A DFT study on the mechanism of Rh-2(II,II)-catalyzed intramolecular amidation of carbamates

The potential-energy surfaces of the reactions of dirhodium tetracarboxylate (Rh-2(II.II)) catalyzed nitrene (NR) insertion into C-H bonds were examined by a DFT computational study. A pure Becke exchange functional (1388) rather than a hybrid exchange functional (133, BHandH) was found to be appropriate for the calculation of the energy difference between the singlet and triplet Rh-2(II.II)-NH nitrene species. Rh-2(II.II)-NR1 (R-1=(S)-2methyl-1-butylformyl) is thermodynamically more favorable with a free energy lower than that of Rh-2(II.II)-N-(PhI)R-1. The singlet and triplet states of Rh2(II,II)-NR1 have similar stability. Singlet Rh-2(II,II)-NR1 undergoes a concerted NR insertion into the C-H bond with simultaneous formation of the N-H and N-C bonds during C-H bond cleavage; triplet Rh-2(II,II)-NR1 undergoes H atom abstraction to produce a diradical, followed by subsequent bond formation by diradical recombination. The singlet pathway is favored over the triplet in the context of the free energy of activation and leads to the retention of the chirality of the C atom in the NR insertion product. The reactivities of the C-H bonds toward the nitrene-insertion reaction follow the order tertiary > secondary > primary. Relative reaction rates were calculated for the six reaction pathways examined in this work.