Synthesis of 2‑(Di/tri/tetraazolyl)‑2H‑azirine-2-carboxylates by Halogen Substitution: Evidence for an SN2′‑SN2′ Cascade Mechanism

An effective and operationally simple method for the preparation of methyl 2-(di/tri/tetraazol-1-yl)-2H-azirine-2-carboxylates from accessible methyl 2-halo-2H-azirine-2-carboxylates and NH-azoles has been developed. The azoles having enhanced NH acidity, react with 2-halo-2H-azirines in the presence of Et3N in a deprotonated form, while the azoles having highly nucleophilic sp2 nitrogen react in neutral form. According to the DFT calculations, the substitution of the halogen by the azole proceeds via an SN2′-SN2′ cascade, with the initial conjugate substitution of the halogen being the rate-determining step. The formation of 2,3-dipyrazolylaziridines in the reaction of pyrazole provides experimental evidence for the cascade mechanism of the reaction.