Visible‐light‐promoted catalyst‐free benzylic C‐H oxidation using molecular oxygen as a green oxidant
Source: PubMed Central Open Access, NCBI / U.S. National Library of Medicine
Abstract 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. A green and simple photo‐oxygenation of benzylic C‐H bond with molecular oxygen was developed for the preparation of a series of carbonyl compounds. The straightforward C‐H carbonylation strategy features catalyst‐ and additive‐free reaction conditions, only oxygen as a green oxidant, broad scope and scalable, and the simpl
Abstract
Abstract 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. A green and simple photo‐oxygenation of benzylic C‐H bond with molecular oxygen was developed for the preparation of a series of carbonyl compounds. The straightforward C‐H carbonylation strategy features catalyst‐ and additive‐free reaction conditions, only oxygen as a green oxidant, broad scope and scalable, and the simple‐operation nature. graphical