An MNDO study of the unimolecular reactions of methyl acetate and its sulphur analogues

The unimolecular reactions of methyl acetate and its sulphur analogues, S-methyl and O-methyl monothioacetate and methyl dithioacetate, were investigated using the semi-empirical MNDO method with respect to ketene–thioketene formation and 1,3-methyl migration. For methyl acetate, 1,3-methyl migration of the methyl ester was calculated to be more favourable than ketene formation. S-Methyl monothioacetate undergoes ketene formation preferentially while O-methyl monothioacetate rearranges via 1,3-methyl migration to the more thermodynamically stable S-methyl isomer. Methyl dithioacetate also prefers methyl migration; however, unlike the other three compounds, the theoretical results condtradict earlier reported experimental results.