Photosensitized oxidation of aryl benzyl sulfoxides. Evidence for nucleophilic assistance to the C-S bond cleavage of aryl benzyl sulfoxide radical cations

The radical cations of a series of aryl benzyl sulfoxides (4-X-C6H4CH2SOC6H4Y+•) have been generated by photochemical oxidation of the parent sulfoxides sensitized by 3-cyano-N-methylquinolinium perchlorate (3-CN-NMQ+ClO4-). Steady-state photolysis experiments showed the prevailing formation of benzylic products deriving from the C-S fragmentation in the radical cations, together with sulfur-containing products. Formation of sulfoxide radical cations was unequivocally established by laser flash photolysis experiments showing the absorption bands of 3-CN-NMQ• (λmax = 390 nm) and of the radical cations (λmax = 500-620 nm). The decay rate constants of radical cations, determined by LFP experiments, decrease by increasing the electron-donating power of the arylsulfinyl Y substituent and to a smaller extent by increasing the electron-withdrawing power of the benzylic X substituent. A solvent nucleophilic assistance to the C-S bond cleavage has been suggested, supported by the comparison of substituent effects on the same process occurring in aryl tert-butyl sulfoxide radical cations. DFT calculations, performed to determine the bond dissociation free energy in the radical cations, the transition state energies associated with the unimolecular C-S bond cleavage, and the charge and spin delocalized on their structures, were also useful to endorse the nucleophilic assistance to the C-S scission.