A silicon-29 NMR study of adduct formation in organosilanes

The reactions of a series of methyl-and phenylsilanes containing one or more electronegative groups such as chloro, triflate (OTf), trifluoroacetate, and trifluoroethoxy with the base HMPA were investigated with Si-29 NMR and conductivity titrations in both methylene chloride and acetonitrile solvents. Formation of both five-coordinate adduct and four-coordinate electrolyte (such as Me3Si(HMPA)(+) OTf-) was observed for some of the strongly electron attracting groups, whereas only adduct formation was observed for the chloro group. Electrolyte formation is also favored by an increase in the base to acid mole ratio and the more polar acetonitrile. The effect of the Si-H bond on the Lewis acidity of organosilanes in intermolecular adduct formation was documented and is believed to be a result of bond strengthening steric effect.