Science of Synthesis

O'Leary, P., Science of Synthesis, (2007) 31, 1391.

Arenediazonium inner salts have a negatively charged group bonded directly to the carbocyclic ring system rather than a “free” anion as, for example, the chloride ion in benzenediazonium chloride. Two types are commonly encountered: (1) those in which the arene unit bears either a sulfonate[‌124‌,‌125‌] or a carboxylate function,[‌126‌] and (2) those where a deprotonated amino or hydroxy group is located ortho or para to the diazonium function.[‌126‌] Where the anionic component is sulfonate or carboxylate the inner salt is best represented as zwitterionic species, even though other resonance structures are possible; benzenediazonium 4-sulfonate (‌10‌) (‌Scheme 39‌) provides a typical example.[‌127‌] However, when 2- or 4-hydroxy or aminoanilines are diazotized and the initial products are deprotonated (either spontaneously or by the addition of a base) the compounds formed differ from conventional diazonium salts both in color and solubility. Here, the products exhibit little ionic character; instead, they show a high degree of oxo or imino character and are best represented as covalent diazocyclohexadienones or diazocyclohexadienimines (‌Scheme 39‌). The formation of inner salts is not confined to the benzene series and some further examples are based upon naphthalene and other aromatic nuclei. Aromatic diazoniophenolates may undergo electrophilic aromatic substitution reactions such as halogenations, nitrations, and sulfonylations;[‌128‌] this topic has already been reviewed (see Houben–Weyl, Vol. 10/3, pp 1–212).

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References