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-butyldiphenylsilylpyrrolidine 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.

Silyl Fluoride Electrophiles for the Enantioselective Synthesis of Silylated Pyrrolidine Catalysts. -(-)-Sparteine-mediated treatment of the pyrrole (I) with the silyl fluorides (II) allows efficient and highly stereoselective access to the products (III). The latter can smoothly be converted into enantiopure pyrroles by recrystallization or trituration. The derivative (IVa) and the free amine (SIL) efficiently promote the asymmetric Michael addition of aldehydes to nitrostyrenes. -(JENTZSCH, K. I.; MIN, T.; ETCHESON, J. I.; FETTINGER, J. C.; FRANZ*, A. K.; J. Org. Chem. 76 (2011) 17, 7065-7075, http://dx.doi.org/10.1021/jo200991q ; Dep. Chem., Univ. Calif., Davis, CA 95616, USA; Eng.) -Jannicke 01-158