Free-radicals arylation, mechanism

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. 

Another free-radical arylation method consists of the photolysis of aryl iodides in an aromatic solvent. Yields are generally higher than in 14-17 or 14-21. The aryl iodide may contain OH or COOH groups. The mechanism is similar to that of 14-17. The aryl radicals are generated by the photolytic cleavage ArI AR + T. The reaction has been applied to intramolecular arylation (analogous to the Pschorr reaction). A similar reaction is photolysis of an arylthallium bis(trifluoroacetate) (12-21) in an aromatic solvent. Here too, an unsymmetrical biaryl is produced in good yields. ... 

The oxidative coupling reactions of certain electron-rich arenes under suitable reaction conditions proceed, at least partially, via free-radical mechanism. Scheme 3. The phenolate anion is oxidized by suitable one-electron oxidant to the phenoxyl-radical whose tautomeric form is aryl-radical on the adjacent carbon atom. The symmetrical biaryl is formed by coupling of the latter, whereas the unsymmetrical one is produced by free-radical arylation of the second arene molecule, usually in an intramolecular... 

Materials that promote the decomposition of organic hydroperoxide to form stable products rather than chain-initiating free radicals are known as peroxide decomposers. Amongst the materials that function in this way may be included a number of mercaptans, sulphonic acids, zinc dialkylthiophosphate and zinc dimethyldithiocarbamate. There is also evidence that some of the phenol and aryl amine chain-breaking antioxidants may function in addition by this mechanism. In saturated hydrocarbon polymers diauryl thiodipropionate has achieved a preeminent position as a peroxide decomposer. 

There is evidence that the mechanism" of the reaetion of alkyllithium compounds with alkyl and aryl iodides involves free radicals." ... 

The mechanism is of the Sn 1 type. That aryl cations are intermediates was shown by the following experiments aryl diazonium chlorides are known to arylate other aromatic rings by a free-radical mechanism (see 14-17). In radical arylation it does not matter whether the other ring contains electron-withdrawing or electron-... 

See Glaser, R. Horan, C.J. Nelson, E.D. Hall, M.K. J. Org. Chem., 1992,57, 215 for the influence of neighboring group interactions on the electronic stmcture of diazonium ions. Aryl iodonium salts Ar2l also undergo substitutions by this mechanism (and by a free-radical mechanism). 

Free-radical substitution at an aromatic carbon seldom takes place by a mechanism in which a hydrogen is abstracted to give an aryl radical. Reactivity considerations here are similar to those in Chapters 11 and 13 that is, we need to know which position on the ring will be attacked to give the intermediate... 

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. 852). However, in neutral or basic solution, diazonium ions are converted to covalent compounds, and these cleave to give free radicals ... 

Aromatic compounds can also be arylated by aryllead tricarboxylates. Best yields ( 70-85%) are obtained when the substrate contains alkyl groups an electrophilic mechanism is likely. Phenols are phenylated ortho to the OH group (and enols are a phenylated) by triphenylbismuth dichloride or by certain other Bi(V) reagents. O-Phenylation is a possible side reaction. As with the aryllead tricarboxylate reactions, a free-radical mechanism is unlikely. ... 

Several studies have been performed on the photodecomposition of diaryl sulfones and polysulfones Khodair, et. al., (21) demonstrated that the photodecomposition of diaryl sulfones proceeds by a free-radical mechanism with initial carbon-sulfur bond cleavage. This gives an aryl radical and an aromatic sulfonyl radical. The latter radical can react with oxygen and a hydrogen donor to eventually form the hydroxyl radical. The hydroxy radical may attack the aromatic nucleus in PET and forms the hydroxyterephthaloyl radical. 

Acyl cyanides1708 can be prepared by treatment of acyl halides with copper cyanide. The mechanism is not known and might be free-radical or nucleophilic substitution. The reaction has also been accomplished with thallium(I) cyanide,1709 with MejSiCN and an SnCl4 catalyst,1710 and with Bu3SnCN,1711 but these reagents are successful only when R = aryl or tertiary alkyl. KCN has also been used, along with ultrasound,1712 as has NaCN with phase transfer catalysts.1713 OS III, 119. 

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