- Mechanistic insights into the oxidation of substituted phenols via hydrogen atom abstraction by a cupric-superoxo complex
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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 k 2 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.
- Lee, Jung Yoon,Peterson, Ryan L.,Ohkubo, Kei,Garcia-Bosch, Isaac,Himes, Richard A.,Woertink, Julia,Moore, Cathy D.,Solomon, Edward I.,Fukuzumi, Shunichi,Karlin, Kenneth D.
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p. 9925 - 9937
(2014/08/05)
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- Dienone tautomers of 4-alkoxy-2,6-di-terf-butylphenols
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Generation and isolation of 4-alkoxycyclohexa-2,5-dienones 9, the tautomeric forms of the title phenols (10), is described. They are generated efficiently by the Ag ion mediated reaction of 4-bromocyclohexa-2,5-dienone 3b with simple alcohols, although they can be irreversibly isomerized into 10 under the reaction conditions. Crude materials with high amounts of 9 can be obtained by conducting the reaction with AgClO4 in the presence of Na2CO3 or with AgOCOCF3 and by interrupting the reaction shortly after the formation of 9 is complete. The AgOCOCF3 reaction produces labile 4-(trifluoroacetoxy)cyclohexa-2,5-dienone 11 also, the formation of which becomes significant as the alcohol becomes bulky. All of 9 prove to be very much susceptible to the prototropic rearrangement into 10 by catalysis with base, acid, or SiO2. Crude dienones 9 can be conveniently prepared directly from phenol 6 by treatment for a short time with Br2 in alcohols containing AgClO4 and Na2CO3. A one-pot synthesis from 6 of 4-oxyfunctionalized 2,6-di-tert-butylphenols, including 10, is also described.
- Omura, Kanji
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p. 7156 - 7161
(2007/10/03)
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- SYNTHESIS OF 4-ALKOXY- AND 4-ACYLOXY-2,6-DI-tert-BUTYLPHENOLS AND QUINOL ETHERS BASED ON THEM
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4-Alkoxy- and 4-acyloxy-2,6-di-tert-butylphenols were obtained by the alkylation and acylation of 2,6-di-tert-butyl-p-dihydroxybenzene.With quinone dioximes under oxidizing conditions the products form quinol ethers.
- Dobronravova, Z. A.,Meshcheryakov, V. I.,Prishchenko, Yu. E.,Gavrilov, L. D.,Vereshchagin, L. I.
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p. 910 - 914
(2007/10/02)
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