Thermoreversible photocyclization of a pyrazolotriazole to a triazasemibullvalene: a novel electrocyclic reaction.

The photochemistry of the pyrazolo[1,2-a]benzotriazole 1b and its dimethyl derivative 1c was studied in argon matrices at 12 K and in solution at 190 K. On irradiation at 365 nm, 1b and 1e undergo ring closure to yield the triazasemibullvalenes 2b and 2c, respectively, which were identified unambiguously by NMR and IR spectroscopy. This novel type of cyclization is reversed on warming or by irradiation at 313 nm. Quantum chemical calculations serve to model the observed IR and UV spectra and to rationalize the mechanism of the photocyclization and its thermal back-reaction.