Autooxidation Mechanism of Pyrogallol,Its Free Radical Effects Studied by Means of ESR,UV, NMR

Pyrogallol(pyrogallic acid) is easy to be auto-oxidized, forming a free radical under alkaline condition, but the mechanism has not been clear. The mechanism of the auto-oxidation of pyrogallol was studied with ESR, UV, NMR techniques, it was found that pyrogallol was autooxidized to a quinone radical under alkaline condition. And at the same time oxygen recieved an electron from the reaction to form superoxide anions, the hydroxyl free radical was then produced by Haber-Weiss reaction. The oxygen free radicals generated from this process promoted the auto-oxidation of pyrogallol.     

邻苯三酚自氧化反应的动力学研究

利用单扫示波极谱法(并辅以紫外-可见光谱法)研究了邻苯三酚在pH7.20～9.10的Tris－HCl缓冲体系中自氧化反应的动力学.结果表明, 在过量溶解O₂存在时, 反应对邻苯三酚为一级.在一定pH范围内, 准一级反应表现速率常数K_app随pH值的增加而增加, 说明H₂PYR-比H₃PYR对溶解O₂具有更高的反应活性.探讨了邻苯三酚自氧化生成半醒自由基和活性氧的反应机理, 用速率方程解释了实验现象.并为酶活力测定和药物抗氧活性研究提供了一种新方法.     

Mechanism of base-catalyzed autooxidation of corticosteroids containing 20-keto-21-hydroxyl side chain

Corticosteroids and related compounds containing the 20-keto-21-hydroxyl side chain such as betamethasone, betamethasone 9,11-epoxide, dexamethasone, and dexamethasone 9,11-epoxide have been found to undergo facile autooxidation on the 1,3-dihydroxyacetone side chain of their D-rings under strong alkaline conditions to yield five main degradants (17-formyloxy-17-acid, 17-acid, 21-aldehyde, 20-hydroxy-21-acid, and 17-ketone). The rate of the autooxidation was correlated with the strength and concentration of the base used in the reaction. A novel mechanism for the observed autooxidation is proposed, in which the facile oxidation of the presumed enolate (resulting from the carbanion at the 21-position) by molecular oxygen is the key step. The proposed autooxidation mechanism, supported by LC-MS isotope experiments using ¹⁸O₂ as the oxidant, can satisfactorily explain the oxidative degradation patterns observed for corticosteroids containing the 20-keto-21-hydroxyl side chain.     

Mechanism of action of base-catalyzed oxygenation of phenol derivatives

Cyclopropyl derivative of 2,6-di-tert-butylphenol is synthesized as a probe to investigate the mechanism of base-catalyzed autooxidation of phenol derivatives. Our study indicates that one electron reduction of molecular oxygen from phenolate gives phenoxyl radical 3, a key intermediate of autooxidation. The coupling of phenoxyl radical and superoxide radical gives peroxylate anion 4 and produces the final epoxy alcohol adduct 6.     

Revising the mechanism of polymer autooxidation

The basic scheme for autooxidation of polymers, originally developed by Bolland, Gee and co-workers for rubbers and lipids, is now widely applied to all types of polymeric materials. According to their scheme, the reaction that makes this process autocatalytic, referred to as the propagation step, is a hydrogen abstraction from the next substrate by the peroxyl radical (ROO˙ + RH → ROOH + R˙). In this study, using advanced quantum-chemical methods, we have shown that this step is actually characterised by largely positive Gibbs free energy (10-65 kJ mol(-1)) for most regular polymers with saturated chains (polypropylene, polyethylene, polyvinyl chloride, polyvinyl acetate, polyurethane, poly(methyl methacrylate) etc.) and even some polymers with unsaturated fragments (polystyrene, polyethylene terephthalate). Neither elevated temperature, nor solvation makes this process thermodynamically favourable. Only when the formed radical centre is conjugated with adjacent double bonds (as in polybutadiene) or captodatively stabilised by two suitable functional groups (such as a carbonyl and a lone pair donor such as oxygen or nitrogen), is the propagation step exoergic. Instead, we show that it is the presence of structural defects, such as terminal or internal double bonds, formed either during polymerisation or in the degradation process itself, that is responsible for the autooxidation of most polyesters and most polyalkenes. Recognition of the real mechanism of autooxidation in polymers is a key to developing strategies for the prevention of their degradation.     

Transformation of Cedrane Skeleton to Noracorane and Decahydroazulene Skeletons via Free Radical Reaction. Autooxidation of 8β-Hydroxycedran-13-Al

8-Cedren-14-ol ( 4d ) was oxidized with PCC, followed by autooxidation to yield 8,14-cedranolide ( 2a ) and 8-cedren-14-carboxylic peracid ( 4e ). 8β-Hydroxycedran-l3-al ( 1d ) was subjected to autooxidation in three different solvents to give eight products, from which the transformation from cedrane skeleton to noracorane and decahydroazulene skeletons were observed. A free radical pathway was proposed.     

Radical formation during phenol oxidation in aqueous media

Kinetics of oxygen consumption, reaction product formation, and chemiluminescence during polyphenol oxidation by molecular oxygen in alkaline aqueous media with additions of l ‐ascorbic acid (AscH₂) and homocysteine (HCys) has been investigated. In these processes, AscH₂ and HCys have been shown to act as typical radical‐reaction inhibitors that can be used for determinations of the radical formation rates. The rates of radical formation during oxidation of hydroquinone (p‐QH₂), chlorohydroquinone (Cl‐QH₂), 2,5‐dichlorohydroquinone (2,5Cl‐QH₂), catechol (PK), 4‐methylcatechol (4CH₃‐PK), pyrogallol (PG), gallic acid (GK) have been estimated. © 2012 Wiley Periodicals, Inc. Int J Chem Kinet 44: 414–422, 2012     

Autooxidized Polyunsaturated Oils/Oily Acids: Post-it Applications and Reactions with Fe(III) and Adhesion Properties

Soybean oil, sesame oil, linoleic acid and linolenic acid were epoxidized, peroxidized and hydroperoxidized via autooxidation under air oxygen and sunlight at room temperature to obtain novel post-it materials. Polymeric soybean oil peroxide and sesame oil peroxide were containing soluble part of 60%(w/w) together with crosslinked part of 40%(w/w) while polymeric linoleic and polymeric linolenic acids were completely soluble. The autooxidized soluble products with Mn varying between 800 and 3100 Daltons were used as post-it adhesive. The highest adhesion was observed in the case of polymeric soybean oil (3.0 Newton), while adhesion of commercial epoxidized soybean oil, polymeric linoleic and polymeric linolenic acid were 0.8, 0.5 and 0.5 Newton, respectively. Reactions of the autooxidized soluble products with Fe(NO₃)₃. 4H₂O in the presence of ethanol, glycerol and diethyleneglycol gave the hydroxy functionalized products with the same M_n values and indicating no adhesive properties. When the commercial epoxidized soybean oil was reacted with Fe(NO₃)₃. 4H₂O in the presence of the alcohols, Mn of the hydroxy functionalized polymeric oil was found to be unchanged. ¹H NMR, FT-IR, SEM and GPC techniques were used in the characterization of the products obtained.     

Kinetics and mechanism of hemoglobin oxidation by nitroethane

In the present paper the process of oxyhemoglobin oxidation by nitroethane has been investigated. The main process is accompanied with numerous side reactions including oxidative denitration of nitroethane resulting in the generation of acetaldehyde and 1,1-dinitroethane. The latter product is formed under the action of nitrite ion which is the product of oxidative denitration of nitroethane. The chain radical mechanism of methemoglobin generation is proposed. The reaction of oxyhemoglobin with nitroethane is regarded as initiated autooxidation of oxyhemoglobin.     

Chemiluminescence of Colloidal MnO2 with Luminol and Determination of Polyhydroxyl Compounds

The reaction between luminol and colloidal MnO₂ (prepared by chemical reduction of KMnO₄ with Na₂S₂O₃ under neutral aqueous condition) produced an intense chemiluminescence (CL) emission in alkaline medium. The CL reaction conditions were carefully optimized and the CL reaction mechanism was thoroughly discussed. Manganese(III) was suggested to be involved in the reaction and 3‐aminophthalate anion was the luminophor. Moreover, the effects of 23 compounds on the colloidal MnO₂‐luminol CL system were investigated to explore its possible analytical applications. Polyhydroxyl compounds were observed to inhibit the signal significantly, whereas sulfhydryl compounds enhance it slightly. The analytical figures for five polyhydroxyl compounds, namely ascorbic acid, rutin, pyrogallol, quercetin, and L‐adrenaline, were presented. As a preliminary application, the method was applied to the determination of rutin in pharmaceutical formulations.     

Electron Transfer‐Initiated Epoxidation and Isomerization Chain Reactions of β‐Caryophyllene

The abundant sesquiterpene β‐caryophyllene can be epoxidized by molecular oxygen in the absence of any catalyst. In polar aprotic solvents, the reaction proceeds smoothly with epoxide selectivities exceeding 70 %. A mechanistic study has been performed and the possible involvement of free radical, spin inversion, and electron transfer mechanisms is evaluated using experimental and computational methods. The experimental data—including a detailed reaction product analysis, studies on reaction parameters, solvent effects, additives and an electrochemical investigation—all support that the spontaneous epoxidation of β‐caryophyllene constitutes a rare case of unsensitized electron transfer from an olefin to triplet oxygen under mild conditions (80 °C, 1 bar O₂). As initiation of the oxygenation reaction, the formation of a caryophyllene‐derived radical cation via electron transfer is proposed. This radical cation reacts with triplet oxygen to a dioxetane via a chain mechanism with chain lengths exceeding 100 under optimized conditions. The dioxetane then acts as an in situ‐formed epoxidizing agent. Under nitrogen atmosphere, the presence of a one‐electron acceptor leads to the selective isomerization of β‐caryophyllene to isocaryophyllene. Observations indicate that this isomerization reaction is a novel and elegant synthetic pathway to isocaryophyllene.     

Off-pathway, oxygen-dependent thiamine radical in the Krebs cycle

The catalytic cofactor thiamine diphosphate is found in many enzymes of central metabolism and is essential in all extant forms of life. We demonstrate the presence of an oxygen-dependent free radical in the thiamine diphosphate-dependent Escherichia coli 2-oxoglutarate dehydrogenase, which is a key component of the tricarboxylic acid (Krebs) cycle. The radical was sufficiently long-lived to be trapped by freezing in liquid nitrogen, and its electronic structure was investigated by electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR). Taken together, the spectroscopic results revealed a delocalized pi radical on the enamine-thiazolium intermediate within the enzyme active site. The radical is generated as an intermediate during substrate turnover by a side reaction with molecular oxygen, resulting in the continuous production of reactive oxygen species under aerobic conditions. This off-pathway reaction may account for metabolic dysfunction associated with several neurodegenerative diseases. The possibility that the on-pathway reaction may proceed via a radical mechanism is discussed.     

Transition-metal complexes as catalysts of active oxygen species formation in autooxidation reactions

Water-soluble Co and Fe phthalocyanines catalyze autooxidation of ascorbic acid and hydroquinone. Co and Fe phthalocyanines with cationic substituents in macroligands and also sulfo- and carboxy-substituted phthalocyanines were found to be the most active. Using the specific radical traps, the formation of active oxygen species (O₂ ^–., OH^–) in the course of the dark reaction of autooxidation was established.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 2105–2111, August, 1996.     

竹红菌素自敏光氧化反应机制的研究

本文对竹红菌素自敏光氧化反应的机制作了较详细的研究,竹红菌素通过自敏光氧化反应生成不稳定的过氧化物,它可以放出¹O₂回到母体化合物,也可以转化为稳定的氧化产物,我们用活泼的单重态氧的接受体捕获到了体系中放出的¹O₂,用吸收光谱的变化证明过氧化物回到了母体化合物。文中还用猝灭实验证实此自敏光氧化反应除涉及¹O₂机制外,还有其它机制起着作用。     

A comparative study of ruthenium(III) catalysed oxidation of L-leucine and L-isoleucine by alkaline permanganate. A kinetic and mechanistic approach

The kinetics of the Ru^III-catalysed oxidation of L-leucine and L-isoleucine by alkaline permanganate were studied and compared, spectrophotometrically using a rapid kinetic accessory. The reaction is first order with respect to [oxidant] and [catalyst] with an apparently less than unit order in [substrate] and [alkali] respectively. The results suggest the formation of a complex between the amino acid and the hydroxylated species of ruthenium(III). The complex reacts further with the alkaline permanganate species in a rate-determining step, resulting in the formation of a free radical, which again reacts with the alkaline permanganate species in a subsequent fast step to yield the products. The reaction constants involved in the mechanism were calculated. There is a good agreement between observed and calculated rate constants under different experimental conditions. The activation parameters with respect to the slow step of the mechanism for both the amino acids were calculated and discussed. Of the two amino acids, leucine is oxidised at a faster rate than isoleucine.     

自由基引发剂用于渣油减黏裂化的研究

基于热裂化按照自由基反应机理进行的特点,在高压釜式反应器中分别添加四种自由基引发剂偶氮二异丁腈(AIBN)、过氧化特二丁基(DTBP)、单质硫(S)和单质碘(I2),考察引发剂在390℃～410℃,引发剂添加量0～3000×10-6条件下对三种减压渣油减黏裂化的影响。通过测定减黏前后渣油的运动黏度和四组分的变化,对引发剂的作用机理进行了初步研究。结果表明,自由基引发剂对不同基属的减压渣油减黏裂化反应均有明显的促进作用,大小依次为:LHVR≥GDVRDQVR,并且这种促进作用在较低的反应温度下更加明显,促进效果由渣油本身的物理化学性质所决定。在同等条件下,I2和S促进渣油减黏的效果是AIBN和DTBP降黏率的两倍,这与S和I2形成氢化物的"活性原子"可以多次循环发生作用有关。与S相比,单质I2减黏促进作用更为明显。     

Kinetics of oxidation of bilirubin and its protein complex by hydrogen peroxide in aqueous solutions

A comparative study of oxidation reactions of bilirubin and its complex with albumin was carried out in aqueous solutions under the action of hydrogen peroxide and molecular oxygen at different pH values. Free radical oxidation of the pigment in both free and bound forms at pH 7.4 was shown not to lead to the formation of biliverdin, but to be associated with the decomposition of the tetrapyrrole chromophore into monopyrrolic products. The effective and true rate constants of the reactions under study were determined. It was assumed that one possible mechanism of the oxidation reaction is associated with the interaction of peroxyl radicals and protons of the NH groups of bilirubin molecules at the limiting stage with the formation of a highly reactive radical intermediate. The binding of bilirubin with albumin was found to result in a considerable reduction in the rate of the oxidation reaction associated with the kinetic manifestation of the protein protection effect. It was found that the autoxidation of bilirubin by molecular oxygen with the formation of biliverdin at the intermediate stage can be observed with an increase in the pH of solutions.     

High performance liquid chromatography/electron spin resonance/mass spectrometry analyses of radicals formed in an anaerobic reaction of 9- (or 13-) hydroperoxide octadecadienoic acids with ferrous ions

The oxidation of linoleic acid yields isomeric acyl hydroperoxides. In order to clarify the relation between the lipid peroxide-derived radicals and the toxicity of the lipid peroxide, identification of the lipid-derived radicals is essential. In this paper, high performance liquid chromatography/electron spin resonance/mass spectrometry (HPLC/EPR/MS) analysis of the radicals was performed for the reaction mixture containing 9-hydroperoxy-(10E,12E)-octadeca-10,12-dienoic acid (9EE-OOH) [or 13-hydroperoxy-(9Z,11E)-octadeca-9,11-dienoic acid (13ZE-OOH)] under an aerobic condition or an anaerobic condition. Following radicals were identified from 9EE-OOH (or 13EZ-OOH) by using high performance liquid chromatography/electron spin resonance spectrometry (HPLC/EPR) and HPLC/EPR/MS: pentyl radical and isomers of epoxylinoleic acid radicals from 13EZ-OOH under an anaerobic condition; 7-carboxyheptyl radical and pentyl radical from 13EZ-OOH under an aerobic condition; 7-carboxyheptyl radical and pentyl radical from 9EE-OOH under an aerobic condition; 7-carboxyheptyl radical from 9EE-OOH under an anaerobic condition. These results showed that the formation of the respective radical species depends on oxygen concentration in the reaction mixtures to a great extent.     

Effect of cationic surfactants on limonene oxidation

Kinetic regularities of free radical formation during limonene oxidation with molecular oxygen in a chlorobenzene solution at 60 °C were studied by the inhibitor method under autooxidation conditions and with additives of surfactants (cetyltrimethylammonium chloride and bromide) using dimer 2,2′-bis[2-(p-dimethylaminophenyl)indane-1,3-dione] and α-tocopherol as acceptors of radicals. In the presence of the surfactants, the rates of radical generation and oxidation increase sharply. The catalytic activity of cetyltrimethylammonium chloride is much higher than that of the corresponding bromide.__________Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 2110–2113, October, 2004.     

Electron Spin Catalysis with Graphene Belts

Here, we report kinetic studies using electron spin resonance spectroscopy on spin catalysis reactions caused by using graphene belts which were synthesized by a radical coupling method. The results show that σ-type free radical species provide the dominant sites for catalytic activity through the spin-spin interaction, although there are some other influencing factors. The spin catalysis mechanism can be applied both in the oxygen reduction reaction (ORR) and in organic synthesis. The graphene belt spin catalyst shows excellent performance with a high ORR half-wave potential of 0.81 V and long-term stability with almost no loss of activity after 50 000 cycles in alkaline media. It also shows excellent performance in a benzylamine coupling with molecular oxygen to generate the corresponding imine at an average conversion of ≈97.7 % and an average yield of ≈97.9 %. This work opens up a new research direction for understanding aerobic processes in the field of spin catalysis.