Electrophilic aromatic substitution 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). 

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). 

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) ... 

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. 

Activation by electron-releasing groups, as well as the evidence of kinetics studies, indicates that coupling is electrophilic aromatic substitution in which the diazonium ion is the attacking reagent ... 

Mechanical addressing can be also used for combinatorial postsynthetical modifications of conductive polymers. Postsynthetical modification was applied to formations of a number of different derivates of polyaniline (Fig. 13.4), the modification was performed by nucleophilic addition (Fig. 13.4), coupling with diazonium salts and by electrophilic aromatic substitution.46... 

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. 

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. 

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)... 

Diazonium salts also participate in another type of aromatic substitution, azo coupling, where the two nitrogen atoms are retained in the product. They can act as electrophiles toward activated aromatic compounds, which are generally substituted at the / electron rich—bearing OR, NRj,... 

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