Mechanistic Discovery of a Nucleophile–Electrophile Coupling Mechanism in Binuclear Gold(I)-Catalyzed Photoredox Alkynylation of Tertiary Aliphatic Amines with 1-Iodoalkynes

Density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations were employed to elucidate the mechanism of the α-amino-C(sp³)-H alkynylation of tertiary aliphatic amines with 1-iodoalkynes catalyzed by [Au₂(μ-dppm)₂]²⁺ under light irradiation. TD-DFT results reveal that excitation proceeds via a metal-to-ligand charge-transfer (MLCT) transition involving low-lying π* orbitals of the dppm ligands rather than a metal-to-metal transition. The excited state oxidizes the amine substrate to generate an amine radical cation and a [Au^I + Au^I]^•+ radical cation, with the latter acting as a strong reductant that transfers an electron to the C-I σ* orbital of 1-iodoalkyne, facilitating oxidative addition with an activation barrier of only 0.2 kcal/mol. The amine radical cation is deprotonated by a second amine, yielding an α-aminoalkyl radical. The resulting [Au^I + Au^II (alkynyl)(I)]^•+ complex is then reduced by the α-aminoalkyl radical to produce an α-aminoalkyl carbocation and a linear Au(I)-alkynyl fragment. The alkynyl ligand subsequently attacks this carbocation in an outer-sphere nucleophile-electrophile coupling (NEC) step to form the C-C bond and regenerate the catalyst. This newly discovered NEC mechanism introduces unprecedented intermediates and provides a foundation for designing future transformations and addressing key mechanistic questions in the binuclear gold photoredox chemistry.