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]PF 6 (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.