Synthesis of Fluorescent Indazoles by Palladium-Catalyzed Benzannulation of Pyrazoles with Alkynes

The synthesis of indazoles from pyrazoles and internal alkynes is described. Instead of complex benzenoid compounds, readily available pyrazoles were used for the preparation of indazoles by reaction of the C-H bonds of the heterocyclic ring. Oxidative benzannulation was also applied to imidazoles, providing benzimidazoles. This convergent strategy enabled alteration of the photochemical properties of benzo-fused diazoles by varying the substituents at the benzene ring, thus leading to the development of tetraarylindazoles as new fluorophores.