Computational Investigation of the Role Played by Rhodium(V) in the Rhodium(III)-Catalyzed ortho-Bromination of...

In this study, M11-L was used to evaluate the feasibility of the formation of rhodium(V) species using the rhodium(III)-catalyzed ortho-bromination of arenes as a model reaction. In most cases for these types of reactions, DFT calculations reveal that the bromination step involves a Br transfer from N-bromosuccinimide to the reacting arylrhodium to form a bromonium intermediate, followed by a Br shift to generate a new C-Br bond, which is more favorable than the previously proposed Rh^III /Rh^V catalytic cycle. The rhodium catalyst remains in its +3 oxidation state throughout. The substituent effects of the reacting arene were studied, and computational results showed that the introduction of electron-donating groups on the reacting arene was favorable for this pathway. In contrast, the inclusion of a strong electron-withdrawing group on the aromatic ring would hinder the formation of a bromonium intermediate. Therefore, the Rh^III /Rh^V catalytic cycle is favorable in cases that involve a Rh^V intermediate, which is generated by oxidative addition with NBS. In this pathway, the C-Br bond is formed by reductive elimination from the Rh^V intermediate. Additionally, a distortion-interaction analysis model along the reaction pathway was used to explain the directing-group effects. The results showed that the interaction energy controlled the reactivity because of the difference in electronic nature of various directing groups.