Thallium bonding

Yields in the above reactions can often be improved by the addition of 1 mole of triphenylphosphine directly to the trifluoroacetic acid solution of the reactants immediately before final work-up. It would appear that the triphenylphosphine functions as a scavenger for TTFA released in the metal-metal exchange reaction, thus protecting the final phenol from further electrophilic thallation and/or oxidation. Validation of the metal-metal exchange mechanism was obtained indirectly by isolation and characterization of an ArTlX2/LTTFA complex directly from the reaction mixture. NMR analysis revealed that this complex still possessed an intact aryl-thallium bond, indicating that it was probably the precursor to the transmetallation products, an aryllead tristrifluoroacetate and TTFA. 

The versatility of ArTlXj compounds as intermediates for the synthesis of substituted aromatic compounds has been substantially extended by the observation that the aryl-thallium bond is extremely labile photochemically. The resulting aryl radical can then be captured by appropriate reagents (see below) to give substituted aromatic compounds. A remarkable feature of these photochemical conversions of ArTlXj compounds to substituted aromatics is that, as before, the new substituent always enters the ring at the position to which thallium was originally attached. 

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. 

Remarkably, ArTlXj compounds also suffer nucleophilic substitution on the carbon-thallium bond with hydride ion to regenerate the parent arom-... 

The aryl-thallium bond is thus apparently capable of displacement either by electrophilic or by suitable nucleophilic reagents. Coupled with its propensity for homolytic cleavage (spontaneous in the case of ArTlIj compounds, and otherwise photochemically induced), ArTlXj compounds should be capable of reacting with a wide variety of reagents under a wide variety of conditions. Since the position of initial aromatic thallation can be controlled to a remarkable degree, the above reactions may be only representative of a remarkably versatile route to aromatic substitution reactions in which organothallium compounds play a unique and indispensable role. 

Heterometallic homo- and heteroleptic porphyrinate dimers with metal-thallium bonds have been described. These include (OEP)Rh Tl(OEP), (TPP)Rh Tl(OEP), (OEP)Rh-Tl(TPP), and (TPP)Rh-Tl(TPP). The ETV-visible spectroscopy confirms the presence of a strong tt-tt interaction between the macrocycles in each metal derivative. 

Several molecular structures, including those of T1(I) nitrate," " iodate," " borate," germ-anate," sulfate, " phosphates, arsenate, chromate," and selenate," have been reported. Heterobimetallic compounds with Tl-0 interactions have been reported. Their metal-thallium bonding and their photophysical properties are of particular interest. 

Data for 2-chloro-3-butanol is unavailable, but the low glycol yield for Tl(III) oxidation is understandable on the basis of steric hindrance to Sn2 attack of water on the carbon-thallium bond. With ethylene and propylene, the thallium (III) is attached to a primary carbon while with 2-butene it is attached to a secondary carbon. The situation with 2-butene is somewhat different from that found with the other olefins. In the oxythallation adduct from this olefin. 

Thus, Criegee 11) prepared the oxythallation adduct of styrene in methanol which he decomposed at 130°C. to give products which result from solvolysis of the carbon-thallium bond with (HI) and without (IV)... 

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 ... 

The conversion of a carbon-thallium bond into a carbon-carbon bond has been performed directly only in a relatively limited number of cases formation of aiylcyanides, reaction with nitroalkane salts as well as the reaction with copper acetylides. Other carbon-carbon bond formation reactions have been reported with a carbon-thallium substrate. However, they all involve palladium catalysis, and this is beyond the scope of this book. 147... 

Alkyl and vinylthallium(in) compounds are mostly obtained by oxythallation of alkenes or alkynes. They can also be formed by transmetallation between, for example, an organomercury compound and thallium(ni) salts. Various types of reactivity have been observed for these compounds. The evolution of the carbon-thallium bond into functionalised products usually takes place homolytically or heterolytically. The ligand coupling process has also been evoked to rationalize a small number of transformations. 

The formation of a carbon-carbon bond by reaction of alkylthallium(III) compounds has been described in the a-nitroalkylation of alkanes. The reaction of alkylthallium (129) with nitroalkane anions leads to the nitroalkyl derivatives (130). In this case, radical intermediates generated by electron transfer activation of the carbon-thallium bond are involved in a non-chain substitution process. 

Selected crystal structures that contain platinum-thallium bonds can be divided into four groups depending on the formal oxidation states ofthe metal ions Pt°-TF (2.86-3.05 A), Pt -TP (2.88-3.15A),Pt -Tl (2.70-2.71 A),i and Pt -TF (2.570-2.628 A). 

R 418 B. le Guennic, K. Matsumoto and J. Autschbach, NMR Properties of Platinum-Thallium Bonded Complexes Analysis of Relativistic Density Functional Theory Results , Magn.Reson.Chem., 2004,42(Spec. Issue),S99... 

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