CH-π Interaction as an Important Driving Force of Host–Guest Complexation. Further Evidence for the Selective Incorporation of Alkyl Groups in the Polyhydroxy Aromatic Cavity of Calix[4]resorcarene Host

Abstract

Abstract The complexation of a calix[4]resorcarene host (2,8,14,20-tetraundecylcalix[4]arene-4,6,10,12,16,18,22,24-octol) with borneol in benzene or alkylbenzene was investigated by circular dichroism (CD) spectroscopy. The binding constants are dramatically solvent-dependent and decrease with respect to the substituents on the benzene ring in the order H > methyl > ethyl > propyl > butyl. The complexation of the same host with alkyl benzoates in limonene as a chiral hydrocarbon solvent was readily monitored by following their competitive inhibition effects on the CD intensities, reflecting the chiral host-solvent interaction. The binding constants for alkyl benzoates were again highly dependent on the alkyl groups, and changed in the order decyl < hexyl < methyl < propyl ≈ pentyl << butyl. There is thus an optimal chain length at butyl. These results provide further evidence for the selective incorporation of alkyl groups in the polyhydroxy aromatic cavity of the host.