Nonplanar Structures of Thioamides Derived from 7-Azabicyclo[2.2.1]heptane. Electronically Tunable Planarity of Thioamides

X-ray crystallographic analysis showed that <i>N</i>-thiobenzoyl-7-azabicyclo[2.2.1]heptane displays marked nonplanarity of the thioamide (<b>1a</b>, α = 167.1° and |τ| = 11.2°) as compared with the corresponding monocyclic pyrrolidine thioamide (<b>2a</b>, α = 174.7° and |τ| = 3.9°). In a series of <i>para</i>-substituted or unsubstituted thioaroyl-7-azabicyclo[2.2.1]heptanes (<b>1a</b>−<b>1h</b>), the planarity of the thioamide depended significantly on the electronic nature of the substituent; for example, in the <i>p</i>-nitro-substituted compound, planarity was substantially restored (<b>1h</b>, α = 175.2° and |τ| = 0.1°). In solution, increasing electron-withdrawing character of the aromatic substituent was associated with a larger rotational barrier of the bicyclic thioamides, as determined by means of variable-temperature <sup>1</sup>H NMR spectroscopy and line shape analysis. The reduced rotational barrier, that is, reduced enthalpy of activation (Δ<i>H</i><sup>⧧</sup>) for thioamide rotation, of <b>1a</b> as compared with that of <b>2a</b> in nitrobenzene-<i>d</i><sub>5</sub> is consistent with the postulate that <b>1a</b> assumes a nonplanar thioamide structure in solution. These results indicate that the planarity of thioamides based on 7-azabicyclo[2.2.1]heptane is controlled by electronic factors in the solid phase and in solution.