Enhancement Of Enantioselectivity Research Articles

Origins of Enhanced Enantioselectivity in the Pd-Catalyzed Decarboxylative Allylic Alkylation of N-Benzoyl Lactams.

We explore the origins of the marked improvement in enantioselectivity in the inner-sphere (PHOX)Pd-catalyzed allylic alkylation of N-benzoyl lactam nucleophiles over their carbocyclic counterparts. We employ density functional theory calculations to aid in the interpretation of experimental results. Ultimately, we propose that the enhancement in enantioselectivity arises primarily from noncovalent interactions between the substrate and ligand rather than secondary substrate chelation, as previously hypothesized.

Engineering residues on C interface to improve thermostability of nitrilase for biosynthesis of Pregabalin precursor

AbstractNitrilase‐mediated bioprocesses exhibited great potential in the production of value‐added carboxylic acids. However, poor thermostability of nitrilases usually restricts their industrial applications. Herein, the thermostability of nitrilase BaNITM0 was significantly improved by engineering the amino acid residues on the intersection of two dimers (C interface). Except for simultaneous enhancement of enantioselectivity and activity, the best variant V82L/M127I/L159M/F166Q/C237S/Q260H (BaNITM4) showed a 10.8‐fold increase in half‐life at 30°C compared with BaNITM0. Structural analysis demonstrated that additional hydrogen bonds were formed between the residues on the C interface, which strengthened the interactions of two symmetrical regions and spirals. Subsequently, the engineered nitrilase was immobilized onto epoxy resins LXTE‐603 and the immobilized nitrilase exhibited excellent stability over 12 repeated cycles, which indicated a great industrial potential for biosynthesis of Pregabalin precursor.

Ligand cooperativity enables highly enantioselective C–C σ-bond hydroboration of cyclopropanes

<h2>Summary</h2> Given the utility of enantioenriched alkylboronates, asymmetric borylation has attracted substantial research interest in recent decades. Herein, a catalytic enantioselective hydroboration through desymmetrization of enantiotopic carbons in cyclopropanes involving C–C bond activation is reported. Treatment of cyclopropanes bearing an amide directing group in the presence of rhodium catalysis using HBpin as a boron source allows the construction of a series of alkyl boronates containing γ-stereogenic amines. The key to high enantioselectivity relies on two distinct ligands that act cooperatively: an exogenous monodentate chiral phosphite that induces enantioselectivity and an endogenous acetylacetonate that exerts a remarkably positive influence on the enhancement of enantioselectivity. Detailed experimental and computational studies elucidate the effect of ligand cooperativity and the key steps involving σ-bond activation process, C–B bond formation, and the absolute configuration of products.

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The development of chiral gold catalysts poses considerable challenges due to the dicoordinated gold(I) complexes with preferred linear geometry, which makes the active reaction site opposite to the chiral ligand and thus limits the capability to transfer asymmetry. We designed a multifunctional ligand (Ming‐Phos), in which hydrogen bonding and fluorine effect played important roles in enhancement of enantioselectivity. Both enantiomers of the cycloadducts with opposite configuration could be obtained in high yields with high regio‐ and enantioselectivity by the employment of either diastereomer of the chiral ligand. More details are discussed in the article by Zhang et al. on page 1407—1412.image

Remarkable enantioselectivity enhancement of the extractors with nonaxial chirality in liquid-liquid extraction of underivatized amino acids by introducing t-butyl group.

A class of carbonyl extractors, (R)-3, (R)-4, and (R)-5, with nonaxial chirality containing asymmetric carbons has been synthesized and studied for their efficiencies in enantioselective liquid-liquid extraction for underivatized amino acids. The bulky t-butyl ketone extractors, (R)-4 and (R)-5, showed the stereoselectivities ranging 5.4-9.4 of l/d ratio much better than those of the aldehyde extractor, (R)-3, ranging 2.4-5.2. The imine formation rates and yields of the t-butyl ketones were not significantly affected by their bulkiness and even in the absence of resonance-assisted hydrogen bond. This work confirms that a bulky t-butyl ketone can be a good choice in the development of an extractor not only with axial chirality but also with nonaxial chirality for the enantioselective extraction of unprotected amino acids.

Enantioselectivity Tunable Gold‐Catalyzed Intermolecular [3 + 2] Cycloaddition of N‐Allenamides with Nitrones

Comprehensive SummaryA highly enantioselective gold‐catalyzed intermolecular [3 + 2] cycloaddition of N‐allenamides with nitrones was realized by the application of Ming‐Phos M6 as a chiral ligand. Both enantiomers of the cycloadducts with opposite configuration could be obtained in high yields with high regio‐ and enantioselectivity by the employment of either diastereomer of the chiral ligand. The acidic N—H bond (hydrogen bonding) of sulfinamide moiety of Ming‐Phos and pentaflurophenyl substituent (fluorine effect) may play important roles in enhancement of enantioselectivity, that is, Ming‐Phos is a multifunctional ligand in this transformation.

Enantioselective luminescence enhancement of chiral Ru(phen)32+ complexes by interaction with deoxyribonucleic acid

Deoxyribonucleic acid (DNA)-based functional materials containing luminescent molecules have attracted widespread attention as opto-electronic materials. In this study, luminescence properties of DNA/chiral Ru(phen)32 + complexes were investigated. The DNA/chiral Ru(phen)32 + complexes showed an enantioselective enhancement of luminescence. The DNA / Δ-Ru(phen)32 + complex showed a higher luminescence intensity and longer luminescence lifetime than the DNA / Λ-Ru(phen)32 + complex. Binding modes between DNA and Δ- / Λ-Ru(phen)32 + are discussed in terms of optical properties. The study shows that Δ-Ru(phen)32 + preferentially intercalates to form a more stable DNA complex than Λ-Ru(phen)32 + , leading to enantioselective luminescence enhancement.

Quinine-Promoted, Enantioselective Boron-Tethered Diels-Alder Reaction by Anomeric Control of Transition-State Conformation.

Diels-Alder reactions of tethered vinyl-metal species offer the opportunity to fashion highly functionalized diol intermediates for synthesis. We have developed the first enantioselective boron-tethered Diels-Alder reaction using quinine as a chiral promoter. Quinine recovery, enantioselectivity enhancement, and manipulation of the cyclohexene core are also investigated. DFT modeling calculations confirm the role of quinine as a bidentate ligand enhancing reaction rates. The enantioselectivity of the cycloaddition is proposed to originate from a boron-centered anomeric effect.

Mechanism-based enhancement of scope and enantioselectivity for reactions involving a copper-substituted stereogenic carbon centre.

A rapidly emerging set of catalytic reactions involves intermediates that contain a copper-substituted stereogenic carbon centre. Here, we demonstrate that an intimate understanding of this distinction provides ways for addressing limitations in reaction scope and explaining why unexpected variations in enantioselectivity often occur. By using catalytic enantioselective Cu-boryl addition to alkenes as the model process, we elucidate several key mechanistic principles. We show that higher electrophile concentration can lead to elevated enantioselectivity. This is because diastereoselective Cu-H elimination may be avoided and/or achiral Cu-boryl intermediates can be converted to allyl-B(pin) rather than add to an alkene. We illustrate that lower alkene amounts and/or higher chiral ligand concentration can minimize the deleterious influence of achiral Cu-alkyl species, resulting in improved enantiomeric ratios. Moreover, and surprisingly, we find that enantioselectivities are higher with the less reactive allylphenyl carbonates as chemoselective copper-hydride elimination is faster with an achiral Cu-alkyl species.

Synthesis and evaluation of pseudopeptide chiral stationary phases for enantioselective resolution

Poly(2-oxazoline)s are regarded as bioinspired polymers due to their structural relation to polypeptides. In this work, a new kind of poly(2-oxazoline)s containing dipeptide segments in the side chains was synthesized through a bottom-up protocol, which involves ring-opening copolymerization of 2-(N-Boc-l-2-pyrrolidinyl)-2-oxazoline (PyOXBoc) with 2-(3-butenyl)-2-oxazoline (BuOX) followed by deprotection and amide coupling with N-protected L-proline. The resulting vinyl-functionalized polymers were subsequently immobilized onto mercaptopropylated silica bead matrices by means of thio-click chemistry and their potential as the chiral stationary phase (CSP) for high-performance liquid chromatography was preliminarily evaluated with a series of structurally different racemates. The results showed that this class of pseudopeptide CSPs is particularly adapted to the enantiomeric separation of 1,1′-bi-2-naphthol and acyloin compounds (such as benzoin) under normal-phase conditions. Moreover, an increase in the length of polymer main chains is beneficial to the enhancement of both enantioselectivity and resolution ability. The chiral discrimination of analytes by the polymeric selectors stems primarily from hydrogen bonding and π-π interactions as well as steric hindrance.

An Overview on the Enhancement of Enantioselectivity and Stability of Microbial Epoxide Hydrolases.

Epoxide hydrolases (EHs; 3.3.2.x) catalyze the enantioselective ring opening of racemic epoxides to the corresponding enantiopure vicinal diols and remaining equivalent unreacted epoxides. These epoxides and diols are used for the synthesis of chiral drug intermediates. With an upsurge in the methods for identification of novel microbial EHs, a lot of EHs have been discovered and utilized for kinetic resolution of racemic epoxides. However, there is still a constraint on the account of limited EHs being successfully applied on the preparative scale for industrial biotransformations. This limitation has to be overcome before application of identified functional EHs on large scale. Many strategies such as optimizing reaction media, immobilizing EHs and laboratory-scale directed evolution of EHs have been adopted for enhancing the industrial potential of EHs. In this review, these approaches have been highlighted which can serve as a pathway for the enrichment of already identified EHs for their application on an industrial scale in future studies.

Molecular mechanism of the spontaneous segregation of enantiomers in liquid nanoblobs

At present, the problem of spontaneous segregation has no satisfactory theoretical description. The main difficulty is a very low probability of the spontaneous formation of a chirally pure nucleus in a racemic mixture if the enantioselectivity energy is below (10–12)kT, which does not correspond to reality. In this paper, we propose a mechanism of the enhancement of enantioselectivity in the chiral phase nucleus, which circumvents the problem of the low probability of its formation.

Optical activity of helical quantum-dot supercrystals

The size of chiral nanoparticles is much smaller than the optical wavelength. As a result, the difference in interaction of enantiomers with circularly polarized light of different handedness is practically unobservable. Due to the large mismatch in scale, the problem of enhancement of enantioselectivity of optical properties of nanoparticles is particularly important for modern photonics. In this work, we show that ordering of achiral nanoparticles into a chiral supercrystal with dimensions comparable to the wavelength of light allows achieving nearly total dissymmetry of optical absorption and demonstrate this using a helical super-crystal made of semiconductor quantum dots as an example. The proposed approach may find numerous applications in various optical and analytical methods used in biomedicine, chemistry, and pharmacology.

1-Hydroxybenzotriazole-Assisted, N-Heterocyclic Carbene Catalyzed β-Functionalization of Saturated Carboxylic Esters: Access to Spirooxindole Lactones.

A 1-hydroxybenzotriazole-assisted, N-heterocyclic carbene catalyzed direct β-functionalization of saturated carboxylic esters is disclosed. This formal [3 + 2] annulation reaction of carboxylic esters with isatins affords optically pure spirooxindole lactones (on gram scale) bearing two vicinal stereogenic centers. A dual role of HOBt is proposed based on controlled experiments to rationalize the enhancement of diastereoselectivity and enantioselectivity.

ChemInform Abstract: A Supramolecularly Tunable Chiral Diphosphine Ligand: Application to Rh and Ir‐Catalyzed Enantioselective Hydrogenation.

A supramolecularly tunable chiral bisphosphine ligand bearing two pyridyl-containing crown ethers, (−) or (+)-Xyl-P16C6-Phos, was fabricated and utilized in the Rh-catalyzed asymmetric hydrogenation of α-dehydroamino acid esters and Ir-catalyzed asymmetric hydrogenation of quinolines in high yields with excellent enantioselectivities (90–99% ee). Up to a 22% enhancement in enantioselectivity was achieved by the addition of certain amounts of alkali ions (Li+, Na+ or K+), which could be selectively recognized and effectively complexed by the crown ethers on the chiral Xyl-P16C6-Phos.

ChemInform Abstract: N‐tert‐Butanesulfinyl Imine and Aromatic Tertiary Amide Derived Non‐Biaryl Atropisomers as Chiral Ligands for Silver‐Catalyzed endo‐Selective [3 + 2] Cycloaddition of Azomethine Ylides with Maleimides.

Simple modifications of our novel ligand (Xing-Phos) were presented in this work, and a series of aromatic tertiary amide derived non-biaryl atropisomers were successfully synthesized in good yields. In addition, it was found that the multifunctional aromatic tertiary amide derived non-biaryl atropisomers exhibited an excellent endo-selectivity in the silver-catalyzed [3+2]-cycloaddition of azomethine ylides with N-aromatic maleimide. And especially, good to high levels of enantioselectivity (up to 98% ee) was obtained with a wide range of substrates in the presence of syn-(R, RS)-2a (Xing-Phos). Furthermore, on the basis of the experimental data, it was demonstrated that a trace amount of water play an important role in the enhancement of enantioselectivity.

Ti-salalen mediated asymmetric epoxidation of olefins with H2O2: Effect of ligand on the catalytic performance, and insight into the oxidation mechanism

The highly enantioselective epoxidation of several conjugated olefins with H2O2 in the presence of five chiral titanium(IV) salalen (dihydrosalen) complexes has been studied, with focus on the effects of substituents (electronic and steric) on the epoxide yield and enantioselectivity. The introduction of electron acceptors at the ligand’s remote positions enhances the catalytic reactivity, without drastic effect on the stereoselectivity and ultimate catalytic efficiency. In turn, the replacement of Ph substituents of the ligand core with o-MeO-Ph and o-Et-Ph substituents leads to the enhancement in enantioselectivity (up to 99.8% ee), conversion (up to 100%) and efficiency (up to 125 TN). Kinetic and Hammett data provide evidence in favour of a stepwise epoxidation mechanism on Ti-salalen catalysts. As compared to Ti-salan protagonists, Ti-salalen catalysts exhibit much higher activity and efficiency, and in most cases comparable enantioselectivity. Kinetic peculiarities of epoxidations on Ti-salalen and Ti-salan complexes are considered.

Chiral quantum supercrystals with total dissymmetry of optical response.

Since chiral nanoparticles are much smaller than the optical wavelength, their enantiomers show little difference in the interaction with circularly polarized light. This scale mismatch makes the enhancement of enantioselectivity in optical excitation of nanoobjects a fundamental challenge in modern nanophotonics. Here we demonstrate that a strong dissymmetry of optical response from achiral nanoobjects can be achieved through their arrangement into chiral superstructures with the length scale comparable to the optical wavelength. This concept is illustrated by the example of the simple helix supercrystal made of semiconductor quantum dots. We show that this supercrystal almost fully absorbs light with one circular polarization and does not absorb the other. The giant circular dichroism of the supercrystal comes from the formation of chiral bright excitons, which are the optically active collective excitations of the entire supercrystal. Owing to the recent advances in assembly and self-organization of nanocrystals in large superparticle structures, the proposed principle of enantioselectivity enhancement has great potential of benefiting various chiral and analytical methods, which are used in biophysics, chemistry, and pharmaceutical science.

N-tert-Butanesulfinyl imine and aromatic tertiary amide derived non-biaryl atropisomers as chiral ligands for silver-catalyzed endo-selective [3+2] cycloaddition of azomethine ylides with maleimides

Simple modifications of our novel ligand (Xing-Phos) were presented in this work, and a series of aromatic tertiary amide derived non-biaryl atropisomers were successfully synthesized in good yields. In addition, it was found that the multifunctional aromatic tertiary amide derived non-biaryl atropisomers exhibited an excellent endo-selectivity in the silver-catalyzed [3+2]-cycloaddition of azomethine ylides with N-aromatic maleimide. And especially, good to high levels of enantioselectivity (up to 98% ee) was obtained with a wide range of substrates in the presence of syn-(R, RS)-2a (Xing-Phos). Furthermore, on the basis of the experimental data, it was demonstrated that a trace amount of water play an important role in the enhancement of enantioselectivity.

Chemical Modification of Lipase for Rational Enhancement of Enantioselectivity

Abstract Chemical modifications of the I287C mutant of a Burkholderia cepacia lipase afforded various I287C-X conjugates, among which I287C-PAA bearing an N-phenylacetamide (PAA) moiety showed excellent enantioselectivity and catalytic activity for secondary alcohols. Site-directed chemical modifications are powerful tools to control enantioselective biocatalysis.