Direct arylation-based synthesis of carbazoles using an efficient palladium nanocatalyst under microwave irradiation

9H-carbazole is a relevant structural scaffold present in a large number of bioactive natural products and drugs with a wide spectrum of therapeutic action. Additionally, 9H-carbazoles are used as building blocks for organic materials, such as polymeric light-emitting diodes (PLED) and organic light-emitting devices (OLED). Therefore, the development of efficient protocols for the generation of structurally diverse carbazoles represents an important synthetic objective in the field of organic synthesis. Many methods have been developed for the preparation of carbazoles. The most widely used approaches to the synthesis of the carbazole nucleus require an intramolecular cyclization forming C-N or C-C bonds from advanced precursors such as substituted biaryls or N,N-diarylamines.

Herein, an eco-friendly palladium-catalyzed tandem reaction for the one-pot synthesis of carbazoles under microwave irradiation is reported. This approach involves an amination and a direct arylation from available and inexpensive anilines and 1,2-dihaloarenes. For the development of this purpose, a novel recoverable palladium nanocatalyst supported on a green biochar under ligand-free conditions is used. Compared to other existing palladium-based protocols, the present synthetic methodology shows a drastic reduction in reaction times and excellent compatibility with different functional groups allowing to obtain a small library of carbazoles with high yields and regioselectivity. The novel heterogeneous palladium nanocatalyst can be recycled and reused up to four 5 times without significant loss activity.