Thallium reagents

This reaction has been used mostly to prepare iodo lactones, but bromo lactones and, to a lesser extent, chloro lactones, have also been prepared. In the case of -y.S-unsaturated acids, 5-membered rings (y-lactones) are predominantly formed (as shown above note that Mar-kovnikov s rule is followed), but 6-membered and even 4-membered lactones have also been made by this procedure. Thallium reagents, along with the halogen, have also been used.653 For a method of iodoacetyl addition, see 5-35. 

The reactions of aromatic substrates with thallium reagents is a fascinating subject which has been reviewed by McKillop and Taylor,two of the prime contributors to this field of chemistry. Two types of reaction are possible, both of which are important for the introduction of oxygen functionality. Tlie fu t is electrophilic aromatic thallation, whilst the second involves one-electron oxidation. 

Numerous transformations in organic chemistry that occur in the presence of thalhum reagents have been described. Many of these reactions are unique and are characterized by manipulative simplicity and high yields. Several review articles describe in detail the use of thallium reagents in organic synthesis. Selected examples are discussed below. 

The reaction of phenylthallium diacetate with the sodium salt of thiophenol did not afford diphenylsulfide. Instead, the product of disproportionation of the thallium reagent was observed. Although not isolated, this product was suggested to be diphenylthallium phenylsulride (110). ... 

The attempt to substitute the thallium moiety of phenylthallium diacetate with acetylacetone failed to give the 3-phenylpentan-2,4-dione. Instead, the presence of the disproportionation product (120) of the thallium reagent was detected by... 

It is of interest to note in passing that In more recent work (ref.72), use has been made in synthesis of the transformation of chalcones to isoflavones with thallium(lll) nitrate. Thus, 6-acetyl-2,2-dimethyl-7-hydroxy-5-methoxychromanone was converted to the chalcone with 2,4-dibenzyloxybenzaldehyde. The O-acetyl derivative by treatment with the thallium reagent followed by acidic cyclisation gave a bischromanone structure. Selective reduction of the least hindered carbonyl group in the bischromanone, acidic dehydration of the resultant alcohol and final debenzylation with boron trichloride gave the linear isofiavone. 

Scheme 27).A 6-arylation of enones using thallium reagents 104... 

For thallium on the other hand, the monovalent state is the stable state and the trivalent state is oxidizing one and prone to reductive elimination, thanks to the inert pair effect. Nevertheless, in spite of their toxicity, trivalent thallium reagents enjoy some unique applications as oxidants in organic chemistry. 

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