Method 4 From Carbyne Complexes

These Fischer-type carbene complexes can be readily converted to Fischer-type carbyne complexes, and this material has been reviewed. Methods to remove the a-alkoxy group include treatment with BBrs, oxide abstraction from acyl ligands, and electrophihc attack on the jS-carbon of... 

Equation 10.52 shows the chemistry used to produce 64 and similar complexes this not only is the original procedure employed by Fischer, but also constitutes perhaps the most versatile method for the preparation of Fischer carbyne complexes. The reaction begins with an electrophilic abstraction of the alkoxy group from starting carbene 69 to give the cationic carbyne 70. Carbyne ligands are such powerful n acceptors that they exert a strong trans effect (Section 7-1-1). If... 

One route to a carbyne (alkylidyne) complex was reaction 23.40 equation 23.91 illustrates the initial method of Fischer. The abstraction of an ot-H atom from a Schrock-type carbene yields the corresponding carbyne complex (equation 23.92). 

An impressive series of carbene- and carbyne-bridged complexes of platinum have been formed by reaction of metal carbene complexes with a platinum complex (equation 141). These complexes have been verified by X-ray crystallography, and the synthetic method appears to be one of some generality. Among the complexes of this type formed from carbenes are ones having metal frameworks with Pt—W,409-413 Pt—Mn414-418 and Pt—Cr413,417 bonds. 

Deviating from the route via nucleophilic attack of the carbanion at the carbon atom of a CO ligand and then reaction of the acylmetallate with an electrophile are those methods which involve (a) addition of the carbanion to the carbon atom of a carbyne ligand, (b) displacement of halides from transition-metal carbonyl halides by cyclohepta-trienyllithium, or derivatives thereof, followed by hydride abstraction or (c) substitution of a coordinated solvent from a metal-carbonyl complex (see also reaction of LiR with carbene complexes). 

In the preparative section 3.2 devoted to metal-carbene complexes, it is shown how the a-elimination reaction from high oxidation state early-transition-metal-alkyl complexes is one of the general methods of synthesis of Schrock s Ta and Nb alkylidene complexes. The other direction, formation of an alkylidene from an alkylidyne complex, can also be a valuable route to metal alkylidenes. For instance, Schrock s arylamino-tungsten-carbynes can be isomerized to imido-tungsten-carbene by using a catalytic amount of NEts as a base. These compounds are precursors of olefin metathesis catalysts by substitution of the two Cl ligands by bulky alkoxides (dimethoxyethane then decoordinates for steric reasons), and this route was extended to Mo complexes ... 

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