Electrophilic aromatic diazo coupling

Diazo coupling follows the rules of orientation of substituents in aromatic systems in accordance with the mechanism of electrophilic aromatic substitution and the concept of resonance. 

The term coupling component referred originally to aromatic hydrocarbons, particularly benzene and naphthalene derivatives, which were required to have a hydroxy or an amino group as a substituent (sometimes, but not necessarily, accompanied by other substituents such as sulfonic acid groups). The presence of the hydroxy or amino group increases the C-nucleophilicity of the coupling component. In most cases this is a necessary requirement because the diazo components are relatively weak electrophiles. If the coupling component is phenol or a derivative of phenol ... 

Another classical electrophilic aromatic substitution reaction is diazo coupling, in which the effective electrophile has been shown to be the diazonium cation (cf. p. 120) ... 

The difference in position of attack on primary and secondary aromatic amines, compared with phenols, probably reflects the relative electron-density of the various positions in the former compounds exerting the controlling influence for, in contrast to a number of other aromatic electrophilic substitution reactions, diazo coupling is sensitive to relatively small differences in electron density (reflecting the rather low ability as an electrophile of PhN2 ). Similar differences in electron-density do of course occur in phenols but here control over the position of attack is exerted more by the relative strengths of the bonds formed in the two products in the two alternative coupled products derivable from amines, this latter difference is much less marked. 

Diazonium Salts as Electrophiles Diazo Coupling Arenediazonium ions act as weak electrophiles in electrophilic aromatic substitutions. The products have the structure Ar—N=N—Ar, containing the —N=N— azo linkage. For this reason, the products are called azo compounds, and the reaction is called diazo coupling. Because they are weak electrophiles, diazonium salts react only with strongly activated rings (such as derivatives of aniline and phenol). 

Diazo coupling often takes place in basic solutions because deprotonation of the phenolic —OH groups and the sulfonic acid and carboxylic acid groups helps to activate the aromatic rings toward electrophilic aromatic substitution. Many of the common azo dyes have one or more sulfonate (—SO3) or carboxylate (—COO-) groups on the molecule to promote solubility in water and to help bind the dye to the polar surfaces of common fibers such as cotton and wool. 

Diazo-l,3-dicarbonyl compounds are electrophilic enough to give azo coupling products with reactive aromatic azo coupling components as well as with CH acidic compounds such as P-diketones and p-keto-esters. 

From this point of view another electrophilic aromatic substitution, diazo coupling, is more suitable. It can be performed in dilute aqueous buffer solutions of low ionic strength the pre-equilibria for the electrophilic reagent and the nucleophilic substrates are relatively simple and have been investigated in detail (compare Zollinger (1961b) for further references). 

Another interesting contribution to the problem of the intermediate in electrophilic aromatic substitutions is Grimison and Ridd s (1958, 1959) investigation of isotope effects in diazo-coupling and iodination of imidazole. Imidazole reacts in the form of its conjugate base (4) which couples with p-diazobenzenesulphonic acid at carbon atom 2 without an... 

Cope elimination A variation of the Hofmann elimination, where a tertiary amine oxide eliminates to an alkene with a hydroxylamine serving as the leaving group, (p. 900) diazo coupling The use of a diazonium salt as an electrophile in electrophilic aromatic substitution. (p. 907)... 

Isotope effects are also useful in providing insight into other aspects of the mechanisms of individual electrophilic aromatic substitution processes. In particular, since primary isotope effects are expected only when the breakdown of the rate-determining, the observation of a substantial kn/ko points to rate-determining deprotonation. Some typical isotope effects are summarized in Table 9.7. While isotope effects are rarely observed for nitration and halogenation, Friedel-Crafts acylation, sulfonation, nitrosation, and diazo coupling provide examples in which the rate of proton abstraction can control the rate of substitution. 

Arenediazonium ions are weak electrophiles they react with highly reactive aromatic compounds—with phenols and tertiary arylamines—to yield azo compounds. This electrophilic aromatic substitution is often called a diazo coupling reaction. 

The nitrogen atoms are retained in the product. This electrophilic aromatic substitution reaction is called diazo coupling, because in the product, two aromatic rings are coupled by the azo, or —N=N—, group. Para coupling is preferred, as in eq. 11.34, but if the para position is blocked by another substituent, ortho coupling can occur. All azo compounds are colored, and many are used commercially as dyes for cloth and in (film-based) color photography ... 

Diazo coupling is an electrophilic aromatic substitution reaction in which phenols and aromatic amines react with aryldiazonium eiectrophiles to give azo compounds. 

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