Hydroperoxide Species Research Articles

Quantum-chemical simulation of the elementary step of the oxidation reactions of styrene and its derivatives involving 1О2 (1Δg)

The results of simulation of the oxidation reaction of styrene and its methyl (two isomers) and phenyl derivatives with molecular oxygen in the excited singlet state (1Δg) have enabled the conclusion that the reaction can proceed through several mechanisms. For styrene and its phenyl derivative, three reaction channels are possible, and for the methyl derivative, there are four possible channels. For the first two substrates, the major channel is 1,2-addition to form dioxetane; for the methyl derivatives, an extra channel to give a hydroperoxide species is possible in addition to the above channel. The multichannel reaction character revealed by calculations makes it possible to qualitatively understand the reason behind the moderate selectivity (no more than 70%) of such reactions in the case of styrene and its derivatives.

Thermochemical Properties and Bond Dissociation Energies for Fluorinated Methanol, CH3-xFxOH, and Fluorinated Methyl Hydroperoxides, CH3-xFxOOH: Group Additivity.

Oxygenated fluorocarbons are routinely found in sampling of environmental soils and waters as a result of the widespread use of fluoro and chlorofluoro carbons as heat transfer fluids, inert materials, polymers, fire retardants and solvents; the influence of these chemicals on the environment is a growing concern. The thermochemical properties of these species are needed for understanding their stability and reactions in the environment and in thermal process. Structures and thermochemical properties on the mono- to trifluoromethanol, CH3-xFxOH, and fluoromethyl hydroperoxide, CH3-xFxOOH (1 ≤ x ≤ 3), are determined by CBS-QB3, CBS-APNO, and G4 calculations. Entropy, S°298, and heat capacities, Cp(T)'s (300 ≤ T/K ≤ 1500) from vibration, translation, and external rotation contributions are calculated on the basis of the vibration frequencies and structures obtained from the B3LYP/6-31+G(d,p) density functional method. Potential barriers for the internal rotations are also calculated from this method and used to calculate hindered rotor contributions to S°298 and Cp(T)'s using direct integration over energy levels of the internal rotational potentials. Standard enthalpies of formation, ΔfH°298 (units in kcal mol(-1)) are CH2FOOH (-83.7), CHF2OOH (-138.1), CF3OOH (-193.6), CH2FOO(•) (-44.9), CHF2OO(•) (-99.6), CF3OO(•) (-153.8), CH2FOH (-101.9), CHF2OH (-161.6), CF3OH (-218.1), CH2FO(•) (-49.1), CHF2O(•) (-97.8), CF3O(•) (-150.5), CH2F(•) (-7.6), CHF2(•) (-58.8), and CF3(•) (-112.6). Bond dissociation energies for the R-OOH, RO-OH, ROO-H, R-OO(•), RO-O(•), R-OH, RO-H, R-O(•), and R-H bonds are determined and compared with methyl hydroperoxide to observe the trends from added fluoro substitutions. Enthalpy of formation for the fluoro-hydrocarbon oxygen groups C/F/H2/O, C/F2/H/O, C/F3/O, are derived from the above fluorinated methanol and fluorinated hydroperoxide species for use in Benson's Group Additivity. It was determined that fluorinated peroxides require interaction terms O/CH2F/O, O/CHF2/O, and O/CF3/O, as opposed to the common (O/C/O) group in hydrocarbons, resulting from interactions of the peroxide oxygen with the fluorines. Hydrogen bond dissociation increment (HBI) groups are also developed.

Kinetic study of propene oxide and water formation in hydro-epoxidation of propene on Au/Ti–SiO2 catalyst

A highly stable and active, Au/TiSiO2 catalyst was used to conduct an extensive kinetic study for the direct epoxidation of propene using hydrogen and oxygen in the non-explosive regime, with an aim to study the combined formation of propene oxide and water. A reaction mechanism was proposed which assumes that there are two types of Au sites: isolated, which forms only water and the ones in the vicinity of Ti which forms propene oxide as well as water. The active intermediate on both was assumed to be the hydro peroxide species, as widely accepted. Based on fundamental reaction steps obtained from this mechanism, rate equations were derived and fitted with the experimental observations. The kinetic model was found to describe well the simultaneous formation of propene oxide and water, thus emphasizing on the importance of Au–Ti synergy to obtain catalysts with higher activity and H2 utilization ability.

ChemInform Abstract: Two‐Phase Oxidations with Aqueous Hydrogen Peroxide Catalyzed by Amphiphilic Pyridinium and Diazinium Salts.

AbstractThis novel oxidation method takes advantage of interim covalent bonding of hydrogen peroxide to an amphiphilic pyridinium or diazinium salt to increase the lipophilicity of the hydroperoxide species which eventually allows for oxidation in a non‐polar medium.

Leonotis nepetifolia Protects against Acetaminophen-Induced Hepatotoxicity: Histological Studies and the Role of Antioxidant Enzymes

Aim of the study: High dose acetaminophen (APAP) increases the risk of liver injury caused by oxidative stress due to accumulation of reactive species. Although N-acetyl cysteine is the standard antidote used to treat acute APAPinduced liver failure, we proposed that known antioxidant phytochemicals in Leonotis nepetifolia extracts would protect against APAP-induced hepatic injury by modulating the activities of antioxidant enzymes. Materials and methods: Methanol and aqueous extracts of L. nepetifolia were orally administered in doses ranging (250 mg/kg to 1000 mg/kg) as pre- and post-treatment with high dose APAP (550 mg/kg) to Swiss albino mice. Twenty-four hours after the final dose, animals were euthanized and blood and liver collected for liver enzymes (ALT and AST), histological assessment and antioxidant enzyme assays. Results: Methanol and aqueous extracts as pre-treatment and post-treatment protected against hepatic injury. Extracts abrogated the 14-fold and 4-fold APAP-induced increases in ALT and AST respectively, including histopathological damage (p<0.05). Extracts reversed APAP-induced 4-fold and 14-fold increases in GR and SOD activities respectively (p<0.05). Additionally, extracts reversed APAP-induced decline in GPx activity; particularly the aqueous extract as pre-treatment increased GPx activity up to 2.2-fold over saline-treated controls (p<0.05). Conclusions: Extracts, as pre-treatment and post-treatment, prevented APAP-induced hepatic injury by modulating the activities of antioxidant enzymes. Of particular interest, is the reversal of APAP-induced decrease in GPx activity and increase in SOD activity? Extract-induced increase in GPx activity would facilitate the scavenging of hydroperoxide and peroxide reactive species generated by high dose APAP. We propose that antioxidant phytochemicals in L. nepetifolia may be acting as free radical scavengers which results in reduced SOD activity.

Two‐Phase Oxidations with Aqueous Hydrogen Peroxide Catalyzed by Amphiphilic Pyridinium and Diazinium Salts

AbstractAmphiphilic pyridinium and diazinium salts were shown to be effective catalysts in two‐phase (water/chloroform or water/dichloromethane) sulfoxidations and N‐oxidations with hydrogen peroxide under mild conditions. This unprecedented oxidation method utilizes covalent bonding of hydrogen peroxide to a simple pyridinium or diazinium nucleus to increase the lipophilicity of the hydroperoxide species and to subsequently activate it for oxidations in a non‐polar medium. The catalytic efficiency was found to depend on the type of heteroarenium core and on the lipophilicity of the catalyst. Five series of heteroarenium catalysts were prepared and investigated: 1‐Alkyl‐3,5‐dicyanopyridinium, 1‐alkyl‐3,5‐dinitropyridinium, 1‐alkyl‐3‐cyanopyrazinium, 1‐alkyl‐4‐cyanopyrimidinium and 1‐alkyl‐4‐(trifluoromethyl)pyrimidinium triflates (alkyl=butyl, hexyl, octyl, decyl, dodecyl and hexadecyl). Among them, the 1‐octyl‐3,5‐dinitropyridinium and 1‐decyl‐4‐(trifluoromethyl)pyrimidinium triflates were found to be superior catalysts, showing the best stability and the highest catalytic activity, achieving acceleration by a factor of 350 relative to the non‐catalyzed reaction. In contrast to other organocatalytic two‐phase oxidations that use hydrogen peroxide, the presented method is characterized by high chemoselectivity and low catalyst loading (5 mol%) and with the reactions being performed under mild conditions, that is, at 25 °C using diluted hydrogen peroxide and a non‐basic aqueous phase. The catalysts have simple structures and are readily available from commercial materials. Practical applications are demonstratedviathe oxidation of several types of sulfides and amines.magnified image

Amine oxidative N-dealkylation via cupric hydroperoxide Cu-OOH homolytic cleavage followed by site-specific fenton chemistry.

Copper(II) hydroperoxide species are significant intermediates in processes such as fuel cells and (bio)chemical oxidations, all involving stepwise reduction of molecular oxygen. We previously reported a Cu(II)-OOH species that performs oxidative N-dealkylation on a dibenzylamino group that is appended to the 6-position of a pyridyl donor of a tripodal tetradentate ligand. To obtain insights into the mechanism of this process, reaction kinetics and products were determined employing ligand substrates with various para-substituent dibenzyl pairs (-H,-H; -H,-Cl; -H,-OMe, and -Cl,-OMe), or with partially or fully deuterated dibenzyl N-(CH2Ph)2 moieties. A series of ligand-copper(II) bis-perchlorate complexes were synthesized, characterized, and the X-ray structures of the -H,-OMe analogue were determined. The corresponding metastable Cu(II)-OOH species were generated by addition of H2O2/base in acetone at -90 °C. These convert (t1/2 ≈ 53 s) to oxidatively N-dealkylated products, producing para-substituted benzaldehydes. Based on the experimental observations and supporting DFT calculations, a reaction mechanism involving dibenzylamine H-atom abstraction or electron-transfer oxidation by the Cu(II)-OOH entity could be ruled out. It is concluded that the chemistry proceeds by rate limiting Cu-O homolytic cleavage of the Cu(II)-(OOH) species, followed by site-specific copper Fenton chemistry. As a process of broad interest in copper as well as iron oxidative (bio)chemistries, a detailed computational analysis was performed, indicating that a Cu(I)OOH species undergoes O-O homolytic cleavage to yield a hydroxyl radical and Cu(II)OH rather than heterolytic cleavage to yield water and a Cu(II)-O(•-) species.

New reaction classes in the kinetic modeling of low temperature oxidation of n-alkanes

Due to the rapid advance in analytical methods, a large detail of intermediate products from the low temperature oxidation of hydrocarbon fuels in jet stirred reactors became recently available in the literature. This new comprehensive information allowed to highlight systematic deviations between model predictions and experimental measurements, both in some oxygenated species and in fuel conversion at very low temperatures (550–650K). These discrepancies are largely reduced by including new reaction classes in the oxidation mechanism. Successive H-abstraction and molecular reactions of hydroperoxide species are proved to play a relevant role at very low temperatures, in competition with the chain branching decomposition reactions. Several recombination and disproportionation reactions of peroxy radicals are also of interest in explaining some intermediate oxygenated components. The aim of this paper is to discuss and analyze the ability of these new reaction classes to explain the formation of oxygenated species, such as organic acids and dicarbonyl species. Rate constants were determined using similarity and analogy rules for consolidated reactions, while theoretical calculations were performed to determine preliminary estimates for reaction channels lacking a reference reaction. Detailed comparisons with several experimental data of propane and n-butane oxidation at low temperatures support these assumptions. This kinetic study points out the need of further research activities in the investigation of successive reactions of hydroperoxide species.

Improved Kinetic Model of the Low-Temperature Oxidation of n-Heptane

The low- and high-temperature oxidation mechanisms of n-heptane have been extensively studied in recent and past literature because of its importance as a primary reference fuel. Recent advanced analytical methods allowed for the identification of several intermediate oxygenated species at very low-temperature conditions in jet-stirred reactors. On these bases, new classes of successive reactions involving hydroperoxide species, already discussed for propane and n-butane oxidation, were included in the low-temperature oxidation mechanism of n-heptane. These new reactions allowed for the improvement of the overall mechanism, not only obtaining a satisfactorily agreement with reaction products, such as organic acids, diones, and ketones, but also in terms of system reactivity. Moreover, deeper attention was also paid to the formation of ketohydroperoxides, rarely experimentally measured. Because of n-heptane importance as a primary reference fuel, the overall POLIMI kinetic mechanism is validated in a wide ...

Au/uncalcined TS-1 catalysts for direct propene epoxidation with H2 and O2: Effects of Si/Ti molar ratio and Au loading

For direct propene epoxidation with H2/O2, TS-1 with blocked micropores (TS-1-B) is recognized as an attractive support for Au catalysts with enhanced stability and activity by suppressing the deactivation caused by micropore blocking. In this work, as a consecutive effort, effect of Si/Ti molar ratio of TS-1-B on catalytic performance is investigated. It is shown that the optimum Si/Ti molar ratio is 40. In other words, both lower and higher ratios are unfavorable for the formation of propylene oxide, which are possibly attributed to the presence of extra-framework Ti species and the difficult transmission of hydroperoxide species from Au surface to nearby Ti sites, respectively. Based on this, effect of Au loading on catalytic performance is further examined. When the Au loading is 0.13wt%, the catalyst shows not only high PO formation rate of 158gPOh−1kgCat−1 comparable to the reported best activity without adding promoters, but also significantly enhanced stability at 200°C over 30h.

Kinetic analysis of the formation and decay of a non-heme ferric hydroperoxide species susceptible to O-O bond homolysis.

The formation of a ferric hydroperoxide species from [Fe(bbpc)(MeCN)2](2+) (bbpc = N,N'-dibenzyl-N,N'-bis(2-pyridylmethyl)-1,2-cyclohexanediamine) and its subsequent decomposition were analyzed kinetically. The rate of decay is not strongly influenced by the presence of either water or substrate, suggesting that the ferric hydroperoxide degrades through O-O bond homolysis and is not the relevant metal-based oxidant in the observed catalysis of C-H activation. The rate law corresponding to the complex's formation from O2 is consistent with the intermediacy of a mononuclear ferric superoxo species.

C–H Oxidation by H2O2 and O2 Catalyzed by a Non-Heme Iron Complex with a Sterically Encumbered Tetradentate N-Donor Ligand

The compound N,N'-dineopentyl-N,N'-bis(2-pyridylmethyl)-1,2-ethanediamine (dnbpn) and its ferrous complex [Fe(dnbpn)(OTf)2] were synthesized. The Fe(II) complex was used to catalyze the oxidation of hydrocarbons by H2O2 and O2. Although the catalyzed alkane oxidation by H2O2 displays a higher preference for secondary over tertiary carbons than those associated with most previously reported nonheme iron catalysts, the catalytic activity is markedly inferior. In addition to directing the catalyzed oxidation toward the less sterically congested C-H bonds of the substrates, the neopentyl groups destabilize the metal-based oxidants generated from H2O2 and the Fe(II) complex. The presence of benzylic substrates with weak C-H bonds stabilizes an intermediate which we have tentatively assigned as a high-spin ferric hydroperoxide species. The oxidant generated from O2 reacts with allylic and benzylic C-H bonds in the absence of a sacrificial reductant; less substrate dehydrogenation is observed than with related previously described systems that use O2 as a terminal oxidant.

Effect of chemical structure on secondary organic aerosol formation from C&amp;lt;sub&amp;gt;12&amp;lt;/sub&amp;gt; alkanes

Abstract. The secondary organic aerosol (SOA) formation from four C12 alkanes (n-dodecane, 2-methylundecane, hexylcyclohexane, and cyclododecane) is studied in the Caltech Environmental Chamber under low-NOx conditions, in which the principal fate of the peroxy radical formed in the initial OH reaction is reaction with HO2. Simultaneous gas- and particle-phase measurements elucidate the effect of alkane structure on the chemical mechanisms underlying SOA growth. Reaction of branched structures leads to fragmentation and more volatile products, while cyclic structures are subject to faster oxidation and lead to less volatile products. Product identifications reveal that particle-phase reactions involving peroxyhemiacetal formation from several multifunctional hydroperoxide species are key components of initial SOA growth in all four systems. The continued chemical evolution of the particle-phase is structure-dependent, with 2-methylundecane SOA formation exhibiting the least extent of chemical processing and cyclododecane SOA achieving sustained growth with the greatest variety of chemical pathways. The extent of chemical development is not necessarily reflected in the oxygen to carbon (O : C) ratio of the aerosol as cyclododecane achieves the lowest O : C, just above 0.2, by the end of the experiment and hexylcyclohexane the highest, approaching 0.35.

Thermal oxidative degradation of styrene-butadiene rubber (SBR) studied by 2D correlation analysis and kinetic analysis

The thermal oxidation degradation of styrene–butadiene rubber (SBR) was investigated by in situ FTIR, 2D-FTIR, and programming heating DSC. The results of analyses suggest that the degradation reaction is an autocatalytic process and mainly occurs on the aliphatic part instead of benzene pendants. Based on the results of in situ FTIR and 2D-FTIR, the oxidation process can be divided into three stages. In stage one, just two carbonyl peaks appear, namely 1,697 cm−1 (conjugate carbonyls) and 1,727 cm−1 (saturated carbonyls), and the generation speed is 1,697 > 1,727 cm−1. Yet the peaks appearing at 1,777 cm−1 belonged to peresters and anhydrides generating in stages two and three. The generation sequences are: 1,698 > 1,727 > 1,777 cm−1 for stage two; and 1,698 < 1,727 < 1,777 cm−1 for stage three. According to DSC results, the thermal oxidation of SBR contains four steps. The first step is the generation of alkyl radicals and the accumulation of hydroperoxide species. The second step is initial oxidation stage mainly producing conjugate carbonyls. The third step is deep oxidation process generating diverse carbonyls. The fourth is chain termination reaction, in which step the generation rates of anhydrides and peresters are the fastest due to bi-radical termination of alkoxy radicals and the consumption of conjugate carbonyl. Furthermore, crosslinking reactions occur during the whole thermal oxidation.

Horner-Wadsworth-Emmons Reactions in THF: Effect of Hydroperoxide Species

Horner–Wadsworth–Emmons (HWE) reactions routinely employ tetrahydrofuran (THF) as the reaction solvent. In this paper we show that THF adducts (derived from THF–hydroperoxide species) of HWE phosphonate ester compounds are formed under microwave irradiation (i.e., under pressure), or in the presence of a reductant [e.g., P(OEt)₃] in a conventionally heated reaction.

Kinetics and mechanism of the manganese(ii) catalysed Calmagite dye oxidation using in situ generated hydrogen peroxide

The kinetics and mechanism for the bleaching of Calmagite (H3CAL, 3-hydroxy-4-(2-hydroxy-5-methylphenylazo)naphthalene-1-sulfonic acid) in aqueous solution at pH 8.00 and 23 ± 1 °C using in situ generated H2O2 is described. Complete mineralisation of H3CAL results with turnover frequencies (TOF = moles of H3CAL bleached per mole of manganese per hour) of 40 h(-1). The monohydroxy azo dyes Me-H2CAL, Orange G and Orange II are not bleached which indicates that a requirement of dye bleaching is the coordination of the dye to the Mn centre. Spectroscopic studies show the formation of Mn(CAL)2 and Mn(CAL) species but in the presence of Tiron (1,2-dihydroxybenzene-3,5-disulfonate, disodium salt, monohydrate, Na2TH2·H2O), [Mn(CAL)(T)] is formed. It is proposed that a Mn(III)-hydroperoxide species is generated, [Mn(O2H)(CAL)(TQ)] from the in situ generated H2O2, where TQ represents the o-quinone form of Tiron, and this is the active species in the bleaching of coordinated CAL; the formation of this hydroperoxide species is supported by UV/VIS and ESI-MS data. The formation of a Mn(III) species is supported by EPR studies which also show some evidence for the presence of a labile d(5) Mn(II) species in the presence of the reducing substrate hydroxylamine (NH2OH). This would enable rapid ligand exchange for both in situ H2O2 generation and dye bleaching to occur; there is no evidence for the presence of Mn(IV)=O species. The virtue of low local concentrations of in situ generated H2O2 is shown to be important in preventing over oxidation of the catalyst and thus contributing to a robust catalytic system.

Coordination Chemistry and Reactivity of a Cupric Hydroperoxide Species Featuring a Proximal H-Bonding Substituent

At -90 °C in acetone, a stable hydroperoxo complex [(BA)Cu(II)OOH](+) (2) (BA, a tetradentate N(4) ligand possessing a pendant -N(H)CH(2)C(6)H(5) group) is generated by reacting [(BA)Cu(II)(CH(3)COCH(3))](2+) with only 1 equiv of H(2)O(2)/Et(3)N. The exceptional stability of 2 is ascribed to internal H-bonding. Species 2 is also generated in a manner not previously known in copper chemistry, by adding 1.5 equiv of H(2)O(2) (no base) to the cuprous complex [(BA)Cu(I)](+). The broad implications for this finding are discussed. Species 2 slowly converts to a μ-1,2-peroxodicopper(II) analogue (3) characterized by UV-vis and resonance Raman spectroscopies. Unlike a close analogue not possessing internal H-bonding, 2 affords no oxidative reactivity with internal or external substrates. However, 2 can be protonated to release H(2)O(2), but only with HClO(4), while 1 equiv Et(3)N restores 2.

Salen copper (II) complex encapsulated in Y zeolite: An effective heterogeneous catalyst for TCF pulp bleaching using peracetic acid

Cu(salen) and NaY encapsulated complexes, were synthesized by the “impregnation” (Cu(salen)/Y-IM) and flexible ligand “ship-in-a-botttle” (Cu(salen)/Y-SB) method. The resulting compounds were characterized by XRD, FTIR, DR UV–vis, BET. The encapsulated complexes show structural properties and chemical behaviour which is different from those of the neat complex. The catalytic bleaching by Cu(salen) and encapsulated Cu(salen) was tested for the eucalyptus oxygen delignified kraft pulp using peracetic acid as an oxidant. Influence of catalysis on TCF (total chlorine free) bleaching viz. delignification, property of bleached pulp was studied. Under the catalytic conditions, the encapsulated copper complexes exhibit enhanced activity or selectivity compared to the neat complex. The reasons for the enhanced activity of Cu(salen) upon encapsulation in NaY are reported by UV–vis techniques. The catalytic performance of encapsulated complex could be attributed to the formation of copper–hydroperoxide species.

Cholesteryl ester acyl oxidation and remodeling in murine macrophages: formation of oxidized phosphatidylcholine

Cholesterol is an essential component of eukaryotic cell membranes, regulating fluidity and permeability of the bilayer. Outside the membrane, cholesterol is esterified to fatty acids forming cholesterol esters (CEs). Metabolism of CEs is characterized by recurrent hydrolysis and esterification as part of the CE cycle; however, since recombinant 15-lipoxygenase (15-LO) was shown to oxidize cholesteryl linoleate of LDL, there has been interest in CE oxidation, particularly in the context atherogenesis. Studies of oxidized CE (oxCE) metabolism have focused on hydrolysis and subsequent reverse cholesterol transport with little emphasis on the fate the newly released oxidized fatty acyl component. Here, using mass spectrometry to analyze lipid oxidation products, CE metabolism in murine peritoneal macrophages was investigated. Ex vivo macrophage incubations revealed that cellular 15-LO directly oxidized multiple CE substrates from intracellular stores and from extracellular sources. Freshly harvested murine macrophages also contained 15-LO-specific oxCEs, suggesting the enzyme may act as a CE-oxidase in vivo. The metabolic fate of oxCEs, particularly the hydrolysis and remodeling of oxidized fatty acyl chains, was also examined in the macrophage. Metabolism of deuterated CE resulted in the genesis of deuterated, oxidized phosphatidylcholine (oxPC). Further experiments revealed these oxPC species were formed chiefly from the hydrolysis of oxidized CE and subsequent reacylation of the oxidized acyl components into PC.

Gold-catalyzed aerobic epoxidation of trans-stilbene in methylcyclohexane. Part II: Identification and quantification of a key reaction intermediate

The gold-catalyzed aerobic oxidations of alkenes are thought to rely on the in situ synthesis of hydroperoxide species, which have however never been clearly identified. Here, we show direct experimental evidence for the presence of 1-methylcyclohexyl hydroperoxide in the aerobic co-oxidation of stilbene and methylcyclohexane catalyzed by the Au/SiO2-R972 optimized catalyst prepared in Part I. Determination of its response in gas chromatography, by triphenylphosphine titration followed by 31P NMR, allows to easily follow its concentration throughout the co-oxidation process and to clearly highlight the simultaneous existence of the methylcyclohexane autoxidation pathway and the stilbene epoxidation pathway.