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 PhN BF acts as a photosensitizer, which can undergo an electronic transition between the gold(I) moiety and PhN BF 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.

Density functional theory (DFT) calculations were performed to investigate the photosensitizer-free visible-light-mediated gold-catalyzed cis-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 cis-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 cis-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.

Density functional theory calculations were performed to understand the mechanism and selectivity for the manganese-catalyzed oxidative C(sp 3 )−H methylation reaction (Nature 2020, 580, 621−627). The calculated results show the detailed mechanisms of several key processes, including preactivation of the catalyst (S,S)-Mn II (CF 3 PDP), formation of the active oxidant species, hydroxylation of the N-heterocycle substrate, and methylation of the hydroxylated intermediate. The present study identifies Mn III −OH and Mn III −OOH as two key intermediates at the catalyst preactivation stage and a Mn III -peracetate complex and its valence tautomer Mn IV O(AcO) as the active oxidants, whose formation involves a fascinating two-state reaction mechanism. The substrate hydroxylation consists of two elementary steps: H-atom abstraction with triplet-to-quintet state intersystem crossing and barrierless OH radical rebound on the quintet surface. Methylation of the hydroxylated product is predicted to be a thermodynamically controlled process, which proceeds predominately through a stepwise mechanism: hydroxyl anion abstract followed by methyl migration. The exclusive α-site selectivity is attributed to the electronic effects (C−H position relative to the lone pair on the N atom).

In a new mechanism for photosensitizer-free visible-light-mediated gold-catalyzed cross-coupling, the π–π complex between aryldiazonium salts and arylboronic acids acts as a photoinitiator.

DFT calculations unveil that the Pd(0)-catalyzed oxidative cross-coupling of 1,2-allenyl ketones with aryl boronic acids may either occur via a Pd(II)-carbenoid intermediate or a -allyl-Pd(II) intermediate, depending on the substituents...