Separation of Enantiomers of sec-Butyl Carboxylicates on β-Cyclodextrin Derivatives and Chiral Recognization Mechanism

Separation of enantiomers of sec-butyl carboxylicates on β-cyclodextrin derivatives chiral column had been studied by gas chromatography. The effect of sec-butyl carboxylicates structure and column temperature on the separation of enantiomers of sec-butyl carboxylicates had been investigated. On the other hand, the thermodynamic parameters in the process of separation of enantiomers on the β-cyclodextrin derivatives chiral column (Cyclodex B and CycloSil B) were determined to discuss driven power in the process of enantiomers separation. Host-guest complexation of permethylated-β-cyclodextrin (PMBCD) with enantiomers of sec-butyl acetates ((R/S)-BAA) was simulated by quantum mechanics PM3 method. Chiral recognization mechanism of (R/S)-BAA enantiomers on PMBCD was investigated. The experimental results showed that the separation factor (α) and separation degree (Rs) of sec-butyl carboxylicates decreased with the increase of chain size of carboxyl group. The separation degree of enantiomers of sec-butyl acetate was best. There is excellent linear relationship between the logarithms of capacity factor (k') and separation factor (α) of enantiomers of sec-butyl carboxylicates and alkyl group parameters on the carboxyl group such as π and Es. And, the capacity factor (k'), the separation factor (α), and the separation degree (Rs) of the enantiomers of sec-butyl carboxylicates decreased with the increase of the column temperature. It was demonstrated that the separation of enantiomers of sec-butyl carboxylicates on the β-cyclodextrin derivatives chiral column (Cyclodex B and CycloSil B) was enthalpy-driven process and there was enthalpy-entropy compensation. The modeling results showed the stabilization partial inclusion complexation structures that formed with (R/S)-BAA enantiomers and PMBCD were different. The stabilization energy of (R)-BAA-PMBCD complex was less than that of (S)-BAA-PMBCD. The former is stable than the latter. The results correspond to the experimental results of separation of (R/S)-BAA on Cyclodex B by gas chromatography.