Benzylic C–H bond functionalization through photo-mediated mesyloxy radical formation

Abstract

Herein, we report a photo-mediated methodology for benzylic C–H bond oxygenation. Our approach employs in situ generated (methylsulfonyloxy)pyridinium mesylate salts to produce mesyloxy radicals apt for benzylic C–H bond cleavage through hydrogen atom transfer (HAT). Subsequent oxidation of the benzylic radical yields a carbocation, functionalized by the mesylate counterion through oxidative radical-polar crossover (ORPC). The reactive benzylic mesylates are converted into stable benzylic alcohols via a straightforward protocol. Reaction optimization utilized modern design of experiment (DoE) techniques for facile setups and rapid reactions. Our proposed mechanistic paradigm is supported by comprehensive investigations, including fluorescence quenching studies, cyclic voltammetry measurements, and determination of kinetic isotope effects (KIEs). Density functional theory (DFT) calculations elucidate the divergent performance of (methylsulfonyloxy)pyridinium salts and (trifluoromethylsulfonyloxy)pyridinium salts.