Speedier Enantiomeric Excess

A copper complex that quickly reveals the composition of mixtures of enantiomers may help organic chemists assess the fruits of their labors more easily (J. Am. Chem. Soc., DOI: 10.1021/ja205775g). Current methods, such as chiral HPLC that are used to determine enantiomeric purity by measuring the ratio of one enantiomer to another, are slow and often can’t keep up with the hundreds or thousands of reactions that chemists need to screen when developing synthetic methods, says Eric V. Anslyn of the University of Texas, Austin. To speed things up, Anslyn and colleagues synthesized an achiral copper-containing molecule that can serve as a host for chiral carboxylic acid guests. When a guest binds to copper, the metal’s quinoline ligands twist into a propeller shape. The direction of the twist depends on the stereochemistry of the guest and causes the complex to absorb circularly polarized light differently. The researchers took circular dichroism spectra of known mixtures of enantiomers to produce a calibration curve and then used it to determine the enantiomeric excess of unknown mixtures. The technique has an error rate of about 3%, Anslyn says, which “is not quite as accurate as HPLC, but it’s up to 100 times faster.”