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.