CONFORMATIONAL-ANALYSIS OF (R,R)-N,N-BIS(1-PHENYLETHYL)ACETAMIDE AND (R,S)-N,N-BIS(1-PHENYLETHYL)ACETAMIDE AND (R,R)-N,N-BIS(1-PHENYLETHYL)THIOACETAMIDE AND (R,S)-N,N-BIS(1-PHENYLETHYL)THIOACETAMIDE A STUDY BYNMR-SPECTROSCOPY AND BY EMPIRICAL FORCE-FIELD AND AM1 CALCULATIONS

The static and dynamic stereochemistry of the diastereoisomeric (R,S)- and (R,R)-N,N-bis(1-phenylethyl)acetamides and their thio analogues h as been studied by H-1 NMR spectroscopy and by the MM2 and AM1 methods . Assignment of the methyl and methine proton resonances was performed by a 2D-NOESY study, which also gave information about exchange proce sses, Two different processes were identified, the normal (thio) amide rotation leading to exchange of the E and Z 1-phenylethyl groups and a process with lower energy barrier exchanging two rotamers differing in the orientation of the l-phenylethyl groups. Rate constants and fre e energy barriers for these processes were determined by bandshape ana lysis. The rotamers were identified with the aid of the chemical shift s and by AM1 and empirical force-held calculations and the interconver sion trajectories were studied by the same theoretical methods. Barrie r differences between the R,S and R,R diastereoisomers were qualitativ ely reproduced by the AM1 calculations. The MM2 calculations utilizing the dipole-dipole interaction scheme were shown to exaggerate the sta bilities of conformers with the Z phenyl group turned towards the (thi o) carbonyl group. Better agreement with experiments was obtained when monopolar charge interactions were used, but the results were found t o depend critically on the choice of the charges.