Photo-oxidation of some benzylic alcohols sensitized by colloidal TiO2 in CH3CN. A kinetic mechanistic study through quantum yield determinations

Abstract

The quantum yields (ϕobs) relating to the colloidal TiO2-sensitized photo-oxidation of ring methoxy-substituted benzylic alcohols (D) were determined. The true quantum yields (ϕ0) were obtained from a Langmuir–Hinshelwood isotherm treatment of ϕobs at different [D]. In the light of the suggested photo-oxidation mechanism, an equation was deduced where ϕ0 is expressed as a function of the electron transfer (ket), back electron transfer (k−et) and benzylic deprotonation (kp) rate constants. In particular, the lower ϕ0 value of 3-CH3O-vs 4-CH3O-benzyl alcohol (1, with lower Ep) should principally depend on the difference in ket, while the decrease in ϕ0 on going from alcohol 1 to the α-methyl derivative (4) should be due to the difference in kp (stereoelectronic effect). The adsorption equilibrium constants under irradiation (KD) were also obtained from the above equation and the values are similar for the considered substrates, except for 4. In fact, this substrate shows a lower KD value, probably because of the steric hindrance of the methyl group to the OH adsorption (preferential site) on TiO2. Finally, both the inter- and intramolecular deuterium isotope effect (kH/kD = 1.8 and 1.6, respectively) are consistent with a kinetically significant Cα—H bond breaking following the electron-transfer step. Copyright © 2000 John Wiley & Sons, Ltd.