title: Mechanistic insights into the oxidation of substituted phenols via hydrogen atom abstraction by a cupric-superoxo complex.
abstract: To obtain mechanistic insights into the inherent reactivity patterns for copper(I)-O2 adducts, a new cupric-superoxo complex [(DMM-tmpa)Cu(II)(O2(•-))](+) (2) [DMM-tmpa = tris((4-methoxy-3,5-dimethylpyridin-2-yl)methyl)amine] has been synthesized and studied in phenol oxidation-oxygenation reactions. Compound 2 is characterized by UV-vis, resonance Raman, and EPR spectroscopies. Its reactions with a series of para-substituted 2,6-di-tert-butylphenols (p-X-DTBPs) afford 2,6-di-tert-butyl-1,4-benzoquinone (DTBQ) in up to 50% yields. Significant deuterium kinetic isotope effects and a positive correlation of second-order rate constants (k2) compared to rate constants for p-X-DTBPs plus cumylperoxyl radical reactions indicate a mechanism that involves rate-limiting hydrogen atom transfer (HAT). A weak correlation of (k(B)T/e) ln k2 versus E(ox) of p-X-DTBP indicates that the HAT reactions proceed via a partial transfer of charge rather than a complete transfer of charge in the electron transfer/proton transfer pathway. Product analyses, (18)O-labeling experiments, and separate reactivity employing the 2,4,6-tri-tert-butylphenoxyl radical provide further mechanistic insights. After initial HAT, a second molar equiv of 2 couples to the phenoxyl radical initially formed, giving a Cu(II)-OO-(ArO') intermediate, which proceeds in the case of p-OR-DTBP substrates via a two-electron oxidation reaction involving hydrolysis steps which liberate H2O2 and the corresponding alcohol. By contrast, four-electron oxygenation (O-O cleavage) mainly occurs for p-R-DTBP which gives (18)O-labeled DTBQ and elimination of the R group.
authors: Department of Chemistry, Johns Hopkins University , Baltimore, Maryland 21218, United States.
authors: Jung Yoon
authors: JY
authors: Lee
authors: Ryan L
authors: RL
authors: Peterson
authors: Kei
authors: K
authors: Ohkubo
authors: Isaac
authors: I
authors: Garcia-Bosch
authors: Richard A
authors: RA
authors: Himes
authors: Julia
authors: J
authors: Woertink
authors: Cathy D
authors: CD
authors: Moore
authors: Edward I
authors: EI
authors: Solomon
authors: Shunichi
authors: S
authors: Fukuzumi
authors: Kenneth D
authors: KD
authors: Karlin
mesh_terms: D003300:Copper / Q000737:chemistry*; D003903:Deuterium; D006859:Hydrogen / Q000737:chemistry*; D007700:Kinetics; D008958:Models, Molecular; D008968:Molecular Conformation; D010084:Oxidation-Reduction; D010104:Oxygen Radioisotopes / Q000737:chemistry; D010636:Phenols / Q000737:chemistry*; D013481:Superoxides / Q000737:chemistry*
chemical_list: D010104:Oxygen Radioisotopes; D010636:Phenols; D013481:Superoxides; D003300:Copper; D006859:Hydrogen; D003903:Deuterium