Spectroscopic and theoretical studies of some N-methoxy-N-methyl-2-[(4′-substituted) phenylsulfonyl]propanamides

The analysis of the IR carbonyl band of the N-methoxy-N-methyl-2-[(4′-substituted)phenylsulfonyl]propanamides Y-PhSO2CH(Me)C(O)N(OMe)Me (Y = OMe 1, Me 2, H 3, Cl 4, NO25), supported by HF/6-31G(d,p) calculations of 3, indicated the existence of two gauche conformers (g1 and g2), the g1 conformer being the most stable and the least polar one (in gas phase and in solution). Both conformers are present in solution of the non polar solvent (CCl4) for 1-5 and in solution of the more polar solvents (CHCl3) for 1, 4, 5 and (CH2Cl2) for 5, while only the g1 conformer is present in solution of the most polar solvent (CH3CN) for 1-5. The g1 and g2 conformers correspond to the enantiomeric pairs of diastereomers (diast1 and diast2) whose relative configurations are [C(3)RNR]/[C(3)SNS] and [C(3)RNS]/[C(3)SNR], respectively. The computed carbonyl frequencies for g1 (diast1) and g2 (diast2) stereoisomers of 3 match well the experimental values. The NBO analysis, for 3 shows the important role of the orbital interactions in conformer stabilization and the overall balance of these interactions corroborates that the g1 conformer is more stable than the g2 one. The observed abnormal solvent effect on the relative intensities of the carbonyl doublet components is attributed to the molecular crowding in the g2 conformer which hinders its solvation in comparison to the g1 conformer (diast1). X-ray single crystal analysis performed for 3 shows the existence of two g2a and g1b conformers of diastereomers (diast2a and diast1b) whose absolute configurations are [C(3)RNS] and [C(3)RNR], respectively. The larger population and, thus, the larger stabilization of the g2a conformer over the g1b form in the crystals may be associated with a larger energy gain deriving from dipole moment coupling in the former conformer along with a series of CH…O electrostatic and hydrogen bond interactions.