Calix[n]arenes. IV. Interaction of tert-Butylcalix[n]arenes with Cyclic Amines

p-tert Butylcalix[n]arenes 1 and 2 react with the cyclic amines 3 – 8 , 10 by proton transfer forming so-called „exo-complexes”︁ between the ammonium and phenolate groups. As well endocyclic as exocyclic amino groups are reactive. The nitroxyl radical 9 does not react with 1 or 2 by the proton transfer reaction. These results demonstrate that an interaction between the nitrogen atom of HALS-compounds, esp. compound 10 , with calix[n]arene hydroxylgroups is possible, but an interaction between the corresponding nitroxyl radicals with calix[n]arene hydroxylgroups does not occur.     

微反应器内基于氮氧自由基催化剂连续氧气/空气氧化反应的研究进展

连续液相氧气氧化技术代替传统氧化技术已经成为氧化反应发展的一大趋势。但是,分子氧通常需要被合适的催化剂进行活化后才能进行高选择性氧化。近年来,氮氧自由基催化剂因其能够在温和条件下高效地催化氧气氧化反应而取得了快速发展。此外,可持续的绿色氧化工艺不仅依赖于高效环保的催化体系,还需要依托能够强化传质和反应性能的反应器技术。本文介绍了连续微反应氧化技术中常用的微反应器,归纳总结了以氮氧自由基及其衍生物为催化剂的空气/氧气氧化反应在连续有机合成中的研究进展。最后,针对现阶段氮氧自由基催化的连续液相氧化技术的潜在挑战,对该技术在精细化工领域中的应用进行了展望。     

The inhibition of autoxidation by aromatic amines

Recent work on the inhibition of autoxidation by aromatic amines is reviewed. The rate controlling step for the inhibition process is abstraction of the amino hydrogen by a peroxy radical. The rate of this reaction is increased by electron donating substituents attached to the aromatic ring. Nitroxide radicals may be produced in the reaction of peroxy radicals with secondary amino radicals. The efficiency of nitroxide formation is greater for tertiary than for secondary peroxy radicals because, in the latter case, the caged nitroxide and alkoxy radical may disproportionate to give a hydroxylamine and a carbonyl compound. Primary aromatic amines do not give nitroxides, presumably because the peroxy radical-amino radical reaction gives an alcohol and a nitroso aromatic. The tertiary amine, N,N,N′,N′-tetramethyl-p-phenylenediamine is shown to be a catalyst of autoxidation rather than an inhibitor, as has been previously reported. The products of the N-phenyl-2-naphthylamine-peroxy radical reaction have been identified. Presented at the AOCS Meeting, San Francisco, April 1969.     

E.s.r. studies on free radicals generated in poly[p-(2-hydroxyethoxy) benzoic acid] fibres at low temperatures

Free radicals generated in stretched and ground poly [p-(2-hydroxyethoxy) benzoic acid] (PEOB) were studied by e.s.r. spectroscopy in an effort to isolate those formed by main chain scission. PEOB fibres stretched at ?86°C in a dry nitrogen atmosphere gave an asymmetric spectrum, which had patterns characteristic of both phenoxy and peroxy radicals in addition to some unknown peaks. The shape of the spectrum changed with increasing temperatures, finally becoming identical to that of phenoxy radicals at room temperature. The phenoxy radical was observed in all PEOB films ground or γ-irradiated in liquid nitrogen. In some cases a small peak of peroxy radical was observed. Theoretical spectra calculated for possible radical species or combinations of them were compared with those observed. No direct evidence was observed of the formation of alkyl type and radicals which were assumed to be formed with phenoxy radicals upon main chain rupture of PEOB. However, the relatively unstable peroxy radical observed in these experiments is thought to arise from them.     

Antioxidant action of a lipophilic nitroxyl radical,cyclohexane-1-spiro-2′-(4′-oxyimidazolidine-1′-oxyl)-5′-spiro-1″-cyclohexane,against lipid peroxidation under hypoxic conditions

Nitroxyl radicals are known to act as radical scavenging antioxidants. In the present study, a lipophilic nitroxyl radical, cyclohexane-1-spiro-2′-(4′-oxyimidazolidine-1′-oxyl)-5′-spiro-1″-cyclohexane (nitroxyl radical I) was synthesized and its antioxidant capacity was assessed in comparison with a hydrophilic nitroxyl radical, 4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl (Tempol). Both nitroxyl radical I and Tempol inhibited methyl linoleate oxidation induced by free radicals, and the efficacy increased with decreasing partial pressure of oxygen, the effect being more pronounced for nitroxyl radical I than Tempol. Their hydroxylamines inhibited lipid peroxidation more effectively than their corresponding parent nitroxyl radicals. In liposomal membranes, a synergistic effect was observed in the combination of nitroxyl radical I with ascorbic acid, whereas only an additive effect was observed between Tempol and ascorbic acid. The present study suggests that nitroxyl radical I and its hydroxylamine may act as potent antioxidants, especially in combination with ascorbic acid under hypoxic conditions.     

New trends in epr and epr kinetics studies in the stabilization mechanism by N-oxyl radicals

The mechanism of the solid state oxidation of isotactic polypropylene films and the the mechanism of inhibition by HALS have been investigated by kinetic-ESR and kinetic modelling with derivation of the rate constants for the sensitive steps in the reaction scheme. The results have shown the inadequacy of the Denisov cycle to account for the observed experimental kinetics of the intermediate nitroxyl radicals. The molecular dynamics associated with the oxidation inhibited by HALS in the polypropylene matrix has been investigated by applying the Liouville stochastic method to the analysis of the temperature effects of the ESR spectra of the intermediate nitroxyl radicals used as spin probes. The investigation on the nature and characteristics of the molecular motions available at the oxidation sites has also been carried out with nitroxyl spin labels bound to the PP chains. For this application a novel method of spin labelling was developed based on reactions with some of the oxidation products, namely the terminal and intrachain ketones (keana method), alcohols, peroxides and macroalkyl radicals.     

Phenothiazine derivatives as new antioxidants for the autoxidation of methyl linoleate and their reaction mechanisms

The effect of new antioxidants, such as phenothiazine derivatives, on the autoxidation of methyl linoleate was evaluated by estimating the induction period using the weighing method. Peroxide values (iodometry and IR), refractive indices, mol wts, and UV and IR spectra were measured to investigate the extent of the autoxidation. The induction period evaluated from the weighing method gives almost the same value as that from the peroxide values. It was shown that some new phenothiazine derivatives are remarkably effective antioxidants, and, besides, the mechanism for the autoxidation of methyl linoleate containing phenothiazine derivatives as antioxidants is probably of the same type as that for the substrate alone. This study also investigated the reaction mechanism of phenothiazine derivative antioxidants by determining the electron spin resonance spectra for the antioxidants in the autoxidation of methyl linoleate. Then, the following mechanism was proposed. That is, within the induction period, these inhibitors hold stable nitroxide radicals (>NO*) in the reaction between the antioxidant amino radical (>N*), produced by the reaction of the antioxidant with ROO* or O₂, and the peroxy radical (ROO*). Besides, the more superior the phenothiazine derivative antioxidant, the more inactive the antioxidant makes oxygen and the peroxy radical for the methyl linoleate autoxidation and also for the antioxidant oxidation. Presented at the 12th World Congress of ISF, Milan, 1974.     

Oxidative radical generation via nitrogen dioxide dimer conversions induced by amide groups of macromolecules

The features of initiation of free radical reactions in polymers by dimers of nitrogen dioxide are considered. The conversion of planar dimers into nitrosyl nitrate in the presence of amide groups of macromolecules has been revealed. Nitrosyl nitrate initiates radical reactions in oxidative primary process of electron transfer with formation of intermediate radical cations and nitric oxide. As a result of subsequent reactions, nitrogen‐containing radicals are produced. The dimer conversion has been exhibited by estimation of the oxyaminoxyl radical yield in characteristic reaction of p‐benzoquinone with nitrogen dioxide on addition of aromatic polyamide and polyvinylpyrrolidone to reacting system. The isomerization of planar dimers is efficient in their complexes with amide groups, as confirmed by ab initio calculations. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

E.p.r. study of radicals produced mechanically in PGMA and their interaction with oxygen

The vibrational grinding of poly(ethyleneglycol methacrylate) (PGMA) in vacuo at the liquid nitrogen temperature gives rise to polymer radicals in high concentrations. Changes in the radical concentration as a function of temperature in the presence and absence of oxygen were followed by means of electron paramagnetic resonance. It was found that polymer radicals reacted at very low temperatures with oxygen with simultaneous formation of polymer peroxy radicals and of a non-paramagnetic polymer tetroxide. This polymer tetroxide, which has been proved indirectly, can decompose to yield polymer peroxy radicals and non-paramagnetic products; the observed anomalies on the curve of the thermal decomposition of radicals may be thus elucidated. The relative participation of polymer tetroxide depends on the oxygen concentration, on the temperature of the sample in contact with oxygen and on the concentration of polymer radicals arising by grinding predominantly on the surface of polymer particles.     

Electron spin resonance studies on graft copolymerization of gaseous styrene onto preirradiated polypropylene. III. Preirradiation under vacuum followed by exposure to oxygen at −78°C

Grafting of styrene vapor was carried out onto polypropylene γ-irradiated under vacuum followed by exposure to oxygen at ?78°C. A comparison with the results on polypropylene irradiated in an oxygen atmosphere leads to an interesting result. Two samples were irradiated at room temperature under vacuum and in an oxygen atmosphere, respectively, the former being followed by exposure to oxygen. Although irradiation was carried out at room temperature for either sample and nearly the same amounts of the peroxy radicals are trapped, the former showed more grafting than the latter. The origin of this difference was investigated by means of electron spin resonance. Significant differences were observed in ESR spectra as well as in isothermal decays of the trapped peroxy radicals. In the former sample, the trapped peroxy radicals had a higher mobility, and consequently a significant hydrogen abstraction by the peroxy radicals was observed during storage at 40°C. This result supports the conclusion of the previous paper that the radicals effective in the grafting reaction of polypropylene preirradiated in an oxygen atmosphere are the carbon radicals which are produced by hydrogen abstraction by the peroxy radicals.     

Readily Accessible Oxazolidine Nitroxyl Radicals: Bifunctional Cocatalysts for Simplified Copper Based Aerobic Oxidation

Using a two‐step procedure a range of bifunctional oxazolidine nitroxyl radicals have been prepared. The application of these co‐catalysts to the copper‐based aerobic oxidation of alcohols was then investigated. From these studies it was found that the parent tetramethyloxazolidine nitroxyl radical L1 was competent for oxidation in the presence of 2,2′‐bipyridine, and the bifunctional pyridyl‐containing nitroxyl radicals L2 and L3 could be used in the absence of additional coordinating ligands. Following optimization, the scope of this simplified transformation was explored, demonstrating that a range of primary and secondary benzylic alcohols are readily oxidized. In addition, the oxidation of allylic alcohols and hydroquinone can be achieved.     

Simplified Free‐Radical Reaction Kinetics for p‐Xylene Oxidation to Terephthalic Acid

The kinetic models based on complex free‐radical mechanisms always involve lots of parameters, which result in model overparameterization. In this work, on the basis of free‐radical reaction mechanisms, a simplified kinetics for liquid‐phase catalytic oxidation of p‐xylene (PX) to terephthalic acid (TPA) was developed. By assuming that different peroxy radicals have equivalent reactivity, all the initiation rate constants are identical, and the differences in the rates of termination between various peroxy radicals are neglected, the kinetic model is simplified to include only six parameters that are to be determined by experiment. The kinetic model established in this paper was shown to have satisfactory precision in predicting the concentration profiles. The kinetic model proposed is even simpler than the first ‐ order kinetic model because the rate constants concerning chain propagation and termination are independent of temperature within the range investigated.     

Chemiluminescence studies of the degradation and stabilization of polymers

In order to predict the safe service life of a polymer from its rate of oxidation, a detailed understanding of degradation and stabilization reactions in the induction period is required. The mechanism by which an oxidizing polymer emits weak visible light (chemiluminescence) is reviewed and the use of the technique to study the early stages of the oxidation of nylon 66 and polypropylene is considered. Steady-state chemiluminescence experiments are described to measure the activation energy of initiation of nylon 66 oxidation and to study the mechanism of the dark reaction of a hindered amine light stabilizer (and its nitroxyl radical) with polypropylene hydroperoxide. Non-stationary experiments, in which the steady state is perturbed by UV irradiation and gas switching, demonstrate the reactions of long lived alkyl radicals in the induction period. The observation of stress-induced chemiluminescence from nylon 66 shows the effect of even a low fiber load is to greatly increase the rate of oxidation.     

ESR study of pressure dependence of the peroxy radical decay in irradiated isotactic polypropylene

By the use of gamma radiation the free radicals were generated in isotactic polypropylene. The polymer was exposed to the action of atmospheric oxygen and the peroxy radicals were formed. Then the decay of peroxy radicals was investigated at varying temperatures as a function of pressure. The rate constants of the free radical decay were determined, and the corresponding activation volumes were calculated. The kinetic characteristics imply that the peroxy radicals predominantly occured in amorphous region the polymer. The results suggest that useful information on molecular mechanism of radical reactions in the solid phase may be obtained from the determination of activation volumes.     

Catalytic oxidation of alcohols using molecular oxygen mediated by poly(ethylene glycol)-supported nitroxyl radicals

The selective catalytic oxidation of alcohols over a mixture of copper(I) chloride and a number of linear ‘linker-less’ or ‘branched’ poly(ethylene glycol)-supported nitroxyl radicals of the 2,2,6,6-tetramethyl-piperidine-1-oxyl (TEMPO) family as a catalyst system has been investigated in the presence of molecular oxygen in a batch reactor. It is found that the activity profile of the polymer-supported nitroxyl radicals is in good agreement with that of low-molecular weight nitroxyl catalysts, for example, allylic and benzylic alcohols are oxidised faster than aliphatic alcohols. The oxidations can be tuned to be highly selective such that aldehydes are the only oxidation products observed in the oxidation of primary alcohols and the oxidations of secondary alcohols yield the corresponding ketones. A strong structural effect of the polymeric nitroxyl species on catalytic activity that is dependent upon their spatial orientation of the nitroxyl radicals is particularly noted. The new soluble macromolecular catalysts can be recovered readily from the reaction mixture by solvent precipitation and filtration. In addition, the recycled catalysts demonstrate a similar selectivity with only a small decrease in activity compared to the fresh catalyst even after five repetitive cycles.     

Crosslink mechanisms of high-solids alkyd resins in the presence of reactive diluents

The drying of alkyds was studied using NMR and mass spectrometry employing model compounds. Crosslinking occurs via oxidation of unsaturated fatty (linoleic) acids in the resin. Hydroperoxides formed as intermediates are degraded to alkoxy and peroxy radicals by cobalt catalyst. These radicals then recombine to form mainly ether and peroxy crosslinks. However, with conjugated fatty acids another mechanism prevails in which addition of radicals to the double bonds occurs. Despite the differences in mechanism the rates of both reactions were found to be similar. High-solids alkyd coatings may employ reactive diluents replacing traditional solvents (white spirit). The rate of incorporation of these reactive diluents into the actual paint film during drying can be followed with NMR. Allyl ether groups appear to react fastest whereas allyl esters show generally little reactivity. Using mass spectrometry it was found that incorporation involves recombination of radicals as well. However, reactive diluents with conjugated double bonds will be incorporated by radical addition.     

Electrochemical oxidation of aliphatic hydrocarbons promoted by inorganic radicals. II. NO3 radicals

The anodic electrolysis of linear alkanes in tert-butanol/H₂O solutions containing HNO₃ and saturated with oxygen at P_{O 2}=1 atm results in the partial oxidation of the hydrocarbons to the corresponding isomeric ketones. The experimental data are in support of a mechanism in which the first steps are: (1) one-electron transfer from the nitrate ion to the anode to give a nitrate radical; (2) hydrogen abstraction by the nitrate radical on the aliphatic substrate to give an alkyl radical; (3) molecular oxygen addition by the alkyl radical and formation of peroxy radicals. These species decay by bimolecular reactions and a carbonyl function is formed. The isomer distribution is in excellent accord with a statistical H abstraction from the secondary C–H bonds.     

Free radical formation in three-component photoinitiating systems

N,N′-diethylthiacarbocyanine iodide (Cy) with two different compounds acting as co-initiators were used in the three-component photoinitiating systems for free radical polymerization of acrylate monomer. The steady state and time-resolved techniques were used to study the fluorescence quenching by co-initiators, as well as laser flash photolysis to investigate an electron transfer process, that occurs in these photoinitiating system (PIS). To investigate the key factors involved with visible-light activated radical polymerizations involving three-component photoinitiators, we used thermodynamic feasibility and kinetic considerations to study photopolymerizations initiated with thiacarbocyanine dye as a photosensitizer. The Rehm–Weller equation was used to verify the thermodynamic feasibility for electron transfer reaction. Experiments show, that the main interaction between the dye and both co-initiators occurs through its excited singlet state. It was concluded, that the key kinetic factors for efficient visible-light activated initiation process are summarized in two ways: (1) to retard back electron transfer and recombination reaction steps and (2) to use a secondary reaction step to consuming dye-based radical and regenerating the original photosensitizer.Basing on the study, it is known that, there are two possible mechanisms of generation of free radicals, which can start polymerization involving photoreducible series mechanism and parallel series mechanism (photoreducible–photooxidizable series mechanism).     

Nitroxyls for scorch suppression,cure control,and functionalization in free-radical crosslinking of polyethylene

This paper describes the use of 2,2,6,6-tetramethylpiperidinyl-oxy (TEMPO) derivatives for scorch suppression, cure control, and functionalization in peroxide crosslinking of polyethylene. When 4-hydroxy 2,2,6,6-tetramethylpiperidin-1-oxyl was used for scorch suppression, there was often a loss in ultimate degree of crosslinking. In contrast, with bis(1-oxyl-2,2,6,6-tetramethylpiperidine-4-yl)sebacate, both scorch suppression and ultimate degree of crosslinking were enhanced. A model study in hexadecane showed that TEMPO radicals terminate with carbon-centered radicals formed as a consequence of peroxide homolysis and propagation steps. This termination occurs preferentially over peroxide-initiated crosslinking and results in TEMPO-grafted polymer. In addition to polymer radical formation, several additional reaction pathways are available following thermal homolysis of the peroxide, including unimolecular disassociation of the peroxy radical to yield a methyl radical and a ketone, and proton extraction from one of several substrates by the peroxy radical to yield an alkyl radical and an alcohol. This study reveals that the reaction rate is limited by the rate of peroxide homolysis, and proceeds to statistical products with little or no preference for any specific species. The implication is that choice of peroxide is a dominant controlling factor over whether the TEMPO derivatives are ultimately grafted to the polymer or are bound to small alkyl radicals. POLYM. ENG. SCI., 47:50–61, 2007. © 2006 Society of Plastics Engineers     

Kinetic Study of Hydroperoxide Degradation in Edible Oils Using Electron Spin Resonance Spectroscopy

Lipid oxidation is a complex phenomenon involving free radicals which are highly reactive molecular species. The life-time of these radical species is extremely short and their detection is therefore difficult. Several electron spin resonance (ESR) spectroscopy methodologies make it possible to identify, quantify and measure the reactivity of radical species formed during oxidation–reduction reactions. In this study we took advantage of the specificity of ESR spectroscopy to detect radical compounds in order to determine the rate constants of hydroperoxide degradation, a key reaction involved in lipid oxidation. The interaction of 5-doxyl stearic acid and lipid-derived radicals was studied by following the intensity of ESR spectra. A kinetic model was developed to simulate data analysis obtained by ESR and values of rate constants for hydroperoxide degradation were determined at 100 and 110 °C. This quantitative approach of ESR spectroscopy has produced useful information about new rate estimates for hydroperoxide degradation in edible oils.