Chemistry

Unexpected Pairing

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Science  06 Jan 2006:
Vol. 311, Issue 5757, pp. 16-17
DOI: 10.1126/science.311.5757.16e

It is well known that when chiral compounds crystallize, they often associate preferentially as like enantiomers or as racemic pairs. This property underlies Pasteur's pioneering elucidation of molecular chirality in tartaric acid, and it has since become a useful purification technique for extracting a homochiral sample from a mixture that is enriched in one enantiomer. However, the phenomenon is commonly attributed to the packing forces pertaining in tightly confined crystals; an analogous effect that would lead to loose aggregation in solution has been less well-studied.

Soloshonok has found that a trifluoromethyl group at a chiral center can have a surprisingly strong effect in inducing such aggregation during chromatography. Samples of a chiral CF3-substituted benzamide derivative were eluted on ordinary, achiral silica gel, and initial enrichment of 67% in one enantiomer induced fractionation into a mostly racemic portion and a portion >99.9% enriched in the major isomer. Systematic variation of the compound's substituents implicated the CF3 group as the critical factor, and further studies confirmed a similar effect in chromatography of several CF3-substituted alcohols. — JSY

Angew. Chem. Int. Ed. 10.1002/anie.200503373 (2005).

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