrahydro-1H-azulene-2,2(1H)-dicarboxylate 

[193,R 1 = Me; X = C(CO 2Me) 2]; Typical Procedure: [180] 

To a soln of [Rh 2Cl 2(CO) 4] (5 mol%) in toluene (4 mL) was added 192 [R 1 = Me; X = C(CO 2Me) 2; 

0.052 g, 0.2 mM] under a N 2 atmosphere. The resultingsoln was heated at 110 8C for 

20 min. After coolingthe mixture, hexane (5 mL) was added, and the soln was passed 

through a column (silica gel 0.5 g, hexane/EtOAc 20:1) to give the product as a colorless 

oil; yield: 0.043 g(82%). 

1.5.6.6.2 Variation 2: 

Intermolecular [5+2] Cycloaddition 

It has been shown that tetracarbonyldi-ì-chlorodirhodium(I) catalyzes the intermolecular 

[5+2] cycloadditions of vinylcyclopropanes and alkynes. [183] Reaction of siloxycyclopropane 

195 with an alkyne in the presence of a catalytic amount of tetracarbonyldi-ì-chlorodirhodium(I) 

at 408C for a few hours affords substituted cycloheptenones 196 in good 

to excellent yield after acid hydrolysis (Scheme 75). The reaction can be conducted in various 

solvents, includingtetrahydrofuran, toluene, and ethanol, although the use of weakly 

coordinatingsolvents such as chloroform and dichloromethane leads to shorter reaction 

times and milder conditions. The reaction is quite general and applicable to electron-rich, 

electron-poor, conjugated, internal, and terminal alkynes as well as acetylene. 

Accordingly, this method provides a practical route to nonconjugated cycloheptenones. 

Scheme 75 Rhodium-Catalyzed [5 + 2] Cycloaddition [183] 

TBDMSO 

195 

R 1 

5 mol% Rh 2Cl 2(CO) 4 

CH 2Cl2, 40 o C, 1−3 h 

R 1 

OTBDMS 

1% HCl/EtOH 

R 1 Yield (%) Ref R 1 Yield (%) Ref 

CO2Et 93 [183] Ph 81 [183] 

Ac 88 [183] iPr84 [183] 

CH2OMe 88 [183] cyclopropyl 88 [183] 

CH2OH 74 [183] cyclohex-1-enyl 75 [183] 

TMS 77 [183] 

4-Substituted Cyclohept-4-en-1-ones 196; General Procedure: [183] 

To an oven-dried, argon-purged Schlenk flask was added [Rh 2Cl 2(CO) 4] (0.05 mmol) and 

anhyd CH 2Cl 2 (10 mL) under an argon atmosphere. Argon was bubbled through the resultant 

yellow solution (1 min) and siloxycyclopropane 195 was added (1 mmol). After an additional 

argon purge (1 min), an alkyne (1.1±1.5 mmol) was added and the flask was placed 

in an oil bath preheated at 408C. The progress of the reaction was monitored by TLC. 

Upon completion, the dark mixture was treated with 1% HCl in EtOH (0.2 mL), and the resulting 

mixture was filtered through a short pad of silica gel (Et 2O eluant), and concentrated 

in vacuo. The residue was purified by flash column chromatography on silica gel to 

give 196. 

R 1 

O 

196 




b1.5.6.7 Method 7: 

Enyne Carbocyclization 

Wilkinson s catalyst, chlorotris(triphenylphosphine)rhodium(I), has been shown to effect 

the carbocyclization of 1-en-6-ynes to methylenecyclohex-2-enes. [184] Reaction of 197 in 

the presence of a catalytic amount of Wilkinson s catalyst in refluxingacetonitrile produces 

exo-methylenecyclohex-2-ene 198 in good yields (Scheme 76). This carbocyclization 

proceeds regioselectively in a 6-exo-trig mode. The nitrogen counterparts of 198 (X = NBz, 

NBn) can be synthesized usingthe same procedure, albeit in much lower yields (20±28%). 

Terminal substitution on the alkene suppresses the carbocyclization significantly, and 

substrates with internal alkynes shut down this catalytic process completely. 

Scheme 76 Rhodium-Catalyzed Carbocyclization of 1-En-6-ynes [184] 

R 1 

X 

R 1 

197 

5 mol% RhCl(PPh3)3, MeCN, 80 o C, 6−24 h 

R1 = H; X = C(CO2Et)2 73% 

R1 = H; X = C(COMe)2 62% 

R1 = Me; X = CHCO2Et 83% 

exo-Methylenecyclohex-2-enes 198; General Procedure: [184] 

To a soln of the 1-en-6-yne 197 in dry MeCN was added [RhCl(PPh 3) 3] (5 mol%). The mixture 

was refluxed under a N 2 atmosphere and the progress of the reaction was monitored by 

GLC. When the reaction was complete, the solvent was removed under reduced pressure 

and the residue filtered through a short column (neutral alumina, petroleum ether/Et 2O 

3:1) to remove the catalyst. The product was purified by preparative TLC to give 198. 

1.5.6.8 Method 8: 

Silylcarbocyclization 

It has been shown that silicon-initiated carbocyclizations of diynes, enynes, and enediynes 

are catalyzed by rhodium complexes. These intramolecular carbocyclization processes 

afford monocyclic, bicyclic, and fused bicyclic ringsystems. Silicon is incorporated 

in the final organic products, thus providing additional handles for further functional 

group manipulation. These processes include silylcarbobicyclization (SiCaB) of 1,6-diynes, 

silylcarbocyclization (SiCaC) of enynes, carbonylative silylcarbocyclization 

(CO-SiCaC) of enynes, and cascade silylcarbobicyclization of endiynes. 

1.5.6.8.1 Variation 1: 

Silylcarbobicyclization of 1,6-Diynes 

Reaction of 1,6-diynes 199 with tert-butyldimethylsilane in the presence of a catalytic 

amount of (acetylacetonato)dicarbonylrhodium(I) under carbon monoxide (11±15 ” 10 3 

Torr) affords 2-silylbicyclo[3.3.0]octenones 200 (Scheme 77). [185,186] Formation of a small 

amount of the more stable isomer 201 is observed in some cases, particularly for substrates 

containingheteroatoms. Rhodium±cobalt, mixed-metal complexes such as dodecacarbonyldicobaltdirhodium(0) 

and tetrakis(tert-butyl isocyanide)tetracarbonylcobaltrhodium(0) 

are also effective catalysts for this carbonylative silylcarbobicyclization process. 

Compounds 200 are readily isomerized to the more stable isomers 201 in quantitative 

yields by reacting 200 with a catalytic quantity of rhodium(III) chloride trihydrate in 

ethanol. Thus, a mixture of 200 and 201 can be readily isomerized to a single product 201 

usingthis protocol. 


R 1 

X 

R 1 

198


bScheme 77 Silylcarbobicyclization of 1,6-Diynes [185±187] 

X 

199 

2 mol% Rh(acac)(CO)2, TBDMSH, CO 

toluene, 50 oC, 15−50 atm, 12−20 h 

TBDMS 

X O 

X Yield (%) of 200 Yield (%) of 201 Ref 

C(CO2Et) 2 92 0 [187] 

CMeCO2Et 70 0 [187] 

CHCO2Et 48 17 [187] 

CHCH2OAc 73 0 [187] 

NBn 10 60 [187] 

O 27 22 [187] 

X O 

200 

TBDMS 

RhCl 3 3H 2O, EtOH, 50 o C, 24 h 

quant 

200 

X O 

201 

TBDMS 

+ 

TBDMS 

X O 

2-(tert-Butyldimethylsilyl)bicyclo[3.3.0]octen-3-ones 200; General Procedure: [187] 

To a round-bottomed flask (25 mL) containinga stirrer bar and [Rh(acac)(CO) 2] (5.2 mg, 

0.02 mmol) under a CO atmosphere was added a soln of TBDMSH (2 mmol) and the hepta-1,6-diyne 

(1 mmol) in toluene (15 mL) and the flask was placed in a stainless steel autoclave. 

The autoclave was charged with CO (7600 Torr) and the pressure released carefully. 

This process was repeated three times. The CO pressure was then adjusted to 11±38 ” 10 3 

Torr, and the mixture was stirred at 508C for 12±20 h. The autoclave was cooled in an ice 

bath, CO was carefully released, and the mixture was submitted to GC and TLC analyses. 

After evaporation of the solvent under reduced pressure, the residue was submitted to 

flash chromatography (silica gel) to afford 200. 

1.5.6.8.2 Variation 2: 

Silylcarbocyclization of Enynes 

Rhodium complexes have been shown to catalyze the silylcarbocyclization (SiCaC) of 1en-6-ynes 

to afford exo-silylmethylenecyclopentane and its congeners. [188] Reaction of 

enyne 202 with a hydrosilane (1.5 equivalents) catalyzed by dodecacarbonyltetrarhodium(0) 

in hexane instantaneously gives the SiCaC products 203 in high to quantitative 

yields (Scheme 78). [189] It is necessary to run the reaction under an atmosphere of carbon 

monoxide to stabilize the active catalyst species, particularly when dodecacarbonyltetrarhodium(0) 

is used as catalyst. The use of dodecacarbonyldicobaltdirhodium(0) as 

catalyst allows the SiCaC reaction to take place under a nitrogen atmosphere. The SiCaC 

reaction is remarkably effective and general, thus providing a highly efficient method for 

the synthesis of carbocycles and heterocycles. The reaction is applicable to a variety of hydrosilanes 

and substrates containingboth terminal and internal alkynes. Functional 

groups such as esters, ethers, and amines are all tolerated under the reaction conditions. 

The formation of six-membered ringproducts from 1-en-7-ynes is also possible, albeit in 

lower yields. 

201 




bScheme 78 Silylcarbocyclization Reaction of 1-En-6-ynes [188,189] 

X 

202 

R 1 

R2 3SiH (1.5 equiv), 0.5 mol% Rh4(CO)12 

CO (1 atm), hexane, rt,

1.5.7 Rhodium±Alkene Complexes 603 

EtO 2C 

EtO 2C 

PhMe2SiH (1 equiv) 

1 mol% Rh(acac)(CO) 2 

CO (1 atm), toluene, 50 oC, 6 h 

b50% 

EtO 2C 

EtO 2C H 

SiMe 2Ph 

EtO2C CO2Et EtO2C CO2Et 207 

208 

Diethyl 3-{[Dimethyl(phenyl)silyl]methylene}-4-methylcyclopentane-1,1-dicarboxylate 

[203,R 1 =H,R 2 3 = PhMe 2; X = C(CO 2Et) 2]; Typical Procedure: [189] 


(3.8 mg, 0.005 mmol, 0.5 mol%). After purging the vessel with CO, hexane (1 mL) was 

added to dissolve the catalyst. After stirringfor 5 min at rt, the mixture was transfered to 

a 10-mL, round-bottomed flask containinga solution of 202 [R 1 = H; X = C(CO 2Et) 2; 238 mg, 

1 mmol], and PhMe 2SiH (205 mg, 1.5 mmol) in hexane (1.5 mL) was added via cannula under 

a CO atmosphere. The mixture was stirred under CO (760 Torr) at rt for 1 min, then all 

volatiles were removed under reduced pressure and the crude product was purified by 

column chromatography (silica gel, hexanes/EtOAc 16:1) to give the product as a viscous, 

colorless liquid; yield: 356 mg(95%). 

Diethyl 3-{[Dimethyl(phenyl)silyl]methylene}-4-(2-oxoethyl)cyclopentane-1,1-dicarboxylate 

[204, X = C(CO 2Et) 2,]; Typical Procedure: [189] 


(3.8 mg, 0.005 mmol, 0.5 mol%). After purging the vessel with CO, 1,4-dioxane (2 mL) was 

added to dissolve the catalyst; 5 min later, (EtO) 3P (17 mg, 0.10 mmol, 5 equiv/Rh) in 1,4dioxane 

(2 mL) was added via a syringe. Over the course of 10 min, the color of the mixture 

turned from bright red to dark red. The resulting catalyst soln was then transfered 

to a 100-mL, round-bottomed flask containinga solution of 202 [R 1 = H; X = C(CO 2Et) 2; 

238 mg, 1 mmol] and PhMe 2SiH (149 mg, 1.1 mmol) in 1,4-dioxane (50 mL) was added via 

cannula under a CO atmosphere. The reaction flask was placed in a 300-mL, stainless steel 

autoclave, pressurized with CO (15 ” 10 3 Torr) and then heated to 105 8C with stirringfor 

48 h. All volatiles were removed under reduced pressure and the crude product was purified 

by column chromatography (silica gel, hexanes/EtOAc 15:1) to give the product as a 

viscous, colorless liquid; yield: 360 mg(90%). 


Rhodium±Alkene Complexes 

Alkene complexes of rhodium are almost exclusively rhodium(I) complexes. In general, 

these complexes are synthesized either by direct reduction of rhodium(III) chloride with 

an alkene in methanol or by replacingethene ligands with other monoalkenes. The latter 

method is convenient for the synthesis of complexes containing high molecular weight 

alkenes since the volatility of the dissociated ethene provides the drivingforce for the reaction. 

The stability of the rhodium-alkene bond decreases with increasingalkyl substitution 

at the alkenyl carbon atoms, but is enhanced with increasingnumbers of electronegative 

substituents on the alkene, presumably owing to the improved back-bonding 

from the metal to the alkene ð* orbital. An extensive review on the synthesis, chemistry, 

and properties of transition-metal±alkene complexes has been published. [192] 

H 




bSynthesis of Product Subclass 7 

1.5.7.1 Method 1: 


Ethene gas is bubbled into an aqueous methanol solution of rhodium(III) chloride trihydrate 

for seven hours to give the chlorine-bridged, dimeric ethene complex di-ì-chlorotetrakis(ethene)dirhodium(I) 

(209), [193] which is historically the first rhodium±alkene 

complex prepared [75] (Scheme 81) (see also Section 1.6.6.3). The yield of 209 can be improved 

if the ethene addition is continued for 22 hours. [99] Complex 209 precipitates 

from the mixture as a red-orange solid which is relatively stable to air at room temperature 

and does not melt. It is sparingly soluble in organic solvents and thus cannot be purified 

by crystallization. It is preferable to store 209 at around 08C. In the presence of lithium 

bromide the reaction gives the corresponding bromide complex di-ì-bromotetrakis(ethene)dirhodium(I), 

which is the result of an anion metathesis reaction between lithium 

bromide and the initially formed 209. [194] Metathesis with other anions affords the 

correspondingiodo, acetate, and nitrate complexes, but these are too unstable to isolate. 

Analogous complexes with deuterated ethene, [195] propene, [75] 2,3-dimethylbut-2-ene, [74] 

cycloheptene, norbornene, and cis-cyclooctene, e.g. to give 210, [60] are also prepared by reduction 

of rhodium(III) chloride trihydrate usingappropriate alkenes in aqueous alcohol. 

Compound 210 is a versatile precursor for a variety of rhodium complexes. [196] 

Scheme 81 Synthesis of Dimeric Rhodium±Alkene Complexes via 

Ligand Substitution [60,99,193,197] 

RhCl3 3H2O Rh Cl H2C CH2, MeOH 

Rh 

Cl 

RhCl 3 3H 2O 

, iPrOH 

209 

Cl 

Rh Rh 

Cl 

Di-ì-chlorotetrakis(ethene)dirhodium (209): [193] 

Reprinted from (Cramer, Inorganic Syntheses), Copyright (1974), p 14, with permission 

from Inorganic Synthesis Inc. 

A soln of RhCl 3 ·3H 2O (10 g, 38 mmol) in H 2O (15 mL) was transferred to a round-bottom 

flask (500 mL) containingMeOH (250 mL) and a stir bar. The flask was degassed by alternately 

evacuating(water pump) and refillingwith ethene to 760 Torr. The MeOH soln was 

stirred at rt under 760 Torr of ethene. The product began to precipitate as a very fine solid 

after ca. 1 h. After 7 h, most of the liquid was decanted and the solid was collected by filtration 

under vacuum (drawingair through the solid was avoided). The solid was washed 

with MeOH (50 mL) and dried in vacuo at rt to give 209; yield: 4.8±5.0 g(60±65% first crop). 

A soln of NaOH (1.5 g) in H 2O (3 mL) was added to the filtrate and washings from the first 

crop. The soln was treated with ethene in the same manner as that mentioned above to 

recover 1.0±1.5 gof 209; combined yield: 6 g(75%); IR (cm ±1 , KBr): 3060 (m), 2980 (m), 1520 

(m), 1430 (vs), 1230 (s), 1215 (vs), 999 (vs), 952 (m), 930 (s). 


210


bDi-ì-chlorotetrakis(cis-cyclooctene)dirhodium (210): [197] 

To a 3-necked, round-bottomed flask (250-mL), equipped with a reflux condenser and a Hg 

bubbler, were sequentially added RhCl 3 ·3H 2O (4.50 g, 17.1 mmol), H 2O (25 mL), propan-2ol 

(50 mL), and cyclooctene (9.0 mL, 67.5 mmol). The resultingdark red soln was refluxed 

for 12 h and the mixture was briefly purged with a slow stream of argon every 2 h during 

the reflux. The resultingyellow soln was stirred for an additional 12 h at rt to form a yellow 

precipitate. The precipitate was collected by filtration, washed (cold MeOH), and 

dried in vacuo to give 210; yield: 11.3±11.8 g(92±96%); mp 150 8C (dec); IR (cm ±1 , Nujol 

mull): 412 (s), 334 (vw), 289 (m), 285 (m), 252 (s, Rh-Cl). 

1.5.7.2 Method 2: 

Monomeric Complexes via Chlorine-Bridge Cleavage Reactions 

The nucleophilic substitution reaction of di-ì-chlorotetrakis(ethene)dirhodium(I) (209) 

with acetylacetonate displaces the chloride ion with concomitant cleavage of the dimeric 

framework. [194,196] Thus, the reaction of a diethyl ether solution of 209 with pentane-2,4dione 

in the presence of potassium hydroxide gives the monomeric complex (acetylacetonato)bis(ethene)rhodium(I) 

(211, R 1 = Me) (Scheme 82). The product, obtained as yelloworange 

crystals, is relatively stable to air and is best stored at 0 8C. Similar reactions with 

other anions of â-diketones give analogous products 211 (R 1 = t-Bu, CF 3, [198] Ph, 4-MeC 6H 4, 

4-ClC 6H 4, 4-O 2NC 6H 4 [199] ) in good to high yields. Alternatively, the dimeric framework can 

be cleaved by the action of cyclopentadienyl anion to produce monomeric alkene(cyclopentadienyl)rhodium 

complexes. Thus, the reaction of 209 with in situ generated sodium 

cyclopentadienide in tetrahydrofuran, followed by sublimation of the solid residue, gives 

pure 212 (R 1 = H). [194,200] Compound 212 (R 1 = H) is a yellow, volatile, air-stable solid and 

soluble in organic solvents. Other complexes 212 (R 1 = CN, CO 2Me, [25,201] CO 2Et, CO 2iPr, 

CHO, COCO 2Et, Me, t-Bu) [25] are also synthesized in a similar manner usinga thallium cyclopentadienide. 

Reaction of 209 with two equivalents of tertiary phosphine per rhodium 

cleaves the chlorine bridge and displaces one molecule of ethene to give monomeric complexes 

of the type chloro(ethene)bis(phosphine)rhodium(I) (213) as yellow crystals after 

recrystallization under an atmosphere of ethene. [202] 

Scheme 82 Monomeric Complexes via Chlorine-Bridge 

Cleavage Reactions [25,194,196,198,200±202] 

1/2 

Rh Cl Rh 

Cl 

209 

β-diketone anion 

R1 = Me 75% 

R1 = t-Bu 88% 

R1 = CF3 66% 

R 1 C 5H 4 − , heat 

R1 = H 47% 

R1 = CO2Me 70% 

R1 = CN 39% 

2L, heat 

− H2C CH2 

Rh 

Rh 

212 

O 

O 

211 

R 1 

R 1 

R 1 

L 

Rh Cl 

L 

213 L = PPh3, P(4-MeC6H4) 3 




b(Acetylacetonato)bis(ethene)rhodium (211,R 1 = Me); Typical Procedure: [196] 

Reprinted from (Van Der Ent; Onderdelinden, Inorganic Syntheses), Copyright (1990), 

p 90, with permission from John Wiley & Sons, Inc. 

A mixture of [Rh 2Cl 2(H 2C=CH 2) 4] (5.5 g, 0.014 mmol) and pentane-2,4-dione (3.0 mL, 

0.030 mmol) in Et 2O (50 mL) was stirred under N 2 at ±208C, and a soln of KOH (10 g) in 

H 2O (30 mL) was added dropwise over a period of 15 min. The mixture was stirred for 

30 min at ±108C, Et 2O (50 mL) was added, and the Et 2O layer was decanted from the aqueous 

phase and passed through a filter. The aqueous layer was extracted with Et 2O 

(5 ” 25 mL) and the extracts were filtered and combined prior to cooling ±808C. About 

4.5 g(60% yield) of orange-yellow platelets crystallized. These were separated by decantingthe 

solvent and were dried under vacuum to give 211 (R 1 = Me). The supernatant liquid 

was concentrated to ca. 250 mL in a stream of N 2 and was chilled to ±80 8C to obtain another 

crop of product (1.0 g); combined yield: 5.5 g (75%); mp 144±1468C (dec). The product 

thus obtained can be further purified by rapid recrystallization from Et 2O or MeOH. IR 

(cm ±1 , KBr): 3060 (w), 2985 (w), 1575 (s), 1558 (s), 1524 (s), 1425 (m), 1372 (m), 1361 (m), 

1267 (m), 1233 (m), 936 (m), 788 (m). 

(Cyclopentadienyl)bis(ethene)rhodium (212, R 2 = H); Typical Procedure: [200] 

A soln of NaCp was prepared under N 2 from Na (17 mL, 30% dispersed in xylene, ca. 

200 mmol) and cyclopentadiene (20 mL, 16 g, 240 mmol) in freshly distilled THF (200 mL) 

by dropwise addition of cyclopentadiene to a slurry of Na dispersion in THF with stirring. 

After all the Na had dissolved, [Rh 2Cl 2(H 2C=CH 2) 4] (16.7 g, 43 mmol) was added and the 

mixture was refluxed for 30 min under N 2 and then stirred for 16 h at rt. The solvent was 

removed under reduced pressure (ca. 30 Torr) to give a brown residue, which was transferred 

under N 2 to a sublimation apparatus and sublimed at 50±1008C/ca. 1 Torr for 4 h 

to a cold finger cooled by running water to give the pure product as a yellow crystalline 

solid; yield: 9.0 g(47%); mp 72±73 8C; 1 H NMR (CS 2): ä 1.00 (br, 4H, CH 2), 2.75 (br, 4H, CH 2), 

5.08 (s, 5H, Cp); IR (cm ±1 , KBr): 3090 (w), 3010 (w), 2950 (w). 

Chloro(ethene)bis(triphenylphosphine)rhodium (213, L = PPh 3); Typical Procedure: [202] 

To a soln of Ph 3P (3.0 g, 11.5 mmol) in toluene (150 mL) was added [Rh 2Cl 2(H 2C=CH 2) 4] 

(1.17 g, 3 mmol) while bubbling H 2C=CH 2 into the soln. H 2C=CH 2-sat. CHCl 3 (150 mL) 

was added and the mixture heated until all the solids dissolved. The soln was pressure filtered 

under H 2C=CH 2 and H 2C=CH 2 was bubbled through the filtrate. After 1.5 h, yellow 

crystals formed which were collected, washed with H 2C=CH 2-sat. petroleum ether, and 

dried in vacuo to give the product; yield: not given. 

1.5.7.3 Method 3: 

Via Displacement of the Ethene Ligand 

It has been demonstrated that the ethene ligand of 16-electron, square-planar rhodium(I) 

complexes such as di-ì-chlorotetrakis(ethene)dirhodium(I) (209) and (acetylacetonato)bis(ethene)rhodium(I) 

(211, R 1 = Me) undergoes rapid exchange with uncoordinated 

ethene in solution. [194] Ethene exchange is a bimolecular reaction that proceeds via an associative 

mechanism involvingan 18-electron tris(ethene)rhodium intermediate. [194,203] 

The rate of exchange decreases for coordinatively saturated, 18-electron complexes such 

as cyclopentadienylbis(ethene)rhodium(I) (212, R 1 = H). [194] Since ethene is very labile and 

volatile, a large number of rhodium complexes, including complexes of other alkenes, 

are readily accessible from 209 by exchanging ethene for other ligands. Acrylonitrile 

and methyl vinyl ketone displace ethene from 209 to give the complexes [Rh 2Cl 2(ç 2 - 

H 2C=CHCN) 4] [75] and [Rh 2Cl 2(ç 2 -H 2C=CHCOMe) 4], [204] respectively. The cyclooctene ligand 

of 210 can also be replaced by acrylonitrile or cis-dimethyl maleate to afford 

[Rh 2Cl 2(H 2C=CHCN) 4] or [Rh 2Cl 2(MeO 2CCH=CHCO 2Me) 4], respectively. [205] Facile displace- 



bment of both ethene ligands of 211 by other monoalkenes provides an effective method 

for the synthesis of rhodium±alkene complexes of type 214 (Scheme 83). The complexes 

214 (R 1 = Me; L = H 2C=CHMe, [206,207] H 2C=CHCl, [206,207] H 2C=CHF, [208] H 2C=CHOAc, 

H 2C=CHCO 2Me, H 2C=CHPh [207] ), 214 (R 1 = t-Bu, CF 3, Ph; L = H 2C=CHMe, H 2C=CHCl, 

H 2C=CHOAc, H 2C=CHCO 2Me, H 2C=CHPh), [198] 214 [R 1 =CF 3; L=H 2C=CHCH 2OH, 

H 2C=CHCH(Me)OH, (Z)-HOCH 2CH=CHCH 2OH], [209] and 214 [R 1 = Me; L = H 2C=CHTMS, 

H 2C=CHSiMe 2(OEt), H 2C=CHSiMe(OEt) 2, H 2C=CHSi(OEt) 3, H 2C=CHCH 2TMS, 

(H 2C=CH) 2SiMe 2,(H 2C=CH) 2SiPh 2] [210] are all prepared usingthis method. 

Scheme 83 Rhodium±Alkene Complexes via Displacement of Ethene Ligand [206±210] 

Rh 

O 

O 

211 

R 1 

R 1 

R 

2L 

1 = Me; L = 

R1 = Me; L = 

R1 = Me; L = 

R1 = CF3; L = 

R1 = CF3; L = 

R1 = Me; L = 

R1 H2C CHCl 76% 

H2C CHMe 73% 

H2C CHF 92% 

H2C CHCH2OH 87% 

HOCH2CH CHCH2OH 84% 

H2C CHTMS 100% 

= Me; L = H2C CHCH2TMS 100% 

Tetrakis(ç 2 -but-3-en-2-one)di-ì-chlorodirhodium (209, alkene = ç 2 -H 2C=CHCOMe): [204] 

To a soln of [Rh 2Cl 2(H 2C=CH 2) 4] (0.1 g, 0.26 mmol) in benzene (10 mL) was added methyl 

vinyl ketone (0.63 g, 9.3 mmol) at rt. The mixture was stirred for 30 min before benzene 

and excess ketone were removed slowly in vacuo. The solid residue was dried in vacuo 

for 3 h to give the product; yield: 0.154 g (100%); mp 68±75 8C (dec). 

(Acetylacetonato)bis(chloroethene)rhodium (214, R 1 = Me; L = H 2C=CHCl); 


A reaction flask was charged with [Rh(acac)(H 2C=CH 2) 2] (0.50 g, 1.93 mmol), evacuated, 

and chilled to ±258C. Chloroethene (ca. 10 mL, 145 mmol) was condensed into the flask, 

which was then warmed slightly to distill off unreacted chloroethene. The residue was 

again treated in a similar fashion with chloroethene to give a yellow oil. This oil crystallized 

when mixed with isobutane at ±508C. The product was obtained by removingisobutane 

at ±50 8C under N 2 through a filter stick, and dried at 208C/1 Torr for 0.5 h; yield: 

0.48 g(76%); mp 42±438C; 1 H NMR (acetone-d 6): ä 1.53, 3.05, 4.50, 4.99. 

(1,1,1,5,5,5-Hexafluoroacetylacetonato)bis(ç 2 -prop-2-en-1-ol)rhodium 

(214,R 1 =CF 3;L=H 2C=CHCH 2OH); Typical Procedure: [209] 

Allyl alcohol (16 mg, 0.275 mmol) in dry Et 2O (1 mL) was added to [Rh(CF 3COCHCOCF 3)- 

(H 2C=CH 2) 2] (50 mg, 0.137 mmol). Removal of the solvent gave yellow crystals which 

were recrystallized (Et 2O/pentane) to give the product; yield: 51 mg (87%); mp 93±948C. 

1.5.7.4 Method 4: 


Ethene can displace one of the ligands of [RhXL 3] to afford monomeric complexes of type 

215 (Scheme 84). Ethene adds rapidly to chlorotris(triphenylphosphine)rhodium(I) in an 

organic solvent at room temperature to form the bright yellow, crystalline complex chloro(ethene)bis(triphenylphosphine)rhodium(I) 

(215, L = PPh 3; X = Cl). [211] Similarly, the 

complexes 215 (L = PPh 3; X = Br) [211] and 215 (L = AsPh 3; X = Cl) [156] are synthesized using 

the same protocol. The two chloro complexes can also be prepared by treating 209 with 

two equivalents of phosphine or arsine in an organic solvent, as discussed in Section 

1.5.7.2. 

L 

L 

Rh 

O 

O 

214 

R 1 

R 1 




bScheme 84 Monomeric Rhodium±Ethene Complexes [156,211] 

L Rh 

L 

X 

L 

H2C CH2 − L 

X = Cl; L = PPh3 70% 

X = Cl; L = AsPh3 80% 

L 

Rh X 

L 

215 

Chloro(ethene)bis(triphenylarsine)rhodium (215, L = AsPh 3; X = Cl); Typical Procedure: [156] 

[RhCl(AsPh 3) 3] (0.2 g, 0.45 mmol) was added to a H 2C=CH 2-sat. soln of CH 2Cl 2 (5 mL) under a 

H 2C=CH 2 amosphere. Et 2O (5 mL) was added to the resultingclear yellow soln while maintaininga 

H 2C=CH 2 atmosphere. A yellow precipitate formed which was collected, washed 

with Et 2O, and dried in vacuo to give the product as yellow crystals; yield: 0.12 g (80%); mp 

1638C (dec); 1 H NMR (CHCl 3): ä 2.74; IR (cm ±1 , Nujol mull): 1304 (m), 962 (m), 949 (m). 


1.5.7.5 Method 5: 

[5+2] Vinylcyclopropane±Alkene Cycloaddition 

Wilkinson s catalyst, chlorotris(triphenylphosphine)rhodium(I), has been shown to efficiently 

promote the intramolecular [5 +2] cycloaddition between vinylcyclopropanes 

and alkenes. [181] Reaction of the ene±vinylcyclopropane 216 catalyzed by Wilkinson s catalyst 

in the presence of silver triflate (0.1±10 mol%) proceeds with complete stereoselectivity 

to afford cycloadducts 217 in good to excellent yields (Scheme 85). Silver triflate is 

found to facilitate the reaction, presumably by freeinga coordination site through removal 

of the chloride ligand. Although silver triflate is not always required as an additive, its 

presence clearly contributes to a clean conversion in some cases. The remarkable stereoselectivity 

may be attributable to the preferential formation and reaction of a cis-fused 

metallacyclopentane intermediate 188 (Scheme 73, Section 1.5.6.6). The formation of 

quaternary centers in 217 (R 1 or R 2 = Me) is also possible in this cycloaddition, albeit a 

higher catalyst loading is required (10 mol%). Methyl substitution on the alkene moiety 

of the vinylcyclopropane 218 is tolerated, but no cyclization is observed for substrates 

bearingsubstituents at the alkene terminus. The cycloaddition is also applicable to the 

synthesis of bicyclo[5.4.0]undecene 219, which is formed in good yield as a single diastereomer 

with trans stereochemistry. An asymmetric variation of this cycloaddition has 

been reported usingdi-ì-chlorotetrakis(ethene)dirhodium(I) as the catalyst precursor and 

(S,S)-Chiraphos as the chiral ligand, [212] which proceeds with a modest enantioselectivity 

(63% ee). 

Scheme 85 Intramolecular [5 +2] Cycloaddition of a Vinylcyclopropane 

and an Alkene [181,212] 

X 

MeO 2C 

MeO 2C 

R 1 

R 2 

RhCl(PPh 3) 3, AgOTf, toluene 

216 217 

218 

5 mol% RhCl(PPh3)3, 5 mol% AgOTf 

toluene, 110 oC, 15 h 

78% 

X 

R 1 

R 2 

MeO2C 

MeO 2C 


MeO 2C 

bMeO 2C 


10 mol% RhCl(PPh3) 3, 10 mol% AgOTf 

toluene, 100 oC, 5 d 

77% 

MeO 2C 

MeO 2C 

X R 1 R 2 Rh, AgOTf (mol%) Temp (8C) Time Yield (%) Ref 

C(CO2Me) 2 H H 0.1 110 17 h 93 [212] 

O H H 5 65 10 h 94 [212] 

C(CO2Me) 2 H Me 10 110 1h 92 [212] 

C(CO2Me) 2 Me H 10 110 1 h 94 [212] 

Dimethyl 3,3a,4,5,6,8a-Hexahydro-1H-azulene-2,2(1H)-dicarboxylate 

[217,R 1 =R 2 = H; X = C(CO 2Me) 2]; Typical Procedure: [212] 

To a base-washed, oven-dried Schlenk flask containingfreshly distilled, oxygen-free toluene 

(2 mL) were added [RhCl(PPh 3) 3] (3.65 mg, 0.004 mmol, 0.001 equiv) and AgOTf 

(1.02 mg, 0.004 mmol, 0.001 equiv) sequentially, each in one batch, under an argon atmosphere. 

The soln was stirred for 5 min at rt, and the ene±vinylcyclopropane 216 

[R 1 =R 2 = H; X = C(CO 2Me) 2] (1.00 g, 3.95 mmol, 1.0 equiv) in toluene (2 mL) was added 

over 10 s. The resultingsoln was heated at 1108C for 17 h, cooled to rt, and the mixture 

was filtered through a pad of alumina and concentrated in vacuo. Purification of the residue 

by flash chromatography (silica gel, EtOAc/hexane 5:95) gave the product as a colorless 

oil; yield: 0.857 g(86%). 

1.5.7.6 Method 6: 

Carbocyclization of 1,6-Dienes 

1,6-Dienes are found to undergo an intramolecular carbocyclization reaction in the presence 

of a rhodium complex to form five-membered carbocycles. [213] Reaction of 1,6-dienes 

220 catalyzed by Wilkinson s catalyst in chloroform affords the corresponding1-methyl- 

2-methylenecyclopentanes 221 in good to excellent yields (Scheme 86). Presaturation of 

the chloroform with dry hydrogen chloride is necessary for this carbocyclization process, 

i.e. no cyclization occurs in the absence of hydrogen chloride. The reaction tolerates functional 

groups such as esters and ketones. Ethanol can be used as an alternative solvent for 

the reaction; however, the product 221 isomerizes to more stable 1,2-dimethylcyclopent- 

1-enes under the reaction conditions. Reaction of substrates bearingsubstitutions at the 

alkene terminuses suffers from the formation of a complex mixture of isomers. The formation 

of larger ring size products is not observed for reactions of 1,7- and 1,8-dienes. 

Scheme 86 Intramolecular Carbocyclization of 1,6-Dienes [213] 

X 

220 

1−2 mol% RhCl(PPh3)3, CHCl3 

heat, 8−48 h 

X = C(CO2Et)2 90% 

X = C(COMe) 2 87% 

X = C(COPh) 2 64% 

O 

X = 

O 

60% 

X 

221 

219 

H 

H 




bDiethyl 3-Methyl-4-methylenecyclopentane-1,1-dicarboxylate [221, X = C(CO 2Et) 2]; 


Dry HCl gas was bubbled through a soln of diethyl diallylmalonate [220, X = C(CO 2Et) 2; 

8.0 g, 0.033 mol] and [RhCl(PPh 3) 3] (0.5 g, 0.54 mmol) in CHCl 3 (50 mL) for 5 min with stirring. 

The mixture was refluxed for 8 h. The solvent was then removed under reduced pressure 

and the residue was dissolved in light petroleum ether (bp 40±60 8C) and filtered 

through a short column of neutral alumina to remove the catalyst. The filtrate was concentrated 

and the residue distilled to afford the product as a colorless oil; yield: 7.2 g 

(90%); bp 106±1078C/ 3 Torr. 

1.5.7.7 Method 7: 

Intramolecular Hydroacylation 

The intramolecular hydroacylation of pent-4-enals to give cyclopentanones is a synthetically 

useful transformation (Scheme 87) which was originally reported to be induced by a 

stoichiometric amount of Wilkinson s catalyst. [214] The effects of various rhodium(I) catalysts 

and reaction variables on this process have been extensively investigated. [215] Much 

attention has been focused on possible mechanisms, [216±218] stereoselectivity, [219] and applications 

to the syntheses of natural products. [220,221] The process initially suffered from low 

yields and low catalyst turnovers owingto the formation of a catalytically inactive carbonylrhodium 

complex. Subsequently, it was found that Wilkinson s catalyst could be 

used in a catalytic quantity under ethene pressure. [222] Later, very high turnover frequencies 

were achieved by usingcationic rhodium(I) complexes bearingchelatingdiphosphine 

ligands. [99,223] 

Reaction of pent-4-enals 222 with a catalytic amount of a dimeric, cationic rhodium 

complex of the type [Rh 2(diphosphine) 2] 2+ in a weakly or noncoordinatingsolvent at room 

temperature affords cyclopentanones 223 in high to quantitative yields (Scheme 87). [99] 

The most effective catalyst is found to be the rhodium(I) complex with 1,2-bis(diphenylphosphino)ethane 

(dppe). The dimeric, cationic catalyst bis[1,2-bis(diphenylphosphino)ethane]dirhodium(I) 

diperchlorate can be generated in situ from bis(norbornadiene)rhodium(I) 

perchlorate and 1,2-bis(diphenylphosphino)ethane or isolated by hydrogenation 

of the complex [1,2-bis(diphenylphosphino)ethane](norbornadiene)rhodium(I) perchlorate. 

The reaction tolerates monosubstition at the 2-, 3-, 4-, and 5-positions and disubstitution 

at the 3-position of pent-4-enal. Disubstitution at the 2-position impedes the process 

considerably, and substrates containingdisubstitution at the alkene terminus (5-position) 

fail to undergo this catalytic process. 

Scheme 87 Intramolecular Hydroacylation of Pent-4-enals [99,223] 

R 3 

R 2 

R 4 O 

222 

R 1 

R 1 = R 2 = H, Me; R 3 = R 4 = H, Me, Ph 

1−2 mol% [Rh2(dppe) 2](ClO4) 2, MeNO2 or CH2Cl2 20 oC, a few min 

88−100% 

R 4 

O 

R3 223 

Asymmetric intramolecular hydroacylation of pent-4-enals has also been investigated. 

[119,224] Reaction of 4-substituted pent-4-enals 224 catalyzed by a chiral diphosphine rhodium 

complex affords the correspondingoptically active 3-substituted cyclopentanones 

225 (Scheme 88). In general, the reaction proceeds with high turnover frequencies at 

room temperature. A cationic rhodium complex bearing the BINAP ligand gives the highest 

level of enantioselection amonga variety of chiral diphosphine catalysts examined. 

Almost complete enantioselectivity is achieved in the reaction of 224 bearingtertiary 


R 2 

R 1


bsubstituents or an ester group at the 4-position. Substrates with acyl substituents also exhibit 

very high asymmetric induction. However, only moderate enantioselectivity is observed 

for the reaction of 224 with aliphatic and aryl substituents. The use of the chiral, 

cationic complex [Rh{(S,S)-Me-DuPHOS}]PF 6, generated in situ by hydrogenation of 

[Rh(nbd){(S,S)-Me-DuPHOS}]PF 6, increases the efficiency and enantioselectivity of the hydroacylation 

reaction, [106] giving 225 in quantitative yield in almost all cases (Scheme 88). 

In contrast to the BINAP-Rh + catalyst, the reactions of pent-4-enals bearinga primary or 

secondary carbon atom substituent catalyzed by this complex exhibit excellent levels of 

asymmetric induction. [106] 

Scheme 88 Enantioselective Intramolecular Hydroacylation 

of 4-Substituted Pent-4-enals [106,119,124] 

R 1 

224 

O 

H 

CH2Cl2, 25 oC, 2 h 

90% 

Catalyst R 1 ee (%) Ref 

[Rh{(S)-BINAP}]ClO 4 (4 mol%) t-Bu, CMe 2OMe, TMS, SiMe 2Ph, SiMePh 2 >99 [119] 

[Rh{(S)-BINAP}]ClO 4 (4 mol%) COMe, COPh, COt-Bu, CO 2Et, CO 2iPr94±99 [119] 

[Rh{(S)-BINAP}]ClO 4 (4 mol%) Me, iPr, cyclo-C 5H 9, Cy 61±81 [119] 

[Rh{(S)-BINAP}]ClO 4 (4 mol%) Ph, 4-MeC 6H 4, 4-MeOC 6H 4, 2-MeOC 6H 4 17±70 [119] 

[Rh{(S,S)-Me-DuPHOS}]PF 6 (4 mol%), 

acetone, 258C, 95% 

R 1 

O 

225 

Me, Et, iPr, Bu, cyclo-C 5H 9,Cy,C 8H 17, Bn 93±98 [106] 

Cyclopentanone (223,R 1 =R 2 =R 3 =R 4 = H); General Procedure: [99] 

All solvents were purged with argon (10 min) immediately prior to use. After purging the 

reaction vessel containing[Rh 2(dppe) 2](ClO) 2 [99] (1.10 mg, 1.8 ìmol, 1 mol%) with argon for 

60 s, the solvent (CH 2Cl 2 or MeNO 2, 0.7 mL) was added. Pent-4-enal (222, 

R 1 =R 2 =R 3 =R 4 = H; 15.87 mg, 0.189 mmol) was added by syringe and the mixture was allowed 

to react at 208C for 6 min to give the product; yield: 15 mg (95%). 

3-Acetylcyclopentanone (225,R 1 = Ac); Typical Procedure: [119] 

Hydrogen gas was bubbled through the soln of [Rh(nbd){(S)-BINAP}]ClO 4 (17.2 mg, 

18.7 ìmol) in dry, degassed CH 2Cl 2 (3 mL) for 3 min. Then, 4-acetylpent-4-enal (224, 

R 1 = COMe; 59.4 mg, 0.469 mmol) was added by syringe, and the soln was stirred for 

4.5 h. The solvent was removed on a rotary evaporator to give the product as a light yellow 

oil; yield: 54.0 mg(90%). 




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