Molecular oxygen, oxidation cationic

Metallic iron is made up of neutral iron atoms held together by shared electrons (see Section 10.7). The formation of rust involves electron-transfer reactions. Iron atoms lose three electrons each, forming Fe cations. At the same time, molecular oxygen gains electrons from the metal, each molecule adding four electrons to form a pair of oxide anions. As our inset figure shows, the Fe cations combine with O anions to form insoluble F 2 O3, rust. Over time, the surface of an iron object becomes covered with flaky iron(ni) oxide and pitted from loss of iron atoms. 

Studies on carotenoid autoxidation have been performed with metals. Gao and Kispert proposed a mechanism by which P-carotene is transformed into 5,8-per-oxide-P Carotene, identified by LC-MS and H NMR, when it is in presence of ferric iron (0.2 eq) and air in methylene chloride. The P-carotene disappeared after 10 min of reaction and the mechanism implies oxidation of the carotenoid with ferric iron to produce the carotenoid radical cation and ferrous iron followed by the reaction of molecular oxygen on the carotenoid radical cation. Radical-initiated autoxidations of carotenoids have also been studied using either radical generators like or NBS.35... 

Addition of 1,5-dithiacyclooctane to zeolite CaY in the presence of molecular oxygen results in spontaneous oxidation to mono- and bis-sulfoxides through formation of the corresponding radical cation characterized by ESR and diffuse reflectance of UV-Vis spectroscopy.51... 

Beyer and coworkers later extended these reactions to platinum clusters Ptn and have demonstrated that similar reaction sequences for the oxidation of carbon monoxide can occur with larger clusters [70]. In addition, they were able to demonstrate poisoning effects as a function of surface coverage and cluster size. A related sequence for Pt anions was proposed by Shi and Ervin who employed molecular oxygen rather than N2O as the oxidant [71]. Further, the group of Bohme has screened the mononuclear cations of almost the entire transition metal block for this particular kind of oxidation catalysis [72,73]. Another catalytic system has been proposed by Waters et al. in which a dimolybdate anion cluster brings about the oxidation of methanol to formaldehyde with nitromethane, however, a rather unusual terminal oxidant was employed [74]. 

The oxidation of CO to CO2 by metal-oxide clusters has received quite some attention, in part due to its relevance for the catalytic converters in automobiles, in part also because carbon monoxide is often used as a probe molecule in surface science and the reasonable simplicity of the system may still permit adequate theoretical treatments. In addition to the various systems involving PtmO cations as well as PtmO anions (see above), considerable efforts have been devoted to cluster anions of silver and gold, as reviewed recently [87]. A particular highlight is a conceptual catalytic cycle for the Au2 -mediated oxidation of CO with molecular oxygen, for which... 

Most mechanistic studies have focused on elucidation of the role of alkali promoters. The addition of Li+ to MgO has been shown to decrease the surface area and to increase both methane conversion and selective C2 production.338,339 As was mentioned, however, besides this surface-catalyzed process, a homogeneous route also exists to the formation of methyl radicals.340-342 The surface active species on lithium-doped catalysts is assumed to be the lithium cation stabilized by an anion vacancy. The methyl radicals are considered to be produced by the interaction of methane with O- of the [Li+0-] center330,343 [Eq. (3.32)]. This is supported by the direct correlations between the concentration of [Li+0 ] and the concentration of CH3 and the methane conversion, respectively. The active sites then are regenerated by dehydration [Eq. (3.33)] and subsequent oxidation with molecular oxygen [Eq. (3.34)] ... 

Tetrahydropterins are highly reactive towards oxidation (e.g. 542 — 544) even molecular oxygen can cause hydroxylation. The autoxidation is due to the electron donating groups such as amino and hydroxy, whereas removal of such substituents enhances the stability of the reduced pteridine nucleus tremendously (96CHEC-li(7)70l). The reaction appears to proceed via single electron transfer. The radical cation (543) can be observed by cyclic voltammetry. 

Oxo-metal species participate in a wide range of biological and chemical oxidation reactions. Representative oxidizing enzyme, cytochrome P-450, which carries iron(III)-porphyrin complex as its active site, catalyzes various O-atom transfer reactions such as epoxidation, hydroxy-lation of C-H bond, and oxidation of sulfides. These reactions have been proven to proceed through cationic oxoiron(IV)-porphyrin species, which are generated by the oxidation of Fe(III) complex with molecular oxygen. This conversion from Fe(III) to 0=Fe(IV) species is a... 

The mechanism given in equation 47 has been proposed for this reaction. The initially formed cation radical reacts with molecular oxygen to generate an intermediate, which may couple with a neutral cyclic silane to form species A. The intermediate A decomposes to the final product B and its cation radical B+", which could also be generated by direct anodic oxidation of the siloxane B. A further oxygen insertion step could take place via intermediate C+. ... 

Oxidation of IAA (2.49) results in cation 2.50, which undergoes decarboxylation and results in the skatolyl radical (2.51). This compound reacts with molecular oxygen to form peroxyl radical 2.52. With IAA or another cellular reductor, the hydroperoxide 2.53 is formed. It is this compound that activates the peroxidase, and thus allows the oxidation of other substrates, such as coniferyl alcohol. Among the degradation products of 2.53, 3-methylene 2-oxindole (2.54) is the most abundant. 

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