Catechol oxidation reactions

The stability constants of the mono- and bis-complexes between Cu(II) and catecholate were determined under anaerobic conditions and were found to be the same as reported earlier, i.e. log p1 = 13.64 (CuC) and log p2 24.92 (CuC2+) (36,39). A comparison of the speciation and oxidation rate as a function of pH clearly indicated that the mono-catechol complex is the main catalytic species, though the effect of other complexes could not be fully excluded. The rate of the oxidation reaction was half-order in [02] and showed rather complex concentration dependencies in [H2C]0, [Cu(II)]0 and pH. The experimental data were consistent with the following rate equation ... 

The kinetic experiments were not performed under true catalytic conditions, i.e. the pre-prepared [FeL(DTBC)] complexes were introduced into the reaction mixtures as reactants and excess substrate was not used. Nevertheless, the results are important in exploring the intimate details of the activation mechanisms of the metal ion catalyzed autoxida-tion reactions of catechols. In excess oxygen the reaction was first-order in the complex concentration and the first-order dependence in dioxygen concentration was also confirmed with the BPG complex. As shown in Table II, the rate constants clearly correlate with the Lewis character of the complex, i.e. the rate of the oxidation reaction increases by increasing the Lewis acidity of the metal center. 

For the cracking of catechol and 3-methylcatechol in the presence of iron oxide, reaction pathways for the secondary products appeared to follow the same trend based on the assumption that the possible identities of the products we proposed above were correct. As presented in Scheme 12.1, catechol and 3-methylcatechol were oxidized to their corresponding quinones, 1,2-benzoquinone, and methylbenzoquinone, respectively. This was followed by an expulsion of CO to form cyclopentadienones and further followed by one more CO expulsion to form possibly vinyl acetylene from catechol and pentenyne from 3-methylcatechol. These products were eventually converted to the tertiary products. The formation of quinones was also observed in other studies where the oxidation of catechols was carried out. The formation of secondary products I in our study is in agreement with the previous studies (e.g., Wornet et Therefore, it is reasonable to propose the reaction pathways de-... 

A more direct, one-step approach to phenazines 84 is possible via condensation of o-phenylenediamines 82 with o-benzoquinones 83 that can be generated in situ from oxidation of the specific catechols. As reactions of substituted o-phenylenediamines with substituted o-benzoquinones inevitably suffer from regioselectivity problems, this method is bound to have a limited range of... 

The discovery of the new titanium silicates and of their catalytic properties in H2O2 oxidation reactions has had a major impact in catalytic science and its industrial applications. One 10,000 ton/year plant for the production of catechol and hydroquinone has been operating since 1986 with excellent results. Moreover, successful tests conducted on a 12,000-ton/year pilot plant for cyclohexanone ammoximation (Notari, 1993b) could be followed soon by an industrial-size plant that would greatly simplify the synthesis of caprolactam. Both these examples are clear indications of the potentials of the new oxidation chemistry made possible by the new materials. 

The positive E° values of the overall redox reactions indicate that the reactions are thermodynamically feasible, and catechol oxidation can thus be accelerated by Fe... 

Catechol melanin, a black pigment of plants, is a polymeric product formed by the oxidative polymerization of catechol. The formation route of catechol melanin (Eq. 5) is described as follows [33-37] At first, 3-(3, 4 -dihydroxyphe-nyl)-L-alanine (DOPA) is derived from tyrosine. It is oxidized to dopaquinone and forms dopachrome. 5,6-Dihydroxyindole is formed, accompanied by the elimination of C02. The oxidative coupling polymerization produces a melanin polymer whose primary structure contains 4,7-conjugated indole units, which exist as a three-dimensional irregular polymer similar to lignin. Multistep oxidation reactions and coupling reactions in the formation of catechol melanin are catalyzed by a copper enzyme such as tyrosinase. Tyrosinase is an oxidase con-... 

Scheme 5.2 The possible reaction pathways in the catalytic cycle of catechol oxidation by dicopper(ll) complexes, as proposed by Casella and co-workers. Redrawn after Casella etal. [38],...

An iron(II) catecholate/hydroquinone/02 system oxidizes phenols to catechols. The reaction mimics the action of tyrosine hydroxylase, which gives dihydroxyphe-nylalanine.161... 

The semiquinone radicals are produced by base-induced oxidation of 1,4-dihydroxy-benzene (hydroquinone) or 1,2-dihydroxybenzene (catechol) by molecular oxygen, present in dissolved form. Radical concentration will increase over a period of time as the oxidation reaction proceeds and then decay as radical-radical reaction and other processes destroy the anions. Rates for these processes will depend on temperature, concentration of the dihydroxybenzene, and other parameters, so some experimentation may be necessary to obtain optimal spectra. 

Mechanistic Study of Oxidation Reactions of Hydroquinone, Catechol, and L-Ascorbic Acid by Dicyanobis(l,10-phenanthroline)iron(III) in Dimethyl Sulfoxide... 

The electron exchange rate constant of the iron(III) complex in DMSO was estimated from the cross reactions with hydroquinone and catechol, which was compared with the rate constant obtained electrochemically. The mechanism of the ascorbic acid oxidation reaction in DMSO is discussed based on the Marcus theory. 

Results for these catechols taken from Reference 139, using inhibited oxidation of styrene initiated by AIBN. Earlier n factors for catechols of 2-3 were attributed to reactions of the initial catechol-oxidation products with peroxyl radical. ... 

Even better results are obtained by a post-synthesis treatment of TS-1 with both hydrogen peroxide and ammonium hydrogen fluoride, NH4HF2. Upon such a treatment (H202/F/Ti = 10 2.5 1 60 °C 4h), a substantial amount of titanium (up to 75% of the initial value) is removed. Nevertheless, the crystalline structure of the zeolite remains unchanged and the catalytic activity does not decrease. On the contrary, it actually increases since the turnover frequency of residual titanium atoms rises from 31 to 80 h . Even more importantly, at 8.6% benzene conversion the selectivities, both on benzene and on hydrogen peroxide, also increase from 83 to 94% and from 67 to 83% respectively, with formation of catechol (4%) and hydroquinone (2%) as the only by-products, without any evidence of further oxidation reactions [19]. 

The catecholamines epinephrine, norepinephrine, and dopamine are inactivated by oxidation reactions catalyzed by monoamine oxidase (MAO) (Figure 15.10). Because MAO is found within nerve endings, catecholamines must be transported out of the synaptic cleft before inactivation. (The process by which neurotransmitters are transported back into nerve cells so that they can be reused or degraded is referred to as reuptake.) Epinephrine, released as a hormone from the adrenal gland, is carried in the blood and is catabolized in nonneural tissue (perhaps the kidney). Catecholamines are also inactivated in methylation reactions catalyzed by catechol-O-methyltransferase (COMT). These two enzymes (MAO and COMT) work together to produce a large variety of oxidized and methylated metabolites of the catecholamines. 

Vanillate is transformed by C. thermoaceticum to catechol and phenol and the C02 produced relieves the requirement for supplemental C02 (Hsu et al. 1990) the metabolically produced C02 is then able to enter a sequence of reactions which results in acetate synthesis. Aromatic aldehydes may be involved in both reductive and oxidative reactions under anaerobic conditions, and in some cases the carboxylic acid is further decarboxylated. 

Studies of the oxidation of ferric catecholate coordination complexes have been useful in exploring mechanistic possibilities for these enzymes. A series of ferric complexes of 3,5-di-r-butyl-catechol with different ligands L have been found to react with O2 to give oxidation of the catechol ligand (Reaction 5.58). 

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