Diazo compounds heterocyclic salts

Oxidative Couplings of Heterocyclic Hydrazones. This method has opened the way to the preparation of azo derivatives of diazo compounds unobtainable by other means, ie, heterocycHc compounds ia which the diazotizable amino group is conjugated with the heterocycHc nitrogen atom as ia 2- and 4-amiQopyridine, compounds which do not normally yield stable diazonium salts (38). The reaction occurs as illustrated by equation 7 for the iateraction of (A/-methylcarbostyryl)hydrazone [28219-37-6] and dimethyl aniline the overall process is oxidation. 

The aza-transfer reaction between 3-hydrazinopyridazines and aromatic diazonium salts or heterocyclic diazo compounds affords the corresponding tetrazolo[l,5-6]pyridazines, while 3-hydrazinopyridazine 1-oxide gives 3-azidopyridazine 1-oxide (76TL3193, 76X725). 

This diazotization is typical of many aminoazoles the diazonium ions formed are relatively strong acids. The pATa values of five di-, tri-, and tetrazolediazonium ions are reported to be between 3 and 4, i. e., about 10 units lower (more acidic) than those of the respective unsubstituted heterocycles (Vilarrasa et al., 1974). Therefore, deprotonation of the diazonium ion is easy and, depending on reaction conditions, yields either the diazonium salt or its conjugate base, the diazo compound. The electrophilic reactivity of the P nitrogen atom in the diazo group of the base is lower than the reactivity of the diazonio group of the cation (Diener and Zollinger, 1986 see Sec. 12.2). 

A quantitative study of the azo coupling reactions of heterocyclic diazo compounds was made by Sawaguchi et al. (1971), who measured the coupling rates of seventeen heterocyclic diazonium ions with R-acid (2-naphthol-3,6-disulfonic acid) and compared them with those of the benzenediazonium ion with the same coupling component. All the heterocyclic diazonium ions investigated react faster with R-acid than does the benzenediazonium salt. More recently, Diener and Zollinger (1986) confirmed Sawaguchi s results and interpreted them in more detail. 

If an aliphatic amino group is to a COOR, CN, CHO, COR, and so on, and has an a hydrogen, treatment with nitrous acid gives not a diazonium salt, but a diazo compound Such diazo compounds can also be prepared, often more conveniently, by treatment of the substrate with isoamyl nitrite and a small amount of acid. Certain heterocyclic amines also give diazo compounds rather than diazonium salts. ... 

The heterocyclic diazo compounds such as diazopyrroles bear the same relation to diazocyclopentadiene that pyridine does to benzene. Just as pyridine is a base and forms a pyridinium ion, in acid solution, so diazopyrroles (5) form pyrrole diazonium salts (6). 

Diazoindazole (11) was first prepared by the diazotization of 3-aminoindazole (9) followed by the treatment of the resulting diazonium salt (10) with base,1,5 This remains the most important method of synthesis for the whole class of heterocyclic diazo compounds. (See p. 4.)... 

Pyrrole and indole diazonium salts are acidic and lose a proton even in dilute acid to yield the diazo compound. Pyrazole diazonium salts, on the other hand, are only feebly acidic and the diazo compound is liberated only in quite strong alkali. In many cases the difficulty in obtaining the heterocyclic diazo compound has mainly centered round the preparation of the preceding amino compound. Nitration... 

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