Electrophilic attack of an arenediazonium ion on an electron-rich aromatic or heteroaromatic carbon constitutes the azo coupling reaction and creates the N=N (azo) linkage of an aryl–N=N–aryl (or heteroaryl) product. The diazonium electrophile is generated from a primary aromatic amine; the nucleophilic partner is typically a phenol, aniline, pyrrole, or indole. Chemoselective rapid azo-coupling can be restricted to a 5-hydroxyindole or 5-hydroxytryptophan nucleus, showing that the electrophile–nucleophile pairing remains the same when the reaction is used as a conjugation method ^[1]. In aqueous polymer chemistry, azo coupling between a diazonium-functional chain and an activated aromatic partner induces macromolecular connection through the newly formed azo bond ^[2]. Regioselective C3-azo coupling of arenediazonium compounds with tryptamines illustrates attack at the most nucleophilic heterocyclic carbon ^[3]. The transformation is an electrophilic aromatic substitution, not a transition-metal cross-coupling: no oxidative addition of a C–X bond is required, and the N=N unit originates entirely from the diazonium reagent.