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DDQ/H(+) system readily oxidizes a variety of electron donors with oxidation potential as high as approximately 1.7 V to the corresponding cation radicals. A re-examination of the controversial arenium-ion versus cation-radical mechanisms for Scholl reaction using DDQ/H(+) together with commonly utilized FeCl(3) as oxidants led us to demonstrate that the reaction proceeds largely via a cation-radical mechanism. The critical experimental evidence in support of a cation-radical pathway for the Scholl reaction includes the following: (i) There is no reaction in Scholl precursors in a mixture of dichloromethane and various acids (10% v/v). (ii) The necessity to use powerful oxidants such as ferric chloride (FeCl(3)) or DDQ/H(+) for Scholl reactions is inconsistent with the arenium-ion mechanism in light of the fact that aromatization of the dihydro intermediates (formed via arenium-ion mechanism) can be easily accomplished with rather weak oxidants such as iodine or air. (iii) Various Scholl precursors with oxidation potentials 1.7 V vs SCE do not react. (iv) Finally, the feasibility of the dicationic intermediate, formed by loss of two electrons, has been demonstrated by its generation from a tetraphenylene derivative using DDQ/H(+) as an oxidant.
Zhai et al. (Thu,) studied this question.