Chiral recognition in the solid state: crystallographically characterized diastereomeric co-crystals between a synthetic chiral selector (Whelk-O1) and a representative chiral analyte

Designed to distinguish between the enantiomers of compounds possessing commonly occurring structural features, the chiral selector used in the chiral stationary phase (CSP) 1 (Whelk-O1) is broadly applicable. In an effort to further the understanding of the mechanism of chiral recognition with this chiral selector, both diastereomeric combinations of selector 1 and a representative analyte, the pivalamide of p-bromo-α-phenylethylamine, 2, were successfully co-crystallized and characterized by single crystal X-ray diffraction. The crystal corresponding to the complex that is more stable in solution is consistent with our previously reported chiral recognition model. The aromatic portion of 2 is in the cleft of selector 1, displaying both face-to-face and face-to-edge π–π interactions as well as a hydrogen bond between the benzamide proton of the selector and the carbonyl oxygen of the analyte. For the crystal corresponding to the complex, which is less stable in solution, the aromatic portion of 2 is not in the cleft of selector 1, having approached from the opposite face of the π-acidic dinitrobenzamide moiety so as to undergo face-to-face π–π and hydrogen bonding interactions. Comparisons of these structures and their relevance to enantioselective chromatography are also discussed.

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