Impact of long-range van der Waals forces on chiral recognition in a Cinchona alkaloid chiral selector system
Abstrakt
Singly-charged complexes of (8S, 9R)-tert-butylcarbamoylquinine (tBuCQN), N-3,5-dinitrobenzoyl-(S, R)-leucine (DNB-S/R-leucine), and alkali metal counter ions (Li+, Na+, K+) were investigated by density-functional theory. It is shown that the cations prefer formation of an ionic pair with the carboxylate group of DNB-Leu over the formation of a cation-pi interaction.
The [tBuCQN center dot DNB-S/R-Leu center dot Na](+) complex is bound by a Coulombic attraction, a hydrogen bond, a pi - pi interaction and van der Waals forces. The tBuCQN chiral selector preferentially complexes with the DNB-S-Leu enantiomer, because the favourable stereochemistry allows the stabilization of the complex by at least one binding mode more compared to the complex containing the DNB-R-Leu molecule.
Weakening of the binding modes is observed using the lithium counter ion compared to the sodium one. The weakening is more pronounced in [tBuCQN center dot DNB-R-Leu center dot Li](+) than in [tBuCQN center dot DNB-S-Leu center dot Li](+).
The exact opposite effect is observed using the potassium counter ion. Hence, the lithium counter ion enhances the enantioselectivity of tBuCQN while the potassium counter ion reduces the enantioselectivity of tBuCQN.