Strategies for chiral separation: from racemate to enantiomer

Chiral separation has become a crucial topic for effectively utilizing superfluous racemates synthesized by chemical means and satisfying the growing requirements for producing enantiopure chiral compounds. However, the remarkably close physical and chemical properties of enantiomers present significant obstacles, making it necessary to develop novel enantioseparation methods. This review comprehensively summaries the latest developments in the main enantioseparation methods, including preparative-scale chromatography, enantioselective liquid-liquid extraction, crystallization-based methods for chiral separation, deracemization process coupling racemization and crystallization, porous material method and membrane resolution method, focusing on significant cases involving crystallization, deracemization and membranes. Notably, potential trends and future directions are suggested based on the state-of-art "coupling" strategy, which may greatly reinvigorate the existing individual methods and facilitate the emergence of cross-cutting ideas among researchers from different enantioseparation domains.

Engineering UiO-68-Typed Homochiral Metal-Organic Frameworks for the Enantiomeric Separation of Fmoc-AAs and Mechanism Study

Homochiral metal-organic frameworks (HMOFs) have been widely investigated in the application of enantiomeric separation. Nonetheless, it remains a significant challenge to explore the effect of multiple weak interactions between HMOF adsorbents and chiral adsorbates on enantiomeric separation performance still. In this work, robust chiral amine-alcohol-functionalized UiO-68-typed Zr-HMOFs 1-3 with the same hydrogen-bonding sites but slightly different π-binding sites were prepared for the enantioseparation of amino acid derivatives (Fmoc-AAs) with large π-binding groups. As a consequence of multiple host-guest interactions, these Zr-HMOFs exhibit speedy adsorption and high adsorption capacity for Fmoc-L/D-AAs and dissimilar enantioselectivity for the adsorption of their enantiomers. Materials 1 and 2 exhibit excellent enantioselective separation performance for Fmoc-valine with a single terminal π-binding group, while material 3 displays excellent enantioselective separation performance for Fmoc-phenylalanine and Fmoc-tryptophan with π-binding groups at both ends. As evidently demonstrated by our experimental and density functional theory (DFT) computational results, when the number of π-binding groups preset in the confined chiral space of adsorbents matches the number of π-binding groups of chiral adsorbates, the synergism of π-π or σ-π interactions will increase enantioselectivity; otherwise, the competition interactions from redundant identical binding sites will weaken enantioselectivity. Our case not only provides a tremendously typical system for investigating the collaborative discrimination of multiple weak interactions and exploring the impact of relatively excessive binding sites of HMOF adsorbents or chiral adsorbates on the enantioselective separation performance but also provides guidance for targeted functional modifications of high-performance chiral porous materials.

Homochiral Porous Metal-Organic Polyhedra with Multiple Kinds of Vertices

Metal-organic polyhedra featuring non-Archimedean/Platonic architectures with multiple kinds of vertices have aroused great attention for their fascinating structures and properties but are yet challenging to achieve. Here, we report a combinatorial strategy to make such nonclassic polyhedral cages by combining kinetically labile metal ions with non-planar organic linkers instead of the usual only inert metal centers and planar ligands. This facilitates the synthesis of an enantiopure twisted tetra(3-pyridyl)-based TADDOL (TADDOL = tetraaryl-1,3-dioxolane-4,5-dimethanol) ligand (L) capable of binding Ni(II) ions to produce a regular convex cage, Ni₆L₈, with two mixed metal/organic vertices and three rarely reported concave cages Ni₁₄L₈, Ni₁₈L₁₂, and Ni₂₄L₁₆ with three or four mixed vertices. Each of the cages has an amphiphilic cavity decorated with chiral dihydroxyl functionalities and packs into a three-dimensional structure. The enantioselective adsorption and separation performances of the cages are strongly dependent on their pore structure features. Particularly, Ni₁₄L₈ and Ni₁₈L₁₂ with wide openings can be solid adsorbents for the adsorptive and solid-phase extractive separation of a variety of racemic spirodiols with up to 98% ee, whereas Ni₆L₈ and Ni₂₄L₁₆ with smaller pore apertures cannot adsorb the racemates. The combination of single-crystal X-ray diffraction analysis of the host-guest adduct and GCMC simulation indicates that the enantiospecific recognition capabilities originate from the well-organized chiral inner sphere as well as multiple interactions within the chiral microenvironment. This work therefore provides an attractive strategy for the rational design of polyhedral cages, showing geometrically fascinating structures with properties different from those of classic assemblies.

Peptide-Based Materials That Exploit Metal Coordination

Metal-ion coordination has been widely exploited to control the supramolecular behavior of a variety of building blocks into functional materials. In particular, peptides offer great chemical diversity for metal-binding modes, combined with inherent biocompatibility and biodegradability that make them attractive especially for medicine, sensing, and environmental remediation. The focus of this review is the last 5 years' progress in this exciting field to conclude with an overview of the future directions that this research area is currently undertaking.

A review on chiral metal-organic frameworks: synthesis and asymmetric applications

Chiral metal-organic frameworks (CMOFs) have the characteristics of framework structure diversity and functional tunability, and have important applications in the fields of chiral identification, separation of enantiomers and asymmetric catalysis. In recent years, the application of CMOFs has also been extended to other research fields, such as circularly polarized fluorescence and chiral ferroelectrics. Compared with achiral MOFs, the design of CMOFs only considers the modes of introduction of chirality, and also takes into account the crystallization and purification. Therefore, the synthesis and characterization of CMOFs face many difficult challenges. This review discusses three effective strategies for constructing CMOFs, including direct synthesis of chiral ligands, spontaneous resolution of achiral ligands or chiral template-induced synthesis, and post-synthetic chiralization of achiral MOFs. In addition, this review also discusses the recent application progress of CMOFs in chiral molecular recognition, enantiomer separation, asymmetric catalysis, circularly polarized fluorescence, and chiral ferroelectrics.

Metal-organic frameworks in chiral separation of pharmaceuticals

Stereoselective chiral molecules are responsible for specific biological functions in nature. At present, more than half of the prescribed drugs are chiral. Living organisms display divergent pharmacological responses to the enantiomers, leading to altered toxicity, pharmacokinetics, and pharmacodynamics. Thus, chiral analysis, separation, and extraction are crucial for ensuring enantiomeric purity to develop safe and effective medication. In recent times, metal-organic frameworks (MOFs) with appealing structures are gaining importance because of their fascinating properties as a sorbent and stationary phase. MOFs are crystalline porous solid materials built by interconnecting metal ions or clusters and organic linkers. This review explores the advancements in MOFs for the isolation and separation of chiral active pharmaceutical drugs.

Construction of a chiral zinc-camphorate framework for enantioselective separation

A chiral metal-organic framework (CMOF) with open chiral channels and multiple recognition sites is constructed from camphoric acid and a dipyridyl ligand. It can act as an efficient chiral solid adsorbent, capable of separating a variety of racemic alcohols and epoxides with excellent enantioselectivities.

Recent progress in organic-inorganic hybrid materials as absorbents in sample pretreatment for pesticide detection

Sample pretreatment is essential for trace analysis of pesticides in complex food and environment matrices. Recently, organic-inorganic hybrid materials have gained increasing attention in pesticide extraction and preconcentration. This review highlighted the common organic-inorganic hybrid materials used as absorbents in sample pretreatment for pesticide detection. Furthermore, the preparation and characterization of organic-inorganic hybrid materials were summarized. To obtain a deep understanding of adsorption toward target analytes, the adsorption mechanism and absorption evaluation were discussed. Finally, the applications of organic-inorganic hybrid materials in sample pretreatment techniques and perspectives in the future are also discussed.

Inversion of Supramolecular Chirality by In Situ Hydrolyzation of Achiral Diethylene Glycol Motifs

Chiral inversion of supramolecular assemblies is of great research interest due to its broad practical applications. However, chiral structure transition induced by in situ regulation of building molecules has remained a challenge. Herein, left-handed fibrous assemblies were constructed by C₂-symmetic l-phenylalanine coupled with diethylene glycol (LPFEG) molecules. In situ hydrolyzing terminal diethylene glycol motifs in LPFEG successfully inverted the chirality of the nanofibers from left- to right-handedness. The transition of right-handed fibers into left-handed fibers could also be achieved via hydrolyzing DPFEG molecules. Circular dichroism (CD) spectroscopy, 1D and 2D nuclear magnetic resonance (NMR) spectroscopy, and Fourier transform infrared (FT-IR) spectroscopy revealed that the back-folded achiral diethylene glycol played a vital role in L/DPFEG molecular arrangements and removing terminal diethylene glycol could induce the opposite rotation of molecular assemblies. Thanks to this merit, the enantioselective separation of racemic phenylalanine was obtained and the enantiomeric excess (ee) values could achieve around ±20% after separation. This study not only provides a new strategy to regulate the chiral structure via dynamic modulation of terminal substituents but also presents a promising application in the field of enantioselective separation.

A biomimetic metal–organic framework with cuboid inner cavities for enantioselective separation

A biomimetic metal–organic framework with cuboid inner cavities and multiple recognition sites was constructed from a phenylalanine-derived ligand. It can enantioselectively separate various racemic alcohols, diols and epoxides with ee up to 99.5%.

Homochiral porous coordination polymer of EuIII for metal ion sensing and enantioselective adsorption

A novel multifunctional homochiral porous metal–organic framework was obtained by combining the luminescent component Eu(iii) with an enantiopure triangular polycarboxylic ligand.

Synthesis of a novel chiral DA-TD covalent organic framework for open-tubular capillary electrochromatography enantioseparation

Herein, a novel chiral covalent organic framework, DA-TD COF, with good chemical/thermal stability was synthesized and used as a chiral stationary phase for open-tubular capillary electrochromatography enantioseparation. The DA-TD COF coated capillary exhibited excellent enantioseparation efficiency and its separation efficiency did not show an obvious decrease over 200 runs. Furthermore, the enantioseparation mechanism was studied.

Enantioselective Recognition and Separation of C2 Symmetric Substances via Chiral Metal-Organic Frameworks

A promising route toward the enantioselective recognition and separation of racemic molecules is the design of chiral metal-organic frameworks (CMOFs) with high enantioselectivity and stability. Herein, we report porous CMOFs Δ- and Λ-RuEu-MOFs constructed from the D₃-symmetry helical chiral Ru(phen)₃-derived tricarboxylate ligand and Eu₂ units, which can be utilized as adsorbents for the enantioselective recognition and separation of 1,1'-bi-2-naphthol (BINOL) derivatives. Investigation of the circular dichroism enantiodifferentiation between the host and guest suggested that Δ- and Λ-RuEu-MOFs can be employed as chiral sensors to discriminate axial enantiomers due to their diastereomeric host-guest relationship. Density functional theory calculations reveal that chiral recognition is attributed to the distinguishing binding affinities stemming from N···H-O hydrogen bonds and π-π stacking between the host and guest. Moreover, the reticulate structure of Δ- and Λ-RuEu-MOFs can be readily recycled and reused for the successive enantioselective separation of BINOL up to 80% ee.

Coordination sphere hydrogen bonding as a structural element in metal-organic Frameworks

In the design of new metal-organic frameworks, the constant challenges of framework stability and structural predictability continue to influence ligand choice in favour of well-studied dicarboxylates and similar ligands. However, a small subset of known MOF ligands contains suitable functionality for coordination sphere hydrogen bonding which can provide new opportunities in ligand design. Such interactions may serve to support and rigidity the coordination geometry of mononuclear coordination spheres, as well as providing extra thermodynamic and kinetic stabilisation to meet the challenge of hydrolytic stability in these materials. In this perspective, a collection of pyrazole, amine, amide and carboxylic acid containing species are examined through the lens of (primarily) inner-sphere hydrogen bonding. The influence of these interactions is then related to the overall structure, stability and function of these materials, to provide starting points for harnessing these interactions in future materials design.

Enantioselective Mixed Matrix Membranes for Chiral Resolution

Most pharmaceuticals are stereoisomers that each enantiomer shows dramatically different biological activity. Therefore, the production of optically pure chemicals through sustainable and energy-efficient technology is one of the main objectives in the pharmaceutical industry. Membrane-based separation is a continuous process performed on a large scale that uses far less energy than the conventional thermal separation process. Enantioselective polymer membranes have been developed for chiral resolution of pharmaceuticals; however, it is difficult to generate sufficient enantiomeric excess (ee) with conventional polymers. This article describes a chiral resolution strategy using a composite structure of mixed matrix membrane that employs chiral fillers. We discuss several enantioselective fillers, including metal-organic frameworks (MOFs), covalent organic frameworks (COFs), zeolites, porous organic cages (POCs), and their potential use as chiral fillers in mixed matrix membranes. State-of-the-art enantioselective mixed matrix membranes (MMMs) and the future design consideration for highly efficient enantioselective MMMs are discussed.

Homochiral metal-organic frameworks for enantioseparation

Obtaining homochiral compounds is of high importance to human health and environmental sustainability. Currently, enantioseparation is one of the most effective approaches to obtain homochiral compounds. Thanks to their controlled synthesis and high efficiency, homochiral metal-organic frameworks (HMOFs) are one of the most widely studied porous materials to enable enantioseparation. In this review, we discuss the chiral pocket model in depth as the key to unlock enantioselective separation mechanisms in HMOFs. In particular, we classify our discussion of these chiral pockets (also regarded as "molecular traps") into: (a) achiral/chiral linker based helical channels as a result of packing modality; and (b) chiral pores inherited from chiral ligands. Driven by a number of mechanisms of enantioseparation, conceptual advances have been recently made in the design of HMOFs for achieving high enantioseparation performances. Herein, these are systematically categorised and discussed. Further we elucidate various applications of HMOFs as regards enantioseparation, systematically classifying them into their use for purification and related analytical utility according to the reported examples. Last but not the least, we discuss the challenges and perspectives concerning the rational design of HMOFs and their corresponding enantioseparations.