A theoretical study of the mechanism of 1,2-migrations in methoxysiloxycarbene

Abstract

Intramolecular rearrangements of methoxysiloxycarbene (CH3OCOSiH3) have been investigated by means of ab initio molecular orbital theory and hybrid density functional theory calculations. Particular attention was paid to 1,2-silyl migration from oxygen to the carbene carbon, and to the analogous 1,2-methyl migration for comparison. A combination of frontier molecular orbital (FMO) theory, natural bond orbital (NBO) analysis, and the theory of atoms in molecules (AIM) were used to shed light on the mechanistic details of these rearrangements. The present analyses clearly indicate that 1,2-silyl migration involves nucleophilic attack by the carbene lone pair at silicon, whereas 1,2-methyl migration seems to involve an anion-like shift of the methyl group from oxygen to the "vacant" carbene p-orbital. Finally, based on the computed relative Gibbs free energy barriers, it is apparent that 1,2-silyl migration is much more favorable than 1,2-methyl migration, in keeping with experimental observations.Key words: carbenes, oxycarbenes, intramolecular rearrangements, 1,2-migrations, quantum chemistry, theory of atoms in molecules, natural bond orbital analysis