Defect-Engineered Chiral Metal-Organic Frameworks for Efficient Asymmetric Aldol Reaction

Defect-engineered chiral metal-organic frameworks

Chirality has an important impact on chemical and biological research, as most active substances are chiral. In recent decades, metal-organic frameworks (MOFs), which are assembled from metal ions or clusters and organic linkers via metal-ligand bonding, have attracted considerable scientific interest due to their high crystallinity, exceptional porosity and tunable pore sizes, high modularity, and diverse functionalities. Since the discovery of the first functional chiral metal-organic frameworks (CMOFs), CMOFs have been involved in a variety of disciplines such as chemistry, physics, optics, medicine, and pharmacology. The introduction of defect engineering theory into CMOFs allows the construction of a class of defective CMOFs with high hydrothermal stability and multi-stage pore structure. The introduction of defects not only increases the active sites but also enlarges the pore sizes of the materials, which improves chiral recognition, separation, and catalytic reactions, and has been widely investigated in various fields. This review describes the design and synthesis of various defective CMOFs, their characterization, and applications. Finally, the development of the materials is summarized, and an outlook is given. This review should provide researchers with an insight into the design and study of complex defective CMOFs.

Metal-Organic Frameworks as Formose Reaction Catalysts with Enhanced Selectivity

The formose reaction is an autocatalytic series of aldol condensations that allows one to obtain monosaccharides from formaldehyde. The formose reaction suffers from a lack of selectivity, which hinders practical applications at the industrial level. Over the years, many attempts have been made to overcome this selectivity issue, with modest results. Heterogeneous porous catalysts with acid-base properties, such as Metal-Organic Frameworks (MOFs), can offer advantages compared to homogeneous strong bases (e.g., calcium hydroxide) for increasing the selectivity of this important reaction. For the very first time, four different Zeolite Imidazolate Frameworks are presented in this work as catalysts for the formose reaction in liquid phase, and their catalytic performances were compared with those of the typical homogeneous catalyst (i.e., calcium hydroxide). The heterogeneous nature of the catalysis, the possible contribution of leached metal or linkers to the solution, and the stability of the materials were investigated. The porous structure of these solids and their mild basicity make them suitable for obtaining enhanced selectivity at 30% formaldehyde conversion. Most of the MOFs tested showed low structural stability under reaction conditions, thereby indicating the need to search for new MOF families with higher robustness. However, this important result opens the path for future research on porous heterogeneous basic catalysts for the formose reaction.

N-Heterocyclic Carbene Silver Complex Modified Polyacrylonitrile Fiber/MIL-101(Cr) Composite as Efficient Chiral Catalyst for Three-Component Coupling Reaction

Complex asymmetric synthesis can be realized by the chiral induction of amino acids in nature. It is of great significance to design a new biomimetic catalytic system for asymmetric synthesis. In this context, we report the preparation and characterization of the composite of polyacrylonitrile fiber (PANF) and metal-organic framework to catalyze the chiral synthesis of propargylamines. A confined microenvironment is established with N-heterocyclic carbene (NHC) silver complex-supported PANF and D-proline-encapsulated MIL-101(Cr). This novel supported catalyst demonstrated high activity in addition to excellent stereoselectivity in the three-component reaction between alkynes, aldehydes, and amines (A3). The regeneration can be realized by adsorption of D-proline again when the stereoselectivity decreases after recycle uses. By regulating the confined microenvironment on the composite, the activity and selectivity of the catalytic system are improved with turnover numbers of up to 2800 and 98% ee. The biomimetic catalytic system to A3 coupling reaction is systematically studied, and the synergistic catalytic mechanism between NHC-Ag and D-proline in the confined microenvironment is revealed.

Creating Tunable Mesoporosity by Temperature-Driven Localized Crystallite Agglomeration

A new synthetic approach for tunable mesoporous metal-organic frameworks (MeMs) is developed. In this approach, mesopores are created in the process of heat conversion of highly mosaic metal-organic framework (MOF) crystals with non-interpenetrated low-density nanocrystallites into MOF crystals with two-fold interpenetrated high-density nanocrystallites. The two-fold interpenetration reduces the volume of the nanocrystallites in the mosaic crystal, and the accompanying localized agglomeration of the nanocrystallites results in the formation of mesopores among the localized crystallite agglomerates. The pore size can be easily modulated from 7 to 90 nm by controlling the heat treatment conditions, that is, the aging temperature and aging time. Various proteins can be encapsulated in the MeM, and immobilized enzymes show catalyst activity comparable to that of the free native enzymes. Immobilized β-galactosidase is recyclable and the enzyme activity of the immobilized catalase is maintained after exposure to high temperatures and various organic solvents.