Inspired by the high efficiency and specificity of enzymes in living systems, the development of artificial catalysts intrinsic to the key features of enzyme has emerged as an active field. Recent advances in supramolecular chemistry have shown that supramolecular coordination cages, built from non-covalent coordination bonds, offer a diverse platform for enzyme mimics. Their inherent confined cavity, analogous to the binding pocket of an enzyme, and the facile tunability of building blocks are essential for substrate recognition, transition-state stabilization, and product release. In particular, the combination of chirality with supramolecular coordination cages will undoubtedly create an asymmetric microenvironment for promoting enantioselective transformation, thus providing not only a way to make synthetically useful asymmetric catalysts, but also a model to gain a better understanding for the fundamental principles of enzymatic catalysis in a chiral environment. The focus here is on recent progress of supramolecular coordination cages for asymmetric catalysis, and based on how supramolecular coordination cages function as reaction vessels, three approaches have been demonstrated. The aim of this review is to offer researchers general guidance and insight into the rational design of sophisticated cage containers for asymmetric catalysis.
Physical separation of C 2 H 2 from CO 2 on metal−organic frameworks (MOFs) has received substantial research interest due to the advantages of simplicity, security, and energy efficiency. However, that C 2 H 2 and CO 2 exhibit very close physical properties makes their separation exceptionally challenging. Previous work appeared to mostly focused on introducing open metal sites that aims to enhance the C 2 H 2 affinity at desired sites, whereas the reticular manipulation of organic components has rarely been investigated. In this work, by reticulating preselected amino and hydroxy functionalities into isostructural ultramicroporous chiral MOFs Ni 2 (L-asp) 2 (bpy) (MOF-NH 2 ) and Ni 2 (L-mal) 2 (bpy) (MOF-OH)we targeted efficient C 2 H 2 uptake and C 2 H 2 /CO 2 separation, which outperforms most benchmark materials. Explicitly, MOF-OH adsorbs substantial amount of C 2 H 2 with record storage density of 0.81 g mL −1 at ambient conditions, which even exceeds the solid density of C 2 H 2 at 189 K. In addition, MOF-OH gave IAST selectivity of 25 toward equimolar mixture of C 2 H 2 /CO 2 , which is nearly twice higher than that of MOF-NH 2 . Notably, the adsorption enthalpies for C 2 H 2 at zero converge in both MOFs are remarkably low (17.5 kJ mol −1 for MOF-OH and 16.7 kJ mol −1 for MOF-NH 2 ), which to our knowledge are the lowest among efficient rigid C 2 H 2 sorbents. The efficiencies of both MOFs for the separation of C 2 H 2 /CO 2 are validated by multicycle breakthrough experiments. DFT calculations provide molecular-level insight over the adsorption/ separation mechanism. Moreover, MOF-OH can survive in boiling water for at least 1 week and can be easily scaled up to kilograms eco-friendly and economically, which is very crucial for potential industrial implementation.
3D covalent organic frameworks (COFs) with well‐defined porous channels are shown to be capable of inducing chiral molecular catalysts from non‐enantioselective to highly enantioselective in catalyzing organic transformations. By condensations of a tetrahedral tetraamine and two linear dialdehydes derived from enantiopure 1,1′‐binaphthol (BINOL), two chiral 3D COFs with a 9‐fold or 11‐fold interpenetrated diamondoid framework are prepared. Enhanced Brønsted acidity was observed for the chiral BINOL units that are uniformly distributed within the tubular channels compared to the non‐immobilized acids. This facilitates the Brønsted acid catalysis of cyclocondensation of aldehydes and anthranilamides to produce 2,3‐dihydroquinazolinones. DFT calculations show the COF catalyst provides preferential secondary interactions between the substrate and framework to induce enantioselectivities that are not achievable in homogeneous systems.
Natural transport channels (or carriers), such as aquaporins, are a distinct type of biomacromolecule capable of highly effective transmembrane transport of water or ions. Such behavior is routine for biology but has proved difficult to achieve in synthetic systems. Perhaps most significantly, the enantioselective transmembrane transport of biomolecules is an especially challenging problem both for chemists and for natural systems. Herein, a group of homochiral zirconium metal−organic cages with four triangular opening windows have been proposed as artificial biomolecular channels for enantioselective transmembrane transport of natural amino acids. These structurally well-defined coordination cages are assembled from six synthetically accessible BINOL-derived chiral ligands as spacers and four n-Bu 3 -Cp 3 Zr 3 clusters as vertices, forming tetrahedral-shaped architectures that feature an intrinsically chiral cavity decorated with an array of specifically positioned binding sites mediated from phenol to phenyl ether to crown ether groups. Fascinatingly, the transformation of single-molecule chirality to global supramolecular chirality within the space-restricted chiral microenvironments accompanies unprecedented chiral amplification, leading to the enantiospecific recognition of amino acids. By virtue of the highly structural stability and excellent biocompatibility, the orientation-independent cages can be molecularly embedded into lipid membranes, biomimetically serving as single-molecular chiral channels for polar-residue amino acids, with the properties that cage-1 featuring hydroxyl groups preferentially transports the L-asparagine, whereas cage-2 attaching crown ether groups spontaneously favor transporting D-arginine. We therefore develop a new type of self-assembled system that can potentially mimic the functions of transmembrane proteins in nature, which is a realistic candidate for further biomedical applications.
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