Reactions of 5-diazo-4-nitroimidazole with hydrochloric acid

Abstract

Reaction of 5-diazo-4-nitroimidazole with concentrated hydrochloric acid leads to 4-chloro-5-diazoimidazole. The same product is formed together with a small quantity of 5-chloro-4-nitroimidazole Previous investigations of the acid--base properties of 5-diazo-4-nitroimidazole (Ia) showed that in 50 and 20% solutions of sulfuric acid (H0 -3.38; -1.01 respectively) this compound exists predominantly in the form of 4-nitroimidazole-5-diazonium sulfate (IIa) (X = HSO4-). At pH 0 an equilibrium between compounds (la), (Ha), and 4-nitro-5-nitrosaminoimidazole (III) was observed. At pH 1-5 diazo compound (Ia) and nitrosamine (III) were present in solution [1]. It is known that heterocyclic nitrosamines are converted in 1-10% hydrochloric acid into the corresponding halo derivatives. An increase in acid concentration leads to a reduction of their yield [2]. Decomposition of aromatic diazonium salts in the absence of catalyst by an S~I mechanism also leads to a haloarene [3].The aim of the present work was to study the reactions of compound (Ia) in concentrated, 1 N, and 0.1 N hydrochloric acid.On solution in concentrated hydrochloric acid (H0 -3.95) diazoimidazole (Ia) is converted practically instantaneously into another diazo derivative of the imidazole series giving a yellow coloration on chromatograms when treated with m-phenylenediamine. 2-Nitrobenzenediazonium chloride reacts with concentrated hydrochloric acid forming 2-chlorobenzenediazonium chloride [4]. By analogy with this we propose that replacement of the nitro group by chlorine occurs in the salt (lib) X = CI-, formed initially from imidazole (Ia), with the formation of 4-chloro-5-diazoimidazole (Ib). To confirm this structure azo couplings with B-naphthol and dimethylamine, characteristic of diazo compounds, were carried out.As a result individual products were obtained and characterized by elemental analysis, PMR and IR spectroscopy data as 4-chloro-5-(3,3-dimethyl-l-triazeno)-imidazole (IV) and 4-chloro-5-(2-hydroxy-1-naphthalene-diazo)imidazole (V). There were bands in the IR spectra of these compounds for the stretching vibrations of N=N and C C! bonds but the bands characteristic of nitro group absorption were absent. In the PMR spectra signals for the imidazole ring protons were observed, in the case of azo compound (V) a multiplet signal for the naphthalene ring protons was also present, and in the spectrum of triazene (IV) there was a singlet for the methyl group protons.Refluxing imidazole (Ia) in 1 N hydrochloric acid gave diazo compound (Ib) together with an insignificant quantity of 5-chioro-4-nitroimidazole (VI) identical (TLC, mp, IR spectrum, and PMR data) with a sample of known structure synthesized by the procedure of [5]. Refluxing of compound (la) in 0.1 N hydrochloric acid for 6 h or leaving the solution at room temperature for a month (until disappearance of the compound (Ia) from the reaction mixture according to TLC) leads mainly to chloronitroimidazole (VI). We also confirmed the presence of a small

Methods of synthesis of halogenonitroimidazoles (review)

“…Transformation of diazonium salt XXII by weak HCl into 4(5)-nitro-5(4)-chloroimidazole (XVII) was reported [42].…”