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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
Author(s) -
Tobler Ernst,
Lämmerhofer Michael,
Mancini Giovanna,
Lindner Wolfgang
Publication year - 2001
Publication title -
chirality
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.43
H-Index - 77
eISSN - 1520-636X
pISSN - 0899-0042
DOI - 10.1002/chir.10015
Subject(s) - chemistry , enantiomer , capillary electrochromatography , enantioselective synthesis , reaction rate constant , atropisomer , high performance liquid chromatography , chromatography , kinetic energy , chiral column chromatography , analytical chemistry (journal) , biphenyl , phase (matter) , capillary electrophoresis , kinetics , stereochemistry , organic chemistry , catalysis , physics , quantum mechanics
The reversible enantiomerization of axially chiral 2′‐dodecyloxy‐6‐nitrobiphenyl‐2‐carboxylic acid was studied in the presence of a brush type chiral stationary phase based on O‐( tert ‐butylcarbamoyl) quinine as chiral selector unit by stopped‐flow high‐performance liquid chromatography (sfHPLC) and capillary electrochromatography (sfCEC). After initial separation of the enantiomers 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 enantiomerization times, kinetic rate constants were calculated. By sfHPLC at a constant temperature of 15°C, kinetic rate constants in the presence of the CSP were found to be 4.1 × 10 −5 s −1 and 2.2 × 10 −5 s −1 for the (−) 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 barriers and kinetic rate constants for both enantiomers is reflected as deracemization. For example, starting from a racemic mixture, an enantiomeric excess (ee) of 14% was seen in the stopped‐flow HPLC experiment described after an enantiomerization time of 220 min at 15°C, and a maximal ee of 17% can be approximated after infinite enantiomerization time. There is good agreement between HPLC and CEC results as well as their experimental errors, confirming that the new sfCEC technique may be a valuable and convenient tool to study interconversion processes. Chirality 13:641–647, 2001. © 2001 Wiley‐Liss, Inc.