γ-Silyl Cyclobutyl Carbocations

A series of 3-trimethylsilyl-1-substituted cyclobutyl trifuloroacetates have been prepared and reacted in CD3CO2D. Rate data indicate that the substrates with the trimethylsilyl group cis to the leaving group react with assistance due to gamma-silyl participation. Rate enhancements range from a factor of 209 for alpha-phenyl-substituted cations to 4.6 x 10(4) for alpha-methyl-substituted cations to > 10(10) for the unsubstituted gamma-trimethylsilylcyclobutyl cation. Acetate substitution products are formed with net retention of stercochemistry. These experimental studies, as well as B3LYP/6-31G* computational studies, are consistent with the involvement of carbocations where the rear lobe of the gamma-Si-C bond interacts strongly with the developing cationic center. Solvolytic rate studies, as well as computational studies, suggest that the secondary gamma-trimethylsilycyclobutyl cation is even more stable than the beta-trimethylsilylcyclobutylcation, i.e., the gamma-silyl effect actually outweighs the potent beta-silyl effect. Although computational studies suggest the existence of certain isomeric cations, where the front lobe of the Si-C bond interacts with the cationic center, solvolytic evidence for the involvement of these front lobe stabilized cations is less compelling.