Gomberg reaction mechanism

Dediazoniation refers to all those reactions of diazo and diazonium compounds in which an N2 molecule is one of the products. The designation of the entering group precedes the term dediazoniation, e. g., azido-de-diazoniation for the substitution of the diazonio group by an azido group, or aryl-de-diazoniation for a Gomberg-Bachmann reaction. The IUPAC system says nothing about the mechanism of a reaction (see Sec. 1.2). For example, the first of the two dediazoniations mentioned is a heterolytic substitution, whereas the second is a homolytic substitution. 

The Pschorr reaction can take place by two different mechanisms, depending on conditions (1) attack by an aryl radical (as in the Gomberg-Bachmann reaction) or (2) attack by an aryl cation (similar to the SnI mechanism discussed on p. 853). Under certain conditions the ordinary Gomberg-Bachmann reaction can also involve attack by aryl cations. 

The thermal decomposition reaction of a diaroylperoxide, (ArCOO)2, in a liquid arene, Ar, furnishes a biaryl, Ar-Ar [101-107]. At elevated temperatures the diaroyl peroxide dissociates by the homolytic pathway to generate the arylcarboxy-radicals, ArCOO, which are prone to rapid decarboxylation forming the aryl radicals, Ar. The latter react with the arene, e.g. benzene, by the free-radical arylation mechanism, closely similar to the classical Gomberg-Bachmann-Hey reaction pathway, to give the biaryls, Ar-Ar, in moderate to good yields [101-103], Scheme 28. 

---