Mechanism studies. Although the mechanism of copper-catalyzed alkene oxidation reactions like the Kharasch reaction has been proposed, some key issues have not been clarified. In particular, the Cu III intermediates frequently mentioned in the mechanism have not been experimentally confirmed . To elucidate the mechanism of the oxidative coupling of C(sp 3 )-H bonds with carboxylic acids, we conducted radical trapping experiments using 2,2,6,6-tetramethyl-1-oxylpiperidine (TEMPO). The inhibition of the reaction by TEMPO indicated that radicals are involved in the reaction (Fig. ). The kinetic isotope effect experiments showed that the kH/kD ratios of the three reactions are 3.1 and 3.3, respectively (Fig. ), suggesting that the hydrogen atom abstraction from the C(sp 3 )-H bond may be a rate-limiting step. We prepared the complexes Cu II -O t Bu and Cu II -OBz, called Int-I and Int-II respectively, and used them to catalyze the reaction to obtain the desired product (Fig. ), which indicates that the reaction went through the intermediates Int-I and Int-II. A series of electron paramagnetic resonance (EPR) measurements were performed to identify the various free radicals in the reaction. The EPR spectra of intermediates Int-I, Int-II, and catalyst prepared in-situ showed the presence of Cu II (Fig. ). We are excited to have detected Cu III intermediate in the reaction solution by low-temperature EPR analysis (Fig. ), which is the first experimental confirmation of the presence of Cu III in a Cu-catalyzed reaction. We also verified by EPR analysis that the tert-butoxy radicals were mainly generated by the photodecomposition of DTBP (Fig. ). Finally, the light on/off experiments showed that ceasing irradiation halted the reaction (Fig. ), indicating that the reaction does not involve the radical chain mechanism. Finally, radical addition of another t BuO radical to Int-IV facilitates its dissociation to afford product 3a while recycling the catalyst to Int-1. Besides, the generation of 1a radical requires an energy barrier of 9.8 kcal/mol, which is the rate-determining step in the proposed mechanism and is consistent with kinetic isotope effect experiments.