Three new chiral pyridino-18-crown-6 ligands have been prepared. These ligands contain either two phenyl, two tert-butyl, or two methyl substituents on chiral macroring carbon atoms. The chiral di-tert-butyl-substituted diester crown analogue was also prepared. The starting chiral di-tert-butyl-substituted tetraethylene glycol needed to prepare the two di-tert-butyl-substituted crowns was obtained from chiral tert-butyl-1,2-ethanediol, which was resolved from its bis(hydrogen phthalate) brucine salt. A high degree of chiral recognition in CD2Cl2 of the enantiomers of [α-(1-naphthyl)ethyl]ammonium perchlorate (NapEt) was shown by the diphenyl- and di-tert-butyl-substituted crowns as measured by differences in the free energy of activation (ΔGc*) values determined by temperature-dependent *H NMR spectroscopy. The diphenyl- and di-tert-butyl-substituted crowns also exhibited high chiral recognition for the enantiomers of NapEt and other chiral organic ammonium salts in methanol and methanol-chloroform mixtures as shown by a large difference in the log K values determined by a direct NMR technique.
ASJC Scopus subject areas
- Organic Chemistry