Studies on Nickel-Catalyzed Photo-Reduction Reaction of Propargylic Derivatives

Nickel-catalyzed coupling reactions of propargylic compounds with nucleophiles or electrophiles have evolved into a powerful method for the preparation of propargylic compounds; however, it is still challenging for its regioselective formation of allenes. We herein report a mild nickel-catalyzed photoreduction of propargylic acetates enabling highly selective synthesis of 1,3-disubstituted and trisubstituted allenes with a broad functional group tolerance, and demonstrate its utility in preparing alkenyl boronic compounds and 1,3-dienes. Careful mechanistic studies unveiled that the protocol relies on a synergistic photoredox/nickel dual SET catalytic cycle, where N,N-diisopropylethylamine functions dually as the reductive quencher of the photocatalyst and the precursor of key α-aminoalkyl radicals, while water acts as the terminal H-atom donor. DFT calculations establish that the allene selectivity originates from a distortion-controlled Ni(II) protodemetalation. Deuterium labeling, radical trapping, and optically active substrate racemization studies fully delineate the radical cascade, proton delivery, and organonickel protodemetallation sequence.