Systematic Repression of β-Silyl Carbocation Stabilization

Solvolysis of 1-(trimethylsilylmethyl)cyclopropyl mesylate. in CD3CO2D gives ring-opened products as well as methylenecyclopropane. The rate enhancement due to the beta-trimethylsilyl group is a factor of about 10(6). The large stabilizing effect of a beta-silyl group (which can cause rate enhancements of up to 1012) on the intermediate cation has been repressed. B3LYP/6-31G* computational studies indicate a carbocation stabilization energy of 16.6 kcal/mol. Rates of solvolyses of 1-phenyl-2-trimethylsilylcyclopropyl chlorides are enhanced by a factor of 10(3)-10(4). The intermediate cyclopropyl cation undergoes substantial ring opening since beta-silyl stabilization is not large (calculated stabilization energy of 12 kcal/mol). Solvolysis rates of 2-trimethylsilylbenzocyclobutyl derivatives are not significantly enhanced by the P-trimethylsilyl group. beta-Silyl stabilization of benzocyclobutenyl carbocations generated in solution has been effectively eliminated due to antiaromatic considerations (calculated stabilization energy of 3.7 kcal/mol when R = Ph). While computational studies parallel solvolytic rate studies, they overestimate the extent of beta-trimethylsilyl stabilization of solvolytically generated carbocations.