2-Pyridylnitrene−1,3-Diazacyclohepta-1,2,4,6-tetraene Rearrangements in the Trifluoromethyl-2-pyridyl Azide Series^1a

Photolysis of Ar matrix isolated trifluoromethyl-substituted 2-pyridyl azides/tetrazolo[1,5-a]pyridines at 12−18 K causes rapid and mostly clean conversion to the corresponding 1,3-diazacyclohepta-1,2,4,6-tetraenes (4D, 5D, 5,6D, and 4,6D) absorbing near 2000 cm^-1 in the IR. In the latter case, the intermediate 3,5-bis(trifluoromethyl)-2-pyridylnitrene (4,6N) was observed by both ESR and IR spectroscopy and converted to the diazacycloheptatetraene 4,6D in the course of 90 min of UV irradiation. The 2-pyridylnitrenes were generally observable by ESR spectroscopy (|D/hc |∼1.05−1.10; |E/hc|∼0.0 cm^-1) following both photochemical and thermal (FVP) generation from the 2-azidopyridines. Irradiation of the Ar matrix isolated mixtures of nitrenes and diazacycloheptatetraenes also caused development of weak carbene transitions (|D/hc|∼0.40−0.45; |E/hc| ∼0.006 cm^-1) in the ESR spectra.