Diazonium salts, coupling from anilines

The most noteworthy reaction of azo-compounds is their behaviour on reduction. Prolonged reduction first saturates the azo group, giving the hydrazo derivative (C NH-NH C), and then breaks the NH NH linkage, with the formation of two primary amine molecules. If method (1) has been employed to prepare the azo-compound, these two primary amines will therefore be respectively (a) the original amine from which the diazonium salt was prepared, and (6) the amino derivative of the amine or phenol with which the diazonium salt was coupled. For example, amino-azobenzene on complete reduction gives one equivalent of aniline, and one of p-phenylene diamine, NHaCeH NH benzene-azo-2-naphthoI similarly gives one equivalent of aniline and one of... 

Elimination of the hydroxyaminomethyl moiety from nitro oxime 15 by treatment with a diazonium salt gave hydrazone 43 (75LA1029) (Scheme 15). The same product was obtained by coupling the diazonium salt with the compound 16. On heating in aniline, oxime 15 was transformed into Schiff base 42. Acylation of the oxime 15 with benzoyl chloride in pyridine led to a mixture of furazan 44 and dinitrile 45. 

The diazonium salt 2 derived from 2-(3-hydroxyphenoxy)aniline (1) undergoes an intramolecular coupling reaction to yield the dibenzoxadiazepinol 3.153... 

The reactions of arenediazonium ions with 7V-alkyl- or 7V-arylhydroxylamines were investigated by Bamberger (1920b, and earlier papers). Mitsuhashi et al. (1965) showed that the l,3-diaryl-3-hydroxytriazenes are tautomeric with 1,3-diaryltriazene-3-oxides (Scheme 6-16). Oxidation of 1,3-diaryltriazenes with peroxybenzoic acid in ether yields the same product as that from diazonium salts and TV-arylhydroxyl-amine. The infrared spectrum of the product obtained by coupling diazotized relabeled aniline with A/-phenylhydroxylamine indicates that the diaryltriazene-oxide is the preponderant tautomer. 

The triazinedione, triazuril (163) is active as a poultry coccidiostat. Diazonium salt 160, prepared from the appropriate aniline, is coupled with the active methylene group of N-carbethoxycyanoacetamide to give 161. Hydrolysis of the cyano group is accompanied by cyclization, and the resulting acid (162) is decarboxylated to triazuril (163) on heating.50... 

This technique is used to diazotise anilines carrying two or more electron-withdrawing substituents, such as 2,4-dinitro- and 2-cyano-4-nitro-aniline, as well as aminoanthra-quinones and heteroaromatic amines [4]. Some diazonium salts prepared by this technique, such as those from 6-halogeno-2,4-dinitroanilines, are unstable in water and have to be added directly to the coupling component solution. 

The coupling of Naphtol AS or its phenyl-substituted derivatives with diazonium salts from variously substituted anilines in aqueous alkaline solution (section 4-11) gave incomplete reactions and impure products in some instances, probably because these coupling components have inadequate solubility in aqueous media. Pure dyes in ca. 90% yields were obtained by reaction in dimethylformamide in the presence of sodium acetate. Metallisation of these o,o -dihydroxyazo ligands with sodium chromium salicylate or a cobalt(II) salt gave metal-complex dyes in 80-100% yields [22]. Specific structural isomers of these complexes were identified by i.r., n.m.r., Raman and UV/visible spectroscopy [23]. 

A sample of the reaction mixture should not become turbid when sodium acetate solution is added. But if now a few drops of a solution of an aniline salt are poured in, there is precipitated yellow diazoaminobenzene, which can be re-dissolved with concentrated hydrochloric acid after a few small pieces of ice have been added. Further, when a few particles of j8-naphthol or of R-acid are dissolved in a small excess of 2 A-sodium hydroxide solution and a sample of the diazo-solution is added, an intense red colour is produced. The development of this colour, which results from coupling , is an infallible test for a diazonium salt, and hence also for the corresponding primary aromatic amine. 

An even more serious disadvantage of this technique is that it often impairs the biological activity of the modified protein. The activities of hen s egg lysozyme (EC 3.2.1.17) and alpha amylase from Aspergillus oryzae (EC 3.2.1.1) were lessened by diazo coupling of glycosides or aniline.12 Whether the decrease in activity was due to the modification of critical residues, or to the introduction of aromatic structures, is not yet clear however, enzymes subjected to the diazocoupling conditions in the absence of the diazonium salts retained their activity, implying that the reaction conditions themselves were not responsible for the loss of activity. 

The use of stabilized diazonium salts is a recent modification, and the exact limits of this reaction have not yet been established. It has been found to be particularly good for nitroanilines (4-nitrobiphenyl is obtained in 70% yield from p-nitroaniline) and for coupling with nitrobenzene. Coupling of the stabilized salt of aniline failed with toluene, biphenyl, naphthalene, and 1-nitronaphthalene, and the stabilized salt of /3-naphthylamine gave no /3-phenylnaphthalene when coupling with ben-... 

Another common laboratory reaction of amines is diazotization to provide unstable and highly reactive diazonium salts. Plimmer et al (14) have isolated an aromatic triazene (XV) from soil containing 3,4-dichloroaniline (XIV) and presented evidence that it is formed by "natural diazotization of the aniline followed by coupling with a second amine molecule (Fig. 5). If this is true— that the natural nitrite commonly found in soil and water can bring about diazotization—a new dimension must be added to both the natural mechanisms of herbicide degradation and the generation of new series of potentially dangerous transformation products. 

The azo compounds are important chromophores because of extended electronic delocalisation between the two aromatic rings via the azo bond. The darkness of the dye is enhanced by extensive delocalisation combined with several sulfonic acid groups which function as auxochromes. An example is provided by Naphthol Blue Black B (10), prepared from 8-amino-l-hydroxynaphthalene-3,6-disulfonic acid (H-acid) (11) by coupling it in the 7-position with diazotised p-nitroaniline in acidic solution and subsequently coupling in the 2-position with diazotised aniline in alkaline solution (Scheme 3). The H-acid (11) is a very versatile component in dye manufacture because it can couple with diazonium salts in either the 2-position or 7-position depending on the pH of the reaction medium, as indicated in Scheme 3. 

The preparation of phenols by hydrolysis of benzediazonium ions is well known in the literature (ref. 10). It involves the preparation of a diazonium salt, e.g. the diazonium hydrogenosulfate by treatment of the aniline hydrogen sulphate with sodium nitrite in dilute aqueous sulphuric acid, followed by hydrolysis in more concentrated aqueous sulphuric acid. The temperature of the hydrolysis is maintained at the boiling point of the aqueous acid by proper adjustment of the concentration of the sulphuric acid and the phenol formed removed from the reaction medium by means of steam distillation in order to minimise the coupling of the formed phenol with the diazonium salt introduced. 

If formed, the triazene can often be converted into the amino azo compound by even gentle warming with dilute acid this conversion is smoother if the triazene is warmed with the amine used for coupling and a small amount of its hydrochloride.368 The acid-catalysed rearrangement of triazenes involves intermediate reformation of the diazonium salt and the amine.369,370 So, if rearrangement of a triazene is carried out in the presence of a different, more readily coupling amine, the diazonium component reacts preferentially with the latter e.g., A,ALdimethyl-/ -(phenylazo)aniline is obtained from diazoaminobenzene and N,N-di-methylaniline.368... 

The azocompound formed in the coupling reaction between a phenol or aniline derivative and a diazonium salt, is reduced at more negative potentials than the corresponding diazonium salt. If the current is measured at potentials corresponding to the limiting current of the diazonium salt, but more positive than the reduction of the azocompound, it is possible to measure the excess of the diazonium salt from the equivalence point. The resulting titration curve possesses the form given in Fig. 47. 

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