Examination of the mechanism of Rh(2)(II)-catalyzed carbazole formation using intramolecular competition experiments.

The use of a rhodium(II) carboxylate catalyst enables the mild and stereoselective formation of carbazoles from biaryl azides. Intramolecular competition experiments of triaryl azides suggested the source of the selectivity. A primary intramolecular kinetic isotope effect was not observed, and correlation of the product ratios with Hammett sigma(+) values produced a plot with two intersecting lines with opposite rho values. These data suggest that electronic donation by the biaryl pi-system accelerates the formation of rhodium nitrenoid and that C-N bond formation occurs through a 4pi-electron-5-atom electrocyclization.