The mechanism of cyclization of 1,4-diaryl-1-azido-2,3-diazabuta-1,3-dienes to tetrazoles. Imidoyl azides stabilized by slow nitrogen inversion

Abstract

The imidoyl azides (9) are stabilized in the open chain azido form because of their configuration (which is Z about the CN bond) and slow ZE isomerisation. The rates of conversion of the azides (9) into the isomeric tetrazoles (12) have been measured and are characterized by a low sensitivity to solvent, a larger substituent effect for the remote substituent Y than for the adjacent X (ρ_Y+ 1.45, ρ_X–0.28 in toluene at 70°), ΔS^‡–6 cal mol^–1, K^–1, and insensitivity to catalysts. All these data point to nitrogen inversion, rather than the azide cyclization step itself, as rate determining. The E-azide was not detected and presumably cyclizes to (12) rapidly. The general applicability of these results to other open-chain azides is discussed.