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)₂phen]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 O₂ 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 O₂ or the excited-state photocatalyst.