Silylzinc reagents

Several catalytic protocols have been developed for the 1,4-addition of silylcopper reagents to enones and other Michael acceptors. Lipshutz et al.183 treated phenyldimethylsilyllithium with dimethylzinc and used the silylzinc reagent PhMe2SiZnMe2Li thus formed as the nucleophile in copper-catalyzed 1,4-additions to various enones and enals, for instance, verbenone 248 which afforded the desired product 249 in almost quantitative yield (Equation (14)). Interestingly, the rather slow addition reaction is strongly accelerated by catalytic amounts of scandium triflate. 

Heteronucleophiles can also be transferred to electrophiles in copper-catalyzed S -substitution reactions. The silylzinc reagent PhMe2SiZnMe2Li formed by treating phenyldimethylsilyllithium with dimethylzinc undergoes substitutions with vinyl oxiranes (e.g., 331) to afford products of the type 332 with high yield and //-stereoselectivity (Equation (17)).183... 

Conjugate addition. Silylzincate reagents are superior to silylcuprates in terms of their efficiency in the addition to the a,jS-unsaturated carbonyl compounds. 

Scheme 3-12. Preparation of silylzincate reagents followed by reactions with...

Silylzinc reagents are also employed for the silylmetalation reaction of alkynes and alkenes. Bis(dimethylphenylsilyl)zinc adds across these unsaturated substrates in the presence of a copper(I) catalyst (Scheme 3-34). The addition reaction proceeds in a stereospecific manner always cis adducts are produced. 

Two regioisomeric c/s-silylzincation products are selectively obtained by proper choice of substituents at both zinc and silicon centers of silylzincate reagents (Scheme 3-35). ... 

Scheme 3-35. Silylzincation of alkynes with silylzincate reagents.

Scheme 3-36. Silylzincation of alkynes and alkenes with higher order silylzincate reagents.

Extremely efficient and convenient ways to prepare alkenyl- and alkylsilanes are transition metal-catalyzed silylation of alkynes and alkenes respectively. A general mechanistic scheme should involve oxidative addition of Si-X (X = H, B, C, Si, Sn, S, and halgen) bonds to transition metal catalysts M followed by insertion of unsaturated bonds into either Si-M or X-M (M = transition metal) bond and reductive elimination (Scheme 3-22). On the other hand, highly reactive silylmetal reagents such as silylzincates and silylcuprates effect similar transformation without the aid of transition metal catalysts. 

Table 13 CuCN-catalyzed silylzincation of allenes Si-Zn reagent (1.1 equiv.) ...

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