- Study on the degradation mechanism and pathway of benzene dye intermediate 4-methoxy-2-nitroaniline: Via multiple methods in Fenton oxidation process
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Benzene dye intermediate (BDI) 4-methoxy-2-nitroaniline (4M2NA) wastewater has caused significant environmental concern due to its strong toxicity and potential carcinogenic effects. Reports concerning the degradation of 4M2NA by advanced oxidation process are limited. In this study, 4M2NA degradation by Fenton oxidation has been studied to obtain more insights into the reaction mechanism involved in the oxidation of 4M2NA. Results showed that when the 4M2NA (100 mg L-1) was completely decomposed, the TOC removal efficiency was only 30.70-31.54%, suggesting that some by-products highly recalcitrant to the Fenton oxidation were produced. UV-Vis spectra analysis based on Gauss peak fitting, HPLC analysis combined with two-dimensional correlation spectroscopy and GC-MS detection were carried out to clarify the degradation mechanism and pathway of 4M2NA. A total of nineteen reaction intermediates were identified and two possible degradation pathways were illustrated. Theoretical TOC calculated based on the concentration of oxalic acid, acetic acid, formic acid, and 4M2NA in the degradation process was nearly 94.41-97.11% of the measured TOC, indicating that the oxalic acid, acetic acid and formic acid were the main products. Finally, the predominant degradation pathway was proposed. These results could provide significant information to better understand the degradation mechanism of 4M2NA.
- Guo, Ying,Xue, Qiang,Cui, Kangping,Zhang, Jia,Wang, Hui,Zhang, Huanzhen,Yuan, Fang,Chen, Honghan
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p. 10764 - 10775
(2018/03/26)
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- Synthesis of α-oxygenated ketones and substituted catechols via the rearrangement of N-enoxy- and N-aryloxyphthalimides
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A common approach to the synthesis of α-oxygenated carbonyl compounds and catechols is the treatment of a carbonyl compound or a phenol with an electrophilic oxygen source. As an alternative approach to these important structures, formal [3,3]-rearrangements of N-enoxyphthalimides, N-enoxyisoindolinones, and N-aryloxyphthalimides have been explored. When used in combination with an initial Chan-Lam coupling, these transformations facilitate the dioxygenation of alkenylboronic acids for the synthesis of α-oxygenated ketones and the dioxygenation of arylboronic acids for the synthesis of catechols. The rearrangements of N-enoxyisoindolinones have also been shown to be diastereoselective.
- Kroc, Michelle A.,Patil, Aditi,Carlos, Anthony,Ballantine, Josiah,Aguilar, Stephanie,Mo, Dong-Liang,Wang, Heng-Yen,Mueller, Daniel S.,Wink, Donald J.,Anderson, Laura L.
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p. 4125 - 4137
(2017/06/29)
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- 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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supporting information
p. 11543 - 11547
(2016/11/17)
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- Efficient Biomimetic Hydroxylation Catalysis with a Bis(pyrazolyl)imidazolylmethane Copper Peroxide Complex
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Bis(pyrazolyl)methane ligands are excellent components of model complexes used to investigate the activity of the enzyme tyrosinase. Combining the N donors 3-tert-butylpyrazole and 1-methylimidazole results in a ligand that is capable of stabilising a (μ-η2:η2)-dicopper(II) core that resembles the active centre of tyrosinase. UV/Vis spectroscopy shows blueshifted UV bands in comparison to other known peroxo complexes, due to donor competition from different ligand substituents. This effect was investigated with the help of theoretical calculations, including DFT and natural transition orbital analysis. The peroxo complex acts as a catalyst capable of hydroxylating a variety of phenols by using oxygen. Catalytic conversion with the non-biological phenolic substrate 8-hydroxyquinoline resulted in remarkable turnover numbers. In stoichiometric reactions, substrate-binding kinetics was observed and the intrinsic hydroxylation constant, kox, was determined for five phenolates. It was found to be the fastest hydroxylation model system determined so far, reaching almost biological activity. Furthermore, Hammett analysis proved the electrophilic character of the reaction. This sheds light on the subtle role of donor strength and its influence on hydroxylation activity.
- Wilfer, Claudia,Liebh?user, Patricia,Hoffmann, Alexander,Erdmann, Hannes,Grossmann, Oleg,Runtsch, Leander,Paffenholz, Eva,Schepper, Rahel,Dick, Regina,Bauer, Matthias,Dürr, Maximilian,Ivanovic-Burmazovic, Ivana,Herres-Pawlis, Sonja
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supporting information
p. 17639 - 17649
(2015/12/08)
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- H2O2 in WEB: a highly efficient catalyst system for the Dakin reaction
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Without using any transition metal catalyst, ligand, base, toxic or hazardous reagent, additives/promoters and organic solvent, green Dakin reactions have been successfully carried out by using H2O2 in a natural feedstock extract. The reaction proceeds in neat 'Water Extract of Banana' (WEB) at room temperature under aerobic conditions in very short reaction times and, therefore, it is an evergreen and environmentally sound alternative to the existing protocols for the Dakin reaction. In our system, the reaction was found to afford excellent yield for the desired product with different electron-withdrawing and electron-donating hydroxylated benzaldehydes.
- Saikia, Bishwajit,Borah, Parinita,Barua, Nabin Chandra
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supporting information
p. 4533 - 4536
(2015/09/15)
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- A new avenue to the Dakin reaction in H2O2-WERSA
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We have developed a novel protocol to realize the Dakin reaction in a more greener way. In fact, by the use of H2O2-WERSA, we can oxidize aromatic arylaldehydes to phenols at room temperature. It is remarkable that the catalytic system does not require activation or any toxic ligand, additive/promoter, transition metal catalyst, base, organic solvent and so on. A range of substituted hydroxylated benzaldehydes were screened to investigate the scope of this protocol.
- Saikia, Bishwajit,Borah, Parinita
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p. 105583 - 105586
(2015/12/30)
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- Selective ortho-hydroxylation-defluorination of 2-fluorophenolates with a Bis(μ-oxo)dicopper(III) species
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The bis(μ-oxo)dicopper(III) species [CuIII 2(μ-O)2(m-XYLMeAN)]2+ (1) promotes the electrophilic ortho-hydroxylation-defluorination of 2-fluorophenolates to give the corresponding catechols, a reaction that is not accomplishable with a (η2:η2-O2) dicopper(II) complex. Isotopic labeling studies show that the incoming oxygen atom originates from the bis(μ-oxo) unit. Ortho-hydroxylation-defluorination occurs selectively in intramolecular competition with other ortho-substituents such as chlorine or bromine. O in, F out: [CuIII2(μ-O) 2(m-XYLMeAN)]2+ is a bis(μ-oxo)dicopper(III) species and promotes the electrophilic ortho-hydroxylation-defluorination of 2-fluorophenolates to give the corresponding catechols. Isotopic labeling shows that the incoming oxygen atom originates from the bis(μ-oxo) unit. Ortho-hydroxylation-defluorination occurs selectively in intramolecular competition with other ortho-substituents such as chlorine or bromine.
- Serrano-Plana, Joan,Garcia-Bosch, Isaac,Miyake, Ryosuke,Costas, Miquel,Company, Anna
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supporting information
p. 9608 - 9612
(2014/10/15)
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- Catalytic phenol hydroxylation with dioxygen: Extension of the tyrosinase mechanism beyond the protein matrix
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A new catalyst (see structure) hydroxylates phenols with O2 via a stable side-on peroxide complex, which is similar to the active site of tyrosinase in terms of the ligand environment and its spectroscopic properties. The catalytic oxidation of phenols to quinones proceeds at room temperature in the presence of NEt3 and even non-native substrates can be oxidized catalytically. The reaction mechanism is analogous to that of the enzyme-catalyzed reaction. Copyright
- Hoffmann, Alexander,Citek, Cooper,Binder, Stephan,Goos, Arne,Ruebhausen, Michael,Troeppner, Oliver,Ivanovic-Burmazovic, Ivana,Wasinger, Erik C.,Stack, T. Daniel P.,Herres-Pawlis, Sonja
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supporting information
p. 5398 - 5401
(2013/06/27)
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- Hydroxylation of p-substituted phenols by tyrosinase: Further insight into the mechanism of tyrosinase activity
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A study of the monophenolase activity of tyrosinase by measuring the steady state rate with a group of p-substituted monophenols provides the following kinetic information: kcatm and the Michaelis constant, KMm. Analysis of these data taking into account chemical shifts of the carbon atom supporting the hydroxyl group (δ) and σp+, enables a mechanism to be proposed for the transformation of monophenols into o-diphenols, in which the first step is a nucleophilic attack on the copper atom on the form Eox (attack of the oxygen of the hydroxyl group of C-1 on the copper atom) followed by an electrophilic attack (attack of the hydroperoxide group on the ortho position with respect to the hydroxyl group of the benzene ring, electrophilic aromatic substitution with a reaction constant ρ of -1.75). These steps show the same dependency on the electronic effect of the substituent groups in C-4. Furthermore, a study of a solvent deuterium isotope effect on the oxidation of monophenols by tyrosinase points to an appreciable isotopic effect. In a proton inventory study with a series of p-substituted phenols, the representation of kcatfn/kcatf0 against n (atom fractions of deuterium), where kcatfn is the catalytic constant for a molar fraction of deuterium (n) and kcatf0 is the corresponding kinetic parameter in a water solution, was linear for all substrates. These results indicate that only one of the proton transfer processes from the hydroxyl groups involved the catalytic cycle is responsible for the isotope effects. We suggest that this step is the proton transfer from the hydroxyl group of C-1 to the peroxide of the oxytyrosinase form (Eox). After the nucleophilic attack, the incorporation of the oxygen in the benzene ring occurs by means of an electrophilic aromatic substitution mechanism in which there is no isotopic effect.
- Munoz-Munoz, Jose Luis,Berna, Jose,Garcia-Molina, Maria del Mar,Garcia-Molina, Francisco,Garcia-Ruiz, Pedro Antonio,Varon, Ramon,Rodriguez-Lopez, Jose N.,Garcia-Canovas, Francisco
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scheme or table
p. 228 - 233
(2012/10/18)
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- Organocatalytic Dakin oxidation by nucleophilic flavin catalysts
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Flavin catalysts perform the first organocatalytic Dakin oxidation of electron-rich arylaldehydes to phenols under mild, basic conditions. Catechols are readily prepared, and the oxidation of 2-hydroxyacetophenone was achieved. Aerobic oxidation is displayed in the presence of Zn(0) as a reducing agent. This reactivity broadens the scope of biomimetic flavin catalysis in the realm of nucleophilic oxidations, providing a framework for mechanistic investigations for related oxidations, such as the Baeyer-Villiger oxidation and Weitz-Scheffer epoxidation.
- Chen, Shuai,Hossain, Mohammad S.,Foss, Frank W.
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supporting information; experimental part
p. 2806 - 2809
(2012/08/07)
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- Electrophilic arene hydroxylation and phenol O-H oxidations performed by an unsymmetric μ-I·1:I·1-O 2-peroxo dicopper(II) complex
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Reactions of the unsymmetric dicopper(II) peroxide complex [Cu II2(μ-I·1: I·1-O2)(m-XYLN3N4)]2+ (1O2, where m-XYL is a heptadentate N-based ligand), with phenolates and phenols are described. Complex 1O2 reacts with p-X-PhONa (X=MeO, Cl, H, or Me) at -90°C performing tyrosinase-like ortho-hydroxylation of the aromatic ring to afford the corresponding catechol products. Mechanistic studies demonstrate that reactions occur through initial reversible formation of metastable association complexes [CuII2(μ- I·1:I·1-O2)(p-X-PhO) (m-XYLN3N4)]+ (1O2·X-PhO) that then undergo ortho-hydroxylation of the aromatic ring by the peroxide moiety. Complex 1O2 also reacts with 4-X-substituted phenols p-X-PhOH (X=MeO, Me, F, H, or Cl) and with 2,4-di-tert-butylphenol at -90°C causing rapid decay of 1O2 and affording biphenol coupling products, which is indicative that reactions occur through formation of phenoxyl radicals that then undergo radical C-C coupling. Spectroscopic UV/Vis monitoring and kinetic analysis show that reactions take place through reversible formation of ground-state association complexes [CuII2(μ- I·1:I·1-O2)(X-PhOH) (m-XYLN3N4)]2+ (1O2·X-PhOH) that then evolve through an irreversible rate-determining step. Mechanistic studies indicate that 1O2 reacts with phenols through initial phenol binding to the Cu2O2 core, followed by a proton-coupled electron transfer (PCET) at the rate-determining step. Results disclosed in this work provide experimental evidence that the unsymmetric 1O2 complex can mediate electrophilic arene hydroxylation and PCET reactions commonly associated with electrophilic Cu2O2 cores, and strongly suggest that the ability to form substrate·Cu2O2 association complexes may provide paths to overcome the inherent reactivity of the O 2-binding mode. This work provides experimental evidence that the presence of a H+ completely determines the fate of the association complex [CuII2(μ-I·1: I·1-O2)(X-PhO(H))(m-XYLN3N4)] n+ between a PCET and an arene hydroxylation reaction, and may provide clues to help understand enzymatic reactions at dicopper sites.
- Garcia-Bosch, Isaac,Ribas, Xavi,Costas, Miquel
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supporting information; body text
p. 2113 - 2122
(2012/04/10)
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- Synthesis of catechols from phenols via Pd-catalyzed silanol-directed C-H oxygenation
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A silanol-directed, Pd-catalyzed C-H oxygenation of phenols into catechols is presented. This method is highly site selective and general, as it allows for oxygenation of not only electron-neutral but also electron-poor phenols. This method operates via a silanol-directed acetoxylation, followed by a subsequent acid-catalyzed cyclization reaction into a cyclic silicon-protected catechol. A routine desilylation of the silacyle with TBAF uncovers the catechol product.
- Huang, Chunhui,Ghavtadze, Nugzar,Chattopadhyay, Buddhadeb,Gevorgyan, Vladimir
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p. 17630 - 17633
(2011/12/16)
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- Efficient ortho-oxidation of phenols with diacyl peroxides
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A stable symmetric diacyl peroxide, m-chlorobenzoyl peroxide (mCBPO), and an asymmetric diacyl peroxide, chloroacetyl m-chlorobenzoyl peroxide (CAMCBPO), were synthesized from m-chloroperbenzoic acid. Both peroxides oxidized phenols selectively at the ortho position predoninantly. CAMCBPO gave para-oxidized compounds as minor products from some phenols. The improvement of the yield of ortho-oxidation of phenols with mCBPO was also reported.
- Tada, Masahiro,Ishiguro, Risa,Izumi, Ryohei
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p. 239 - 242
(2008/09/21)
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- Significant enhancement of monooxygenase activity of oxygen carrier protein hemocyanin by urea
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Oxygenation of a series of p-substituted phenols to the corresponding catechols (phenolase activity) by the (μ-η2:η2-peroxo)dicopper(II) species of Octopus hemocyanin has been directly examined for the first time by using a UV-vis spectroscopic method in a 0.5 M borate buffer solution containing 8 M urea under anaerobic conditions. Preliminary kinetic studies have indicated that the reaction involves an electrophilic aromatic substitution mechanism as in the case of phenolase reaction of tyrosinase. The oxygenation of phenols by hemocyanin also proceeded catalytically when the reaction was carried out under aerobic conditions. Copyright
- Morioka, Chiyuki,Tachi, Yoshimitsu,Suzuki, Shinnichiro,Itoh, Shinobu
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p. 6788 - 6789
(2007/10/03)
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- A new approach to sesquiterpene arenes of the 9,11-drimenyl type (=[(1E,2RS,4aRS,8aRS)-octahydro-2,5,5,8a-tetramethylnaphthalen-1(2H)-ylidene] methyl type)
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A new reaction sequence for the synthesis of the sesquiterpene arenes (±)-wiedendiol B ((±)-1) and the siphonodictyal B derivative (±)- 21 consists in the coupling of (±)-drimanoyl chloride ((±)-3) with lithiated and appropriately substituted aromatic synthons to furnish the ketones (±)-7 and (±)-17 which were reduced to the benzyl alcohols (±)-8a,b and (±)-18a,b, respectively (Schemes 5, 4, and 12). The 9,11-double bond of the drimenes (±)-9 and (±)-19 was formed by elimination of H2O from the benzyl alcohols (±)-8a,b and (±)-18a,b (Schemes 6 and 12). New alternatives were applied to this elimination reaction involving either the pyridine · SO3 complex or chloral as reagents.
- Bernet, Andreas,Seifert, Karlheinz
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p. 784 - 796
(2007/10/03)
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- Effects of electron-withdrawing substituents on DPPH radical scavenging reactions of protocatechuic acid and its analogues in alcoholic solvents
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The DPPH (2,2-diphenyl-1-picrylhydrazyl) radical scavenging activity of protocatechuic acid (3,4-dihydroxybenzoic acid) and its related catechols was examined. Compounds possessing strong electron-withdrawing substituents showed high activity. NMR analysis of the reaction mixtures of catechols and DPPH radical in methanol showed the formation of methanol adducts. The results suggest that high radical scavenging activity of catechols in alcohol is due to a nucleophilic addition of an alcohol molecule on o-quinones, which leads to a regeneration of a catechol structure. Furthermore, the radical scavenging activity in alcohols would largely depend on the electron-withdrawing/donating substituents, since they affect the susceptibility toward nucleophilic attacks on o-quinone.
- Saito, Shizuka,Kawabata, Jun
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p. 8101 - 8108
(2007/10/03)
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- Nitration and hydroxylation of substituted phenols by peroxynitrite. Kinetic feature and an alternative mechanistic view
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The reaction of peroxynitrite (ONOO-) with a series of para-substituted phenols has been examined in aqueous phosphate buffer and acetonitrile solutions. Major products were the corresponding 2-nitro derivative and the 4-substituted catechol. Kinetic study showed good correlation with Hammett σ(p)+ parameters and reduction potentials, suggesting the possible one-electron transfer process involving the nitrosoniun ion (NO+) as initial electrophile generated from peroxynitrous acid.
- Nonoyama, Nobuaki,Chiba, Kazuhiko,Hisatome, Kaori,Suzuki, Hitomi,Shintani, Futoshi
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p. 6933 - 6937
(2007/10/03)
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- Comparison of Substituted 2-Nitrophenol Degradation by Enzyme Extracts and Intact Cells
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The first catabolic pathway enzyme, nitrophenol oxygenase, transforms o-nitrophenol (ONP) to catechol. Thirteen of 16 substituted nitrophenols tested were actively transformed by both enzyme preparations and intact cells yielding a wide range of Km (Ks) and Vmax. Individual chemicals in binary mixtures demonstrated competitive inhibition. Chemical and physical characteristics (electron withdrawal, size, and position of substitution on the 2-nitrophenol ring) affected degradation kinetics. The strongest correlation were between Km or Vmax values and electron withdrawal, though there was also evidence for effects relating to position and size of substitution on the aromatic ring. Kinetic parameters determined for enzyme preparations did not correlate to those determined for intact cells. Though enzyme reactivity ultimately determined whether a given chemical would be transformed, the transformation by intact cells was apparently affected by factors other than those directly impacting the initial catabolic enzyme.
- Folsom, Brian R.,Stierli, Ruth,Schwarzenbach, Rene P.,Zeyer, Josef
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p. 306 - 311
(2007/10/03)
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- Mechanistic Studies of Selective Catechol Formation from o-Methoxyphenols using a Copper(II)-Ascorbic Acid-Dioxygen System
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Mechanistic details of selective conversion of o-methoxyphenols to the corresponding catechols using a Cu2+-ascorbic acid-O2 system, were studied. 2,5-Dimethoxyphenol was converted predominantly to the o-demethylated compound and partially to the m-demethylated one.Anisole with no phenolic hydroxy group was much less reactive.When guaiacol (1). guaiacol and 2-methoxy-6-methoxyphenol was used as substrates, moderate intermolecular and intramolecular kinetic isotope effects were observed (1.4-1.9). Catechol was derived from 1 in nine-fold excess over catechol when the reaction was run in an 18O2 atmosphere with natural water as a solvent, though no catechol was formed when using natural O2 and H218O.It was determined that the Cu2+-ascorbic acid-O2 system operates in a monooxygenase mode because the oxygen atom of dioxygen (not water) was incorporated into the products, and this oxidative conversion proceeded mainly via ipso-substitution at the methoxy position, probably with hydroxyl radical coordinated to the cupric ion as the active oxygen species.
- Aihara, Kazuhiro,Urano, Yasuteru,Higuchi, Tsunehiko,Hirobe, Masaaki
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p. 2165 - 2170
(2007/10/02)
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- Sodium Percarbonate: A Convenient Reagent for the Dakin Reaction
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Sodium percarbonate, a readily available, inexpensive and easy to handle reagent efficiently oxidizes hydroxylated benzaldehydes and hydroxylated acetophenones to hydroxyphenols.
- Kabalka, G. W.,Reddy, N. K.,Narayana, C.
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p. 865 - 866
(2007/10/02)
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- Evaluation of the cytotoxic potential of catechols and quinones structurally related to butylated hydroxyanisole
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The cytotoxicity of 2- and 3-butylated hydroxyanisole (BHA) and 18 related aromatic compounds has been determined employing cultured P388 and KB cells. The phenolic compounds, 3-BHA and 2-BHA, had moderately low cytotoxic activity. Their corresponding catechols had ED50 values that were much lower than those of the parent compounds. This substantial increase in the cytotoxic activity is attributed to the presence of the catechol group, which is known to undergo one-electron oxidation readily to give the corresponding semiquinone radical. Other related catechols had similar cytotoxic activity. In general, derivatization of the catechol functionality resulted in a decrease of the cytotoxic potential of the compounds. Monoacetylation or monomethylation of the catechols gave products that were less potent cytotoxic agents than the parent compounds. Further loss of activity was observed when both hydroxy groups of the catechol function were blocked. Substitution of a methoxy group in place of a hydrogen atom in these compounds resulted in a significant increase of cytotoxicity, whereas the replacement of a methoxy group with a methyl group reduced the cytotoxicity. The catechols and quinones derived from 2-BHA were more active when compared with those derived from 3-BHA. The t-butyl group adjacent to the catechol or quinone moiety in the 3-BHA derivatives appeared to exert a significant steric effect toward the cytotoxic potential of these compounds. These results suggest the potential use of o-quinones and catechols as cytotoxic and antitumor agents.
- Lam,Garg,Swanson,Pezzuto
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p. 393 - 395
(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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- ORTHO-HYDROXYLATION SELECTIVE DES PHENOLS : II - UN NOUVEAU SYSTEME CATALYTIQUE A CARACTERE PREPARATIF.
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A direct synthetic transformation of phenols into copper(II) catecholates (i.e. catechols) is described, which involves a copper-catalyzed activation of molecular oxygen.The mechanism and scope of this new reaction are discussed.
- Capdevielle, Patrice,Maumy, Michel
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p. 1577 - 1580
(2007/10/02)
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- Hair dyeing composition containing an aryldiamine and a substituted catechol
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A composition for use in the dyeing of keratinous fibre such as hair includes an aqueous anaerobic solution of an aryldiamine and a substituted catechol. Optionally, an aromatic coupling agent can also be incorporated in the composition to modify the shade of color produced. Anaerobic storage conditions can, for example, be maintained by packing the composition in an aerosol container with a halocarbon propellant.
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