Asymmetric hydrogenation of ketones: Tactics to achieve high reactivity,enantioselectivity, and wide scope

Ru complexes with chiral diphosphines and amine-based ligands achieve high catalytic activity and enantioselectivity for the hydrogenation of ketones under neutral to slightly basic conditions. The chiral environment is controllable by changing the combination of these two ligands. A concerted six-membered transition state is proposed to be the origin of the high reactivity. The η⁶-arene/TsDPEN–Ru and MsDPEN–Cp*Ir catalysts effect the asymmetric reaction under slightly acidic conditions. A variety of chiral secondary alcohols are obtained in high enantiomeric excess.     

Rhodium‐catalysed hydrogenation of enamides with monodentate phosphorous ligands. A density functional theory study

The hydrogenation of enamides catalysed by rhodium complexes of monodentate phosphorous ligands has been studied by density functional theory. The role of trans intermediates, made accessible by the non‐chelating nature of the ligand, has been taken into account. The findings here reported show that cis intermediates play the major role in the mechanism of the reaction, suggesting that data obtained with chiral monodentate phosphorous ligands or with a mixture of them should be interpreted excluding the intervention of structures with trans phosphine arrangement. Thus, results observed with monodentate phosphorous ligands must be interpreted in the light of the exclusive intervention of cis intermediates, without involvement of structures with trans phosphine arrangements. Copyright © 2010 John Wiley & Sons, Ltd.     

Precision synthesis,structure and function of helical polymers

Helical structures are chiral, which means that if we can synthesize a polymer having a stable one-handed helicity, the polymer is optically active. In 1979, we succeeded in the synthesis of a one-handed helical polymer from an optically inactive achiral monomer, triphenylmethyl methacrylate (TrMA). This is the first example of the asymmetric synthesis of an optically active one-handed helical polymer. The polymer (PTrMA) exhibited an unexpected high chiral recognition ability and afforded a practically useful chiral stationary phase (CSP) for high-performance liquid chromatography (HPLC) by coating it on silica gel. In addition, we also succeeded in the development of very useful CSPs for HPLC using the phenylcarbamate derivatives of polysaccharides, cellulose and amylose. These CSPs can efficiently resolve a broad range of chiral compounds, and have been used all over the world for separating and analyzing chiral compounds.     

Giant Asymmetric Transmission and Circular Dichroism with Angular Tunability in Chiral Terahertz Metamaterials

Inspired by the chiral structure from mode interaction, a chiral terahertz metamaterial and its complementary structure with the coexistence of asymmetric transmission and circular dichroism response are designed. The asymmetric transmission happens in the presence of electric and magnetic modes perpendicular to each other, resulting from intrinsic chirality under normal incidence. Circular dichroism exists in the presence of electric and magnetic modes parallel to each other under oblique incidence because of extrinsic chirality. Both asymmetric transmission and circular dichroism are enhanced by complementary structures, benefitting from the coupling between adjacent units. The maximum of chiral parameter κ can achieve 450, which is one hundred times higher than previously reported. The chiral response can be tuned by the incident angle and is sensitive to the environmental refractive index. The results highlight the potential applications of these metamaterials in chiral sensing and polarization transformation devices.     

Polymer-supported and polymeric chiral guanidines: Preparation and application to the asymmetric Michael reaction of iminoacetate with methyl vinyl ketone

Summary Polymer-supported and polymeric chiral guanidines carrying an imidazolidine skeleton are designed as polymer-based base catalysts. Thus, (R)-2-[(S)-1-hydroxymethyl-2-phenylethylimino]-4-phenylimidazolidine was newly prepared from (R)-phenylglycine as the key functional unit of these guanidines. Application of the polymer-based chiral guanidines to the asymmetric Michael reaction of t-butyl diphenyliminoacetate with MVK led to expected asymmetric induction in the Michael adduct with moderate enantioselectivity in the use of the latter polymeric chiral guanidines.     

Optical properties of a three-dimensional chiral metamaterial

A three-dimensional chiral metamaterial with four-fold rotational symmetry is designed, and its optical properties are investigated by numerical simulations. The results show that this chiral metamaterial has the following features: high polarization conversion, perfect circular dichroism, and asymmetric transmission of circularly polarized light. A comparison of the results of chiral metamaterials without and with weak coupling between the constituent nanostructures enables us to confirm that the optical properties of our proposed nanostructure are closely related to the coupling between the single nanoparticles. This means that the coupling between nanoparticles can enhance the polarization conversion, circular dichroism, and asymmetric transmission. Due to the excellent optical properties, our metamaterial might have potential applications in the development of future multi-functional optical devices.     

Room Temperature Synthesis of Phosphine-Capped Lead Bromide Perovskite Nanocrystals without Coordinating Solvents

The room temperature synthesis of perovskite nanocrystals (NCs) is typically achieved by employing a ligand-assisted reprecipitation (LARP) method, which can be handled in air, and its products are comparable to what is obtained using the traditional hot-injection method. However, the LARP method typically requires the use of coordinating polar solvents such as dimethylformamide, which are not appropriate for large-scale production due to toxicity concerns and can also degrade or form defective perovskite NCs. Herein, an amine and oleic-acid-free room temperature synthesis of lead bromide perovskite NCs is reported that uses a combination of trioctylphosphine oxide and diisooctylphosphinic acid ligands. This combination of ligands provides a stable platform for the polar-solvent-free synthesis in air of fully inorganic CsPbBr₃ (fwhm ≈ 14 nm, emission = 519 nm) and hybrid organic-inorganic FAPbBr₃ (fwhm ≈ 19 nm) NCs with photoluminescence emission between 530 and 535 nm, which is in line with the Rec. 2020 color standards. In addition, it is shown that compared to a traditionally used ligand combination, phosphine ligands can be easily removed from the surface of the NCs, which is important for the future development of this technology in optoelectronic devices.     

Novel Tb(III) complexes with two different structures of phosphine oxides and their properties

Novel Tb(III) complexes with two different structures of phosphine oxide ligands and three β-diketonates were synthesized. Characteristic of the excitation spectra mainly depend on the substituents of β-diketonates, and remarkable correlation among molecular structures, maximum wavelengths and emission intensity could be found. Namely, the shorter the maximum wavelengths when electron-donating groups are substituted in β-diketonates, the larger the emission intensity. These results were explained on the basis of extent of back energy transfer from excited Tb(III) ions to the ligand. From the analysis of the branching ratio of emission spectra, that is defined as the ratio of the intensity of the electric-dipole transition to the magnetic-dipole transition, complex 2 with β-diketonates having small substituent (methyl group) has more asymmetric ligand fields than others. It was found that introduction of two different phosphine oxide was effective for increasing solubility of Tb(III) complexes.     

Immobilizing a single pybox ligand onto a library of solid supports

Chiral pyridinebis(oxazoline) (pybox) ligands can be efficiently immobilized onto silica through position 4 of the pyridine ring. The crucial intermediate in this strategy is 4-chloropybox, which is prepared in good yield from chelidamic acid. 4-Chloropybox reacts with p-hydroxybenzaldehyde and p-aminophenol to give two intermediates (pybox-CHO and pybox-NH2) that allow to introduce the formyl and amino groups able to link to spacers with triethoxysilyl groups. The modified ligands and their ruthenium complexes are immobilized by grafting onto preformed silicas or, alternatively, the support is created by sol-gel synthesis using the functionalized chiral ligand as a silica monomer. In this way it is possible to create a library where the variation involves the support rather than the catalyst. The aim of this approach is to study the influence of different parameters, such as the textural properties of the support and the immobilization method, on the functionalization and catalytic performance. Some of the immobilized complexes are compared as heterogeneous catalysts in the cyclopropanation reaction of styrene with ethyl diazoacetate.     

Chiral asymmetry of anti-symmetric coordinates studied by the Raman differential bond polarizability of S-phenylethylamine

<正>The Raman optical activity(ROA) study on 5-phenylethylamine is presented by the intensity analyses via bond polarizability and differential bond polarizability.Ample information concerning the physical picture of this chiral system is obtained,and its ROA mechanism is constructed.Especially,we propose that the asymmetric modes and/or the off-diagonal elements of the electronic polarizability tensor are the potential keys to the exploration of ROA.     

Isomer Structural Transformation in Au–Cu Alloy Nanoclusters: Water Ripple‐Like Transfer of Thiol Ligands

Structure isomerism is observed in noble metal nanoclusters; however, the mechanism study of this process is rarely reported. Herein, by using the biphosphine ligands instead of phosphine ligands, the high symmetry of the Au₂Cu₆(PPh₃)₂(SAdm)₆ nanocluster is successfully broken and [Au₄Cu₄(L₁)₂(SAdm)₅]Br (where, L₁ is bis(diphenylphosphine)methane, DPPM) nanocluster is obtained with chiral arrangement. This newly obtained nanocluster contains a total of four isomers in crystal unit cell. Interestingly, these isomers undergo rapid isomorphism in solution, which is confirmed by ¹H‐¹H COSY spectrum. Density functional theory calculations demonstrate that the water ripple–like transfer of the thiol ligands results from the isomorphism of nanoclusters.     

Experimental study of the radiation mechanism of the chiral action

The first results of the RAMBAS (Radiation Mechanism of Biomolecular Asymmetry) experiment on the study of the radiation mechanism of the effect on chiral molecules as a factor leading to chiral asymmetry formation are presented. It was found that irradiation with an electron flux from a radioactive source of a mixture of simple achiral materials initiates synthesis of the simplest amino acids; in a racemic mixture of complexes of amino acids and metals irradiation leads to asymmetric decomposition and chrial asymmetry formation. The results obtained can be essential for solving the problem of the origin of life and the formation of chiral asymmetry of the biosphere.     

Asymmetric MRI Magnet Design Using a Hybrid Numerical Method

This paper describes a hybrid numerical method for the design of asymmetric magnetic resonance imaging magnet systems. The problem is formulated as a field synthesis and the desired current density on the surface of a cylinder is first calculated by solving a Fredholm equation of the first kind. Nonlinear optimization methods are then invoked to fit practical magnet coils to the desired current density. The field calculations are performed using a semi-analytical method. A new type of asymmetric magnet is proposed in this work. The asymmetric MRI magnet allows the diameter spherical imaging volume to be positioned close to one end of the magnet. The main advantages of making the magnet asymmetric include the potential to reduce the perception of claustrophobia for the patient, better access to the patient by attending physicians, and the potential for reduced peripheral nerve stimulation due to the gradient coil configuration. The results highlight that the method can be used to obtain an asymmetric MRI magnet structure and a very homogeneous magnetic field over the central imaging volume in clinical systems of approximately 1.2 m in length. Unshielded designs are the focus of this work. This method is flexible and may be applied to magnets of other geometries.     

On the quantification of asymmetry in nature

A new concept of quantitative measure of the amount of structural asymmetry is introduced and defined by maximal overlap between two enantiomers of the same chiral body. This concept is to be called chiral coefficient, and it can readily be extended to physical properties such as mass or electronic wave-function distributions over chiral objects such as molecules or unit cells in crystals. The mathematical aspects of these coefficients are discussed. There exist two cases of maximal overlaps, symmetric and asymmetric, where for the asymmetric case there exist two positions of maximal overlap. It is also shown that there always exists a natural directionz for every asymmetric body for any physical property treated. The possibility of quantizing structural asymmetry is discussed, and, if it exists, it may be associated with the naturalz axis of the chiral system. Possible applications of asymmetry quantification are discussed.     

Kaon and antikaon optical potentials in isospin asymmetric hyperonic matter

The medium modifications of the energies of kaons and antikaons in isospin asymmetric hyperonic matter are investigated using a chiral SU(3) model. The isospin-dependent medium effects are important for asymmetric heavy-ion collision experiments, as well as relevant for the neutron star phenomenology as the bulk matter in the interior of the neutron star is highly isospin asymmetric. The effects of hyperons on the medium modifications of the kaons and antikaons in the strange hadronic matter are investigated in the present work and are seen to be appreciable for hadronic matter with large strangeness fractions. The study of the K -mesons in the asymmetric strange hadronic matter can be especially relevant for the compressed strange baryonic matter which can result from asymmetric heavy-ion collision experiments in the future accelerator facility FAIR at GSI.     

Catalytic enantioselective synthesis of chiral organic compounds of ultra-high purity of >99% ee

Shortly after the discovery of Zr-catalyzed carboalumination of alkynes in 1978, we sought expansion of the scope of this reaction so as to develop its alkene version for catalytic asymmetric C–C bond formation, namely the ZACA (Zr-catalyzed asymmetric carboalumination of alkenes). However, this seemingly easy task proved to be quite challenging. The ZACA reaction was finally discovered in 1995 by suppressing three competitive side reactions, i.e., (i) cyclic carbometalation, (ii) β-H transfer hydrometalation, and (iii) alkene polymerization. The ZACA reaction has been used to significantly modernize and improve syntheses of various natural products including deoxypolypropionates and isoprenoids. This review focuses on our recent progress on the development of ZACA–lipase-catalyzed acetylation–transition metal-catalyzed cross-coupling processes for highly efficient and enantioselective syntheses of a wide range of chiral organic compounds with ultra-high enantiomeric purities.     

Synthesis of Atom‐Precise Chiral Ag14 Clusters Protected by Penicillamine Ligands

A pair of atom‐precise chiral silver(I) nanocluster enantiomers ( Ag₁₄‐d and Ag₁₄‐l ) protected by d ‐ and l ‐penicillamine ligands is reported. Crystallographic structures reveal that the nanoclusters consist of a S^2? template and a chiral Ag₁₄ core stabilized by 12 penicillamine ligands. The penicillamine ligands show two binding fashions: (i) only thiolate coordination, and (ii) thiolate and carboxylate co‐coordination. Meanwhile, the two enantiomers show strong circular dichroism with opposite signals (mirror image relationship) owing to the chiral metallic core induced by chiral ligands, suggesting that the nanoclusters have well‐defined stereostructures as common chiral molecules do. The proton conductivity is also explored due to the existence of both amino groups and carboxylate groups from the penicillamine ligands, which is beneficial to construct H‐bond network for proton transfer.     

High performance of ultrasonic-assisted synthesis of two spherical polymers for enantioselective catalysis

Chiral polymers have aroused great attention in among chiral supramolecular materials based on their features. Herein, for the first time, the synthesis of chiral polymeric composites (CMNPs/1,4-Zbtb & 1,3-Zbtb) have been reported with entrapment through three strategies: ultrasonic irradiation, solvothermal, and mechanical stirring. According to the obtained results, it is found that ultrasound-assisted synthesis can be considered as an inexpensive and efficient method than the others, from the point of view of energy and time consuming. In this strategy, encapsulation of chiral magnetic nanoparticles (CMNPs) by using tetrazole-based polymers (Zbtbs) happens, in-situly. These chiral sphere-like inorganic–organic polymers can be considered as core and shell composites with catalytic activity due to their acidic (semi unsaturated Zn: open metal sites) and basic (abundant basic nitrogens) centers. In these structures, the unprecedented chirality induction can happen from the core to shell by non-covalent interaction, easily. They could catalyze symmetric oxidation and asymmetric henry condensation to give chiral β-nitroalkanol. Circular dichroism and chiral gas chromatography were used to characterize the produced enantiomers. These chiral polymeric materials can be considered as unique acid-base bifunctional catalysts with efficient properties such as high stability, enantiomeric excess, enantioselectivity to the main product, and protecting from CMNPs leaching.     

Soliton molecules and mixed solutions of the(2+1)-dimensional bidirectional Sawada-Kotera equation

In this paper, we investigate some new interesting solution structures of the(2+1)-dimensional bidirectional Sawada–Kotera(bSK) equation. We obtain soliton molecules by introducing velocity resonance. On the basis of soliton molecules, asymmetric solitons are obtained by changing the distance between two solitons of molecules. Based on the N-soliton solutions,several novel types of mixed solutions are generated, which include the mixed breather-soliton molecule solution by the module resonance of the wave number and partial velocity resonance,the mixed lump-soliton molecule solution obtained by partial velocity resonance and partial long wave limits, and the mixed solutions composed of soliton molecules(asymmetric solitons), lump waves, and breather waves. Some plots are presented to clearly illustrate the dynamic features of these solutions.     

Melting and wetting transitions in the three-state chiral clock model

The melting transition of the two-dimensional, three-state, asymmetric or chiral clock model is examined. Evidence from scaling arguments and analysis of perturbation series is presented, indicating that the chiral symmetry-breaking operator is relevant at the symmetric (or pure Potts) critical point with a crossover exponent of ø ≈ 0.2. The remainder of the commensurate-disordered phase boundary therefore appears to be in a new universality class, distinct from the pure three-state Potts transition. An interfacial wetting transition that plays an important role in the crossover between the two types of critical behavior is discussed. The location and exponents of this wetting transition are obtained both in a low-temperature limit using generating function techniques and in a systematic low-temperature expansion of the transfer matrix.