Arylthallium compounds

Thallation of aromatic compounds with thallium tris(trifluoroacetate) proceeds more easily than mercuration. Transmetallation of organothallium compounds with Pd(II) is used for synthetic purposes. The reaction of alkenes with arylthallium compounds in the presence of Pd(Il) salt gives styrene derivatives (433). The reaction can be made catalytic by use of CuCl7[393,394], The aryla-tion of methyl vinyl ketone was carried out with the arylthallium compound 434[395]. The /9-alkoxythallium compound 435, obtained by oxythallation of styrene, is converted into acetophenone by the treatment with PdCh[396]. 

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. 

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... 

Conversion of Arylthallium Compounds to Phenols Hydroxy-de- bistrifluoroacetoxy)thallation... 

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). 

Now, these arylthallium compounds are useful, not in themselves, but as intermediates in the synthesis of a variety of other aromatic compounds. Thallium can be replaced by other atoms or groups which cannot themselves be introduced directly into the aromatic ring - or at least not with the same regiospecificity. In this way one can prepare phenols (ArOH, Sec. 24.5) and aryl iodides (Sec. 25.3). Direct iodination of most aromatic rings does not work very well, but the process of thallation followed by treatment with iodide ion gives aryl iodides in high yields. 

Oxidation of arylthallium compounds. Discussed in Sec. 24.5. ArTl(OOCCFj)2 ArOOCCFi ArO ArOH... 

Aryl iodides can be prepared by simple treatment of arylthallium compounds with iodine. As in the synthesis of phenols (Sec. 24.5) the thallation route has the advantages of speed, high yield, and orientation control (see Secs. 11.7 and 11.13)... 

The direct carbonylation of arylthallium compounds usually requires high temperatures and pressures, but in the presence of palladium catalysts the reaction proceeds in high yield at room temperature and atmospheric pressure (equation 47). ... 

An interesting alternative which combines both boron and thallium chemistry has been developed. The arylthallium compound is treated with diborane to provide the arylboronic acid which, by oxidation under standard conditions, yields the phenolic compound in good yield (equations 58 and 59)". ... 

When the reaction of the arylthallium compound ArTlX2 with the nucleophile Nu" leads to a too stable intermediate AtT1Nu2, competing protonolysis may become predominant. This is circumvented by using copper salts, which favours the removal of a ligand from the thallium metal. According to Uemura et al, this reaction leads to substitution in a concerted process by reductive displacement of... 

The aryl chlorides and aryl bromides are easily prepared by treatment of the arylthallium(III) bis(trifluoroacetates) with the corresponding copper(I) or copper(II) halides. The best yields were obtained with the copper(II) halides in dioxane under reflux. 2 These reactions have been successfully applied by Somei et al to the synthesis of indole derivatives by reaction of the arylthallium compounds with copper(II) salts in DMF. 127,133,134... 

However, aryl cyanides are more directly obtained by reaction of the arylthallium compound with various copper cyanides. Different reaction systems have been reported, such as CuCN or Cu(CN)2 in acetonitrile or pyridine with arylthallium acetate perchlorate monohydrates. Other systems involve the arylthallium bistrifluoroacetate with CuCN in acetonitrileor in DMF. With these systems, a number of aryl cyanides (114), heteroaryl cyanides (115) 25,126 indolyl cyanides (116) and (117)124 148 have been prepared. 

With other halides, the reaction is conveniently performed by treatment of the alkylthdlium compound with the appropriate copper(I) halide. The product yield is increased by the addition of potassium halide.jhe thallium moiety can also be replaced by other groups, similarly to the arylthallium compounds. The cyano group is introduced by reaction with copper(I) cyanide.lSi the thiocyano group by reaction with potassium and/or copper thiocyanate>5i.i52 and the selenocyano group by treatment with potassium selenocyanate. 

Other monovinylthallium(ni) compounds such as (132), the product of oxythallation of acetylenes, react analogously to the arylthallium compounds. Halogenodethallation and pseudohalogeno-dethallation reactions result from the interaction of the oxyalkenylthallium compound with either potassium iodide or with the appropriate copper derivatives. 6... 

For examples of palladium-catalysed reactions involving arylthallium compounds, see ref. 90 and 91. 

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