Enantiomeric recognition by chiral pyridino-18-crown-6 for 1-naphthylethylamine. The effect of alkyl substituents on the macrocycle ring

Molecular recognition of three chiral pyridino-18-crown-6 macrocycles with various dialkyl substituents for (R)- and (S)-α-(1-naphthyl)ethyl]ammonium perchlorate (NapEt) has been determined by isoperibol titration calorimetry and compared to values already determined for the chiral di-methyl- and di-tert-butyl-substituted crowns. Possible mechanisms of recognition for these systems are discussed using the thermodynamic values log K, ΔH, and TΔS. Macrocycles with the bulky substituents tert-butyl, isobutyl, and sec-butyl showed better recognition than those with the less bulky isopropyl and methyl substituents. In the case of macrocycles with bulky substituents, recognition of the enantiomers of NapEt was mainly the result of differences in entropy value changes. With macrocycles having less bulky substituents, reognition was mainly the result of differences in enthalpy value changes.