The Gomberg-Bachmann-Hey reaction

The arylation of aromatic compounds with aryldiazonium salts (III) in the presence of base affording biaiyls (II) is called the Gomberg-Bachmann-Hey (GBH) reaction [46]. First observations were published by Bamberger [47] and Kuhling [48] more than a century ago. However, the first practical synthesis of biaryls was discovered by the former authors, and was further developed by Hey. The original 

When an arenediazonium salt solution was added to a mixture of the liquid arene and sodium hydroxide solution, the excess of base forms the corresponding sodium diazotate, Ar-N=N-0 Na, also an alternative intermediate, which is easily 

Alternative substrates which are capable of generating the aryl radicals are A-nitroso acetanilides [49,62-65], aryltriazenes [66,67], arylazo-triphenylmethanes [68], and arylhydrazines under oxidative conditions. The nitroso-acetanilides (V), prepared by 

the GBH reactions are performed by using aryldiazonium tetrafluoroborate salts. An aromatic amine is diazotized at about -10 to 0 C in the presence of at least 3 equivalents of concentrated hydrochloric acid by adding the solution of sodium nitrite [61]. To the aryldiazonium chloride, 50% aqueous solution of tetrafluoroboric acid is added with additional cooling to precipitate the corresponding aryldiazonium 

As all free-radical arylation reactions, the ptGBH reaction produces a mixture of all three isomers if the arene is unsymmetrical [61,76,79]. The isomer distribution can be seen from the following examples of monosubstituted benzene arylation. Table 3. A number of instances have shown that isomer distribution is indifferent to the catalyst used, this strongly suggests that an aryl radical as an actual arylating agent is formed under either phase transfer or classical GBH conditions. 

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Unsymmetrically substituted biphenyls may be prepared from diazonium salts by means of the Gomberg-Bachmann-Hey reaction.2 Some preparative examples of the classical method and of a PTC procedure are described in Expt 6.79.3... 

Of preparative interest is the improvement effected in the Gomberg-Bachmann-Hey arylation of thiophene by conducting the reaction in a non-aqueous solvent with 18-crown-6 as catalyst (80T3327). 

The reaction (the Gomberg reaction or Gomberg-Bachmann-Hey reaction) probably involves the intermediate formation of the diazohydroxide, which... 

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. 

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