Quinone formation

Dunlap, T. Chandrasena, R. E. P. Wang, Z. Sinha, V. Wang, Z. Thatcher, G. R. J. Quinone formation as a chemoprevention strategy for hybrid chugs balancing cytotoxity and cytoprotection. Chem. Res. Toxicol. 2007, 20, 1903-1912. 

LaVoie, M.J., Hastings, T.G. Dopamine quinone formation and protein modification associated with the striatal neurotoxicity of methamphetamine evidence against a role for extracellular dopamine. J. Neurosci. 19 1484, 1999. 

The catalytic cycle of laccase includes several one-electron transfers between a suitable substrate and the copper atoms, with the concomitant reduction of an oxygen molecule to water during the sequential oxidation of four substrate molecules [66]. With this mechanism, laccases generate phenoxy radicals that undergo non-enzymatic reactions [65]. Multiple reactions lead finally to polymerization, alkyl-aryl cleavage, quinone formation, C> -oxidation or demethoxylation of the phenolic reductant [67]. 

Metabolism of BP mediated by the cytochrome P-450 monooxygenase system forms three classes of products phenols, dihydrodiols and quinones. Formation of phenols and dihydrodiols is obtained by an initial electrophilic attack of an enzyme-generated oxygen atom. 

Support for this conclusion is provided by the hydroperoxide specificity of BP oxidation. The scheme presented in Figure 6 requires that the same oxidizing agent is generated by reaction of h2°2/ peroxy acids, or alkyl hydroperoxides with the peroxidase. Oxidation of any compound by the iron-oxo intermediates should be supported by any hydroperoxide that is reduced by the peroxidase. This is clearly not the case for oxidation of BP by ram seminal vesicle microsomes as the data in Figure 7 illustrate. Quinone formation is supported by fatty acid hydroperoxides but very poorly or not at all by simple alkyl hydroperoxides or H2C>2. The fact that... 

In a thorough study by Balia and coworkers, it was shown that in slightly acidic aqueous solution (25 °C, /jl = 1.0 KN03), the mechanism is very similar to that followed in aprotic media (36). Parallel measurement of the 02 consumption and quinone formation confirmed the following stoichiometry up to 35% conversion ... 

The mechanism shown in Scheme 5 postulates the formation of a Fe(II)-semi-quinone intermediate. The attack of 02 on the substrate generates a peroxy radical which is reduced by the Fe(II) center to produce the Fe(III) peroxide complex. The semi-quinone character of the [FeL(DTBC)] complexes is clearly determined by the covalency of the iron(III)-catechol bond which is enhanced by increasing the Lewis acidity of the metal center. Thus, ultimately the non-participating ligand controls the extent of the Fe(II) - semi-quinone formation and the rate of the reaction provided that the rate-determining step is the reaction of 02 with the semiquinone intermediate. In the final stage, the substrate is oxygenated simultaneously with the release of the FemL complex. An alternative model, in which 02 attacks the Fe(II) center instead of the semi-quinone, cannot be excluded either. 

The 02-oxidation of hydroquinone into quinone, which is very slow in the absence of a catalyst, was found to be accelerated by the addition of the ce-pyrrolinonate-bridged Pt(2.5 + )4 (19) (117). The detailed kinetic investigation revealed that the Pt(2.0+)2 species formed according to Eq. (1) plays a major role as the catalyst. The reaction rate of the quinone formation is higher than that of 02 oxidation of Pt(2.0+)2 into Pt(3.0 + )2 and was found to be rather linear to the hydroquinone concentration. Therefore, it was suggested that the quinone formation proceeds via a certain intermediate formed between the Pt(2.0+)2 species and molecular oxygen (e.g., peroxo species). The possible schematic mechanism is illustrated in Eq. (12). 

Complex iron(III) salts are frequently used in oxidative arene coupling reactions and quinone formation and tetra-n-butylammonium hexacyanoferrate(III) has several advantages in it use over more conventional oxidative procedures. When used as the dihydrogen salt, Bu4N[H2Fe(CN)6], it oxidizes 2,6-di-z-buty 1-4-methylphenol (1) to the coupled diarylethane (2), or aryl ethers (3) and (4) (Scheme 10.4), depending on the solvent. It is noteworthy that no oxidation occurs even after two days with the tris-ammonium salt. 

Anthracene (404) and its derivatives are reported to yield 9,10-sndoperoxides (405). No mechanistic studies were made with respect to the influence of substituents and their positions on the reactivity of the anthracenes toward oxygen, except those discussed earlier. However, substituent effects have been observed with regard to the thermal stability of the anthracene endoperoxide and its thermal transformation reactions, which either lead to quinone formation or to evolution of oxygen and reformation of the hydrocarbon (Table XIII). 

This product suggests a singlet oxygen mediated mechanism for quinone formation (Lee-Ruff et al., 1986). 

There have been only a few reports of direct hydroxylation362 by an electrophilic process (see, however, 2-26 and 4-5).363 In general, poor results are obtained, partly because the introduction of an OH group activates the ring to further attack. Quinone formation is common. However, alkyl-substituted benzenes such as mesitylene or durene can be hy-droxylated in good yield with trifluoroperacetic acid and boron trifluoride.364 In the case of mesitylene, the product is not subject to further attack ... 

Leo Duffy Does oxidation result in broadening of the C=0 band If so, what is the possibility of quinone formation during oxidation ... 

Dr. Murchison Oxidation of the resinite from lignitous coal does seem to cause broadening of the CO absorption and makes the appearance of this band very similar to that in the spectrum of the oxidized layer from a weathered sample. It is too early to say whether or not this a general feature that affects all oxidized resinite. The possibility of quinone formation during oxidation has not been studied. 

The equilibria governing semi-quinone formation from quinones are similar to those for the flavin semiquinones which were discussed in Section B,6. Two consecutive one-electron redox steps can be defined. Their redox potentials will vary with pH because of a pfCa for the semiquinone in the pH 4.5 -6.5 region. For ubiquinone this pKa is about 4.9 in water and 6.45 in methanol. A pKa of over 13 in the... 

The formation of the products 43 and 45 has also been studied from a mechanistic point of view. Labeling studies with H2180 revealed that two molecules of acetyl-2-iodoxybenzoic acid (formed by the reaction between Dess-Martin Peri-odinane 8 and water) are involved in para-quinone formation. It is suspected that the substituent in 2-position in 42 blocks another molecule of acetyl-2-iodoxybenzoic acid attacking the initially formed product leading to the formation of ortho-imidoquinones. Anilides substituted in the 3-position does lead to complex mixtures in the oxidation reaction. 

This quinone formation can be suppressed by the use of pyridinium acetate as additive [85] or by using MTO immobilized on PVP or PVP oxide, to yield the desired seven-membered lactone ring in moderate to good yields [103],... 

These quinone derivatives, by further reduction, can produce only arnidophenols. If the quinone formation is prevented from taking place, for instance by esterifying the hydroxyl-group, the normal reaction to azoxy-bodies occurs, o- and p-Nitroanisol pass smoothly into azoxy- or azo-derivatives. The acylizing of the amido-group in the case of o- and p-nitroamines hinders likewise the quinone, and. therewith the amine, formation. The azoxy-body is smoothly formed, thus ... 

Refiner mech. pulp o-Quinones Formation of oxyphosphorane adduct 10-12 Lebo et al. 1988... 

---