MOLECULAR MECHANICS CALCULATIONS AND COMPARISON OF PROTON, FLUORINE, AND CARBON NMR DIASTEREOMER DISCRIMINATION VIA NONBONDING INTERACTIONS BETWEEN FLUORINE-LABELED ENANTIOMERIC AMIDES AND ENANTIOMERICALLY PURE CHIRAL SOLVATING AGENTS

Diastereomer discrimination of fluorine-labeled enantiomers in chloroform solutions was studied with and without two chiral solvating agents (IS and 2S) using H-1, C-13, and F-19 NMR spectroscopy. Although by C-13 NMR spectroscopy the diastereomer discrimination is not observable, changes of chemical shifts for some carbon atoms unambiguously show formation of nonbonding interactions between the enantiomers and the chiral solvating agents. The position and the ratio of signal sets in both hydrogen and fluorine NMR spectra correspond to the enantiomeric composition in the solution. On the basis of changes in the chemical shifts of enantiomers in chloroform solutions of chiral solvating agent, binding constants and binding energy differences were calculated. Using the MM2 force field, calculations were performed on binding complexes between the chiral solvating agent 2S and enantiomers 6. It was shown that demand for energy differences between diastereomeric nonbonding complexes of racemic amides and the chiral solvating amides necessary to obtain their NMR diastereomer discrimination is low for H-1, intermediary for F-19, and high for C-13 NMR.