Diazonium salts tautomerization

The tautomeric character of the pyrazolones is also illustrated by the mixture of products isolated after certain reactions. Thus alkylation normally takes place at C, but on occasion it is accompanied by alkylation on O and N. Similar problems can arise during acylation and carbamoylation reactions, which also favor C. Pyrazolones react with aldehydes and ketones at to form a carbon—carbon double bond, eg (41). Coupling takes place when pyrazolones react with diazonium salts to produce azo compounds, eg (42). 

Synthesis. Almost without exception, azo dyes ate made by diazotization of a primary aromatic amine followed by coupling of the resultant diazonium salt with an electron-rich nucleophile. The diazotization reaction is carried out by treating the primary aromatic amine with nitrous acid, normally generated in situ with hydrochloric acid and sodium nitrite. The nitrous acid nitrosates the amine to generate the N-nitroso compound, which tautomerizes to the diazo hydroxide. 

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. 

Whilst azo compounds prepared from diazonium salts and phenolic or keto-enol coupling components are often depicted in the hydroxyazo form (4.11), an alternative tautomeric structure can be drawn for such compounds (Scheme 4.19). This ketohydrazone tautomer (4.21) can, in cases where the azo and hydroxy groups are located on adjacent carbon atoms, exhibit hydrogen bonding between the two groups as shown. Similar pairs of structures, but without hydrogen bonding, can be drawn for p-hydroxyazo compounds. 

Comparison of the ultraviolet absorption and ionization constants of aminopyrazine, methylaminopyrazine, and dimethylaminopyra-zine indicates that aminopyrazine exists as such and not in the tautomeric imino form.147 Aminopyrazines, like their pyridine analogs, form diazonium salts, which readily decompose to the corresponding pyrazinones. For example, nitrous acid treatment of aminopyrazine318 and 2-aminopyrazine-5-carboxylic acid210 gives the corresponding pyrazinones in 30 and 59% yield, respectively. The diazonium salt from aminopyrazine cannot be converted into bromo-pyrazine under the conditions of the Sandmeyer reaction. 

As already mentioned, hydroxypyrazines exist in tautomeric equilibria with the corresponding pyrazinones which are normally the predominant species in the equilibria. Some of the reactions of hydroxypyrazines are reminiscent of those of phenols they can, for example, be coupled with diazonium salts and brominated and nitrated in either the ortho or para position to the hydroxyl group. Coupling with diazonium salts occurs in neutral or weakly alkaline solution, but if the reaction is carried out in 1 M sodium hydroxide solution, arylation of the pyrazine ring takes place. From hydroxy-pyrazine and benzenediazonium chloride 47% 2-hydroxy-3-phenyl-and 4% 2-hydroxy-3,6-diphenylpyrazine are obtained. 

Biaryls are produced in low yields by several related coupling reactions. When benzenediazonium chloride is treated with aqueous sodium hydroxide in the presence of benzene, nitrogen is eliminated and biphenyl is formed in 22% yield. Better results are sometimes obtained when the sodium hydroxide is replaced by sodium acetate or when stabilized diazonium salts are employed. In the former modification the aryl-diazoacetate, ArN = NOCOCHj, is an intermediate product. The tautomeric forms of the aryldiazoacetates are N-nitrosoamides, ArN(N0X30CH, which are obtained by the action of nitrous fumes or nitrosyl chloride on... 

The dyes formed when imidazoles couple with diazonium salts (Section 4.07.1.4.8) in alkaline medium could exist as the usually accepted azo form (95 R = H) or the hydrazone form (96). Comparison of the UV and visible spectra of the dyes with a non-tautomeric model (95 R = Me) demonstrated unequivocally that if such an equilibrium as shown in Scheme 32 exists, then the azo structure predominates (79BAP249). 

In reactions of the imidazole neutral molecule or cation, attack by electrophiles at C-5 is preferred. However, when there is a free NH in the ring, tautomerism makes the 4- and 5-positions equivalent. Conditions in which the imidazole anion exists are reported to be characterized by electrophilic substitution at C-2. There would appear to be exceptions to this generalization, e.g. halogenation (Section 4.07.1.4.5). When the preferred positions are blocked it is usually possible to induce electrophiles to enter alternative ring positions and multiple substitution is commonly possible. In fact it is difficult to prepare mono-bromoimidazole, and imidazole can be dinitrated. More than one benzeneazo group can couple when imidazole reacts with diazonium salts. 

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