Probing the Arenium-Ion (ProtonTransfer) versus the Cation-Radical (Electron Transfer) Mechanism of Scholl Reaction Using DDQ as Oxidant

DDQ/H⁺ system readily oxidizes a variety of electron donors with oxidation potential as high as ∼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 FeCl₃ 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 (FeCl₃) 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 undergo ready oxidative C−C bond formation with DDQ/H⁺ as oxidant, whereas Scholl precursors with oxidation potentials greater than >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.