Pd loaded TiO2 as recyclable catalyst for benzophenone synthesis by coupling benzaldehyde with iodobenzene under UV light

The benzophenone structural unit is one of the most important functional groups in organic chemistry, which has a wide range of applications in drugs, light-curing agent, UV-light absorbent and so on. However, the traditional synthetic methods of benzophenone generally involve additives or toxic regents, leading to a large amount of waste and non-recyclable of [Pd] catalyst. Herein, we report Pd loaded carbon-modified TiO2 (Pd/C-TiO2) as an efficient and recyclable catalyst realized the generation of benzophenone through cross-coupling of benzaldehyde and iodobenzene under UV light irradiation, with the yield of 98% and selectivity up to 99%. Based on density functional theory (DFT) calculation and electron paramagnetic resonance (EPR) analysis, the reaction undergoes the Pd0-PdII-PdIII catalytic pathway, that is, benzaldehyde directly activated to acyl radical under UV light and then attracted by Pd-iodobenzene complex to proceed the coupling reaction for benzophenone generation. Owing to the in-situ reduction of dissolved [Pd] through TiO2 photocatalyst in the reaction system, good recyclability of metallic Pd can be achieved. This work not only shed light on a facile method for the synthesis of carbonyl-containing compounds but also offered a practical approach for minimizing the leaching of active metals in transition metal-catalyzed coupling reactions.