Titanium dioxide photosensitised oxidation of α,β-dihydroxybenzyl derivatives in CH3CN

The TiO2-photosensitised oxidation in CH3CN of a series of 1-(X-phenyl)-1,2-ethanediols (1) and of 2-(X-phenyl)-1,2-propanediols (2) gives the corresponding benzaldehyde and acetophenone, respectively, accompanied by formaldehyde, whereas a series of symmetrically X-ring-substituted 1,2-diphenyl-1,2-ethanediols (3) yields the corresponding benzaldehyde (substrate/product molar ratio = 0.5). The adsorption constant values (2400–3900 M−1) are significantly higher than those of benzylic alcohols and can be explained by assuming that the TiO2 surface adsorption occurs through both OH groups (chelation effect). The relative rates, determined for each series from competitive kinetic experiments, remained nearly constant when the oxidation potential increased, instead of decreasing as observed in homogeneous phase. This behaviour has been attributed to the preferential adsorption of the OH groups, that increases their oxidizability until they favourably compete, as regards electron abstraction, with the aromatic site. The reaction rate thereby becomes much less sensitive to the ring substituent effect. Based on the comparison of true quantum yield values (obtained in the photo-oxidation in the presence of colloidal TiO2) of ring unsubstituted 1 and 2, the deprotonation pathway of the cation radical was excluded. Analogous to what happens with 2-aminoalcohol radical cations, the most probable path is a heterolytic, concerted and base-catalysed (by basic sites at the TiO2 surface) Cα–Cβ fragmentation that produces a radical and a final product (carbonyl compound).