Dynamic (1)H NMR spectroscopic study of the ring inversion in N-sulfonyl morpholines--studies on N-S interactions

• Published in: Journal of organic chemistry Vol. 74; no. 13; pp. 4740 - 4746
• Main Authors: Modarresi-Alam, Ali Reza, Amirazizi, Homeyra Alsadat, Bagheri, Hajar, Bijanzadeh, Hamid-Reza, Kleinpeter, Erich
• Format: Journal Article
• Language: English
• Published: United States 03.07.2009
• Subjects: Crystallography, X-Ray; Magnetic Resonance Spectroscopy; Molecular Conformation; Molecular Structure; Morpholines - chemistry; Stereoisomerism; Structure-Activity Relationship; Sulfonamides - chemistry
• ISSN: 1520-6904
• DOI: 10.1021/jo900454a

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 (1)H 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 C(aryl)-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 -CH(2)-N-CH(2)- moieties in staggered conformation with the S-O bonds of the SO(2) group.