The reaction of C3-deuterated 1d-d (97%D) under standard conditions provided 3da with no D-atom incorporation (Eq. 2). Additionally, there was no difference in the rate of formation of 3da when 1d-d was exchanged for 1d (refer to the graph below Eqs. 1-3). This observation suggested that aromatization of the indole moiety is fast. In sharp contrast, when 2a-d (97%D) replaced 2a (Eq. 3), the rate of the formation of 3da slowed down substantially (green square), but the rate of disappearance of 1d remained the same. In fact, in the reaction with 2a-d, we observed a reaction intermediate (Int, purple triangle) in the crude 1 H NMR spectrum (Figure ). This species had a broad singlet at 8.32 ppm (N-H of indole) and two doublets (J = 9.9 Hz) at 6.48 and 7.68 ppm (corresponding to α-and β-protons, ), which is also in accord with the radical clock experiments (Figure ). Alternatively, a closed-shell pathway with a transition We have described an enantioselective coupling of N-carboxyindoles with naphthols for the synthesis of diverse indolyl-naphthol biaryls. The current Cu(I)-bisphosphine-catalyzed reaction was proposed to have a closed-shell mechanism, rather than a ligated naphthyl radical. Although the reaction was proposed to proceed through a mono-dentate naphthol-Cu(I) complex, the formation of the indolyl-naphthol was remarkably enantioselective and overcomes previous limitations in the scope and functional groups in naphthols. Current effort in our laboratory is directed toward the utilization of this novel mechanism for ddevelopment of new Cu(I)-catalyzed reactions.