A comparative study of Rh2-catalyzed intermolecular nitrene transfer reactions: mechanism and chemoselectivity

Abstract

The mechanism and chemoselectivity of Rh₂-catalyzed intermolecular nitrene transfer reactions of carvone have been investigated using the M06L method. By comparing Rh₂(esp)₂vs. Rh₂(OAc)₄ catalysis, the origin of aziridination-to-amination chemoselectivity is revealed. The stepwise radical pathways are preferred for aziridination and amination by the two Rh₂ catalysts. However, the Rh₂(esp)₂ and Rh₂(OAc)₄ catalysts have a large difference in their aziridination-to-amination chemoselectivity, and this difference stems from the steric effect. Exclusive aziridines are obtained by Rh₂(OAc)₄ because the π-bond is more electron-rich and has a lower bond dissociation energy (BDE) compared to that of the σ_C–H bond. However, for Rh₂(esp)₂ catalysis, the disadvantage from the larger steric congestion compensates for the advantage from the lower BDE of the π-bond in aziridination, leading to a similar yield of aziridine and amine. The stepwise mechanisms and ligand-dependent chemoselectivities of the intermolecular nitrene transfer reactions are strikingly different from those of the intramolecular ones. This work offers valuable information for the further development of highly efficient and selective catalysts, which can be applied to intermolecular nitrene transfer reactions.