Selectively Aerobic Oxidation of Benzylic C—H Bonds Enabled by Dual Anthracene and Cerium Catalysis under Continuous-Flow Conditions★

The benzyl oxidation reaction serves as a crucial functional group transformation method in the field of organic synthesis.Regrettably, traditional benzyl oxidation reactions frequently necessitate harsh conditions, such as elevated temperatures and potent oxidizing agents.In contrast, this article showcases a highly selective catalytic benzylic oxidation executed within a continuous-flow microreactor.By harnessing the previously established cerium-alcohol complex's ligand to metal charge transfer (LMCT)-hydrogen atom transfer (HAT) activation mechanism and the anthracene-cerium synergistic catalytic system, a diverse array of aromatic ketones was synthesized from aryl alkanes with remarkable efficiency under ambient and aerobic conditions.The continuous-flow technology, endowed with unique advantages such as heightened illumination efficiency, superior gas-liquid mass transfer, repeatability, and scalability, has emerged as a powerful instrument for scaling-up photocatalytic reactions.In this process, under flow conditions, ethyl acetate solutions comprising Ce(NO3)3•6H2O, tetrabutylammonium bromide (TBABr), 9,10-dibromoanthracene (DBA), trichloroethanol (TCE), and ethylbenzene encountered and mixed with oxygen within the microreactor.Subsequently, a photocatalytic aerobic oxidation reaction occurred under visible light irradiation at room temperature, achieving complete conversion within a mere 5 min, and rapidly generated a series of aromatic ketones with good to excellent yields.Mechanistic studies indicated the paramount importance of the anthracene-derived catalyst DBA in achieving the heightened efficiency.Under visible light irradiation, the excited state DBA was initially oxidatively quenched with oxygen or peroxide species generated in the system, resulting in the formation of the DBA cationic free radical.Subsequently, the DBA cationic free radical underwent a single electron transfer (SET) process with the low-valent cerium (III) complex, consequently expediting the oxidative regeneration of the cerium (IV) catalyst and markedly boosting its catalytic efficacy.Eventually, this highly efficient catalytic system is characterized by its simplicity, mild reaction conditions, elevated selectivity, minimal waste production, and extensive applicability.Furthermore, it is effortlessly scalable and amenable to continuous production.