Asymmetric Supramolecular Primary Amine Catalysis in Aqueous Buffer: Connections of Selective Recognition and Asymmetric Catalysis

A new approach of asymmetric supramolecular catalysis has been developed by combining the supramolecular recognition of beta-cyclodextrin (beta-CD) and the superior property of a chiral primary amine catalyst. The resulted beta-CD enamine catalysts could effectively promote asymmetric direct aldol reactions with excellent enantioselectivity in an aqueous buffer solution (pH = 4.80). The identified optimal catalyst CD-1 shows interesting characteristics of supramolecular catalysis with selective recognition of aldol acceptors and donors. A detailed mechanistic investigation on such supramolecular catalysis was conducted with the aid of NMR, fluorescence, circular dichroism, and ESI-MS analysis. It is revealed that the reaction is initialized first by binding substrates into the cyclodextrin cavity via a synergistic action of hydrophobic interaction and noncovalent interaction with the CD-1 side chain. A rate-limiting enamine forming step is then involved which is followed by the product-generating C-C bond formation. A subsequent product release from the cavity completes the catalytic cycle. The possible connections between molecular recognition and asymmetric catalysis as well as their relevance to enamine catalysis in both natural enzymes and organocatalysts are discussed based on rational analysis.