Role of Hypervalent Silicon Species

It is well known that strong electrophiles such as carbocations are reduced by organosilicon hydrides (Eq. 1).3,70,71 On the other hand, simple mixtures of organosilicon hydrides and compounds with weakly electrophilic carbon centers such as ketones and aldehydes are normally unreactive unless the electrophilicity of the carbon center is enhanced by complexation of the carbonyl oxygen with Brpnsted acids3,70 73 or certain Lewis acids (Eq. 2).1,70,71,74,75 Using these acids, hydride transfer from the silicon center to carbon may then occur to give either alcohol-related or hydrocarbon products. 

Alternatively, unreactive mixtures of organosilicon hydrides and carbonyl compounds react by hydride transfer from the silicon center to the carbon center when certain nucleophilic species with a high affinity for silicon are added to the mixture.76 94 This outcome likely results from the formation of valence-expanded, pentacoordinate hydrosilanide anion reaction intermediates that have stronger hydride-donating capabilities than their tetravalent precursors (Eq. 6).22,95 101 

A diaryldihydrosilane with a hexacoordinated silicon center, produced through intramolecular coordination, is reported not to react with benzaldehyde, although the silane is capable of reducing silver ion to silver metal.113 There is also a report of a heptacoordinate silicon hydride species with the ability to transfer hydride to trityl cation while remaining inactive toward methanol.108114 

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Belot, V. et al., Sol-gel chemistry of hydrogenosiliconates The role of hypervalent silicon species, in Better Ceramics Through Chemistry IV, Zelinski, B.J.J. et al., Eds., Materials Research Society Symposium Proceedings, Warrendale, PA, 1990, p. 3. 

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