Practical Epoxidation of Olefins Using Air and Ubiquitous Iron-Based Fluorous Salen Complex

The epoxidation of olefins by substituting "air" for potentially harmful oxidants was achieved using an oxidation method that integrated a fluorous iron(III) salen catalyst derived from common metals and pivalaldehyde. Several aromatic disubstituted olefins were converted into their corresponding epoxides with high efficiency and quantitative yields. This reaction represents an environmentally friendly oxidation process that utilizes an abundant source of air and employs a readily available metal, iron, in the form of salen complexes, making it an environmentally conscious oxidation reaction.

Preparative and Catalytic Properties of MoVI Mononuclear and Metallosupramolecular Coordination Assemblies Bearing Hydrazonato Ligands

A series of polynuclear, dinuclear, and mononuclear Mo(VI) complexes were synthesized with the hydrazonato ligands derived from 5-methoxysalicylaldehyde and the corresponding hydrazides (isonicotinic hydrazide (H2L1), nicotinic hydrazide (H2L2), 2-aminobenzhydrazide (H2L3), or 4-aminobenzhydrazide (H2L4)). The metallosupramolecular compounds obtained from non-coordinating solvents, [MoO2(L1,2)]n (1 and 2) and [MoO2(L3,4)]2 (3 and 4), formed infinite structures and metallacycles, respectively. By blocking two coordination sites with cis-dioxo ligands, the molybdenum centers have three coordination sites occupied by the ONO donor atoms from the rigid hydrazone ligands and one by the N atom of pyridyl or amine-functionalized ligand subcomponents from the neighboring Mo building units. The reaction in methanol afforded the mononuclear analogs [MoO2(L1-4)(MeOH)] (1a-4a) with additional monodentate MeOH ligands. All isolated complexes were tested as catalysts for cyclooctene epoxidation using tert-butyl hydroperoxide (TBHP) as an oxidant in water. The impact of the structure and ligand lability on the catalytic efficiency in homogeneous cyclooctene epoxidation was elucidated based on theoretical considerations. Thus, dinuclear assemblies exhibited better catalytic activity than mononuclear or polynuclear complexes.

Preparation of Bio-Based Polyurethane Coating from Citrullus colocynthis Seed Oil: Characterization and Corrosion Performance

In this study, a new epoxidized oil from Citrullus colocynthis seed oil (CCSO) was obtained for a potential application in the formulation of polyurethane coatings. Initially, the fatty acid composition of CCSO was determined by gas chromatography-mass spectrometry (GC-MS). Subsequently, the epoxidation of CCSO was performed with in situ generated peracetic acid, which was formed with hydrogen peroxide (30 wt.%) and glacial acetic acid and catalyzed with sulfuric acid. The reaction was continued at a molar ratio of 1.50:1.0 of hydrogen peroxide to double bond (H2O2:DB) for 6 h at a controlled temperature of 60 °C. The resulting epoxidized oil was then used to produce a bio-based polyol by hydroxylation. The molar ratio of epoxy groups to methanol and distilled water was maintained at 1:11:2, and the reaction was carried out for 2 h at a controlled temperature of 65 °C. The major functional groups of the epoxidized oil and its polyol were validated by Fourier-transform infrared (FT-IR) and proton nuclear magnetic resonance (1H NMR) spectroscopies. A polyurethane (PU) coating was produced from the synthesized polyol and 3HDI isocyanurate, keeping the molar ratio of NCO:OH at 1:1. The resulting PU coating was then applied to glass and aluminum panels (Al 1001). After the film was cured, the properties of the PU coating were evaluated using various techniques including pencil hardness, pendulum hardness, adhesion, gloss, chemical resistance, and EIS tests. The results show that the PU coating obtained from CCSO is a promising new raw material for coating applications.

A Safe One-Pot Synthesis and Characterization of Epoxidized Moringa Oleifera Oil

Global demand for epoxidized vegetable oil has been steadily growing. Epoxidized vegetable oils are typically produced using a two-pot synthesis process in which the oxidation and epoxidation reactions are carried out sequentially. This two-pot synthesis method, however, has a major drawback in industrialscale production, particularly when it comes to operational and process safety issues. A laboratory-scale one-pot synthesis method was attempted in this study with the aim to safely synthesize epoxidized Moringa Oleifera oil (eMOo) by avoiding the occurrence of undesired exothermic runaway reaction. The oil extracted from Moringa Oleifera oil seed kernel (MOo) was used as a starting component due to its high degree of unsaturation and also because the Moringa Oleifera plant can be freely grown in any soil conditions. Two parallel oxidation and epoxidation reactions were carried out simultaneously in this one-pot synthesis method to produce eMOo. The effect of five different mole ratios of MOo, acetic acid and hydrogen peroxide (1:1:1, 1:1:2, 1:1.5:2, 1:1.75:2 and 1:2:2, respectively) on reaction mechanism was investigated at the controlled temperature range of 43 - 55°C and reaction time of 0 - 120 min. The physicochemical properties of MOo as well as the oxirane oxygen content (OOC) of the resulting eMOo were characterized. In addition, GC-MS and FTIR analysis were performed to verify the molecular composition of MOo and also to identify the epoxy group of the resulting eMOo respectively. Among the five different mole ratios studied, the 1:1.5:2 mole ratio has the highest unsaturation conversion of 79.57% and OOC of 4.12%.

Influence of Equatorial Co-Ligands on the Reactivity of LFeIIIOIPh

Previous biomimetic studies clearly proved that equatorial ligands significantly influence the redox potential and thus the stability/reactivity of biologically important oxoiron intermediates; however, no such studies were performed on FeIIIOIPh species. In this study, the influence of substituted pyridine co-ligands on the reactivity of iron(III)-iodosylbenzene adduct has been investigated in sulfoxidation and epoxidation reactions. Selective oxidation of thioanisole, cis-cyclooctene, and cis- and trans-stilbene in the presence of a catalytic amount of [FeII(PBI)3](OTf)2 with PhI(OAc)2 provide products in good to excellent yields through an FeIIIOIPh intermediate depending on the co-ligand (4R-Py) used. Several mechanistic studies were performed to gain more insight into the mechanism of oxygen atom transfer (OAT) reactions to support the reactive intermediate and investigate the effect of the equatorial co-ligands. Based on competitive experiments, including a linear free-energy relationship between the relative reaction rates (logkrel) and the σp (4R-Py) parameters, strong evidence has been observed for the electrophilic character of the reactive species. The presence of the [(PBI)2(4R-Py)FeIIIOIPh]3+ intermediates and the effect of the co-ligands was also supported by UV-visible measurements, including the color change from red to green and the hypsochromic shifts in the presence of co-ligands. This is another indication that the title iron(III)-iodosylbenzene adduct is able to oxygenate sulfides and alkenes before it is transformed into the oxoiron form by cleavage of the O-I bond.

Study on the epoxidation of chain olefins using biquaternary ammonium phosphotungstic acid phase transfer catalysts under no-solvent condition

In this study, we have developed a novel catalyst synthesized by phosphotungstic acid and a gemini quaternary ammonium cation salt. This quaternary ammonium salt not only reduces the interfacial tension between olefins and hydrogen peroxide but also forms a notably stable structure with phosphotungstic acid. Dodecene was successfully epoxidized to epoxy dodecane with a selectivity of 82.9 %. The impact of initial conditions was systematically investigated such as molar ratio, temperature, reaction time, and catalyst dosage on the catalytic performance. Characterization of the catalyst morphology was performed by SEM, TEM and SAXS. Raman spectra, FT-IR and XPS spectra were employed to perform the catalyst transformation during the epoxidation reaction. This catalytic mechanism study could provide the industrial application in the epoxidation of long-chain olefins.

Synthesis and Characterization of Cardanol-Based Non-Isocyanate Polyurethane

This paper describes the synthesis of NIPU by using cardanol as starting material. A cardanol formaldehyde oligomer was first prepared through the reaction of cardanol and formaldehyde, catalyzed by citric acid. The resulting oligomer was then subjected to epoxidation with m-chloroperbenzoic acid to obtain an epoxide compound, which was subsequently used to fix carbon dioxide (CO2) and form a cyclic carbonate. Using this cyclic carbonate, along with an amine, cardanol-based isocyanate polyurethane (NIPU) was prepared. Different characterization methods, such as Fourier transform infrared spectroscopy (FTIR), proton nuclear magnetic resonance (NMR), gel permeation chromatography (GPC), and thermogravimetric analysis (TGA), were used to confirm the synthesis of the four intermediate products and NIPU in the reaction process. This study highlights the promise of bio-based NIPU as a sustainable alternative in a number of applications while offering insightful information on the synthesis and characterization of the material.

An Operationally Simple Approach to Indole Derivatives from 2-Alkenylanilines Utilizing an Oxidation-Intramolecular Cyclization-Elimination Sequence

Herein we describe a novel route to indole derivatives from a variety of N-substituted 2-alkenylanilines. This route features three operationally simple steps: (1) oxidation to convert N-substituted 2-alkenylanilines into epoxide intermediates, (2) intramolecular cyclization, and (3) the acid-catalyzed elimination of water.

Optimization of sample clean-up for the determination of small amounts of MOSH and MOAH in edible oils - method DGF C-VI 22 (20)

Methods for determining MOSH and MOAH in edible oils showed major problems with interlaboratory comparability of analytical results, especially in the lower concentration range below 10 mg/kg. However, a method with improved sensitivity and reproducibility is urgently needed to obtain a valid data basis for minimization efforts. To cope this problem a new method was created in 2020. The method was established as the standard method DGF C-VI 22 (20) of the German Society for Fat Science e.V. (DGF). For the development of this method different sample epoxidation approaches have been performed, evaluated and improved. Additionally, a saponification, a decision tree for sample preparation, an upstream clean-up column and a system suitability test were introduced. The focus was on reliability and interlaboratory comparability over all edible oil matrices up to a LOQ of 1 mg/kg. The optimized method was validated in terms of trueness and precision in a collaborative trail with 11 laboratories. The achieved recovery rates of 89-105% MOSH and 70-105% MOAH met the JRC requirements. Method and validation results were obtained with HorRat values between 1.3 and 1.8 for MOSH and MOAH.

A Lewis Acid-Controlled Enantiodivergent Epoxidation of Aldehydes

Two epoxidation catalysts, one of which consists of two VANOL ligands and an aluminum and the other that consists of two VANOL ligands and a boron, were compared. Both catalysts are highly effective in the catalytic asymmetric epoxidation of a variety of aromatic and aliphatic aldehydes with diazoacetamides, giving high yields and excellent asymmetric inductions. The aluminum catalyst is effective at 0 °C and the boron catalyst at -40 °C. Although both the aluminum and boron catalysts of (R)-VANOL give very high asymmetric inductions (up to 99% ee), they give opposite enantiomers of the epoxide. The mechanism, rate- and enantioselectivity-determining step, and origin of enantiodivergence are evaluated using density functional theory calculations.

The Effect of Milling on the Ethanolic Extract Composition of Dried Walnut (Juglans regia L.) Shells

This study investigates the ethanolic extract of dried walnut (Juglans regia L.) shells upon hammer milling (HM) and ball milling (BM) grinding processes. Marked differences were observed in the attenuated total reflection Fourier-transform infrared (ATR-FTIR) spectra. The two extracts were investigated by reversed-phase liquid chromatography coupled with electrospray ionization and high-resolution mass spectrometry (RPLC-ESI-HRMS). Following enzymatic digestion, the fatty acids (FAs) were examined, and tandem MS of epoxidized species was applied to establish the C-C double bond position; the most abundant species were FA 18:2 Δ9,12, FA 18:1 Δ9, and FA 18:3 Δ9,12,15. However, no significant qualitative differences were observed between FAs in the two samples. Thus, the presence of potential active secondary metabolites was explored, and more than 30 phenolic compounds, including phenols, ellagic acid derivatives, and flavonoids, were found. Interestingly, the HM samples showed a high concentration of ellagitannins and hydrolyzable tannins, which were absent in the BM sample. These findings corroborate the greater phenolic content in the HM sample, as evaluated by the Folin-Ciocalteu test. Among the others, the occurrence of lanceoloside A at m/z 391.1037 [C19H20O9-H]-, and a closely related benzoyl derivate at m/z 405.1190 (C20H22O9-H]-), was ascertained. The study provides valuable information that highlights the significance of physical pre-treatments, such as mill grinding, in shaping the composition of extracts, with potential applications in the biorefinery or pharmaceutical industries.

Hierarchical Zeolites Containing Vanadium or Tantalum and Their Application in Cyclohexene Epoxidation Reaction

The aim of this study was the synthesis, characterization, and catalytic application of new hierarchical materials modified with tantalum and vanadium ions. These materials exhibit secondary porosity, thus allowing the reactant molecules to access the active sites of the material while maintaining the acidity and crystallinity of the zeolites. Based on the results, these systems were found to be highly active and selective in the oxidation of cyclohexene. The performance of the catalysts was compared in oxidation processes carried out by conventional and microwave-assisted methods. Microwave-assisted experiments showed that in the presence of a hierarchical FAU zeolite containing Ta, long reaction times could be shortened with increased activity and selectivity under the same residual experimental conditions.

Oxygen-Transfer Mechanism

The mechanisms of sulfoxidation and epoxidation mediated by previously synthesized and characterized iron(III)-iodosylbenzene adduct, Fe^III(OIPh) were investigated using para-substituted thioanisole and styrene derivatives as model substrates. Based on detailed kinetic reaction experiments, including the linear free-energy relationships between the relative reaction rates (logk_rel) and the σ_p (4R-PhSMe) with ρ = -0.65 (catalytic) and ρ = -1.13 (stoichiometric), we obtained strong evidence that the stoichiometric and catalytic oxidation of thioanisoles mediated by Fe^III(OIPh) species involves direct oxygen transfer. The small negative slope -2.18 from log k_obs versus E_ox for 4R-PhSMe gives further clear evidence for the direct oxygen atom transfer mechanism. On the contrary, with the linear free-energy relationships between the relative reaction rates (logk_rel) and total substituent effect (TE, 4R-PhCHCH₂) parameters with slope = 0.33 (catalytic) and 2.02 (stoichiometric), the stoichiometric and catalytic epoxidation of styrenes takes place through a nonconcerted electron transfer (ET) mechanism, including the formation of the radicaloid benzylic radical intermediate in the rate-determining step. On the basis of mechanistic studies, we came to the conclusion that the title iron(III)-iodosylbenzene complex is able to oxygenate sulfides and alkenes before it is transformed into the oxo-iron form by cleavage of the O-I bond.

A DFT and kinetic study: Is it possible to prepare epoxides without catalysts using the in-situ generated peroxy radicals or peroxides by one-step method?

DFT calculations and kinetic analysis have been employed to comprehensively explore the possibility to prepare epoxides by one-step method using the in-situ generated peroxy radicals or hydroperoxides as epoxidizing agents. Computational studies demonstrated that the selectivities for the reaction systems of O₂ /R2/R1, O₂ /CuH/R1, O₂ /CuH/styrene, O₂ /AcH/R1 were 68.2%, 69.6%, 100% and 93.3%, respectively. The in-situ generated peroxide radicals, such as HOO˙, CuOO˙ and AcOO˙, could react with R1 or styrene by attacking the CC double bond to form a CO bond and subsequently undergoing a cleavage of OO bond to yield epoxides. Peroxide radicals could abstract a hydrogen atom from methyl group on R1, forming unwanted by-products. It should be noted that the hydrogen atoms of HOO˙ is easy to be abstracted by CC double bond and simultaneously the oxygen atom is connected to the CH moiety to form an alkyl peroxy radical (Rad11), greatly limiting the selectivity. The comprehensive mechanistic studies provide a deep understanding on preparing epoxides by one-step method.

Enzymatic and Synthetic Routes of Castor Oil Epoxidation

Epoxidation of castor oil in synthetic and enzymatic routes was carried out in order to promote a system with less environmental impact. The epoxidation reactions of castor oil compounds upon addition of lipase enzyme with and without acrylic immobilization and with reaction times of 24 and 6 h, as well as the synthetic compounds upon addition of Amberlite resin and formic acid, were investigated using Fourier transform infrared spectroscopy (FTIR) and nuclear magnetic resonance in hydrogen molecules (¹H-NMR). The analysis indicated that the enzymatic reactions (6 h) and synthetic reactions provided a conversion from 50 to 96% and epoxidation from 25 to 48%, resulting from peak stretching and signal disintegration in the hydroxyl region due to the appearance of H₂O in the interaction of peracid with catalyst. In systems without toluene, a dehydration event with a peak absorbance of 0.02 AU, indicating a possible vinyl group at 2355 cm^-1 in enzymatic reactions without acrylic immobilization, was observed and resulted in a selectivity of 2%. In the absence of a solid catalyst, an unsaturation conversion of castor oil above 90% was achieved; however, this catalyst is necessary for the epoxidation to take place, whereas the lipase enzyme becomes able of epoxidizing and dehydrating the castor oil upon changing the time or reaction system. The conversation from 28 to 48% of solid catalysts (Amberlite and lipase enzyme) displays their importance to the instauration conversion of castor oil into oxirane rings.

Green Aromatic Epoxidation with an Iron Porphyrin Catalyst for One-Pot Functionalization of Renewable Xylene, Quinoline, and Acridine

Sustainable functionalization of renewable aromatics is a key step to supply our present needs for specialty chemicals and pursuing the transition to a circular, fossil-free economy. In the present work, three typically stable aromatic compounds, representative of products abundantly obtainable from biomass or recycling processes, were functionalized in one-pot oxidation reactions at room temperature, using H₂O₂ as a green oxidant and ethanol as a green solvent in the presence of a highly electron withdrawing iron porphyrin catalyst. The results show unusual initial epoxidation of the aromatic ring by the green catalytic system. The epoxides were isolated or evolved through rearrangement, ring opening by nucleophiles, and oxidation. Acridine was oxidized to mono- and di-oxides in the peripheral ring: 1:2-epoxy-1,2-dihydroacridine and anti-1:2,3:4-diepoxy-1,2,3,4-tetrahydroacridine, with TON of 285. o-Xylene was oxidized to 4-hydroxy-3,4-dimethylcyclohexa-2,5-dienone, an attractive building block for synthesis, and 3,4-dimethylphenol as an intermediate, with TON of 237. Quinoline was directly functionalized to 4-quinolone or 3-substituted-4-quinolones (3-ethoxy-4-quinolone or 3-hydroxy-4-quinolone) and corresponding hydroxy-tautomers, with TON of 61.

Converging Stereodivergent Reactions - Highly Stereoselective Formal anti-Markovnikov Addition of H2O to Mixtures of Olefins

We describe a highly diastereo- and enantioselective two-step formal anti-Markovnikov addition of H2O to diastereomeric mixtures of olefins. Our approach relies on the highly enantioselective syn-specific Shi-epoxidation of both olefin isomers and a stereoconverging epoxide hydrosilylation featuring a directional isomerization step via a configurationally labile radical intermediate that is reduced by a syn-selective intramolecular hydrogen atom transfer from a Ti-H bond.

Synthesis of Biorenewable Terpene Monomers Using Enzymatic Epoxidation under Heterogeneous Batch and Continuous Flow Conditions

A commercially available Lipase B from Candida antarctica immobilized onto a macroporous support (Novozym 435) has been employed in the presence of H2O2 as a benign oxidant for the epoxidation of various biorenewable terpenes. This epoxidation protocol was explored under both heterogeneous batch and continuous flow conditions. The catalyst recyclability was also investigated demonstrating good activity throughout 10 cycles under batch conditions, while the same catalyst system could also be productively used under continuous flow operation for more than 30 h. This practical and relatively safe sustainable flow epoxidation of di- and trisubstituted alkenes by H2O2 allows for the production of gram quantities of a range of terpene epoxides. As a proof of principle, the same protocol can also be applied to the epoxidation of biobased polymers as a means to post-functionalize these macromolecules and equip them with cross-linkable epoxy groups.

Epoxidation of Methyl Esters as Valuable Biomolecules: Monitoring of Reaction

The paper is focused on the epoxidation of methyl esters prepared from oil crops with various profiles of higher fatty acids, especially unsaturated, which are mainly contained in the non-edible linseed and Camelina sativa oil (second generation). The novelty consists in the separation and identification of all products with oxirane ring formed through a reaction and in the determination of time course. Through the epoxidation, many intermediates and final products were formed, i.e., epoxides with different number and/or different position of oxirane rings in carbon chain. For the determination, three main methods (infrared spectroscopy, high-pressure liquid chromatography and gas chromatography with mass spectrometry) were applied. Only gas chromatography enables the separation of individual epoxides, which were identified on the base of the mass spectra, molecule ion and time course of products. The determination of intermediates enables: (i) control of the epoxidation process, (ii) determination of the mixture of epoxides in detail and so the calculation of selectivity of each product. Therefore, the epoxidation will be more environmentally friendly especially for advanced applications of non-edible oil crops containing high amounts of unsaturated fatty acids.

Synthesis and Characterization of New Lutidinium Ionic Liquid-Supported Vanadylsalicylaldoxime Complex for Catalytic Application in Epoxidation Reaction

A new chelating task-specific ionic liquid (TSIL), lutidinium-based salicylaldoxime (LSOH), and its square pyramidal vanadyl(II) complex (VO(LSO)₂ ) have been successfully synthesized and structurally characterized using elemental (CHN), spectral, and thermal analyses. The catalytic activity of the lutidinium-salicylaldoxime complex (VO(LSO)₂ ) in the alkene epoxidation reactions was studied under various reaction conditions, such as solvent effect, alkene/oxidant molar ratio, pH, reaction temperature, reaction time, and the catalyst dose. The results demonstrated that the CHCl₃ solvent, 1 : 3 of the cyclohexene/H₂ O₂ ratio, pH 8, temperature of 340 K, and catalyst dose of 0.012 mmol are assigned as the optimum conditions for achieving maximum catalytic activity for VO(LSO)₂ . Moreover, the VO(LSO)₂ complex has the potential for application in the effective and selective epoxidation of alkenes. Notably, under optimal VO(LSO)₂ conditions, cyclic alkenes convert more efficiently to their corresponding epoxides than linear alkenes.

Continuous Flow Epoxidation of Alkenes Using a Homogeneous Manganese Catalyst with Peracetic Acid

Epoxidation of alkenes is a valuable transformation in the synthesis of fine chemicals. Described herein are the design and development of a continuous flow process for carrying out the epoxidation of alkenes with a homogeneous manganese catalyst at metal loadings as low as 0.05 mol%. In this process, peracetic acid is generated in situ and telescoped directly into the epoxidation reaction, thus reducing the risks associated with its handling and storage, which often limit its use at scale. This flow process lessens the safety hazards associated with both the exothermicity of this epoxidation reaction and the use of the highly reactive peracetic acid. Controlling the speciation of manganese/2-picolinic acid mixtures by varying the ligand:manganese ratio was key to the success of the reaction. This continuous flow process offers an inexpensive, sustainable, and scalable route to epoxides.

Stereoselective Epoxidation of Triterpenic Allylic Alcohols and Cytotoxicity Evaluation of Synthesized Compounds

The epoxidation process of semi-synthetic triterpenoids 2-methyl-3-oxo-19β,28-epoxy- 18α-olean-1-ene, and its allylic alcohol derivatives were examined. 1,2α-epoxide, as the main product, was found to be formed from the starting enone exposed to m-chloroperbenzoic acid (mCPBA). In the case of hydroxy-directed mCPBA-oxidation of triterpenic allyl alcohols and their 3α-alkyl-substituted derivatives, inversion of C1 and C2 asymmetric centers with the formation of 1,2β-epoxyalcohols took place. The synthesis of 2,3α-epoxides was fulfilled from 2,3-dialkyl-substituted C(3) allyl alcohols by the action of pyridinium chlorochromate under [1,3]-oxidative rearrangement conditions. The transformations brought about enabled chiral oleanane derivatives with an oxygen-containing substituent at the C1, C2, and C3 atoms to be obtained. The study also provides information on in silico PASS prediction of pharmacological effects and in vitro evaluation of the cytotoxic activity of the synthesized compounds.

Hemp Seed Oil and Oilseed Radish Oil as New Sources of Raw Materials for the Synthesis of Bio-Polyols for Open-Cell Polyurethane Foams

We report on the development of open-cell polyurethane foams based on bio-polyols from vegetable oils: hemp seed oil, oilseed radish oil, rapeseed oil and used rapeseed cooking oil. The crude oils were pressed from seeds and subjected to an optimal solvent-free epoxidation process. Bio-polyols were obtained by a ring-opening reaction using diethylene glycol and tetrafluoroboric acid as catalysts. The resultant foams were analysed in terms of their apparent density, thermal conductivity coefficient, mechanical strength, closed cell content, short-term water absorption and water vapour permeability, while their morphology was examined using scanning electron microscopy. It was found that regardless of the properties of the oils, especially the content of unsaturated bonds, it was possible to obtain bio-polyols with very similar properties. The foams were characterized by apparent densities ranging from 11.2 to 12.1 kg/m3, thermal conductivity of <39 mW/m∙K, open cell contents of >97% and high water vapour permeability.

W-SBA-15 as an Effective Catalyst for the Epoxidation of 1,5,9-Cyclododecatriene

The results of a study on the epoxidation of 1,5,9-cyclododecatriene (CDT) on a W-SBA-15 catalyst using the batch and half-periodic methods are presented. During this study, the activity of the W-SBA-15 catalyst was compared to that of the Ti-SBA-15 catalyst, and the W-SBA-15 catalyst was found to be about 20 times more active than the Ti-SBA-15 catalyst. The highest CDT conversion so far, amounting to 86 mol%, was obtained after carrying out the 4 h epoxidation process. Conducting the studied process using the semi-batch method did not result in the significant improvement in value functions describing this process (CDT conversion and selectivity of CDT transformation to ECDD), but the fastest H₂O₂ dosing rate (246 µL/h) allowed us to obtain 9 mol% higher CDT conversion in comparison to the batch method.

Sunflower Oil as a Renewable Resource for Polyurethane Foams: Effects of Flame-Retardants

Currently, polyurethane (PU) manufacturers seek green alternatives for sustainable production. In this work, sunflower oil is studied as a replacement and converted to a reactive form through epoxidation and oxirane opening to produce rigid PU foams. Confirmatory tests such as Fourier-transform infrared spectroscopy (FT-IR), gel permeation chromatography (GPC), and hydroxyl value among others were performed to characterize the synthesized polyol. Despite the versatility of rigid PU foams, they are highly flammable, which makes eco-friendly flame retardants (FRs) desired. Herein, expandable graphite (EG) and dimethyl methyl phosphonate (DMMP), both non-halogenated FR, were incorporated under different concentrations to prepare rigid PU foams. Their effects on the physio-mechanical and fire-quenching properties of the sunflower oil-based PU foams were elucidated. Thermogravimetric and compression analysis showed that these foams presented appreciable compressive strength along with good thermal stability. The closed-cell contents (CCC) were around 90% for the EG-containing foams and suffered a decrease at higher concentrations of DMMP to 72%. The burning test showed a decrease in the foam's flammability as the neat foam had a burning time of 80 s whereas after the addition of 13.6 wt.% of EG and DMMP, separately, there was a decrease to 6 and 2 s, respectively. Hence, our research suggested that EG and DMMP could be a more viable alternative to halogen-based FR for PU foams. Additionally, the adoption of sunflower polyol yielded foams with results comparable to commercial ones.

In Situ Nucleation-Decoupled and Site-Specific Incorporation of Å-Scale Pores in Graphene Via Epoxidation

Generating pores in graphene by decoupled nucleation and expansion is desired to achieve a fine control over the porosity, and is desired to advance several applications. Herein, epoxidation is introduced, which is the formation of nanosized epoxy clusters on the graphitic lattice as nucleation sites without forming pores. In situ gasification of clusters inside a transmission electron microscope shows that pores are generated precisely at the site of the clusters by surpassing an energy barrier of 1.3 eV. Binding energy predictions using ab initio calculations combined with the cluster nucleation theory reveal the structure of the epoxy clusters and indicate that the critical cluster is an epoxy dimer. Finally, it is shown that the cluster gasification can be manipulated to form Å-scale pores which then effectively sieve gas molecules based on their size. This decoupled cluster nucleation and pore formation will likely pave the way for an independent control of pore size and density.

Enhancing the Peroxygenase Activity of a Cofactor-Independent Peroxyzyme by Directed Evolution Enabling Gram-Scale Epoxide Synthesis

Peroxygenases selectively incorporate oxygen into organic molecules making use of the environmentally friendly oxidant H₂ O₂ with water being the sole by-product. These biocatalysts can provide 'green' routes for the synthesis of enantioenriched epoxides, which are fundamental intermediates in the production of pharmaceuticals. The peroxyzyme 4-oxalocrotonate tautomerase (4-OT), catalysing the epoxidation of a variety of α,β-unsaturated aldehydes with H₂ O₂ , is outstanding because of its independence from any cost-intensive cofactor. However, its low-level peroxygenase activity and the decrease in the enantiomeric excess of the corresponding α,β-epoxy-aldehydes under preparative-scale conditions is limiting the potential of 4-OT. Herein we report the directed evolution of a tandem-fused 4-OT variant, which showed an ∼150-fold enhanced peroxygenase activity compared to 4-OT wild type, enabling the synthesis of α,β-epoxy-aldehydes in milligram- and gram-scale with high enantiopurity (up to 98 % ee) and excellent conversions. This engineered cofactor-independent peroxyzyme can provide new opportunities for the eco-friendly and practical synthesis of enantioenriched epoxides at large scale.

Quantitative Analysis and Structural Elucidation of Fatty Acids by Isobaric Multiplex Labeling Reagents for Carbonyl-Containing Compound (SUGAR) Tags and m-CPBA Epoxidation

In this study, a novel analytical method was developed to investigate fatty acids (FAs) for relative quantification, carbon-carbon double-bond localization, and cis-/trans-geometry differentiation by isobaric multiplex labeling reagents for carbonyl-containing compound (SUGAR) tag conjugation and meta-chloroperoxybenzoic acid (m-CPBA) epoxidation. FAs are essential components of cells and have diverse functions in energy storage and as complex lipid constituents. It has been reported that FAs play different roles in various biological processes such as the functional development of the brain. The comprehensive characterization and quantification of FAs are crucial to further elucidate their biological roles. However, it is challenging to perform relative quantification and structural elucidation of FAs using integrated mass spectrometry (MS)-based methods. Recently, our group developed isobaric multiplex SUGAR tags for quantitative glycomics. Besides aldehyde/ketone groups on glycans, hydrazide groups also possess reactivity toward carboxylic acids on FAs. In this study, we extended SUGAR tag labeling with FAs for the quantitative analysis by liquid chromatography (LC)-MS/MS in the positive ion mode and applied this strategy for the comparative analysis of FAs hydrolyzed from oil samples. In addition, to comprehensively elucidate the structures of unsaturated FAs, epoxidation by m-CPBA was performed before SUGAR tag labeling to enable carbon-carbon double-bond localization. Moreover, the cis- and trans-geometries of carbon-carbon double bonds in multiple pairs of monounsaturated FAs could also be differentiated in higher-energy collisional dissociation (HCD)-MS/MS. This study developed a high-throughput comprehensive FA analysis platform, which could be widely applied and utilized in biological and clinical studies.

Innovative Closed-Loop Recyclable Bio-Based Composites from Epoxidized Waste Flour and Recycled Carbon Fibers

Epoxy-based composites are designed for long-lasting applications, though their wide use is in contrast with their poor recyclability, which poses serious end-of-life issues. In order to reduce their environmental impact, precursors derived from fossil fuel based raw materials should be replaced with eco-friendly sources. This can be attained by using naturally derived epoxy matrices, or by finding a suitable solution for recycling at the end of life. In this paper, both strategies were analyzed, by replacing traditional monomers with epoxidized waste flour (EWF), an innovative bio-precursor derived from the organic waste stream, and a cleavable hardener, which allowed the recyclability of the matrix. The recyclable matrix was reinforced with recycled carbon fibers, derived from pyrolysis. DSC measurements were carried out in order to optimize the curing steps of the matrix, then flexural tests were performed in order to evaluate the mechanical response of the composite. A green recycling procedure was then investigated, which involved the use of non-toxic solvents and mild working conditions, and allowed recovery of the matrix while still preserving the properties of the carbon fibers. The components obtained after recycling were analyzed by FTIR analysis, which revealed the presence of the epoxy ring on the recycled waste flour. Hence, recycled waste flour was again used as a precursor and mixed with the cleavable hardener, thus, obtaining a closed-loop recycling.

Sulfonates as Versatile Structural Counterions of Epoxidized Salts

Ionic liquids have recently emerged as monomers to synthesize multifunctional polymeric materials. Among such species, ionic epoxy-based networks represent promising but underdeveloped materials that are hindered by tricky access to the functionalized ionic liquid monomers. To date, the reported epoxidized imidazolium salts have focused on highly toxic epichlorohydrin. This study concerns flexible and efficient methods to synthesize versatile building blocks with sulfonates as valuable anions. The judicious combination of an aliphatic or aromatic sulfonate with an imidazolium leads to new epoxidized salts with high structural variability and good chemical and thermal stability (>300 °C).

Comparative Study of Binding Pockets in Human CYP1A2, CYP3A4, CYP3A5, and CYP3A7 with Aflatoxin B1, a Hepato-Carcinogen, by Molecular Dynamics Simulation & Principal Component Analysis

BACKGROUND

Aflatoxin B1 is a harmful hepatocarcinogen which is metabolized in our body by Cytochrome P450 enzymes, namely CYP1A2, CYP3A4, CYP3A5, and CYP3A7, into toxic (exo-8, 9-epoxide) and non-toxic (AFQ1, endo-epoxide) products. We have found from the literature that due to cooperativity, the rate of metabolic reactions increases in CYP1A2 and CYP3A4 involving more than one site of proteins to form two products at a given time, whereas the interaction of CYP3A5 and CYP3A7 is still unknown. Our work aims to study these four enzymes with AFB1 based on binding site pocket characterization and to find the probable resultant products at each binding site.

METHODS

We used computational approaches like homology modeling, molecular docking to form mono and double ligated systems, molecular dynamic simulations to analyze the potential energies (vdW & electrostatic), PCA, RMSF, and residue-wise interactions at the active as well as allosteric sites of these four enzymes.

RESULTS

We found that CYP1A2, CYP3A4, and CYP3A5 were more hydrophobic at the first site and may induce epoxidation reaction to form toxic products, whereas the second site would be expected to be more polar and comprising charged interactions, thus enhancing non-toxic hydroxylated products. However, in CYP3A7, the first site favors hydroxylation, whereas the second site is involved in higher hydrophobic interactions.

CONCLUSION

Thus, in the fetus where AFB1 is metabolized only by CYP3A7, a lower concentration of toxic metabolites will be expected, while in adults exhibiting CYP1A2, CYP3A4 and CYP3A5 may increase the concentration of the toxic metabolites due to the combined effect of these enzymes, consequently increasing liver toxicity. We believe that AFB1 binding characteristics will be helpful for medicinal chemists in the process of designing a new drug.

Sustainable Epoxidized Guayule Natural Rubber, Blends and Composites with Improved Oil Resistance and Greater Stiffness

Production of petroleum-based synthetic rubbers (SRs) causes an enormous carbon footprint for the rubber industry. Carbon footprint would be reduced if natural rubber (NR) could take a larger market share and replace significant quantities of SR. However, some SRs have higher oil resistance than NRs, and, in applications where these properties are needed, chemically modified NR will be required. Epoxidation is a chemical modification of NR which partially converts unsaturated bonds on the backbone of NR to epoxy groups. In this research, epoxidized guayule natural rubber (EGNR)/guayule natural rubber (GNR) blends and GNR were used to make carbon black (CB) filled composites. The processability, mechanical properties, swelling behaviors and dynamic mechanical properties were characterized at various epoxide fractions. Composites made with EGNR/GNR had higher oil resistance, wet traction and stiffness than GNR composites, although tensile strength and elongation at break were reduced by epoxidation. EGNR is expected to lead to the development of new NR products with similar properties to SR, eroding SR markets and increasing the sustainability of the rubber industry.

Engineering Collariella virescens Peroxygenase for Epoxides Production from Vegetable Oil

Vegetable oils are valuable renewable resources for the production of bio-based chemicals and intermediates, including reactive epoxides of industrial interest. Enzymes are an environmentally friendly alternative to chemical catalysis in oxygenation reactions, epoxidation included, with the added advantage of their potential selectivity. The unspecific peroxygenase of Collariella virescens is only available as a recombinant enzyme (rCviUPO), which is produced in Escherichia coli for protein engineering and analytical-scale optimization of plant lipid oxygenation. Engineering the active site of rCviUPO (by substituting one, two, or up to six residues of its access channel by alanines) improved the epoxidation of individual 18-C unsaturated fatty acids and hydrolyzed sunflower oil. The double mutation at the heme channel (F88A/T158A) enhanced epoxidation of polyunsaturated linoleic and α−linolenic acids, with the desired diepoxides representing > 80% of the products (after 99% substrate conversion). More interestingly, process optimization increased (by 100-fold) the hydrolyzate concentration, with up to 85% epoxidation yield, after 1 h of reaction time with the above double variant. Under these conditions, oleic acid monoepoxide and linoleic acid diepoxide are the main products from the sunflower oil hydrolyzate.

Asymmetric Epoxidation of Olefins with Sodium Percarbonate Catalyzed by Bis-amino-bis-pyridine Manganese Complexes

Asymmetric epoxidation of a series of olefinic substrates with sodium percarbonate oxidant in the presence of homogeneous catalysts based on Mn complexes with bis-amino-bis-pyridine ligands is reported. Sodium percarbonate is a readily available and environmentally benign oxidant that is studied in these reactions for the first time. The epoxidation proceeded with good to high yields (up to 100%) and high enantioselectivities (up to 99% ee) using as low as 0.2 mol. % catalyst loadings. The epoxidation protocol is suitable for various types of substrates, including unfunctionalized alkenes, α,β-unsaturated ketones, esters (cis- and trans-), and amides (cis- and trans-). The reaction mechanism is discussed.

Aquivion perfluorosulfonic superacid as an effective catalyst for selective epoxidation of vegetable oils

The acid-promoted epoxidation of vegetable oils was studied using a variety of acidic ion exchange resins as heterogeneous acid catalysts. Quantitative and selective epoxidation of a series of vegetable oils with different composition of saturated, mono-, di- and tri-unsaturated fatty acids was obtained upon identification of the more efficient catalyst and experimental conditions. Furthermore, optimized reaction conditions were successfully applied to the epoxidation of a waste cooking oil, thus extending our procedure to the valorization of a biowaste, an area of increasing importance within a more sustainable society. The use of quantitative ¹HNMR besides making accurate evaluation of the amounts of reagents to be employed and of the selectivity, allowed facile and rapid quantification of mono-, di- and tri-epoxides, thus providing an indirect indication on the fatty acid composition of the vegetable oils, even in the presence of very low quantities of linolenic acid.

Site-Selective Catalytic Epoxidation of Alkene with Tunable, Atomic Precision by Molecularly Imprinted Artificial Epoxidases

Distinction of chemical functionality by their local chemical environment is a skill mastered by enzymes, evident from the selective synthesis, cleavage, and transformation of peptides, nucleic acids, and polysaccharides that abound with the same type of functional groups. In contrast, synthetic catalysts are generally better at differentiating functional groups based on their electronic and steric properties. Here we report artificial epoxidases prepared through molecular imprinting of surface-core doubly cross-linked micelles, followed by efficient functionalization of the imprinted site in the micellar core via photoaffinity labeling. The size and shape of the active sites are tuned by the modularly synthesized templates, with the oxygen-delivering peroxy acid group positioned accurately. These catalysts are used in epoxidation of alkene in water with hydrogen peroxide under mild conditions, without any additional additives. Most importantly, atomic precision is achieved in the catalysis and enables alkenes to be distinguished that differ in the position of the carbon-carbon double bond by a single carbon.

Nucleotide Analogues Bearing a C2' or C3'-Stereogenic All-Carbon Quaternary Center as SARS-CoV-2 RdRp Inhibitors

The design of novel nucleoside triphosphate (NTP) analogues bearing an all-carbon quaternary center at C2′ or C3′ is described. The construction of this all-carbon stereogenic center involves the use of an intramoleculer photoredox-catalyzed reaction. The nucleoside analogues (NA) hydroxyl functional group at C2′ was generated by diastereoselective epoxidation. In addition, highly enantioselective and diastereoselective Mukaiyama aldol reactions, diastereoselective N-glycosylations and regioselective triphosphorylation reactions were employed to synthesize the novel NTPs. Two of these compounds are inhibitors of the RNA-dependent RNA polymerase (RdRp) of SARS-CoV-2, the causal virus of COVID-19.

Peroxide-Mediated Oxygenation of Organic Compounds by Fungal Peroxygenases

Unspecific peroxygenases (UPOs), whose sequences can be found in the genomes of thousands of filamentous fungi, many yeasts and certain fungus-like protists, are fascinating biocatalysts that transfer peroxide-borne oxygen (from H2O2 or R-OOH) with high efficiency to a wide range of organic substrates, including less or unactivated carbons and heteroatoms. A twice-proline-flanked cysteine (PCP motif) typically ligates the heme that forms the heart of the active site of UPOs and enables various types of relevant oxygenation reactions (hydroxylation, epoxidation, subsequent dealkylations, deacylation, or aromatization) together with less specific one-electron oxidations (e.g., phenoxy radical formation). In consequence, the substrate portfolio of a UPO enzyme always combines prototypical monooxygenase and peroxidase activities. Here, we briefly review nearly 20 years of peroxygenase research, considering basic mechanistic, molecular, phylogenetic, and biotechnological aspects.

Catalytic Enantioselective Access to Dihydroquinoxalinones via Formal α-Halo Acyl Halide Synthon in One Pot

An enantioselective one-pot catalytic strategy to dihydroquinoxalinones, featuring novel 1-phenylsulfonyl-1-cyano enantioenriched epoxides as masked α-halo acyl halide synthons, followed by a domino ring-opening cyclization (DROC), is documented. A popular quinine-derived urea served as the catalyst in two out of the three steps performed in the same solvent using commercially available aldehydes, (phenylsulfonyl)acetonitrile, cumyl hydroperoxide and 1,2-phenylendiamines. Medicinally relevant 3-aryl/alkyl-substituted heterocycles are isolated in generally good to high overall yield and high enantioselectivity (up to 99 % ee). A rare example of excellent reusability of an organocatalyst at higher scale, subjected to oxidative conditions, is demonstrated. Mechanistically, labile α-ketosulfone has been detected as the intermediate involved in the DROC process. Theoretical calculations on the key epoxidation step rationalize the observed stereocontrol, highlighting the important role played by the sulfone group.

Electrophilic Epoxidation of α,β-Unsaturated Oximes with Dioxiranes and Ring Opening of the Epoxides

α,β-Unsaturated oximes underwent electrophilic epoxidation with in-situ-generated dimethyldioxirane to give the corresponding epoxides in good yields. This reaction is an example of "carbonyl umpolung" by transformation of α,β-unsaturated ketones to their oximes. Nucleophilic ring-opening reactions of the epoxides afforded α-substituted products. Shi asymmetric epoxidation of the oximes proceeded with moderate enantioselectivity.

Stereospecific Epoxidation of Limonene Catalyzed by Peroxygenase from Oat Seeds

Limonene is one of the most abundant naturally occurring cyclic monoterpenes and has recently emerged as a sustainable alternative to petroleum-based solvents as well as a chemical platform for the production of value-added compounds. The biocatalytic epoxidation of both enantiomers of limonene was carried out in the presence of a peroxygenase-containing preparation from oat (Avena sativa) flour. Different reaction profiles were observed depending on the starting enantiomer of limonene, but in both cases the 1,2-monoepoxide was obtained as the main product with excellent diastereoselectivity. Trans-1,2-monoepoxide and cis-1,2-monoepoxide were isolated from the reaction of (R)-limonene and (S)-limonene, respectively, and the reactions were scaled-up to 0.17 M substrate concentration. The process is valuable for operational simplicity, lack of toxic metal catalysts, and cost-effectiveness of the enzymatic source. Pure stereoisomers of 1,2-monoepoxides of limonene constitute a useful starting material for biorenewable polymers, but can be also converted into other chiral derivatives by epoxide ring opening with nucleophiles. As a proof of concept, a tandem protocol for the preparation of enantiopure (1S,2S,4R)-1,2-diol from (R)-limonene and (1R,2R,4S)-1,2-diol from (S)-limonene was developed.

Replacement of Volatile Acetic Acid by Solid SiO2@COOH Silica (Nano)Beads for (Ep)Oxidation Using Mn and Fe Complexes Containing BPMEN Ligand

Mn and Fe BPMEN complexes showed excellent reactivity in catalytic oxidation with an excess of co-reagent (CH3COOH). In the straight line of a cleaner catalytic system, volatile acetic acid was replaced by SiO2 (nano)particles with two different sizes to which pending carboxylic functions were added (SiO2@COOH). The SiO2@COOH beads were obtained by the functionalization of SiO2 with pending nitrile functions (SiO2@CN) followed by CN hydrolysis. All complexes and silica beads were characterized by NMR, infrared, DLS, TEM, X-ray diffraction. The replacement of CH3COOH by SiO2@COOH (100 times less on molar ratio) has been evaluated for (ep)oxidation on several substrates (cyclooctene, cyclohexene, cyclohexanol) and discussed in terms of activity and green metrics.

Conversion of Limonene over Heterogeneous Catalysis: An Overview

The natural terpene limonene is widely found in nature. The (R)-limonene (the most abundant enantiomer) is present in the essential oils of lemon, orange, and other citrus fruits, while the (S)-limonene is found in peppermint and the racemate in turpentine oil. Limonene is a low-cost, low toxicity biodegradable terpene present in agricultural wastes derived from citrus peels. The products obtained from the conversion of limonene are valuable compounds widely used as additives for food, cosmetics, or pharmaceuticals. The conversion of limonene to produce different products has been the subject of intense research, mainly with the objective to improve catalytic systems. This review focused on the application of heterogeneous catalysts in the catalytic conversion of limonene.

Fully Recyclable Bio-Based Epoxy Formulations Using Epoxidized Precursors from Waste Flour: Thermal and Mechanical Characterization

Organic wastes represent an increasing pollution problem due to the exponential growth of their presence in the waste stream. Among these, waste flour cannot be easily reused by transforming it into high-value-added products. Another major problem is represented by epoxy-based thermosets, which have wide use but also poor recyclability. The object of the present paper is, therefore, to analyze both of these problems and come up with innovative solutions. Indeed, we propose a completely new approach, aimed at reusing the organic waste flour, by converting it into high-value epoxy-based thermosets that could be fully recycled into a reusable plastic matrix when added to the waste epoxy-based thermosets. Throughout the research activity, the organic waste was transformed into an epoxidized prepolymer, which was then mixed with a bio-based monomer cured with a cleavable ammine. The latter reactant was based on Recyclamine™ by Connora Technologies, and in this paper, we demonstrate that this original approach could work with the synthetized epoxy prepolymers derived from the waste flour. The cured epoxies were fully characterized in terms of their thermal, rheological, and flexural properties. The results obtained showed optimal recyclability of the new resin developed.

Detection and Structural Characterization of SFAHFA Homologous Series in Mouse Colon Contents by LTQ-Orbitrap-MS and Their Implication in Influenza Virus Infection

Fatty acid esters of hydroxy fatty acids (FAHFAs) are a new class of endogenous lipids with promising physiological functions in mammals. We previously introduced a new type of lipids to this family called short-chain fatty acid esters of hydroxy fatty acids (SFAHFAs), branching specific to the C2 carbon of a long-chain fatty acid (≥C20). In this study, we discovered a homologous series of SFAHFAs comprising C16-C26 hydroxy fatty acids esterified with short-chain fatty acids (C2-C5) in mouse colon contents. The detected SFAHFAs were characterized by high-resolution mass spectrometry with MSn analysis. The double-bond position of monounsaturated SFAHFAs was determined by the epoxidation reaction of samples with m-chloroperoxybenzoic acid and their MSn analysis. Further, the measurement of SFAHFA concentration in the colon contents of mice infected with influenza A/Puerto Rico/8/34 (H1N1; PR8) virus revealed a significant increase in their levels compared to native control. A strong correlation was observed between hydroxy fatty acid and SFAHFAs. Detection, characterization, and profiling of these new SFAHFA levels in relation with pandemic H1N1; PR8 influenza virus will contribute to the in-depth study of their function and metabolism.

Precisely Engineered Supported Gold Clusters as a Stable Catalyst for Propylene Epoxidation

Designing a stable and selective catalyst with high H₂ utilisation is of pivotal importance for the direct gas-phase epoxidation of propylene. This work describes a facile one-pot methodology to synthesise ligand-stabilised sub-nanometre gold clusters immobilised onto a zeolitic support (TS-1) to engineer a stable Au/TS-1 catalyst. A non-thermal O₂ plasma technique is used for the quick removal of ligands with limited increase in particle size. Compared to untreated Au/TS-1 catalysts prepared using the deposition precipitation method, the synthesised catalyst exhibits improved catalytic performance, including 10 times longer lifetime (>20 days), increased PO selectivity and hydrogen efficiency in direct gas phase epoxidation. The structure-stability relationship of the catalyst is illustrated using multiple characterisation techniques, such as XPS, ³¹ P MAS NMR, DR-UV/VIS, HRTEM and TGA. It is hypothesised that the ligands play a guardian role in stabilising the Au particle size, which is vital in this reaction. This strategy is a promising approach towards designing a more stable heterogeneous catalyst.

Effect of the Ligand Backbone on the Reactivity and Mechanistic Paradigm of Non-Heme Iron(IV)-Oxo during Olefin Epoxidation

The oxygen atom transfer (OAT) reactivity of the non-heme [Fe^IV (2PyN2Q)(O)]²⁺ (2) containing the sterically bulky quinoline-pyridine pentadentate ligand (2PyN2Q) has been thoroughly studied with different olefins. The ferryl-oxo complex 2 shows excellent OAT reactivity during epoxidations. The steric encumbrance and electronic effect of the ligand influence the mechanistic shuttle between OAT pathway I and isomerization pathway II (during the reaction stereo pure olefins), resulting in a mixture of cis-trans epoxide products. In contrast, the sterically less hindered and electronically different [Fe^IV (N4Py)(O)]²⁺ (1) provides only cis-stilbene epoxide. A Hammett study suggests the role of dominant inductive electronic along with minor resonance effect during electron transfer from olefin to 2 in the rate-limiting step. Additionally, a computational study supports the involvement of stepwise pathways during olefin epoxidation. The ferryl bend due to the bulkier ligand incorporation leads to destabilization of both d z 2 and d x 2 - y 2 orbitals, leading to a very small quintet-triplet gap and enhanced reactivity for 2 compared to 1. Thus, the present study unveils the role of steric and electronic effects of the ligand towards mechanistic modification during olefin epoxidation.

Catalytic Performance of Zr-Based Metal-Organic Frameworks Zr-abtc and MIP-200 in Selective Oxidations with H2 O2

The catalytic performance of Zr-abtc and MIP-200 metal-organic frameworks consisting of 8-connected Zr₆ clusters and tetratopic linkers was investigated in H₂ O₂ -based selective oxidations and compared with that of 12-coordinated UiO-66 and UiO-67. Zr-abtc demonstrated advantages in both substrate conversion and product selectivity for epoxidation of electron-deficient C=C bonds in α,β-unsaturated ketones. The significant predominance of 1,2-epoxide in carvone epoxidation, coupled with high sulfone selectivity in thioether oxidation, points to a nucleophilic oxidation mechanism over Zr-abtc. The superior catalytic performance in the epoxidation of unsaturated ketones correlates with a larger amount of weak basic sites in Zr-abtc. Electrophilic activation of H₂ O₂ can also be realized, as evidenced by the high activity of Zr-abtc in epoxidation of the electron-rich C=C bond in caryophyllene. XRD and FTIR studies confirmed the retention of the Zr-abtc structure after the catalysis. The low activity of MIP-200 in H₂ O₂ -based oxidations is most likely related to its specific hydrophilicity, which disfavors adsorption of organic substrates and H₂ O₂ .

Proton Switch in the Secondary Coordination Sphere to Control Catalytic Events at the Metal Center: Biomimetic Oxo Transfer Chemistry of Nickel Amidate Complex

High-valent metal-oxo species are key intermediates for the oxygen atom transfer step in the catalytic cycles of many metalloenzymes. While the redox-active metal centers of such enzymes are typically supported by anionic amino acid side chains or porphyrin rings, peptide backbones might function as strong electron-donating ligands to stabilize high oxidation states. To test the feasibility of this idea in synthetic settings, we have prepared a nickel(II) complex of new amido multidentate ligand. The mononuclear nickel complex of this N5 ligand catalyzes epoxidation reactions of a wide range of olefins by using mCPBA as a terminal oxidant. Notably, a remarkably high catalytic efficiency and selectivity were observed for terminal olefin substrates. We found that protonation of the secondary coordination sphere serves as the entry point to the catalytic cycle, in which high-valent nickel species is subsequently formed to carry out oxo-transfer reactions. A conceptually parallel process might allow metalloenzymes to control the catalytic cycle in the primary coordination sphere by using proton switch in the secondary coordination sphere.

Hydroxylation, Epoxidation, and Dehydrogenation of Capsaicin by a Microbial Promiscuous Cytochrome P450 105D7

Cytochrome P450 enzymes (P450s) are versatile biocatalysts, which insert a molecular oxygen into inactivated C-H bonds under mild conditions. CYP105D7 from Streptomyces avermitilis has been reported as a bacterial substrate-promiscuous P450 which catalyzes the hydroxylation of 1-deoxypentalenic acid, diclofenac, naringenin, compactin and steroids. In this study, CYP105D7 catalyzes hydroxylation, epoxidation and dehydrogenation of capsaicin, a pharmaceutical agent, revealing its functional diversity. The kinetic parameters of the CYP105D7 oxidation of capsaicin were determined as K_m =311.60±87.30 μM and k_cat =2.01±0.33 min^-1 . In addition, we conducted molecular docking, mutagenesis and substrate binding analysis, indicating that Arg81 plays crucial role in the capsaicin binding and catalysis. To our best knowledge, this study presents the first report to illustrate that capsaicin can be catalyzed by prokaryotic P450s.