Evaluation of the Relative Magnitude of the [beta]-Effect of Silicon and the [gamma]-Effect of Tin by Intermolecular Competition

The relative magnitude of the β-effect of silicon and the γ-effect of tin was evaluated by intermolecular competition. An acetal was allowed to react with a 1 : 1 mixture of an allylsilane and a homoallylstannane in the presence of TMSOTf (trimethylsilyl trifluoromethanesulfonate); the former was found to be more reactive than does the latter, indicating that the β-silyl group activates the carbon-carbon double bond more effectively than the γ-stannyl group. This is consistent with the results of molecular-orbital calculations which indicate that the HOMO level of the allylsilane is higher than that of the homoallylstannane. The intermolecular competition between a homoallylstannane and a terminal alkene toward electrophilic reactions with an acetal have revealed that the former is more reactive than the latter. This result indicates that the stannyl group at the γ-position definitely activates the carbon-carbon double bond toward electrophiles.