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Abstract:
It is expected that a chiral solvent can induce an asymmetry in the ternary solubility phase diagram of two enantiomers. Despite this concept was considered as a relevant tool since the beginning of the 20th century, only a few studies can be found in the literature [1]. For example, Yamamoto et al. reported solubility differences in the case of a chiral cobalt salt in pure enantiomeric diethyl tartrate [2]. However, no general conclusions can be deduced from the reported studies.
We propose to establish the complete ternary phase diagrams both for compound and conglomerate forming systems, in order to quantify and understand how chiral solvent-solute interactions can affect them. General conclusions regarding the feasibility of chiral resolutions with the help of optically active solvents are intended. Preliminary work involved the investigation of the effect of ethyl lactate as chiral solvent on the enantiomeric mandelic acid system. Up to now, several solubility isotherms in the ternary phase diagram as a function of the enantiomeric excess of the solvent ((S)-ethyl lactate / (R)-ethyl lactate) have been determined. However, no exploitable effect was observed in this system.
In order to detect the impact of the chiral solvent interactions in general, currently we screen a variety of chiral solvents. For this purpose, we consider that the best solvent candidate should be able to recognize the solute chirality by means of selective or specific non-covalent interactions. There is large experience in using chiral solvents as a chemical shift agent in NMR spectroscopy [3]. We use a similar approach in order to detect the existence of specific chiral interactions in solution, which might provide a more suitable situation for modifying the solid-liquid equilibria.
[1] J. Jacques, A. Collet, S. H. Wilen, Enantiomers, Racemates, and Resolutions, Krieger Publishing, Florida, 1994.
[2] M. Yamamoto and Y. Yamamoto, Stereospecific solute-solvent interaction between ∆-(+)D or ∆-(-)D –
Co (en) 3 3+ and L- (+)D –diethyltartrate appeared in solubility and viscosity, Inorg.Nuclear Chem. Lett., 1975, 11, 833-836.
[3] Y. Kobayashi, N. Hayashi, C. H. Tan, and Y. Kishi, Toward the creation of NMR databases in chiral solvents for assignments of relative and absolute stereochemistry: proof of concept, Org. Lett., 2001, 3, 2245-2248
