Abstract
A significant improvement of the chemical yields (up to 88%), stereoselectivity (> 99:1) and enantiomeric excesses (up to 98%) of (L)-proline catalyzed direct asymmetric aldol reaction was found when proline based chiral ionic liquids (CILs) were added as additives. Different ratios of DMSO/H2O as solvent and chiral ionic liquids (CILs) with chiral cations of different chain length were investigated
Highlights
Since the first example of chiral ionic liquid reported by Howarth et al.[3] in 1997, a large number of chiral ionic liquids (CILs) bearing chiral cations, anions or seldom both were reported by other groups.[4,5]
A novel asymmetric catalytic system with N-substituted pyrrolidine-ionic liquids as chiral additives was developed and their potentials of providing chiral environment by direct asymmetric aldol reaction were demonstrated. This catalytic system can lead to high yields, good stereoselectivities (> 99:1) and superior enantiomeric excesses for syn-selective aldol reactions
This is the first use of chiral environment as chiral source to lead asymmetric reaction to our knowledge
Summary
Room temperature ionic liquids (RTILs) including chiral ionic liquids (CILs) have received growing attentions both from theoretical research and actual productions due to their tuneable features for various chemical tasks and their advantages as homogeneous support, reaction media, etc.[1,2] Since the first example of chiral ionic liquid reported by Howarth et al.[3] in 1997, a large number of CILs bearing chiral cations, anions or seldom both were reported by other groups.[4,5] Previous studies have shown that in many asymmetric reactions the use of CILs as reaction media or catalyst can enhance the yield and/or selectivity significantly.[6,7,8] As most of the asymmetric reactions were carried out using metal/chiral catalysts as chiral sources, which are relatively expensive or environmental unfriendly, searching for a catalyst which is easy to obtain and environmentally benign for the asymmetric reactions is of great significance. Synthesis of CILs. of (L)-proline even with a slight amount the reactions proceeded at room temperature and gave the aldol product. Using different aromatic aldehyde in the (L)proline catalyzed aldol reaction, adding 2b as additive and as well good yield and selectivity, the enantiomeric excesses can have up to 24% enhancement.
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