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4.4.28.9 Method 9: From Silyl Anions and Esters

DOI: 10.1055/sos-SD-004-00626

Aizpurua, J. M.; Palomo, C.Science of Synthesis, (20024612.

α,α-Bis(dimethylphenylsilyl) alcohols 60 are conveniently prepared from linear or α-branched alkanoic acid methyl esters and 2 equivalents of dimethylphenylsilyllithium reagent (Scheme 21).[‌44‌] The method is neither applicable to the methyl esters of aromatic carboxylic acids, which are unreactive, nor to those of α,β-unsaturated acids, which give mixtures of products arising from 1,2- and 1,4-additions. Using only 1 equivalent of dimethylphenylsilyllithium at 110°C, followed by aqueous quench gives clean transformation of, for example, methyl acetate, into the corresponding acetyl(dimethyl)phenylsilane (70% isolated yield), which constitutes clear evidence supporting the intermediacy of acylsilanes in the reaction. This fact is helpful to account for an additional complication of the method: the formation of byproducts 61 when methyl esters with a single branch at the α position are used as substrates. These α-(alkoxydimethylsilyl) alcohols are easily identified as the result of a [1,2] SiC phenyl shift taking place in the intermediate hemiketal alkoxide [R1C(OR2)(OLi)SiMe2Ph], before the second equivalent of the silyllithium reagent attacks the intermediate acylsilane. This pathway is effectively avoided by using the phenyl esters of alkanoic acids as substrates, in place of the methyl esters.

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