Perturbing Effects of Chiral Stationary Phase on Enantiomerization Second-Order Rate Constants Determined by Enantioselective Dynamic High-Performance Liquid Chromatography: A Practical Tool to Quantify the Accessible Acid and Basic Catalytic Sites Bonded on Chromatographic Supports

Second-order rate constants of the diethylamine-promoted enantiomerization of 2-[2-(1-methyl-1H-pyrrol-2-yl)-2-oxo-1-phenylethyl]-isoindole-1,3-dione, a chiral alpha-substituted ketone endowed with high anti-MAO activity type-A, were measured by dynamic high-performance liquid chromatography (DHPLC), stopped-flow high-performance liquid chromatography (sf-HPLC), and a classical method based on enantioselective HPLC as the monitoring tool. The chiral column used in all determinations was the commercial Chiralpak AD. By comparison of the obtained data, perturbing effects of the stationary phase on the DHPLC and sf-HPLC determinations were highlighted and distinguished in indirect (SP(IPC)) and direct (SP(DPC)) type. It was evidenced that SP(DPC) noise effects may be completely erased by simple mathematical treatment of data obtained at different concentrations of the basic catalyst. Perturbations of type SP(IPC) may instead only be partially kept down by modulating the concentration of the basic catalyst. An estimation of the density distribution of catalytic sites covalently bonded to the stationary phase (SP) of the Chiralpak AD was performed exploiting the quantified SP(DPC) effects. Such an approach might be of general application, supplying a useful way to characterize the attitude of SPs to speed acid- or base-catalyzed equilibria possibly active during chromatographic separations.