Azo coupling with aromatic components

Much the most important application of azo coupling is that to aromatic or heterocyclic components, above all in relation to production of dyes.355 It has, however, also preparative interest for the synthesis of primary amines, for these can be obtained by reductive cleavage of azo compounds (see page 568). 

Phenols. In the benzene series the azo group enters almost wholly at the /rara-position. The small amount of ortho-product can sometimes be removed by distillation in steam.356 When the /wra-position is already occupied, coupling occurs at the arf/w-position, e.g., in /7-chlorophenol357 or/ -cresol 358 when the orf/w-positions are also occupied, coupling either does not occur or, as with also /7-hydroxybenzoic acid,359 occurs with removal of the blocking substituent in addition, O-azo coupling may then result, as with 2,4,6-tri- 

If formed, the triazene can often be converted into the amino azo compound by even gentle warming with dilute acid this conversion is smoother if the triazene is warmed with the amine used for coupling and a small amount of its hydrochloride.368 The acid-catalysed rearrangement of triazenes involves intermediate reformation of the diazonium salt and the amine.369,370 So, if rearrangement of a triazene is carried out in the presence of a different, more readily coupling amine, the diazonium component reacts preferentially with the latter e.g., A,ALdimethyl-/ -(phenylazo)aniline is obtained from diazoaminobenzene and N,N-di-methylaniline.368 

Triazene formation can be avoided ab initio by using the JV-(sulfomethyl) derivative instead of the free amine and subsequently hydrolysing off the sulfomethyl group 372 these derivatives are easily obtained from the amine, formaldehyde, and sodium hydrogen sulfite 371 

Acylation of an amine generally hinders coupling, although some exceptions to this are known, e.g., A-acetyl- and A-benzoyl-1 -naphthylamine,373 N- p-toluenesulfonyl)-1 -and -2-naphthylamine.374 

---