Aryl Nitrile Imines and Diazo Compounds. Formation of Indazole, Pyridine N-Imine, and 2-Pyridyldiazomethane from Tetrazoles

Both photolysis and flash vacuum pyrolysis (FVP) of tetrazoles (1/5) are known to generate nitrile imines (13, 19, and 38), which rearrange to 1H-diazirines, imidoylnitrenes, and carbodiimides. Moreover, FVP of 5-aryltetrazoles is a convenient source of aryldiazo compounds (30/47) and arylcarbenes, including pyridylcarbenes. The factors that determine which path is followed are poorly understood. Calculations at the density functional theory and CASPT2 levels now examine cyclization of N-phenylnitrile imine 13 to indazole 17. A corresponding cyclization of C-phenylnitrile imine 19 can also lead to indazole, but this reaction, which passes through a carbenic nitrile imine, requires a much higher activation energy and is therefore not competitive with the known rearrangements to phenyldiazirines, ring expansion to diazenylcycloheptatetraene, or a new, potential rearrangement to cyanoazepine. C-(2-Pyridyl)nitrile imine 38 is predicted to undergo a new rearrangement to cyanopyridine N-imide 40 with an activation energy of 43 kcal/mol. The experimental observation that 2-pyridyldiazomethane 47 is actually formed requires a reaction with an energy barrier below 43 kcal/mol. This is found in the H-transfer from the tetrazole ring in 5-(2-pyridyl)tetrazole to the pyridine ring with a subsequent formation of 1H-2-(diazomethylene)pyridine and elimination of N2 with a barrier of ca. 26 kcal/mol. This new, facile mechanism has not previously been considered.