Asymmetric Diethyl‐ and Diphenylzinc Additions to Aldehydes by Using a Fluorine‐Containing Chiral Amino Alcohol: A Striking Temperature Effect on the Enantioselectivity, a Minimal Amino Alcohol Loading, and an Efficient Recycling of the Amino Alcohol

Abstract

A chiral pyrrolidinylmethanol derivative containing perfluoro‐ponytails (5) was prepared from (S)‐proline. The use of this perfluoro‐substituted amino alcohol in catalytic asymmetric additions of organozinc reagents to aldehydes affords products with high enantioselectivities in both pure hexane and a mixture of hexane and FC‐72 (perfluorohexane). Enantiomeric excesses up to 94 and 88 % ee have been achieved in Et₂Zn and Ph₂Zn additions, respectively. For the reactions in the biphasic solvent system a striking temperature effect was observed. Thus, when the temperature was raised from 0 to 40 °C the ee value of the product increased from 81 to 92 %. Furthermore, the catalyst loading could be remarkably low, and with only 0.1 mol % of amino alcohol 5 a product with 90 % ee was obtained in the Et₂Zn addition to benzaldehyde in hexane. The perfluoro‐ligand was easily recovered by simple phase separation, and until the ninth repetition its reuse proceeded without significant loss of enantioselectivity and reactivity.