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

• Published in: ACS catalysis 2015-08, Vol.5 (8), p.4796-4802
• Main Authors: Choi, Sungkyu, Chatterjee, Tanmay, Choi, Won Joon, You, Youngmin, Cho, Eun Jin
• Format: Article
• Language: English
• ISSN: 2155-5435; 2155-5435
• DOI: 10.1021/acscatal.5b00817

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 PdII species, to the photoexcited Ir catalyst. This step triggers reductive elimination in a PdIII-containing palladacycle to produce the carbazole and a PdI species. The one-electron-reduced photocatalyst is reoxidized by O2 to generate the original form of the photocatalyst, and the PdI species can be oxidized to the resting state through oxidative electron transfer to O2 or the excited-state photocatalyst.