Ab initio and DFT investigations on the stereochemistry of ring opening of episulfides

Cleavage of cis and trans-2,3-dimethylepisulfides by triethyl phosphite have been experimentally accomplished by Neureiter and Bordwell and they have reported that the reaction proceeded with complete retention of stereochemistry. This reaction has been modeled here at ab initio and DFT level to look at the mechanism of the reaction and to understand how the stereochemistry of the reaction is controlled. Calculations reveal that the reaction takes place in a single step passing through a concerted synchronous transition state. In this step, cleavage of two C-S bonds takes place with the simultaneous formation of a new P-S bond and this results in the formation of the alkene with the same stereochemistry, in total agreement with the experiment. Intrinsic reaction coordinate analysis explains the synchronous nature of the transition state very well and smoothly connects the reactants and products. For comparison, single point energy calculations have been performed at higher levels and that also confirms the same trend. High exothermicity of the reaction indicates the release of the ring strain during the reaction in the episulfide.