Selective visible-light-induced photooxidation of benzylic alcohols to corresponding carbonyl compounds over titanium dioxide: A study of the structure-reactivity relationship

We herein report the selective photooxidation of primary and secondary benzylic alcohol to the corresponding carbonyl compounds over titanium dioxide in acetonitrile under visible light irradiation. To enhance the understanding in which way parameters influence the reaction rate, several factors, such as initial concentration, catalyst amount, light intensity, and its spectral characteristics were investigated. The use of titanium dioxide in anatase modification as a photo-catalyst was also considered with a significant change in the reaction rate. Further studies on benzylic alcohols substituted with different electron-donating (EDG) and electron-withdrawing groups (EWG) showed that the Hammett plot provided an excellent structure-reactivity relationship between substitution and rate constant for primary substituted alcohols. It displayed an upward curvature indicating that the reaction mechanism changed as the substituents became more electron-withdrawing. The kinetic isotope effect (KIE) revealed that the alpha-H abstraction was the rate-determining step for benzylic alcohols substituted with EDGs. When comparing the reaction rate between primary and secondary benzylic alcohols, it was found that the rate of reaction for secondary benzylic alcohols bearing EDGs decreased compared to primary benzylic alcohols, an increase was observed when the aromatic ring was substituted with EWGs.