Arylthallium trifluoroacetates

Specifically, it has recently been found 149) that diarylthallium tri-fluoroacetates may be converted into aromatic iodides by refluxing a solution in benzene with an excess of molecular iodine. Yields are excellent (74-94%) and the overall conversion represents, in effect, a procedure for the conversion of aromatic chlorides or bromides into aromatic iodides via intermediate Grignard reagents. The overall stoichiometry for this conversion is represented in Eq. (10), and it would appear that the initial reaction is probably formation of 1 mole of aromatic iodide and 1 mole of arylthallium trifluoroacetate iodide [Eq. (8)] which subsequently spontaneously decomposes to give a second mole of aromatic iodide and thallium(I) trifluoroacetate [Eq. (9)]. Support for this interpretation comes from the... 

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. 

Similarly, unactivated arenes readily react with thallium(III) trifluoroacetate in TFA to give the corresponding arylthallium trifluoroacetates, ArTl(02CCF3)2, which are stable and do not readily decompose to aryl trifluoroacetates and T1(I) 282-286 The rate of aromatic mercuration is increased by a factor of 7 X 10s in TFA relative to acetic acid as solvent.292... 

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). 

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]. 

Arylthallium bis(trifluoroacetate)s (10.70) are versatile synthons for various reactions, e.g., acylations (Larock and Fellows, 1982) and photolytic cyanations (Taylor et al., 1970), as shown in Scheme 10-93. Copper-catalyzed cyanations (Uemura et al., 1972) can be carried out at 115 °C with arylthallium (acetate)(perchlorate) (Scheme 10-94). 

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. 

The 1,1-dimetallic compounds, R2C(SnMe3)ZnBr, were oxidized by dry air at —10 to 0°C in the presence of Me3SiCl to give aldehydes or ketones, R2C=0. In a related indirect method, arylthallium bis(trifluoroacetates) (prepared by 12-21) can be converted to phenols by treatment with lead tetraacetate followed by triphenylphosphine, and then dilute NaOH. Diarylthallium trifluoroacetates undergo the same reaction. ... 

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. ... 

Arylthallium bis(trifluoroacetates) (see 12-21) can be converted to aryl nitriles by treatment with copper(I) cyanide in acetonitrile. Another procedure uses excess aqueous KCN followed by photolysis of the resulting complex ion ArTl(CN)3 in the presence of excess KCN. Alternatively, arylthallium acetates react with Cu(CN)2 or CuCN to give aryl nitriles. Yields from this procedure are variable, ranging from almost nothing to 90 or 100%. 

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. ... 

It has been reported previously (147), and will be discussed in detail later, that arylthallium ditrifluoroacetates may be converted in good yield to phenols by treatment with lead tetraacetate in trifluoroacetic acid solution followed by addition of 1 mole of triphenylphosphine. The immediate product of the above reaction, an aryl trifluoroacetate, is hydrolyzed... 

This phenol synthesis complements the analogous reaction (see below) from arylthallium ditrifluoroacetates (147). Although yields are only moderate, the procedure represents a viable conversion of aryl Grignard reagents to phenols. It is a practical method, however, only when the diarylthallium trifluoroacetate precursor is formed via the Grignard route the alternative synthesis via symmetrization of arylthallium ditrifluoroacetates is obviously circuitous, since the latter compounds may be converted directly to phenols. 

It has been reported that photolysis of arylthallium ditrifluoroacetates in benzene suspension results in replacement of the thallium substituent by a phenyl group (i.e., phenylation) to give unsymmetrical biphenyls in excellent yield (152) this reaction is summarized in Section III,C. An analogous reaction occurs upon photolysis of diarylthallium trifluoroacetates in benzene suspension unsymmetrical biphenyls are formed in comparable yield (40-95%). The mechanism of this conversion is undoubtedly similar to that... 

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. 

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... 

The enhancement of the electrophilic properties of thallium(III) trifluoroacetate makes it a very important thallation reagent. The products of thallation, eg, arylthallium bis(trifluoracetate), undeigo a variety of substitution reactions, yielding iodides, fluorides, nitriles, thiophenols, phenols, and biaryls. 

Arylthallium bis(trifluoroacetates) (see 2-22) can be carbonylated with CO, an alcohol, and a PdCl2 catalyst to give esters 387... 

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). 

Several examples of the arylation of vinyl ketones with arylthallium bis(trifluoroacetate) and lithium tetrachloropalladate(II) have been reported (equation 14).40... 

Aryl nitriles.5 Arylthallium bis(trifluoroacetates) react with CuCN in refluxing CH3CN to give aryl nitriles in 60-85% yield. A one-pot synthesis is possible by thallation of the arene in CH3CN. 

Whilst direct electrophilic hydroxylation of the arylthallium species can be effected using peroxytriflu-oroacetic acid, further oxidation of the phenol to a quinone accompanies this process. This over-oxidation can be avoided by initial transmetallation to a lead species with concomitant reduction of the thallium trifluoroacetate (TTFA) by triphenylphosphine, followed by displacement of the lead by trifluo-roacetate to give the aryl trifluoroacetate. lliis hydroxylation method has yet to find use in the synthesis of molecules which are more complex than simple arenes. 

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. ... 

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... 

Treatment of arylthallium(III) compounds with metal nitrites in trifluoroacetic acid affords the corresponding nitrosoarenes, which are oxidised to the nitroarenes. The reaction was suggested to occur by electrophilic attack of NO+ on the carbon-thallium bond. l ... 

Aryldiazonium tetrahaloborates, 16 Aryl methyl ethers, 115 Arylthallium ditrifluoroacetates, 287-288 Aryl trifluoroacetates, 287 Asparagine, 102 Aspidosperma alkaloids, 196 (+)-Aspidospermidine, 196 Asymmetric synthesis, 199—200 Aurantiocladin, 7 Autoxidation, 245... 

Synthesis of phenols and aromatic nitriles.3 In a new phenol synthesis, an aromatic hydrocarbon is first thallated with the reagent in trifluoroacetic acid to give an arylthallium ditrifluoroacetate (which can be isolated) and then oxidized with lead tetraacetate in the presence of 1 equivalent of triphenylphosphine. The resulting aryl trifluoroacetate is then hydrolyzed with dilute base. The triphenyl-... 

Thallation of species deactivated to electrophilic substitution requires more vigorous conditions, but Tl(III) trifluoroacetate decomposes above 100°C. The thallating ability of Tl(III) trifluoroacetate can be improved by the addition of electron-pair acceptor acids, such as BFj or SbFj. Another approach is the use of the even more powerful thallating agent, Tl(III) trifluoromethanesulfonate, formed from Tl(III) oxide and trifluoromethane sulfonic acid. It reacts with even unreactive polyfluoroarenes to produce arylthallium bis(trifluoromethanesulfonate) ° ... 

Easier and faster is reaction of arylthallium bis(trifluoroacetate) in the presence of trimethylphosphite. A rapid, exotherm results, yielding 65-95% of the diarylthallium trifluoroacetate. Methyl- and phenyl thallium diacetates react similarly. Although the role of the trimethylphosphite is uncertain, the synthesis is useful for diarylthallium(III)s from the accessible arylthallium(III) bis(trifluoroacetates). 

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