Visible-Light-Driven Benzyl C–H Bond Aerobic Oxidation with Nitroarene in Aqueous Medium

Abstract

The pursuit of green and efficient methods for benzylic oxidation remains a long-standing goal and an area of intense interest in organic synthesis. Herein, a visible-light-driven photocatalytic system has been developed for the selective oxidation of benzylic C–H bonds to carbonyl compounds using molecular oxygen as the sole oxidant in an aqueous medium. This strategy employs commercially available nitroarenes, such as 4-nitro-2-(trifluoromethyl)benzonitrile. Water plays a crucial role in enhancing both the reaction rate and selectivity. The protocol features a broad substrate scope with excellent functional group tolerance, accommodating alkylbenzenes bearing diverse side chains, including sensitive amine moieties. Mechanistic investigations reveal a dual activation pathway involving both hydrogen atom transfer (HAT) from the photoexcited nitroarene and single-electron transfer (SET) via an electron donor–acceptor (EDA) complex between the substrate and the nitroarene. The promoting effect of water is attributed to π–hydrogen bond interactions, which thermodynamically facilitate C–H bond cleavage. Beyond benzylic oxidation, the methodology is successfully applied to sulfide oxidation, oxidative formylation, and Povarov-type reactions. Furthermore, it demonstrates significant potential for sustainable applications, including the photodegradation of lignin model compounds and polystyrene, as well as efficient operation in a continuous-flow system. This work presents a green and practical photocatalytic platform that merges the advantages of a benign aqueous medium, a low E-factor, and the use of molecular oxygen as the sole oxidant.