Abstract
Several gauche interactions between sulfur and nitrogen functions are determined from measurement of conformational equilibria of 3-(X)-thiacyclohexanes (X = NH 2, NMe 2, NHPh and NHCO 2Bu t), their epimeric sulfoxides, and the corresponding sulfones, by low-temperature carbon-13 and proton NHR spectroscopy. The (RNH/S) gauche interaction is 0.18 (R=H), -0.04 (R=Ph) and -0.31 (R=CO 2Bu t) kcal/mol. The (Me 2N/S) gauche interaction is 0.70 kcal/mol, of which 0.26 kcal/mol is estimated to arise from the gauche repulsive effect between the electron lone pairs. The (RNH/SO x) gauche interactions in the trans-sulfoxides (x=1) are approximately as destabilizing as expected on simple steric grounds, whereas in their cis epimeres and in the sulfones they are 0.65 (R=H, x=1), 0.28 (R=H, x=2), -0.78 (R=Ph, x=1), -0.22 (R=Ph, x=2), < -1.94 (R=Co 2Bu t, x=1) and <-1.41 (R=CO 2Bu t, x=2) kcal/mol. Concentration and solvent effects in the RNH- series are small, intermediate and large, respectively, when R is H, Ph or CO 2Bu t. The effect of protonation upon conformational equilibria is also discussed.
Published Version
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