Thallium proton exchange

Oxidation of the steroidal olefin (XXVII) with thallium(III) acetate gives mainly the allylic acetates (XXXI)-(XXXIII) (Scheme 15), again indicating that trans oxythallation is the preferred reaction course (19). Addition of the electrophile takes place from the less-hindered a-side of the molecule to give the thallinium ion (XXVIII), which by loss of a proton from C-4 would give the alkylthallium diacetate (XXIX). Decomposition of this intermediate by a Type 5 process is probably favorable, as it leads to the resonance-stabilized allylic carbonium ion (XXX), from which the observed products can be derived. Evidence in support of the decomposition process shown in Scheme 15 has been obtained from a study of the exchange reaction between frawr-crotylmercuric acetate and thallium(III) acetate in acetic acid (Scheme 16) (142). 

Rapid exchange of alkyl groups occurs in thallium(III) alkyls as demonstrated by the proton NMR study of Maher and Evans (81). At low temperatures ( —85°) the multiplet structure expected for proton-thallium coupling is superimposed on the proton spectra of thallium alkyls. On warming, the multiplets collapse. Analysis of this collapse region [Eq. (29)] yielded a value of 6 1 kcal mole-1 for the exchange-activation energy... 

Rapid intermolecular exchange of groups is also occurring in organo-thallium compounds at room temperature, since metal-proton spin-spin... 

Trimethylthallium (Figure 13(b)) has a similar crystal structure, though in solution both thallium trialkyls and triaryls are monomeric. A proton magnetic resonance study of MeaTl in PhCD3 shows that methyl exchange is second-order and consistent with an electron-deficient transition state only 6 kcal/mole above that of two monomers ... 

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