Mechanistic Insights into Photoredox/Nickel Dual-Catalyzed Difunctionalization of Alkynes: Revealing Roles of Noninnocent P-Ligand and Origins of Selectivities

Photoredox/nickel cooperative catalysis has advanced greatly to accomplish many challenging cross-coupling reactions. Nevertheless, the detailed mechanistic pathways of photoredox/nickel dual catalysis could be complicated in terms of conversions of oxidation states of the Ni complex and origins of chemo- and regioselectivities of products, which deserve further clarification. Herein, we conduct a combined computational and experimental study to gain mechanistic insights into the photoredox/Ni dual-catalyzed difunctionalization of alkynes. First, a novel route to transform the NiCl₂(PCy₃)₂ precatalyst to a Ni(0) species is proposed under photoredox conditions. The noninnocent PCy₃ ligand could play an important role in quenching the excited Ir(III) photocatalyst via a single-electron transfer (SET) step to yield a PCy₃^•+ species + Ir(II). Then, the reduced Ir(II) species could undergo a second SET with Ni(II) to afford a Ni(I) species. Meanwhile, the formed PCy₃^•+ species could combine with the hydroxide anion to yield a Cy₃P(OH)^• radical, which subsequently undergoes an H atom transfer step with the formed Ni(I) to furnish a Ni(0) species. In addition, the single-electron oxidation of Ni(II)ArCl by the excited photocatalyst to form a Ni(III)ArCl⁺ intermediate and the coordination of the alkynyl moiety of the substrate with the Ni(III) species are important factors to account for the chemoselective generation of Cl^•, instead of Ar^•, in the photoinduced ligand-to-metal charge transfer (LMCT) process. The generated Cl^• could then undergo radical attack to the alkynyl moiety of the substrate, and a radical sorting mechanism is proposed to rationalize the origin of regioselectivity of the product.