Induced Chirality in a Metal–Organic Framework by Postsynthetic Modification for Highly Selective Asymmetric Aldol Reactions

Abstract

A straightforward synthetic route to chiral metal-organic frameworks is proposed that relies on an acid-base interaction between an acid linker and a chiral primary amino acid derived diamine organocatalyst. High ee values for the aldol condensation of linear ketones and aromatic aldehydes are reported with this heterogeneous catalyst. Three consecutive catalyst reuse experiments demonstrated that the majority of the activity was preserved, as was the enantioselectivity.Although organocatalysis with amino acids and their derivatives has proven very successful towards highly enantioselective condensation reactions, there is an urgent need to immobilize these elegant and bioinspired chiral catalysis for easy reuse. Many immobilization protocols that employ different organic and inorganic support materials have been discussed in the literature, and most of them follow a covalent anchoring strategy, sometimes with the prospect of using them in a flow setup. [1] Metal-organic frameworks (MOFs) [2] are among the most promising supporting materials: distinct from traditional inorganic materials, MOFs can be synthesized from a large variety of building blocks. In spite of the large degree of unpredictability in synthesis, [3] thousands of MOF structures have been reported to date. [4] Different strategies have been employed to prepare homochiral MOFs, including chiral-template and direct synthesis methods, either starting with enantiopure ligands or through self-resolution from achiral or racemic molecules. [5] The current methods for the synthesis of homochiral MOFs have shown limited flexibility: on the one hand, if the organocatalyst is used as a linker for MOF synthesis, catalysts with limited stability are formed owing to linker dimensions that are usually large. On the other hand, the templated approach is limited by the stability of the organocatalyst. [6] In this work, we present a straightforward postfunctionalization based on the noncovalent immobilization of a phenylalanine-derived chiral ligand on a sulfated achiral MOF derivative through acid-base pair interaction. The presence of the organocatalyst in the MOF structure was confirmed with N 2 sorption measurements, FTIR spectroscopy, and thermal gravimetrical analyses. The morphology and framework of the solid chiral catalyst was characterized by SEM and X-ray diffraction, respectively. Very high enantioselectivities were obtained in the direct asymmetric aldol reactions of various linear ketones with aromatic aldehydes. The presented postsynthetic modification strategy is very practical, straightforward, and flexible towards the synthesis of a variety of chiral MOF structures with very high stability.Postsynthetic modification [7] of a preassembled achiral MOF offers the advantage that a single, stable parental framework can be converted into an active chiral MOF by carefully selecting the suitable catalytic chiral units. In 2009, Banerjee et al. reported the synthesis of a chiral MOF by the attachment of l-proline-derived chiral ligands to the open metal...

Enantioselective Hydrogenation of Olefins Enhanced by Metal–Organic Framework Additives

“…There are several asymmetric catalysts based on homochiral MOFs, but their expensive synthesis combined with their catalytic performance, which is often lower than that obtained with analogous homogeneous catalysts, have hampered their practical use. Alternatively, an achiral MOF can be functionalized with a chiral molecule, which is a modular and cost‐efficient solution also employed in this work. Given that MOFs feature a high surface area and big pores, they can be used as additives in catalysis to adsorb big molecules .…”

The Application of Covalent Organic Frameworks for Chiral Chemistry

“…CCOFs prepared in this way are only constructed via achiral blocks, but the spatial organization of them endowed frameworks with chirality property. This method is mature in the synthesis of chiral MOFs, [ 35 ] but there was only one report on the synthesis of CCOFs published by Cui's group in 2018. [ 36 ] The researchers prepared a series of imine‐linked CCOFs in the presence of ( R )‐ or ( S )‐1‐phenylethylamine as chiral inducers.…”

Simple and Robust Immobilization of a Ruthenium Olefin Metathesis Catalyst Inside MOFs by Acid–Base Reaction

“…Surprisingly, however, there are very few examples of direct, noncovalent immobilization of discrete catalysts inside MOFs. Positively charged metal complexes were successfully incorporated into negatively charged frameworks by ion exchange. , Basic organocatalysts were immobilized in acidic MOFs by acid–base reactions. , Recently, we have also shown that cationic olefin metathesis (OM) catalysts can be easily immobilized inside (Al)­MIL-101-NH 2 and (Al)­MIL-101 by simple physisorption from solution. Although in these two cases the catalysts were held inside the MOFs by only noncovalent forces, no leaching was observed even in relatively polar solvents, such as dichloromethane (DCM) and ethyl acetate (EA).…”

“…The heterogenized catalysts thus obtained can be used even in very polar “green” solvents, such as i -PrOH and dimethyl carbonate (DMC), , without any appreciable leaching of Ru into the solution. Although this straightforward acid–base strategy has been used for the immobilization of various catalysts on conventional supports, − to the best of our knowledge the only precedents for a similar approach in MOFs are limited to the two organocatalysts mentioned above. , An alternative approach toward robust immobilization of OM catalysts for use in “green” solvents is presented in an accompanying paper by Grela and co-workers …”

“…3,4 Basic organocatalysts were immobilized in acidic MOFs by acid−base reactions. 5,6 Recently, we have also shown that cationic olefin metathesis (OM) catalysts can be easily immobilized inside (Al)MIL-101-NH 2 7 and (Al)MIL-101 8 by simple physisorption from solution. Although in these two cases the catalysts were held inside the MOFs by only noncovalent forces, no leaching was observed even in relatively polar solvents, such as dichloromethane (DCM) and ethyl acetate (EA).…”