Chemo- and Regioselective Nickel-Catalyzed Reductive 1,4-Alkylarylation of 1,3-Enynes through an L2NiAr Intermediate

Three-component reductive dicarbofunctionalization reactions of 1,3-enynes have been rarely reported because of the intricate control of chemo- and regioselectivity required, coupled with a limited understanding of radical and catalytic species involved. Herein, we report a nickel-catalyzed reductive 1,4-alkylarylation method for 1,3-enynes to yield tri- and tetrasubstituted allenes using readily accessible alkyl and aryl iodides, featuring a simple operational protocol and mild reaction conditions. In our mechanistic studies, the formation of a propargyl/allenyl radical was substantiated by the isolation of a propargyl dimer, the detection of the corresponding TEMPO–radical adduct, and radical probe experiments. Two reduced L2NiAr complexes, expected to act as catalytic intermediates, were generated and characterized by EPR spectroscopy as NiI complexes. The stoichiometric reaction of L2Ni(p-NCC6H4) with 1,3-enyne and alkyl iodide showed conversion into the corresponding propargyl dimer and allene, suggesting that the reaction encompasses the same key mechanistic steps as the catalytic reaction, i.e., activation of alkyl iodide, generation of a propargyl/allenyl radical, and selective coupling of this radical with the aryl component.