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–π interactions between the organic cations and the π‐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)‐α‐methylbenzyltrimethylammonium iodides undergoes chiral resolution by 2 with high selectivity. Cavitand–salen and uranyl–cavitand–salen receptors were synthesized and used for the molecular recognition of chiral ammonium ion pairs. The best selectivity values were observed for ion pairs possessing an amino acid quaternary cation. The presence of the uranyl moiety ensures that a binding site is available for the anion of the ion pair, which increased the affinity of the receptor.