Asymmetric Phase Transfer Catalysis with Chiral Crown Ethers

Asymmetric phase transfer catalysis exploits chiral crown ethers as molecular shuttles to transfer ionic reactants between immiscible phases, enabling the enantioselective construction of carbon–carbon and carbon–heteroatom bonds under mild conditions. The macrocyclic architecture of crown ethers, often endowed with chiral backbones derived from sugars or hydrobenzoin units, provides a stereochemically defined cavity that binds cationic counter-ions and their associated nucleophiles. Upon complexation in the aqueous phase, the chiral host transports the nucleophile into the organic phase, where it engages with an electrophile to deliver chiral products with high enantiomeric excess. Systematic variations in ring size, heteroatom incorporation, lariat-type side arms and saccharide stereochemistry have been shown to fine-tune binding strength, ion-pair geometry and chiral induction. Applications span asymmetric alkylations, Michael additions, epoxidations, Darzens condensations and cyclopropanations, furnishing valuable enantioenriched intermediates for pharmaceuticals and agrochemicals. Recent strategies to anchor crown ethers on polymer supports have enhanced catalyst recovery and reuse, while studies of rare monosaccharide frameworks have expanded the structural toolkit and deepened mechanistic insight into ion-pair dynamics. Together, these advances underscore the global significance of chiral crown-ether catalysis as a sustainable route to enantioenriched compounds without reliance on stoichiometric chiral auxiliaries.

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Asymmetric Phase Transfer Catalysis with Chiral Crown Ethers publication trend

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Technical terms

Asymmetric phase transfer catalysis: A method in which a chiral catalyst ferries ionic reactants between two immiscible phases to induce enantioselective reactions.

Chiral crown ethers: Macrocyclic compounds with ether linkages and a chiral framework that selectively binds cationic species and their accompanying anions.

Lariat ether: A crown ether bearing an additional side arm that enhances binding specificity and chiral induction.

Enantiomeric excess (ee): The percentage difference between two enantiomers in a chiral mixture, indicating product purity in asymmetric synthesis.

Biphasic system: A reaction medium composed of two immiscible solvents, typically aqueous and organic, in which phase transfer catalysts operate.

References

1. Reusable Glucose-Based Crown Ethers Anchored to PVC. Molecules (2023).
2. Carbohydrate-Based Azacrown Ethers in Asymmetric Syntheses. Chemistry (2021).
3. d-Idose-Based Monoaza-15-Crown-5 Lariat Ethers: Synthesis of an Elusive d-Hexose and Application of Derived Macrocycles in Enantioselective Syntheses. Symmetry (2023).