Isotope studies of photocatalysis: Dual mechanisms in the conversion of anisole to phenol

The partial TiO2-mediated photocatalytic degradation of anisole has been st udied using isotopically labeled substrates and comparisons to Fenton and h v/H2O2 chemistry. An H/D isotope selectivity of 2.7 is observed for hydroge n abstraction from the methyl group in all cases. An O-18 tracer study show s that anisole is simultaneously converted to phenol by both ipso arrack an d degradation of me methyl, The isotope selectivity for hydroxylation of an isole is reported to be about 1.3 under all conditions. This is attributed to an O-2-dependent competition along the: reaction pathway that can be avo ided by use of benzoquinone,instead of O-2 as an electron acceptor with TiO 2. Under those conditions, the H/D isotope selectivities are between 0.9 an d. 1.0, as would be expected for inverse secondary kinetic isotope effects.