Enantioselective Molecular Recognition of Chiral Organic Ammonium Ions and Amino Acids Using Cavitand–Salen-Based Receptors

Two new receptors, a cavitand-salen (2) and a uranyl-cavitand-salen (3), for the selective molecular recognition of chiral ammonium ion pairs, where the amino acid is the countercation or counteranion of the ion pair, were designed and synthesized. UV/Vis measurements indicate the formation of 1: 1 host-guest complexes with high association constants and good to excellent enantiomeric discriminations. NMR spectroscopic experiments confirmed cation-p interactions between the organic cations and the p-electron-rich cavity, leading to the stability of the complexes, as well as the coordination of the amino acid carboxylate anion to the uranyl metal center. Extraction experiments showed that a racemic mixture of (R,S)-alpha-methylbenzyltrimethylammonium iodides undergoes chiral resolution by 2 with high selectivity.