Peterson alkenation silicon substitution

In the Peterson reaction of an a-silyl carbanion bearing a phosphorus substituent on the anionic carbon atom, there are two possibilities for alkene formation, that is, the Peterson reaction to form phosphorus-substituted alkenes and the Wittig-Horner reaction and the Wadsworth-Emmons reaction to form silicon-substituted alkenes. Most of the reports on these competing reactions have been focused on the reactions of a-silyl phosphonates with carbonyl compounds. It is noteworthy that the alkenylphosphonates have been exclusively obtained in almost every case. That is, the Peterson reactions override the Wittig and Wadsworth-Emmons reactions. 

Reaction of DMSB with triphenylsilyl-substituted oxiranyllithium leads to the formation of an olefinic silanol via sequential (1) coordination to the silicon, (2) Si-C bond migration, and (3) Peterson-type Si-O elimination to furnish the alkene. A pentacoordinate siliconate intermediate is presumably involved in this transformation. Therefore, it was reasonable to expect that addition of a nucleophile (methyllithium or lithium t>-propoxide) to an oxiranyl-substi-tuted SCB, which could generate a similar intermediate, would induce the C-Si bond migration to form the same silacyclopentane. Indeed, this alternative order of addition sequence provides the corresponding silanol with better efficiency (84% yield vs. 44%, Scheme 36). 

As a general mle, unless an anion-stabilizing group, such as phenyl, or a heteroatom such as sulfur is present, the alkylsilane is not readily deprotonated. The a-halosilane can be deprotonated but, unlike the readily available chloromethyltrimethylsilane, there are few general methods to this approach. Al-kyllithium reagents add to vinylsilanes ( ) to produce the carbanion (287). Silyl derivatives with heteroatoms, such as sulfur, selenium, silicon or tin, in the a-position (288) may be transmetallated (Scheme 41). Besides the difficulty in synthesizing the anion, alkene formation lacks specificity for simple di- and tri-alkyl-substituted alkenes. As a result, the Peterson reaction of an a-silyl carbanion with a carbonyl has found the greatest utility in the synthesis of methylene derivatives, (as discussed in Section 3.1.3), heterosubstituted alkenes and a,p-unsaturated esters, aldehydes and nitriles. 

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