Monatshefte fu ¨r Chemie 138, 1115–1123 (2007) DOI 10.1007/s00706-007-0759-2 Printed in The Netherlands Invited Review Asymmetric Synthesis in Ionic Liquids Attila Paczal and Andra ´s Kotschy  Institute of Chemistry, Eo ¨tvo ¨s Lora ´nd University, Budapest, Hungary Received June 25, 2007; accepted June 28, 2007; published online October 27, 2007 # Springer-Verlag 2007 Summary. In the last decades catalytic enantioselective transformations have became one of the most studied fields in synthesis chemistry. With the advancement of biphasic ca- talysis several of these reactions have been adapted to be run in ionic liquids. The variability of ionic liquids provides the means to fine tune catalytic processes, and even more impor- tantly, the ionic liquids might be used to immobilize the cata- lyst and allow its easy recycling. The present review aims to provide an overview of the state of the art of enantioselective homogeneous catalytic transformations in ionic liquids. Keywords. Metal catalysis; Organocatalysis; Stereoselectivity. Introduction Ionic liquids (ILs) are ionic compounds that have a melting point below 100  C. Most of the commonly used ILs are liquid at room temperature. Ionic liq- uids have a high polarity (usually between aceto- nitrile and methanol) and low (negligible) vapor pressure. These features combined with the fact that most ionic liquids are immiscible with less polar organic solvents led to their burgeoning use as media or co-solvent in catalysis. The importance of this area is highlighted by the increasing number of re- views and books dedicated to the topic [1]. The first example of the use of ILs in biphasic asymmetric catalysis was reported by Chauvin et al. [2] in 1995, while the first examples of their use as media in such transformations appeared five years later [3]. Since then a multitude of reports appeared in literature using several different classes of ionic liquids. The most common ionic liquids in asymmet- ric transformations are the N,N 0 -disubstituted imida- zolium salts (C x C y im, where C x and C y are normal alkyl groups of n carbon atoms). Ammonium, phos- phonium, pyridinium (py), and pyrrolidinium (pyr) salts have also been used, although less frequently. ILs can be chiral in the cation or anion alike. Such ILs offer a unique approach to asymmetric synthesis through the presence of a chiral medium. There are three major classes of catalytic enantio- selective transformations in ILs: biocatalysis, organo- catalysis, and metal catalysis. Due to the limits of this review we only present here an arbitrary selection of representative examples of the two latter classes. Organocatalysis in Ionic Liquids It is not surprising that proline, the grandfather of organocatalysts [4], is one of the first and most fre- quently used compounds that was used as organocat- alyst in ILs. In the aldol reaction of benzaldehyde derivatives and acetone (Scheme 1) interestingly the nature of the IL used was found to have a significant influence on the chemoselectivity of the process. The most efficient media was [C 4 C 1 im][PF 6 ], in which enantioselectivities were acceptable (58–82%) and the catalyst could be recycled several times without apparent degradation of the ee [5]. The use of immo- bilized ILs in the same transformation led to a mod- erate decrease of the efficiency [6]. Extension of this  Corresponding author. E-mail: kotschy@elte.hu