To confirm that the reaction proceeded via a free-radical mechanism, the radical scavenger 5-dimethyl-1-pyrroline (DMPO) was used to inhibit the reactions. The addition of DMPO to a reaction mixture containing 2, Cu(CF3COO)2, and DMAP, leads to a significant decrease in the isolated yield of 3a. In the presence or absence of DMPO, the yields of 3a were 17% and 85%, respectively. Similarly, on using DMPO and switching the catalytic base from DMAP to LiOH, the product 5a was not obtained, indicating that a free-radical mechanism is involved. Next, electron paramagnetic resonance (EPR) studies were performed to study the signal for free radicals. Under light irradiation, a solution of 2 did not generate any signal to indicate the presence of a free radical species. On the addition of Cu(CF3COO)2 and LiOH, and subsequent reaction progress, the EPR signal indicating the presence of free radical species significantly increased. Large amounts of free radicals, including peroxide, carbon, and trifluoroacetate radicals, were generated in a solution of 2, Cu(CF3COO)2, and LiOH (see Supplementary Information Fig. ). Under light irradiation, only a few peroxide-based free radicals were generated in a solution of 2, Cu(CF3COO)2, and DMAP which was consistent with the results of the hydrogen peroxide test (see Supplementary Information Figs. ).