DFT Study on Photosensitizer-Free Visible-Light-Mediated Au-Catalyzed -Difunctionalization of Alkynes: Mechanism and Selectivities as Compared to Rh Catalysis.

Density functional theory (DFT) calculations were performed to investigate the photosensitizer-free visible-light-mediated gold-catalyzed -difunctionalization of alkynes with aryl diazonium salts. The detailed reaction mechanism is established, and the observed regio- and chemoselectivities are rationalized. The results are compared to those of the rhodium-catalyzed -difunctionalization of alkynes. It is indicated that the excitation of the aryl diazonium salt initiates the photocatalytic cycle, and the following single-electron transfer between the Au(I) catalyst and the excited aryl diazonium salt affords the key aryl radical. Both gold- and rhodium-catalyzed reactions involve two major steps: alkyne insertion into the M-N or M-C bond (M = Au, Rh), and C-C or C-N reductive elimination from the M(III) center. The -difunctionalized product can be obtained by the trimethylsilyl (TMS)-substituted alkyne through the gold catalysis or by the Ph-substituted alkyne through the rhodium catalysis. The catalyst-dependent reactivity switch of TMS- and Ph-substituted alkynes is attributed to the catalyst-induced shift of the rate-determining step.