Nitrene–Nitrene Rearrangement under Thermal, Photochemical, and Electron‐Impact Conditions: The 2‐Azidopyridines/Tetrazolo[1,5‐a]pyridines

Abstract

15N‐Labeling demonstrates that the two nitrogen atoms in the 2‐pyridylnitrene radical cation 2·+ become equivalent prior to fragmentation in the mass spectrometer. Furthermore, the mass spectra of 6‐ and 7‐tetrazolo[1,4‐a]pyridine are identical, as are those of 5‐ and 8‐tetrazolo[1,5‐a]pyridine, thereby again demonstrating interconversion of the nitrogen atoms in 2‐pyridylnitrenes. These rearrangements parallel the reactions established under thermal (flash vacuum pyrolysis) and photochemical condition. Calculations of the energies of ground and transition states at the CASPT2(7,8) level support the notion that 2‐pyridylnitrenes undergo very easy and exothermic ring expansion to 1,3‐diazacycloheptatetraene 3, both in the neutrals and the radical cations. In addition, the ring opening to 4‐cyanobutadienylnitrene 4 can take place in both the neutrals and the radical cations with modest activation barriers.