Proline Functionalization of the Mesoporous Metal−Organic Framework DUT-32

Abstract

The linker functionalization strategy was applied to incorporate proline moieties into a metal−organic framework (MOF). When 4,4′-biphenyldicarboxylic acid was replaced with a Boc-protected proline-functionalized linker (H 2 L) in the synthesis of DUT-32 (DUT = Dresden University of Technology), a highly porous enantiomerically pure MOF (DUT-32-NHProBoc) was obtained, as could be confirmed by enantioselective high-performance liquid chromatography (HPLC) measurements and solid-state NMR experiments. Isotope labeling of the chiral side group proline enabled highly sensitive one-and two-dimensional solid-state 13 C NMR experiments. For samples loaded with (S)-1-phenyl-2,2,2-trifluoroethanol [(S)-TFPE], the proline groups are shown to exhibit a lower mobility than that for (R)-TFPE-loaded samples. This indicates a preferred interaction of the shift agent (S)-TFPE with the chiral moieties. The high porosity of the compound is reflected by an exceptionally high ethyl cinnamate adsorption capacity. However, postsynthetic thermal deprotection of Boc−proline in the MOF leads to racemization of the chiral center, which was verified by stereoselective HPLC experiments and asymmetric catalysis of aldol addition. ■ INTRODUCTIONIt has been widely accepted that enantiomers of the same chiral compound may show quite different bioactivities, as well as different physical, chemical, pharmacological, and toxicological properties. Therefore, their preparation and especially analysis are important in many fields of science. To obtain a chiral heterogeneous selector/catalyst, usually the porous substrates are coated or immobilized with proteins, enzymes, oligosaccharides, or polysaccharides. Because important progress has been made in the field of porous materials through the development of metal−organic frameworks (MOFs), this material class is also considered to be very promising for the design and development of a new generation of chiral porous materials. The incorporation of chiral groups by ligand or cluster functionalization or the synthesis of chiral MOFs from achiral building blocks generates homochiral materials. Inspired by homogeneous chiral catalysis, the implementation of wellestablished chiral catalytically active groups as linkers or as side groups of the framework backbone has been reported for a variety of MOFs. 1−5 One common organocatalytic group for a wide range of asymmetric reactions is proline. Up to now, there are only a few homochiral MOFs with proline functionalities reported. So far, the incorporation of proline could be performed by coordination on open metal sites in MIL-101, 6 postsynthetic click reaction, 7 or postsynthetic amide coupling. 8 Furthermore, the incorporation of proline by chiral functionalization of the linker before MOF synthesis was carried out by Telfer et al. 9 The reported homochiral material could be used as heterogeneous chiral catalysts in asymmetric aldol reactions. The important point in the synthesis of enantiopure (prolinecontaining) MOFs is to avoid the racemization that can ...

Synthesis of Chiral MOF‐74 Frameworks by Post‐Synthetic Modification by Using an Amino Acid

“…Thisw as not expected due to the mild synthesis conditions used and racemisation of l-Pro upon PSMw as previously reported only for the post-synthetic thermal deprotection of DUT-32-NHPro. [60] The other resonances for l-Pro are also split with the C2 signal at 62.0 and 60.5 ppm, C5 at 50.2 and 48.1 ppm, and C3 and C4 at 37.9, 33.7, 31.4, 29.6 and 28.7 ppm.…”

Mesoporous Metal–Organic Frameworks: Synthetic Strategies and Emerging Applications

“…This one‐fell‐swoop assembly of primitive ligands and their fragments not only introduces functional groups in originally microporous MOFs, but also functionalized cavities, bringing considerable improvement to the mass transport of the resulting hierarchical micro‐ and mesoporous MOFs. Kutzscher et al reported the proline functionalization of the linker of mesoporous DUT‐32 (DUT refers to Dresden University of Technology), and the product exhibits the high liquid‐phase adsorption capacity with ethyl cinnamate . Liu et al proposed strain‐promoted “click” chemistry, which can be used to introduce different functional groups into the mesopores of MOFs.…”

Porous aromatic framework (PAF-1) as hyperstable platform for enantioselective organocatalysis

“…Among the applications, immobilization of homogeneous catalysts, especially chiral catalysts, on cavity surfaces of the solid porous materials is of great importance. In this field, the latest developments involved enantioselective catalysis based on metal-organic frameworks (MOFs) [1][2][3][4][5][6][7][8][9][10][11][12][13][14], covalent organic frameworks (COFs) [15][16][17][18][19][20][21][22][23][24] and porous organic polymers (POPs) [25][26][27][28][29][30][31][32]. Many advantages of MOFs, COFs and POPs for heterogeneous enantioselective catalysis have been elaborated by scientists.…”

“…Meanwhile, owing to the appropriate size as well as the easy preparation, the proline-type organocatalysts are one of the best candidates to investigate if a new solid material could be used as the platform for enantioselective catalysis. On the other hand, the Aldol reaction is an important reaction which is frequentlyused to investigate the catalytic activity of the catalyst supported by porous materials such as MOFs, COFs and POPs [1][2][3][4][5][6][7][8][9]23,24,32]. Thus in this paper we make an attempt to introduce the chiral catalytically active L-prolinamide unit into the PAF-1 material by stepwise post-synthetic modifications and investigate the catalytic performance of the obtained material for catalyzing the Aldol reaction.…”