Organothalliums

Stereoselective cis-dihydroxylation of the more hindered side of cycloalkenes is achieved with silver(I) or copper(II) acetates and iodine in wet acetic acid (Woodward gly-colization J.B. Siddall, 1966 L. Mangoni, 1973 R. Criegee, 1979) or with thallium(III) acetate via organothallium intermediates (E. Glotter, 1976). In these reactions the intermediate dioxolenium cation is supposed to be opened hydrolytically, not by Sn2 reaction. 

Thallium(III) acetate reacts with alkenes to give 1,2-diol derivatives (see p. 128) while thallium(III) nitrate leads mostly to rearranged carbonyl compounds via organothallium compounds (E.C. Taylor, 1970, 1976 R.J. Ouelette, 1973 W. Rotermund, 1975 R. Criegee, 1979). Very useful reactions in complex syntheses have been those with olefins and ketones (see p. 136) containing conjugated aromatic substituents, e.g. porphyrins (G. W. Kenner, 1973 K.M. Smith, 1975). 

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]. 

In contrast to other organothallium(I) compounds, cyclopentadienyl-thallium(I) is a remarkably stable compound. Samples can be stored in sealed bottles for months without appreciable decomposition occurring it is unaffected by water and dilute alkali and it is only slowly oxidized by air at room temperature. Cyclopentadienyltballium(I) was first prepared by Meister in 1956 by addition of freshly distilled cyclopentadiene to a suspension of thallium(I) sulfate in dilute potassium hydroxide solution 101, 102). A number of variations of this procedure have been described (5, 25, 34, 56), and the compound has been made in other ways 35, 56,110, 164), but Meister s preparation, in which the yield of crude product is greater than 90%, remains the method of choice. Purification of crude cyclopenta-dienylthallium(I) is best accomplished by vacuum sublimation, and purity of samples can readily be assessed by gas-liquid chromatography on silicone oil at 170° C using hydrogen as carrier gas (7). 

Very little is known as yet of the chemistry of cyclopentadienylthallium(I) and the related compounds listed in Table I. The parent compound gives tribromocyclopentane on treatment with bromine and the hexabromo derivative with potassium hypobromite 112). By far the most important use discovered so far for these organothallium(I) compounds is the preparation of metallocenes and cyclopentadiene-transition metal complexes. These preparations are, in general, characterized by manipulative simplicity and high yields, and details of the reactions reported thus far are summarized in Tables II-IV. 

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. 

From the evidence currently available it appears that the mechanism of oxythallation is similar to that of oxymercuration. That is, initial rapid formation of a 7r-complex—a thallinium ion—followed by rearrangement of this species to give a o-bonded organothallium derivative (54). Decomposition of this latter intermediate by one or more of the processes shown in Scheme 8 then leads to products. The results obtained from a number of kinetic studies are in broad general agreement with this interpretation (52,79). 

The recently reported (757) conversion of 5-pyrazolones directly to a,j8-acetylenic esters by treatment with TTN in methanol appears to be an example of thallation of a heterocyclic enamine the suggested mechanism involves initial electrophilic thallation of the 3-pyrazolin-5-one tautomer of the 5-pyrazolone to give an intermediate organothallium compound which undergoes a subsequent oxidation by a second equivalent of TTN to give a diazacyclopentadienone. Solvolysis by methanol, with concomitant elimination of nitrogen and thallium(I), yields the a,)S-acetylenic ester in excellent (78-95%) yield (Scheme 35). Since 5-pyrazolones may be prepared in quantitative yield by the reaction of /3-keto esters with hydrazine (168), this conversion represents in a formal sense the dehydration of /3-keto esters. In fact, the direct conversion of /3-keto esters to a,jS-acetylenic esters without isolation of the intermediate 5-pyrazolones can be achieved by treatment in methanol solution first with hydrazine and then with TTN. 

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