Recent advances of optically active helical polymers as adsorbents and chiral stationary phases for chiral resolution

Chiral resolution is very important and still a big challenge due to different biological activity and same physicochemical property of one pair (R)- and (S)-isomer. There is no doubt that chiral selectors are essentially needed for chiral resolution, which can stereoselectively interact with a pair of isomers. To date, a large amount of optically active helical polymers as chiral selectors have been synthesized via two strategies. First, the target helical polymers are derived from natural polysaccharide such as cellulose and amylose. Second, they can be synthesized by polymerization of chiral monomers. Alternatively, an achiral polymer is prepared first followed by static or dynamic chiral induction. Furthermore, a part of them is harnessed as chiral stationary phases for chromatographic chiral separation and as chiral adsorbents for enantioselective adsorption/crystallization, resulting in good enantioseparation efficiency. In summary, the present review will focus on recent progress of the polymers with optical activity for chiral resolution, especially the literature published in the past 10 years. In addition, development prospects and future challenges of optically active helical polymers will be discussed in detail.

K A. Enantioselective Separation of Amino Acids Using Chiral Polystyrene Microspheres Synthesized by a Post-Polymer Modification Approach

The enantioselective separation of a racemic mixture of amino acids was achieved by chiral amino acid-modified polystyrene (PS) that was developed by a post-polymer modification approach. Styrene was polymerized using the reversible addition-fragmentation chain-transfer (RAFT) polymerization technique and further post-polymer modification was applied by Friedel-Crafts acylation reaction with chiral N-phthaloyl-l-leucine acid chloride to obtain the protected PS-l-Leu. The chiral PS (protected PS-l-Leu) was assembled into microspheres using a surfactant and was used for carrying out the enantioselective separation of amino acid racemic mixtures by enantioselective adsorption followed by a simple filtration process. Compared to as-precipitated chiral PS (protected PS-l-Leu) powder, the protected PS-l-Leu microspheres exhibited a better enantioselective separation efficiency (ee %). Furthermore, the protected PS-l-Leu was deprotected to obtain the amine-functionalized deprotected PS-l-Leu chiral PS, which was also assembled into microspheres and used for carrying out enantioselective separation. Deprotected PS-l-Leu-functionalized chiral PS microspheres could achieve up to 81.6 ee % for the enantioselective separation of a racemic mixture of leucine. This is one of the first reports of the synthesis of amino acid-modified chiral PS microspheres and their application to the simple filtration-based enantioselective separation of native amino acids from their racemic mixtures.

Inversion of Supramolecular Chirality by In Situ Hydrolyzation of Achiral Diethylene Glycol Motifs

Chiral inversion of supramolecular assemblies is of great research interest due to its broad practical applications. However, chiral structure transition induced by in situ regulation of building molecules has remained a challenge. Herein, left-handed fibrous assemblies were constructed by C₂-symmetic l-phenylalanine coupled with diethylene glycol (LPFEG) molecules. In situ hydrolyzing terminal diethylene glycol motifs in LPFEG successfully inverted the chirality of the nanofibers from left- to right-handedness. The transition of right-handed fibers into left-handed fibers could also be achieved via hydrolyzing DPFEG molecules. Circular dichroism (CD) spectroscopy, 1D and 2D nuclear magnetic resonance (NMR) spectroscopy, and Fourier transform infrared (FT-IR) spectroscopy revealed that the back-folded achiral diethylene glycol played a vital role in L/DPFEG molecular arrangements and removing terminal diethylene glycol could induce the opposite rotation of molecular assemblies. Thanks to this merit, the enantioselective separation of racemic phenylalanine was obtained and the enantiomeric excess (ee) values could achieve around ±20% after separation. This study not only provides a new strategy to regulate the chiral structure via dynamic modulation of terminal substituents but also presents a promising application in the field of enantioselective separation.

Temperature-induced solid-to-solid transformation in helical homochiral coordination polymers

The reactions of (R)- and (S)-4-(1-carboxyethoxy)benzoic acid (H₂CBA) with 1,3-bis(2-methyl-1H-imidazol-1-yl)benzene (1,3-BMIB) ligands afforded a pair of homochiral coordination polymers (CPs), namely, poly[[[μ-1,3-bis(2-methyl-1H-imidazol-1-yl)benzene][μ-(S)-4-(1-carboxylatoethoxy)benzoato]zinc(II)] monohydrate], {[Zn(C₁₀H₈O₅)(C₁₄H₁₄N₄)]·H₂O}_n or {[Zn{(S)-CBA}(1,3-BMIB)]·H₂O}_n (1-L), and poly[[[μ-1,3-bis(2-methyl-1H-imidazol-1-yl)benzene][μ-(R)-4-(1-carboxylatoethoxy)benzoato]zinc(II)] monohydrate] (1-D). Three kinds of helical chains exist in compounds 1-D and 1-L, which are constructed from Zn^II atoms, 1,3-BMIB ligands and/or CBA^2- ligands. When the as-synthesized crystals of 1-L and 1-D were further heated in the mother liquor or air, poly[[μ-1,3-bis(2-methyl-1H-imidazol-1-yl)benzene][μ-(S)-4-(1-carboxylatoethoxy)benzoato]zinc(II)], [Zn(C₁₀H₈O₅)(C₁₄H₁₄N₄)]_n or [Zn{(S)-CBA}(1,3-BMIB)]_n (2-L), and poly[[μ-1,3-bis(2-methyl-1H-imidazol-1-yl)benzene][μ-(R)-4-(1-carboxylatoethoxy)benzoato]zinc(II)] (2-D) were obtained, respectively. The single-crystal structure analysis revealed that 2-L and 2-D only contained one type of helical chain formed by Zn^II atoms and 1,3-BMIB and CBA^2- ligands, which indicated that the helical chains were reconstructed though solid-to-solid transformation. This result not only means the realization of helical transformation, but also gives a feasible strategy to build homochiral CPs.

Chiral metal–organic frameworks constructed from four-fold helical chain SBUs for enantioselective recognition of α-hydroxy/amino acids

Three chiral 3D metal–carboxylate frameworks have been successfully synthesized, featuring four-fold helical metal chains as SBUs. Co-MOFs could recognize enantio-selectively α-hydroxy/amino acids by the change of CD signals.

Orientation effect induced selective chelation of Fe2+to a glutamic acid appended conjugated polymer for sensing and live cell imaging

We have demonstrated the first example of highly selective sensing and bioimaging of Fe²⁺in living HeLa cells using a biocompatible glutamic acid appended polyfluoreneviaselective chelation of amine and acid groups with Fe²⁺.