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

The reaction of p-chlorophenyldiazonium chloride with azide ion at -80 degrees C has been examined by H-1 and N-15 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 beta-atom.