Which Radical Abstracts Hydrogen Atom? Alkylperoxy versus Alkoxy in Cu-Catalyzed Benzylic C-H Oxidation

Selective alkane oxidation remains a central objective in industrial chemistry, with hydrogen-atom transfer (HAT) widely recognized as the key elementary step. In copper-catalyzed C(sp³)-H oxygenation using hydroperoxides, however, multiple oxygen-centered radicals could be generated, bearing uncertainty of the species responsible for HAT. Here, we employ an external-circulation online multispectroscopic system (ECOMS) to elucidate the radical mechanism of benzylic C(sp³)-H oxidation using cumene hydroperoxide as the oxidant. This operando approach enables direct and quantitative tracking of the time evolution of the cumylperoxyl radical (CumylOO^•) throughout the reaction, unambiguously identifying it as the operative HAT agent. Further theoretical studies clarify the mechanistic origin of the dominance of alkylperoxy over alkoxy radical. Moreover, increasing counteranion basicity promotes Cu(II)-OOR formation, resulting in elevated CumylOO^• steady-state concentrations and faster oxidation rates.