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Hydroperoxide Intermediate (hydroperoxide + intermediate)
Selected AbstractsInsertion of Molecular Oxygen in Transition-Metal Hydride Bonds, Oxygen-Bond Activation, and Unimolecular Dissociation of Metal Hydroperoxide Intermediates.HELVETICA CHIMICA ACTA, Issue 3 2008Short Communication Abstract Thermal activation of molecular oxygen is observed for the late-transition-metal cationic complexes [M(H)(OH)]+ with M=Fe, Co, and Ni. Most of the reactions proceed via insertion in a metalhydride bond followed by the dissociation of the resulting metal hydroperoxide intermediate(s) upon losses of O, OH, and H2O. As indicated by labeling studies, the processes for the Ni complex are very specific such that the O-atoms of the neutrals expelled originate almost exclusively from the substrate O2. In comparison to the [M(H)(OH)]+ cations, the ionmolecule reactions of the metal hydride systems [MH]+ (M=Fe, Co, Ni, Pd, and Pt) with dioxygen are rather inefficient, if they occur at all. However, for the solvated complexes [M(H)(H2O)]+ (M=Fe, Co, Ni), the reaction with O2 involving OO bond activation show higher reactivity depending on the transition metal: 60% for the Ni, 16% for the Co, and only 4% for the Fe complex relative to the [Ni(H)(OH)]+/O2 couple. [source] Studies on the oxygen atom transfer reactions of peroxomonosulfate: Oxidation of glycolic acidINTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 3 2009S. Shailaja The kinetics of oxidation of glycolic acid, an ,-hydroxy acid, by peroxomonosulfate (PMS) was studied in the presence of Ni(II) and Cu(II) ions and in acidic pH range 4.05,5.89. The metal glycolate, not the glycolic acid (GLYCA), is oxidized by PMS. The rate is first order in [PMS] and metal ion concentrations. The oxidation of nickel glycolate is zero-order in [GLYCA] and inverse first order in [H+]. The increase of [GLYCA] decreases the rate in copper glycolate, and the rate constants initially increase and then remain constant with pH. The results suggest that the metal glycolate ML+ reacts with PMS through a metal-peroxide intermediate, which transforms slowly into a hydroperoxide intermediate by the oxygen atom transfer to hydroxyl group of the chelated GLYCA. The effect of hydrogen ion concentrations on kobs suggests that the structure of the metal-peroxide intermediates may be different in Ni(II) and Cu(II) glycolates. © 2008 Wiley Periodicals, Inc. Int J Chem Kinet 41: 160,167, 2009 [source] H2O2 -mediated oxidation of tetrahydrobiopterin: Fourier transform Raman investigations provide mechanistic implications for the enzymatic utilization and recycling of this essential cofactorJOURNAL OF RAMAN SPECTROSCOPY, Issue 8 2002Jeremy Moore The oxidation of (6R)- L - erythro -5,6,7,8-tetrahydrobiopterin (6BH4) by H2O2 was examined by Fourier transform Raman spectroscopy. Initial investigations indicated that oxidation proceeds by incorporation of the H2O2 into the 6BH4 molecule without the formation of any additional water. In addition, the pyrimidine ring is affected with the shift of the double bond from the N1,C2 to the C2,N3 position. Such rearrangements of this double bond are observed after the production of either a carbinolamine or quinonoid species. Using deuterium exchange experiments, it was possible to substantiate that the oxidation of 6BH4 initially proceeds by the formation of a 4a-OH-carbinolamine intermediate prior to its spontaneous dehydration yielding the quinonoid dihydro species (qBH2). Furthermore, the hydrogen on the hydroxyl group of the carbinolamine interacts with the oxygen of the carbonyl group at the C4 position of the pyrimidine ring. It is proposed that this interaction facilitates the dehydration of the carbinolamine, thus explaining its instability. Furthermore, a mechanism for the dehydration reaction is suggested, wherein the 4a-hydroxyl group forms an H-bond to the carbonyl group, thus making the oxygen of the hydroxyl group more susceptible to attack by the proton at position N5 of the pyrazine ring, resulting in qBH2 production concomitant with the loss of a water molecule. Upon increasing the concentration of H2O2 the qBH2 converts to 7,8-BH2, which is further oxidized to L -biopterin. Taken together, our results do not support an earlier proposed mechanism implicating a hydroperoxide intermediate in this oxidation reaction. Copyright © 2002 John Wiley & Sons, Ltd. [source] Oxidation and chemiluminescence of catechol by hydrogen peroxide in the presence of Co(II) ions and CTAB micellesLUMINESCENCE: THE JOURNAL OF BIOLOGICAL AND CHEMICAL LUMINESCENCE, Issue 5 2007Jan Lasovsky Abstract The oxidation of catechol in neutral and slightly alkaline aqueous solutions (pH 7,9.6) by excess hydrogen peroxide (0.002,0.09 mol/L) in the presence of Co(II) (2.10,7,2.10,5 mol/L) is accompanied by abrupt formation of red purple colouration, which is subsequently decolourized within 1 h. The electron spectra of the reaction mixture are characterized by a broad band covering the whole visible range (400,700 nm), with maximum at 485 nm. The reaction is initiated by catechol oxidation to its semiquinone radical and further to 1,2-benzoquinone. By nucleophilic addition of hydrogen peroxide into the p -position of benzoquinone C=O groups, hydroperoxide intermediates are formed, which decompose to hydroxylated 1,4-benzoquinones. It was confirmed by MS spectroscopy that monohydroxy-, dihydroxy- and tetrahydroxy-1,4-benzoquinone are formed as intermediate products. As final products of catechol decomposition, muconic acid, its hydroxy- and dihydroxy-derivatives and crotonic acid were identified. In the micellar environment of hexadecyltrimethylammonium bromide the decomposition rate of catechol is three times faster, due to micellar catalysis, and is accompanied by chemiluminescence (CL) emission, with maxima at 500 and 640 nm and a quantum yield of 1 × 10,4. The CL of catechol can be further sensitized by a factor of 8 (maximum) with the aid of intramicellar energy transfer to fluorescein. Copyright © 2007 John Wiley & Sons, Ltd. [source] | ||||||||||