Detection and identification of a novel carbon-centered quinone ketoxy radical

Abstract

We have shown recently that halogenated quinones could enhance the decomposition of hydroperoxides and formation of alkoxyl/hydroxyl radicals through a metal-independent nucleophilic substitution coupled with hemolytic decomposition mechanism. However, neither the proposed quinone enoxy radical intermediate, nor the major reaction products were unambiguously identified. In the present study, one of the major reaction products between 2,5-dichloro-1,4-benzoquinone (DCBQ) and t -butylhydroperoxide ( t -BuOOH) was isolated and purified by semipreparative HPLC, and identified as 2-hydroxy-3- t -butoxy-5-chloro-1,4- benzoquinone [CBQ(OH)-O- t -Bu], which is the rearranged isomer of the postulated quinone-peroxide reaction intermediate. The formation of CBQ(OH)-O- t -Bu was found to be inhibited by the spin trapping agent 5,5-dimethyl-1-pyrroline N -oxide (DMPO), and concurrently, a new DMPO adduct with 1-chlorine isotope peak clusters at m / z 268 was observed. Further ESR spin trapping, 1H NMR and HPLC/Fourier transform ion cyclotron resonance (FTICR) mass spectrometric studies with O-17-labeled and unlabeled H2O2 strongly suggest that the radical trapped by DMPO is a carbon-centered quinone ketoxy radical, which is the spin isomer of the proposed oxygen-centered quinone enoxy radical. This study represents the first detection and identification of an unusual carbon-centered quinone ketoxy radical, which provides direct experimental evidence to further support and expand our previously proposed mechanism for metal-independent decomposition of hydroperoxides by halogenated quinones.