Transmetallation reactions with thallium

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]. 

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). 

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. 

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