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Reductions with hydrosilanes are chemical reactions that involve the combination of an organosilane (R3SiH) with an organic substrate containing unsaturated or electron-withdrawing functionality. Products in which the electron-withdrawing group has been replaced by hydrogen or the unsaturated group has been hydrogenated result.〔Larson, G. L.; Fry, J. L. ''Org. React.'' 2008, ''71'', 1. 〕 ==Mechanism== Silicon (1.90) is less electronegative than hydrogen (2.20); as a result, the silicon-hydrogen bond possesses some hydridic character. In the presence of a strong electrophile, organosilanes containing an Si-H bond (hydrosilanes) can serve as hydride donors to highly electrophilic organic substrates. Alcohols, alkyl halides, acetals, orthoesters, alkenes, aldehydes, ketones, and carboxylic acid derivatives may be reduced in good yield using hydrosilanes in conjunction with either a Brønsted or Lewis acid or an activating nucleophile (most commonly fluoride). Because only reactive electrophiles undergo reduction, selectivity is possible in reactions of substrates with multiple reducible functional groups. Chiral Lewis acids and metal complexes may be used for the enantioselective reduction of ketones with hydrosilanes.〔Hayashi, T.; Hayashi, C.; Uozumi, Y. ''Tetrahedron: Asymmetry'' 1995, ''6'', 2503.〕 ''(1)'' Hydrosilanes are not intrinsically nucleophilic; thus, they react only with highly electrophilic functional groups that have carbocationic character. However, many resonance-stabilized carbocations are insufficiently electrophilic to react with hydrosilanes.〔Fry J. L.; Ott, R. A. ''J. Org. Chem.'' 1981, ''46'', 602.〕 Upon the generation of a carbocation, rate-determining hydride transfer from the organosilane occurs to yield a reduced product and a neutral silane in which the counterion of the carbocation has replaced hydrogen. Prior to hydride transfer, the carbocation intermediate is liable to undergo Wagner-Meerwein rearrangements.〔Doyle, M. P.; McOsker, C. C.; West, C. T. ''J. Org. Chem.'' 1976, ''41'', 1393.〕 This simple picture is complicated by experimental observations that point to more complex mechanisms. For instance, retention of configuration at silicon has been observed in silane reductions of chiral triaryl methyl chlorides in benzene. This result suggests that the exchange of chlorine for hydrogen occurs through σ-bond metathesis, without the formation of a planar silicenium ion.〔Austin, J. D.; Eaborn, C. ''J. Chem. Soc.'' 1964, 2279.〕 Reductions in more polar solvents may involve silicenium ions, although the extent of racemization at silicon (expected to be complete if free silicenium ions are involved) depends on the polarity of the solvent.〔Sommer, L. H.; Bauman, D. L. ''J. Am. Chem. Soc.'' 1969, ''91'', 7045.〕 ''(2)'' Hypervalent silicon species are likely the active reducing agent when nucleophiles with a high affinity for silicon, such as fluoride, are added to the reaction mixture.〔Chuit, C.; Corriu, R. J. P.; Perz, R.; Reyé, C. ''Synthesis'' 1982, 981.〕 Coordination of an anionic nucleophile such as fluoride to silicon increases its reducing power substantially, such that reductions of aldehydes and ketones can occur without the help of a Lewis acid. ''(3)'' 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Reductions with hydrosilanes」の詳細全文を読む スポンサード リンク
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