AB-INITIO CALCULATIONS ON THE PREFERRED MODE OF RING-OPENING IN SILACYCLOPROPANE

CASSCF and CASPT2N/6-31G calculations on silacyclopropane (1) find th at the transition state for hydrogen migration in concert with ring op ening, leading to ethylsilylene (2), has an energy that is significant ly lower than the energies of the 1-sila- and 2-silatrimethylene dirad icals (6 and 7), formed by ring opening alone. (12/12)CASPT2N calculat ions, corrected for differences in zero-point energies, give barriers of 24.7 and 13.2 kcal/mol, respectively, for the silacyclopropane to e thylsilylene reaction and its reverse. These values are in reasonable agreement with experimental estimates of 23-28 and 10.4 kcal/mol, resp ectively, for the corresponding reactions of alkylsilacyclopropanes. D iradicals 6 and 7, which are transition states, are computed to be ca. 20 kcal/mol higher in energy than the transition state leading to eth ylsilylene (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 th e opening of cyclopropane to the trimethylene diradical, is interprete d in terms of the relative Si-H and C-H BDE's in primary silyl and alk yl radicals.