C–H oxidation using graphite oxide Jia, Hong-Peng; Dreyer, Daniel R.; Bielawski, Christopher W.
Tetrahedron,
06/2011, Letnik:
67, Številka:
24
Journal Article
Recenzirano
Graphite oxide was found to be an effective oxidant for use in a broad range of reactions, including the oxidation of olefins to their respective diones, methyl benzenes to their respective ...aldehydes, diarylmethanes to their respective ketones, and various dehydrogenations. The temperatures used in the reactions were typically mild (100–120 °C), and the heterogeneous nature of the oxidant facilitated isolation and purification of the desired products. In most cases, no by-products were observed and the desired products were isolated in good to excellent yields.
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Boron‐containing materials, and in particular boron nitride, have recently been identified as highly selective catalysts for the oxidative dehydrogenation of alkanes such as propane. To date, no ...mechanism exists that can explain both the unprecedented selectivity, the observed surface oxyfunctionalization, and the peculiar kinetic features of this reaction. We combine catalytic activity measurements with quantum chemical calculations to put forward a bold new hypothesis. We argue that the remarkable product distribution can be rationalized by a combination of surface‐mediated formation of radicals over metastable sites, and their sequential propagation in the gas phase. Based on known radical propagation steps, we quantitatively describe the oxygen pressure‐dependent relative formation of the main product propylene and by‐product ethylene. Free radical intermediates most likely differentiate this catalytic system from less selective vanadium‐based catalysts.
High propylene selectivity during oxidative dehydrogenation catalyzed by hexagonal boron nitride originates from surface‐initiated radical reactions that propagate via gas‐phase chemistry. This reaction network contrasts with previously studied vanadium‐based catalysts where surface reactions predominate and lower selectivity. An experimental and computational approach was used to probe this complex surface–gas‐phase reaction network.
In a photoelectrochemical (PEC) cell, the production of solar fuels such as hydrogen is often accompanied either by the oxidation of water or by the oxidation of organic substrates. In this study, we ...report bromide-mediated PEC oxidation of alkenes at a mesoporous BiVO4 photoanode and simultaneous hydrogen evolution at the cathode using water as an oxygen source. NaBr as a redox mediator was demonstrated to play a dual role in the PEC organic synthesis, which facilitates the selective oxidation of alkenes into epoxides and suppresses the photocorrosion of BiVO4 in water. This method enables a near-quantitative yield and 100% selectivity for the conversion of water-soluble alkenes into their epoxides in H2O/CH3CN solution (v/v, 4/1) under simulated sunlight without the use of noble metal-containing catalysts or toxic oxidants. The maximum solar-to-electricity efficiency of 0.58% was obtained at 0.39 V vs Ag/AgCl. The obtained epoxide products such as glycidol are important building blocks of the chemical industry. Our results provide an energy-saving and environment-benign approach for producing value-added chemicals coupled with solar fuel generation.
The effect of a 30% predeformation treatment on the high‐temperature oxidation behavior of 436 stainless steel is investigated by oxidizing it at 900 °C for 100 h in air. After 48 h of oxidation, the ...undeformed sample primarily exhibits bilayer structures consisting of Cr‐rich oxide and outer Fe oxide, while the predeformed sample predominantly displays a single‐layer structure composed of Cr oxide. The oxidation mechanisms of predeformed and undeformed samples are clarified from the point of recrystallization through oxidation kinetic curves, surface and section micrographs, X‐ray diffraction (XRD), electron probe microanalyzer (EPMA), scanning electron microscope (SEM), energy‐dispersive spectroscopy (EDS), electron backscattered diffraction (EBSD), and line scanning characterization methods. The results show that after 1 h of oxidation, the grain size of the undeformed sample is 34.6 μm, and the grain size of the predeformed sample is 16.0 μm. This reduces the critical content required for the generation of protective Cr oxides and provides more diffusion channels for Cr elements, resulting in the formation of a greater amount of Cr oxides. Cr oxides can inhibit the formation of Fe and Mn oxides, thus delaying the onset of breakaway oxidation and improving its resistance to oxidation.
Predeformation can refine grains size and produce a continuous dense Cr‐rich oxide layer at the initial stage of oxidation. Compared with the undeformed sample, the predeformed sample can significantly delay the appearance of the time point in the breakaway oxidation process, thus improving the oxidation resistance of stainless steel.
The oxidation of catecholamine at a microelectrode, following its release from individual vesicles, allows interrogation of the content of single nanometer vesicles with vesicle impact ...electrochemical cytometry (VIEC). Previous to this development, there were no methods available to quantify the chemical load of single vesicles. However, accurate quantification of the content is hampered by uncertainty in the proportion of substituent molecules reaching the electrode surface (collection efficiency). In this work, we use quantitative modeling to calculate this collection efficiency. For all vesicles except those at the very edge of the electrode, modeling shows that ∼100% oxidation efficiency is achieved when employing a 33 μm diameter disk microelectrode for VIEC, independent of the location of the vesicle release pore. We use this to experimentally determine a precise distribution of catecholamine in individual vesicles extracted from PC12 cells. In contrast, we calculate that when a nanotip conical electrode (∼4 μm length, ∼1.5 μm diameter at the base) is employed, as in intracellular VIEC (IVIEC), the current–time response depends strongly on the position of the catecholamine-releasing pore in the vesicle membrane. When vesicle release occurs with the pore opening occurring far from the electrode, lower currents and partial oxidation (∼75%) of the catecholamine are predicted, as compared to higher currents and ∼100% oxidation, when the pore is close to/at the electrode surface. As close agreement is observed between the experimentally measured vesicular content in intracellular and extracted vesicles from the same cell line using nanotip and disk electrodes, respectively, we conclude that pores open at the electrode surface. Not only does this suggest that electroporation of the vesicle membrane is the primary driving force for catecholamine release from vesicles at polarized electrodes, but it also indicates that IVIEC with nanotip electrodes can directly assess vesicular content without correction.
Antioxidant Activity of Proteins and Peptides Elias, Ryan J; Kellerby, Sarah S; Decker, Eric A
Critical reviews in food science and nutrition,
05/2008, Letnik:
48, Številka:
5
Journal Article
Recenzirano
Proteins can inhibit lipid oxidation by biologically designed mechanisms (e.g. antioxidant enzymes and iron-binding proteins) or by nonspecific mechanisms. Both of these types of antioxidative ...proteins contribute to the endogenous antioxidant capacity of foods. Proteins also have excellent potential as antioxidant additives in foods because they can inhibit lipid oxidation through multiple pathways including inactivation of reactive oxygen species, scavenging free radicals, chelation of prooxidative transition metals, reduction of hydroperoxides, and alteration of the physical properties of food systems. A protein's overall antioxidant activity can be increased by disruption of its tertiary structure to increase the solvent accessibility of amino acid residues that can scavenge free radicals and chelate prooxidative metals. The production of peptides through hydrolytic reactions seems to be the most promising technique to form proteinaceous antioxidants since peptides have substantially higher antioxidant activity than intact proteins. While proteins and peptides have excellent potential as food antioxidants, issues such as allergenicity and bitter off-flavors as well as their ability to alter food texture and color need to be addressed.
PhI(OTf)2 and related ArI(OTf)2 species have been incorrectly invoked as intermediates in oxidation reactions for many years. We recently established that such compounds did not yet exist but remain ...an attractive target. Here we describe the synthesis, isolation, and structural characterization of NO2‐PhI(OTf)2, which is resistant to decomposition and more reactive than PhI(OTf)(OAc), the species previously misidentified as PhI(OTf)2.
After many years of being invoked as an intermediate, the existence of PhI(OTf)2 was recently disproven. Taking advantage of a nitro substituent that suppresses decomposition; an NO2‐PhI(OTf)2 molecule has been synthesized and structurally characterized.
Work remains to develop an aerobic catalyst that will selectively oxidize secondary alcohols in the presence of primary alcohols. In principle, this is possible when there is an oxidation potential ...preference for doing so. One potential strategy is the use of a low oxidation potential catalyst. Recently, we reported a dimeric vanadium(V)–(μ-O)2 perfluoropinacolate (pinF) complex with the ability to oxidize alcohols via dehydrogenation that was sensitive to the alcohol redox potential. Herein, we report the scope, chemoselectivity, and insights into the mechanism by which V(V) cat performs oxidations on activated primary and secondary alcohols.
In this paper, the role of periodate and persulfate as inorganic oxidants were studied in presence of ultraviolet radiation and titanium dioxide nanoparticles as a hybrid advanced oxidation ...photocatalytic processes for degradation and mineralization of the pirimicarb insecticide in aqueous media. The effects of several factors such as the ultraviolet irradiation, initial oxidant concentration, titanium dioxide nanoparticles dosage, and pH on the process performance were investigated. The process optimization was performed by the central composite design as a tools of response surface methodology for 30 mg L−1 of the insecticide initial concentration at 25°C and 10 min of degradation process. A degradation efficiency of about 86% and 85% have been obtained for the persulfate and periodate processes, respectively, in the optimum conditions. The mineralization efficiency of the process using persulfate and periodate were about 35% and 46% after 60 min, respectively. The kinetic studies show that both processes follow a pseudo‐first‐order kinetic model and the rate constants were 0.1483 min−1 for the persulfate and 0.1152 min−1 for the periodate process. Generally, it can be concluded that this method is suitable for the degradation and mineralization of toxic aromatic compounds.
We disclose a highly regioselective, catalytic one‐step dehydrogenation of α‐substituted cyclic ketones in the presence of 2,3‐dichlorobenzo‐5,6‐dicyano‐1,4‐benzoquinone (DDQ). The high ...regioselectivity originates from a phosphoric acid‐catalyzed enolization, selectively affording the thermodynamically preferred enol, followed by the subsequent oxidation event. Our method provides reliable access to several α‐aryl and α‐alkyl substituted α,β‐unsaturated ketones.
A highly regioselective, one‐step dehydrogenation of α‐substituted cyclic ketones in the presence of 2,3‐dichlorobenzo‐5,6‐dicyano‐1,4‐benzoquinone is described. The reaction proceeds via phosphoric acid‐based enol catalysis and provides access to several α‐aryl and α‐alkyl substituted α,β‐unsaturated ketones.
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