Introduction

Chirality is the most common feature of the natural world and chiral compounds have crucial and increasing applications in many fields, such as pharmaceutical and agrochemical sciences, fine chemicals, and advanced materials, which great greatly promote the development of asymmetric synthesis. Asymmetric catalysis is one of the most efficient tools to obtain chiral compounds by selectively making and breaking the chemical bonds in an organic transformation using a small amount of asymmetric catalyst. From the economic and ecological point of view, the use of chiral MOFs and chiral COFs as asymmetric catalysts is considered to be a potential eco-friendly method due to their numerous advantages, including mild reaction conditions, high enantiomeric excess, easy purification of the products, and reusability of the catalysts. The most important advantage typical of chiral MOFs and chiral COFs, is their good selectivity for the size and shape of reacting molecules, which can be realized by the different size and topology of pores of the chiral MOFs and chiral COFs.

Synthetic Methods of Chiral MOFs and Chiral COFs

Chiral MOFs and chiral COFs can be built by using numerous distinct strategies, there are now four main ways to access chiral MOFs and chiral COFs^[1].

• Direct synthesis: The chiral porous frameworks can be constructed by using chiral organic monomers.
• Postsynthetic modification: By decorating the previously synthesized achiral frameworks with a variety of chiral auxiliaries, chiral porous framework catalysts can be fabricated.
• Encapsulation: Encapsulation of chiral catalytic species in the cavities of framework materials acquire the chiral porous framework catalysts.
• Chiral induction: Chiral porous frameworks can be constructed absolutely from achiral building blocks through chiral induction by using chiral templates.

Fig.1 The typical synthetic strategies of chiral porous framework catalysts: a) direct synthesis, b) postsynthetic modification, c) encapsulation, and d) chiral induction (the red ball represents chiral molecules)

Application of Chiral MOFs in Asymmetric Catalysis

Numerous chiral ligands, such as 2,2'-bis(diphenylphosphino)-1,1'-binaphthlyl (BINAP), 1,1'-binaphthol (BINOL), metallo-salen, chiral phosphoric acid and organoamine molecules, contributing to the enantioselectivity during asymmetric catalysis, have been incorporated either onto the metal connecting nodes or into the organic linkers of MOFs through direct synthesis or postsynthetic modification. Chiral MOFs can catalyze a variety of asymmetric organic reactions such as Dialkylzinc additions to carbonyl compounds, Diels-Alder reactions, carbonyl-ene and cyclization reactions, hydrogenation reactions, amine addition/cyclization reactions, Friedel-Crafts reactions, Michael-type reactions and 1,2-additions to imines, aldol and nitroaldol reactions, cyanosilylations of carbonyl compounds, cyclopropanations, α-Alkylations of aldehydes, sulfoxidations and many others^[2].

Fig.2 Synthesis of chiral Cd-MOF for the addition of diethylzinc to aromatic aldehydes

Application of Chiral COFs in Asymmetric Catalysis

Although compared to the prosperous COFs-based achiral catalysis, the COF-based asymmetric catalysis has been drawn much less attention. There also are many examples for the asymmetric organic reactions catalyzed by the chiral COFs. For instances, Wang and coworkers reported the synthesis of chiral pyrrolidine-based COFs, denoted as LZU-76, followed by the deprotection of Boc groups, LZU-76 had a superior acid resistance was assessed in the asymmetric aldol reactions of acetone to aromatic aldehydes in the presence of trifluoroacetic acid and it also showed good recyclability and comparable enantioselectivity with ee values up to 88%^[3]. In addition, chiral COFs can catalyze other asymmetric organic reactions such as aldol reaction, α-aminooxylation of aldehydes, Diels-Alder reactions, steglich rearrangement reactions, amination of β-Ketoesters reactions, Henry reactions, coupling reactions, Michael reactions and so on.

Fig.3 Synthesis of LZU-76 for asymmetric aldol reactions

What Can Alfa Chemistry Do

Alfa Chemistry provides various chiral MOFs and chiral COFs with high-activity which can be used in different types of asymmetric organic reactions. And our professional technology team also provide customers with high-quality chiral MOFs and chiral COFs design and customization services, no matter what design ideas you have, we will implement them together with you. In addition, Alfa Chemistry is committed to supporting customers a series of solutions in asymmetric catalysis fields by using MOFs and COFs. Please contact us immediately to order or cooperate in research and development with high quality and reasonable price.