Peroxide Intermediates Research Articles

Operando Surface Enhanced Infrared Spectroscopic Investigations of Interfacial Restructuring and Oxygen Electrochemistry in Ionic Liquid Electrolytes for Metal-Air Batteries

Alkali metal-oxygen (M-O2) chemistry has received significant research interest as a promising next-generation battery, but many challenges need addressing before the technology can become viable in practice. Key among these obstacles are the understanding and control of the mechanisms of the oxygen reduction and evolution reactions (ORR and OER), and the tendency for the electrolyte materials to degrade due to attack by reactive intermediates. As such, a greater understanding of the ORR/OER in M-O2 cells, and how these factors contribute to the poor reversibility on cycling, is required in order to develop stable electrolytes.Attenuated total reflection surface enhanced infrared absorption spectroscopy (ATR-SEIRAS) is highly sensitive to the interfacial region and can probe sub-monolayer adsorbates and species within ca. 10 nm of the electrode surface, which makes it valuable for studying interfacial processes. Our group demonstrated the application of ATR-SEIRAS to identify ORR products in nonaqueous M-O2 batteries, wherein a ZnSe prism internal reflection element (IRE) was utilised to extend the measurement range down to ca. 700 cm-1.1 While ZnSe offers enhanced optical properties over some conventional IREs, the lower wavenumber cut-off means that the sub-700 cm-1 vibrational frequencies of some superoxide and peroxide species and intermediates generated during ORR/OER cannot be observed. Recently, a micromachined Si wafer IRE has been developed to extend the measurement range of ATR-SEIRAS to well below 700 cm-1.2 In this work, we report ATR-SEIRAS investigations into the O2 electrochemistry and the potential dependent interfacial restructuring of 1-methyl-1-propylpyrrolidinium bis{(trifluoromethyl)sulfonyl}imide ([Pyrr13][TFSI]) utilising a thin-layer Au electrode on the micromachined Si wafer IRE to extend the measurement range towards the far-IR region. While the potential-dependent restructuring of the IL/Au electrode interface has been investigated by ATR-SEIRAS previously,3 the optical transmission of the Si wafer IRE used here reveals previously unreported changes in the 1000–500 cm-1 spectral region (Figure 1). We also discuss here how the redox couple of dissolved O2 strongly alters the double-layer restructuring, and the associated activation energy barriers, in the IL electrolyte. The ORR in the IL electrolyte in the presence of Li+ or Na+ cations is also explored, wherein the identification of sub-700cm-1 (su)peroxy intermediates/products, critical to understanding the overall mechanisms, is made possible using the Si wafer IRE. References J. P. Vivek, N. G. Berry, J. Zou, R. J. Nichols, and L. J. Hardwick, J. Phys. Chem. C, 121, 19657–19667 (2017).T. A. Morhart, B. Unni, M. J. Lardner, and I. J. Burgess, Anal. Chem., 89, 11818–11824 (2017).K. Motobayashi, Y. Shibamura, and K. Ikeda, Electrochem. Commun., 100, 117–120 (2019). Figure 1. Operando ATR-SEIRAS measurements of [Pyrr13][TFSI] (at a thin-layer Au electrode) at 0.9 V vs reference spectra at -2.7 V (dark blue and dark red spectra) at a) 1500-900 cm-1 and b) 860-500 cm-1. The Si wafer IRE (red spectrum) allows detection of [TFSI]- peaks below 1000 cm-1, which are not visible using the Si prism (light blue spectrum) due to the poor IR transmission in this region. Wafer spectra intensities are multiplied by a factor of 2 and the grey trace is an ex situ ATR-FTIR spectrum of the bulk IL. Figure 1

(Invited) Oxygen Reduction at Low-Pt-Content-Catalysts in Acid Media: Development of Systems and Electroanalytical Diagnostic Methodology

The proton-exchange membrane fuel cell (PEMFC) technology is one of the most promising approaches for energy conversion in automotive applications. Despite the use of expensive noble metal electrocatalysts, the sluggish kinetics of the oxygen reduction reaction (ORR) in acid media, and formation of the undesirable hydrogen peroxide intermediate are still the main drawbacks, on top of the stability problems, when it comes to the widespread commercialization of PEMFC devices. The progress in this subject is greatly hindered by the high cost and scarcity of the state-of-the-art platinum-based materials which are regarded as the most effective cathode catalysts. Therefore, most of the current research studies have been devoted to the optimization of active centers and maximization of their utilization, which should allow the lowering of the cathode Pt loadings without loss of performance and durability. Unfortunately, the problem of electrochemical stability and the danger of generation of higher quantities of the undesirable hydrogen peroxide intermediate may become even more serious in the case of systems utilizing lower amounts of the Pt catalystAmong important strategies is hybridization, activation, and stabilization of carbon-supported Pt catalysts by functionalization through admixing with certain nanostructured and typically substoichiometric metal oxides. Special attention is paid to application of the bi- or multi-metallic Pt-based alloys in their various forms and structures. Additionally, doping of carbon carriers with heteroatoms would produce surface functional groups and active centers capable of not only improving the catalysts’ performance, but also affecting their stability. Among other important issues are such features as porosity, hydrophilicity, and degree of graphitization of carbon components, in addition to the existence of metal–support interactions, high electrochemical active surface area, electronic structure of interfacial Pt, and the feasibility of adsorptive or activating interactions with oxygen molecules.Voltammetric approaches, particularly the rotating ring-disk electrode (RRDE) methodology, are typically applied to diagnose mechanisms and dynamics of ORR. A tempting alternative for recording the electrochemical responses is to integrate the current and to report charge passed as a function of time. Chronocoulometry offers important advantages including good signal-to-noise ratio because the act of integration smooths random noise on the current transients. Furthermore, by integrating the responses, it is possible to separate surface phenomena (e.g. interfacial oxidation of carbon or platinum) more readily from bulk electrochemical responses (ORR). The double-potential-step chronocoulometry permits, in principle, estimation of the number of electrons (n) involved in the oxygen reduction and the percent of formation of the H2O2 intermediate. The problem lies in understanding the reaction kinetics and mechanisms in order to define properly experimental parameters (potential steps and pulse times). Comparative measurements are performed using RRDE voltammetry. Hybrid supports, which utilize metal oxides (e.g., CeO2, WO3, or ZrO2) have been demonstrated to stabilize Pt and carbon nanostructures and diminish their corrosion while exhibiting high activity toward the four-electron (most efficient) reduction of oxygen. Acknowledgements: This work was supported by the National Science Center (Poland) under Opus Project (2018/29/B/ST5/02627) and under auspices of the European Union EIT Raw Materials ALPE 19247 Project (Specific Grant Agreement No. EIT/RAW MATERIALS/SGA2020/1).

Tuning the Intramolecular Chemiexcitation of Neutral Dioxetanones by Interaction with Ionic Species.

The intramolecular chemiexcitation of high-energy peroxide intermediates, such as dioxetanones, is an essential step in different chemi- and bioluminescent reactions. Here, we employed the Time-Dependent Density Functional Theory (TD-DFT) methodology to evaluate if and how external stimuli tune the intramolecular chemiexcitation of model dioxetanones. More specifically, we evaluated whether the strategic placement of ionic species near a neutral dioxetanone model could tune its thermolysis and chemiexcitation profile. We found that these ionic species allow for the “dark” catalysis of the thermolysis reaction by reducing the activation barrier to values low enough to be compatible with efficient chemi- and bioluminescent reactions. Furthermore, while the inclusion of these species negatively affected the chemiexcitation profile compared with neutral dioxetanones, these profiles appear to be at least as efficient as anionic dioxetanones. Thus, our results demonstrated that the intramolecular chemiexcitation of neutral dioxetanones can be tuned by external stimuli in such a way that their activation barriers are decreased. Thus, these results could help to reconcile findings that neutral dioxetanones could be responsible for efficient chemi-/bioluminescence, while being typically associated with high activation parameters.

Abstract 2166: Ibuprofen induces ferroptosis to potentiate antitumor immunity

Abstract Ferroptosis is an iron-dependent regulated cell death pathway that has recently emerged as a promising target for cancer therapy. PTGS2 has been identified as a marker of ferroptosis due to its upregulation in cells after treatment with ferroptosis inducers. Nonsteroidal anti-inflammatory drugs (NSAIDs) inhibit the protein product of PTGS2 and have been shown to enhance antitumor immunity. We hypothesized that NSAIDs induce ferroptosis and that this mechanism is essential for their effect on antitumor immunity. To test this, we first determined that ibuprofen and other NSAIDs induce cell death in YUMMER-1.7 melanoma and MC38 colorectal cancer cells in vitro. Cell death occurred progressively over 24 hours and could be blocked by multiple ferroptosis inhibitors. Treatment with NSAIDs also increased intracellular lipid peroxides and decreased levels of reduced glutathione and ferrous iron. This indicated that ibuprofen and other NSAIDs induce ferroptosis in vitro, so we next evaluated whether ibuprofen also induces ferroptosis in vivo. Analysis of ibuprofen-treated YUMMER-1.7 or MC38 tumors showed increased levels of lipid peroxides and lipid peroxide intermediates, 4-hydroxynonenal and malondialdehyde, compared to untreated tumors. Treatment of tumor-bearing mice with ibuprofen alone resulted in approximately 60% tumor clearance and survival, whereas a combination of multiple ferroptosis inhibitors with ibuprofen completely abrogated this effect. Since it has been demonstrated by that CD8+ T cells can release IFN-γ to induce ferroptosis in tumor cells, we next tested the dependence of the antitumor effect of ibuprofen on CD4+ T cells, CD8+ T cells, and IFN-γ using depleting antibodies. Depletion of any of these was sufficient to prevent the antitumor effect of ibuprofen. Given the need to identify pathways that enhance responses to existing immunotherapies, we determined whether treatment with ibuprofen synergizes with PD-1 checkpoint blockade. The combination of α-PD-1 with ibuprofen resulted in a 100% tumor clearance and survival rate, whereas α-PD-1 or ibuprofen alone resulted in 40% and 60% tumor clearance and survival rates, respectively. Furthermore, treatment with ibuprofen also sensitized α-PD-1-resistant tumors to PD-1 in two distinct models of α-PD-1 resistance. Overall, these findings establish ibuprofen and other NSAIDs as inducers of ferroptosis and underscore the potential for ferroptosis inducers to be used in combination with existing immunotherapy regimens. Citation Format: Ronan Talty, Michelle Ferreira, Irina Krykbaeva, Marcus Bosenberg. Ibuprofen induces ferroptosis to potentiate antitumor immunity [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 2166.

Mechanistic Insights into Cobalt-Based Water Oxidation Catalysis by DFT-Based Molecular Dynamics Simulations.

We report the mechanistic details of the water oxidation process by the complex, [CoII(bpbH2)Cl2], where bpbH2 = N, N'-bis(2'-pyridinecarboxamide)-1,2-benzene. An experimental study reported the complex as the efficient catalyst for the water oxidation process. We performed density functional theory calculations at the M06-L level and first-principles molecular dynamics simulations to study the catalytic nature of the complex. We investigated the energetics of the total catalytic cycle, which combines the oxygen-oxygen bond formation, proton-coupled electron transfer (PCET) steps, and release of oxygen molecule. The formed peroxide and superoxide intermediates in the catalytic cycle were characterized with the help of the Mulliken spin density parameters. Mulliken spin densities of the metal-oxo bond reveal that the triplet state of CoV═O has a double-bond nature, but the quintet state of the complex has a radical nature (CoIV-O•-). In an alternative way, the deprotonation of the amide groups of the ligand is also considered. The deprotonation and formation of higher oxidation metal-oxo intermediates are also possible. In addition to this, we have considered the effect of phosphate buffer on water nucleophilic addition. The oxygen-oxygen bond formation is favorable by the catalyst with the deprotonated form of the ligand, with the addition of water as the nucleophile. In the oxidation process, the C═O bonds of the ligand transfer the electron density to nitrogen atoms, stabilizing the higher order oxo, peroxide, and superoxide bonds. The oxygen-oxygen bond formation is the rate-determining step in the overall water oxidation process. This bond was further investigated using first-principles molecular dynamics at the PBE-D2 level. The dynamics of proton, hydroxide ion, and the nature of the ligand structure on the oxygen-oxygen bond were examined. We find that the oxygen molecule is released from the superoxide complex with the addition of water molecules.

Ginger and curcumin can inhibit heterocyclic amines and advanced glycation end products in roast beef patties by quenching free radicals as revealed by electron paramagnetic resonance

This study investigated the inhibitory effects of ginger and curcumin on the formation of free and bound heterocyclic amines (HAs)1 and advanced glycation end products (AGEs) in roast beef patties. The underlying mechanism related to quenching free radical, alleviating lipid peroxidation and scavenging carbonyl intermediates was explored by electron paramagnetic resonance (EPR) and UPLC-MS/MS. Ginger (0.5%, 1.0%, 1.5%) and curcumin (0.005%, 0.010%, 0.015%) dose-dependently inhibited free and bound HAs and AGEs simultaneously. The maximum inhibition rates for free and bound HAs were 59.97% (0.015% curcumin) and 27.42% (1.5% ginger), and those for free and bound AGEs were 71.57% and 35.64% (1.5% ginger). EPR result indicated alkyl free radical and 1O2 were the pivotal free radicals for HAs and AGEs, and ginger and curcumin also dose-dependently reduced these radicals. Lipid peroxidation, active carbonyl intermediates-phenylacetaldehyde, glyoxal, and methylglyoxal were also inhibited by ginger and curcumin in a dose-dependent manner, probably due to quenching of free radical.

The Effect of an External Magnetic Field on the Electrocatalytic Activity of Heat-Treated Cyanometallate Complexes towards the Oxygen Reduction Reaction in an Alkaline Medium.

This work focuses on the development of an electrocatalytic material by annealing a composite of a transition metal coordination material, iron hexacyanoferrate (Prussian blue) immobilized on carboxylic-acid-functionalized reduced graphene oxide. Pyrolysis at 500 °C under a nitrogen atmosphere formed nanoporous core–shell structures with efficient activity, which mostly included iron carbide species capable of participating in the oxygen reduction reaction in alkaline media. The physicochemical properties of the iron-based catalyst were elucidated using transmission electron microscopy, X-ray diffraction, Mössbauer spectroscopy, and various electrochemical techniques, such as cyclic voltammetry and rotating ring–disk electrode (RRDE) voltammetry. To improve the electroreduction of oxygen over the studied catalytic material, an external magnetic field was utilized, which positively shifted the potential by ca. 20 mV. The formation of undesirable intermediate peroxide species was decreased compared with the ORR measurements without an external magnetic field.

Insights into the carbonate effect on water oxidation over metal oxide photocatalysts/photoanodes.

Photocatalytic/photoelectrochemical water splitting using metal oxide semiconductors is a promising technology for direct and simple solar-energy conversion. The addition of carbonate salts to an aqueous reaction solution has been known to promote stoichiometric O2 evolution and H2O2 production via H2O oxidation. To elucidate the effect of carbonates, density functional theory calculations are performed to study the photoinduced H2O and H2CO3 oxidation mechanisms on TiO2 and BiVO4. The oxidation reactions proceeded via peroxide intermediates, such as H2O2 for H2O, H2C2O6 for H2CO3, and H2CO4 for the coexistence of H2O and H2CO3 molecules. Regardless of the reactant and metal oxide, the free energy changes in the four proton-coupled electron-transfer (PCET) steps of the oxidation mechanism indicate that the first PCET requires the highest energy input and is the rate-limiting step. All PCET steps of the H2O oxidation, except the second one, are more endergonic than those of the H2CO3 oxidation. The H2O reactant requires a larger energy barrier at the highest energy profile, as well as at the final state, than the H2CO3 reactant. The computational results verify that the adsorbed H2CO3 molecule is easily photo-oxidized compared with the adsorbed H2O molecule, facilitating the formation of the peroxide intermediate and improving O2 evolution and H2O2 production.

Laboratory investigation of the spontaneous combustion characteristics and mechanisms of typical vegetable oils

To effectively prevent and control the autoignition of vegetable oil, the spontaneous combustion characteristics and oxidation mechanisms of three typical vegetable oils were investigated. Microcal, TG-DSC, Raman spectroscopy and GC-MS were used to analyze the self-ignition characteristics of the samples at both macro- and micro-scales. The results showed that the initial exothermic temperatures were between 65 and 90 °C, after which vigorous oxidation began to release abundant heat. DSC data showed that the entire combustion process of the samples includes 4 typical stages: low-temperature oxidation stage, thermal decomposition stage, combustion stage, and after-combustion stage. The activation energy of the low-temperature oxidation stage reached as low as 33.13 kJ/mol, indicating that the vegetable oil is extremely prone to self-oxidation even when the temperature <200 °C. And the self-ignition tendency was in the order linseed oil > perilla seed oil > safflower seed oil. Based on analysis of the organic molecular structures, it was found that the unsaturated groups in vegetable oil are responsible for its autoignition behavior. Taking linseed oil as an example, 3 peroxide intermediates, namely 10,12-Octadecadienoicacid,9-hydroperoxy-, (10E,12Z)-;9,11-Octadecadienoicacid, 13-hydroperoxy-, (9Z,11E, 13S)-, and 9-Octadecenoic acid, 13-hydroperoxy-, (Z)-(8CI, 9CI) were marked as significant indicators to trace the low temperature oxidation reaction path of the oil samples.

Catalase protects against nonenzymatic decarboxylations during photorespiration in Arabidopsis thaliana.

Photorespiration recovers carbon that would be otherwise lost following the oxygenation reaction of rubisco and production of glycolate. Photorespiration is essential in plants and recycles glycolate into usable metabolic products through reactions spanning the chloroplast, mitochondrion, and peroxisome. Catalase in peroxisomes plays an important role in this process by disproportionating H2O2 resulting from glycolate oxidation into O2 and water. We hypothesize that catalase in the peroxisome also protects against nonenzymatic decarboxylations between hydrogen peroxide and photorespiratory intermediates (glyoxylate and/or hydroxypyruvate). We test this hypothesis by detailed gas exchange and biochemical analysis of Arabidopsis thaliana mutants lacking peroxisomal catalase. Our results strongly support this hypothesis, with catalase mutants showing gas exchange evidence for an increased stoichiometry of CO2 release from photorespiration, specifically an increase in the CO2 compensation point, a photorespiratory‐dependent decrease in the quantum efficiency of CO2 assimilation, increase in the 12CO2 released in a 13CO2 background, and an increase in the postillumination CO2 burst. Further metabolic evidence suggests this excess CO2 release occurred via the nonenzymatic decarboxylation of hydroxypyruvate. Specifically, the catalase mutant showed an accumulation of photorespiratory intermediates during a transient increase in rubisco oxygenation consistent with this hypothesis. Additionally, end products of alternative hypotheses explaining this excess release were similar between wild type and catalase mutants. Furthermore, the calculated rate of hydroxypyruvate decarboxylation in catalase mutant is much higher than that of glyoxylate decarboxylation. This work provides evidence that these nonenzymatic decarboxylation reactions, predominately hydroxypyruvate decarboxylation, can occur in vivo when photorespiratory metabolism is genetically disrupted.

Toward Understanding the Mechanism of Wine Oxidation

The electronic configuration of oxygen (O₂) does not allow it to react directly with wine reductants such as polyphenols. It relies on the catalytic intervention of iron (Fe), which redox cycles between its ferrous (Fe(II)) and ferric (Fe(III)) states. O₂ oxidizes Fe(II) to Fe(III), and Fe(III) then oxidizes polyphenols. Low concentrations of copper accelerate oxidation, and nucleophiles, especially sulfite, promote polyphenol oxidation. In wine that is protected from air, Fe exists mainly as Fe(II), but the Fe(III):Fe(II) concentration ratio increases immediately on air exposure, stabilizing at varying speeds and values. The oxidation of Fe(II) in air-saturated model wine and the reduction of Fe(III) by a catechol under nitrogen in model wine were examined separately to better understand the oxidative process. The Fe(III) produced when Fe(II) reacted with O₂ slows the reaction. As in wine, it was important to include sulfite to remove the intermediate hydrogen peroxide, which also oxidizes Fe(II). The reaction was pseudosecond-order in Fe(II), indicating that the transfer of both electrons to O₂ is rate determining. Similarly, when Fe(III) was reduced by the catechol, the Fe(II) produced inhibited the reaction, which overall followed a pseudosecond-order rate law in Fe(III). The rate of Fe(II) oxidation was slower than the rate of Fe(III) reduction, but when the reactions occurred together, as in wine oxidation, Fe(III) and Fe(II) concentrations equilibrated such that their rates equalized. Under the conditions studied, this occurred at 32% Fe(III). This equilibrium was attained quickly, as is the case in red wine. These findings on the oxidative process should help explain the relationships between wine composition, redox state, and Fe(III):Fe(II) concentration ratios.

(Invited) How the Surface Chemistry of WO3 Influences Its Activity for the Oxygen-Evolution Reaction

Tungsten(VI) oxide (WO3) remains prominent in photoelectrochemical approaches toward the oxygen evolution reaction (OER) from water because of its stability in acidic solution (desirably for its redox partner, the hydrogen evolution reaction). Instability in the photocurrent density during the OER has previously been attributed to formation of destructive peroxide intermediates. Under constant illumination, repeated cycles of poising electrodes at 0.98 V vs Ag/AgCl in pH 1 sulfate solution followed by rest periods at open-circuit potential show reversibility in the photocurrent decay. This behavior is attributed to reversible proton deintercalation/intercalation in H x WO3. H+ intercalation with concomitant reduction of W6+ begins during the synthesis from annealing an ammonium metatungstate sol that is spin coated onto fluorinated tin oxide. The net result is that H x WO3 shows an increased donor density (1021 cm–3). The change in surface composition during the OER—specifically tungsten reduction, is measurable by XPS analysis, Mott-Schottky analysis of electrochemical impedance spectra, and a broad near-IR absorption band. The Faradaic efficiency for the OER (ηOER) is ~70% on H x WO3. When an amorphous FeOOH electrocatalyst layer is deposited on the surface, the photocurrent density stabilizes and ηOER increases to near 100%. Moreover, fast oxidation reactions such as chloride oxidation also show very stable photocurrent density on WO3, hinting that other redox partners for hydrogen evolution under acidic conditions are plausible.

Effect of &lt;i&gt;Thymus marschallianus&lt;/i&gt; extract on lipid peroxidation processes &lt;i&gt;in vivo&lt;/i&gt; and &lt;i&gt;in vitro&lt;/i&gt; experiments

BACKGROUND: Currently, fundamental and experimental data confirming the role of free radicals in physiological and pathological processes have been accumulated. The literature describes conflicting information about the effect of antioxidants on the organism: they are able to protect not only normal cells from the alterative effect of free radicals, but also tumor cells, thereby supporting the survival and growth of the latter. Therefore, the study of the effect of Thymus marchallianus extract on the processes of lipoperoxidation in vivo against the background of the tumor process is relevant.&#x0D; AIM: To study the effect of Thymus marchallianus extract on the quantity of lipid peroxidation intermediates in the blood plasma of mongrel white rats with liver cancer PC-1 and compare with in vivo antioxidant activity.&#x0D; MATERIALS AND METHODS: The object of research was the dried Thymus marchallianus grass. Water and alcohol extracts were used in the experiment. Fifteen male white mongrel rats with a tumor were included in the experiment: the first group was intact, the second group received the water extract of Thymus marchallianus, and the third group received the alcohol extract of Thymus marchallianus. The activity of lipoperoxidation processes was evaluated, the content of intermediate lipid peroxidation intermediates malon dialdehyde, lipid hydroperoxides and medium-mass molecules in the blood plasma of experimental rats were estimated by means of conventional spectrophotometric methods. Determination of the antioxidant activity of the infusion and alcohol extraction in the in vitro experiment was carried out by the titrimetric method.&#x0D; RESULTS: In the control groups and the group of animals that received intraperitoneal water and alcohol extracts of Thymus marchallianus, increase in the intermediate products of lipid peroxidation was not revealed. The amount of malon dialdehyde, lipid hydroperoxides and medium-mass molecules in the blood plasma of experimental rats did not differ significantly from the level of similar indicators of intact animals. In the in vitro experiment, water and alcohol extracts from raw Thymus marchallianus showed antioxidant activity.&#x0D; CONCLUSIONS: For the first time, the experiment was conducted to study the effect of water and water-alcohol extracts of Thymus marchallianus on lipoperoxidation processes in mongrel white rats with alveolar liver cancer PC-1. The activity of formation of intermediate products of lipid peroxidation did not change, since their content in the blood plasma of experimental animals did not differ from the level of similar indicators of intact animals. In vitro the infusion and alcohol extract demonstrated antioxidant activity.

1,5,7‐Triazabicyclo[4.4.0]dec‐5‐ene Enhances Activity of Peroxide Intermediates in Phosphine‐Free α‐Hydroxylation of Ketones

The critical role of double hydrogen bonds was addressed for the aerobic α-hydroxylation of ketones catalyzed by 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD), in the absence of either a metal catalyst or phosphine reductant. Experimental and theoretical investigations were performed to study the mechanism. In addition to initiating the reaction by proton abstraction, a more important role of TBD was revealed, that is, to enhance the oxidizing ability of peroxide intermediates, allowing DMSO to be used rather than commonly used phosphine reductants. Further characterizations with nuclear Overhauser effect spectroscopy (NOESY) confirmed the presence of double hydrogen bonds between TBD and the ketone, and kinetic studies suggested the attack of dioxygen on the TBD-enol adduct to be the rate-determining step. This work should encourage the application of TBD as a catalyst for oxidations.

1,5,7‐Triazabicyclo[4.4.0]dec‐5‐ene Enhances Activity of Peroxide Intermediates in Phosphine‐Free α‐Hydroxylation of Ketones

AbstractThe critical role of double hydrogen bonds was addressed for the aerobic α‐hydroxylation of ketones catalyzed by 1,5,7‐triazabicyclo[4.4.0]dec‐5‐ene (TBD), in the absence of either a metal catalyst or phosphine reductant. Experimental and theoretical investigations were performed to study the mechanism. In addition to initiating the reaction by proton abstraction, a more important role of TBD was revealed, that is, to enhance the oxidizing ability of peroxide intermediates, allowing DMSO to be used rather than commonly used phosphine reductants. Further characterizations with nuclear Overhauser effect spectroscopy (NOESY) confirmed the presence of double hydrogen bonds between TBD and the ketone, and kinetic studies suggested the attack of dioxygen on the TBD‐enol adduct to be the rate‐determining step. This work should encourage the application of TBD as a catalyst for oxidations.

Oxygen Reduction by Iron Porphyrins with Covalently Attached Pendent Phenol and Quinol.

Phenols and quinols participate in both proton transfer and electron transfer processes in nature either in distinct elementary steps or in a concerted fashion. Recent investigations using synthetic heme/Cu models and iron porphyrins have indicated that phenols/quinols can react with both ferric superoxide and ferric peroxide intermediates formed during O2 reduction through a proton coupled electron transfer (PCET) process as well as via hydrogen atom transfer (HAT). Oxygen reduction by iron porphyrins bearing covalently attached pendant phenol and quinol groups is investigated. The data show that both of these can electrochemically reduce O2 selectively by 4e-/4H+ to H2O with very similar rates. However, the mechanism of the reaction, investigated both using heterogeneous electrochemistry and by trapping intermediates in organic solutions, can be either PCET or HAT and is governed by the thermodynamics of these intermediates involved. The results suggest that, while the reduction of the FeIII-O2̇- species to FeIII-OOH proceeds via PCET when a pendant phenol is present, it follows a HAT pathway with a pendant quinol. In the absence of the hydroxyl group the O2 reduction proceeds via an electron transfer followed by proton transfer to the FeIII-O2̇- species. The hydrogen bonding from the pendant phenol group to FeIII-O2̇- and FeIII-OOH species provides a unique advantage to the PCET process by lowering the inner-sphere reorganization energy by limiting the elongation of the O-O bond upon reduction.

The specific features of the liquid-phase oxidation of saturated esters. Kinetics, reactivity and mechanisms of formation of destruction products

Data on kinetics, reactivity, and mechanisms of formation of destruction products of the liquid-phase oxidation of saturated esters by molecular oxygen have been systematized and discussed. The ester group has both an activating and deactivating effects on the reactivity of the CH bonds of the alkoxyl (α′-, β′-, γ′-, and δ′-) and acyl (α-, β-, and γ-) groups. The activating effect in esters is weaker than in alcohols, ketones, and ethers and extends to the α- and α′-CH bonds. The reactivity of the β-, β′-, and γ′-CH bonds is significantly lower than that of the secondary CH bonds of hydrocarbons. Oxidation of the β-CH bonds of the acyl moieties of esters leads to HOO· radical, α,β-epoxy-, α,β-dihydroxy-, and α,β-unsaturated esters, but not to hydroperoxides. Upon the oxidation of esters, the highly active α-carboxy-alkylperoxy-, α-acyloxyperoxy-, and HOO· -radicals are formed. The recombination reactions involving these radicals occur both with and without chain termination. The specificity of the decomposition of ester hydroperoxides are due to intra- and intermolecular interactions of the hydroperoxide groups with the ester groups. Labile peroxide intermediates (α-hydroxyperoxy-esters, peroxyesters, and peroxylactones) are formed through the corresponding tetrahedral intermediates and decay predominantly by the non-radical pathway. The recombination of the peroxyl radicals and the decay of labile peroxides result in the parallel accumulation of peroxide and non-peroxide products during the oxidation of esters.

Content of lipid peroxidation products in pig meat at industrial and domestic slaughter

The nutritional value of meat, its organoleptic characteristics and therefore the shelf life depend on the content of lipid peroxidation products in it. Intermediates of peroxidation can have a toxic effect on the human body and are one of the main factors of non-microbial degradation of meat and various products. Based on this, the aim of this work was to study the content of lipid peroxidation products in pig meat of different origins. Furthermore, the aim was to establish the level of primary and secondary products of lipid peroxidation in meat, selected from different parts of the carcass. Meat samples were taken immediately after the pigs were slaughtered. A total of 3 meat samples were taken from 10 pig carcasses. One sample of meat was taken from each of the carcasses from the cervical, dorsal and femoral parts. The first 5 pigs were kept in industrial conditions and were slaughtered within the industrial slaughterhouse, and the second 5 were kept and slaughtered in personal farms. The results of the research showed that the highest level of primary and secondary products of lipid peroxidation is registered in the meat obtained from the dorsal part of pork carcass, which is primarily due to its fat-acid composition. In meat selected from the cervical and femoral parts, the level of the studied indicators is probably lower. During storage, lipid peroxidation activity increases from the first to the fourth day and decreases slightly until the seventh. The increase in the level of all studied indicators on the fourth day of meat storage is due to the fact that natural components contained in muscle tissue, such as ferrum, myoglobin, hydrogen peroxide and ascorbic acid can cause lipid oxidation, acting as catalysts or promoting the formation of reactive oxygen species. In the case of domestic keeping and slaughter, pig meat was found to have significantly higher concentrations of diene conjugates, lipid hydroperoxides and TBA-active products compared to industrial. The main prerequisite for this is an unbalanced diet, especially in terms of the content of substances that have antioxidant properties.

Facet Engineered α-MnO2 for Efficient Catalytic Ozonation of Odor CH3SH: Oxygen Vacancy-Induced Active Centers and Catalytic Mechanism.

The oxygen vacancy in MnO2 is normally proved as the reactive site for the catalytic ozonation, and acquiring a highly reactive crystal facet with abundant oxygen vacancy by facet engineering is advisable for boosting the catalytic activity. In this study, three facet-engineered α-MnO2 was prepared and successfully utilized for catalytic ozonation toward an odorous CH3SH. The as-synthesized 310-MnO2 exhibited superior activity in catalytic ozonation of CH3SH than that of 110-MnO2 and 100-MnO2, which could achieve 100% removal efficiency for 70 ppm of CH3SH within 20 min. The results of XPS, Raman, H2-TPR, and DFT calculation all prove that the (310) facets possess a higher surface energy than other facets can feature the construction of oxygen vacancies, thus facilitating the adsorption and activate O3 into intermediate peroxide species (O2-/O22-) and reactive oxygen species (•O2-/1O2) for eliminating adjacent CH3SH. In situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS) revealed that the CH3SH molecular was chemisorbed on S atom to form CH3S-, which was further converted into intermediate CH3SO3- and finally oxidized into SO42- and CO32-/CO2 during the process. Attributed to the deep oxidation of CH3SH on 310-MnO2 via efficient cycling of active oxygen vacancies, the lifetime of 310-MnO2 can be extended to 2.5 h with limited loss of activity, while 110-MnO2 and 100-MnO2 were inactivated within 1 h. This study deepens the comprehension of facet-engineering in MnO2 and presents an efficient and portable catalyst to control odorous pollution.

Nouryon slates peroxide intermediates

Nouryon plans to build a 35,000-metric-ton-per-year facility in Ningbo, China, to produce t-butyl alcohol and t-butyl hydroperoxide, intermediates for its organic peroxides business. Raw material will be supplied by a joint venture of Huntsman and Sinopec in Nanjing, China. Nouryon says it currently imports t-butyl hydroperoxide to make organic peroxides at the site.