Aromatic compounds from aryl diazonium salts

Gomberg-Bachmann reaction. Formation of diaryl compounds from aryl diazonium salts and aromatic compounds in the presence of alkali. 

Unsymmetrical diaryls may be prepared by treating an aryl diazonium salt solution with sodium hydroxide or sodium acetate in the presence of a liquid aromatic compound. Thus 2-chlorodiphenyl is readily formed from o-chloro phenyl diazonium chloride and sodium hydroxide solution (or sodium acetate solution) in the presence of benzene ... 

Arylamines are converted by diazotization with nitrous acid into arenediazonium salts, ArN2+ X-. The diazonio group can then be replaced by many other substituents in the Sandmeyer reaction to give a wide variety of substituted aromatic compounds. Aryl chlorides, bromides, iodides, and nitriles can be prepared from arenediazonium salts, as can arenes and phenols. In addition to their reactivity toward substitution reactions, diazonium salts undergo coupling with phenols and arylamines to give brightly colored azo dyes. 

Although it is possible to prepare aryl chlorides and aryl bromides by electrophilic aromatic substitution, it is often necessary to prepare these compounds from an aromatic amine. The amine is converted to the corresponding diazonium salt and then treated with copper(I) chloride or copper(I) bromide as appropriate. 

Finally, aryl radicals Ar, which in the presence of hypophosphoric acid (H3P02) are generated from diazonium salts and Cu(I), undergo reduction to the aromatic compounds Ar-H in a radical chain reaction (Figure 5.54). 

In each case, the mechanism involves generation of an aryl radical from a covalent azo compound. In acid solution, diazonium salts are ionic and their reactions are polar. When they cleave, the product is an aryl cation (see p. 856). However, in neutral or basic solution, diazonium ions are converted to covalent compounds, and these cleave to give free radicals (Ar and Z"). Note that radical reactions are presented in Chapter 14, but the coupling of an aromatic ring with an aromatic compound containing a leaving group prompted its placement here. Note the similarity to the Suzuki reaction in 13-12. 

Aryl radicals may also be prepared from the respective diazonium salts by electrochemical reduction. The radicals obtained from this single electron transfer then react with the diamond surface, arylating it in the course of the process. Varying a bit with the reagent chosen, the resultant surface covering is about 13% of a monolayer. Depending on the substitution pattern of the aromatic compound, the most different functionalizations of diamond films can be achieved this way. For example, doubly meta-chlorinated or para-nitrated diazonium salts can be... 

The radical-generating step is a special case of the decomposition of an azo compound. An important use for this reaction is in the synthesis of biphenyls, by reactions in which a second aromatic molecule is attacked by the aryl radical. Under these conditions, hydrogen abstraction from the intermediate arylcyclohexadienyl radical becomes part of the chain mechanism, with the aryl diazonium ion oxidizing the radical intermediate to give the biphenyl. Aryl diazonium ions generated in the usual way by diazotization of aryl amines can also serve as sources of aryl radicals. Substituted biphenyls can be synthesized by base-catalyzed decomposition of the diazonium salt, usually in the presence of an excess of the aromatic substrate. 

Conversion of diazonium compounds to aryl chlorides, bromides, or cyanides is usually accomplished using cuprous salts, and is known as the Sandmeyer reaction. Since a CN group is easily converted to a CO2H group (eq. 10.13), this provides another route to aromatic carboxylic acids. The reaction with KI gives aryl iodides, usually not easily accessible by direct electrophilic iodination. Similarly, direct aromatic fluorination is difficult, but aromatic fluorides can be prepared from diazonium compounds and tetrafluoroboric acid, HBF4. 

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