Enantiomeric recognition of organic ammonium salts by chiral crown ethers based on the pyridino-18-crown-6 structure

Enantiomeric recognition by several chiral dimethyl-substituted macrocycles of the pyridino-18-crown-6 type for chiral organic ammonium salts has been studied by titration calorimetry in CH/sub 3/OH, temperature-dependent /sup 1/H NMR spectroscopy in CD/sub 2/CL/sub 2/, and selective crystallization. Results from the three procedures are consistent in demonstrating either host-guest recognition or nonrecognition in the systems investigated. Futhermore, enaniomeric recognition by one chiral host for a pair of chiral guests is correlated with X-ray crystallographic data for the same system. The chiral dimethyl-substituted ligands used in the study include three dimethyl diester pyridino-18-crown-6 ligands, dimethyl thiono diester pyridino-18-crown-6, and dimethyl-pyridino-18-crown-6 ligands. All of these ligands exhibited chiral recognition. Dimethylpyridino-18-crown-6 in complexation with (R)- and (S)-(..cap alpha..-(1-naphthyl)ethyl)ammonium perchlorate exhibited the largest ratio of ..delta..G/sub c/+ yet observed by the /sup 1/H NMR technique. A diphenyl-substituted diester pyridino-18-crown-6 where the phenyl substituents are in less rigid portion of the macrocycle failed to show chiral recognition.