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

The quantum yields (phi (obs)) relating to the colloidal TiO2-sensitized ph oto-oxidation of ring methoxy substituted benzylic alcohols (D) were determ ined. The true quantum yields (phi (0)) were obtained from a Langmuir-Hinsh elwood isotherm treatment of phi (obs) at different [D]. In the light of th e suggested photo-oxidation mechanism, an equation was deduced where phi (0 ) is expressed as a function of the electron transfer (k(et)), back electro n transfer (k(-et)) and benzylic deprotonation (k(p)) rate constants. In pa rticular, the lower phi (0) value of 3-CH3O-vs 4-CH3O-benzyl alcohol (1, wi th lower E-p) should principally depend on the difference in k(et), while t he decrease in phi (0) on going from alcohol 1 to the alpha -methyl derivat ive (4) should be due to the difference in k(p) (stereoelectronic effect). The adsorption equilibrium constants under irradiation (K-D) were also obta ined from the above equation and the values are similar for the considered substrates, except for 4. In fact, this substrate shows a lower K-D value, probably because of the steric hindrance of the methyl group to the OH adso rption (preferential site) on TiO2. Finally, both the inter- and intramolec ular deuterium isotope effect (k(H)/k(D) = 1.8 and 1.6, respectively) are c onsistent with a kinetically significant C-alpha-H bond breaking following the electron-transfer step. Copyright (C) 2000 John Wiley & Sons, Ltd.