Ab Initio Calculations on the Preferred Mode of Ring Opening in Silacyclopropane

CASSCF and CASPT2N/6-31G* calculations on silacyclopropane ( 1 ) find that the transition state for hydrogen migration in concert with ring opening, leading to ethylsilylene ( 2 ), has an energy that is significantly lower than the energies of the 1-sila- and 2-silatrimethylene diradicals ( 6 and 7 ), formed by ring opening alone. (12/12)CASPT2N calculations, corrected for differences in zero-point energies, give barriers of 24.7 and 13.2 kcal/mol, respectively, for the silacyclopropane to ethylsilylene reaction and its reverse. These values are in reasonable agreement with experimental estimates of 23−28 and 10.4 kcal/mol, respectively, for the corresponding reactions of alkylsilacyclopropanes. Diradicals 6 and 7, which are transition states, are computed to be ca. 20 kcal/mol higher in energy than the transition state leading to ethylsilylene ( 2 ). The barrier to the conversion of 2 to 1-silapropene ( 3 ) is calculated to be 30.7 kcal/mol. The strong preference for opening of silacyclopropane ( 1 ) to ethylsilylene ( 2 ), which contrasts with the opening of cyclopropane to the trimethylene diradical, is interpreted in terms of the relative Si−H and C−H BDE's in primary silyl and alkyl radicals.