Mechanistic Insights into the Oxidation of Substituted Phenols via Hydrogen Atom Abstraction by a Cupric–Superoxo Complex
Mechanistic Insights into the Oxidation of Substituted Phenols via Hydrogen Atom Abstraction by a Cupric–Superoxo Complex
† Department of Chemistry, Johns Hopkins University, Baltimore, Maryland 21218, United States
‡ Department of Material and Life Science, Graduate School of Engineering, and ALCA, Japan Science and Technology Agency (JST), Osaka University, Suita, Osaka 565-0871, Japan
§ Department of Chemistry, Stanford University, Stanford, California 94305, United States
J. Am. Chem. Soc., Article ASAP
DOI: 10.1021/ja503105b
Publication Date (Web): June 22, 2014
Copyright © 2014 American Chemical Society
ACS AuthorChoice + 12, Open Access on 06/22/2015
Abstract
To obtain mechanistic insights into the inherent reactivity patterns for copper(I)–O2 adducts, a new cupric–superoxo complex [(DMM-tmpa)CuII(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 (kBT/e) ln k2 versus Eox 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–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 18O-labeled DTBQ and elimination of the R group.
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