Flash vacuum pyrolysis of azolylacroleins and azolylbutadienes

2-Aryl-5-acroleinyl-1,2,3,4-tetrazoles (1a-d) and 2-aryl-5-butadienyl-1,2,3,4-tetrazoles (1e-g) were subjected to flash vacuum pyrolysis. Acroleinyl derivatives resulted in nitrogen extrusion to give nitrilimines followed by ring closure to give the corresponding indazoles 3a-d in good yields. On the other hand, butadiene derivatives underwent ring fragmentation to give p-substituted anilines without formation of the expected indazoles. Differences between thermal behaviour of 2-(4-chlorophenyl)-5-acroleinyl-1,2,3,4-tetrazole (1c) and 1-(4-chlorophenyl)-4-acroleinyl-1,2,3-triazole (2) were studied in details. DFT calculations have been used to examine the nitrilimine and carbene nature of the intermediates involved in the thermal reactions of azolyl derivatives.