Carbanions silicon-stabilized, alkylation

Generation of the silicon stabilized benzyl carbanion 153 from 149 required the presence of hexamethylphosphotriamide. In its absence lithiation occurred at least in part in the aromatic ring. Alkylation of 153 with 152 produced 154 in 94% yield as a 2 1 isomer mixture at the benzylic site. Methiodide formation, addition of caesium fluoride, and refluxing in acetonitrile for 1.5 hours then afforded ( )-estrone methyl ether 28 (containing 7-8% of the C-9 p-H isomer) in 86% yield. Pure 28 was obtained on recrystallization. 

The pattern of results suggest that, if the substituent at the acyl group is alkyl, and hence relatively carbanion-destabilizing, rearrangement is inhibited relative to the alternative normal Wittig reaction pathway when the substituent is aromatic, however, and therefore capable of stabilizing incipient carbanion formation as the silicon-carbon bond... 

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. 

This chapter deals with the formation of new C—C bonds by alkylation of sp -hybridized carbanions stabilized by a range of heteroatoms, including oxygen, boron, silicon, phosphorus and the halogens. The properties of the same carbanions with additional stabilization provided by an adjacent double bond are also discussed. 

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