Chiral recognition for the complexation dynamics of β-cyclodextrin with the enantiomers of 2-naphthyl-1-ethanol

Abstract

The focus of this study is to understand the origin of the chiral recognition for a host-guest system containing complexes with different stoichiometries. Each enantiomer of 2-naphthyl-1-ethanol forms two different 1 : 1 complexes with β-cyclodextrin, leading to the formation of three different 2 : 2 complexes. One of these 2 : 2 complexes leads to excimer emission of the guest. Fluorescence studies were employed to determine the binding isotherms for the 1 : 1 and 2 : 2 complexes. No chiral discrimination was directly observed for the formation of the 1 : 1 complexes, while higher equilibrium constants (29% from binding isotherms and 40% from kinetic studies) were observed for the formation of the 2 : 2 complexes with (R)-2-naphthyl-1-ethanol when compared to the formation of the 2 : 2 complexes formed from (S)-2-naphthyl-1-ethanol. The relaxation kinetics was studied using stopped-flow experiments. The formation of the 2 : 2 complexes was followed by detecting the excimer emission from one of the 2 : 2 complexes. The relaxation kinetics was faster for (S)-2-naphthyl-1-ethanol, where a higher dissociation rate constant, by 47%, was observed, suggesting that the chiral discrimination occurs because the interaction between two cyclodextrins is more favorable for the complexes containing (R)-2-naphthyl-1-ethanol when compared to (S)-2-naphthyl-1-ethanol. The same overall equilibrium constants were observed for the 1 : 1 complexes with both enantiomers showing that at a given cyclodextrin concentration the sum of the two types of 1 : 1 complexes is the same for both enantiomers. However, analysis of the binding isotherms indicates that the ratio between the two different 1 : 1 complexes for each enantiomer was different for (R)- and (S)-2-naphthyl-1-ethanol.

Insight into functionalized-macrocycles-guided supramolecular photocatalysis

“…Thus, this review highlights the supramolecular photocatalytic substrate transformation involving various stoichiometric host-guest complexes (1:1, 2:1, 1:2, and 2:2). For the 1:1, 2:1, and 1:2 host-guest complexes, the thermodynamic association (K a ) and dissociation constants (K d ) for the reactive guests and photoproducts are crucial for the fundamental understanding of supramolecular noncovalent interactions [12,13]. In addition, during the catalytic process, the reactive guest should have a greater binding affinity or the photoproducts should have the lowest binding affinity with the supramolecular host to avoid product inhibition, so as to produce a better catalytic turnover efficiency [14,15].…”