RESUMO
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
RESUMO
DDQ in the presence of an acid is known to oxidize a variety of aromatic donors to the corresponding cation radicals. Herein, we now demonstrate that the DDQ/H(+) system can be effectively utilized for the oxidative C-C bond formations or biaryl synthesis. The efficient preparation of a variety of polyaromatic hydrocarbons including graphitic hexa-peri-hexabenzocoronenes, ease of isolation of the clean products, and ready regeneration of DDQ from easily recovered reduced DDQ-H(2) advances the use of DDQ/H(+) for Scholl reactions.
RESUMO
The isolation and X-ray crystal structure determination of octamethoxydibenzochrysene () cation radical together with DFT calculations allow us to delineate evidence that the complex structural changes (i.e. elongation and shortening of various bonds) in a polyaromatic hydrocarbon can be predicted based on the positioning of the largest bonding and antibonding character of the HOMO.