Coupling reactions of diazonium salts

In coupling reactions of diazonium salts, nitrogen is retained and actually bonds to an activated aromatic ring in an electrophilic aromatic substitution reaction. This reaction is used to make azo dyes. 

Coupling reactions with diazonium salts to yield intensely colored azo derivatives have often been used for the detection of phenols, primary aromatic amines and electron-rich heterocyclics. 

The azo produets obtained have an extended conjugate system having both the aromatie rings joined through the -N=N- bond. These compormds are often coloured and are used as dyes. Benzene diazonium chloride reacts with phenol in which the phenol molecule at its para position is coupled with the diazonium salt to formp-hydroxyazobenzene. This type of reaction is known as coupling reaction. Similarly the reaction of diazonium salt with anUtne yields p-aminoazobenzene. This is an example of electrophilic substitution reaction. 

Aryldiazonium salts, usually obtained from arylamines, undergo replacement of the diazonium group with a variety of nucleophiles to provide advantageous methods for producing aryl halides, cyanides, phenols and arenes by reductive removal of the diazo group. Coupling reaction of aryldiazonium salts with phenols or arylamines give rise to the formation of azo dyes. 

Reaction of Diazonium Salts with Quinones and with Phenols. Aryl-diazonium salts couple with quinones in the presence of sodium acetate to... 

The preparation of unsymmetrical biaryls,6 the Sandmeyer reaction,6 the replacement of an aromatic diazonium group by hydrogen,7 and the reaction of diazonium salts with ,0-unsaturated carbonyl compounds 8 appear to proceed by free radical mechanisms and will not be discussed. Diazonium coupling reactions, however, have the characteristics of ionic processes. 

The existence of S /ace-N=N-aryl bonds are more difficult to establish because azo bonds are also formed during the growth of the film. A C(OH)-C-N=N-C6H5 has been observed by TOF-SIMS [126] and assigned to an electrophilic attack of the diazonium cation on an -OH-substituted aromatic ring at the surface of carbon. This is the well-known azo coupling reaction between diazonium salts and, for example, naphthol that has been the basis of the dye chemistry. It is different from the reaction of Figure 3.1 that involves a radical. 

The most important color reaction of diazonium salts is their coupling with a suitable passive component, i.e., with a phenol in alkaline medium, or with an aromatic amine in acid medium (see p. 341). Among phenols, resorcinol and 2-naphthol or R-salt are suitable passive components among amines, 1-naphthylamine or N-l-naphthylethylenediamine are suitable. 

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. 

P-coupling occurs in the formation of azophosphonic esters [ArN2PO(OCH3)2] from diazonium salts and dimethyl phosphite [HPO(OCH3)2] (Suckfull and Hau-brich, 1958). P-coupled intermediates are formed in the reaction between diazonium salts and tertiary phosphines, studied by Horner and Stohr (1953), and by Horner and Hoffmann (1956). The P-azo compound is hydrolyzed to triphenylphosphine oxide, but if a second equivalent of the tertiary phosphine is available, phenyl-hydrazine is finally obtained along with the phosphine oxide (Scheme 6-26 Horner and Hoffmann, 1958). It is likely that an aryldiazene (ArN = NH) is an intermediate in the hydrolysis step of the P-azo compounds. 

In the first paper on arenediazonium salt/crown ether complexes, Gokel and Cram (1973) mention that they were not able to synthesize the rotaxane 11.14 by an azo coupling reaction of the complexed diazonium ion with Af,Af-dimethylaniline. 

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. 

More recently, Bagal and coworkers (Luchkevich et al., 1991) obtained similar results in a kinetic investigation of the coupling reactions of some substituted benzenediazonium ions with 1,4-naphtholsulfonic acid, and with 1,3,6-, 2,6,8-, and 2,3,6-naphtholdisulfonic acids. The kinetic results are consistent with the transient formation of an intermediate associative product. The maximum concentration of this product reaches up to 94% of the diazonium salt used in the case of the reaction of the 4-nitrobenzenediazonium ion with 1,4-naphtholsulfonic acid (pH 2-4, exact value not given). The authors assume that this intermediate is present in a side equilibrium, i. e., the mechanism of Scheme 12-77 mentioned above rather than that of Scheme 12-76, and that the intermediate is the O-azo ether. 

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