Visible-light-promoted catalyst-free benzylic C-H oxidation using molecular oxygen as a green oxidant.

Herein, we report a catalyst- and additive-free strategy for the visible-light-promoted carbonylative oxidation of benzylic C-H bonds employing molecular oxygen as the sole oxidant. This operationally simple protocol enables the conversion of a broad range of aromatic alkanes into valuable ketones, including complex drug derivatives and intermediates under mild conditions using ethyl acetate as a green solvent. Mechanistic studies reveal that the reaction proceeds via a substrate-oxygen charge-transfer (CT) complex upon photoexcitation, generating reactive oxygen species (singlet oxygen and superoxide anion) and a key benzylic radical. Interestingly, the carbonyl product can further accelerate the transformation by participating in a separate CT complex with the starting material. To address the scalability limitations, a novel solvent-free continuous-flow photoreactor was developed, which demonstrated a significant efficiency enhancement of over 150-fold compared to batch processes. This work presents a green, practical, and scalable method for benzylic oxidation, underpinned by a detailed mechanistic understanding of the photoinduced CT process.