Pentazole chemistry: the mechanism of the reaction of aryldiazonium chlorides with azide ion at –80 °C: concerted versus stepwise formation of arylpentazoles, detection of a pentazene intermediate, a combined 1H and 15N NMR experimental and ab initio theoretical study

Abstract

The reaction of p-chlorophenyldiazonium chloride with azide ion at –80 °C has been examined by ¹H and ¹⁵N NMR spectra. The main product, p-azidochlorobenzene, was present in the earliest spectra. Spectra were obtained before the appearance of the second product, p-chlorophenylpentazole and an intermediate was observed. Correlated ab initio calculations at the MP2/6-31G* level on 1H-pentazole and 1-phenylpentazole agree with the NMR spectra. An (E,Z)-arylpentazene 3 is the key intermediate leading to the 1-arylpentazole products. A (Z,E)-arylpentazene 2 leads directly to the aryl azide and is not convertible to the E-isomer. Three isomeric arylpentazenes, Z,E, E,E and E,Z are formed from initial azide ion attack at the diazonium β-atom.