Asymmetric Catalysis in Chiral Solvents: Chirality Transfer with Amplification of Homochirality through a Helical Macromolecular Scaffold

Abstract

Use of chiral solvents in asymmetric synthesis as a sole source of enantioselection remains largely unexplored in organic synthesis. We found that the use of a helical macromolecular catalyst of which helical chirality is dynamically formed in chiral solvents allowed several mechanistically different reactions to proceed with high enantioselectivity. In this system, the chirality of the solvent, such as limonene, induces a configurational imbalance to the helical macromolecular scaffold of the catalyst, and in turn to the reaction products through palladium-catalyzed asymmetric reactions including Suzuki-Miyaura cross-coupling (up to 98% ee), styrene hydrosilylation (up to 95% ee), and silaboration (up to 89% ee). Not only enantiomerically pure limonene but also limonene with low enantiomeric excesses induce single-handed helical structures with majority-rule-based amplification of homochirality. The helical conformation of the macromolecular catalyst was retained even in the absence of limonene in the solid state, enabling asymmetric cross-coupling in achiral solvent with high enantioselectivity.

Helicity Induction and Its Static Memory of Poly(biphenylylacetylene)s Bearing Pyridine N‐Oxide Groups and Their Use as Asymmetric Organocatalysts

“…Further modification of the pendant groups with catalytically active, but achiral units, produces functional helical polyisocyanides while maintaining the helicity memory, which catalyzes the asymmetric direct aldol reaction, although its enantioselectivity was rather low, providing the first polymer‐based asymmetric catalysts based on the helicity memory . This “helicity induction and memory” concept has recently been applied to dynamic helical poly(quinoxaline‐2,3‐diyl)s to develop powerful asymmetric catalysts by Nagata et al …”

“…Further modification of the pendant groups with catalytically active, but achiral units, produces functional helical polyisocyanides while maintaining the helicity memory, which catalyzes the asymmetric direct aldol reaction, although its enantioselectivity was rather low, providing the first polymerbased asymmetric catalysts based on the helicity memory. 39 This "helicity induction and memory" concept has recently been applied to dynamic helical poly(quinoxaline-2,3-diyl)s to develop powerful asymmetric catalysts by Nagata et al 49 Recently, we also found unique helical polymers, poly(biphenylylacetylene)s (PBAs), such as poly-A (Chart 1), to which the "helicity induction and memory" principle can be applied. 13,50 In fact, poly-A formed a preferred-handed helical conformation through noncovalent chiral interactions with the optically active 1-phenylethanol ((R)-1 and (S)-1) both in solution and the solid state, and the macromolecular helicity induced in the PBA backbones could be memorized after complete removal of the optically active alcohols.…”

Chirality Induction through Nano‐Phase Separation: Alternating Network Gyroid Phase by Thermotropic Self‐Assembly of X‐Shaped Bolapolyphiles

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The single gyroid phase as well as the alternating double network gyroid, composed of two alternating single gyroid networks,hold asignificant place in ordered nanoscale morphologies for their potential applications as photonic crystals,m etamaterials and templates for porous ceramics and metals.Here,wereport the first alternating network cubic liquid crystals.T hey form through self-assembly of X-shaped polyphiles,w here glycerol-capped terphenyl rods lie on the gyroid surface while semiperfluorinated and aliphatic sidechains fill their respective separate channel networks.This new self-assembly mode can be considered as at wo-color symmetry-broken double gyroid morphology,p roviding at ailored way to fabricate novel chiral structures with sub-10 nm periodicities using achiral compounds.New routes to chirality from initially achiral systems are of particular contemporary interest for obtaining chiral templates in asymmetric synthesis and catalysis. [1] This is important for the use in different fields of material-and nanoscience [2] as well as for the understanding of fundamental principles of the emergence of biological homochirality. [3] Creating chirality in liquids and liquid crystals (LCs), having no fixed positions of individual molecules,i se specially challenging.

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Dynamically Chiral Helical Polymers: A New Frontier in Asymmetric Catalysis?

“…In the never-ending search for new methods and concepts to effect asymmetric synthesis, Suginome et al have introduced a novel approach that leverages the cumulative effect of small perturbations provided by chiral solvents to induce a helical conformation in a polymer that subsequently acts as an enantioselective catalyst for a number of useful transformations. 1 …”

“…This observation, together with the known influence of chiral solvents on polymer helicity, 7 inspired the work described in this publication. 1 …”