Formation of new 4-isocyanobut-2-enenitriles by thermal ring cleavage of 3-pyridyl azides

A new thermal ring cleavage of 3-pyridyl nitrenes for the formation of 4-isocyanobut-2-enenitrile products is reported. Thermolysis of 4-(thien-3-yl)-3-pyridyl azide 1 and 3-azido-4-(1-TIPS-1H-pyrrol-3-yl)pyridine 5 afforded two new isonitrile–nitrile products by ring cleavage; 4-isocyano-2-(thiophen-3-yl)but-2-enenitrile (3, 27%) and 4-isocyano-2-(1-TIPS-1H-pyrrol-3-yl)but-2-enenitrile (7, 20%), in addition to our previously reported pyrido[3,4-b]thienopyrrole (2, 29%) and pyrido[3,4-b]pyrrolo[3,2-d]pyrrole (6, 71%) products. Minor amounts of 2-(thien-3-yl)-1H-pyrrole-3-carbonitrile (4, 6%), formed by ring contraction, were also isolated after thermolysis of azide 1. Isonitriles 3 and 7 underwent degradation into amine 3b and formamide 7a by acidic hydrolysis. The nature and chemistry of compounds 3, 4 and 7 were investigated.