NMR and Theoretical Studies on the Conformational Preferences of Some Non-metal Coordinated N-Enoyl Systems Attached to Common Chiral Auxilaries
<p>We report a systematic study of a series of N-enoyl systems attached to common oxazolidin-2-ones, oxazolidine-2-thiones and thiazolidine-2-thiones chiral auxiliaries in order to determine the most stable conformation of these compounds. <sup>1</sup>H NMR studies show an <em>anti</em>-<em>s</em>-<em>cis</em> structure as the most stable conformation for these series of compounds. Density Functional Theory geometry optimizations and vibrational analysis using the b3lyp exchange-correlation functional with the standard 6-31g** basis sets were done, including solvent effects (chloroform and toluene). Gibbs free energy differences show that the <em>anti</em>-<em>s</em>-<em>cis</em> structures are the most stable conformers lying, on average, ca. 6 kcal/mol lower in energy than the <em>syn</em>-<em>s</em>-<em>cis</em> conformers, widely used to explain the structure and reactivity of <em>N</em>-enoyl systems.</p>