New Orbital Symmetry-Allowed Route for Cycloreversion of Silacyclobutane and Its Methyl Derivatives.

The [2+2] cycloreversion of silacyclobutane (SCB) and its two methyl-substituted derivatives, 1-methyl-1-silacyclobutane (MSCB) and 1,1-dimethyl-1-silacyclobutane (DMSCB), were studied using ab initio quantum chemistry calculations. The second-order Møller-Plesset (MP2) perturbation theory, complete active space self-consistent field (CASSCF), and coupled clusters methods were used to explore both the concerted and the stepwise cycloreversions of the three molecules. In addition to the orbital symmetry-forbidden supra-supra [2s+2s] transition state, a new orbital symmetry-allowed supra-antara [2s+2a] transition state was discovered for the concerted route for all three molecules. Both methyl substitution and temperature play a role in the kinetic competition between the [2s+2s] and [2s+2a] routes. At 0 and 298 K, the concerted [2s+2a] cycloreversion is kinetically more favorable than the [2s+2s] cycloreversion for SCB, but the opposite is true for MSCB and DMSCB. With increasing temperatures to above 600 and 1800 K, the [2s+2a] cycloreversion becomes more favorable for MSCB and DMSCB, respectively. The methyl substitutions on Si atoms also affect the stability of the diradical intermediate formed by Si-C bond rupture, leading to a less stable diradical with increasing methyl groups.