Mechanism Of Epoxidation Research Articles

Mechanistic insight into the cyclohexene epoxidation with VO(acac)2 and tert-butyl hydroperoxide

The epoxidation reaction of cyclohexene is investigated for the catalytic system vanadyl acetylacetonate (VO(acac)2) with tert-butyl hydroperoxide (TBHP) as oxidant with the aim to identify the most active species for epoxidation and to retrieve insight into the most plausible epoxidation mechanism. The reaction mixture is composed of various inactive and active complexes in which vanadium may either have oxidation state+IV or+V. Inactive species are activated with TBHP to form active complexes. After reaction with cyclohexene, each active species transforms back into an inactive complex that may be reactivated again. The reaction mixture is quite complex containing hydroxyl, acetyl acetonate, acetate, or a tert-butoxide anion as ligands, and thus, various ligand exchange reactions may occur among active and inactive complexes. Also, radical decomposition reactions allow transforming V+IV to V+V species. To obtain insight into the most abundant active complexes, each of previous transformation steps has been modeled through thermodynamic equilibrium steps. To unravel the nature of the most plausible epoxidation mechanism, first principle chemical kinetics calculations have been performed on all proposed epoxidation pathways. Our results allow to conclude that the concerted Sharpless mechanism is the preferred reaction mechanism and that alkylperoxo species V+IVO(L)(OOtBu) and V+VO(L1)(L2)(OOtBu) species are most abundant. At the onset of the catalytic cycle, vanadium+IV species may play an active role, but as the reaction proceeds, reaction mechanisms that involve vanadium+V species are preferred as the acetyl acetonate is readily oxidized. Additionally, an experimental IR and kinetic study has been performed to give a qualitative composition of the reaction mixture and to obtain experimental kinetic data for comparison with our theoretical values. The agreement between theory and experiment is satisfactory.

Epoxidation of Soybean Oil with Tert-Butyl Hydroperoxide over Manganese (III) Tetraphenylporphyrin Chloride Catalyst

A new catalytic route was reported for the epoxidation of soybean oil (SBO) with tert-butyl hydroperoxide (TBHP, 65 wt. %) as the clean oxidant over manganese (III) tetraphenylporphyrin chloride (Mn (TPP) Cl) catalyst without carboxylic acid. Effect of tetraphenylporphyrin chloride with various metals (Mn, Fe, Co, Ni and Zn) on SBO epoxidation has been investigated. As biomimetic catalysts, the optimum reaction temperature of the system existed for catalytic activity, which was about 40 °C. The optimum reaction time was 6 h and the suitable promoter was pyrrole considering various factors. The suitable oxidant was TBHP of 65 wt. %. A mechanism for SBO epoxidation was discussed, and a role for high-valent metal peroxide intermediate with double bond directly as part of this mechanism was proposed, which was verified by in situ UV-Vis spectroscopy. The optimized reaction conditions were acquired.

Imparting permanent antimicrobial activity onto viscose and acrylic fabrics

Viscose and acrylic fabrics were aminated to enhance metal chelation of Cu, Zn and Ni for the purpose of imparting fabrics antimicrobial activity against Escherichia coli and Staphylococcus aureus. Fabrics were firstly epoxidized using epichlorohydrin prior amination. Optimization of the reaction conditions, e.g., temperature, medium pH, amine type and metal type and their concentrations, are reported. Aminated fabrics of viscose and acrylic were shown to be viable for chelation with divalent metal cations. The overall results revealed that antibacterial resistance of metalized aminated fabrics that the activity trend of metals is in the order Cu-complex>Zn-complex>Ni-complex with regard to fighting of microorganisms and permanent even after 10 washing. Reaction mechanism of epoxidation, amination and metal chelation of fabrics are offered supported by FT-IR structural study, nitrogen content and atomic absorption spectroscopy.

Mechanistic insights into β-oxygen atom transfer in olefinepoxidation mediated by W(vi) complexes and H2O2

A DFT investigation of the olefin epoxidation mechanism catalysed by the [Cp*WO(OH)(2)](+) complex with H(2)O(2) reveals that the outer sphere transfer of the hydroperoxido O(β) atom to the olefin is considerably more favourable than that of the hydroperoxido O(α) atom. The reasons for this unusual pathway are discussed.

Ultrasound and ionic liquid: An efficient combination to tune the mechanism of alkenes epoxidation

In this proof of concept study, the advantageous properties of both H 2O 2/NaHCO 3/imidazole/Mn(TPP)OAc oxidation system and MOPyrroNTf 2 ionic liquid have been combined under ultrasonic irradiation to give an exceptionally favorable environment for Mn(TPP)OAc catalyzed olefin oxidations. The results reveal the crucial role played by the ultrasonic irradiations that influence drastically the oxidation process. In MOPyrroNTf 2 and under ultrasonic irradiation, the mechanism probably involves an oxo-manganyl intermediate at the expense of the classical bicarbonate-activated peroxide route.

Exploration Research on High Selectivity of Propylene Epoxidation over TS-1 Catalyst

In the direct epoxidation of propylene to propylene oxide over TS-1, the effects of reaction pressure, reaction temperature, catalyst quality and concentration of hydrogen peroxide on conversion and effective utilization of hydrogen peroxide, and selectivity of propylene oxide were systematically investigated in acetonitrile solvent. The experimental results show that the selectivity of propylene oxide is not related to technological conditions at all, and is always 100%. Then the mechanism of propylene epoxidation in acetonitrile solvent, compared with that in methanol solvent, was preliminarily researched in order to provide theoretical guidance for selecting an advanced solvent in propylene epoxide technology.

Mechanism of Al3+-Catalyzed Oxidations of Hydrocarbons: Dramatic Activation of H2O2 toward O−O Homolysis in Complex [Al(H2O)4(OOH)(H2O2)]2+ Explains the Formation of HO• Radicals

A radical mechanism of hydrocarbon oxidations with the environmentally friendly and cheap homogeneous nontransition metal system [Al(H(2)O)(6)](3+)/H(2)O(2)/MeCN-H(2)O was proposed for the first time on the basis of DFT calculations. A dramatic activation of H(2)O(2) toward homolysis in the key intermediate [Al(H(2)O)(4)(OOH)(H(2)O(2))](2+) due to the presence of the easily oxidizable OOH coligand provides, without a change of metal oxidation state, the generation of HO(•) radicals, which then oxidize hydrocarbons. Nonradical mechanisms of the olefin epoxidation with the same catalytic system were also investigated.

Switching on the Electrocatalytic Ethene Epoxidation on Nanocrystalline RuO2

Ruthenium-based oxides with rutile structure were examined regarding their properties in electrocatalytic ethene oxidation in acid media. A possible promoting effect of chloride ions toward oxirane formation was explored. Online differential electrochemical mass spectrometry combined with electrochemical polarization techniques were used to monitor the potential dependence of organic products resulting from ethene oxidation as well as the reaction solution decomposition products. Quantum chemical modeling by means of density functional theory was employed to study key reaction steps. The ethene oxidation in acid media led to CO(2), whereas oxirane was formed in the presence of 0.3 M Cl(-). In the Cl(-) promoted oxidation on RuO(2), oxirane and a small amount of CO(2) were the only detected electro-oxidation products at potentials below the onset of Cl(2) and O(2) evolution, resulting from Cl(-) and water oxidation. It is demonstrated here that the epoxidation is a surface-related electrocatalytic process that depends on the surface properties. Cl acts as the epoxidation promoter that switches off the combustion pathway toward CO(2) and enables the epoxidation reaction channel by surface reactive sites blocking. The proposed epoxidation mechanism implies binuclear (recombination) mechanism for O(2) evolution reaction on considered surfaces.

Nature of the Reaction Intermediates in the Flavin Adenine Dinucleotide-Dependent Epoxidation Mechanism of Styrene Monooxygenase

Styrene monooxygenase (SMO) is a two-component flavoenzyme composed of an NADH-specific flavin reductase (SMOB) and FAD-specific styrene epoxidase (NSMOA). NSMOA binds tightly to reduced FAD and catalyzes the stereospecific addition of one atom of molecular oxygen to the vinyl side chain of styrene in the enantioselective synthesis of S-styrene oxide. In this mechanism, molecular oxygen first reacts with NSMOA(FAD(red)) to yield an FAD C(4a)-peroxide intermediate. This species is nonfluorescent and has an absorbance maximum of 382 nm. Styrene then reacts with the peroxide intermediate with a second-order rate constant of (2.6 ± 0.1) × 10(6) M(-1) s(-1) to yield a fluorescent intermediate with an absorbance maximum of 368 nm. We compute an activation free energy of 8.7 kcal/mol for the oxygenation step, in good agreement with that expected for a peroxide-catalyzed epoxidation, and acid-quenched samples recovered at defined time points in the single-turnover reaction indicate that styrene oxide synthesis is coincident with the formation phase of the fluorescent intermediate. These findings support FAD C(4a)-peroxide being the oxygen atom donor and the identity of the fluorescent intermediate as an FAD C(4a)-hydroxide product of the styrene epoxidation. Overall, four pH-dependent rate constants corresponding to peroxyflavin formation (pK(a) = 7.2), styrene epoxidation (pK(a) = 7.7), styrene oxide dissociation (pK(a) = 8.3), and hydroxyflavin dehydration (pK(a) = 7.6) are needed to fit the single-turnover kinetics.

Theoretical investigations of the catalytic role of water in propene epoxidation on gold nanoclusters: A hydroperoxyl-mediated pathway

We report a comprehensive theoretical investigation of the catalytic reaction mechanisms of propene epoxidation on gold nanoclusters using density functional theory (DFT). We have shown that water acts as a catalytic promoter for propene epoxidation on gold catalysts. Even without reducible supports, hydroperoxyl (OOH) and hydroxyl (OH) radicals are readily formed on small-size gold clusters from co-adsorbed H2O and O2, with energy barriers as low as 4–6 kcal/mol (1 cal = 4.186 J). Propene epoxidation occurs easily through reactions between C3H6 and the weakened O-O bond of the OOH radicals on the surfaces of gold clusters.

The detailed kinetics and mechanism of ethylene epoxidation on an oxidised Ag/α-Al 2O 3 catalyst

Selectivity in the epoxidation of ethylene over Ag/α-Al 2O 3 catalysts has been shown to be a function of the Ag–O bond strength. The weaker the Ag–O bond is, the more selective it is. Promotion by Cl atoms, which are held under the surface of the Ag, is effected by their weakening the Ag–O bond. Promotion by Cs is effected by its being adsorbed on the Ag sites which form the stronger Ag–O bond, so preventing the formation of the unselective Ag–O bond. The reaction proceeds by the ethylene adsorbing on surface O atoms, forming two types of Ag–O–CH 2–CH 2 intermediates, whose fate is a function of the Ag–O bond strength. The Ag–O–CH 2–CH 2 intermediate formed on the area of the Ag surface having the weaker Ag–O bond (the Ag(1 1 1) surface) has a greater probability of cyclising to form EO than being involved in unselective H abstraction reactions. Conversely, the Ag–O–CH 2–CH 2 intermediate formed on the area of the Ag surface having the stronger Ag–O bond (a stepped surface) will have a greater probability of being involved in unselective H abstraction reactions by the more electronegative surface O atoms than cyclising to form EO. The unselective reaction on both surfaces is a collective reaction of the sea of surface O atoms forming H bonds of different strength to the adsorbed intermediate. The same adsorbed O atom is therefore involved in both the selective and unselective reactions, the extent of which it is involved in each is determined by the Ag–O bond strength. This is an entirely new concept in the partial oxidation of ethylene to ethylene oxide.

ChemInform Abstract: Ruthenium-oxo and -Tosylimido Porphyrin Complexes for Epoxidation and Aziridination of Alkenes

Dioxoruthenium(VI) porphyrins including those containing chiral porphyrinato ligands can be readily prepared by oxidation of (Ru II (Por)(CO)(MeOH)) with PhIO or meta- chloroperoxybenzoic acid. Similar reactions with PhINTs gave (Ru VI (Por)(NTs)2) isolated as air stable solids at room temperature. The (Ru VI (Por)O2) complexes are competent oxidants for epoxidation of alkenes with high selectivities. Enantioselective epoxidation of alkenes giving the corresponding organic epoxides with moderate to good enantioselectivites have also been achieved using chiral (Ru VI (D4-Por*)O2 )( D4-H2Por* o 5,10,15,20-tetrakis-(1S,4R,5R,8S- 1,2,3,4,5,6,7,8-octahydro-1,4:5,8-dimethanoanthracen-9-yl)porphyrin) and (Ru VI (D2-Por*)O2) complexes. The mechanism of alkene epoxidation by (Ru VI (Por)O2) has been examined. Procedures for catalytic epoxidation of alkenes using ruthenium porphyrin catalysts has also been developed. (Ru VI (Por)(NTs)2) was found to undergo aziridination of alkenes and amidation

A Computational Study of the Olefin Epoxidation Mechanism Catalyzed by Cyclopentadienyloxidomolybdenum(VI) Complexes

A DFT analysis of the epoxidation of C(2)H(4) by H(2)O(2) and MeOOH (as models of tert-butylhydroperoxide, TBHP) catalyzed by [Cp*MoO(2)Cl] (1) in CHCl(3) and by [Cp*MoO(2)(H(2)O)](+) in water is presented (Cp*=pentamethylcyclopentadienyl). The calculations were performed both in the gas phase and in solution with the use of the conductor-like polarizable continuum model (CPCM). A low-energy pathway has been identified, which starts with the activation of ROOH (R=H or Me) to form a hydro/alkylperoxido derivative, [Cp*MoO(OH)(OOR)Cl] or [Cp*MoO(OH)(OOR)](+) with barriers of 24.9 (26.5) and 28.7 (29.2) kcal mol(-1) for H(2)O(2) (MeOOH), respectively, in solution. The latter barrier, however, is reduced to only 1.0 (1.6) kcal mol(-1) when one additional water molecule is explicitly included in the calculations. The hydro/alkylperoxido ligand in these intermediates is eta(2)-coordinated, with a significant interaction between the Mo center and the O(beta) atom. The subsequent step is a nucleophilic attack of the ethylene molecule on the activated O(alpha) atom, requiring 13.9 (17.8) and 16.1 (17.7) kcal mol(-1) in solution, respectively. The corresponding transformation, catalyzed by the peroxido complex [Cp*MoO(O(2))Cl] in CHCl(3), requires higher barriers for both steps (ROOH activation: 34.3 (35.2) kcal mol(-1); O atom transfer: 28.5 (30.3) kcal mol(-1)), which is attributed to both greater steric crowding and to the greater electron density on the metal atom.

Quantum chemical modeling of the epoxidation with hydrogen peroxide catalyzed by Mn(III): Mechanistic insight

Styrene epoxidation with hydrogen peroxide catalyzed by Mn(III) complex was studied using DFT calculation method. The previous mechanisms of metal complex-catalyzed epoxidation were found unsuitable for the Mn(III) complex used in this work. Reasonable mechanisms were proposed and investigated. Lowering of barrier was found in catalyzed reaction. The side reaction, oxidation of styrene, was also investigated. The Gibbs energy changes of epoxidation and oxidation were calculated to give a general prediction of product distribution.

First-principles study of the mechanism of ethylene epoxidation over Ag–Cu particles

Silver–copper alloys have been proposed as catalysts for ethylene epoxidation due to their superior selectivity compared to pure silver, the predominant catalyst for this reaction. Under reaction conditions it has been previously shown that, rather than a two-dimensional (2D) Ag–Cu alloy, a thin copper oxide-like layer forms on top of silver, and several possible surface structures were identified (Phys. Rev. Lett., 2010, 104, 035503). By means of density-functional theory calculations, we study the mechanism of ethylene epoxidation catalyzed by the thin oxide-like surface structures. We identify different reaction pathways that will compete and/or synergetically interplay in the catalysis. In general, the reaction mechanism is structure-dependent and the reaction does not always proceed through the formation of (meta)stable intermediates, in contrast to clean Ag and the 2D alloy. Analyzing the competing reactions, we discuss how the addition of Cu improves the selectivity and stress the overall importance of accounting for the effect of ambient conditions.

Biomimetic kinetics and mechanism of cyclohexene epoxidation catalyzed by metalloporphyrins

The cyclohexene epoxidation by molecular oxygen in the presence of tetraphenylporphyrin chloride of Mn, Fe, Co and Ru and isobutyraldehyde have been investigated. The results showed that high yield of epoxide was obtained by the liquid phase epoxidation of cyclohexene in the presence of isobutyraldehyde using Mn(TPP)Cl as catalyst. The reaction mechanism has been proposed, and the results indicated (according to the mechanism) that cyclohexene epoxide was mainly formed from the reaction of high-valent metal oxo intermediate with olefin directly, which was verified by in situ EPR and in situ UV–vis spectroscopy respectively. The factors influencing epoxide generation rate, e.g. the concentration of cyclohexene, manganese porphyrin catalyst and isobutyraldehyde, have been well investigated. The kinetics of cyclohexene aerobic epoxidation developed from the proposed mechanism was consistent with the experimental kinetics accurately.

In situ Fourier transform infrared spectroscopic studies of limonene epoxidation over PW-Amberlite

In situ attenuated total reflection infrared spectroscopy (ATR-IR) in the liquid phase and in situ transmission IR spectroscopy in the gas phase were used to validate a previously proposed mechanism for limonene epoxidation with aqueous hydrogen peroxide on PW-Amberlite. Catalyst activation by the oxidant was evidenced by ATR-IR through the presence of multiple bands in the 1220–910 cm −1 range, which are attributed to metal-oxygen interactions on the catalytic surface. The presence of reaction intermediates is linked to bands in the 840–820 cm −1 region. Gas phase experiments indicate that acetonitrile is easily adsorbed and desorbed from the catalyst, while limonene is adsorbed on the solvent or oxidant saturated PW-Amberlite and not on the fresh catalyst, as proposed in the mechanistic pathway.

Kinetics and mechanism of the catalytic epoxidation of cyclohexene with tert-butyl hydroperoxide in the presence of molybdenum–squarate complex

The kinetics of the catalytic epoxidation of cyclohexene with tert-butyl hydroperoxide (TBHP) in the presence of a novel molybdenum–squarate complex was studied. Both the kinetics and mechanism of the process were found to be similar to those established with the “classical” homogeneous catalysts for epoxidation.

Anti-Selective Epoxidation of Methyl α-Methylene-β-tert-butyldimethylsilyloxycarboxylate Esters. Evidence for Stereospecific Oxygen Atom Transfer in a Nucleophilic Epoxidation Process

Methyl alpha-methylene-beta-tert-butyldimethylsilyloxycarboxylate esters are found to undergo diastereoselective epoxidation in the presence of potassium tert-butoxide-tert-butyl hydroperoxide to form anti products. In an effort to better understand mechanistic details of the transformation and the basis of diastereoselectivities observed, we studied the epoxidation of substrates with alpha-methylene groups containing (trans) deuterium labels and discovered that oxygen-atom transfer proceeds with > or = 95% stereospecificity in all cases examined. These and other experiments suggest that the mechanism of epoxidation is not distinguishable from a concerted process.

Synthesis, Characterization, and Application of Vanadium−Salan Complexes in Oxygen Transfer Reactions

We report the synthesis and characterization of several chiral salen- and salan-type ligands and their vanadium complexes, which are derived from salicylaldehyde or salicylaldehyde derivatives and chiral diamines (1R,2R-diaminocyclohexane, 1S,2S-diaminocyclohexane, and 1S,2S-diphenylethylenediamine). The structures of H(2)sal(R,R-chan)(2+) x 2 Cl(-) x (CH(3))(2)CHOH x H(2)O (1c; H(2)sal(R,R-chan) = N,N'-salicyl-R,R-cyclohexanediaminium), Etvan(S,S-chen) (3c; Etvan(S,S-chen) = N,N'-3-ethoxy-salicylidene-S,S-cyclohexanediiminato), and naph(R,R-chen) (6c; naph(R,R-chen) = N,N'-naphthylidene-R,R-cyclohexanediiminato) were determined by single-crystal X-ray diffraction. The corresponding vanadium(IV) complexes and several other new complexes involving different salicylaldehyde-type precursors were prepared and characterized in the solid state and in solution by spectroscopic techniques: UV-vis, circular dichroism, electron paramagnetic resonance, and (51)V NMR, which provide information on the coordination geometry. The salan complexes oxidize in organic solvents to V(V) species, and this process was also studied using spectroscopic techniques. Single crystals suitable for X-ray diffraction were obtained for [{V(V)O[sal(S,S-dpan)]}(2)(mu-O)] x H(2)O x 2(CH(3))(2)CHOH (14c; sal(S,S-dpan) = N,N'-salicyl-S,S-diphenylethylenediaminato) and [{V(V)O[t-Busal(R,R-chan)]}(2)(mu-O)] x 2 (CH(3))(2)CHOH (15c), both containing an OV(V)(mu-O)V(V)O moiety (V(2)O(3)(4+) core) with tetradentate ligands and one mu-oxo bridge. Both structures are the first examples of dinuclear vanadium complexes involving the V(V)(2)O(3)(4+) core with tetradentate ligands, the configuration of the V(2)O(3) unit being twist-angular. The V-salen and V-salan complexes are tested as catalysts in the oxidation of styrene, cyclohexene, cumene, and methyl phenyl sulfide with H(2)O(2) and t-BuOOH as oxidants. Overall, the V-salan complexes show higher activity and normally better selectivity in alkene oxidation and higher activity and enantioselectivity for sulfoxidation than their parent V-salen complexes, therefore being an advantageous alternative ligand system for oxidation catalysis. The better performance of V-salan complexes probably results from their significantly higher hydrolytic stability. Mechanisms for the alkene oxidation with these newly obtained V-salan compounds are discussed, including the use of DFT for the comparison of several alternative mechanisms for epoxidation.