Mechanistic Insights into the Oxidation of Substituted Phenols via Hydrogen Atom Abstraction by a Cupric鈥揝uperoxo Complex

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• 作者：Jung Yoon Lee ; Ryan L. Peterson ; Kei Ohkubo ; Isaac Garcia-Bosch ; Richard A. Himes ; Julia Woertink ; Cathy D. Moore ; Edward I. Solomon ; Shunichi Fukuzumi ; Kenneth D. Karlin
• 刊名：Journal of the American Chemical Society
• 出版年：2014
• 出版时间：July 16, 2014
• 年：2014
• 卷：136
• 期：28
• 页码：9925-9937
• 全文大小：582K
• ISSN：1520-5126

文摘

To obtain mechanistic insights into the inherent reactivity patterns for copper(I)鈥揙₂ adducts, a new cupric鈥搒uperoxo complex [(DMM-tmpa)Cu^II(O₂^{鈥⑩€?/sup>)]+ (2) [DMM-tmpa = tris((4-methoxy-3,5-dimethylpyridin-2-yl)methyl)amine] has been synthesized and studied in phenol oxidation鈥搊xygenation reactions. Compound 2 is characterized by UV鈥搗is, 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_BT/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, 18O-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 CuII鈥揙O鈥?ArO鈥? 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鈥揙 cleavage) mainly occurs for p-R-DTBP which gives 18O-labeled DTBQ and elimination of the R group.}