On-column deracemization of an atropisomeric biphenyl by quinine-based stationary phase and determination of rotational energy barrier by enantioselective stopped-flow HPLC and CEC

The reversible enantiomerization of axially chiral 2'-dodecyloxy-6-nitrobip henyl-2-carboxylic acid was studied in the presence of a brush type chiral stationary phase based on O-(tert-butylcarbamoyl) quinine as chiral selecto r unit by stopped-flow high-performance liquid chromatography (sfHPLC) and capillary electrochromatography (sfCEC). After initial separation of the en antiomers in the first section of the column, the flow was stopped and the resolved species allowed to enantiomerize on-column. From this conversion, which could be determined from the enantiomeric ratios at different enantio merization times, kinetic rate constants were calculated. By sfHPLC at a co nstant temperature of 15 degreesC, kinetic rate constants in the presence o f the CSP were found to be 4.1 X 10(-5) s(-1) and 2.2 x 10(-5) s(-1) for th e (-) and (+)-enantiomers, respectively, corresponding to half-lives of 279 and 530 min. Thus, apparent activation energies of enantiomerization were calculated to be 93.0 and 94.6 kJ mol(-1) for the (-) and (+)-enantiomers. On the macroscopic level, the apparent difference of rotational energy barr iers and kinetic rate constants for both enantiomers is reflected as derace mization. For example, starting from a racemic mixture, an enantiomeric exc ess (ee) of 14% was seen in the stopped-flow HPLC experiment described afte r an enantiomerization time of 220 min at 15 degreesC, and a maximal ee of 17% can be approximated after infinite enantiomerization time. There is goo d agreement between HPLC and CEC results as well as their experimental erro rs, confirming that the new sfCEC technique may be a valuable and convenien t tool to study interconversion processes. (C) 2001 Wiley-Liss, Inc.