Temperature-induced inversion of elution order in the chromatographic enantioseparation of 1,1′-bi-2-naphthol on an immobilized polysaccharide-based chiral stationary phase

In this work, the enantioseparations of 1,1′-bi-2-naphthol (BINOL) and its three derivatives were performed on an immobilized polysaccharide-based chiral stationary phase, Chiralpak IA, under normal-phase mode. The effects of the content of polar modifier in the mobile phase and the column temperature on the retention and enantioseparation were investigated in detail. Temperature-induced inversion of elution order for BINOL was observed directly when n-hexane/2-propanol (92/8, v/v) was used as mobile phase. The isoenantioselective temperature (T_iso) was calculated to be 31.4 °C. When n-hexane/2-propanol/THF (93/2/5, v/v/v) was used as mobile phase, the T_iso value decreased to −8.2 °C. Entropically driven enantioseparation which had practical application was obtained successfully (separation factor being 1.189 and 1.332 at 25 °C and 50 °C, respectively). The corresponding thermodynamic parameters for other three binaphthyl compounds were compared with that for BINOL. Some inferences about chiral recognition mechanism were stressed.