Arylthallium trifluoroacetates aryl iodides

Mercuration of aromatic compounds can be accomplished with mercuric salts, most often Hg(OAc)2 ° to give ArHgOAc. This is ordinary electrophilic aromatic substitution and takes place by the arenium ion mechanism (p. 675). ° Aromatic compounds can also be converted to arylthallium bis(trifluoroacetates), ArTl(OOCCF3)2, by treatment with thallium(III) trifluoroacetate in trifluoroace-tic acid. ° These arylthallium compounds can be converted to phenols, aryl iodides or fluorides (12-28), aryl cyanides (12-31), aryl nitro compounds, or aryl esters (12-30). The mechanism of thallation appears to be complex, with electrophilic and electron-transfer mechanisms both taking place. 

Aryl iodides and fluorides can be prepared from arylthallium bis(trifluor-oacetates) (see 12-21), indirectly achieving the conversions ArH —> Arl and ArH ArF. The bis(trifluoroacetates) react with KI to give Arl in high yields. Aryllead triacetates, ArPb(OAc)3, can be converted to aryl fluorides by treatment with BFs-etherate. ... 

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 conversion of diarylthallium trifluoroacetates to aromatic iodides by treatment with molecular iodine is thus analogous to the well-known conversion of diarylmercury derivatives with iodine to a mixture of an aromatic iodide and an arylmercury iodide (134), but it is much more effective as a synthetic tool because of the spontaneous disproportionation to product of the intermediate arylthallium trifluoroacetate iodide. The present procedure thus provides a practical synthetic method for the ultimate conversion of aryl Grignard reagents to aromatic iodides. 

Thallium(III), particularly as the trifluoroacetate salt, is also a reactive electrophilic metallating species, and a variety of synthetic schemes based on arylthallium intermediates have been devised.75 Arylthallium compounds are converted to chlorides or bromides by reaction with the appropriate cupric halide.76 Reaction with potassium iodide gives aryl iodides.77 Fluorides are prepared by successive treatment with potassium fluoride and boron trifluoride.78 Procedures for converting arylthallium compounds to nitriles and phenols have also been described.79... 

Mercuration- Thallation. Mercuric acetate and thallium trifluoroacetate react with benzene to yield phenylmercuric acetate [62-38-4] or phenylthallic trifluoroacetate. The arylthallium compounds can be converted into phenols, nitriles, or aryl iodides (31). 

The formation of iodobenzene by treatment of phenylthallium(III) compounds with potassium iodide was reported, without experimental details, by Challenger et al in the 1930 s. 2,113 jhe potential and synthetic interest of this iododethallation reaction was extensively studied by McKillop and Taylor in the early 1970 s.8 7-89 Although arylthallium(III) compounds prepared by reaction of the arenes with thallium tris(trifluoroacetate) (84) can be isolated, they can also be directly converted into aryl iodides by addition of aqueous potassium iodide to the thallation reaction mixture. An intermediate arylthallium(III) diiodide (91) was suggested to be formed and to decompose intramolecularly to lead to the aryl iodide. ... 

Aryl radicals, that is, CeHs, which have been generated thermally by the decomposition of, for example, aryl diazonium salts (ArN2 ) in the presence of copper(I) salts (Equation 6.113) and the decomposition of diacyl peroxides (Equation 6.114), or photolytically from aryl iodides (e.g., iodobenzene, CeHsl) (Equation 6.115) and arylthallium trifluoroacetates as shown in Equation 6.116, react with benzene (CeHe) to give biphenyl (CeHs-CeHs) (Scheme 6.97). 

For example, photolysis of a suspension of an arylthallium ditrifluoro-acetate in benzene results in the formation of unsymmetrical biphenyls in high yield (80-90%) and in a high state of purity 152). The results are in full agreement with a free radical pathway which, as suggested above, is initiated by a photochemically induced homolysis of the aryl carbon-thallium bond. Capture of the resulting aryl radical by benzene would lead to the observed unsymmetrical biphenyl, while spontaneous disproportionation of the initially formed Tl(II) species to thallium(I) trifluoroacetate and trifluoroacetoxy radicals, followed by reaction of the latter with aryl radicals, accounts for the very small amounts of aryl trifluoroacetates formed as by-products. This route to unsymmetrical biphenyls thus complements the well-known Wolf and Kharasch procedure involving photolysis of aromatic iodides 171). Since the most versatile route to the latter compounds involves again the intermediacy of arylthallium ditrifluoroacetates (treatment with aqueous potassium iodide) 91), these latter compounds now occupy a central role in controlled biphenyl synthesis. 

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