Enantiomeric recognition of chiral ammonium salts by chiral pyridino- and pyrimidino-18-crown-6 ligands: Effect of structure and solvents

Chiral pyridino18-crown-6 ligands interact with chiral primary organic ammonium salts by hydrogen bonding from the ammonium cation to the pyridino nitrogen and two alternate ring oxygen atoms. Enantiomeric recognition in these interactions are caused by the steric bulk of the substituents at chiral macrocycle ring positions. Recognition is best for the interaction of chiral pyridino-18-crown6 hosts with the enantiomers of a-( 1-naphthylethy1)ammonium perchlorate (NapEtHC10,) over (a-phenylethy1)ammonium perchlorate (PhEtHClO,) possibly because of a greater x-x overlap between the naphthalene ring of the guest and pyridine ring of the host. Solvents play an important role in the degree of recognition. A binary solvent composed of 7/3 &H4C12/CH,0H (v/v) gave an enhanced degree of recognition. A new chiral pyrimidino18-crown-6 ligand exhibited recognition for the enantiomers of NapEtHC10,.