Catechols, autoxidation

BIETHYLENE (106-99-0) QH Extremely flammable, polymerizable gas extremely low ignition temperature makes it very dangerous [explosion limits in air (vol %) 2.0 to 11.5 flash point -105°F/-76°C autoignition temp 788°F/420°Ct Fire Rating 4]. Self-reactive. In absence of an inhibitor (e.g., ferf-butyl catechol), autoxidation forms heat-, mechanical shock-, and impact-sensitive peroxides with air. Fires, explosions, or hazardous polymerization may result from contact with strong oxidizers, aliuninumtetrahydroborate, copper, high copper alloys, chlorine dioxide. 

A recent study on catechol autoxidation reported by Sayre and coworkers demonstrated, however, that conjugate addition of water to o-quinone... 

When catechol was oxidized with Mn04 under aprotic conditions, a semiquinone radical ion intermediate was involved. For autoxidations (i.e., with atmospheric oxygen) a free-radical mechanism is known to operate. 

A brief overview on why most of the autoxidation reactions develop complicated kinetic patterns is given in Section II. A preliminary survey of the literature revealed that the majority of autoxidation studies were published on a small number of substrates such as L-ascor-bic acid, catechols, cysteine and sulfite ions. The results for each of these substrates will be discussed in a separate section. Results on other metal ion mediated autoxidation reactions are collected in Section VII. In recent years, non-linear kinetic features were discovered in some systems containing dioxygen. These reactions form the basis of a new exciting domain of autoxidation chemistry and will be covered in Section VIII. 

In non-aqueous solution, the copper catalyzed autoxidation of catechol was interpreted in terms of a Cu(I)/Cu(II) redox cycle (34). It was assumed that the formation of a dinuclear copper(II)-catecholate intermediate is followed by an intramolecular two-electron step. The product Cu(I) is quickly reoxidized by dioxygen to Cu(II). A somewhat different model postulated the reversible formation of a substrate-catalyst-dioxy-gen ternary complex for the Mn(II) and Co(II) catalyzed autoxidations in protic media (35). 

Catalytic Activity of Co(II) and Co(III) Complexes in Autoxidation of DI-ferf-P,UTYI,CATECHOL (52) ... 

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. 

Radical 80 has been prepared as its perchlorate salt by anodic oxidation in ethyl acetate in the presence of hthium perchlorate. The reactivity toward nucleophiles of material so prepared was investigated nitrite and nitrate ions give 2-nitrodibenzo[l,4]dioxin although the mechanisms of the reactions are not clear. Pyridine gives 7V-(2-dibenzo[l,4]dioxinyl)pyridinium ion (84). Other nucleophiles acted as electron donors and largely reduced 80 back to the parent heterocycle they included amines, cyanide ion and water. In an earlier study, the reaction of 80 with water had been examined and the ultimate formation of catechol via dibenzo[l,4]dioxin-2,3-dione was inferred. The cation-radical (80) has been found to accelerate the anisylation of thianthrene cation-radical (Section lII,C,4,b) it has been found to participate in an electrochemiluminescence system with benzo-phenone involving phosphorescence of the latter in a fluid system, and it has been used in a study of relative diffusion coefficients of aromatic cations which shows that it is justified to equate voltammetric potentials for these species with formal thermodynamic redox potentials. The dibenzo[l,4]dioxin semiquinone 85 has been found to result from the alkaline autoxidation of catechol the same species may well be in-... 

The autoxidation of hydroquinone is accompanied by the fonnation of the p-semiquinone radical this was shown as early as 1938 by Michaelis and coworkers Since then numerous additional examples of this type of reaction have been described, relating not only to hydroquinones but also to catechols, resorcinols, pyrogallols, naphthols and substituted phenols. Most of this material has been reviewed including that relating to synthetic application of phenol oxidation and to phenoxyl radicals involved in the biosynthesis of natural products -... 

Fomstedt, B. Bmn, A. Rosengren, E. and Carlsson, A. The apparent autoxidation rate of catechols in dopamine-rich regions of human brains increases with the degree of depigmentation of substantia nigra. J Neural Trans 1 279-295, 1989. 

In a few cases, catechols and hydroquinones have been found to undergo further oxidation to quinones (reactions 1-G and l-I). Such reactions occur by two single-electron steps and can be either enzymatic or nonenzy-matic (i.e., resulting from autoxidation and yielding as by-product the superoxide anion-radical 0, -). The intermediate in this reaction is a semiquinone. Both quinones and semiquinones are reactive, particularly to-... 

Oleuropein appears to interfere with some biological processes such as lipoprotein oxidation, platelet aggregation, platelet and leukocyte eicosanoid production and cardiovascular control too. As previously described, oleuropein and hydroxy-tyrosol are characterised by a catechol moiety that appears to be needed for their scavenger and antioxidant activities. In fact, it was demonstrated that these compounds prevent thermally initiated autoxidation of methyl linoleate in homogenous solutions [56], protect LDL from oxidation [57] and inhibit production of... 

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