Azo coupling

In organic chemistry, an azo coupling is an organic reaction between a diazonium compound (R–N≡N+
) and another aromatic compound that produces an azo compound (R–N=N–R’). In this electrophilic aromatic substitution reaction, the aryldiazonium cation is the electrophile and the activated carbon (usually from an arene which is called coupling agent) act as a nucleophile.^[1] In most cases, including the examples below, the diazonium compound is also aromatic.

Diazotization

The process of conversion of primary aromatic amines into its diazonium salt is called diazotization. Diazonium salts are important synthetic intermediates that can undergo coupling reactions to form azo dyes and electrophilic substitution reactions to introduce functional groups.

Uses of the reaction

Aromatic azo compounds tend to be brightly colored due to the extended conjugated systems. Many are used as dyes (see azo dye).^[2] Important azo dyes include methyl red and pigment red 170. Azo printing exploits this reaction as well. In this case, the diazonium ion is degraded by light, leaving a latent image in undegraded diazonium salt which is made to react with a phenol, producing a colored image: the blueprint.^[3]

Prontosil, a first sulfa drug, was once produced by azo coupling. The azo compound is a prodrug that is activated in-vivo to produce the sufanilamide, which is active.

The reaction is also used in the Pauly reaction test to detect tyrosine or histidine residues in proteins.

Examples of azo C-coupling reactions

Many procedures have been described, which re.^[4][5] Phenol reacts with benzenediazonium chloride to give a Solvent Yellow 7, a yellow-orange azo compound. The reaction is faster at high pH.^[2]

The related dye called aniline yellow is produced from the reaction of aniline and the diazonium salt. In this case the C- and N-coupling compete.^[2] When the para position to the orientating group (for aromatic electrophilic substitution) is substituted, coupling occurs at the ortho position, albeit at a slower rate.

Naphthols are popular coupling agents. One example is the synthesis of the dye "organol brown" from aniline and 1-naphthol:

Similarly β-naphthol couples with phenyldiazonium electrophile to produce an intense orange-red dye.

Besides activated carbons in aromatic rings, other nucleophilic carbons could also be coupled with diazonium salt:

Synthesis of C.I. Pigment Yellow 12, a diarylide pigment.

Examples of azo N-coupling reactions

Azo N-coupling

In alkaline media, diazonium salt can react with most primary and secondary amines that exist as a free base and produce triazene.^[6] This chemical reaction is called azo N-coupling^[7] or the synthesis of azoamines.^[8]

References

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