Enantioseparation of binaphthol and its mono derivatives by cyclodextrin-modified capillary zone electrophoresis.

Chiral binaphthols belong to the group of most effective ligands for asymmetrical catalysis. In this context, various binaphthols presenting original substituents have been synthesized. Their study through capillary electrophoresis is the object of this work. The literature dedicated to the separation of atropisomers by capillary electrophoresis, corresponding only to binaphthol, reveals that its enantioseparation is always delicate because of the influence of many factors and the resolutions obtained are weak. Therefore, for a structured optimization, we first successfully evaluated the acidity constants of different binaphthols by means of capillary electrophoresis. With these known physicochemical characteristics, we could successfully carry out enantiomeric separations of the different binaphthols at pH 11.5, practically in completely ionized form, in phosphate medium, and in the presence of cyclodextrin (CD), with analysis times lower than 8min. The nature of CDs (alpha-CD, beta-CD, gamma-CD, hydroxypropyl-alpha-cyclodextrin (HP-alpha-CD), HP-beta-CD, HP-gamma-CD and trimethyl-beta-CD (TM-beta-CD)) and other factors in relation to enantiomeric resolution (applied voltage, nature and concentration of the electrolyte, and concentration of cyclodextrin) were optimized. These studies allowed us to determine the optimal conditions of separation (concentration and nature of CD) for each of the studied binaphthols. It is necessary to mention that, for the 1,1'-binaphthyl-2,2'-diol (Binol) at pH 11.5, the S atropisomer always migrated first, regardless of the nature and concentration of the cyclodextrin used. Moreover, an inversion in elution order of the two atropisomers as a function of pH was observed with gamma-CD (pH range: 10-11.5). The R atropisomer migrated first at pH 10. At pH 10.8 the migration order of the two atropisomers of Binol was reversed as a function of gamma-CD concentration. Finally, the addition of chiral ionic liquids (R(-)-1-hydroxy-N,N,N-trimethylbutan-2-aminium bis(trifluoromethylsulfonyl)imide and S(+)-tetrabutylammonium camphorsulfonate) was conducted. In the case of S(+)-tetrabutylammonium camphorsulfonate, a weak antagonistic effect was observed with modeling the evolution of enantiomeric resolution by means of the experimental design, while in the case of R(-)-1-hydroxy-N,N,N-trimethylbutan-2-aminium bis(trifluoromethylsulfonyl)imide the effect was neutral.