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Abstract:
Usually, it is expected that a chiral solvent can discriminate two enantiomers by creating some weak interactions between the solvent and the substrate molecules forming diastereomers which have different physical properties. This may lead to asymmetry in the solubility phase diagrams and can be employed for resolution purposes [1]. Based on these expectations much work has been done in the past in this field with limited outcome. However, recently Gausepohl et al. reported a first example of an enantioselective asymmetric synthesis using a chiral ionic liquid as a reaction medium [2].
In this work, we will report results obtained by studying the potential of chiral solvents in enantioselective crystallization on the example of two chiral systems. The selected systems are the compound forming system mandelic acid and the conglomerate forming system N-methylephedrine. In the case of mandelic acid, we performed a comprehensive resolution study based on the preliminary metastable zone width results for primary nucleation, which show that the (R)-mandelic acid is inhibited with the (S)-ethyl lactate as chiral solvent in the measured range.
For N-methylephedrine the effect of the chiral solvent on the ternary solubility phase diagram, on the metastable zone width and on a possible resolution with chiral solvents will be outlined.
Further, first results concerning the application of a chiral ionic liquid as an example of a non-classical chiral solvent will be shown for selective crystallization of enantiomers.
[1] J. Jacques, A. Collet, S. H. Wilen, Enantiomers, Racemates, and Resolutions, Krieger Publishing, Florida, 1994.
[2] R. Gausepohl, P. Buskens, J. Kleinen, A. Bruckmann, C. W. Lehmann, J. Klankermayer, W. Leitner, Highly Enantioselective Aza-Baylis-Hillman Reaction in a Chiral Reaction Medium, Angew. Chem. Int. Ed., 2006, 45, 3689-3692.
