Diazonium salts, coupling compounds

Diazonium salts couple readily with aromatic primary amines, giving diazoamino compounds. If for instance an aqueous solution of aniline sulphate is diazotised with a deficiency of nitrous acid, only part of it is converted into benzenediazonium sulphate and the latter then couples with the unchanged aniline to give diazoaminobenzene. The reaction is carried out at the opti-CeHsNHj.HjSO + HONO = CbHsNjHSO, + zHaO... 

The reaction is very common in pyrazolone chemistry. Since alkoxypyrazoles and tautomerizable pyrazolones undergo this reaction and 3-pyrazolin-5-ones, like antipyrine, do not, it is assumed that the reaction takes place at C-4 of the OH tautomer. Pyrazolone diazo coupling is an important industrial reaction since the resulting azo derivatives are used as dyestuffs. For instance, tartrazine (Section 4.04.4.1.3) has been prepared this way. 3,5-Pyrazolidinediones react with aryldiazonium salts resulting in the introduction of a 4-arylazo group. As has been described in Section 4.04.2.1.4(v), diazonium salts couple in the 3-position with indazole to give azo compounds. 

Diazonium salts couple to hydroxy-substituted vicinal triazoles (101) with subsequent rearrangement of the hydroxy arylazo compounds (102) to the carbamoyl tetrazole (104).170 An open-chain intermediate (103) has been proposed.169 This rearrangement is similar to that of the benzoyl... 

Compared to diazonium salts, diazo compounds are generally much less reactive towards nucleophiles than towards electrophiles. As a result of this azo coupling reactions of diazo compounds are the exception rather than the rule. Electron withdrawing substituents on the diazo carbon increase the reactivity towards nucleophiles. Consequently the ability to undergo azo coupling reactions increases from diazomethane to diazocarbonyl- and 2-diazo-l, 3-dicarbonyl compounds. Among the earliest reactions known were those with cyanide and sulfite ions Tertiary phosphines, as opposed to amines, can form stable addition complexes with diazoalkanes probably due to the ability of phosphorus to stabilize the betaine with its empty d orbitals (6). 

When the diazotization is conducted in the presence of N-(l-naphthyl) ethy-lenediamine, the reactive diazonium salt couples with the amine to form an intenselycolored, reddish purple azo compound (Eq. 4.9). 

H Chemical shifts of azobenzene-like compounds,8-26 ortho-, ortho -dihydroxyazopyrazole27 and diazonium salts coupling products with guaiacol,28 5-aminopyrazole,29 2-phenyl-5-oxazolone,30 3-(4-methoxyphenyl)-1 -oxo- 1H,5H-pyrido[l,2-a]benzimidazole,31 2 - ortho, meta, paru-aminophenyl)oxazolo-... 

Table 9 gives the 13C chemical shifts of the compounds77 (3—40) listed in Table 5. Depending on the nature and orientation of the substituents in the dyes, different conformers, resultant from inner rotation around the C-N bonds, could be identified. The 13C chemical shifts of azobenzene-like compounds and diazonium salts coupling products with heterocycles are reported in Refs. 9, 18, 25, 26, 36, 39, 48, 78-87. The 13C chemical shifts of dyes derived from 3-methyl-l-phenylpyrazol-5-one (of type 78 Table 10),88 pyridone (81),39 and acetoacetanilide (82)65 existing completely in hydrazone forms were assigned.

A related process that involves diazonium salts is the Pschorr reaction, hich also couples aryl diazo-nium compounds to other aromatic rings. This diazonium salt coupling can he done under acidic conditions, but addition of copper powder usually promotes the radical process. Aryl amines generate aryl diazonium salts upon treatment with nitrous acid.l An example is the reaction of 233 to give an aryl diazonium salt, which cyclized in the presence of copper to give thaliporphine (234) in 43% yield. Kupchan called this transformation an improved Pschorr reaction. 68... 

Like other alkaloids having a free position para to a hydroxyl or methoxyl in an aromatic ring, papaverine can be sulfonated with cold sulfuric acid. The interpretation of the position of the sulfonic acid group in papaverine-6 -sulfonic acid is supported by the inability of the compound to couple with diazonium salts. The compound probably has an inner salt structure... 

First attempts to prepare poly(phenylene) date back to 1842 [78]. Riese describes a process in which poly(phenylene) ( =13) is synthesized from 1,4-dibromobenzene and sodium. Further methods are the Ullmann reaction, thermal decomposition of diazonium salts, coupling of phenylene dihalogenide - Grignard compounds. 

Section 2 of this review covers the literature dealing with NMR spectra of azo dyes measured in solution since the publication of last review in 1993 and focuses on azo compounds. Hydrazo compounds have been included either when prepared by diazonium salt coupling reactions or when they are important for azo-hydrazone tautomerism description. The C and N CP/MAS NMR spectra are discussed in Section 3. Azo-hydrazone tautomerism, a property that is indivisibly linked to azo dyes, is the topic of Section 4. NMR spectra of metal complexes of azo dyes are reviewed in Section 5. 

NH2) = —276.4 ppm), whereas in the major tautomer it is considerably higher (6( NH2) = —228.9) and closer to 6( NH—) of the imino group. 4-Dimethylaminopent-3-ene-2-one undergo a subsequent diazonium salt coupling reaction to give compound 50. ... 

Azo-compounds can be obtained by reduction of nitro-compounds, or by oxidation of hydrazo-compounds. They are usually prepared, however, by reacting a phenol or amine with a diazonium salt. The coupling usually takes place in the position para to the hydroxyl or amino group, but if this position is occupied it goes to the ortho position, e.g. 

The most important reaction of the diazonium salts is the condensation with phenols or aromatic amines to form the intensely coloured azo compounds. The phenol or amine is called the secondary component, and the process of coupling with a diazonium salt is the basis of manufacture of all the azo dyestuffs. The entering azo group goes into the p-position of the benzene ring if this is free, otherwise it takes up the o-position, e.g. diazotized aniline coupled with phenol gives benzeneazophenol. When only half a molecular proportion of nitrous acid is used in the diazotization of an aromatic amine a diazo-amino compound is formed. 

When an aqueous solution of a diazonium salt is added to an alkaline solution of a phenol, coupling occurs with formation of an azo-compound (p. 188). If ho vc cr the ntiueous solution of the diazonium salt, t. . ., />-bromohenzene diazonium chloride, is mixed with an excess of an aromatic hydrocarbon, and aqueous sodium hydroxide then added to the vigorously stirred mixture, the diazotate which is formed, e.g., BrC,H N OH, dissolves in the hydrocarbon and there undergoes decomposition with the formation of nitrogen and two free radicals. The aryl free radical then reacts with the hydrocarbon to give a... 

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... 

Attention has previously (see Diazonium Sails) been drawn to the fact that unless an excess of hydrochloric (or mineral) acid is used in the diazotisation process, coupling occurs between the diazonium salt and the amine to give diazoamino compounds. Thus phenyldiazonium chloride and aniline yield diazoaminobenzene. This substance may be conveniently prepared by dissolving two equivalents of aniline in three equivalents of hydrochloric acid, and adding one equivalent of sodium nitrite in aqueous solution followed by two equivalents of sodium acetate ... 

Diazo coupling involves the N exocyclic atom of the diazonium salt, which acts as an electrophilic center. The diazonium salts of thiazoles couple with a-naphthol (605). 2-nitroresorcinol (606), pyrocatechol (607-609), 2.6-dihydroxy 4-methyl-5-cyanopyridine (610). and other heteroaromatic compounds (404. 611) (Scheme 188). The rates of coupling between 2-diazothicizolium salts and 2-naphthol-3.6-disulfonic acid were measured spectrophotometrically and found to be slower than that of 2-diazopyridinium salts but faster than that of benzene diazonium salts (561 i. The bis-diazonium salt of bis(2-amino-4-methylthiazole) couples with /3-naphthol to give 333 (Scheme 189) (612). The products obtained from the diazo coupling are usuallv highly colored (234. 338. 339. 613-616). 

Azo coupling (Section 22 18) Formation of a compound of the type ArN=NAr by reaction of an aryl diazonium salt with an arene The arene must be strongly activated toward... 

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). 

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. 

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. 

A diazonium salt is a weak electrophile, and thus reacts only with highly electron-rich species such as amino and hydroxy compounds. Even hydroxy compounds must be ionized for reaction to occur. Consequendy, hydroxy compounds such as phenols and naphthols are coupled in an alkaline medium (pH > of phenol or naphthol typically pH 7—11), whereas aromatic amines such as N,N diaLkylamines are coupled in a slightly acid medium, typically pH 1—5. This provides optimum stabiUty for the dia2onium salt (stable in acid) without deactivating the nucleophile (protonation of the amine). 

B) Coupling Orange II.—Seventy-two grams of -naphthol (0.5 mole) is dissolved in the warm solution obtained by dissolving no g. (2.75 moles) (Note 2) of sodium hydroxide in 600 cc. of water in a s-1. flask, and the solution is cooled to about 5 by the addition of 400 g. of ice. The suspension of the diazonium salt then is added and the mixture is stirred well and allowed to stand without external cooling for one hour (Note 3). The azo compound soon separates from the red solution and eventually forms a stifli paste. 

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