30 years of chiral ligand exchange

The history of the discovery and some benchmark achievements of enantioselective ligand exchange chromatography (LEC) are briefly reviewed. Some of the most important results of investigations into the structure of Cu(II) complexes with nitrogen-substituted alpha-amino acids are summarized, and the role of water molecules that are coordinated in two axial positions of these complexes in the mediation of between-ligand interactions is underlined. UV and circular dichroism spectroscopy are shown to be very convenient for studying and quantifying the thermodynamic and kinetic enantioselectivity phenomena in the complex formation. Polystyrene-based chiral ligand exchange resins served as the first highly selective column packing in the development of enantioselective liquid chromatography. These resins were then followed by more efficient silica bonded monomeric and polymeric HPLC phases. The latter proved to be especially stable in aqueous media at elevated temperatures. Finally, chiral coated ligand exchanging RP phases presented the most inexpensive, but highly efficient and selective phases for the resolution of chelate-forming racemic solutes in LC and TLC. The mechanism of chiral resolution on these last phases involves an essential interaction of both the selector and selectand with the achiral hydrophobic surface. The same is valid for chiral resolutions according to chiral mobile phase techniques. Perspectives on further development of enantioselective LEC are discussed.