Kinetic Resolution Of Terminal Epoxides Research Articles

Mechanochemical synthesis of unsymmetrical salens for the preparation of Co-salen complexes and their evaluation as catalysts for the synthesis of α-aryloxy alcohols via asymmetric phenolic kinetic resolution of terminal epoxides.

In this paper, we report the mechanochemical synthesis of unsymmetrical salens using grinding and ball milling technologies, respectively, both of which were afforded in good yield. The chelating effect of the unsymmetrical salens with zinc, copper, and cobalt was studied and the chiral Co–salen complex 2f was obtained in 98% yield. Hydrolytic kinetic resolution (HKR) of epichlorohydrin with water catalyzed by complex 2f (0.5 mol %) was explored and resulted in 98% ee, suggesting complex 2f could serve as an enantioselective catalyst for the asymmetric ring opening of terminal epoxides by phenols. A library of α-aryloxy alcohols 3 was thereafter synthesized in good yield and high ee using 2f via the phenolic KR of epichlorohydrin.

Asymmetric Synthesis of Methoxylated Ether Lipids: A Glyceryl Glycidyl Ether Key Building Block Design, Preparation, and Synthetic Application.

The report describes the preparation and use of a double-C3 building block intended as a head group synthon in the synthesis of saturated, mono-, and polyunsaturated 1-O-alkyl-sn-glycerol type methoxylated ether lipids (MELs). The resulting head piece, an enantiopure isopropylidene-protected glyceryl glycidyl ether diastereomer, was accomplished in 49% yield (max 50%) from a 1:1 diastereomeric mixture obtained from R-solketal and racemic epichlorohydrin after treatment with the Jacobsen (S,S)-Co(III)salen catalyst for the hydrolytic kinetic resolution of terminal epoxides. The diol hydrolytic product obtained in 47% yield from the unwanted diastereomer was reconverted into epoxide with an inversion of configuration in a three-step operation involving a highly regioselective lipase. This enabled the recovery of a substantial amount of diastereopure material after a subsequent treatment with the Jacobsen catalyst to furnish the oxirane head piece in altogether 72% yield of higher than 99% diastereomeric purity. A modified synthesis of a monounsaturated 16:1 MEL confirmed the correct stereochemistry and excellent enantiopurity of the head piece and resulted in a dramatic improvement in yields, efficiency, and economy of the synthesis.

Cobalt(III) Complexes as Bifunctional Hydrogen‐Bond Donor Catalysts Featuring Halide Anions for Cyclic Carbonate Synthesis at Ambient Temperature and Pressure: A Mechanistic Insight

AbstractThe metal‐templated hydrogen bond donors (HBD) are prospective catalytic systems for the activation of organic molecules in chemical reactions. Here we report the application of the chiral cationic Co(III) complexes based on commercially available (S,S)‐1,2‐diaminocyclohexane and (R,R)‐1,2‐diphenylethylendiamine and salicylaldehydes with an iodide counter‐anion as bifunctional one‐component hydrogen bond donor/nucleophilic catalysts for the conversion of carbon dioxide with epoxides into valuable cyclic carbonates under solvent‐ and co‐catalyst free conditions. We demonstrated that (R,R)‐1,2‐diphenylethylendiamine based complex Λ(R,R)‐2 is superior to (S,S)‐1,2‐diaminocyclohexane based Δ(S,S)‐1 a catalyzing the reaction at ambient conditions (RT and 1 bar CO2). The TON and TOF values of 850 and 35 h−1, respectively, were achieved at low catalyst loading (0.1 mol %) at 10 bars of CO2 and 100 °C. Furthermore, the Co(III) complexes catalyzed the reaction with diluted air/CO2 mixture (15 vol % of CO2) producing the desired styrene carbonate in up to 75% yield. A plausible catalytic cycle consistent with all experimental observations was proposed based on DFT calculations. The DFT calculations elucidated the difference in the ring opening step in cases of propylene oxide and styrene oxide. In addition, the kinetic resolution of terminal epoxides was observed with selectivity factor (s) of up to 1.6 in case of the complex Λ(R,R)‐2.

Chiral Cobalt-Salen Complexes: Ubiquitous Species in Asymmetric Catalysis.

Since the discovery of their extraordinary reactivity in the hydrolytic kinetic resolution of terminal epoxides about twenty years ago, chiral cobalt-salen complexes have been shown to be essential for many other asymmetric catalytic reactions. This account summarizes the inspiring works dedicated to the discovery of their new reactivity and their mode of action, as well as the new processes towards the optimization of their cooperativity for bimetallic activation and the implementation of their effective immobilization, including also our contribution on these topics.

Catalytic and Efficient Synthesis of Optically Active Terminal Epoxides and 1,2-Diols using a New Lanthanum Triflate Assisted C1-Symmetric Bimetallic Chiral Salen Cobalt Complex

The catalytic asymmetric synthesis of terminal epoxides and 1,2-diols via a hydrolytic kinetic resolution of racemic terminal epoxides using a new bimetallic chiral salen CoIII complex incorporated with La(OTf)3 as a catalyst is reported. The new bimetallic chiral salen CoIII complex bearing La(OTf)3 was prepared in a one-pot procedure by activation and bimetallic complex formation from the inactive Jacobsen chiral salen CoII complex (salen = N, N′-bis(3,5-di-tert-butylsalicylidene)-1,2- cyclohexane-diamine). We observed that C1-symmetric chiral bimetallic Co-complex coordinated with La(OTf)3 works as a chiral catalyst and remarkably increased the reactivity and enantioselectivity in the hydrolytic kinetic resolution of terminal epoxides compared to its monometallic analogues. La(OTf)3 plays a crucial role as an achiral Lewis acid in this process, which is supported by the experimental observation presented. To explore the absolute configuration of glycidyl butyrate a vibrational circular dichroism (VCD) spectra was calculated using density functional theory. Keywords: Chiral epoxides, asymmetric catalysis, bimetallic chiral salen complex, hydrolytic kinetic resolution, lewis acid, VCD.

속도론적 분할법을 통한 말단 에폭사이드로부터 고광학순도의 아렌술폰산 2-하이드록시 에스터의 합성

속도론적 분할법을 통한 말단 에폭사이드로부터 고광학순도의 아렌술폰산 2-하이드록시 에스터의 합성

Eric N. Jacobsen

Eric N. Jacobsen

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.

Aluminum(salen) Complexes as Catalysts for the Kinetic Resolution of Terminal Epoxides via CO2 Coupling

The development of inexpensive and sustainable aluminum(salen) complexes as catalysts for the kinetic resolution of terminal epoxides is described. The kinetic resolution is carried out under mild conditions (0–25 °C and 1 bar of CO2 pressure) in the presence of tetrabutylammonium bromide as co-catalyst in the absence of solvent. The relative rate of reaction of the two epoxide enantiomers (krel) is substrate dependent, and the highest krel obtained was 15.4, using N-(2,3-epoxypropyl)diphenylamine as substrate.

Enantioselective ring opening of epoxides with TMSN3 by macrocyclic oligomer-supported Cr(III)–salen catalysts

Three Cr(III)–salen catalysts supported onto macrocyclic oligomeric cyclooctene through linkers of different lengths were synthesized. The influences of the linker length and the oligomeric support on the activity and the enantioselectivity of catalysts were investigated in the asymmetric ring opening of a meso-epoxide and the kinetic resolution of terminal epoxides. For both reactions all three catalysts exhibited significantly higher catalytic efficiency than the unsupported monomeric Cr(III)–salen catalyst. The catalyst with the shortest linker 1 showed the highest reaction rate while the catalyst with the longest linker 3 afforded the highest enantioselectivity in both reactions. The recoverability of catalyst 1 was also studied.

Counterion and substrate effects on barrier heights of the hydrolytic kinetic resolution of terminal epoxides catalyzed by Co(III)-salen.

Density functional theory (DFT) has been applied to the proposed rate-limiting step of the hydrolytic kinetic resolution (HKR) of terminal epoxides as catalyzed by Co-salen-X (X = counterion) in order to resolve questions surrounding the mechanism. The present results indicate that the bimetallic mechanism proposed by Jacobsen shows nonadditive, cooperative catalysis with a larger reduction in barrier height than the sum of the barrier height reductions from the two monometallic reaction pathways. We computed barrier heights for the reaction using several counterions (chloride, acetate, tosylate, and hydroxide). For the three counterions that are experimentally active (chloride, acetate, and tosylate) the barrier heights are 35, 38, and 34 kJ mol(-1), respectively, while for hydroxide it is 48 kJ mol(-1). The similarity of the barrier heights for chloride, acetate, and tosylate is in agreement with their similar peak reaction rates. The finding that Co-salen-X with these counterions leads to rather different overall reaction profiles suggests that they have quite different rates of reaction with epoxide to form the activated Co-salen-OH required for the bimetallic mechanism. Co-salen-OH is inactive as the sole catalyst for HKR, and this inactivity is ascribed to its larger barrier height for the ring-opening step, rather than to any inability to activate epoxide. Barrier heights were also computed using propylene oxide, 1-hexene oxide, and epichlorohydrin; propylene oxide and 1-hexene oxide have similar barrier heights, 35.5 and 33.2 kJ mol(-1), respectively, and epichlorohydrin has a significantly lower barrier height of 18.8 kJ mol(-1), which is qualitatively consistent with experiments showing faster reactions for epicholorohydrin than propylene oxide when catalyzed by Co-salen-OAc.

A broadly applicable and practical oligomeric (salen)Co catalyst for enantioselective epoxide ring-opening reactions

The (salen)Co catalyst (4a) can be prepared as a mixture of cyclic oligomers in a short, chromatography-free synthesis from inexpensive, commercially available precursors. This catalyst displays remarkable enhancements in reactivity and enantioselectivity relative to monomeric and other multimeric (salen)Co catalysts in a wide variety of enantioselective epoxide ring-opening reactions. The application of catalyst 4a is illustrated in the kinetic resolution of terminal epoxides by nucleophilic ring-opening with water, phenols, and primary alcohols; the desymmetrization of meso epoxides by addition of water and carbamates; and the desymmetrization of oxetanes by intramolecular ring opening with alcohols and phenols. The favorable solubility properties of complex 4a under the catalytic conditions facilitated mechanistic studies, allowing elucidation of the basis for the beneficial effect of oligomerization. Finally, a catalyst selection guide is provided to delineate the specific advantages of oligomeric catalyst 4a relative to (salen)Co monomer 1 for each reaction class.

Heterobimetallic dual-catalyst systems for the hydrolytic kinetic resolution of terminal epoxides

A heterobimetallic dual-catalyst system based on the preparation and use of various salen complexes has been developed for the hydrolytic kinetic resolution (HKR) of terminal epoxides. A combination of cobalt-salen and manganese-salen complexes, generated from ligands with the same configuration possessing thiophene or pyrrole moieties, produced indeed highly selective catalysts for the hydrolysis of epibromohydrin. This effect could also be extended to other terminal epoxides and to the more challenging ring opening of cyclohexene oxide. Kinetic studies indicated that only one CoIII salen complex was involved in the rate-determining step, which supported a heterobimetallic highly enantioselective pathway based on the crucial existence of in situ generated CoIII-OH species, previously postulated in the literature. The beneficial effect of the presence of additional Mn-complexes was ascribed to the inhibition of the alternative less enantioselective monometallic reaction pathway by epoxide activation.

Formal Total Synthesis of (+)-Neopeltolide

This paper describes the formal total synthesis of (+)-neopeltolide, a cytotoxic macrolide isolated from the marine sponge Neopeltidae. The key features of the synthesis include an asymmetric Evans alkylation to fix the C9-methyl center, Jacobsen hydrolytic kinetic resolution of terminal epoxides followed by their regioselective opening to fix the stereocenters at the C11 and C13 positions, respectively, a Pd-catalyzed oxa-Michael reaction to construct the tetrahydropyran ring, and Yamaguchi macrolactonization to form the macrocyclic core of the molecule.

Electrogenerated Polymers as Efficient and Robust Heterogeneous Catalysts for the Hydrolytic Kinetic Resolution of Terminal Epoxides

AbstractThiophene‐ and pyrrole‐modified chiral cobalt–salen complexes were found to be efficient, enantioselective homogeneous catalysts for the dynamic hydrolytic kinetic resolution (HKR) of epibromohydrin. Their corresponding electrogenerated thiophene–salen–cobalt polymers were prepared by anodic oxidation and could be recovered as insoluble powders for heterogeneous catalysis. Their activity and selectivity were evaluated according to the polymerization conditions. A polymer that was formed from the direct oxidation of a CoII complex in a CH2Cl2/AcOH/nBu4NBF4 electrolytic mixture promoted the most‐efficient synthesis of the expected bromoacetal (up to >99 % yield and 87 % ee). This heterogeneous catalyst was easily recovered, could be reused without the need for a reactivation step, and was highly stable, even after 11 consecutive catalytic runs. Moreover, this catalyst was engaged in a multi‐substrate recovery procedure with various terminal epoxides, thereby demonstrating its usefulness for the transformation of diverse substrates.

Mechanistic Basis for High Reactivity of (salen)Co–OTs in the Hydrolytic Kinetic Resolution of Terminal Epoxides

The (salen)Co(III)-catalyzed hydrolytic kinetic resolution (HKR) of terminal epoxides is a bimetallic process with a rate controlled by partitioning between a nucleophilic (salen)Co-OH catalyst and a Lewis acidic (salen)Co-X catalyst. The commonly used (salen)Co-OAc and (salen)Co-Cl precatalysts undergo complete and irreversible counterion addition to epoxide during the course of the epoxide hydrolysis reaction, resulting in quantitative formation of weakly Lewis acidic (salen)Co-OH and severely diminished reaction rates in the late stages of HKR reactions. In contrast, (salen)Co-OTs maintains high reactivity over the entire course of HKR reactions. We describe here an investigation of catalyst partitioning with different (salen)Co-X precatalysts and demonstrate that counterion addition to epoxide is reversible in the case of the (salen)Co-OTs. This reversible counterion addition results in stable partitioning between nucleophilic and Lewis acidic catalyst species, allowing highly efficient catalysis throughout the course of the HKR reaction.

Highly crosslinked polycyclooctyl-salen cobalt (iii) for the hydrolytic kinetic resolution of terminal epoxides

A new support structure for Co(III)salen catalysts has been developed to improve the kinetics for the hydrolytic kinetic resolution (HKR) of terminal epoxides. The new support consists of a copolymer composed of crosslinked salen-containing cyclic oligomers. Previous studies show that higher molecular weight cyclic oligomers are more active HKR catalysts than lower weight oligomers. The crosslinking reaction forms high molecular weight oligomers using a similar support structure and in significantly greater synthetic yield making the presented protocol synthetically more viable.

Promoted activity of Cr(Salen) in a nanoreactor for kinetic resolution of terminal epoxides

Download options Please wait... Supplementary files Supplementary information PDF (529K) Article information DOI https://doi.org/10.1039/C2SC20653F Article type Edge Article Submitted 23 May 2012 Accepted 20 Jun 2012 First published 21 Jun 2012 Download Citation Chem. Sci., 2012,3, 2864-2867 BibTex EndNote MEDLINE ProCite ReferenceManager RefWorks RIS Permissions Request permissions Promoted activity of Cr(Salen) in a nanoreactor for kinetic resolution of terminal epoxides S. Bai, B. Li, J. Peng, X. Zhang, Q. Yang and C. Li, Chem. Sci., 2012, 3, 2864 DOI: 10.1039/C2SC20653F To request permission to reproduce material from this article, please go to the Copyright Clearance Center request page. If you are an author contributing to an RSC publication, you do not need to request permission provided correct acknowledgement is given. If you are the author of this article, you do not need to request permission to reproduce figures and diagrams provided correct acknowledgement is given. If you want to reproduce the whole article in a third-party publication (excluding your thesis/dissertation for which permission is not required) please go to the Copyright Clearance Center request page. Read more about how to correctly acknowledge RSC content. Social activity Tweet Share Search articles by author Shiyang Bai Bo Li Juan Peng Xiaoming Zhang Qihua Yang Can Li

Highly active oligomeric Co(salen) catalysts for the asymmetric synthesis of α-aryloxy or α-alkoxy alcohols via kinetic resolution of terminal epoxides

A mixture of Co(salen) macrocycles, prepared via the ring expansion metathesis oligomerization of salen-functionalized cyclooctene monomers, among the most active soluble catalysts for the hydrolytic kinetic resolution (HKR) of terminal epoxides, is exploited as the catalyst in the ring-opening of epoxides using aliphatic alcohols or phenols as nucleophiles, leading to the direct synthesis of optically active α-aryloxy alcohols or α-alkoxy alcohols. The catalyst is compared to other dimeric, oligomeric and monomeric Co(salen) complexes including a pimelate-linked macrocyclic Co(salen) catalyst and a dimeric Co(salen) catalyst referred to as a bisalen. The catalysts that contain multiple Co(salen) units within a single molecular framework allow for substantial decreases in catalyst loading compared with the monomeric catalyst. The cyclooctene-based Co(salen) macrocycle catalyst allows for good activity and enantioselectivity in the ring-opening of terminal epoxides with phenols as nucleophiles, giving enhanced turnover frequencies relative to many literature catalysts. The cyclooctene-based Co(salen) macrocycle catalyst and the bisalen catalysts are shown to be the most active in the asymmetric ring-opening of (±)1,2-epoxyhexane with methanol, out-performing the other catalysts tested. The Co(salen) macrocycle catalyst is recycled 3 times in this reaction with some loss in activity but no noteworthy change in selectivity.