ca01 only detailed ToC 1..24

116 Science of Synthesis 2.6 Complexes of Cr, Mo, and W without CO Ligands
2.6.4.4 Method 4:
By Protonation of Carbene and Carbyne Ligands
Under suitable conditions, carbene and carbyne ligands can take up protons to generate
alkyl derivatives. The proton source must be of low acidity to avoid further protonolysis
of the alkyl product. This methodology is of rather limited synthetic utility. Two examples
are shown in Scheme 27. [115,116] The formation of 65 involves loss of the pyridine ligand
and proton transfer from the silanol to the carbene ligand, while even more extensive
changes accompany the protonation of the carbyne ligand to give alkyl complex 66.
Intramolecular proton transfer from other ligands (e.g., other alkyl groups) is also possible.
[65,66]
Scheme 27 Protonation of Carbene and Carbyne Ligands [115,116]
ON
Mo
CHBut py
W( CR 1 )(OBu t ) 3
R 1 = t-Bu, TMS
Ph3SiOH, benzene
rt, 90 min
75%
Et4NOH, THF
62−82%
ON
Mo
CH2But OSiPh3 65
[Et 4N][W(CH 2R 1 )O 3]
ç 5 -Cyclopentadienyl(2,2-dimethylpropyl)nitrosyl(triphenylsilanolato)molybdenum(II)
(65); Typical Procedure: [115]
In a glovebox, [Mo(=CHt-Bu)Cp(pyridine)(NO)] (102 mg, 0.30 mmol) and Ph 3SiOH (83 mg,
1.0 equiv) were weighed into the reaction vessel. Benzene (20 mL) was vacuum-transferred
onto the solids. The mixture was then warmed to rt and stirred for 1.5 h. Over the
course of the reaction a color change from amber to dark red-brown occurred. The solvent
was removed from the final mixture in vacuo, and the residue was extracted with Et 2O
(2 ” 25 mL). The extracts were filtered through Celite and the filtrate was concentrated under
reduced pressure to incipient precipitation. Well-defined red blocks formed overnight
and were isolated by cannulation; yield: 121 mg (75%); IR (Nujol) í~ max: (NO) 1607
(vs) cm –1 ; 1 H NMR (benzene-d 6, ä): 3.79 (d, 1H, CHH, J HH = 9.9 Hz), 0.99 (d, 1H, CHH,
J HH = 9.9 Hz).
Applications of Product Subclass 4 in Organic Synthesis
Alkylchromium(III) compounds are involved in the large-scale commercial polymerization
of ethene and propene. Well-defined complexes that mimic the activity and selectivities
of the commercial catalyst have been obtained. [117] An application of group 6 alkyl
and aryl complexes that has been successfully applied to organic synthesis is the addition
reaction to carbonyl compounds (see Sections 2.6.4.5 and 2.6.4.6). Other chromium-based
systems have been developed for single-electron-transfer chemistry, [118] oxidation of alkanes
via hydrogen atom abstraction, [119] and asymmetric ring opening of meso-epoxides.
[120] Although these latter systems are of interest for synthetic organic chemistry,
they do not involve the formation of direct Cr-C bonds, and consequently these applications
are not treated here. [121]
66