Substituent effects on the TiO2 photosensitized oxidation reaction of benzyl thioethers and thiols in deaerated acetonitrile

• The TiO2- and MeOP+-sensitized photooxidation of benzyl sulphides and thiols occurs by an electron transfer mechanism.
• Both sulfide+ and thiol+ deprotonates to give the benzyl radical, which oxidizes to produce benzaldehyde with sulfides or dimerizes to disulfides with thiols.
• The adsorption involvement in the TiO2-photooxidation is not significant because of the poor attitude of the sulfur to form hydrogen bonds with the TiO2 hydroxyl groups.
• DFT calculation indicates a significant charge and spin delocalization in the sulfur radical cations involving both the ring and the sulfur.

The TiO2 photosensitized oxidation of benzyl methyl sulfides (X-C6H4CH2SCH3) and benzyl thiols (X-C6H4CH2SH) has been investigated in Ar-saturated CH3CN. Steady-state irradiation produced benzaldehydes or dibenzylsulfides as oxidation products with sulfides and thiols, respectively. The results obtained through kinetic competitive experiments, aimed to evaluate the ring substituent effect on the reactivity, suggested the involvement of radical cation intermediates, formed by the favorable electron transfer from the substrate to the TiO2 photogenerated hole, which reasonably deprotonate to give the final products. This process was poorly affected by the adsorption of the substrate at the TiO2 surface, as demonstrated by similar results, both in terms of products and reactivity, collected for the homogeneous photooxidation of the same substrates sensitized by N-methoxyphenanthridinium hexafluorophosphate (MeOP+PF6−). This behavior is likely due to the low hydrogen-bond acceptor ability of divalent sulfur systems. Density functional theory calculations pointed out that the most stable conformations of X-C6H4CH2SH+ are characterized by having the CS bond almost collinear with the π system of the aromatic ring and by a significant charge and spin delocalization involving both the phenyl ring and the sulfur atom.

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