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Aizpurua, J. M.; Palomo, C., Science of Synthesis, (2002) 4, 612.
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] Si → C 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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M8 | M8 | Meeee (%) | Mee | |
---|---|---|---|---|
88 | 88 | |||
Me | Me | 88 | – | [88] |
eMe | Me | 88 | – | [88] |
e-Me | Me | 88 | – | [88] |
(MM8)8Me | Me | 88 | – | [88] |
MM8MM8Me | Me | 88 | – | [88] |
Me | Me | 88 | 88 | [88] |
Me | Me | 88 | – | [88] |
Meeeeeeeeeee Meeeeeeee
8,8-Mee(eeeeeeeeeeeeeeeeeee)-8,8-eeeeeeeeeeeeee-8-ee (88, M8 = e-Me); Meeeeee Meeeeeeee:[88]
M 8 M eeee ee MeMe8MeMe ee MMM (8.8 eM, 8.8 eeee) eee eeeee eeeeeeee ee e eeeeeee eeee ee eeeeee eeeeeeee (8.88 e, 8 eeee) ee eee MMM (8 eM) eeeee eeeee ee −88°M eeee 88 eee, eeeee eeeee eee eeeeeee eee eeeeee ee eee eeee eeeeeeeeeee eee 8 e. Meeee eeeeeeee ee eee. ee MM8Me (8 eM) ee eee eeeeeee ee eee eeeeeeeee eeee Me8M (8 × 88 eM). Mee eeeeeeee eeeeeeee eeee eeeeee eeee eeeee, eeeee (MeMM8), eee eeeeeeeeee eeeee eeeeeee eeeeeeee. Mee eeeeeee eee eeeeeeee ee eeeeeeeeeeeeee (eeeeee eee, eeeeee/Me8M 88:8); eeeee: 8.88e (88%).
References
[44] | Meeeeee, M.; Meeee, M., M. Meee. Mee., Meeeee Meeee. 8, (8888), 888. |