Examination of the Mechanism of Rh2(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 σ+ values produced a plot with two intersecting lines with opposite ρ values. These data suggest that electronic donation by the biaryl π-system accelerates the formation of rhodium nitrenoid and that C−N bond formation occurs through a 4π-electron-5-atom electrocyclization.