Accelerating effect of imidazolium ionic liquids on the singlet oxygen promoted oxidation of thioethers: A theoretical study

A computational study of the catalytic activity exhibited by imidazolium based ionic liquids on the singlet oxygen promoted oxidation of thioethers is presented. The reaction of singlet oxygen with thioanisole and diethyl sulfide, in the absence and in the presence of 1-butyl-3-methylimidazolium cation, was followed along the reaction coordinate up to the formation of the persulfoxide intermediate that is widely recognized as the key intermediate in this process. In line with previous experimental observations, the results obtained show that the imidazolium cation strongly stabilizes the persulfoxide intermediate by hydrogen bonding thus depressing the unproductive intersystem crossing and favoring product formation. Evidence for a significant role played by exciplex intermediates in this reaction was also found. Surprisingly, the results for the singlet oxygen-sulfide system in the absence of imidazolium show that whereas diethyl sulfide can readily form the persulfoxide intermediate, from which S-oxidation products ensue, for thioanisole the persulfoxide formation is characterized by a high activation energy. A detailed investigation on the role played by the aromatic ring in this dramatic activation energy increase is presented suggesting, for the first time, a rationale for the well known low reactivity exhibited by aromatic sulfides in reactions with singlet oxygen.