Effects of substitution on nitrogen on barriers to rotation of amides. 2—Evaluation of the importance of resonance effects

Analogs of N-acetyl- and N-benzoyl-azacyclohex-2-enes having an oxygen atom, a methylene-d₂ group or a carbonyl group in place of the C-4 methylene group have been synthesized. The amide rotational barriers in these compounds have been measured by the total line-shape analysis of variable-temperature ¹H NMR spectra. The free energies of activation for both the N-acetyl and N-benzoyl series decrease in the sequence O?CD₂ > C?O. The barriers for the first two compounds in each series are similar to those of the corresponding saturated analogs, a result which confirms recent reports that the long accepted barrier-decreasing effect of an α-olefinic substituent on nitrogen is counteracted. Evidence is presented that resonance stabilization of the rotational transition state in the unsaturated compounds still obtains. It is suggested that the introduction of an α-olefinic substituent on nitrogen has a negligible effect on the amide rotational barriers in the cases of the oxygen and methylene-d₂ analogs, since any increase in the stabilization of the transition state by resonance is offset by the accompanying decrease in the sp³ character of the nitrogen atom. In cases in which a substituent on the olefinic group can interact directly with the lone pair on nitrogen, for example in the carbonyl analogs, then the resonance stabilization of the transition state is dominant.