Visible-Light-Promoted and Electron Donor–Acceptor Complex-Driven Double Csp2–H Bond Functionalization of 2-Arylindoles: A Strategy for the Synthesis of Benzo[a]carbazoles

A green and efficient protocol for the preparation of benzo[a]carbazoles via visible-light-promoted and electron donor-acceptor (EDA) complex-driven intermolecular cyclization of 2-arylindoles with Z-α-bromocinnamaldehydes in the absence of external photocatalysts, transition metals, and oxidants was reported. This new approach demonstrates an intermolecular cyclization model using indole derivatives as electron donors under visible light. Mechanistic investigations have showed that 2-arylindoles with Z-α-bromocinnamaldehydes form EDA complexes, which undergo sequential single-electron transfer, radical coupling, 6π-electrocyclization, and dehydroaromatization to generate benzo[a]carbazoles under visible light irradiation. The current photochemical method features readily accessible starting materials, mild conditions, simple operation, and a broad substrate scope.