Triazoloazine-Diazomethylazine Valence Isomerization. [1,2,3]Triazolo[1,5-a]pyridines and 2-Diazomethylpyridines

Abstract

2-Diazomethylpyridines 1D and 6D, the valence isomers of [1,2,3]triazolo[1,5-a]pyridines 1T and 6T, have been observed directly at ∼2080 cm(-1) by a combination of mild flash vacuum pyrolysis (FVP) at 200-600 °C with low temperature IR spectroscopy. Calculations confirm a ca. 17 kcal/mol barrier for the formation of 2-diazomethylpyridine 1D from [1,2,3]triazolo[1,5-a]pyridine 1T, the diazo compound lying ca. 5 kcal/mol above the triazole. In the higher temperature range (400-600 °C) 2-diazomethylpyridine 1D eliminates N2 with formation of 2-pyridylcarbene 2 and rearrangement to 1-cyanocyclopentadiene 4. 2-Diazomethylpyridine 1D undergoes 1,3-dipolar cycloaddition with tetracyanoethylene (TCNE) at 20-90 °C to yield 3-(2-pyridyl)cyclopropanetetracarbonitrile 11 and 3-(tricyanovinyl)-[1,2,3]triazolo[1,5-a]pyridine 13T via unobserved pyrazolines 10 and 12. FVP of triazole 13T affords an IR absorption at 2080 cm(-1) ascribed to the corresponding diazo compound 13D.