Temperature dependent inversion of enantiomer selectivity in the complexation of optically active azophenolic crown ethers containing alkyl substituents as chiral barriers with chiral amines

Azophenolic crown ethers (S,S)-1, (R,R)-2 and (S,S)-3 have been prepared in enantiomerically pure forms by using (S)-1-(1'-adamantyl)ethane-1,2-diol,(R)-3,3-dimethylbutane-1,2- diol and (S)-propane-1,2-diol, respectively, as chiral subunits, and the association constants for their complexes with chiral amines have been determined by H-1 NMR or UV-VIS spectroscopic methods at various temperatures, The enantiomer selectivities of crown ethers (S,S)-1 and (R,R)-2 in complexation with 2-aminopropan-1-ol are reversed at ca. 6 degrees C and increase with increasing temperature above the isoenantioselective temperature.