Theoretical study on the reduction behaviour of sulphur containing heterocycles. I Cleavage reaction of the C–S bond in the dibenzo[b,e]thiepinonesulphone class

Our previous experimental data on the reduction mechanism of two sulphones in the benzothiepinone class, dibenz[b,e]-thiepinone-5,5-dioxide (1) and dibenz[b,e]-thiepin-thione-5,5-dioxide (2) were analysed by ab initio and solvent dependent semiempirical methods. The calculations were performed on the neutral molecules and the corresponding charged species present in the system during the electrochemical reduction, the anion radicals and the dianions. The main feature of the reduction mechanism, the cleavage of the sulphur containing median ring, was accounted for by the profile of sections through the potential energy surfaces along the two possible reaction coordinates, the CH2–SO2 and Caromatic–SO2 bonds. An estimation of the energy required for the ring cleavage shows that for both compounds the reaction occurs at the CH2–SO2 bond. According to the theoretical predictions the bond breaking occurs for compound 1 after the first reduction wave starting from the anion radical, while for compound 2 after the second wave, i.e. starting from the dianion, in agreement with the experimental data. The use of solvent dependent optimizations led to a decrease of all the energy barriers as compared to the in vacuo calculations attesting the importance of considering the solvation effects in modeling the reaction mechanisms of charged species.