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[摘要]：Chiral silylated pyrrolidine catalysts are obtained in high yield and enantioselectivity by sparteine-mediated lithiation of N-Boc-pyrrolidine and addition to silyl fluoride electrophiles. The activity and enantioselectivity of a new tert-butyldiphenylsilyipyrrolidine catalyst has been demonstrated for various asymmetric Michael reactions at 5 mol % catalyst loading and affords up to 99% ee for asymmetric Michael reactions with aldehydes and nitro-olefins. Acetaldehyde donors proceed with yields up to 77% and enantioselectivities up to 96% ee, avoiding common side reactions that often lower yields. Insight into the mechanism of pyrrolidine-based catalysts is provided by demonstrating ESI mass spectrometry evidence for activation of a nitro acceptor by formation of a hydrogen-bonding adduct with the catalyst amine. Analysis of reaction intermediates using mass spectrometry provides evidence that the pyrrolidine catalyst also plays a role in activating nitro-olefins through hydrogen-bonding. |
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