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The dynamic 1H NMR study of some primary carbamates in the solvents CDCl3 and CD3COCD3 between 183 and 298 K is reported. The free energies of activation, thus obtained (12.4 to 14.3 kcal mol-1), were attributed to the conformational isomerization about the N-C bond. These barriers to rotation show solvent dependence in contrast to the tertiary analogues and are lower in free energy by ca. 2-3 kcal mol-1.
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.
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 H-1 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 le 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 -CH2-N-CH2- moieties in staggered conformation with the S-O bonds of the SO2 group.
Dynamic 1H NMR (500 MHz) investigation of aryl-N-(arylsulfonyl)-N-(triphenylphosphoranylidene)imidocarbamates in CDCl3, CD3COCD3, and CD3OD at the temperature range of 183-298 K is reported. The observed free energy barriers (almost 12 kcal mol;1) are attributed to conformational isomerization about the NùS bond and these barriers show very little solvent dependence.