Dynamic ¹H NMR Spectroscopic Study of the Ring Inversion in <i>N</i>-Sulfonyl MorpholinesStudies on N−S Interactions

The effect of the exocyclic conjugation, via d−p orbital interaction and/or negative hyperconjugation (anomeric effect) of the N−S bond, on the inversion of the morpholine ring in some N-arylsulfonyl morpholines is studied by variable-temperature 1H NMR spectroscopy in different solvents. The observed free energy barriers are 9.2−10.3 kcal mol−1; the lower values were obtained with increasing conjugation (substituents of higher electron withdrawing power) along the series. The barrier to ring inversion of 1e was solvent independent. X-ray data of compounds 1b,d reveal the chair conformation of the six-membered ring, the flattened pyramidal orientation of the ring nitrogen atom, and the sulfonyl group in equatorial position with the plane containing the Caryl−S−N bond perpendicular to the plane of the benzene ring. In addition, the sulfonamide group prefers a conformation with the S−C bond antiperiplanar with respect to the nitrogen atom lone pair and the −CH2−N−CH2− moieties in staggered conformation with the S−O bonds of the SO2 group.