Temperature’s Effects on Chromatographic Enantiomer Disproportionation Examined

Rzeszów University of Technology (Poland) scientists recently studied how temperature can affect chromatographic separation driven by the rare occurrence of enantiomer self-disproportionation. Their findings and procedures were published in the Journal of Chromatography A (1).

Enantiomer self-disproportionation (SDE) is a phenomenon where, after a physicochemical process is applied, scalemic material spontaneously fractionates into enantio-enriched and -depleted fractions (2). This rare occurrence relies on the formation of homochiral and heterochiral associates between molecules of the same and opposite enantiomers of a chiral compound. A number of SDE-phoric groups have been identified, and when they are present in the structure of a molecule, either hydrogen or halogen bonding, dipole-dipole, or aromatic π-π interactions can occur, which activates the SDE phenomenon. Among the chiral compounds possessing these groups are sulfoxides, CF₃-derived substances, amines, amides, carboxylic acids, and amino acids, among others.

The efficiency of SDE-driven achiral chromatographic separation is altered by the concentration of enantiomers in the separated mixture, its enantiomeric excess (ee) and the composition of the mobile phase. Strong concentration overload enhances the SDE effect, as the association kinetics accelerate with increasing concentration of interacting molecules. The increase in ee enlarges differences between retentions of the concentration fronts of the enantiomers, which facilitates their separation. The presence of a polar modifier in the mobile phase suppresses the SDE effect, which stems from competitive interactions modifier-enantiomer and enantiomer-enantiomer. The effects exerted by these process parameters can be quantitatively described through the use of mechanistic modeling.

In this study, researchers examined the influence of temperature on the extent of SDE and the resulting efficiency of separation of enantiomers by achiral chromatography. Three chiral components of distinctly different structures and chemical properties were selected for the study: citalopram (CT), methyl p-tolyl sulfoxide (MTSO), and methyl 2-(acetylamino)propanoate (MAAP). CT enantiomers formed associates only in the adsorbed phase; this compares to MTSO and MAAP, which were associated in both the liquid and adsorbed phases. The temperature dependence of the association behavior in the liquid phase was determined on the basis of optical rotation measurements, whereas that in the adsorbed phase was determined on the basis of chromatographic elution profiles, which were interpreted using a mechanistic model.

For every chiral compound that was examined, association in the adsorbed phase had a dominating influence on the extent to which SDE occurred. In the case of the MTSO and MAAP enantiomers, the association processes were enhanced when temperatures were increased; with CT, it weakened. The opposite was true for the ratio of the equilibrium constants of the heterochiral and homochiral association, which determined the separation selectivity. CT association occurred with negative changes in both enthalpy and entropy, implying that their adsorption was driven by enthalpy changes. The ratio of the equilibrium constants of the heterochiral and homochiral association decreased with increasing temperatures. Further, the coexistence of the effects of temperature on the separation selectivity and the peak shapes entailed a complex dependence of the separation yield of the enantiomers versus temperature.

With these results, temperature was determined to be viable as an operating variable to enhance SDE and accelerate elution. However, the temperature effect is specific to the chemical structure of chiral compounds and the composition of their enantiomeric mixtures.

References

(1) Gumieniak, J.; Olbrycht, M.; Balawejder, M.; Antos, D. Effect of Temperature on Chromatographic Separation Driven by Enantiomer Self-Disproportionation. J. Chromatogr. A 2025, 1758, 466175. DOI: 10.1016/j.chroma.2025.466175

(2) Han, J.; Kitagawa, O.; Wzorek, A.; et al. The Self-Disproportionation of Enantiomers (SDE): a Menace or An Opportunity? Chem. Sci. 2018, 9, 1718–1739. DOI: 10.1039/C7SC05138G