Isotope Studies of Photocatalysis:  Dual Mechanisms in the Conversion of Anisole to Phenol

The partial TiO₂-mediated photocatalytic degradation of anisole has been studied using isotopically labeled substrates and comparisons to Fenton and hν/H₂O₂ chemistry. An H/D isotope selectivity of 2.7 is observed for hydrogen abstraction from the methyl group in all cases. An ¹⁸O tracer study shows that anisole is simultaneously converted to phenol by both ipso attack and degradation of the methyl. The isotope selectivity for hydroxylation of anisole is reported to be about 1.3 under all conditions. This is attributed to an O₂-dependent competition along the reaction pathway that can be avoided by use of benzoquinone instead of O₂ as an electron acceptor with TiO₂. Under those conditions, the H/D isotope selectivities are between 0.9 and 1.0, as would be expected for inverse secondary kinetic isotope effects.