Helical Polymers Showing Inverse Helicity and Synergistic Effect in Chiral Catalysis: Catalytic Functionality Determining Enantioconfiguration and Helical Frameworks Providing Asymmetric Microenvironment

Abstract

This article reports optically active helical polymers showing inverse helicity, which is controlled by copolymer composition. L‐Proline‐derived monomer and an achiral monomer have been copolymerized, affording copolymers with optical activity and remarkable catalytic ability. By adjusting relative content of the two monomers, a series of copolymers with varied optical activity has been obtained. More notably, the handedness of the macromolecular helices inverses at specific monomer feed ratios. Copolymers with opposite helicity have been applied as chiral catalysts for asymmetric aldol reaction to investigate the exact role of helical structures in catalysis. The variation in the stereoselectivity of product has demonstrated an intriguing synergistic effect occurring between the helical macromolecular frameworks and pendent prolineamide groups. The macromolecular helices provide asymmetric microenvironment for the reaction while enantioconfiguration of the product is determined by the specific catalytic functionality. This study opens up new opportunities for manufacturing macromolecules as mimetic enzymes catalyzing asymmetric reactions.

Optically Active Helical Polyacetylene Bearing Ferrocenyl Amino-Acid Derivative in Pendants. Preparation and Application as Chiral Organocatalyst for Asymmetric Aldol Reaction

“…For the corresponding helical isomer of poly(L-MTFc 0.1 -co-M1 0.9 ), namely, poly(D-MTFc 0.1 -co-M1 0.9 ), it gave similar results only with the product showing the opposite chirality (Table 2, run 14). On the basis of both the present investigations and our earlier studies, 48 we propose that ferrocenyl amino acid derivatives in the side chains played key roles in controlling the chiral configuration of the product in the aldol reactions, while the helical main chains played assistant roles. In particular, when the helical polymer chains are in well matching with the pendent catalytic groups in chirality, the asymmetric products may achieve a good enantioselectivity with high ee values.…”

Chirality-Switchable 2,2′-Bipyridine Ligands Attached to Helical Poly(quinoxaline-2,3-diyl)s for Copper-Catalyzed Asymmetric Cyclopropanation of Alkenes

“…However, because of the molecular symmetry, along with the tedious synthesis, fine-tuning of the ligand structure is often difficult. It seems likely that C 1 -symmetrical chiral bipyridine ligands would allow higher flexibility in the molecular design and easier access to a wide variety of ligand structures, although C 1 -symmetrical bipyridine ligands have seldom afforded high enantioselectivities in catalytic asymmetric reactions. − ,, We envisioned that helical macromolecular scaffolds could serve as a platform for incorporation of the 2,2′-bipyridine group as a pendant for use in asymmetric catalysis. , In spite of the C 1 symmetry, the huge helical macromolecular structure is expected to provide an effective chiral reaction environment around the coordinated metal centers. The effect of a helical macromolecular structure on 2,2′-bipyridine/copper-mediated asymmetric reactions has been clearly demonstrated recently; natural DNA was used as a chiral scaffold to which 2,2′-bipyridine was noncovalently incorporated …”

Recent advances in asymmetric organocatalysis based on helical polymers

“…In addition, Deng et al developed two other categories of helical polyacetylenes containing thiourea and proline on the pendants through the copolymerization of a thiourea or proline derived monomer and another monomer without the catalytic moiety. 61,62 They could be used as organocatalysts for catalyzing the asymmetric Michael addition reaction and aldol reaction, respectively. Both of them showed excellent asymmetric catalytic ability.…”