Brønsted acid-catalysed aerobic photo-oxygenation of benzylic C-H bonds

Photo-oxygenation represents an appealing strategy for the effective production of high value-added carbonyls by directly manipulating the ubiquitous C-H bonds. Nevertheless, the conventional transition-metal complexes or organic photosensitizers employed in light-driven oxygenation are often deemed cost-ineffective, thus detracting from the potential application in industrial production. Herein, we report a Brønsted acid-promoted aerobic photo-oxygenation protocol for the expedient construction of aromatic acids and ketones. The direct elaboration of bulk alkyl arenes is highly desirable, whereas avoiding the use of transition metals or photosensitizers adds another merit for this strategy. Notably, the acidic ionic liquid [BSPy][OTf] turns out to be an appropriate choice considering the viewpoints of eco-friendliness and reaction efficiency. Furthermore, singlet oxygen (¹O₂) is demonstrated to be integral to the success of this transformation based on electron paramagnetic resonance (EPR) studies and control experiments.