Oxidation of catechols

Less is known about the mechanism than is the case for 19-3, but, as in that it seems to vary with the oxidizing agent. For oxidation of catechol with NaI04, it was found that the reaction conducted in H2 0 gave unlabeled quinone, " so the following mechanismwas proposed ... 

Szewzyk R, N Pfennig (1987) Complete oxidation of catechol by the strictly anaerobic sulfate-reducing Desulfobacterium catecholicum sp. nov. Arch Microbiol 147 163-168. 

Kuever J, J Kuhner, S Janssen, U Eischer, K-H Blotevogel (1993) Isolation and characterization of a new spore-forming sulfate-reducing bacterium growing by complete oxidation of catechol. Arch Microbiol 159 282-288. 

The development of catalysts for the efficient oxidation of catechol and its derivatives in water is topic of ongoing work in this laboratory. Towards this end, polyethylene glycol side-chains were incorporated in a pentadentate salen ligand to enhance the water solubility of the complexes derived thereof. A dinuclear copper(II) complex is found to catalyze the oxidation of 3,5-di-tert.-butylcatechol into 3,5-di-tert-butyl-o-benzoquinone more than twice as fast in aqueous organic solution as in purely organic solvents (ly,at/knon= 140,000). Preliminary data are discussed. 

The electrochemical oxidation of catechols in the presence of 6-methyl-1,2,4-triazin-3-thion-5-one and 4-amino-6-methyl-l,2,4-triazin-3-thion-5-one as nucleophiles in aqueous solutions provided an efficient electrosynthesis of thiazolo[3,2-6][l,2,4]triazin-7-one and l,2,4-triazino[3,4-/)J 1,3,4-thiadiazinc derivatives respectively <06TL1713> <06TL8553>. 

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-... 

D. Nematollahi, and Z. Forooghi, Electrochemical oxidation of catechols in the presence of 4-hydroxy-6-methyl-2-pyrone, Tetrahedron 58(24), 4949 953 (2002). 

Cycloaddition Reactions with Other Nucleophiles The anodic two-electron oxidation of catechol affords o-quinone that may react with the enolates of 4-hydroxycoumarine or 5,5-dimethyl-1,3-cyclohexanedione (dimedone). The resulting adducts undergo a second anodic oxidation leading to benzofuran derivatives in good yields (90-95%) (Scheme 53) [75, 76]. 

The combination of anodic oxidation of benzene using the Ag(I)/Ag(II) mediator with cathodic oxidation of benzene using the Cu(I)/Cu(II) mediator in a single electrolytic cell produces p-benzoquinone selectively in both the anodic and the cathodic chambers [242]. Silver-mediator promoted electrooxidation of hydrocarbon has been attempted [243]. The kinetics of indirect oxidation of catechol and L-dopa with IrCl6 has been studied in polymer-coated glassy carbon [244]. 

Mechanistic aspects of the intermolecular cyclization reaction in the anodic oxidation of catechol in the presence of 4-hydroxycoumarin were discussed in Sect. 2.2. This reaction is a synthetically simple and versatile method for the preparation of formally [3 + 2] cycloadducts between a -diketo compound and catechol [44,45]. Anodic oxidation of catechol using controlled potential electrolysis (E = 0.9-1.1 V vs SCE) or constant current electrolysis (i = 5 mA/cm ) was performed in water solution containing sodium acetate (0.15 mol/1) in the presence of various nucleophiles such as 4-hydroxycoumarin,... 

The anodic oxidation of catechol in the presence of 1,3-dimethylbarbituric acid was carried out in aqueous solution containing sodium acetate in an undivided cell at graphite and nickel hydroxide electrodes [114]. The results did not fit with the expected structure (Scheme 47, path A) but a dis-piropyrimidine was isolated in 35% yield (Scheme 47, path B). It seems that the initial attack of 1,3-dimethylbarbituric acid on the anodically formed o-quinone does not occur through the carbon-oxygen bond formation but rather through the carbon-carbon bond formation, giving rise to the final product via several consecutive reaction steps. 

Oxidation of catechol with Ag20 in presence of /3-alanine methyl ester gave a phenoxazine-2,3-dione in low yield, and further condensation with o-phenylenediamine gave the pentacyclic compound 153 [78ZN(C)912],... 

A values have been obtained for oxidation of benzenediols by [Fe(bipy)(CN)4], including the effect of pH, i.e., of protonation of the iron(III) complex, and the kinetics of [Fe(phen)(CN)4] oxidation of catechol and of 4-butylcatechol reported. Redox potentials of [Fe(bipy)2(CFQ7] and of [Fe(bipy)(CN)4] are available. The self-exchange rate constant for [Fe(phen)2(CN)2] has been estimated from kinetic data for electron transfer reactions involving, inter alios, catechol and hydroquinone as 2.8 2.5 x 10 dm moF s (in dimethyl sulfoxide). 

Benzoquinones are conveniently prepared in solution by the anodic oxidation of catechols. 1,2-Quinones are unstable in solution but they have a sufficient lifetime for the redox process to be reversible at a rotating disc electrode. Reaction involves two electrons and two protons and the half-wave potential varies with pH at 25 °C according to Equation 6.1. Some redox potentials for catechols and hy-droquinones are given m Table 6.6. 

Anodic oxidation of catechols enables the unstable quinones to be prepared and reacted in situ. Reaction of the 1,2-quinone with a 1,3-dicarbonyi compound gives a high yield of a benzofuran [123, 124]. Both 1,2- and 1,4-quinones, prepared electrochemically in nitromethane, are efficiently topped in Diels-Alder reactions with butadienes [125]. 

Oxidation of catechols in the presence of a protein may lead to extensive catechol-protein covalent coupling (Figure 9.4) as demonstrated in the case of the chlorogenic acid-BSA couple. Autoxidation of EGCG at pH 4.9 in the presence of Zn(II) cations was shown to generate semiquinone radicals (stabilized by Zn(II) binding) mainly on the B ring moiety. 

Hemocyanin [30,31], tyrosinase [32] and catechol oxidase (2) [33] comprise this class of proteins. Their active sites are very similar and contain a dicopper core in which both Cu ions are ligated by three N-bound histidine residues. All three proteins are capable of binding dioxygen reversibly at ambient conditions. However, whereas hemocyanin is responsible for O2 transport in certain mollusks and arthropods, catechol oxidase and tyrosinase are enzymes that have vital catalytic functions in a variety of natural systems, namely the oxidation of phenolic substrates to catechols (Scheme 1) (tyrosinase) and the oxidation of catechols to o-quinones (tyrosinase and catechol oxidase). Antiferromagnetic coupling of the two Cu ions in the oxy state of these metalloproteins leads to ESR-silent behavior. Structural insight from X-ray crystallography is now available for all three enzymes, but details... 

These studies are part of a rare family of examples of the chemoselec-tive oxidation of catechols (150). The identification of the catalyst and the interception of the catalyst-dioxygen adduct are of particular relevance when the chemistry of catechol dioxygenase and tyrosinase enzymes is concerned. 

Fate of quinone intermediates formed by pH-dependent oxidation of catechols by molecular oxygen. 

Artificial Catechol Humic Acid. This was prepared by K. B. Roy, Department of Macromolecules, Indian Association for the Cultivation of Science, Calcutta, India, by the air oxidation of catechol in the presence of various amino acids. 

Tyrosinase, a copper-containing oxidoreductase, catalyzes the orthohydroxy-lation of monophenols and the aerobic oxidation of catechols. The enzyme activity will be assayed by monitoring the oxidation of 3,4-dihydroxyphenyl-alanine (dopa) to the red-colored dopachrome. The kinetic parameters Ku and Vmax will be evaluated using Lineweaver-Burk or direct linear plots. Inhibition of tyrosinase by thiourea and cinnamate will also be studied. Two stereoisomers, L-dopa and D-dopa, will be tested and compared as substrates. 

Figure 2-4. Auto-oxidation of catechol can result in the formation of different dimers.

The formation of o-quinones by the above oxidative methods is less reliable since the ortho quinonoid system is more susceptible to attack by electrophilic and nucleophilic species mild conditions are therefore essential. The use of the silver carbonate/Celite reagent noted above for phenol coupling reactions is particularly suitable the conditions are those described in Expt 6.129, and they have been applied to the oxidation of catechol, 4-methylcatechol, 4-t-butylcatechol, and 3,5-di-t-butylcatechol to yield the corresponding o-quinones in almost quantitative yield. 

There are other modes of inactivation of pressor amines that probably are not highly significant factors from our present standpoint. These include the effect of the cytochrome C-cytochrome oxidase oxidation of catechol derivatives to the corresponding ortho-quinone (41, 103), the oxidation of the phenolic nucleus by the ascorbic-dehydroascorbic acid system (18), and the deamination of pressor amines in the presence of aldehydes (120, 121,152). One may refer to the reviews by Hartung (89) and by Beyer (23) for discussions of these systems. 

It has been demonstrated that the oxidation of alcohols with hexacyanoferrate(III) (HCF) shows a hyperbolic variation with HCF concentration, and the reaction order varies from one to zero on increasing the HCF concentration. This rate law is obeyed during the initial moments of the reaction and at any subsequent time. These results rule out the possibility that any substance produced during the course of the reaction acts as an activator or inhibitor of the reaction rate. The mixed order has been attributed to the comparable rates of complex decomposition and catalyst regeneration steps.86 HCF acts as a selective oxidizing agent for the oxidation of catechols even in the presence of 2-mercaptobenzoxazole, as an easily oxidizable thiol, to produce related catechol thio ethers.87 Hexacyanoferrate(II) has a retarding effect on the oxidation of vanillin with HCF in alkaline solutions. A mechanism based on the observed kinetics has been proposed 88... 

Other reactions show an even greater resemblance to those which occur in biological systems. A typical example is seen in the smooth oxidation of catechol by dioxygen in the presence of mixed pyridine/methanol solutions containing copper(i) chloride (Fig. 9-28). The cleavage products in this reaction are derived from an intermediate 1,2-quinone. 

Figure 9-28. The ring-opening oxidation of catechol by dioxygen in the presence of copper salts. The first step presumably involves an electron transfer type of process to generate the quinone, followed by the oxygen atom transfer in the second step.

There is evidence that quinone methides form as intermediates in the metabolic oxidation of catechol derivatives, a key step in a variety of biosynthetic processes such as melanization and sclerotization of animal cells. Tyrosinase from mushrooms catalyzes the oxidation of a-methyldopa methyl ester 54a. It has been proposed that this reaction observed in vitro is part of a metabolic pathway for the metabolism of 54a. This reaction proceeds by oxidation of ct-methyl dopa methyl ester 54a to give 54b, which cyclizes and is further oxidized to quinone methide 54c (Scheme 26).101 This quinone methide was identified by comparison to authentic 54c, which was prepared by chemical oxidation of 54a to 54c.102... 

Catechol oxidase catalyzes the oxidation of catechols to the respective quinones through a four-electron reduction of dioxygen to water. Whereas the exact mechanism of the enzymatic conversion remains uncertain, the commonly accepted mechanism is that proposed by Krebs and co-workers [3, 21] (Figure 5.4). The catalytic cycle begins with the met form of catechol oxidase, which is the resting form of the enzyme. The dicopper(II) center of the met form reacts with one... 

A number of copper) I) and copper) 11) complexes with [22]py4pz and [22]pr4pz have been isolated and structurally characterized [47—49]. Their structural and catalytic properties, as well as studies on the mechanism of the catalytic oxidation of catechol performed by some of these compounds, are discussed below. 

The mechanism of the catalytic reaction proved indeed to be very different from that found for [Cu2([22]py4pz)( r-0H)](C104)3 H20. Thus, in the first step of the reaction, a stoichiometric oxidation of catechol by the dicopper(II) complex takes place however, only one electron is transferred in this stoichiometric reaction, resulting in the formation of a semiquinone radical and a mixed-valence Cu"Cu species. Interestingly, the dicopper(II) complex was found to be essentially dinuclear in solution nevertheless, only one of the two copper(II) ions was found to participate in the redox process, whereas the second one played a purely structural... 

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