r-Silyl Cyclobutyl Carbocations

摘要

Aseries of 3-trimethylsilyl-1-substituted cyclobutyl trifluoroacetates 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 γ-silyl participation. Rate enhancements range from a factor of 209 for R-phenyl-substituted cations to 4.6 104 for R-methyl-substituted cations to >1010 for the unsubstituted r-trimethylsilylcyclobutyl cation. Acetate substitution products are formed with net retention of stereochemistry. These experimental studies, as well as B3LYP/6-31G* computational studies, are consistent with the involvement of carbocations where the rear lobe of the r-Si-C bond interacts strongly with the developing cationic center. Solvolytic rate studies, as well as computational studies, suggest that the secondary r-trimethylsilylcyclobutyl cation is even more stable than the β-trimethylsilylcyclobutyl cation, i.e., the r-silyl effect actually outweighs the potent b-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.