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 (1a−1h), 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.