Stereochemical and electronic interaction studies of some N-methoxy-N-methyl-2-[(4′-substituted)phenylsulfinyl]propanamides

The 1H NMR spectra of N-methoxy-N-methyl-2-[(4′-substituted)phenylsulfinyl]-propanamides [Y-Ph-S(O)CH(Me)C(O)N(OMe)Me; Y = OMe 1, Me 2, H 3, Cl 4, NO25] along with the X-ray diffraction analysis of the nitro-derivative (5), have shown the existence of two pairs of diastereomers (racemic mixture) [CRSS/CSSR (diast1) and CRSR/CSSS (diast2)] in the ratio of ca. 7:3, respectively. The νCO IR analysis of the title compounds supported by HF and B3LYP/6-31G∗∗ calculations of 3 and of the parent N-methoxy-N-methyl-propanamide (6) by HF, have shown that diast1 exists in an equilibrium between the two more polar and more stable quasi-cis (q-c1 and q-c2) conformers and the gauche(g) conformer. The population of the g conformer in the equilibrium increases with the increase in the solvent polarity, which is attributed to a larger solvation effect on the carbonyl and sulfinyl groups. Diast2 of compound 3 occurs in the gas phase as an equilibrium between the most stable quasi-gauche (q-g) conformer and the quasi-cis (q-c) conformer, both presenting very similar dipole moments. The former is stabilized by electrostatic and charge transfer interactions, which results in a less solvated spatial arrangement. Moreover, all conformers of both diastereomers are stabilized by several intramolecular hydrogen bonds. X-ray single crystal analysis performed for diast1 and for diast2 of 5 indicates that both stereoisomers assume, in the solid state, the anti-clinal (gauche) conformation. For the crystal packing, diast1 of 5 is made up of three molecules joined through two centro-symmetric H...O hydrogen bonds.