The Dual Role of Gold(I) Complexes in Photosensitizer-Free Visible-Light-Mediated Gold-Catalyzed 1,2-Difunctionalization of Alkynes: A DFT Study

Recently, a photosensitizer-free visible-light-mediated gold-catalyzed 1,2-difunctionalization of alkynes has been developed. However, mechanistic aspects of this unconventional photocatalytic reaction remain largely obscure. With the aid of density functional theory (DFT) and time-dependent (TD)DFT calculations, we mimicked the photosensitizer-free visible-light-mediated gold-catalyzed 1,2-difunctionalization of 1-phenyl-1-hexyne and focused on two fundamental questions: how does photoredox catalysis occur without assistance of an exogenous photosensitizer under visible light irradiation, and what is the detailed mechanism of the gold-catalyzed 1,2-difunctionalization of alkynes? The results reveal the dual role of the gold(I) complex in light-harvesting and catalysis, where a charge-transfer (CT) complex formed by the association of gold(I) catalyst with PhN2BF4 acts as a photosensitizer, which can undergo an electronic transition between the gold(I) moiety and PhN2BF4 of the CT complex into an excited electronic state and afford a charge-transfer exciplex. The oxidative quenching of the exciplex generates the gold(II) species and diazobenzene radical. The subsequent catalytic cycle proceeds via two parallel pathways, involving the radical addition to gold(II) and gold(I) centers, respectively, and in these two pathways the reductive elimination of gold(III) species is identified as the rate-determining step of the whole reaction. The present study could provide a new understanding for exogenous-photosensitizer-free visible-light-mediated gold-catalyzed processes.