ENANTIOMERIC RECOGNITION OF ORGANIC AMMONIUM-SALTS BY CHIRAL DIALKYL-SUBSTITUTED, DIALKENYL-SUBSTITUTED, AND TETRAMETHYL-SUBSTITUTED PYRIDINO-18-CROWN-6 AND TETRAMETHYL-SUBSTITUTED BIS-PYRIDINO-18-CROWN-6 LIGANDS - COMPARISON OF TEMPERATURE-DEPENDENT H-1-NMR AND EMPIRICAL FORCE-FIELD TECHNIQUES

Six new chiral pyridino-18-crown-6 and one chiral bis-pyridino-18-crown-6 ligands have been prepared. The pyridino-crowns contain either two isopropyl, two isobutyl, two (S)-sec-butyl, two benzyl, two 3-butenyl, or four methyl substituents on chiral macroring carbon atoms. The starting chiral dialkyl-substituted tetraethylene glycols needed for the preparation of the chiral dialkyl-crowns were prepared starting from the appropriate amino acid. The bis(3-butenyl-substituted) tetraethylene glycol was prepared from D-mannitol. The chiral tetramethyl-substituted tetraethylene glycol needed to prepare the chiral tetramethyl-substituted pyridino-18-crown-6 was prepared from monobenzyl-blocked (2R,3R)-2,3-butanediol. Chiral tetramethyl-bis-pyridino-18-crown-6 was prepared by the cyclization of 2 mol of (2R,3R)-2,3-butanediol with 2,6-bis[(tosyloxy)methyl]pyridine. Energy of activation (ΔGc*) values determined by the temperature-dependent 1H NMR spectroscopy technique indicated that these chiral ligands exhibited chiral recognition for the enantiomers of various organic ammonium salts. Differences in ΔΔgc* values (ΔΔGC*) were compared in several cases with the corresponding energy differences calculated from empirical energy functions. The observed and calculated Δ ΔGC* values were in good agreement in most cases. An X-ray analysis of a crystal of the tetramethyl-bis-pyridino-18-crown-6 ligand (11) shows that the two pyridine rings are bent out of the plane of the four oxygen atoms. © 1990, American Chemical Society. All rights reserved.