Mechanism, reactivity, and selectivity in Rh(III)-catalyzed phosphoryl-directed oxidative C-H activation/cyclization:...

Density functional theory calculations (DFT) have been performed on Rh(III)-catalyzed phosphoryl-directed oxidative C-H activation/cyclization to investigate the detailed mechanism, including four basic steps: C-H activation, alkyne insertion, reductive elimination, and catalyst recycling, each of which consists of different steps. Interestingly, the Rh(III)-AgOAc catalyst system was found to be more favorable in the C-H activation step in comparison with the Rh(III)-Ag2CO3 system, whereas the Rh(I)-Ag2CO3 catalyst system was more efficient for catalyst recycling. Importantly, our calculations suggest that the alkyne insertion process is a reversible step. Reductive elimination is the rate-determining step with an activation energy of 25.0 kcal/mol. In addition, the origin of the reactivity and selectivity difference between diarylacetylenes and dialkylacetylenes or electron-rich and electron-deficient diarylacetylenes was probed by means of comparative DFT calculations. The calculation results show that the electronic effects of alkynes play a key role in the reactivity and selectivity, in line with the experimental observations that diarylacetylenes and electron-rich diarylacetylenes are more reactive than dialkylacetylenes and electron-deficient diarylacetylenes, respectively. Our findings should be useful for further developments of transition-metal-catalyzed C-H activation reactions.