Visible-Light-Promoted\nand Electron Donor–Acceptor\nComplex-Driven Double Csp<sup>2</sup>–H Bond Functionalization\nof 2‑Arylindoles: A Strategy for the Synthesis of Benzo[<i>a</i>]carbazoles

A green and efficient protocol for the preparation of\nbenzo[<i>a</i>]carbazoles via visible-light-promoted and\nelectron donor–acceptor\n(EDA) complex-driven intermolecular cyclization of 2-arylindoles with <i>Z</i>-α-bromocinnamaldehydes in the absence of external\nphotocatalysts, transition metals, and oxidants was reported. This\nnew approach demonstrates an intermolecular cyclization model using\nindole derivatives as electron donors under visible light. Mechanistic\ninvestigations have showed that 2-arylindoles with <i>Z</i>-α-bromocinnamaldehydes form EDA complexes, which undergo sequential\nsingle-electron transfer, radical coupling, 6π-electrocyclization,\nand dehydroaromatization to generate benzo[<i>a</i>]carbazoles\nunder visible light irradiation. The current photochemical method\nfeatures readily accessible starting materials, mild conditions, simple\noperation, and a broad substrate scope.