Benzene thallation

Mercuration-Thallation. Mercuric acetate and thallium ttifluoroacetate react with benzene to yield phenyHnercuric acetate [62-38-4] or phenylthaHic ttifluoroacetate. The arylthalHum compounds can be converted iato phenols, nitriles, or aryl iodides (31). 

Ethylbenzene, Thallium triacetate Ucmura, S. et al., Bull. Chem. Soc., Japan., 1971, 44, 2571 Application of a published method of thallation to ethylbenzene caused a violent explosion. A reaction mixture of thallium triacetate, acetic acid, perchloric acid and ethylbenzene was stirred at 65°C for 5 h, then filtered from thallous salts. Vacuum evaporation of the filtrate at 60°C gave a pasty residue which exploded. This preparation of ethylphenylthallic acetate perchlorate monohydrate had been done twice previously and uneventfully, as had been analogous preparations involving thallation of benzene, toluene, three isomeric xylenes and anisole in a total of 150 runs, where excessive evaporation had been avoided. 

Another general approach to benzene-ring haloindoles involves thallation chemistry. Hollins and co-workers demonstrated that C-4 thallation occurs readily in a series of 3-acylindoles 31, affording 4-iodoindoles 32 following treatment of the thallated intermediates with KI [38]. 

Methyl-substituted benzenes react with mercury(II) and thallium(III) electrophiles in thermal reactions to yield mercuration and thallation products, respectively [113], (Eqs 24, 25) ... 

The mechanism of thallation appears to be complex, with electrophilic and electron-transfer mechanisms both taking place. Transient metalated aryl complexes can be formed that react with another aromatic compound. Aryl iodides reacted with benzene to form a biaryl in the presence of an iridium catalyst. Aniline derivatives reacted with TiCLj to give the para-homo coupling product (RaN-Ar-Ar-NRj). ... 

The distribution rates for iodination of monosubstituted benzene derivatives have been reported.2 Under conditions of thermodynamic control (elevated temperature), me la substitution is observed. Under conditions of kinetic control (room temperature), a significant preference for para substitution is observed for compounds containing ortho-ptira-direcung substituent groups. Ortho substitution results when chelation of TTFA with the directing substituent permits intramolecular delivery of the electrophile. For example, methyl benzoate gives almost exclusively ordio-thallation (95%). 

Controlled fluorination of benzene is difficult, but it can be accomplished by a two-step thallation procedure. Benzene reacts with thallium tris(trifluoroacetate), TI(0C0CF3)3, to give an organothallium intermediate. Further reaction with potassium fluoride and boron trifluoride gives the aryl fluoride. Propose a mechanism for the first step, the thallation of benzene. 

While the chemistry of aluminum is largely that of the trivalent state, the monovalent state becomes increasingly stable as one goes down group 13 for thallium, the monovalent state is the common state. Trivalent T1 salts are reactive and used as oxidants in organic chemistry, as shown below for the thallation of benzene ... 

Consider thallation, an electrophilic substitution of an aromatic compound by a trivalent thallium compound, depicted below for benzene and T1(02CCF3)3 ... 

The thallated benzene is a valuable synthetic intermediate. With two equivalents of potassium iodide, it provides a convenient route to aromatic iodides, as shown below ... 

Complex 13 undergoes electrophilic substitution with aromatic substrates. Thus, treatment with benzene in dichloromethane at ambient temperature results in the formation of the diphenyl complex 15 (Scheme V. Reaction of 13 with pyridine (5-6 equivs) in dichloromethane affords a new complex that is the result of pyridine a-CH activation. The NMR data clearly show two chemically equivalent coordinated pyridines and pyridine that has lost one of the a-hydrogens. Structure 16 is proposed from the preliminary data. The formation of 15 and 16 was quantitative by NMR monitoring, but these compounds are reactive and have not been isolated as pure solids. While main group Lewis acids are well known to undergo aromatic substitutions (e.g., mercurations, thallations, etc.) (33), relatively little is known about the ability of transition metal complexes to undergo electrophilic aromatic substitution (34). 

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