Upper-Rim Urea-Derivatized Calix[4]arenes as Neutral Receptors for Monocarboxylate Anions

Several new anion receptors have been synthesized by introducing one or two urea or thiourea units at the upper rim of calix[4]arenes, blocked in the cone conformation. The difunctionalized receptor 13 shows selectivity in the recognition of acetate anion, which interacts with the two urea units through four hydrogen bonds. The monofunctionalized receptors 21 and 22 complex more strongly aromatic carboxylates or butyrate anion since the primary hydrogen bonding contacts are strengthened by weak pi/pi or CH3/pi host-guest interactions