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

X-ray crystallographic analysis showed that N-thiobenzoyl-7-azabicyclo[2.2.1]heptane displays marked nonplanarity of the thioamide (1a, α = 167.1° and |τ| = 11.2°) as compared with the corresponding monocyclic pyrrolidine thioamide (2a, α = 174.7° and |τ| = 3.9°). In a series of para-substituted or unsubstituted thioaroyl-7-azabicyclo[2.2.1]heptanes (1a1h), the planarity of the thioamide depended significantly on the electronic nature of the substituent; for example, in the p-nitro-substituted compound, planarity was substantially restored (1h, 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 1H NMR spectroscopy and line shape analysis. The reduced rotational barrier, that is, reduced enthalpy of activation (ΔH) for thioamide rotation, of 1a as compared with that of 2a in nitrobenzene-d5 is consistent with the postulate that 1a 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.