Chiral Thin Films of Metal Oxide

In this paper, we describe for the first time the synthesis of new chiral nanosized metal oxide surfaces based on chiral self‐assembled monolayers (SAMs) coated with metal oxide (TiO₂) nanolayers. In this new type of nanosize chiral surface, the metal oxide nanolayers enable the protection of the chiral self‐assembled monolayers while preserving their enantioselective nature. The chiral nature of the SAM/TiO₂ films was characterized by variety of unique techniques, such as second‐harmonic generation circular dichroism (SHG‐CD), quartz crystal microbalance, and chiral adsorption measurements with circular dichroism spectroscopy. The chiral resolution abilities of the SAMs coated with metal oxide (TiO₂) nanolayers were investigated in the crystallization of a racemic mixture of threonine and glutamic acid. Our proposed methodology for the preparation of nanoscale chiral surfaces described in this article could open up opportunities in other fields of chemistry, such as chiral catalysis.     

Asymmetric zinc-Reformatsky reaction of Evans chiral imide with acetophenones and its application to the stereoselective synthesis of triazole antifungal agents

The Ni(acac)₂ catalytic ZnEt₂-mediated asymmetric Reformatsky-type reaction of Evans chiral imide with various acetophenones was studied. The chiral imido zinc enolate, which was formed through the metal–halogen exchange reaction of chiral α-bromopropionyl-2-oxazolidinones 2 with diethyl zinc under the catalysis of Ni(acac)₂, performed the asymmetric zinc-Reformatsky reaction with activated α-haloacetophenones 3 to give the chiral β-hydroxyamide 4 in good yields and high ratios of syn-(2R,3R)-isomers (up to >97%). This new asymmetric synthesis technology affords a practical method to synthesize the versatile chiral building block 5 for triazole antifungal agents, such as Voriconazole, Ravuconazole, TAK-187, and RO-0094815.     

Asymmetric synthesis of new chiral long chain alcohols

Sixteen new chiral alcohols with alkyl (C₁₁–C₁₉) and aryl, substituted aryl, hetero aryl and biaryl groups 2a–2t were synthesized by three different asymmetric reduction methods from their corresponding ketones 1a–1t. Chiral NaBH₄ (method A), chiral BH₃ (method B) and chiral AIP (method C) were used as asymmetric reduction catalysts. Chiral NaBH₄ was modified by four different ligands 3a–3d, chiral BH₃ and chiral AIP by four different ligands 4a–4d. Ligand 4c was synthesized for the first time in this work. Chiral NaBH₄ generated chiral alcohols of (R)-configuration and chiral BH₃ and chiral AIP of (S)-configuration with high enantiomeric excesses, were analysed by chiral HPLC. In order to determine the ee values by chiral HPLC, sixteen corresponding racemic alcohols, synthesized by reducing their corresponding ketones via NaBH₄, were used for chiral resolution on a Daicel OD HPLC column. The sixteen starting ketones were synthesized in this study by Friedel–Craft acylation. The new chiral alcohols were characterized by IR, NMR, (¹H and ¹³C), MS, elemental analyses and specific rotation. The reduction methods A, B and C were applied to these ketones for the first time in this study and were compared with each other. The relationship between the structure of the ketone and the yield and the enantiomeric excess was discussed.     

Enantioselective Crystallization of Chiral Inorganic Crystals of ϵ-Zn(OH)2 with Amino Acids

Many inorganic materials can form crystals, but little is known about their enantioselective crystallization. Herein, we report on the enantioselective crystallization of ϵ-Zn(OH)₂ (Wulfingite) chiral crystals by using amino acids. Crystals of ϵ-Zn(OH)₂ were crystallized from supersaturated sodium hydroxide and zinc nitrate aqueous solutions in the presence of l - or d -arginine. All of the chiral measurements, such as selective chiral adsorption by circular dichroism (CD), chiral chromatography, and polarimetry measurements, clearly show chiral discrimination during the crystallization of ϵ-Zn(OH)₂. In addition, a new method has been developed for identifying chirality in crystals by using electron paramagnetic resonance (EPR). Although the values of chiral induction of the ϵ-Zn(OH)₂ crystals obtained are somewhat low, these values are still significant because they demonstrate that enantioselectivity during the crystallization of chiral inorganic crystals with chiral additives can be achieved. The method can be applied to many chiral inorganic systems. Understanding and controlling the crystallization of chiral inorganic crystals is important for gaining knowledge on the interaction of chiral molecules with inorganic surfaces. This knowledge can lead to an understanding of basic scientific questions such as the evolution of homochirality in biomolecules and the development of chiral inorganic crystals for a variety of purposes such as asymmetric catalysis and optical applications.     

Chiral Carbonaceous Nanotubes Modified with Titania Nanocrystals: Plasmon‐Free and Recyclable SERS Sensitivity

Chiral carbonaceous nanotubes (CNT) were successfully used in plasmon‐free surface‐enhanced Raman scattering (SERS) for the first time. Further modification of TiO₂ nanocrystals on the chiral CNTs successfully realized the recycling of SERS substrate as chiral CNT/TiO₂ hybrids. The high SERS sensitivity of methylene blue (MB) over the chiral CNT/TiO₂ hybrids is ascribed to the laser‐driven birefringence induced by the helical structure, which provides much more opportunities for the occurrence of Raman scattering. The TiO₂ nanocrystals highly dispersed on the surface and inside the hollow cavity of chiral CNTs can completely degrade the MB under the solar light irradiation, leading to the self‐cleaning of SERS substrate. The present research opens a new way for the application of chiral inorganic materials in plasmon‐free SERS detection.     

A new C2-symmetric azolium compound for Cu-catalyzed asymmetric conjugate addition of R2Zn to cyclic enone

A new chiral N-heterocyclic carbene (NHC) ligand was designed. Thus, an efficient synthetic route to C₂-symmetric bis(hydroxyamide)-functionalized benzimidazolium salts from chiral β-amino alcohols was developed. The combination of Cu(OTf)₂ and the chiral azolium compound efficiently promoted the conjugate addition reaction of cyclic enone with dialkylzinc to give the corresponding adduct in good yield. Among a series of chiral NHC proligands, the functionalized benzimidazolium chloride possessing a tert-butyl group as a stereodirecting group was found to be the best choice of ligand. Under optimized reaction conditions, an excellent enantioselectivity (96% ee) was realized by allowing 2-cyclohepten-1-one to react with Bu₂Zn at room temperature.     

From MOF-74-Zn to Triazolate-Directed Nonsymmetric Assembly of Chiral Zn6@Zn6 Clusters

The creation of new cluster building blocks, as well as new ligand coordination modes, are among the most effective ways to develop new framework materials. Yet, large and chiral clusters are both difficult to create and relatively few. Here, by studying the competing coordination of different azolates against carboxylate and combined carboxylate/phenolate, it is shown that the impact of azolates in the MOF-74 synthesis system differs dramatically, leading to the synthesis of MOF-74, UTSA-74, and CPM-72 for 2-methylimidazole, 1,2,4-triazole, and 1,2,3-triazole, respectively. The new CPM-72 contains a novel chiral Zn₁₂ triazolate cluster, which features a trigonal-prismatic Zn₆ core inside an octahedral Zn₆ shell. In contrast with MOF-74 with fully deprotonated and symmetrically bonded 2,5-dihydroxyterephthalic acid (H₄DOBDC), H₄DOBDC adopts an unusual nonsymmetric bonding mode in CPM-72 (carboxylate only at one end and carboxylate/phenolate at the other), resulting in a highly porous and intrinsically chiral 3D framework. The nonsymmetric bonding mode by H₄DOBDC, apparently dictated by the chiral Zn₁₂ cluster, can be replicated with 2-hydroxyterephthalic acid (H₃OBDC), leading to the synthesis of porous isoreticular CPM-73.     

On the use of dispersed nanoparticles modified with single layer beta-cyclodextrin as chiral selecor to enhance enantioseparation of clenbuterol with capillary electrophoresis

A new strategy for chiral separation by capillary electrophoresis employing modified-nanoparticles as chiral selector is described for clenbuterol analysis. Nanoparticles modified with β-cyclodextrin (β-CD) form a large surface area platform to serve as a pseudostationary chiral phase, which can be applied for the enhancement of the enantioseparation. The application of four kinds of nanoparticles was investigated (multi-walled nanotubes (MWNTs), polystyrene (PS), TiO₂ and Al₂O₃) modified with single layer β-CD as chiral selector in the enantioseparation of clenbuterol by capillary electrophoresis (CE). Successful clenbuterol enantioseparation could be achieved with the β-CD-modified MWNTs as chiral selector. X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) confirmed the β-CD modification of the nanoparticles. The effects of nanoparticles, surfactant, chiral selector (β-CD) and run buffer were studied in relation to the enantiomeric separation of clenbuterol. This study opens attractive perspectives for the use of modified nanoparticles for chiral separational purposes in CE.     

Design and synthesis of fused-ring chiral ionic liquids from amino acid derivatives

Six novel imidazolium salts, which contain a chiral moiety as well as a fused-ring system, have been designed, synthesized, and fully characterized. The synthesis of these ionic liquids is concise and practical due to the commercial availability of the starting materials. These imidazolium compounds were readily prepared from 1-methyl-2-imidazoliumcarboxaldehyde and chiral amino alcohols. Salts that contain the PF₆ anion were solids, but salts with the NTf₂ anion were liquids at room temperature. We envision that these new chiral imidazolium compounds can serve as effective reaction media as well as chiral catalysts for asymmetric reactions, which are presently being investigated in our lab.     

Copper(II)-containing C2-symmetric bistetracarboline amides in enantioselective Henry reactions

A series of new C₂-symmetric chiral diamides were synthesized from l-tryptophan and used as chiral ligands chelated with Cu(II) in enantioselective Henry reactions. Ligand 4a with CuCl₂·2H₂O (5%) showed effective catalytic efficiency in Henry reaction. High yields (up to 99%) and excellent enantioselectivities (up to 98%) were achieved for both aromatic and aliphatic aldehyde substrates.     

A new chiral sulfinyl-NH-pyridine ligand for Ir-catalyzed asymmetric transfer hydrogenation reaction

A new flexible C₁-symmetric tridentate ligand (S)-N-(2-(tert-butylsulfinyl)benzyl)-1-(pyridin-2-yl)methanamine sulfoxide (L1) was successfully prepared and utilized as a chiral ligand for Ir(I)-catalyzed ATH (asymmetric transfer hydrogenation) reactions. Without any cooperation of other chiral center, encouraging ee and conversion values have been achieved, which provide us a better understanding on these types of ligands and a new strategy to develop new high-efficiency chiral catalysts for asymmetric reaction.     

Ferroelectric liquid crystal materials synthesized from 2(S)-[2(S)-methylbutyloxy]propanol

Abstract

A new chiral alcohol, 2(S)-[2(S)-methylbutyloxy]propanol (3), containing two chiral centres has been synthesized from ethyl lactate and (S)-1-iodo-2-methylbutane. It was used as a chiral building block for the preparation of ferroelectric liquid crystal materials. Several of the new materials exhibit an enantiotropic S*_c phase with a wide temperature range. The results indicate that the molecular structure of 3 is useful for synthesizing ferroelectric liquid crystal materials.     

Catalytic Enantioselective Reactions. Part VIII. Catalytic Enantioselective Addition of Diethylzinc to Aldehydes Using (2S,3S)-(-)-1,4-Diamino-2,3-Butanediol Derivatives as Chiral Catalysts

A series of new chiral β-N,N-dialkylaminoalcohol derivatives with an axis of C₂ symmetry was prepared from diethyl L-tartrate and their enantioselectivities as chiral catalysts for enantioselective addition of diethylzinc to aldehydes were compared.     

Exploration of Chiral Organic–Inorganic Hybrid Semiconducting Lead Halides

Organic–inorganic lead halides have recently emerged as promising alternatives to conventional optoelectronic materials, considering their intriguing physical properties. However, organic–inorganic lead halides featuring chirality are seldom explored. Here, a pair of enantiomorphic organic–inorganic hybrid semiconducting lead halides, (R‐C₅H₁₄N₂)PbBr₄ ( 1R ) and (S‐C₅H₁₄N₂)PbBr₄ ( 2S ), were successfully obtained with the templating of chiral amines. These compounds adopt distinct one‐dimensional infinite quantum helices formed by edge‐shared transformative lead bromide octahedra. Notably, 1R and 2S present mirror circular dichroism (CD) signals due to the chirality transfer of the enantiopure amines. Furthermore, 1R and 2S exhibit phase‐matchable quadratic nonlinear response and typical semiconducting behaviours. This work highlights the potential of lead halides as a new kind of chiral semiconducting materials in spintronic and chiral optical applications.     

Chiral Self‐Discrimination and Guest Recognition in Helicene‐Based Coordination Cages

Chiral nanosized confinements play a major role for enantioselective recognition and reaction control in biological systems. Supramolecular self‐assembly gives access to artificial mimics with tunable sizes and properties. Herein, a new family of [Pd₂L₄] coordination cages based on a chiral [6]helicene backbone is introduced. A racemic mixture of the bis‐monodentate pyridyl ligand L¹ selectively assembles with Pd^II cations under chiral self‐discrimination to an achiral meso cage, cis‐[Pd₂ L^1P ₂ L^1M ₂]. Enantiopure L¹ forms homochiral cages [Pd₂ L^1P/M ₄]. A longer derivative L² forms chiral cages [Pd₂ L^2P/M ₄] with larger cavities, which bind optical isomers of chiral guests with different affinities. Owing to its distinct chiroptical properties, this cage can distinguish non‐chiral guests of different lengths, as they were found to squeeze or elongate the cavity under modulation of the helical pitch of the helicenes. The CD spectroscopic results were supported by ion mobility mass spectrometry.     

Separation Performance of MOFs Zn(ISN)2·2H2O as Stationary Phase for High-Resolution GC

Metal–organic frameworks (MOFs) as a new type of porous materials have attracted tremendous attention for their potential applications in recent years. Here, we used a simple chiral MOF, Zn(ISN)₂·2H₂O, as chiral stationary phase for high-resolution gas chromatography. A Zn(ISN)₂·2H₂O-coated fused-silica capillary column (3 m long × 75 μm i.d.) was fabricated by using a dynamic coating method with ethanol suspension of Zn(ISN)₂·2H₂O (1 mg mL⁻¹⁾. Then, the coating properties, polarity, and column efficiency of the column were studied. The column efficiency of Zn(ISN)₂·2H₂O-coated column was up to 3,020 plates m⁻¹. The separation performance was investigated by separating a wide range of organic compounds such as Grob test mixtures, alkanes, alcohols, isomers, and racemates. The relative standard deviations for the five replicate separations of alanine were 0.32 and 2.6 % for retention time and peak area, respectively. The experimental results showed that the new stationary phase possesses high column efficiency, good reproducibility, excellent selectivity, and chiral recognition ability.

     

On the way to biodegradable poly(hydroxy butyrate) from propylene oxide and carbon monoxide via β-butyrolactone: Multisite catalysis with newly designed chiral indole-imino chromium(III) complexes

Enantioenriched poly(hydroxy butyrate) (PHB) is a biodegradable polyester of significant commercial interest as an environmentally benign substitute of commodity polyolefines. We report on the design and development of new chiral indole-based ligand families and on their chromium(III) complexes as enantioselective catalysts for the conversion of propylene oxide and carbon monoxide to enantioenriched β-butyrolactone, the key monomer for the production of PHB by ring-opening polymerization. The enantioselective carbonylation catalysts are based on new chiral tri- and tetradentate [N₂O] and [N₄] chromium(III) complexes containing chiral indolaldimine ligand scaffolds. The conceptual design of these ligands is inspired by Jacobsen’s salicylaldimine lead structure; the key difference is an exchange of the salicyl-O-donor against an indole-N-donor, allowing additional structural diversity and stereoelectronic tuning by the indole substitution pattern. Synthetically, chiral indolealdimines are easily accessible from 7-formylindoles by standard Schiff base condensation with chiral amine building blocks; the 7-formylindoles in turn are synthesized from the corresponding 7-bromoindoles by the Rapoport synthesis, and the starting 7-bromoindoles are accessible from 2-bromoaniline by the classical Fischer indole synthesis. Three generations of chiral [N₂O] and [N₄] chromium(III) catalysts have been developed and evaluated in the enantioselective carbonylation of racemic propylene oxide with carbon monoxide using tetracarbonylcobaltate as the nucleophilic reagent for the insertion of carbon monoxide into the activated propylene oxide/chiral Lewis acid complex. The best catalyst out of 10 candidates showed at a temperature of 80 °C an activity of 37% conversion, 100% chemoselectivity, and 19% stereoselectivity.     

Asymmetric synthesis of substituted 1-aminocyclopropane-1-carboxylic acids from a new chiral glycine equivalent with 3,6-dihydro-2H-1,4-oxazin-2-one structure

Condensation of the new chiral glycine equivalent 10 with aldehydes at room temperature in the presence of K₂CO₃ under solid–liquid phase-transfer-catalysed conditions afforded stereoselectively new chiral (Z)-α,β-didehydroamino acid (DDAA) derivatives with oxazinone structure 14. These systems have been used in diastereoselective cyclopropanation reactions for the synthesis of enantiomerically pure 1-aminocyclopropanecarboxylic acids (ACCs) such as (−)-allo-norcoronamic and (−)-allo-coronamic acids.     

A NEW EFFICIENT PROTOCOL FOR THE PREPARATION OF CHIRAL BISPHOSPHINES OF THE BICP FAMILY

A new procedure for the synthesis of chiral BICP ligands is described. The protocol involves the reaction of sodium diarylphosphide–borane complex with chiral 2,2′-bisbenzenesulfonyl-1,1′-dicyclopentane in toluene, followed by HBF₄·OMe₂-mediated BH₃ decomplexation.     

Separation Performance of MOFs Zn(ISN)2·2H2O as Stationary Phase for High-Resolution GC

Metal–organic frameworks (MOFs) as a new type of porous materials have attracted tremendous attention for their potential applications in recent years. Here, we used a simple chiral MOF, Zn(ISN)₂·2H₂O, as chiral stationary phase for high-resolution gas chromatography. A Zn(ISN)₂·2H₂O-coated fused-silica capillary column (3 m long × 75 μm i.d.) was fabricated by using a dynamic coating method with ethanol suspension of Zn(ISN)₂·2H₂O (1 mg mL^?1). Then, the coating properties, polarity, and column efficiency of the column were studied. The column efficiency of Zn(ISN)₂·2H₂O-coated column was up to 3,020 plates m^?1. The separation performance was investigated by separating a wide range of organic compounds such as Grob test mixtures, alkanes, alcohols, isomers, and racemates. The relative standard deviations for the five replicate separations of alanine were 0.32 and 2.6 % for retention time and peak area, respectively. The experimental results showed that the new stationary phase possesses high column efficiency, good reproducibility, excellent selectivity, and chiral recognition ability.