Oxidative couplings of arenes to biaryls and polyaryls

Oxidative couplings of simple arenes, whose reactivity is comparable with benzene itself, to biaryls have been performed with stoichiometric palladium salts [2-5], Biphenyl (8) is obtained in high yield from two molecules of benzene by the influence of palladium(II) acetate or mixture of palladium(II) chloride (1 eq.) and sodium acetate (2 eq.). The reaction apparently involves the electrophilic palladation of benzene to give phenylpalladium(ll) acetate (370) which further reacts with the 

Since the oxidative coupling of two arene molecules actually involves the elimination of two electrons by Tl(lll), the reaction stoichiometrically requires one molar equivalent of TTFA per two equivalents of arene, but, TTFA has been often employed in slight excess. The efficacy of thaUium(III) trifluoroacetate-mediated oxidative coupling of relatively electron-rich benzenes is presented in the Table 2. 

Beside the biaryls, TTFA / TFA reagent has been applied in the synthesis of polyarenes by oxidative polymerization. Thus 3-alkylthiophenes were polymerized to the corresponding poly(3-alkyl)thiophenes [24] in fair yields. 

In addition, several other one- and two-electron oxidants have been involved in the oxidative coupling reactions of mainly activated benzenes such as phenols, phenol-ethers, alkylated phenol-ethers, etc. 

Additionaly, inexpensive and easily available permanganate reagents allow to scale-up the potential oxidative coupling products. 

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Mechanisms of oxidative couplings of arenes to biaryls and polyaryls... 

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