Thallium in Organic Synthesis. 58. Regiospecific Intermolecular Oxidative Dehydrodimerization of Aromatic Compounds to Biaryls Using Thallium (III) Trifluoroacetate

Article abstract of DOI:10.1021/ja00541a020

Treatment of a variety of aromatic substrates with thallium(III) trifluoroacetate (TTFA) in trifluoroacetic acid (TFA), or in carbon tetrachloride or acetonitrile containing boron trifluoride etherate, results in smooth, rapid, and direct regiospecific oxidative dehydrodimerization to give symmetrical biaryls in good to excellent yield.The method is particularly useful when applied to substrates in which the ring substituents are either electron donating or mildly electron withdrawing.Aromatic substrates which contain powerful electron-withdrawing groups (CN, COOR, NO2) fail to react.The reaction is postulated to proceed via (a) reaction of TTFA with the aromatic substrate and generation of the radical cation Ar⁺; (b) reaction of this electrophile with the aromatic substrate; (c) oxidative aromatization of the intermediate thus produced by TTFA.Biaryls can be obtained similarly by oxidation of the same substrates with either mercury(II) trifluoroacetate in TFA containing boron trifluoride, lead(IV) acetate in acetonitrile containing boron trifluoride, iron(III) chloride in methylene chloride, or cobalt(III) fluoride in TFA.Yields in the Hg(II) and Fe(III) reactions are generally inferior to those obtained with TTFA, but those obtained in the Pb(IV) and Co(III) oxidations are in many instances comparable to, or even better than, the TTFA results.The oxidations with Hg(II), Pb(IV), Fe(III), and CO(III) are also postulated to proceed via a radical cation mechanism.