Chiral ionic liquids, starting from (S)-proline, have been prepared and evaluated the ability of a chiral catalyst. In Michael reaction of trans-β-nitrostyrene and cyclohexanone, all the reactions were carried out under homogeneous conditions without any solvent except for excess cyclohexanone. The chiral ionic liquid catalyst with the positive charge delocalized bulky pyrrolidinium cation shows excellent yields (up to 92%), diastereoselectivities (syn/anti = 96/4), and enantioselectivities (up to 95% ee) and could be reused at least three times without any loss of its catalytic activity. Such results demonstrated a promising new approach for green and economic chiral synthesis by using the chiral ionic liquids as a chiral catalyst and a chiral medium. 1. Introduction Ionic liquids have been widely used in organic syntheses as catalysts or as reaction media because they are useful and have environmentally benign chemical and physical properties [1–3]. The high designability of ionic liquids is their most attractive and unique property. Also a vast number of possible anion-cation combinations and the introduction of various functional groups onto the structure of ionic liquids produce diverse so-called task-specific ionic liquids for special purposes [4, 5]. Among many kinds of task-specific ionic liquids, chiral ionic liquids have gained considerable attention not only for chiral synthesis [6] and chiral extraction [7] but also for stationary phases in chiral chromatography [8, 9] and chiral NMR shift reagent [10]. There are several strategies to gain access to chiral ionic liquids. One is the racemic ionic liquids synthesis and its subsequent chiral separation of two enantiomers by means of the chiral column chromatography [11] or by chiral cocrystallization [12]. In either case, the chiral separation from the racemic ionic liquids is complicated task because optimising the chiral separation conditions is quite a difficult and the chiral separation also requires an enormous amount of labour and is costly. Another useful methodology is the enantioselective chiral ionic liquids synthesis that produces the pure enantiomer from the achiral precursor using chiral reagents or chiral catalysts. However the chiral catalysts are often expensive and cannot be applied to all kinds of chiral ionic liquids. In addition, the more economic and simpler method are required for the large-scale synthesis of the chiral ionic liquids than ever because the chiral ionic liquids are expected to use as not only the chiral catalyst but also the chiral medium. Therefore, we
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