To investigate whether the radical intermediates were involved in the current transformation, the reaction was conducted in the presence of various radical scavengers (Figure ). Notably, 2,6-tButoluene (BHT) and 1,1-diphenylethylene had no inhibitory effect. In contrast, 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) slowed down the reaction, but still yielded 3da in 74% yield. Only TEMPO exhibited distinct inhibition of catalysis, but in this case, TEMPO may oxidize Cu(I), rather than trapping with a radical intermediates. In the pursuit of direct evidence regarding the presence or absence of radical intermediates from N-carboxyindole 1 and 2-naphthol 2 in the catalytic cycle, substrates bearing radical clocks were designed (Figure ). The introduction of a vinyl group in the N-carboxy leaving group (10) resulted in the formation of product 3da (73%, 83:17 er) and 2-vinylbenzoic acid (11, 98%) as a side product, with no observation of 12. This ruled out the intermediacy of a benzoyloxy radical. In the reaction of 13 bearing a cyclopropyl group at C2 of indole, no ring opened product 15 was observed, supporting the absence of indole-derived radical involvement. Interestingly, incorporation of a 4-pentenyl group in 2-naphthol resulted in a dearomatized indolylation product 17 (44%, 88:12 er), with no 5-exo-trig cyclization product 18 observed. In addition, the reaction of cyclopropyl-2-naphthol 19 gave 20, without the ring-opened byproduct 21. These experiments helped clary the absence of C1 radicals from 2naphthols in the catalytic cycle.