Metal-independent decomposition of hydroperoxides by halogenated quinones: detection and identification of a quinone ketoxy radical.
We have shown recently that halogenated quinones could enhance the decomposition of hydroperoxides and formation of alkoxyl/hydroxyl radicals through a metal-independent 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 electron spin resonance (ESR) spin-trapping, (1)H-NMR and HPLC/Fourier transform ion cyclotron resonance (FTICR) mass spectrometric studies with oxygen-17-labeled and unlabeled hydrogen peroxide 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. Analogous results were observed when DCBQ was substituted by other halogenated quinones. 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.
Zhu
Ben-Zhan
BZ
State Key Laboratory of Environmental Chemistry and Ecotoxicology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, People's Republic of China. mail: bzhu@rcees.ac.cn
Shan
Guo-Qiang
GQ
Huang
Chun-Hua
CH
Kalyanaraman
Balaraman
B
Mao
Li
L
Du
Yu-Guo
YG
D016227:Benzoquinones / Q000737:chemistry*; D002851:Chromatography, High Pressure Liquid; D003497:Cyclic N-Oxides; D004578:Electron Spin Resonance Spectroscopy; D006846:Hydrocarbons, Halogenated / Q000737:chemistry*; D013058:Mass Spectrometry; D008956:Models, Chemical; D015394:Molecular Structure; D020122:tert-Butylhydroperoxide / Q000737:chemistry*
D016227:Benzoquinones; D003497:Cyclic N-Oxides; D006846:Hydrocarbons, Halogenated; C071926:2,6-dichlorobenzoquinone; C017245:5,5-dimethyl-1-pyrroline-1-oxide; D020122:tert-Butylhydroperoxide