Enantioseparations by high-performance liquid chromatography based on chiral ligand exchange

Although the first application of chiral ligand-exchange chromatography (CLEC) in HPLC dates back to late 1960s, this enantioselective strategy still represents the elective choice for the direct analysis of compounds endowed with chelating moieties. As a specific feature of the CLEC mechanism, the interaction between the chiral selector and the enantiomer does not take place in direct contact. Indeed, it is mediated by a central metal ion that, acting as a Lewis acid, simultaneously coordinates the two species, selector and analyte, through the activation of dative bonds. As a consequence, two diastereomeric mixed ternary complexes are generated in the column, ultimately leading to the stereoisomeric discrimination. CLEC applications can be carried out both with the chiral selector included in the mobile phase (chiral mobile phase, CMP), or as a part of the stationary phase. In the latter case, the chiral selector can be either covalently immobilized onto a solid support (bonded CSP, B-CSP) or physically adsorbed onto a conventional packing material, coated chiral stationary phase (C-CSP). In this chapter, a selection of CLEC applications with CMP- and C-CSP-based chiral systems is presented.