Twenty-Five Years’ History, Mechanism, and Generality of Preferential Enrichment as a Complexity Phenomenon
Résumé
Described here are the history, mechanism, and generality of ‘preferential enrichment (PE)’ which is a spontaneous chiral resolution phenomenon observed upon recrystallization of certain kinds of chiral racemic organic compounds from organic or aqueous solvents. The use of dimorphic cocrystals of chiral racemic compounds and achiral coformers could greatly expand the scope of PE. A thorough mechanistic investigation through (i) the direct observation of crystallization processes by using in situ techniques such as attenuated total reflectance infrared spectroscopy, temperature-controlled video-microscopy, and time-resolved X-ray powder diffraction measurement, (ii) the graph set analysis of crystal structures of deposited crystals, (iii) the kinetic/thermodynamic studies on the unique solubility properties of two enantiomers, and (iv) the observation of equilibrium behavior of an added labeled compound in solution and in the solid phases under PE conditions, together with (v) the construction of the corresponding ternary phase diagram, has indicated that PE is ascribed to an event of nonequilibrium ‘complexity system.’