Azirine-triazole hybrids: selective synthesis of 5-(2H-azirin-2-yl)-, 5-(1H-pyrrol-2-yl)-1H-1,2,3-triazoles and 2-(5-(2H-azirin-2-yl)-1H-1,2,3-triazol-1-yl)pyridines

Azirine-triazole hybrids, 1-R-5-(3-aryl-2H-azirin-2-yl)-1H-1,2,3-triazoles, were selectively synthesized by reaction of 1-(3-aryl-2H-azirin-2-yl)-2-(triphenylphosphoranylidene)ethanones with tosyl and (E)-2-benzoylvinyl azides in high yields at rt. The reaction with 2-azidopyridine makes it possible to obtain azirine-triazole-pyridine hybrids, albeit in moderate yields, at 170 °C. The mechanism and selectivity of the reaction of α-carbonylphosphoranes with azides are discussed on the basis of DFT calculations. According to the calculation, the reaction of α-carbonylphosphoranes with model mesyl azide, leading to 1,5-disubstituted triazoles proceeds via a non-concerted cycloaddition, while the reaction leading to 1,4-disubstituted triazoles proceeds via a concerted azide cycloaddition, but through the transition state which has much higher energy. In contrast to the reaction of α-(triphenylphosphoranylidene)ketones with TsN₃, the reaction with TfN₃ yields the α-diazo ketones. Ni-Catalyzed reaction of azirinyl-1,2,3-triazoles with acetylacetone provides pyrrole-triazole and pyrrole-triazole-pyridine hybrids in good yields under mild conditions.