Rhodium(III)-Catalyzed Indazole Synthesis by C-H Bond Functionalization and Cyclative Capture

An efficient, one-step, and highly functional group-compatible synthesis of substituted N-aryl-2H-indazoles is reported via the rhodium(III)-catalyzed C-H bond addition of azobenzenes to aldehydes. The regioselective coupling of unsymmetrical azobenzenes was further demonstrated and led to the development of a new removable aryl group that allows for the preparation of indazoles without N-substitution. The 2-aryl-2H-indazole products also represent a new class of readily prepared fluorophores for which initial spectroscopic characterization has been performed.