Chiral discrimination in the complexation of heptakis-(2,6-di-O-methyl)-β-cyclodextrin with 2,3-diazabicyclo[2.2.2]oct-2-ene derivatives

Abstract

The chiral discrimination of enantiomeric camphanate esters of 2,3-diazabicyclo[2.2.2]oct-2-ene by heptakis-(2,6-di-O-methyl)-β-cyclodextrin was studied by means of induced circular dichroism, UV spectrophotometry, and fluorescence spectroscopy. The first two spectroscopic techniques were employed to study the thermodynamics, while the kinetics of complexation was determined by using steady-state fluorescence quenching experiments. The formation of 1:1 and 2:1 inclusion complexes was monitored through opposite induced circular dichroism effects and an increase of the near-UV extinction coefficient of the azo chromophore, from which the binding constants (K) were determined by means of titrations. The binding constants for 1:1 complexation (ca. 1500 M⁻¹) were more than 1 order of magnitude larger than those for 2:1 complexation (ca. 40 M⁻¹). An insignificant chiral discrimination was found for the thermodynamics of 1:1 complexation, but a significant effect on the association kinetics, which was ca. 20% faster for the (−)-enantiomer. The association rate constants for the formation of the 2:1 complex were found to be too small ( × 10⁷ s⁻¹) to allow determination by the fluorescence quenching method.