Photosensitized Oxidation of Aryl Benzyl Sulfoxides. Evidence for Nucleophilic Assistance to the C–S Bond Cleavage of Aryl Benzyl Sulfoxide Radical Cations
2015
Del Giacco, Tiziana | Lanzalunga, Osvaldo | Lapi, Andrea | Mazzonna, Marco | Mencarelli, Paolo
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.
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