- A Catalyst-Controlled Aerobic Coupling of ortho-Quinones and Phenols Applied to the Synthesis of Aryl Ethers
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ortho-Quinones are underutilized six-carbon-atom building blocks. We herein describe an approach for controlling their reactivity with copper that gives rise to a catalytic aerobic cross-coupling with phenols. The resulting aryl ethers are generated in high yield across a broad substrate scope under mild conditions. This method represents a unique example where the covalent modification of an ortho-quinone is catalyzed by a transition metal, creating new opportunities for their utilization in synthesis.
- Huang, Zheng,Lumb, Jean-Philip
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p. 11543 - 11547
(2016/11/17)
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- Multiple oxo-vanadium schiff base containing cyclotriphosphazene as a robust heterogeneous catalyst for regioselective oxidation of naphthols and phenols to quinones
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Grafting of multiple oxo-vanadium Schiff base moieties to cyclotriphosphazene provided an efficient and recyclable heterogeneous catalyst for the regioselective oxidation of naphthols and phenols to quinones by using tbutylhydroperoxide as oxidant. One of the main advantages of the developed catalyst was the presence of multiple oxovanadium moieties in close proximity which made the developed catalyst more active as compared to its homogeneous oxo-vanadium Schiff base. After the reaction, the catalyst was easily recovered from the reaction mixture by simple filtration and reused for five runs without loss in activity.
- Khatri, Praveen K.,Jain, Suman L.
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p. 1020 - 1025
(2012/10/29)
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- Manganese(II) catalyzed oxidation of 2,4-xylidine by periodate - A kinetic-mechanistic study
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The kinetics of the periodate oxidation of 2,4-xylidine (XYL) in acetone-water medium has been followed by monitoring the increase in the absorbance of reaction intermediate, C4, and the main reaction product 3,5-dimethyl-1,2-benzoquinone. Results under pseudo-first order conditions, [IO4-] >> [XYL], are in agreement with the rate law : d[C]/dt = kK3K4Kw[Mn II] [S] [IO4-]0 [H +]/{K2Kw + (Kw + KbK 2)[H+] + Kb[H+]2} where kK3K4 is the empirical composite rate constant, K w is ionic product of water, K2 is acid dissociation constant of H4IO6- and Kb is base dissociation constant of XYL. In agreement with the rate law the 1/k cat versus [H+] profile passes through the minimum. Free radical scavengers do not affect the reaction rate. The values of thermodynamic parameters are : Ea = 4.9 kcal mol-1, A = 8.7 × 103 dm3 mol-1 s-1; ΔS # = -47.2 cal mol-1 K-1, ΔG# = 18.9 kcal mol-1 and ΔH# = 4.3 kcal mol -1.
- Kaushik,Kumar, Dharmendra,Kumar, Anuj,Kumar, Ajay
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experimental part
p. 811 - 816
(2011/04/22)
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- Redox Interactions of Cr(VI) and Substituted Phenols: Products and Mechanism
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The mechanisms of aqueous oxidation-reduction interactions between Cr(VI) and substituted phenols (RArOH) were characterized by kinetic analysis and determinations of reaction products and intermediates. A rapid, peroxidative equilibrium between HCrO4(-) and RArOH forms chromate ester intermediates, as verified by spectroscopy. The subsequent rate-limiting ester decomposition proceeds via innersphere electron transfer. The overall rate dependence on [H(+)] is well accounted for by three parallel redox pathways involving zero, one, and two protons. The two-proton pathway dominates at pH = 5. The parallel reaction rate expression was fitted to data for 4-methyl-, 4-methoxy-, 2,6-dimethoxy-, and 3,4-dimethoxyphenol for pH 1-6. Beside accurately predicting rates for the calibrated conditions, the model predicts a sharp decline in rates at pH >= 6. Rates subsequently measured at pH 7 agreed well with those calculated a priori. Such predictions suggest that the proposed mechanism is robust and accurate. Rate constants were correlated with Hammett-type substituent parameters. Reaction products indicated both one- and two-electron pathways.
- Elovitz, Michael S.,Fish, William
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p. 1933 - 1943
(2007/10/03)
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- Multiple Electron Oxidation of Phenols by an Oxo Complex of Ruthenium(IV)
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The kinetics of oxidation of phenol and alkylated phenol derivatives by (2+) and (2+) (bpy is 2,2'-bipyridine and py is pyridine) to give the corresponding quinones have been studied in aqueous solution and in acetonitrile.The reactions are first order in both phenol and Ru(IV)=O(2+) or Ru(III)-OH(2+).They proceed via a detectable intermediate, which is a Ru(II) complex. 18O isotopic labeling experiments show that transfer of the oxo group from Ru(IV)=O(2+) to phenol is quantitative.The reactions are facile.With Ru(IV)=O(2+) as oxidant k(25 deg C, CH3CN) = 1.9 (+/-0.4) E2M-1s-1; with Ru(III)-OH(2+) as oxidant k(25 deg C, CH3CN) = 4.0 (+/-0.4) E1M-1s-1.On the basis of the rate laws, the magnitude of OH/OD and CH/CD kinetic isotope effects, and the 18O labeling results, the most reasonable mechanisms for oxidation of phenol by Ru(IV)=O(2+) appears to be electrophilic attack on the aromatic ring.For Ru(III)-OH(2+), oxidation appears to occur by CH/H atom transfer in CH3CN and OH/H atom transfer in water.
- Seok, Won K.,Meyer, Thomas J.
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p. 7358 - 7367
(2007/10/02)
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- Mechanism of the Oxidation of NADH by Quinones. Energetics of One-Electron and Hydride Routes
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The kinetics of NADH oxidation by 7 o-benzoquinones and 14 p-benzoquinones were studied by using buffered aqueous solutions and UV/vis spectroscopy.For each quinone the rate law was first order in NADH and first order in quinone.The rate constants varied from 0.0745 to 9220 M-1s-1.Variation of the pH from 6 to 8 gave no change in rate.The use of 4-D and 4,4-D2NADH revealed kinetic isotope effects.The dideutero data gave kH/kD in the range 1.6-3.1 for p-quinones and 4.2 for 3,5-di-tert-butyl-o-quinone.When p-quinones were used, the log k was a linear function of Eo for the quinone/hydroquinone monoanion (Q/QH(1-)) couple with a slope of 16.9 V-1. o-Quinones reacted about 100 times more rapidly, but the same linear relationship with a slope of 16.4 V-1 was observed.Comparisons to data for one-electron-transfer reactions indicate that such mechanisms are not involved.A hydride-transfer mechanism accommodates all the data, and rate-limiting hydrogen atom transfer followed by electron transfer cannot be ruled out.
- Carlson, Brian W.,Miller, Larry L.
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p. 479 - 485
(2007/10/02)
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- Quassinoid Synthesis via o-Quinone Diels-Alder Reactions
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The reaction of 3,5-disubstituted o-quinones (2a, 2c) and 4-chloro-3,5-disubstituted o-quinones (2b, 2d) with simple dienes was investigated as a potential route to the quassinoid skeleton.Quinones 2a and 2c reacted in high yield at the 3,4-position with only a small excess of diene.Attempted equilibration of cis-fused cycloadducts to the trans-fused system failed due to the intervention of a stable enol form, as in 20.Compound 2c with ethyl 3,5-hexadienoate gave 15a, which upon reduction and lactonization provided BCD-ring tricyclic quassinoid analogues 18a and 19a.Again isomerization to the BC trans-fused system was not possible.The chloroquinones showed some preference for Diels-Alder reaction at the 5,6-position, but the additions were characterized generelly by low yields, side reactions, and lessened stereoselectivity.
- Weller, Dwight D.,Stirchak, Eugene P.
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p. 4873 - 4879
(2007/10/02)
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