Fig. 5: Mechanistic investigations.

a The radical-clock substrate (±)-9 underwent ring opening before the C–C bond formation, thus indicating the initial generation of a benzylic radical. b The reaction of 1aa with copper acetylide did not occur in the absence of L5, indicating that both chiral ligand and terminal alkyne are indispensable for reaction initiation. c The reaction was proposed to proceed through sequential single-electron reduction of substrate 1, 1,5(6)-HAA, and copper-catalyzed C(sp3)–C(sp) coupling.