Attack at the a-Position of an Alkyl Group

In some cases, electrophiles react at the carbon of an alkyl group attached to the metal to abstract a group bound to this carbon. This process typically forms a metal-carbene 

Alternative routes to the same or related carbene complexes also involve electrophilic abstractions at the a-carbon. As shown in Equations 12.38 and 12.39, trimethylsilyl triflate can be used to abstract alkoxo or hydroxo groups from the a-carbon to generate cationic carbene complexes. 

Electrophilic reagents react not only at the a-carbon of alkyl groups, but at the a-carbon of carbene and carbyne complexes. In these cases, the electrophile can either form a stable adduct by coordination to the nucleophilic carbon, or it can abstract a labile group. As noted in the section of Chapter 13 on carbene complexes, nucleophilic early metal alkylidene complexes, such as Cp HCH, coordinate Lewis acids at the carbene carbon. CpjTiCHj coordinates Me AlCl to form Tebbe s reagent. 

Examples of the electrophilic modification of a-carbons also include electrophilic attack at carbyne complexes. Calculations suggest that carbyne carbons bear a significant negative charge, but reports of electrophilic attack upon these carbons are rare. In Equation 12.40, protonation of CUMejPl WsCR gives a distorted alkylidene complex the C-H bond interacts with the coordinatively unsaturated metal in an agostic fashion. 

Additions of electrophiles to metal-alkyl complexes can also induce abstraction of a hydride from the -position. Reactions of electrophiles at the 3 Carbons of alkyl, vinyl, and alkynyl groups have all been reported. Abstraction of a hydride at the -position by 

---