ca01 only detailed ToC 1..24

110 Science of Synthesis 2.6 Complexes of Cr, Mo, and W without CO Ligands
Applications of Product Subclass 2 in Organic Synthesis
The reactivity of Schrock-type carbyne complexes has to date been little exploited in organic
synthetic applications in comparison with that of the corresponding carbene complexes
(Section 2.6.1.5). For the most part, the interest has been limited to stoichiometric
transformations to other organometallic products. Some carbyne compounds ([M]”CR)
undergo Wittig-like reactions with X”Y molecules, affording [M]”X and RC”Y products.
[63] As seen in Section 2.6.2.3, the addition of an alkyne (R 2 C”CR 2 ) to a carbyne complex
([M]”CR 1 ) may afford the metathetical products (R 2 C”CR 1 +[M]”CR 2 ). Some of these
complexes catalyze the alkyne metathesis reaction. [87]
2.6.2.7 Method 7:
Alkyne Metathesis
Medium-large cycloalkynes 53 have been synthesized from diyne precursors by a ringclosing
metathesis process in which a tungsten carbyne complex acts as a catalyst
(Scheme 20). [46] The cycloalkyne obtained by this methodology can be partially reduced
to obtain the cycloalkene exclusively as the Z-isomer. The stereoselectivity of this route
is particularly notable, since current molybdenum- and ruthenium-based ring-closing metathesis
catalysts usually provide mixtures of E- and Z-configured cycloalkenes from diene
precursors (see Section 2.6.1.5.3). The catalyst tolerates various functional groups: lactones,
lactams, and silyl ethers have been obtained by this method. The catalyst is incompatible
with terminal acetylenes; thus, it is necessary to use precursors with R 1 ,R 2 „ H.
The use of a high boiling point solvent (e.g., 1,2,4-trichlorobenzene) allows the removal
of the alkyl byproduct (R 1 C”CR 2 ) under reduced pressure, with a positive effect on the
conversion.
Scheme 20 Cycloalkynes by Ring-Closing Metathesis of Diynes [46]
X
R 1
R 1
W( Ct-Bu)(Ot-Bu) 3
52−97%
R 1 = Me, Et; X = (CH 2) 2OC(O)(CH 2) 4C(O)O(CH 2) 2
1,6-Dioxacyclotetradec-9-yne-2,5-dione [53,X=(CH 2) 2OC(O)(CH 2) 4C(O)O(CH 2) 2];
Typical Procedure: [46]
A soln of the diyne [MeC”C(CH 2) 2O(O)C(CH 2) 2] 2 (121 mg, 0.43 mmol) and [W(”Ct-Bu)(Ot-
Bu) 3] (12 mg, 6 mol%) in chlorobenzene (20 mL) was stirred under argon at 808C for 2 h.
The solvent was removed in vacuo, and the residue purified by flash chromatography
(Merck silica gel, hexane/EtOAc 4:1). This led to the recovery of some unchanged starting
material (12 mg, 10%) and afforded the cycloalkyne product as colorless crystals; yield:
70 mg (73%); mp 106–1078C; 13 CNMR(ä): 173.0 (alkyne).
2.6.3 Product Subclass 3:
Metal–ó-Alkyl and –ó-Aryl Homoleptic Complexes
Compounds of this class are rather limited, their syntheses are generally low yielding,
and have no general applications in organic synthesis. As is usually the case for any metal,
the more robust complexes are those without â-hydrogens and aryl complexes. Well-characterized
members of this class have the stoichiometries MR 6,MR 4,[MR 6] 3– ,[MR 5] 2– ,
[MR 4] – ,MR 3, and [MR 4] 2– . The MR 6 complexes are known only for tungsten and have a
X
53
+
R 1
R 1