Electrophilic aromatic substitution reaction diazonium coupling

Other typical electrophilic aromatic substitution reactions—nitration (second entr-y), sul-fonation (fourth entry), and Friedel-Crafts alkylation and acylation (fifth and sixth entries)—take place readily and are synthetically useful. Phenols also undergo electrophilic substitution reactions that are limited to only the most active aromatic compounds these include nitrosation (third entry) and coupling with diazonium salts (seventh entry). 

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) ... 

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. 

Diazonium coupling reactions are typical electrophilic aromatic substitutions in which the positively charged diazonium ion is the electrophile that reacts with the electron-rich, ring of a phenol or arylamine. Reaction usually occurs at the para position, although ortho reaction can take place if the para position is blocked. 

The replacement of an electrofugic atom or group at a nucleophilic carbon atom by a diazonium ion is called an azo coupling reaction. By far the most important type of such reactions is that with aromatic coupling components, which was discovered by Griess in 1861 (see Sec. 1.1). It is a typical electrophilic aromatic substitution, called an arylazo-de-hydrogenation in the systematic IUPAC nomenclature (IUPAC 1989c, see Sec. 1.2). 

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). 

A more interesting problem than the influence of substituents in the electrophilic reagent of azo coupling is the extremely high selectivity of the C-coupling reactions, relative to other electrophilic aromatic substitutions. Unsubstituted benzene does not react with any arenediazonium ion, 1,3,5-trimethoxybenzene reacts very slowly with strongly electrophilic diazonium ions only aromatic amines (e.g. N,N-dimethyl-aniline) or phenolate ions react very fast, in some cases close to diffusion control. 

Reaction with phenols and naphthols are usually carried out in the pH range 8-11, when the coupling species is the phenoxide ion. A cold, acidic solution of the diazonium salt is added to an alkaline solution of the phenol, when a fast electrophilic aromatic substitution occurs at the 4-position (Scheme 8.25). If this position is already occupied, attack occurs at the 2-position. 2-Naphthol couples at the 1-position. 

Azo coupling (Section 22.19) 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 electrophilic aromatic substitution that is, it must bear a powerful electron-releasing substituent such as —OH or —NR2. 

Instead, these heterocycles and their derivatives most commonly undergo electrophilic substitution nitration, sulfonation, halogenation. Friedel-Crafts acylation, even the Reimer-Tiemann reaction and coupling with diazonium salts. Heats of combustion indicate resonance stabilization to the extent of 22-28 kcal/ mole somewhat less than the resonance energy of benzene (36 kcal/mde), but much greater than that of most conjugateci dienes (about Tlccal/mole). On the basis of these properties, pyrrole, furan, and thiophene must be considered aromatic. Clearly, formulas I, II, and III do not adequately represent the structures of these compounds. 

Aryl diazonium salts can also undergo coupling reactions with phenol or aromatic amines, which possess nucleophilic OH or NH2 groups, respectively. This electrophilic substitution reaction (with the diazonium salt as the electrophile) produces highly coloured azo compounds. 

Initially, it was decided that the donor substituent should contain the aromatic diamine functionality necessary for polymerization (27). However, the target monomer M-5 proved impossible to prepare by classical electrophilic substitution reactions. All attempts to couple triaiylamines substituted with peripheral electron donor groups with diazonium salts failed and resulted instead in deaiylation 18). The desired monomer M-5 was finally prepared by the Mills reaction of tris-p-aminophenylamine with p-nitronitrosobenzene 12 as shown in Scheme 2. This is an exceptionally versatile synthetic procedure limited only by the ability to prepare substituted nitrosobenzene partners. 

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