Photoredox copper-catalyzed regioselective syntheses of allenols via HAT-mediated propargyl radical generation†

Selectivity control of the reactions of propargylic and/or allenylic radicals remains particularly elusive. Here, a visible light-mediated Cu-catalyzed protocol for the highly selective syntheses of tri- and tetra-substituted allenols from alkynol derivatives has been developed. The synthetic potential of the products has been demonstrated. The control experiments, Stern–Volmer quenching, and cyclic voltammetry studies support a reaction pathway involving the generation of allenyl radicals via photo-mediated N–O bond scission, an intramolecular 1,5-HAT process, the resonance to propargylic radicals, and coupling. A non-common intramolecular 1,6-HAT process has also been observed.