Free radicals as intermediates in catalytic oxidation of light alkanes: New opportunities

The analysis of catalytic partial oxidation of light alkanes indicates that processes involving this group proceed via the formation and consecutive transformations of free radicals. Depending on the properties of the catalytic system and reaction conditions the same primary radical can give different final products, olefins, oxygenated organic substances and carbon oxides. An approach to design a complex catalytic system for efficient alkane partial oxidation based on separation of zones where free radicals are formed and where they are transformed into desired products is suggested and examples of its implementation are presented.     

Identification of oxidation products and free radicals of tryptophan by mass spectrometry

New oxidation products and free radicals derived from tryptophan (Trp) oxidation under Fenton reaction conditions were identified using mass spectrometry. After the oxidation of tryptophan using hydrogen peroxide and iron (II) system (Fenton reaction), mono- and dihydoxy tryptophans and N-formylkynurenine were identified using electrospray mass spectrometry (ES-MS) and ES-MS/MS. Besides these products, new products resulting from the reaction of tryptophan and oxidized tryptophan and 3-methyl indole derivatives were also identified. The 3-methyl indole derivatives resulted, most probably, from the oxidation process and not from in-source processes. A dimer formed by cross-linking between two Trp radicals (Trp-Trp), similar to the previously described tyrosine dimer was observed, as well as the corresponding monohydroxy-dimer (Trp-Trp-OH). Tandem mass spectrometry was used to identify the structures of these new oxidation products. Free radicals derived from tryptophan oxidation under Fenton reaction were detected using as spin trap the DMPO. The free radical species originated during the oxidation reaction formed stable adducts with the spin trap, and these adducts were identified by ES-MS. New adducts of oxidized tryptophan radicals, namely monohydroxy-tryptophan and dihydroxy-Trp dimer radicals, with one and two DMPO spin trap molecules where identified. Tandem mass spectrometry was used to confirm the proposed structure of the observed adducts.     

The effect of 1,3-diphenylisobenzofuran on the photo-oxidative degradation of cis-1,4-polybutadiene

1,3-Diphenylisobenzofuran (DPBF) is easily photo-oxidized by two mechanisms viz free radical oxidation and singlet oxygen oxidation. The final products of DPBF oxidation by these two mechanisms are the same. Using light in the range 280–480 nm, DPBF is an effective sensitizer of photooxidative degradation of polybutadiene in the solid and in solution. In a system with methylene blue (MB) in methanol-benzene solution (0.5:9.5) where free radicals from MB and ¹O₂ are formed during irradiation with visible light, DPBF is oxidized by both ¹O₂ and free radical mechanisms. DPBF cannot stop free radical degradation of PB initiated by MB radicals in MB-methanol-benzene solution. These results show that the DPBF is an ineffective stabilizer for polydienes against ¹O₂ and free radical oxidation. It rather acts as a sensitizer for photo-oxidation of polydienes.     

Catalytic activity of CuI/CuII cyanide based phenanthroline-bicarbonate system for enhancing aerobic oxidation of 2,6-di-tert-butylphenol

The metal organic framework {[Cu₂(CN)₃(phen)₃]5H₂O} MOF1- bicarbonate system was investigated as an efficient catalyst for aerobic oxidation of 2, 6-di-tert-butylphenol (2,6- DTBP). The catalytic system showed very efficient catalytic behavior for the oxidation of selective coupling of 2,6- DTBP to 3,3′,5,5′-tetra-tert-butyl-4,4′-diphenoquinone (DPQ) in excellent yield. The influence of reaction parameters on the selective oxidation of 2, 6-DTBP to DPQ had been investigated. Photoluminescence probing technology of Disodium salt of terephthalic acid as well as scavenging experiments revealed the creation of the hydroxyl radicals as the main active oxidation radicals produced by the MOF1/O₂/basic bicarbonate system. The oxidation reaction mechanism was also discussed. The recycled catalytic system retained its activity for eight successive runs.     

Synthesis of highly substituted pyrroles via oxidative free radical reactions of β-aminocinnamates

Oxidative free radical reactions of β-aminocinnamates are described. Imine radicals produced by tetra-n-butylammonium cerium(IV) nitrate (TBACN) oxidation of enamines undergo efficient addition to the C-C double bond of β-aminocinnamates. This TBACN mediated free radical reaction between β-aminocinnamates and enamines provides a novel method for the synthesis of highly substituted pyrroles. The direct TBACN oxidation of β-aminocinnamates gave the dimerization products effectively.     

Catalysis of radical reactions in mixed micelles of surfactants with hydroperoxides

The peculiarities of the catalytic action of cationic surfactants (CSurf) in combination with hydroperoxides on the generation of radicals and the influence of various factors on this process (transition metal compounds, oxygen, and external magnetic field) were considered. In the oxidized hydrocarbons (RH), hydroperoxides (ROOH), which are the primary amphiphilic products of oxidation, form mixed micelles {mROOH…nCSurf} with CSurf, in which fast decomposition of ROOH into radicals occurs and other polar components (metal compounds, inhibitors, etc.) can concentrate, which significantly affects the rate and mechanism of oxidation. The cationic surfactants immobilized on a solid support retain the ability to catalyze the decomposition of hydroperoxides, forming radicals, and to initiate radical oxidation and polymerization. It was found that acetylcholine, which is the most important neurotransmitter that plays an important role in the neuromuscular and cognitive activity of living beings, like cationic surfactants, catalyzes the radical decomposition of hydroperoxides in organic media, and the yield of radicals in this process decreases in a magnetic field and in the presence of oxygen.     

Temperature effect on the rate of formation of free radicals in CTAB-catalyzed decomposition of hydroperoxides

The temperature effect on the rate of the decomposition of hydroperoxides and the rate of the formation of free radicals in the oxidation of ethylbenzene with molecular oxygen in the presence of -phenylethyl hydroperoxide—cetyltrimethylammonium bromide (CTAB) as a catalytic system for free radical generation was studied by kinetic methods (from the oxygen consumption and hydroperoxide decomposition rates) and the inhibition method involving different acceptors of free radicals.     

Oxidation of alkylarene C-H Bonds by tert-butyl hydroperoxide in the presence of cobalt, chromium, and vanadium acetylacetonates

Cobalt, chromium, and vanadium acetylacetonates catalyze efficient hydroperoxide oxidation of alkylarenes under mild conditions (20°C). The process takes place with either conservation, or destruction of carbon backbone of the hydrocarbon. Oxidizing agents are oxygen generated in the system, as well as tert-butylperoxy radicals. In the formation of end products alkylperoxy radicals originating in the substrates, tert-butylperoxy radicals, as well as metal-containing peroxides are involved. For the oxidative degradation processes a recombination reaction of the alkylperoxy radicals is suggested with the intermediate formation of 1,2-dioxetanes.     

Liquid-phase oxidation of cyclohexane. Elementary steps in the developed process,reactivity, catalysis,and problems of conversion and selectivity

The literature data concerning features of the kinetics and mechanisms of elementary steps of liquid-phase oxidation of cyclohexane and its oxygen derivatives are considered and analyzed. A comparison of rates of intermolecular and intramolecular reactions of cyclohexylperoxyl radicals under the industrial conditions indicated a necessity to take into account intramolecular interactions. The occurrence of cross recombination of hydroperoxyl and α-hydroxyperoxyl radicals without chain termination in the course of cyclohexanol and 2-hydroxycyclohexanol oxidation was proved. A significance of degenerate branching reactions involving cyclohexyl hydroperoxide in the industrial process of cyclohexane oxidation at 423 K was evaluated. The influence of the electron-withdrawing functional groups on the reactivity of carbon–hydrogen bonds of organic compounds in the reactions with electrophilic peroxyl radicals was studied. The low conversion of a substrate in the industrial process are mainly caused by the radicalchain oxidation of cyclohexanone leading only to by-products. The catalysts of cyclohexane oxidation, viz., compounds of variable valence metals, affect the reaction rate and ratio of the yields of the target products (cyclohexyl hydroperoxide, cyclohexanol, and cyclohexanone) but exert no effect on their relative reactivity. The use of the catalytic additives increasing the yield of cyclohexanone in the step of cyclohexane oxidation in the production of caprolactam is revealed to be inexpedient.     

Sensitive luminescence techniques to study the early stages of polymer oxidation

This paper describes recent developments in the use of chemiluminescence (CL) and profluorescent nitroxides (PFNs) in probing the “induction period” of polymer oxidation. CL measures the instantaneous rate of reaction of hydroperoxides responsible for initiating degradation and the spreading of oxidation, while PFNs can be used to measure the concentration of alkyl radicals produced in oxidation events and thus provide an integrating sensor for the extent of cumulative damage. The PFN additive acts as an oxidation retarder by competing with oxygen to scavenge the alkyl radicals that generate chain carrying peroxy radicals and so mirrors the performance of hindered amine stabilisers (HAS) in one part of their stabilisation cycle. Using the example of polypropylene (PP) and cis-polyisoprene (PIP) as substrates which can rapidly spread oxidative damage, the factors controlling the reaction of PFNs can be determined from CL and fluorescence as well as infra-red (IR) spectroscopy through the detection of oxidation products as measured by the carbonyl index. Matrix effects on the reactivity are demonstrated using a polyethylene-norbornene copolymer (TOPAS) as carrier for both PIP and the PFN and it is seen that the PFN is a radical scavenger only above T_g of the carrier. When PIP alone is oxidized, the PFN is an integrating sensor for free radical production under ambient conditions for up to twelve months while also stabilizing the polymer. Critically, it is thus able to determine the underlying rate of radical production in the oxidation induction period.     

Radiation-induced oxidation of solid poly(ethylene oxide). II. Mechanism

The mechanism of γ-initiated oxidation of solid poly(ethylene oxide) (PEO) has been investigated, and an overall reaction scheme has been developed which accounts for most of the experimental observations. Data are correlated on the basis that the oxidation process is the sum of two reactions that are first-order and half-order in rate of initiation. They provide evidence that a significant fraction of the interactions of α-alkoxyalkylperoxy radicals is nonterminating at ambient temperature and yield free alkoxy radicals that are very subject to β scission. The unimolecular decomposition of secondary peroxy radicals, which has been invoked previously for the oxidation of PEO in solution, is not needed to explain the products of the oxidation of PEO in the solid phase. Approximately 90% of the total oxygen consumption has been accounted for by the observed products of oxidation. The radiochemical yield for backbone radical formation in irradiated PEO was estimated to be 6.5 ± 1.5.     

γ射线辐射医用级超高分子量聚乙烯自由基的演变

采用γ射线对医用级超高分子量聚乙烯(UHMWPE)进行辐照处理, 利用电子自旋共振波谱仪(ESR)研究了辐照诱导自由基的种类及其在氩气和不同氧分压下的衰减行为. 在氩气中, 辐射诱导UHMWPE主要产生烷基自由基和烯丙基自由基, 总的辐射化学产额约为0.48/100 eV. 室温下烷基自由基的稳定性差, 其寿命仅有 1 d左右. 在含氧气氛中, 自由基主要通过氧化反应而衰减, 其衰减速率随氧分压的增加而增加, 半衰期则由1×10⁵ Pa氩气中的224.0 h降至5×10⁵ Pa O₂气中的1.8 h. 根据此结果推算, 室温下陷落在晶区的自由基迁移至微晶表面的速率非常快, 仅需小时量级.     

Di(tert-butylperoxy)triphenylbismuth and the triphenylbismuth—tert-butyl hydroperoxide system as efficient oxidants of alcohols

Di(tert-butylperoxy)triphenylbismuth and the triphenylbismuth—tert-butyl hydroperoxide system react with aliphatic alcohols and cyclohexanol to give carbonyl compounds in high yields. The oxidation occurs as the radical dehydrogenation of alcohols; Bi derivatives serve as the sources of free radicals.     

Hydrocarbon Oxidation by a Porphyrin-π-Cation Radical Complex

Heme and chlorin π-cation radical oxidants are widely implicated in biological and synthetic oxidation catalysis. Little insight into the role of π-cation radicals in proton coupled electron transfer (PCET) oxidation is available. We prepared a Ni^II-porphyrin-π-cation complex ([Ni^II(P⋅⁺)]) and found it to be capable of the oxidation of a variety of simple hydrocarbon substrates. Interestingly, some of the products were hydroxylated, with ([Ni^II(P⋅⁺)]) working in concert with atmospheric O₂ to yield hydroxylated hydrocarbons. Kinetic data suggested that the porphyrin-π-cation radical species oxidised substrates through a concerted PCET mechanism, where the porphyrin-π-cation radical accepted the electron, and the proton was transferred to a free anion. Our findings highlight the potential role of π-cation radicals as hydrocarbon activators, demonstrating that porphyrin ligand non-innocence could be a readily manipulated resource for oxidation catalyst development.     

Laser flash photolysis in a O3/Cl2 mixture at 266 nm in a very low-pressure flow system

The laser flash photolysis in a very low-pressure flow system with mass spectrometry detection technique was developed for the study of oxidation reactions of chlorofluorocarbons. In this work, we have studied the UV photolysis of O3 in the presence of Cl2 at room temperature, which presents two catalytic cycles of O3 depletion with efficiencies dependent on the partial pressures in the photoreactor. The ozone dissociation was initiated with fourth harmonic pulses of a Nd:YAG laser. The detection of the reactants and the final and intermediate reaction products was performed with real-time mass spectrometry. The variations of the O3, Cl2 and ClO concentration were measured. The equations system associated to a proposed kinetic scheme was solved numerically and excellent agreement with the experimental results was obtained. The results from this work allowed the determination of the wall loss rates of the O(1D), Cl and ClO radicals.     

Fe~(2+)协同热活化过一硫酸氢钾盐诱导自由基脱除NO的研究

在气液撞击流反应器中,研究了Fe~(2+)协同热活化过一硫酸氢钾盐诱导自由基脱除模拟烟气中的NO。考察了主要工艺参数(溶液温度、Fe~(2+)浓度、过一硫酸氢钾盐浓度、溶液pH值、NO入口浓度)对NO脱除效率的影响。分析检测了反应产物和自由基。基于不同系统的对比研究、反应产物检测和活性自由基的捕获,揭示了NO脱除过程的机制和反应路径。结果表明,提高溶液温度、Fe~(2+)浓度和过一硫酸氢钾盐浓度均提高了NO的脱除效率,而提高溶液pH值和NO入口浓度均降低了NO的脱除效率。Fe~(2+)和热对活化过一硫酸氢钾盐产生自由基有显著的协同效应。自由基氧化是NO脱除的主要路径,而过一硫酸氢钾盐直接氧化是次要的脱除路径。Fe~(2+)和热的协同活化体系具有比其他体系高得多的NO脱除率。     

Perylene-based profluorescent nitroxides for the rapid monitoring of polyester degradation upon weathering: An assessment

A profluorescent nitroxide possessing an isoindoline nitroxide moiety linked to a perylene fluorophore was developed to monitor radical mediated degradation of melamine-formaldehyde crosslinked polyester coil coatings in an industry standard accelerated weathering tester. Trapping of polyester-derived radicals (most likely C-radicals) that are generated during polymer degradation leads to fluorescent closed-shell alkoxy amines, which was used to obtain time-dependent degradation profiles to assess the relative stability of different polyesters towards weathering. The nitroxide probe couples excellent thermal stability and satisfactory photostability with high sensitivity and enables detection of free radical damage in polyesters under conditions that mimic exposure to the environment on a time scale of hours rather than months or years required by other testing methods. There are indications that the profluorescent nitroxide undergoes partial photo-degradation in the absence of polymer-derived radicals. Unexpectedly, it was also found that UV-induced fragmentation of the NO–C bond in closed-shell alkoxy amines leads to regeneration of the profluorescent nitroxide and the respective C-radical. The maximum fluorescence intensity that could be achieved with a given probe concentration is therefore not only determined by the amount of polyester radicals formed during accelerated weathering, but also by the light-driven side reactions of the profluorescent nitroxide and the corresponding alkoxy amine radical trapping products. Studies to determine the optimum probe concentration in the polymer matrix revealed that aggregation and re-absorption effects lowered the fluorescence intensity at higher concentrations of the profluorescent nitroxide, but too low probe concentrations, where these effects would be avoided, were not sufficient to trap the amount of polyester radicals formed upon weathering. The optimized experimental conditions were used to assess the impact of temperature and UV irradiance on polymer degradation during accelerated weathering.     

Chemiluminescence kinetic analysis on the oxidative degradation of poly(lactic acid)

The oxidation of polymers is characterized by the generation of free radicals, and the kinetic parameters depicting the progress of oxidation are specific for each material structure and formulation. The durability of materials depends intrinsically on the molecular structure, and degradation mechanism influences the long-term stability of products. The intimate transformations of macromolecules can be reliably characterized by chemiluminescence (CL), which depicts the evolution of oxidation state by the reaction of free radicals with molecular oxygen. The isothermal and nonisothermal CL spectra are complementary proofs for the interpretation of oxidation behavior involving hydroperoxide as the initiators of oxidation. The degradation of PLA takes place by decomposition and fragmentation to lactide. The values of five kinetic parameters (initial CL intensity, CL intensity at the first peak, temperature corresponding to the first peak, oxidation induction time and oxidation rate) obtained for oxidative degradation of PLA lead to the activation energies ranged between 49 and 99 kJ mol^?1. The evolution of thermal degradation for poly(lactic acid) is an excellent example for the explanation of the decay fate of radical intermediates.     

Electrospray mass spectrometric analysis of 5-hydroperoxy and 5-hydroxyeicosatetraenoic acids generated by lipid peroxidation of red blood cell ghost phospholipids

Recent evidence suggests that generation of hydroxyl radicals in the presence of lipid membranes can lead to oxidation of arachidonic acid esterified to glycerophospholipids and the production of compounds isomeric to prostaglandins, thromboxanes, and leukotrienes. Liquid chromatography tandem mass spectrometry and multiple reaction monitoring were employed to quantitate the production of 5-hydroxyeicosatetraenoic acid (5-HETE), 5-hydroperoxyeicosatetraenoic acid (5-HPETE), and 5-oxo-eicosatetraenoic acid (5-oxo-ETE) in red blood cells ghosts treated with t-butylhydroperoxide (tBuOOH). Untreated red blood cell ghosts were found to contain low, but measurable quantities of these three 5-oxygenated eicosanoids as phospholipid esters. Following treatment, there was approximately a 53- and 22. 5-fold increase in 5-HETE and 5-HPETE, respectively, and an 8. 5-fold increase in 5-oxo-ETE. The formation of these compounds was inhibited nearly 90% by the antioxidants butylated hydroxytoluene, ascorbic acid, and resveratrol providing further evidence for free radical mediated oxidation of arachidonic acid. This analytical protocol provided sufficient sensitivity for detection of these compounds in studies in which previous analysis by high-pressure liquid chromatography with UV detection failed to detect their presence. These results reveal that the biologically active eicosanoids 5-HETE, 5-HPETE, and 5-oxo-ETE are formed esterified to phospholipids following exposure of cellular membranes to reactive oxygen species and free radicals in a model system where intracellular antioxidant mechanisms were depleted.     

Oxidative nucleophilic substitution of hydrogen in nitroarenes with phenylacetic acid derivatives

Oxidative nucleophilic substitution of hydrogen (ONSH) in nitroarenes with carbanion of isopropyl phenyl acetate gives various products depending on the conditions and oxidant. The reaction carried out in liquid ammonia and KMnO₄ oxidant gives iso-propyl α-hydroxy-α-nitroarylphenylacetates formed via hydroxylation of the initial ONSH products. In some cases additionally dimeric, trimeric and tetrameric products are formed. In THF and Bu₄N⁺MnO₄⁻ or DDQ oxidants simple ONSH products are formed whereas oxidation by dimethyl dioxirane (DMD) gave iso-propyl hydroxyaryl phenyl acetates. The dimeric and trimeric products are apparently formed via coupling of nitrobenzylic radicals generated in course of oxidation with nitrobenzylic carbanions of the ONSH products.