Synthesis of carbazoles: light-promoted tandem coupling of nitroarenes with Grignard reagents†

In this study, an efficient method for one-pot synthesis of carbazoles from readily available nitroarenes and Grignard reagents by intermolecular thermal C–N bond coupling followed by a photoinduced aza-6π electrocyclization reaction is reported. The photochemical pathway only requires irradiation of the reaction mixture with purple light (390–395 nm), without any external catalysts and additives, thus providing a novel and step-economic route for the synthesis of carbazoles. Mechanistic studies suggest that nitrosoarene and diarylamine are important intermediates in the reaction process.