Brønsted acid-mediated annulation of carbazoles with diaryl-1,2-diones: access to furo[3,2-c]carbazoles.

A Brønsted acid-mediated annulation of carbazoles with diaryl-1,2-diones has been developed for the synthesis of furo[3,2-]carbazoles, a previously unexplored fused carbazole framework. The metal-free transformation proceeds through sequential C-C and C-O bond formation and affording the desired products in good to excellent yields. Both electron-donating and electron-withdrawing substituents on the diaryl-1,2-diones were well tolerated, and the reaction was also applicable to -substituted as well as -carbazoles. The synthetic utility of the resulting furo[3,2-]carbazoles was demonstrated through bromination, nitration, and formylation reactions. The synthesized compounds exhibited blue fluorescence with quantum yields of up to 77%, while photophysical, electrochemical, thermal, and computational studies provided insight into their electronic structures and excited-state properties.