Are Pi-ligand exchange reactions of thiirenium and thiiranium ions feasible? An ab initio investigation

Ab initio calculations at the G2 level have been employed to investigate the hitherto unobserved π-ligand exchange reactions of thiirenium and thiiranium ions. Two pathways for exchange and one pathway for insertion are presented. The lower energy exchange pathway has barriers of 19.8-46.6 kJ mol^-1. The barriers for the second exchange pathway and for the insertion reaction are considerably larger. The calculations predict that π-ligand exchange is indeed feasible for thiirenium and thiiranium ions, that the exchange reaction is stereospecific with respect to the configuration at sulfur, and that insertion will not compete with exchange despite a high thermodynamic driving force for the former. These conclusions closely parallel those reached in a recent theoretical investigation of the corresponding phosphorus systems for which π-ligand exchange was observed experimentally.