Nucleophilic abstraction from carbene complexes

A further general route to heteroatom-substituted carbene complexes is based on the a-abstraction of nucleophiles from alkyl complexes (electrophilic abstraction Figure 2.13). 

A further synthetic approach to carbon-metal double bonds is based on the acid-catalyzed abstraction of alkoxy groups from a-alkoxyalkyl complexes [436 -439] (Figure 3.11). These carbene complex precursors can be prepared from alk-oxycarbene complexes (Fischer-type carbene complexes) either by reduction with borohydrides or alanates [23,55,63,104,439-445] or by addition of organolithium compounds (nucleophilic addition to the carbene carbon atom) [391,446-452]. 

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). 

The last example of nucleophilic abstraction shows removal of a methyl group from a Fischer carbene complex (equation 8.56).75 The course of this reaction is unusual because normally nucleophiles add to the carbene carbon, as we shall see in Chapter 10. In this case, however, the apparent steric bulk of the abstracting... 

In 1964, Fischer prepared and characterized unambiguously the first metal carbene complex 3 obtained by nucleophilic attack of phenyl lithium at tungsten hexacarbonyl followed by 0-alkylation. This was followed by Schrock s synthesis of a high oxidation state metal alkylidene complex 4 obtained by a-hydrogen abstraction from tris(neopentyl) tantalumfv) dichloride (Scheme 1.1)/ ... 

Main routes to 71-donor-substituted allenylidene complexes include (1) the nucleophilic addition of secondary amines to Fischer-type carbenes [M =C(OR ) C=CR (C0)5] (M = Cr, W) [9], (2) the Lewis-acid induced abstraction of NR2 groups from anionic complexes [M C=CC(NMe2)3](CO)5] (M = Cr, W) [9], and (3) the regioselective addition of protic nucleophiles to metallacumulenylidenes with more extended unsaturated carbon chains, such as butatrienylidenes or penta-tetraenylidenes [10]. In the following sections updated syntheses are presented by Periodic Group. 

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