Thalliums transmetallation

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]. 

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. 

Cyanamides are pseudo-halide nitrogen ligands that are readily coordinated to metals. A novel compound is 2-cyanaminofluoren-9-one (HL4)24. Its thallium salt T1+(L4)-(Scheme 7) is useful as a transmetallating agent in a reaction with trimethyltin chloride to produce the corresponding tin cyanamide complex [SnMe3L4] (Scheme 8). 

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. 

Although R2Cd compounds cannot be prepared by transmetalation with R2Hg, bis(perfluorophenyl)cadmium has been obtained by transmetalation with the thallium derivative (C6F5)2TlBr (equation 8). (C6Fs)2Cd can also be prepared... 

The most general route to organolead tricarboxylates, such as aryllead, vinyllead or alkynyllead tricarboxylates, is the metal-metal exchange which can involve mercury-lead, thallium-lead, silicon-lead, tin-lead, boron-lead or zinc-lead transmetallation. 

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 reactivity of arylboronic acids is mainly effected by the steric factors. Bulky substituents in orr/zo-positions significantly retard the formation of the afe-eomplex XXII, common arylboronic species which undergoes the transmetallation process to arylpalladium- or arylnickel complexes. However, as mentioned earlier, the use of much stronger bases such as potassium r-butoxide in dry solvents (DME, DME / r-BuOH) [15] or the use of thallium-bases [19] led to the successful SM reaction with reasonable bulky boronic acids. 

Aqueous TlOH solution, which produces highly insoluble salts similar to the barium base, completes the coupling reaction of mesitylboronic acid with iodobenzene within 30 min at room temperature (Table 2 and the procedure in the text)." However, this excellent base for mesitylboronic acid, unfortunately, does not work well for other less substituted arylboronic acids. Aryl- and 1-alkenylboronic acids transmetallate to thallium salts. 

Recent investigations outline the involvement of this class of polymetallic compounds in reactions of fundamental importance to catalysis, such as inner-sphere oxidation or transmetalation. Given the widespread use of copper(l) and silver(l) salts - and, to a lesser extent, thallium(l) - as additives in palladium- and platinum-based catalytic systems, a systematic study of these reactions is warranted. Additionally, gathering further knowledge on the properties and reactivity of metal-metal-bound species is a critical step toward the rational design of truly bimetallic catalytic cycles. 

Larock reported the use of stoichiometric amounts of thallium salts for direct C H metallation and cyclocarbonylation for the synthesis of benzofiiranones and benzo-hexanones (Scheme 2.10). Surprisingly, this reaction occurs in the absence of external oxidant because Tl(III) salts generated via transmetallation can reoxidize Pd(0) to Pd(II). Lithium chloride and magnesium oxide were added to suppress... 

As shown in Scheme 17, thallation of an aromatic alcohol gives the o-metallated species, which then transmetallates with PdCl2 and undergoes cyclocarbonylation to furnish the lactone products, hi this protocol, the thallium salt also serves to reoxidize Pd(0) to the requisite Pd(Il) species. 

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