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+center dot) 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 CS 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 center dot (lambda(max) = 390 nm) and of the radical cations (lambda(max) = 500620 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 CS 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.