Epoxidation Of Allylic Alcohols Research Articles

Mechanism of the Sharpless Epoxidation Reaction: A DFT Study.

The Sharpless reaction is an enantioselective epoxidation of prochiral allylic alcohols that employs a Ti(IV) catalyst formed from titanium tetra(isopropoxide), Ti(O-i-Pr)4, diethyl tartrate (DET), and the oxidizing agent tert-butyl hydroperoxide. The M06-2X DFT functional with the 6-311+G(d,p) basis set has been employed to model the structures and energetics of the Sharpless epoxidation reaction. The monomeric tetracoordinate titanium(IV) diethyltartrate is thermodynamically strongly favored to dimerize, producing a pentacoordinate catalyst, [Ti(DET)(O-i-Pr)2]2, that is a more reactive chiral epoxidation catalyst. The rapid ligand exchange reactions needed to generate the "loaded" catalyst and to repeat the overall catalytic cycle have been examined and are found to have activation energies that are much lower than the epoxidation barriers. The transition structures and activation energies for the enantioselective epoxidation of allyl alcohol, trans-methyl-allyl alcohol and trans-tert-butyl-allyl alcohol with the "loaded" Sharpless catalyst, [Ti2(DET)2(O-i-Pr)2-(OAllyl)-(OOt-Bu)], are presented. The effect of the C═O···Ti interactions on the activation energies and the significance of the O-C-C═C dihedral angle on the enantioselectivity of the epoxidation reaction are discussed.

Hydrophilic Ti-MWW for catalyzing epoxidation of allyl alcohol

Titanosilicates are widely applied in the alkene epoxidation reactions with high reaction rate and selectivity to desired products. Their catalytic performance depends on the structure topology, the micro-environment of Ti active sites, and the hydrophobicity/hydrophilicity of zeolite framework. Herein, we focus on a hydrophilic substrate of allyl alcohol (AAL) and investigated catalytic performance of four titanosilicates (TS-1, Ti-MOR, Ti-MWW, and Re-Ti-MWW) in the AAL epoxidation reaction with hydrogen peroxide as the oxidant. Among them, Re-Ti-MWW, synthesized via the post-modification of Ti-MWW with piperidine, exhibited the highest activity. Moreover, the preferred solvent changed from MeCN for Ti-MWW to H2O for Re-Ti-MWW. The relative diffusion rate of AAL over Re-Ti-MWW was up to 466 × 10-7 s-1, much larger than those of other zeolites. The higher diffusion rate of Re-Ti-MWW was probably derived from the higher framework hydrophilicity as revealed by the smaller water contact angle of Re-Ti-MWW compared to the other zeolites, which contributed to the high activity in AAL epoxidation. In the continuous slurry reactor, Re-Ti-MWW achieved a high catalytic lifetime of 163 h, with the selectivity of desirable glycidol product maintained at > 97% in the H2O solvent system, showing high potential as an industrial catalyst for the AAL epoxidation reaction.

Systematic Development of Vanadium Catalysts for Sustainable Epoxidation of Small Alkenes and Allylic Alcohols.

The catalytic epoxidation of small alkenes and allylic alcohols includes a wide range of valuable chemical applications, with many works describing vanadium complexes as suitable catalysts towards sustainable process chemistry. But, given the complexity of these mechanisms, it is not always easy to sort out efficient examples for streamlining sustainable processes and tuning product optimization. In this review, we provide an update on major works of tunable vanadium-catalyzed epoxidations, with a focus on sustainable optimization routes. After presenting the current mechanistic view on vanadium catalysts for small alkenes and allylic alcohols' epoxidation, we argue the key challenges in green process development by highlighting the value of updated kinetic and mechanistic studies, along with essential computational studies.

Nickel Nanoparticles Decorated on Glucose-Derived Carbon Spheres as a Novel, Non-Palladium Catalyst for Epoxidation of Olefin

Carbon spheres supporting nickel nanoparticles (NPs), generated by the integration of hydrothermal and microwave irradiation techniques, catalyzed the epoxidation of 1-octene, cyclooctene, styrene, allyl alcohol, and cyclohexene. The average particle sizes of the carbon spheres (CSs) and nickel oxide species immobilized on the CSs were 240 nm and 26 nm, respectively. The fabricated composites incorporating nickel NPs showed higher activity in the cyclohexene epoxidation process. The cyclohexene conversion was enhanced by raising the Ni loading to 10%. Within 14 h, the cyclohexene conversion had grown to 98%. This robust catalytic activity can be attributed to the efficient distribution of Ni species on the CSs, the facile lowering of the surface, and the development of uniformly nanosized species. The composite exhibited good recyclability across at least five cycles (which is not a simple task involving nickel-nanoparticle-based catalysts that are employed in water), and no nickel species leached into the solution, making the total system environmentally benign and cost-effective.

Investigation of catalytic activity of vanadyl sulphate immobilized on prepared UiO-66 modified with urea and melamine as allyl alcohol epoxidation catalysts

Investigation of catalytic activity of vanadyl sulphate immobilized on prepared UiO-66 modified with urea and melamine as allyl alcohol epoxidation catalysts

Total Synthesis, Revised Structure, and Cytotoxic Activities of Iubol

To unambiguously establish the stereostructure of the marine cytotoxic bromotriterpenoid iubol, a member of the thyrsiferol family, asymmetric chemical synthesis has been carried out. The synthesis features a one-pot process for the tetrahydropyranyl D ring construction through a stoichiometric Sharpless asymmetric epoxidation of allylic alcohols followed by titanium chelation-assisted 6-exo oxacyclization. In this paper, we report the asymmetric total synthesis of iubol, revision of the proposed structure to the C22-epimer, and preliminary cytotoxic activities of synthetic compounds against some tumor cells.

Mechanistic Details of the Sharpless Epoxidation of Allylic Alcohols—A Combined URVA and Local Mode Study

In this work, we investigated the catalytic effects of a Sharpless dimeric titanium (IV)–tartrate–diester catalyst on the epoxidation of allylalcohol with methyl–hydroperoxide considering four different orientations of the reacting species coordinated at the titanium atom (reactions R1–R4) as well as a model for the non-catalyzed reaction (reaction R0). As major analysis tools, we applied the URVA (Unified Reaction Valley Approach) and LMA (Local Mode Analysis), both being based on vibrational spectroscopy and complemented by a QTAIM analysis of the electron density calculated at the DFT level of theory. The energetics of each reaction were recalculated at the DLPNO-CCSD(T) level of theory. The URVA curvature profiles identified the important chemical events of all five reactions as peroxide OO bond cleavage taking place before the TS (i.e., accounting for the energy barrier) and epoxide CO bond formation together with rehybridization of the carbon atoms of the targeted CC double bond after the TS. The energy decomposition into reaction phase contribution phases showed that the major effect of the catalyst is the weakening of the OO bond to be broken and replacement of OH bond breakage in the non-catalyzed reaction by an energetically more favorable TiO bond breakage. LMA performed at all stationary points rounded up the investigation (i) quantifying OO bond weakening of the oxidizing peroxide upon coordination at the metal atom, (ii) showing that a more synchronous formation of the new CO epoxide bonds correlates with smaller bond strength differences between these bonds, and (iii) elucidating the different roles of the three TiO bonds formed between catalyst and reactants and their interplay as orchestrated by the Sharpless catalyst. We hope that this article will inspire the computational community to use URVA complemented with LMA in the future as an efficient mechanistic tool for the optimization and fine-tuning of current Sharpless catalysts and for the design new of catalysts for epoxidation reactions.

Syn‐Selective Epoxidation of Chiral Terminal Allylic Alcohols with a Titanium Salalen Catalyst and Hydrogen Peroxide

In the Sharpless asymmetric epoxidation of chiral secondary allylic alcohols, one substrate enantiomer is predominantly converted to the anti‐epoxy alcohol. We herein report the first highly syn‐selective epoxidation of terminal allylic alcohols using a titanium salalen complex as catalyst, at room temperature, and aqueous hydrogen peroxide as oxidant. With enantiopure terminal allylic alcohols as substrates, the epoxy alcohols were obtained with up to 98 % yield and up to >99 : 1 dr (syn). Catalyst loadings as low as 1 mol % can be applied without eroding the syn‐diastereoselectivity. Modification of the allylic alcohol to an ether does not affect the diastereoselectivity either [>99 : 1 dr (syn)]. Inverting the catalyst configuration leads to the anti‐product, albeit at lower dr (ca. 20 : 1). The synthetic potential is demonstrated by a short, gram‐scale preparation of a tetrahydrofuran building block with three stereocenters, involving two titanium salalen catalyzed epoxidation steps.

Syn‐Selektive Epoxidierung von chiralen, terminalen Allylalkoholen mit einem Titan‐Salalen‐Katalysator und Wasserstoffperoxid

AbstractIn der asymmetrischen Sharpless‐Epoxidierung von chiralen, sekundären Allylalkoholen wird ein Substratenantiomer bevorzugt zum anti‐Epoxyalkohol umgesetzt. In dieser Arbeit stellen wir die erste hoch syn‐selektive Epoxidierung von terminalen Allylalkoholen vor. Mit einem Titan‐Salalen‐Katalysator und H2O2 als Oxidationsmittel werden enantiomerenreine, terminale Allylalkohole mit bis zu 98 % Ausbeute und >99 : 1 dr (syn) zu Epoxyalkoholen umgesetzt. Diese syn‐Selektivität bleibt auch bei niedrigeren Katalysatorbeladungen (bis zu 1 mol‐%) erhalten. Auch die Modifizierung der Allylalkoholfunktion zu einem Ether beeinflusst die Diastereoselektivität nicht [>99 : 1 dr (syn)]. Wird die Katalysatorkonfiguration invertiert, so wird bevorzugt das anti‐Produkt gebildet, allerdings mit niedrigerem dr (ca. 20 : 1). Das Anwendungspotential wird am Beispiel der Synthese eines THF‐Bausteins mit drei Stereozentren im Grammmaßstab demonstriert, in welcher zwei Ti‐Salalen‐katalysierte Epoxidierungen durchlaufen werden.

Epoxidation vs. dehydrogenation of allylic alcohols: heterogenization of the VO(acac)2 catalyst in a metal-organic framework.

Allylic alcohol epoxidation and dehydrogenation reactivity is distinguished when VO(acac)2 is used in solution or anchored in a metal-organic framework (MOF). The chemical mechanism depends on the electronic profile of alkene substituents when the vanadyl complex is used in the homogenous phase. However, confinement effects imparted by MOF channels allow gaining control of the chemoselectivity toward the dehydrogenation product.

The AHP method in the optimization of the epoxidation of allylic alcohols

The article presents the results of work on the application of the AHP method in the selection of optimal parameters of the epoxidation process of unsaturated compounds, which significantly affect the course of the research process, while allowing the user to ignore those factors that have a negligible impact on the results. AHP is a multi-criteria decision technique that can assist in making an overall decision by breaking down a complex problem into a multi-level hierarchical structure of goal, criteria, and alternatives. It has been found that this method greatly simplifies the testing process, reducing the time to obtain reliable results and reducing testing costs as it reduces the number of tests required.

Mechanism of Orbital Interactions in the Sharpless Epoxidation with Ti(IV) Peroxides: A DFT Study.

The M06-2X DFT functional has been employed to examine monomeric titanium(IV) hydroperoxo catalysts that model the individual steps in the dimeric titanium(IV)-catalyzed Sharpless reaction. This is the first example of a transition structure for titanium(IV) tert-butyl hydroperoxide-catalyzed epoxidation that describes the molecular motion required for oxygen atom transfer. These epoxidation catalysts have been examined for both bimolecular reactions with E-2-butene and the intramolecular epoxidation of allyl alcohol. The transition structure for the bimolecular peroxyacetic acid epoxidation of E-2-butene has been shown to be spiro in nature, and likewise, the intramolecular epoxidation of allyl alcohol is also nearly spiro. The significance of the O-C-C═C dihedral angle of allyl alcohol is examined for the Ti(IV) tert-butyl hydroperoxide epoxidation mechanism. Evidence is presented that supports a hexacoordinate titanium peroxo environment that exists in the dimeric form of the Sharpless catalyst. The mechanism for a 1,3-rearrangement of the alkoxide ligand in a titanium hydroperoxide to the Ti center in concert with oxygen atom transfer of the proximal oxygen to the C═C bond of the substrate is presented. The dimerization of Ti(IV)-(R,R)-diethyl tartrate-diisopropoxide and its hydrolysis have been calculated. The mechanism for rapid ligand exchange with alkyl hydroperoxides involving the Ti(O-i-Pr)4 precursor is examined to show how the active epoxidation catalyst is produced.

Kinetics and Mechanism of Allyl Alcohol Epoxidation with Hydrogen Peroxide on a Titanium Silicalite Catalyst TS-1. Formulation and Discrimination between Hypothetical Mechanisms

Kinetics and Mechanism of Allyl Alcohol Epoxidation with Hydrogen Peroxide on a Titanium Silicalite Catalyst TS-1. Formulation and Discrimination between Hypothetical Mechanisms

Can Brønsted acids catalyze the epoxidation of allylic alcohols with H2O2? With a little help from the proton, the H3PMo12O40 acid did it and well

In this work, we have explored the catalytic activity of typical Brønsted acid in epoxidation reactions of terpene alcohols with hydrogen peroxide. Keggin heteropolyacids (HPAs) (i.e., H3PW12O40, H3PMo12O40, and H4SiW12O40) were compared to the common Brønsted acids (i.e., sulfuric, and p-toluenesulfonic acids) in epoxidation reactions of nerol, the model molecule selected. The effects of the main reaction variable, such as temperature, time, load and sort of acid catalyst were evaluated. Among the catalysts investigated, H3PMo12O40 was the most active and selective catalyst toward epoxide nerol, the goal product. The highest conversion obtained was 99%, with a selectivity of 97% towards nerol epoxide. A discussion of the reaction mechanism was performed as the basis of experimental results and acidity properties of the catalysts. A comparison with a non-protic catalyst (i.e., Na3PMo12O40) allowed demonstrate that both Molybdenum and protons present in the H3PMo12O40 are the main active sites in this epoxidation reaction.

Sharpless Asymmetric Epoxidation: Applications in the Synthesis of Bioactive Natural Products

This review discusses an important synthetic tool proposed by K.B. Sharpless in 1980, known as the Sharpless asymmetric epoxidation of allylic alcohols, and examines its use in the total synthesis of representative exponents of biologically active natural products. Focus is given to the synthesis of simple to highly complex secondary metabolites, including lactones, amino acids, diterpenes, and macrolides. The Sharpless approach involves the use of a catalyst, titanium tetra isopropoxide [Ti(OiPr)4], dialkyl tartrate as a chiral ligand, and tert-butyl hydroperoxide (TBHP) as an oxidizing agent. The method allows converting allylic alcohols to epoxides, which are chiral building blocks and versatile intermediates in the synthesis of natural products. The biological and synthetic importance of epoxides lies in the susceptibility of the three-membered heterocyclic ring to stereoand regioselective opening by nucleophilic or acidic reagents, providing oxygen adducts.

Role of Organic Fluoride Salts in Stabilizing Niobium Oxo-Clusters Catalyzing Epoxidation.

We present here that easily available organic salts can stabilize/modify niobium (Nb) oxo-clusters. The as-synthesized Nb oxo-clusters have been characterized by various methods. These Nb oxo-clusters were catalytically active for the epoxidation of allylic alcohols and olefins with H2O2 as an oxidant. Notably, Nb-OC@TBAF-0.5 appeared as highly dispersed nanosized particles and showed the highest catalytic activity, which can be attributed to the following reasons on the basis of characterization. First, the strong coordination of fluorine ions with Nb sites and the steric protection with bulky organic cations led to high stabilization and dispersion of the oxo-clusters in the course of the reaction. Second, a hydrogen-bond interaction between the coordinated fluorine atom and the -OH group of allylic alcohol favored the epoxidation reaction. Third, the electron density of Nb sites decreased due to the strong electron-withdrawing ability of F- adjacent to Nb sites, thus promoting the electrophilic oxygen transfer to the C═C bond.

Post-synthetic modification of porous [Cu3(BTC)2] (BTC = benzene‐1,3,5‐tricarboxylate) metal organic framework with molybdenum and vanadium complexes for the epoxidation of olefins and allyl alcohols

Post-synthetic modification of porous [Cu3(BTC)2] (BTC = benzene‐1,3,5‐tricarboxylate) metal organic framework with molybdenum and vanadium complexes for the epoxidation of olefins and allyl alcohols

Immobilized some of vanadium compounds on modified graphene oxide as nanofiber network for epoxidation of allyl alcohols

VO(acac)2 and the in situ generated VO(Salen) complex were immobilized on the covalently modified graphene oxide (GO) with ethylenediamine (en) and designated as VO(acac)2@en‐GO and VO@Salen@en‐GO, respectively. Characterization of the prepared compounds was carried out with FT‐IR, XRD, ICP, CHN, Raman, SEM, TEM, and BET techniques. The SEM results confirmed the formation a nanofiber network and nanointerconnected needles. The prepared immobilized complexes were then used as heterogeneous catalysts for epoxidation of some allyl alcohols such as geraniol, trans‐2‐hexene‐1‐ol, and 1‐octen‐3‐ol. Based on the results obtained by GC and GC–mass analyses, epoxidation of geraniol with 97% conversion and 100% selectivity seemed promising. Readily recycling of the catalyst and reusing for four successive catalytic runs without significant loss of activity revealed that this catalysis system deserves nomination for practical application.

Hybrid chemoenzymatic heterogeneous catalyst prepared in one step from zeolite nanocrystals and enzyme-polyelectrolyte complexes.

The combination of inorganic heterogeneous catalysts and enzymes, in so-called hybrid chemoenzymatic heterogeneous catalysts (HCEHCs), is an attractive strategy to effectively run chemoenzymatic reactions. Yet, the preparation of such bifunctional materials remains challenging because both the inorganic and the biological moieties must be integrated in the same solid, while preserving their intrinsic activity. Combining an enzyme and a zeolite, for example, is complicated because the pores of the zeolite are too small to accommodate the enzyme and a covalent anchorage on the surface is often ineffective. Herein, we developed a new pathway to prepare a nanostructured hybrid catalyst built from glucose oxidase and TS-1 zeolite. Such hybrid material can catalyse the in situ biocatalytic formation of H2O2, which is subsequently used by the zeolite to trigger the epoxidation of allylic alcohol. Starting from an enzymatic solution and a suspension of zeolite nanocrystals, the hybrid catalyst is obtained in one step, using a continuous spray drying method. While enzymes are expectedly unable to resist the conditions used in spray drying (temperature, shear stress, etc.), we leverage on the preparation of “enzyme–polyelectrolyte complexes” (EPCs) to increase the enzyme stability. Interestingly, the use of EPCs also prevents enzyme leaching and appears to stabilize the enzyme against pH changes. We show that the one-pot preparation by spray drying gives access to hybrid chemoenzymatic heterogeneous catalysts with unprecedented performance in the targeted chemoenzymatic reaction. The bifunctional catalyst performs much better than the two catalysts operating as separate entities. We anticipate that this strategy could be used as an adaptable method to prepare other types of multifunctional materials starting from a library of functional nanobuilding blocks and biomolecules.

Olefin epoxidation with ionic liquid catalysts formed by supramolecular interactions

This work demonstrated that the specific ionic liquids (ILs) have been designed via the supramolecular complexation between 18-crown-6 (CE) and ammonium peroxoniobate (NH4-Nb). The resultant ILs have been characterized by elemental analysis, FT-IR, Raman, NMR, DSC, conductivity measurement and MALDI-TOF, etc. The IL (CE-1) consisting of CE and ammonium peroxoniobate can be further coordinated with GLY to generate a new IL (CE-2), which showed both high catalytic activity in epoxidation with H2O2 and good recyclability. The characterization of 93Nb NMR spectra revealed that the peroxoniobate anions has demonstrated a structural evolution in the presence of hydrogen peroxide, in which NbO species can be easily oxidized into the catalytically active niobium−peroxo species. Especially, the supramolecular complexation can provide suitable hydrophobicity, which ensured that the hydrophobic olefins and allylic alcohols were easily accessible to the catalytically active anions, and thus facilitated the epoxidation reaction. Notably, the supramolecular IL catalysts in this work exhibited a huge advantage of the easy availability, as compared with the previously reported peroxoniobate-based ILs. As far as we know, this is the first example of the highly selective epoxidation of olefins and allylic alcohols by using supramolecular ILs as catalysts.