Asymmetric Ring Opening Of Epoxides Research Articles

Layered Double Hydroxide Catalysts Preparation, Characterization and Applications for Process Development: An Environmentally Green Approach

The adage of new generation of fine chemicals process is the best process applied in the absence of conventional methods. However, many methods use different reaction parameters, such as basic and acidic catalysts, for example oxidation, reduction, bromination, water splitting, cyanohydrin, ethoxylation, syngas, aldol condensation, Michael addition, asymmetric ring opening of epoxides, epoxidation, Wittig and Heck reaction, asymmetric ester epoxidation of fatty acids, combustion of methane, NOx reduction, biodiesel synthesis, propylene oxide polymerization. Layered Double Hydroxides (LDHs) have received considerable attention due their potential applications in flame retardant and has excellent medicinal property for reducing acidity. These catalysts are characterized using analytical techniques, such as: X-ray diffraction (XRD), Fourier-transform infrared (FT-IR), Raman spectroscopy, Thermogravimetric-Differential Thermal Analyzer (TG-DTA), Scanning electron microscope (SEM), Transmission electron microscopes (TEM), Brunauer-Emmett-Teller (BET) surface area, N2 Adsorption-desorption, Temperature programmed reduction (TPR), X-ray photoelectrons spectroscopy (XPS), which gives its overall picture of its structure, porosity, morphology, thermal stability, reusability, and activity of catalysts. LDHs catalysts have proven to be economic and environmentally friendly. The above discussed applications make these catalysts unique from Green Chemistry point of view since they are reusable, and eco-friendly catalysts. Copyright © 2021 by Authors, Published by BCREC Group. This is an open access article under the CC BY-SA License (https://creativecommons.org/licenses/by-sa/4.0).

Asymmetric Synthesis of N,O-Heterobicyclic Octanes and (-)-Geissman-Waiss Lactone.

A short, asymmetric synthesis of tetrahydro-2H-furo[3,2-b]pyrrole derivatives and (−)-Geissman–Waiss lactone starting from meso-cyclohexadiene epoxide is described. Pivotal transformations in the developed synthetic procedure include asymmetric epoxide ring opening to install the requisite 1S,5S stereocenters and oxidative lactonization/lactamization sequences. This route provides a streamlined synthetic pathway toward necine alkaloids.

Asymmetric Ring-Opening of Epoxides Catalyzed by Metal–Salen Complexes

The asymmetric ring-opening of epoxides is an important reaction in organic synthesis, since it allows for the enantioselective installation of two vicinal functional groups with specific stereochemistry within one step from a highly available starting material. An effective class of catalysts for the asymmetric ring-opening of epoxides is metal–salen complexes. This review summarizes the development of metal–salen catalyzed enantioselective desymmetrization of meso-epoxides and kinetic resolution of epoxides with various nucleophiles, including the design and application of both homogeneous- and heterogeneous epoxide-opening catalysts as well as multi-metallic covalent and supramolecular catalytic systems.

Significant improvement of the enantioselectivity of a halohydrin dehalogenase for asymmetric epoxide ring opening reactions by protein engineering.

Halohydrin dehalogenases (HHDHs) have attracted much attention due to their ability to synthesize enantiomerically enriched epoxides and β-haloalcohols. However, most of the HHDHs exhibit low enantioselectivity. Here, a HHDH from the alphaproteobacteria isolate 46_93_T64 (AbHHDH), which shows only poor enantioselectivity in the catalytic resolution of rac-PGE (E = 9.9), has been subjected to protein engineering to enhance its enantioselectivity. Eight mutants (R89K, R89Y, V137I, P178A, N179Q, N179L, F187L, F187A) showed better enantioselectivity than the wild type. The best single mutant N179L (E = 93.0) showed a remarkable 9.4-fold increase in the enantioselectivity. Then, the single mutations were combined to produce the double, triple, quadruple, and quintuple mutants. Among the combinational mutants, the best variant (R89Y/N179L) showed an increased E value of up to 48. The E values of the variants N179L and R89Y/N179L for other epoxides 2-7 were 12.2 to > 200, which showed great improvement compared to 1.2 to 10.5 for the wild type. Using the variant N179L, enantiopure (R)-PGE with > 99% ee could be readily prepared, affording a high yield and a high concentration.

Pickering Emulsion-Derived Liquid-Solid Hybrid Catalyst for Bridging Homogeneous and Heterogeneous Catalysis.

We describe a novel method to prepare a liquid-solid hybrid catalyst via interfacial growth of a porous silica crust around Pickering emulsion droplets, which allowed us to overcome the current limitations of both homogeneous and heterogeneous catalysts. The inner micron-scaled liquid (for example, ionic liquids) pool of the resultant catalyst can host free homogeneous molecular catalysts or enzymes to create a true homogeneous catalysis environment. The porous silica crust of the hybrid catalyst has excellent stability, which makes it amenable to packing directly in fixed-bed reactors for continuous flow catalysis. As a proof of concept, the enzymatic kinetic resolution of racemic alcohols, CrIII(salen) complex-catalyzed asymmetric ring opening of epoxides and Pd-catalyzed Tsuji-Trost allylic substitution reactions were used to verify the generality and versatility of our strategy for bridging homogeneous and heterogeneous catalysis. The hybrid catalyst-based continuous flow system exhibited a 1.6∼16-fold enhancement in activity relative to homogeneous counterparts even over1500 h, and the afforded enantioselectivities were completely equal to those obtained in the homogeneous counterpart systems. Interestingly, the catalytic efficiency can be tuned through rational engineering of the porous crust and the dimensions of the liquid pool, resulting in features of an innovatively designed catalyst. This contribution provides a new method to design efficient catalysts that can bridge the conceptual and technical gaps between homogeneous and heterogeneous catalysis.

Aromatic Donor-Acceptor Interaction-Based Co(III)-salen Self-Assemblies and Their Applications in Asymmetric Ring Opening of Epoxides.

Aromatic donor-acceptor interaction as the driving force to assemble cooperative catalysts is described. Pyrene/naphthalenediimide functionalized Co(III)-salen complexes self-assembled into bimetallic catalysts through aromatic donor-acceptor interactions and showed high catalytic activity and selectivity in the asymmetric ring opening of various epoxides. Control experiments, nuclear magnetic resonance (NMR) spectroscopy titrations, mass spectrometry measurement, and X-ray crystal structure analysis confirmed that the catalysts assembled based on the aromatic donor-acceptor interaction, which can be a valuable noncovalent interaction in supramolecular catalyst development.

Superior Effect of Ultrasonic Homogenization to Mechanical Agitation on Accelerating Reaction Rates in Asymmetric Ring Opening of Epoxides

Double‐layered mesoporous silica was prepared for use as an efficient support for the encapsulation of chiral salen Co‐BF3 complexes in the pore channels. The heterogenized chiral salen complex was applied successfully to the asymmetric ring opening (ARO) of terminal epoxides using water, alcohols, and carboxylic acids as nucleophiles. Although the homogeneous salen catalyst exhibited higher catalytic activity than the heterogenized catalyst in ARO reactions as expected, the heterogeneous salen catalysts exhibited high catalytic activities that were sufficient for practical applications. This study evaluated the unique contribution of ultrasonic irradiation as a powerful mixing medium to improve the kinetics in comparison to conventional mechanical agitation during the ARO reactions. Both the homogeneous and heterogeneous chiral salen complexes catalyzed enantioselectively the racemic or chiral epoxides under ultrasonication. The adoption of ultrasonic irradiation in the catalysis resulted in a superior mixing effect to vigorous mechanical stirring. The reaction time was shortened to obtain the highest enantiomeric excess (%ee) when ultrasonic irradiation was applied to the reaction system. Through the ARO reaction of epoxide by phenol derivatives, the highly enantio‐enriched β‐blockers could be synthesized easily from racemic terminal epoxides or chiral ones using chiral cobalt–salen catalysts. The regioselective ARO reaction of (R)‐ECH with propionic or butyric acid proceeded smoothly at room temperature under ultrasonic exposure in the presence of the heterogenized catalyst. The corresponding glycidyl alkylates were produced with excellent optical purity of up to 98%ee. The chiral cobalt–salen complexes encapsulated in MCM‐41 retained its catalytic activity and enantioselectivity for three recycles. Consequently, the heterogeneous chiral catalysts anchored in MCM‐41 offered the practical advantages of facile reuse through simple recovery from the reactant and products.

Application of sulfonyl chlorides and chiral amines in the efficient synthesis of nonracemic sulfinamides

A simple protocol that allows the preparation of separable epimeric sulfinamides from chiral amines and sulfonyl chlorides reduced in situ with triphenylphosphine in the presence of KOH is described. Using this method, tosyl chloride and nosyl chloride were successfully reacted with α-substituted primary amines to give five diastereomeric pairs, in certain cases accompanied by a small amount of the corresponding sulfonamide. Enantiopure products were isolated by column chromatography. The obtained enantiomerically pure sulfinamides were tested as organocatalysts in the asymmetric epoxide ring opening.

Asymmetric Ring Opening of Epoxides with a Supported Cobalt–Salen Complex

Asymmetric Ring Opening of Epoxides with a Supported Cobalt–Salen Complex

ChemInform Abstract: Metal‐Catalyzed Directed Regio‐ and Enantioselective Ring‐Opening of Epoxides

AbstractReview: 48 refs.

Metal-Catalyzed Directed Regio- and Enantioselective Ring-Opening of Epoxides.

Control of regio- and stereoselectivity of chemical reactions is the central theme in synthetic chemistry. Regioselective and enantiospecific ring opening of readily available enantioenriched epoxides precursors provides a straightforward access to diverse highly functionalized molecules which can serve as chiral building blocks for synthesis of biologically active compounds. However, prior to our research work, the scope of catalytic highly regioselective ring opening of epoxides is limited to structurally or electronically biased epoxides, such as terminal and aromatic epoxides. Regioselective ring-opening of epoxides with substituents on both sides demonstrating similar steric and electronic effects is still a formidable challenge for organic chemists. To address this challenge, our approach is to use the readily available functional moiety incorporated in an epoxide as a directing group to realize the regioselective nucleophilic attack on the oxirane ring. Alternatively, asymmetric ring-opening of epoxides can also provide the ring-opening products in highly enantioenriched form. However, excellent results are usually obtained in the case of the kinetic resolution of terminal epoxides or the desymmetrization of meso-epoxides. In these cases, the issue of regiocontrol of the ring-opening is circumvented or minimized. Based on our successful results of regioselective ring-opening of functionalized epoxides by implementing the directed ring-opening strategy, we also investigated the enantioselective ring-opening of internal epoxides bearing a functional moiety as directing group. This Account summarizes our research on metal-catalyzed directed ring-opening reactions of epoxides, which encompasses the following breakthroughs: (1) highly regioselective ring-opening of various substrates including epoxy allylic alcohols, epoxy homoallylic alcohols and epoxy allylic sulfonamides with a variety of N-, O-, and halide-nucleophiles catalyzed by W-, Mo-, or Ni-salt; (2) first kinetic resolutions of epoxy allylic alcohols, epoxy homoallylic alcohols, and epoxy allylic sulfonamides with various amines as nucleophiles, which were catalyzed by W-bishydroxamic acid (W-BHA), nickel-BINAM, and Gd-N,N'-dioxide catalytic system, respectively; (3) successful implementation of the strategy of combined asymmetric syntheses by the combination of the enantioselective epoxidation and the enantioselective ring-opening of 2,3-epoxy alcohols establishing a new entry to prepare amino alcohols in regio-, diastereo-, and enantiomerically pure form.

Dinuclear salen cobalt complex incorporating Y(OTf)3: enhanced enantioselectivity in the hydrolytic kinetic resolution of epoxides

Yttrium triflate present in dinuclear salen cobalt complex not only shows a promoting effect on electron transfer, but also assists in the asymmetric ring opening of epoxides with water efficiently.

ChemInform Abstract: Asymmetric Ring Opening of Epoxides with Cyanides Catalyzed by Chiral Binuclear Titanium Complexes.

AbstractTitanium(IV) catalysts generated in situ from Ti(OiPr)4 and BINOL‐based bis salen‐compounds are developed and successfully used as recyclable catalysts for the asymmetric ring‐opening reaction of epoxides with TmsCN.

Inside Cover: Design of Peptidyl Thiourea Derivatives as Organocatalysts for the Asymmetric Ring Opening ofmeso-Stilbene Oxides (Asian J. Org. Chem. 6/2014)

Organocatalysis Organocatalytic asymmetric ring opening of epoxides with aliphatic amines has always been a challenge. In their Communication on page 700 ff., Swapandeep Singh Chimni report the design and synthesis of chiral peptidyl thiourea organocatalysts for the asymmetric ring opening of epoxides with N-phenylpiperazines. A tripeptide scaffold with a β-turn nucleating element provides favorable stereochemistry by positioning the nucleophile close to the epoxide. Chiral β-amino alcohols were obtained with up to 95% ee after crystallization. The configuration at the N-terminal residue of the catalyst affects the stereochemical outcome of the reaction.

Asymmetric ring opening of epoxides with cyanides catalysed by chiral binuclear titanium complexes

A series of Schiff bases obtained from salicylaldehydes and 3,3′-diformyl-BINOL were synthesized. The complexes of these Schiff bases with Ti(IV) were active for the asymmetric ring opening of epoxides with TMSCN. A mixture of unpurified ligands was found to be as effective as the best one. The influence of temperature, solvent polarity and structural modification of the pre-catalysts on the enantioselectivity of the process has also been investigated.

Dendritic Catalysis in Asymmetric Synthesis

In this review, we have focused our attention on the application of dendrimers as asymmetric catalyst in organic synthesis, which appeared in the literature. Dendrimers have been identified as a promising candidate for catalysis application. The combination of high surface area and high solubility makes dendrimers as useful nanoscale catalysts. The main advantage of the dendritic catalysts is they can be separated from the reaction mixture by simple solvent precipitation and reused for several times with similar catalytic activity. Importance of catalytically active substances like metal salts or organic molecule like pyrrolidinylmethanol thatare attached either to the core or to the periphery of the dendrimers have been highlighted and their applications in important organic reactions such as Baylishillman, Mannich-type, Diels-Alder, Wittig reaction, Michael addition, addition of dialkylzinc to aldehyde, asymmetric epoxide ring opening, asymmetric hydrogenation and asymmetric epoxidation are also presented. Use of dendrimer based salts and chiral compounds in asymmetric syntheses are discussed and the advantages and disadvantages of those catalysts are brought out in this review. Keywords: Dendritic catalyses, Asymmetric syntheses, Organic transformations, Chiral catalysts, residues, boundchiral ligands, Dendrimers, catalysts, pyrrolidinylmethanol, ethylvinyl ketone

ChemInform Abstract: Synthesis of New Bimetallic Chiral Salen Catalyst Bearing Co(BF4)2 Salt and Its Application in Asymmetric Ring Opening of Epoxide.

AbstractA new bimetallic chiral salen catalyst containing the Co(BF4)2 moiety is non‐covalently anchored on the acidic sites of mesoporous Al‐SBA‐15 and used for the asymmetric ring opening of epoxides.

The bigger, the better: Ring-size effects of macrocyclic oligomeric Co(iii)-salen catalysts

Macrocyclic oligomeric Co(III)-salen complexes derived from cyclooctene salen monomers are among the most active catalysts for asymmetric epoxide ring-opening reactions. Due to the uncontrollable feature of the ring-expanding olefin metathesis step during catalyst synthesis, the macrocyclic oligomeric Co(III)-salen complexes are produced as mixtures of oligomers with different ring sizes. We rationalize that the ring size of the Co(III)-salen oligomers might have a significant effect on the catalytic efficiency and selectivity and report here a purification protocol to isolate macrocyclic dimers, trimers, tetramers and oligomeric mixtures with larger size rings. Hydrolytic kinetic resolution (HKR) tests using allyl glycidyl ether as substrate show that the dimer is inactive at a catalyst loading of 0.01 mol%. Increasing ring size shows a remarkable effect on reaction rates with the largest ring-size species exhibiting superior selectivities and activities. NMR studies reveal that the dimeric catalyst is strained which is not observed for the larger ring-size catalysts. Computational modeling studies indicate that the dimer is lacking the flexibility to allow adjacent Co(III)-salen groups to form a bimetallic complex. Further catalytic tests of larger ring-size Co(III)-salen complexes (tetramer to hexamer mixture) by investigating the HKR of various racemic terminal epoxides and the asymmetric epoxide ring-opening with different nucleophiles demonstrate the superior catalytic activity of large ring-size macrocyclic catalysts. Furthermore, this study demonstrates again the structural (or configurational) sensitivity of Co(III)-salen catalyst towards the selectivity and efficiency of cooperative bimetallic reactions.

ChemInform Abstract: Chiral Lewis Acids in Catalytic Asymmetric Reactions

This review examines the recent advances in the use of chiral Lewis acids, usually generated in situ from a Lewis acid and a chiral auxiliary, in the Diels-Alder reaction, the ene reaction, the hydrocyanation of aldehydes, the ring-opening of epoxides and various miscellaneous reactions. Structural studies of the complex formed between carbonyl compounds and Lewis acids are also reported. The Sharpless epoxidation is not covered by this review. 1. Introduction 2. Asymmetric Diels-Alder Reactions 2.1. Chiral Titanium Reagents 2.2. Chiral Aluminum Reagents 2.3. Chiral Boron Reagents 2.4. Chiral Ruthenium Reagents 3. Asymmetric [2+2] Cycloaddition Reactions 4. Asymmetric Ene Reaction 5. Asymmetric Hydrocyanation of Aldehydes 6. Asymmetric Ring Opening of Epoxides 7. Miscellaneous Asymmetric Reactions 8. Structural Studies on the Complexes of Carbonyl Compounds and Lewis Acids

Asymmetric synthesis of α-aryloxy alcohols via kinetic resolution of terminal epoxides catalyzed by (R,R)-N,N′-bis(2-hydroxy-5-tert-butylsalisilidine)-1,2-cyclohexenediamino cobalt

New chiral Co-salen complexes with tert-butyl group in position 5 of the aromatic ring in the structure have been synthesized and they were applied as a chiral catalyst. A dimeric chiral salen having a transition metal salt such as AlCl3, displayed very high catalytic reactivity and enantioselectivity for the asymmetric ring opening of epoxides to synthesize optically pure α-aryloxy alcohols via phenolic kinetic resolution.