Formation of Carbon---Tl Bonds

For reviews of chemical approaches to radical cyclization reactions see a) Motherwell WB, Crich D (1992) Free radical chain reactions in organic synthesis, Academic Press, London, b) Giese B (1986) Radicals in organic synthesis formation of carbon-carbon bonds, Pergamon, Oxford, and c) Jasperse CP, Curran DP, Fevig TL (1991) Chem Rev 91 1237... 

For example, photolysis of a suspension of an arylthallium ditrifluoro-acetate in benzene results in the formation of unsymmetrical biphenyls in high yield (80-90%) and in a high state of purity 152). The results are in full agreement with a free radical pathway which, as suggested above, is initiated by a photochemically induced homolysis of the aryl carbon-thallium bond. Capture of the resulting aryl radical by benzene would lead to the observed unsymmetrical biphenyl, while spontaneous disproportionation of the initially formed Tl(II) species to thallium(I) trifluoroacetate and trifluoroacetoxy radicals, followed by reaction of the latter with aryl radicals, accounts for the very small amounts of aryl trifluoroacetates formed as by-products. This route to unsymmetrical biphenyls thus complements the well-known Wolf and Kharasch procedure involving photolysis of aromatic iodides 171). Since the most versatile route to the latter compounds involves again the intermediacy of arylthallium ditrifluoroacetates (treatment with aqueous potassium iodide) 91), these latter compounds now occupy a central role in controlled biphenyl synthesis. 

The tail of j/i in the direction of j/2 becomes much less pronounced at the TS (central column of orbitals). This orbital resembles a distorted carbon 2p AO in the rightmost column, but it begins to attain the form of a distorted sp hybrid at the TS. Orbital /6 is reshaped in a similar fashion and becomes less distorted towards /5 and more towards j/i. The changes in j/i and tj/e are mirrored by identical changes in /4 and /5. All of this indicates that the bonds t /i-t /2, t /5 6 and /3- i/4 are breaking up at the TS, in parallel with the formation of new bonds tj/i-tl/g and tl/4-tl/5. The shapes of orbital /2 and its symmetry-related counterpart, /3, are particularly interesting These orbitals appear to be almost equally distorted towards one another (as the reaction proceeds further, these distortions lead to the formation of the cyclohexene n bond) and towards their former partners in the butadiene tz bonds ( /i and /4, respectively). Consequently, j/2 and j/3 closely resemble in form the SC orbitals for benzene [8-10]. 

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