Visible light-induced Minisci reaction through photoexcitation of surface Ti-peroxo species

Abstract

Photocatalytic Minisci-type functionalization of pyridine with tetrahydrofuran (THF) proceeded using hydrogen peroxide (H₂O₂) and a TiO₂ photocatalyst under acidic conditions. Under UV light (λ = 360 nm), the reaction selectivity based on pyridine (S_py) was >99% while the selectivity based on THF (S_THF) was low such as 19%. In contrast, under visible light (λ = 400 or 420 nm) S_py was similarly high (>99%) and S_THF was two times higher than that under UV light. A surface peroxo complex formed upon contact of hydrogen peroxide with the TiO₂ surface can be selectively photoexcited by visible light to inject the photoexcited electron to the conduction band of TiO₂. The electron can reduce H₂O₂ to a reactive oxygen species (ROS) and promote selectively the Minisci-type cross-coupling reaction between pyridinium ions and THF. A reaction test with a hole scavenger (methanol) evidenced that the hole oxidation of H₂O₂ under UV light is responsible for the lower selectivity, in other words, the higher selectivity under visible light would be due to suppression of the hole oxidation of H₂O₂. These results demonstrate a novel way to improve the selectivity of the photocatalytic cross-coupling reaction by using H₂O₂ as an oxidant with the photoexcitation of surface Ti-peroxo species on TiO₂.