ALL-trans β-CAROTENE-5,6-EPOXIDE IN THYLAKOID MEMBRANES

All- trans β-carotene-5,6-epoxide has been found in the thylakoid membranes of spinach and of the cyanobacterium Synechococcus vulcanus Copeland. The epoxide was extracted from the thylakoid membranes with acetone, and was isolated by high-performance liquid chromatography (HPLC). The structure of the epoxide was identified by means of mass, Raman, and electronic absorption spectroscopy. Changes in the amount of the epoxide, as a result of epoxidation and (apparent) de-epoxidation reactions in the membranes, were traced by analysis of extracts on HPLC. In isolated thylakoid membranes, only the epoxidation reaction took place. The reaction was caused by irradiation or by the addition of ferricyanide, suggesting that electron transport reactions in the membranes are involved in the epoxidation. In intact spinach leaves, however, both epoxidation and de-epoxidation took place; the extent of epoxidation correlated with the intensity of light incident on the leaves. The epoxidation and de-epoxidation of all- trans β-carotene are contrasted with those of xanthophylls (in the violaxanthin cycle).     

Effects of Epoxidation Content of ENR on Morphology and Mechanical Properties of Natural Rubber Blended PVC

This research work has concerned a study on toughness of PVC/natural rubber (NR) blends compatibilized with epoxidized natural rubber (ENR). The aim of this work was to investigate the effect of degree of epoxidation on morphology and mechanical properties of the blends. Epoxidized natural rubber with a variety of epoxidation contents were prepared by reacting the NR latex with formic acid and hydrogen peroxide at various chemical contents. Chemical structure and epoxidation content of epoxidized natural rubber were evaluated by FTIR and ¹H-NMR techniques. After that, three grades of ENR with epoxidation contents of 15, 25 and 42 % (by mole) were further used for blending with PVC and NR in an internal mixer at 60 rpm and at 170 °C. From tensile and impact tests, it was found that tensile elongation and impact strength of the materials remarkably increased with degree of epoxidation. On the other hand, tensile strength and modulus of the materials rarely changed with the epoxidation content. An increase in toughness of the blends with epoxidation content was related to a better molecular interaction between PVC and ENR as suggested by torque-time curves of the materials.     

Epoxidation of 4-methyloct-4-ene: identification of reaction products and kinetic study

The performic acid epoxidation of 4-methyloct-4-ene, a model compound for 1,4-polyisoprene, is the subject of this study. The purpose was the optimization of the conditions for the epoxidation reaction by this oxidation agent. The first part of this work is concerned with the identification and characterization of the reaction products through NMR and FTIR spectroscopies. Four main products were characterized. Among them, an epoxide results from the epoxidation of 4-methyloct-4-ene by performic acid and three others, namely a ketone, a diol and a glycol ester, are the result of epoxide rearrangements. The second part is devoted to kinetic studies of the epoxidation reaction through high performance liquid chromatography (HPLC) technique. This investigation allowed an insight into the influence of the reaction parameters (temperature and concentration of hydrogen peroxide) on the conversion rate. The determining step of the epoxidation reaction was the formation of performic acid. The enthalpy of activation was found to be ΔH^* = 91 kJ/mol and the entropy of activation ΔS^*=−34.6 J/mol K.     

Epoxidation of syndiotactic 1,2-polybutadiene with peracids

The epoxidation of syndiotactic 1,2-polybutadiene (84 and 16% 1,2 and 1,4 units, respectively) with carboxylic peracids prepared in situ and m-chloroperbenzoic acid was studied. In the course of epoxidation in the presence of carboxylic peracids, oxirane groups are formed only through epoxidation of double bonds in the macromolecular backbone, whereas m-chloroperbenzoic acid is responsible for the chemical modification of 1,2 and 1,4 units of polybutadiene. The basic kinetic parameters of 1,2-polybutadiene epoxidation with peracids of various chemical structures were determined.     

Epoxidation of Soybean Oil in the Course of Cooxidation with Hydrogen Peroxide in the Presence of Propanoic Acid and Chlorinated KU-2 × 8 Cation Exchanger

Pattern of soybean oil epoxidation catalyzed by chlorinated KU-2 × 8 cation exchanger in the presence of H₂O₂ and propanoic acid was studied. The epoxidation rate and the selectivity of formation and yield of the epoxidation product were studied as influenced by the concentration of H₂O₂, C₂H₅COOH, and the catalyst. Some kinetic and activation parameters of soybean oil epoxidation were determined.     

Poly(ethylene glycol)-supported α,α,α-trifluoroacetophenone in dioxirane mediated alkene epoxidation reactions

Poly(ethylene glycol) (PEG) was used for the immobilization of α,α,α-trifluoroacetophenone and the utility of this supported ketone has been examined in dioxirane mediated epoxidation of alkenes. The PEG-ketone reagent was found to be an effective homogeneous catalyst for the epoxidation of a variety of alkenes in the presence of Oxone^® and was readily recovered from the reaction mixtures and reused.     

Asymmetric organocatalytic epoxidation of alpha,beta-unsaturated aldehydes with hydrogen peroxide

The first asymmetric organocatalytic epoxidation of alpha,beta-unsaturated aldehydes is presented. A chiral bisaryl-silyl-protected pyrrolidine acts as a very selective epoxidation organocatalyst using simple oxidation agents, such as hydrogen peroxide and tert-butyl hydroperoxide. The asymmetric epoxidation reactions proceed under environmental friendly reaction condition in, for example, water mixtures of alcohols, and the scope of the reaction is demonstrated by the formation of optically active alpha,beta-epoxy aldehydes in high yields and enantioselectivities >94% ee. Furthermore, the direct synthesis of the sex pheromone from an acaric mite by asymmetric epoxidation of citral is presented.     

An Overview of Heterogeneous Transition Metal-Based Catalysts for Cyclohexene Epoxidation Reaction

Cyclohexane epoxide, which contains highly active epoxy groups, plays a crucial role as an intermediate in the preparation of fine chemicals. However, controlling the epoxidation pathway of cyclohexene is challenging due to issues such as the allylic oxidation of cyclohexene and the ring opening of cyclohexane epoxide during the cyclohexene epoxidation process to form cyclohexane oxide. This review focuses on the structure-activity relationships and synthesis processes of various heterogeneous transition metal-based catalysts used in cyclohexene epoxidation reactions, including molybdenum(Mo)-based, tungsten(W)-based, vanadium(V)-based, titanium(Ti)-based, cobalt(Co)-based, and other catalysts. Initially, the mechanism of cyclohexene epoxidation by transition metal-based catalysts is examined from the perspective of catalytic active centers. Subsequently, the current research of cyclohexene epoxidation catalysts is summarized based on the perspective of catalyst support. Additionally, the differences between alkyl hydroperoxide, hydrogen peroxide (H₂O₂), and oxygen (O₂) as oxidants are analyzed. Finally, the main factors influencing catalytic performance are summarized, and reasonable suggestions for catalyst design are proposed. This work provides scientific support for the advancement of the olefin epoxidation industry.     

Molecule-induced homolysis of N-hydroxyphthalimide (NHPI) by peracids and dioxirane. A new, simple, selective aerobic radical epoxidation of alkenes

Evidences are reported concerning the molecule-induced homolysis of NHPI by peracids and dioxirane; their combination can be utilized for the aerobic free-radical epoxidation of alkenes with selectivity quite different from the well-known epoxidation by peracids.     

Kinetic study of hydroperoxide epoxidation of 1‐octene in the presence of molybdenum disilicide as a catalyst in a low concentration range of olefin

The kinetic regularities of the influence of low 1‐octene concentrations on its epoxidation by tert‐butyl hydroperoxide in the presence of molybdenum disilicide (MoSi₂) as a catalyst were investigated. The minimum in the dependence of the initial rate of hydroperoxide consumption on 1‐octene concentration was observed. The kinetic scheme of epoxidation, which includes the competition between hydroperoxide and olefin for the catalytic active centers, was proposed. The equation for the reaction rate was derived according to the kinetic scheme. The kinetic parameters of epoxidation were calculated. © 2009 Wiley Periodicals, Inc. Int J Chem Kinet 41: 623–628, 2009     

Asymmetric total synthesis of rhinacanthin A

Starting from a reduced lapachol compound, the total synthesis of rhinacanthin A in both racemic and enantioenriched forms is achieved in eight steps without forming any undesired β-lapachone derivatives. For the synthesis of enantioenriched rhinacanthin A, the introduction of the asymmetric center was carried out by using the catalytic asymmetric epoxidation of an unfunctional trisubstituted olefin using Shi’s epoxidation diketal catalyst. The acidic treatment of a derived enantioenriched epoxynaphthol and the following CAN oxidation afforded the target molecule with high enantiomeric purity.     

Microwave-assisted epoxidation of hindered homoallylic alcohols using VO(acac)2: application to polypropionate synthesis

The VO(acac)₂ catalyzed epoxidation of hindered homoallylic alcohols was conducted under microwave irradiation in an open vessel using toluene as solvent. The reaction time for the epoxidation of a series of cis- and trans-2-methyl-3-alkenols was dramatically reduced from 6 to 10 days to less than 3 h when compared to conventional heating. The cis alkenols exhibited very high diastereoselectivity. The more elaborated polypropionate precursors 12, 14 and 16 were epoxidized in good yield and excellent diastereoselectivities using the microwave-assisted epoxidation technique described here, which is safe and suitable for multi-gram scales.     

Direct Epoxidation of Propylene over Stabilized Cu+ Surface Sites on Titanium‐Modified Cu2O

Direct propylene epoxidation by O₂ is a challenging reaction because of the strong tendency for complete combustion. Results from the current study demonstrate that by generating highly dispersed and stabilized Cu⁺ active sites in a TiCuO_x mixed oxide the epoxidation selectivity can be tuned. The TiCuO_x surface anchors the key surface intermediate, an oxametallacycle, leading to higher selectivity for epoxidation of propylene.     

Olefin epoxidation by alkyl hydroperoxide with a novel cross-bridged cyclam manganese complex: demonstration of oxygenation by two distinct reactive intermediates

Olefin epoxidation provides an operative protocol to investigate the oxygen transfer process in nature. A novel manganese complex with a cross-bridged cyclam ligand, MnIV(Me2EBC)(OH)2(2+) (Me2EBC = 4,11-dimethyl-1,4,8,11-tetraazabicyclo[6.6.2]hexadecane), was used to study the epoxidation mechanism with biologically important oxidants, alkyl hydroperoxides. Results from direct reaction of the freshly synthesized manganese(IV) complex, [Mn(Me2EBC)(OH)2](PF6)2, with various olefins in neutral or basic solution, and from catalytic epoxidation with oxygen-labeled solvent, H2 18O, eliminate the manganese oxo moiety, Mn(IV)=O, as the reactive intermediate and obviate an oxygen rebound mechanism. Epoxidations of norbornylene under different conditions indicate multiple mechanisms for epoxidation, and cis-stilbene epoxidation under atmospheric 18O2 reveals a product distribution indicating at least two distinctive intermediates serving as the reactive species for epoxidation. In addition to alkyl peroxide radicals as dominant intermediates, an alkyl hydroperoxide adduct of high oxidation state manganese(IV) is suggested as the third kind of active intermediate responsible for epoxidation. This third intermediate functions by the Lewis acid pathway, a process best known for hydrogen peroxide adducts. Furthermore, the tert-butyl peroxide adduct of this manganese(IV) complex was detected by mass spectroscopy under catalytic oxidation conditions.     

A New Generation of Titanosilicate Catalyst: Preparation and Application to Liquid-Phase Epoxidation of Alkenes

A novel titanosilicate with the MWW topology, Ti-MWW, has been prepared by direct hydrothermal synthesis using boric acid as a structure-supporting agent, and also by post-incorporation of tetrahedral Ti species into MWW silicalite through controlled structural conversions between three-dimensional crystalline silicalite and the lamellar precursor. Ti-MWW is further converted by delamination into a thin sheet material applicable to the reaction of bulky reactants. Both direct hydrothermal synthesis and postsynthesis methods make it possible to introduce a controllable amount of Ti species into the MWW structure. An acid treatment of uncalcined samples is essentially important for the removal of the extraframework octahedral Ti species located on the exterior layer surface. The catalytic properties of Ti-MWW have been compared with those of conventional titanosilicates (TS-1, TS-2, Ti-Beta, Ti-MOR, Ti-MCM-41, etc.) in the epoxidation of various alkenes with hydrogen peroxide. Hydrothermally synthesized Ti-MWW proves to be more effective in the epoxidation of linear alkenes including functionalized ones, and also exhibits considerable activity for cycloalkenes. Moreover, it shows a unique shape selectivity not shared with other titanosilicates in the epoxidation of cis/trans geometric alkene isomers. Postsynthesized Ti-MWW, nearly free of boron, catalyses the alkene epoxidation more effectively as a result of the tetrahedral Ti species different from those resulting from the direct synthesis, which turns out to be the most active epoxidation titanosilicate catalyst so far. Delaminated Ti-MWW, possessing an extremely open and accessible surface area but maintaining the basic structure of zeolite, catalyses the epoxidation of various cycloalkenes more actively than large pore titanosilicates including mesoporous Ti-MCM-41.     

Properties of HTPB based polyurethane membrane prepared by epoxidation method

Hydroxyl-terminated polybutadiene (HTPB) based polyurethane (PU) was synthesized by solution polymerization. The PU was then cast into membrane. The epoxidation of HTPB based PU membrane by an in situ generated peracid method is discussed. The chemical composition of the epoxidized PU membrane was studied by infrared spectroscopy. The absorption peak at 970 cm⁻¹ for the CC double bond decreased with epoxidation time whereas the absorption peak at 1183 cm⁻¹ for oxirane group increased. The absorption peak at about 1700–1740 cm⁻¹ for the CO group and –OH group at about 3200–3700 cm⁻¹ increased with epoxidation time that indicated the side reaction of epoxidation took place. The oxirane weight content was determined by titration method. The density, tensile strength, elongations, and decomposition temperature of the epoxidized PU membranes were measured. The molecular weight between crosslinking points PU membrane was calculated. Contact angle and protein absorption of fibrinogen and albumin experiments were also determined. It was found that the density and the tensile strength of epoxidized PU membrane increased with increasing epoxidation time whereas the molecular weight between crosslinking points, elongation and the amount of protein adsorption on the epoxidized PU membrane decreased. By using Kaelble’s equation and the contact angle data, the surface tension of epoxidized PU membrane was determined. It was found that the surface tension of epoxidized PU membrane increased whereas the contact angle decreased with epoxidation time. The property changes reduced the permeability of gas through epoxidized PU membrane, but increased the gas selectivity between oxygen and nitrogen. The activation energies (E_p) for gas diffusing through various epoxidized PU membranes were obtained by the Arrhenius law; it is evident that E_p increased with the extent of epoxidation.     

Ethylene Epoxidation in Low-Temperature AC Dielectric Barrier Discharge: Effect of Electrode Geometry

In this work, the epoxidation of ethylene under a cylindrical dielectric barrier discharge (DBD) reactor and a parallel DBD reactor was comparatively studied. The effects of important operating parameters—feed O₂/C₂H₄ molar ratio, applied voltage, input frequency, and residence time—were investigated on the reaction performance in terms of reactant conversions, product selectivities, product yields, and power consumptions per molecule of ethylene converted and per molecule of ethylene oxide produced. The optimum conditions obtained from the operating parameter investigation were used for a comparative performance evaluation of both DBD reactor systems. It was found that under the optimum conditions of each system, the cylindrical DBD system exhibited superior epoxidation performance for ethylene oxide production compared to the parallel DBD system, indicating that the electrode geometry (electrode edge length-to-electrode surface area ratio) plays a significant role in the ethylene epoxidation.     

Castor oil modified by epoxidation,transesterification, and acrylation processes: Spectroscopic characteristics

In the present study, castor oil (CO) was modified by epoxidation, transesterification, and acrylation processes. In situ epoxidation method was used to prepare epoxidized castor oil (ECO) in acetic acid with hydrogen peroxide in the presence of Seralite SRC-120 catalyst. Transesterified epoxidized castor oil was synthesized from the reaction of methanol in the presence of sodium methoxide catalyst. The acrylated epoxidized castor oil was synthesized from the reaction of ECO with acrylic acid containing hydroquinone. Chemical structures of modified CO were analyzed by Fourier transform infrared spectroscopy and proton nuclear magnetic resonance spectra analysis.     

Diastereoselectivity in epoxidation of carbohydrate fused [13]-macro-dilactones

DMDO epoxidation of carbohydrate fused [13]-macro-dilactones was found to be highly diastereoselective. Facial selectivity of the epoxidation depended on the identity of the fused carbohydrate. Gluco-configured macro-dilactones gave the R, R epoxide, whereas the galacto- configuration gave the S, S epoxide. The epoxide stereochemisty was confirmed by independent syntheses of dimethyl 4 R,5R-epoxyoctanedioate via Shi epoxidation of dimethyl E-oct-4-enedioate and by transesterification of the epoxide derived from the gluco-[13]-macro-dilactone. We demonstrate diastereoselectivity in alkene reactivity driven by remote rather than adjacent stereocenters.