Synthesis of Carbazoles by a Merged Visible Light Photoredox and Palladium-Catalyzed Process

Carbazoles have attracted great interest in recent years for a variety of applications in organic and medicinal chemistry as well as in materials science. In this work, an efficient method for the synthesis of carbazoles through the intramolecular C-H bond amination of N-substituted 2-amidobiaryls has been developed. Under visible light and an aerobic atmosphere, the transformation requires only catalytic amounts of Pd(OAc)(2) and [Ir(dFppy)(2)phen]PF6 (dFppy = 2-(2,4-difluorophenyl)pyridine; phen = 1,10-phenanthroline), the latter of which is utilized in synthetic chemistry for the first time. Spectroscopic and electrochemical studies revealed that the reaction is initiated by photoinduced electron transfer from a palladacyclic intermediate, formed from the 2-amidobiaryl and Pd-II species, to the photoexcited Ir catalyst. This step triggers reductive elimination in a Pd-III-containing palladacycle to produce the carbazole and a Pd-I species. The one-electron-reduced photocatalyst is reoxidized by O-2 to generate the original form of the photocatalyst, and the Pd-I species can be oxidized to the resting state through oxidative electron transfer to O-2 or the excited-state photocatalyst.