The Mechanism of Chromatographic Retention - Chiral Chromatography > Chiral Polysiloxane Stationary Phases > Page 79

 

There are many possible derivatives that can be prepared and almost every one is likely to possess some unique property that will enhance the separation of certain enantiomeric pairs. Those depicted in figure 41 are those more broadly useful for chiral separations.

The cyclodextrin cavity readily accepts certain functional groups as well as the aromatic nucleus. For chiral selectivity one of the enantiomers must fit closer to the neighboring groups to allow stronger interaction with either the surface diol groups or the groups of a derivative. If solute penetration into the cavity is significant, then the more dispersive groups that are situated within the cavity become available for interaction. The separation of the enantiomers of a rather complex molecule is shown in figure 42.

The separation was achieved on a b-cyclodextrin that carried the hydroxy propyl ether group attached to the lip of the cavity in the 2 and 3-positions. A possible, and rational, explanation for the chiral selectivity is suggested in the diagram. The suggested interactive mechanism has not been proved although the explanation is quite plausible.

 

Courtesy of ASTEC Inc.

 

Figure 42. A Chiral Separation that is Interpreted to Result from Molecular Group Inclusion