title: Photosensitized oxidation of aryl benzyl sulfoxides. Evidence for nucleophilic assistance to the C-s bond cleavage of aryl benzyl sulfoxide radical cations.
abstract: 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.
authors.affiliation: Dipartimento di Chimica, Biologia e Biotecnologie and Centro di Eccellenza Materiali Innovativi Nanostrutturati, Università di Perugia , via Elce di sotto 8, 06123 Perugia, Italy.
authors.forename: Tiziana
authors.initials: T
authors.lastname: Del Giacco
authors.forename: Osvaldo
authors.initials: O
authors.lastname: Lanzalunga
authors.forename: Andrea
authors.initials: A
authors.lastname: Lapi
authors.forename: Marco
authors.initials: M
authors.lastname: Mazzonna
authors.forename: Paolo
authors.initials: P
authors.lastname: Mencarelli
mesh_terms: D001593:Benzyl Compounds / Q000737:chemistry*; D002412:Cations / Q000737:chemistry; D005609:Free Radicals / Q000737:chemistry; D015394:Molecular Structure; D010084:Oxidation-Reduction; D055668:Photochemical Processes; D017319:Photosensitizing Agents / Q000737:chemistry*; D013454:Sulfoxides / Q000737:chemistry*
chemical_list: D001593:Benzyl Compounds; D002412:Cations; D005609:Free Radicals; D017319:Photosensitizing Agents; D013454:Sulfoxides