Metal-Organic Frameworks for Asymmetric Catalysis and Chiral Separations
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Frameworks for Asymmetric Catalysis and Chiral Separations Wenbin Lin
Abstract Metal-organic frameworks (MOFs) are an interesting class of molecule-based hybrid materials built from metal-connecting points and bridging ligands. MOFs have received much attention, owing to their potential impact on many technological areas, including gas storage, separation, and heterogeneous catalysis. The modular nature of MOFs endows them with facile tunability, and as a result, properly designed MOFs can yield ideal heterogeneous catalysts with uniform active sites through judicious choice of the building blocks. Homochiral MOFs, which can be prepared by numerous approaches (construction from achiral components by seeding with a chiral single crystal, templating with coordinating chiral co-ligands, and building from metal-connecting nodes and chiral bridging ligands), represent a unique class of materials for the economical production of optically pure compounds, whether through asymmetric catalysis or enantioselective inclusion of chiral guest molecules in their porous frameworks. As such, homochiral MOFs promise new opportunities for developing chirotechnology. This contribution provides a brief overview of recent progress in the synthesis of homochiral porous MOFs and their applications in asymmetric catalysis and chiral separations.
Introduction Zeolites are an important class of microporous aluminosilicate materials used for the production of a variety of consumer products.1 Crude oil, for example, is refined via size-selective catalytic transformations inside zeolite pores to improve fuel quality and reduce air pollution.2 The presence of cations in zeolites also makes them ideal materials for ion exchange processes. Many molecules exist as a pair of enantiomers, which are nonsuperimposable mirror images of each other—much like one’s left and right hands. Pure enantiomers are termed homochiral, chiral, or optically pure. (The terms chiral and homochiral in this article imply that the bulk sample of a metalorganic framework is optically pure, and both terms are used interchangeably.)
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Because of the importance of chirality in biological processes (and thus pharmaceuticals), significant research efforts have been devoted to developing chiral zeolites and related porous solids with the goals of using them for the production of optically pure fine chemicals via chiral separations and asymmetric catalysis. Chiral zeolites, however, are difficult to obtain in optically pure form, because high-temperature calcination, which is necessary for removal of the surfactant templates to generate the porous architecture of the zeolite, inevitably destroys the chirality of the aluminosilicate–surfactant assemblies.3,4 The resulting achiral or racemic mixtures of zeolites cannot be used in chiral separations and asymmetric catalysis. This has prompted exploration of new strategies
for the preparation of chiral porous solids that may find practical applications in chirotechnology. Porous metal-organic frameworks (MOFs) have received s
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