Mixed hetero-/homogeneous TiO2/N-hydroxyimide photocatalysis in visible-light-induced controllable benzylic oxidation by molecular oxygen

Homogeneous and heterogeneous types of catalysis are frequently considered as separate disciplines or even opposed to each other. In the present work, a new type of mixed hetero-/homogeneous catalysis was demonstrated for the case of selective alkylarene oxidation by molecular oxygen. The proposed catalytic system consists of two widely available components: N-hydroxyphthalimide (NHPI, a homogeneous organocatalyst for free-radical chain reactions) and nanosized TiO2 (heterogeneous UV-active photoredox catalyst). The interaction of NHPI with TiO2 allows for a shift from UV to visible light photoredox activity and generation of phthalimide-N-oxyl (PINO) radicals that diffuse into the solution where NHPI/PINO-catalyzed free-radical chain reaction can proceed without the additional light input providing a fundamental increase in energy efficiency. The NHPI/TiO2 ratio controls the selectivity of oxidation affording preferential formation of hydroperoxide or ketone from alkylarene.