Spectroscopy method measures enantiomeric excess on the fly

By combining two spectroscopy methods, researchers have come up with a noninvasive way to monitor real-time changes in enantiomeric excess (ee) during the course of asymmetric catalytic transformations (ACS Catal. 2018, DOI: 10.1021/acscatal.8b01411). Information derived from the new combo technique may help deduce reaction mechanisms, optimize flow processes, and monitor catalyst deactivation. To follow the progress of a reaction, pharmaceutical chemists can make use of various methods that measure changes in reactant and product concentrations. But a technique for measuring changes in ee on the fly, especially in complex, multistep asymmetric catalytic reactions, has remained elusive. So Matthew P. Mower and Donna G. Blackmond of Scripps Research Institute California devised a flow-cell system that simultaneously measures vibrational circular dichroism and Fourier transform infrared spectra. The techniques provide complementary information about the concentrations of pairs of enantiomers. In a proof-of-concept demonstration, Mower and Blackmond applied the method to monitor temporal changes in ee in a salen cobalt-catalyzed reaction of racemic epoxides, a well-studied, industrially relevant reaction. Mechanistic details deduced via the new method closely match those reported previously on the basis of experimental kinetic measurements and computational studies.