Empirical observations and mechanistic insights on the first boron-containing chiral selector for LC and supercritical fluid chromatography.

Boromycin is a macrodiolide that exists as a hydrophobic Boeseken complex formed from boric acid and a chiral polyhydroxy macrocyclic ligand. It was covalently bonded to silica gel through a urea linkage to an attached d-valine ester. When evaluated as a chiral stationary phase, it shows pronounced enantioselectivity toward primary amine-containing racemates, separating 98% of those tested. The selectivity is most pronounced in the presence of organic solvents and supercritical CO2 + methanol. The enantioselective binding site and mechanistic factors are examined. Analytes can be complexed as either the free base or their protonated analogues, with the free base being more strongly associated with the chiral selector.