Cyclisation of α-diazonitriles to 5-halo-1,2,3-triazoles: A computational study

Density functional theory [B3LYP/6-31G(d) with inclusion of solvent effects by the Onsager SCRF approximation] is used to investigate the mechanism of the hydrogen halide induced cyclisation of α-diazonitriles to 5-halo-1,2,3-triazoles. The reaction occurs stepwise. The first step, addition of the hydrogen halide onto the nitrile group to form diazo imidoyl halides, is rate determining. Barriers are calculated to be in the range 20-30 kcal mol−1. The largest ones were found for 2-diazomalononitrile and 2-trifluoroacetyl-2-diazoacetonitrile. The second step, cyclisation of these intermediates to the triazoles, has a significantly lower barrier, <15 kcal mol−1.