Oxidation iron -catalysed

NO was shown by Kanner et al. [120] to inhibit iron-catalysed oxidation reactions by binding to ferrous complexes. It was also shown that NO inhibited the superoxide driven Fenton reaction which, in the presence of iron, generates hydroxyl radical (OH) in vitro. By adding varying amounts of NO to a Fenton reaction process the hydroxylation of benzoic acid was reduced. This demonstrates that depending on the fluxes of the different reactive species, NO may have an antioxidant capability. 

The method based on iron catalysed oxidation of o-phenylenediamine with H2O2 has been developed for speciation of iron in river and tap waters [11]. Xylenol Orange was applied to simultaneous determination of iron and nickel in alloys and industrial waste water [12]. The chelate complexes of Fe and Co with PAR made a basis of simultaneous determination of both metals by the second derivative spectrophotometry [13]. 

A possible stabilisation step under these conditions has been suggested which may occur by the reaction of iron ions such as Fe /Fe with this complex to form the corresponding iron salts [39, 96], Subsequent conversion to iron polyphosphates means that the iron-catalysed oxidation is suppressed. 

Interestingly, coupling of phenols and jS-ketoesters under iron-catalysed oxidative conditions leads selectively to the formation of substituted benzofurans. Notably, the iron catalyst acts as a transition-metal catalyst in the oxidative coupling step and as Lewis acid in the condensation step. The presence of water coordinated to the catalyst and/or protic solvents accelerates the process, likely favouring the (final) tautomerisation step (Scheme 13.12). ... 

Scheme 13.13 Ligand- and solvent-free iron-catalysed oxidative CDC coupling of tertiary amines and terminal alkynes.

Although camosic acid and carnosol are readily oxidized at 60°C and higher temperatures, their antioxidant activities are maintained. The oxidation products formed are apparently active antioxidants at high temperatures and protect oils during frying. They also have carry-over activity by protecting the fried foods. This carry-over activity can be attributed to the formation of various oxidation and isomerization products. Indeed, the radical [2,2 -azobis-(2,4-dimethylvalerate)] initiated or iron-catalysed oxidation of camosic acid in the... 

A number of recent papers have also focussed on the identification of paramagnetic transient reactive intermediates in bio-inspired non-heme iron catalysed oxidations. Makhlynets and Rybak-Akimova investigated the mechanism of the substrate oxidation, using H2O2 as oxidant, in an iron aminopyridine complex for aromatic hydro qrlation (Scheme 1). EPR with stopped-flow kinetic measurements, was used to identify the formation of the key Fe" (OOH) intermediate, with rhombic g... 

Metals of transient valency, particularly copper and iron, catalyse the lipid oxidation because they decompose lipid hydroperoxides with formation of free radicals [15.8] and [15.9] ... 

Iron (II) oxide and especially that made by reducing the other oxides, combusts spontaneously if it is heated to 200°C. It also strongly catalyses the combustion of carbon in air. This behaviour can explain the spontaneous inflammable property of the products of burning iron oxalate, which contain this oxide and carbon. When they are placed on the hand and thrown into the air, they form very spectacular showers of sparks. It combusts in contact with liquid oxygen in the presence of carbon. 

This oxide catalyses the violent or even explosive decomposition of hydrogen peroxide. This reaction explains the numerous accidents mentioned involving the contact of hydrogen peroxide with rusted iron. Two accidents of this nature dealt with mixtures of hydrogen peroxide with ammonia and an alkaline hydroxide The detonations took place after a period of induction of respectively several hours and four minutes. Iron (III) oxide also catalyses the explosive decomposition of calcium hypochlorite. 

The phenothiazines, chlorpromazine and promethazine, have been described as inhibitors of CCU-induced lipid peroxidation at relatively high concentrations in rat liver microsomes (Slater, 1968). Structural modifications of chlorpromazine were undertaken to try to increase antioxidant activity and maintain molecular lipophilicity. The 2-N-N-dimethyl ethanamine methanesulphonate-substituted phenothiazine (3) was found to be a potent inhibitor of iron-dependent lipid peroxidation. It was also found to block Cu -catalysed oxidation of LDL more effectively than probucol and to protect primary cultures of rat hippocampal neurons against hydrogen peroxide-induced toxicity in vitro (Yu et al., 1992). 

This filler is mined, ground and sieved to a particle size less than 100 mesh and used as an inert diluent and cheapening filler for rubber compounds. It is usually off-white to cream in colour. Depending upon source, the filler can be contaminated with metal ions, e.g., iron, copper, manganese, which can catalyse oxidation. It can be used in very high loadings with great effect on compound hardness. 

Transition metals (iron, copper, nickel and cobalt) catalyse oxidation by shortening the induction period, and by promoting free radical formation [60]. Hong et al. [61] reported on the oxidation of a substimted a-hydroxyamine in an intravenous formulation. The kinetic investigations showed that the molecule underwent a one-electron transfer oxidative mechanism, which was catalysed by transition metals. This yielded two oxidative degradants 4-hydroxybenzalde-hyde and 4-hydroxy-4-phenylpiperidine. It has been previously shown that a-hydroxyamines are good metal ion chelators [62], and that this can induce oxidative attack on the a-hydroxy functionality. 

The principal iron oxides used in catalysis of industrial reactions are magnetite and hematite. Both are semiconductors and can catalyse oxidation/reduction reactions. Owing to their amphoteric properties, they can also be used as acid/base catalysts. The catalysts are used as finely divided powders or as porous solids with a high ratio of surface area to volume. Such catalysts must be durable with a life expectancy of some years. To achieve these requirements, the iron oxide is most frequently dis-... 

Ferrous iron catalyses the oxidation of luminol (see below) by hydrogen peroxide. The intensity of the chemiluminescence which follows increases linearly with the concentration of Fe(II) as it rises from 10-10 M to 10-8 M. To measure a solution of unknown content in Fe(II), 2 ml of the solution was extracted and then introduced to 1 ml of water, followed by the addition of 2 ml of hydrogen peroxide and finally 1 ml of an alkali solution of luminol. The luminescence signal is emitted and integrated over a period of 10 seconds giving a value of 16.1 (arbitrary units). In a second attempt, 2 ml of the... 

Iron-catalysed homo-coupling of aryl Grignard reagents has been successfully developed.96 A variety of aryl Grignard reagents have been efficiently converted into the corresponding symmetrical biaryls in the presence of FeCl3 and a stoichiometric amount of 1,2-dichloroethane as oxidant (Scheme 18). 

It is ironic that while most organisms rely on oxygen for respiration, its mere presence can have damaging effects. Oxidation occurs slowly in an oxygen rich atmosphere but is accelerated by many transition metals such as those regularly encountered in biological systems. When oxidation of lipids, DNA, RNA, proteins and other biomolecules occurs a major problem is encountered as it affects their functions. There are, of course, many necessary enzymes that catalyse oxidation... 

This review is concerned with the quantitative aspects of metal-catalysed oxyradical reactions. As such one will find discussions of structures of metal complexes, rate constants and reduction potentials, not unlike our review of 1985 [34], Two areas related to the role of transition metals in radical chemistry and biology have been reviewed recently these are the metal-ion-catalysed oxidation of proteins [35] and the role of iron in oxygen-mediated toxicities [36]. These topics will not be discussed in detail in this review. Related to this work is a review on the role of transition metals in autoxidation reactions [37]. Additional information can be obtained from Afanas ev s two volumes on superoxide [38,39], This subject is also treated in a more general and less quantitative manner by Halliwell and Gutteridge [40],... 

Recently, we have suggested an alternative to the iron-catalysed HO -dependent mechanism to explain the cleavage and consequent loss of viscosity of synovial fluid HA during rheumatoid arthritis [16]. This mechanism involves the haem enzyme myeloperoxidase (MPO) and the production of the strong oxidant hypochlorous acid (HOC1) by the following reaction ... 

The iron or haemoprotein catalysed oxidative reactions may mediate the responses associated with acute and chronic inflammation. In the post-ischaemic, reperfused heart the role of oxidant stress has been linked with increases in leucocyte adhesion and transendothelial cell migration from oxidant production within the microcirculation [115]. This is probably caused by an increased expression in adhesion molecules or the fixation of transiently expressed adhesion molecules by the peroxidation of membrane lipids which reduces membrane fluidity [117,118]. This oxidant stress may also lead to apoptosis induction, DNA damage, inflammatory mediator synthesis and regulate gene expression [119,120,121]. 

Various precautions should be taken during manufacture and storage to minimise oxidation. The oxygen in pharmaceutical containers should be replaced with nitrogen or carbon dioxide contact of the drug with heavy-metal ions such as iron, cobalt or nickel, which catalyse oxidation, should be avoided and storage should be at reduced temperatures. 

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