Oxidation dioxygen

Ca.ta.lysis, Iridium compounds do not have industrial appHcations as catalysts. However, these compounds have been studied to model fundamental catalytic steps (174), such as substrate binding of unsaturated molecules and dioxygen oxidative addition of hydrogen, alkyl haHdes, and the carbon—hydrogen bond reductive elimination and important metal-centered transformations such as carbonylation, -elimination, CO reduction, and... 

P.J. Goddard, and R.M. Lambert, Basic studies of the oxygen surface chemistry of silver Oxygen, dioxygen, oxide and superoxide on rubidium-dosedAg(l 1 ),Surf. Sci. 107,519-532(1981). 

The stereochemistry of dioxygen oxidation has been studied for the di-f-butyldimesityldisilene (3).93 Oxidation of (E)-3 produced ( )-59a, ( > 60a, and ultimately ( )-61a exclusively, showing that all the steps in the sequence are stereospecific with retention of configuration. (Similar oxidation of a mixture of ( > and (Z)-3 gave isomeric mixtures of 59a, 60a, and 61a having the same proportions of stereoisomers as in the starting material.) Oxidation of 3 was found to be stereospecific both in solution and in the solid state. 

Dioxygen oxidizes transition metal ions in the lower valence state generating the hydroxyperoxyl radicals or superoxide ions [155,156]. The thermodynamic characteristics of these reactions are presented in Table 10.6. It is seen that all cited reactions are endothermic, except for the reaction of the cuprous ion with 02. The reaction of the ferrous ion with dioxygen has a sufficiently low enthalpy (28 kJ mol 3). 

Extensive efforts have been made to establish and rationalize the kinetics of dioxygen oxidation of iron(II) polyaminocarboxylate complexesThe kinetics of dioxygen oxidation of iron(II) complexes of 1,2- and of 1,3-propylenetetraacetate show the influence of steric factors on these electron transfer reactions7 " ... 

Dioxygen oxidizes VC13 or VCl(salen) to vanadium(IV) with the intermediate formation of dioxygen adducts,235 with equilibrium constants of the order of 102 they are involved in the catalysis of oxygenation of 3,5-di-r-butylpyrocatechol.236... 

The general profile of the dioxygen oxidation of tetraaryldisilenes is shown in Eq. (63).7 Both in the solid state and in solution, the initial oxidation product of a tetraaryldisilene is the corresponding 1,2-disiladioxetane 145, whose intramolecular isomerization gives the thermodynamically more stable 1,3-disiladioxetane 146. All the steps of the oxidation occur intramolecularly and with the retention of stereochemistry around the Si-Si bond. While a small amount of disilaoxirane 147 is produced in the oxidation in low-temperature solution, 147 is converted to 146 smoothly in the presence of excess oxygen. 

Dioxygen oxidation of tetrasila-1,3-butadiene 64 provides unusual oxidation product 151, in which even the sterically well-protected central Si-Si single bond is oxidized together with the two Si = Si double bonds [Eq. (65)].106 In contrast, when m-chloroperoxybenzoic acid (mCPBA) is used as a milder oxidant for the reaction, a mixture of 152 and 153, whose central Si-Si bonds remain intact, is obtained. 

In fact, a closer examination of the products of decarboxylation from compound 4 and 10 led Zeng to conclude that, unless dioxygen is rigorously excluded, there is always perhaps 10-15% carboxylic acid product formed (along with the normal turnover products as discussed in Scheme 7) that can best be explained as resulting from dioxygen oxidation of the intermediate enamine in each case66, similar to the proposal by Abell and Schloss. 

It is relatively stable in very basic solutions and forms insoluble precipitates with alkaline earth, lead, and cadmium dications. Dioxygen oxidizes the trioxodinitrate anion. 

Salts of [Mn(bpy)3] + have proved to be of some interest as catalysts for the periodate oxidation of toluidine (911), the dioxygen oxidation of tso-propylbenzene to cumene hydroperoxide (881), the oxidation of tet-rahydronaphthalene to tetralone (756), and aldol condensations (419), and as photocatalysts for the dimerization of phenyl vinyl ether (638). 

This procedure usually leads to very active films for many electrocatalytic reactions reduction of protons, or of dioxygen, oxidation of dihydrogen, and oxygenated molecules. 

N. Kitajima, H. Fukui, Y. Morooka, A model for methane mono-oxygenase Dioxygen oxidation of alkanes by use of a /e-oxo binuclear iron complex, ]. Chem. Soc. Chem. Commun. (1988) 485. 

It was originally postulated that the methyl groups at C-4 were removed as COj -a suggestion that has proved to be correct. These groups are hydroxylated by a mixed-function oxidase which is NAD(P)H and Oj dependent. First, the 4a-methyl is attacked, yielding the 4 -hydroxymethyl-4 -methyl sterol. This reaction is catalyzed by a methyl sterol oxidase which has been solubilized and partially purified in Gaylor s laboratory [108]. The same enzyme preparation will, with reduced pyridine nucleotide and dioxygen, oxidize the C-30 carbon to a carboxylic acid. The 4a-methyl-4/8-hydroxymethyl-5a-cholestan-3j8-ol is not a substrate for sterol biosynthesis while its epimer is [5]. The detailed mechanisms for the enzymatic removal of C-30 and C-31 are not fully understood. The initial reaction yields a 4a-hydroxy-methyl sterol by inference however, neither the isolation nor the enzymatic formation of a 4a-hydroxymethyl sterol has been demonstrated in animal tissues. This may well result from the fact that the hydroxylation reaction is the slow step in the demethylation process [5]. 

In nature, oxidation reactions are essential for aerobic life. Energy for cells is provided by the combustion of carbohydrates and fatty acids with dioxygen. Oxidation reactions are also involved in biosynthesis, metabolism reactions, and the detoxification of harmful compounds. In several of these reactions, iron or manganese enzymes are involved. These manganese and iron enzymes have frequently been used as a source of inspiration for the development of manganese- and iron-based oxidation catalysts. 

D. P. Riley and D. F. Fields, Electron-transfer agents in metal-catalyzed dioxygen oxidations effective catalysts for the interception and oxidation of carbon radicals , J. Amer. Chem. Soc., 114, 1881 (1992). 

DIOXYGEN OXIDATION BY THE METAL-IMMOBIUZED CATALYST DERIVED FROM SDJCA AND MONTMORILLONTTE MODIFIED BY SILANE COUPLING... 

The silica and montmorillonite modified with silane coupling reagents(SCR) were treated by metal ions(Fe,Cu,Co) in order to obtain effective dioxygen oxidation catalyst. The immobilized catalyst derived from silica was useful for the dioxygen oxidation of cyclohexanes. Also, the immobilized catalyst from montmorillonite was effective for oxidation of ethylbenzene. 

In contrast, a coordinatively unsaturated a-ketoglutarate ferrous complex Fe(II)(BF)(HB-3,5-Me2pz)3 was reported very recently [90]. The dioxygen oxidation of this ferrous complex forms a reactive species capable of epoxidizing olefin. The reaction mechanism of this system has not yet been elucidated, while the reactivity of this system is markedly high and is expected to be a clue in understanding the mechanism of a-keto acid dependent nonheme iron monooxygenase. 

The dioxygen oxidation system utilizing a dinuclear j,-oxo iron(III) complex catalyst (HB(pz)3)2Fe( X-0)(X)2, (X = acetate or hexafluoroacetylacetonate) has been reported [92, 93]. This system also needs zinc powder as an electron donor and acetic acid as a proton source. Different from the Gif system, the hydroxylation of arene proceeds in the latter case, suggesting that the reactive intermediate of this system is different from that of the Gif system. Replacing X in the complex by hexafluoroacetylacetone enhances the reaction rate and the turnover frequently of benzene hydroxylation at room temperature reaches more than 10. Typical results are shown in eq. (9). Unfortunately, these dinuclear iron complexes are not so stable for elongated reaction time and decomposes to afford an inert complex, (HB(pz)3)2Pe which makes further mechanistic investigation difficult. 

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