n-π-Interaction in Enamines and Enaminoketones. A 15N-NMR. Study†

15N-Chemical shifts of 32 enamines, 11 enaminoketones and 28 closely related amines have been determined with the isotope in natural abundance. In order to eliminate substituent effects, differential chemical shifts Δδ(N) are defined as δN(amine)-δN(enamine). This parameter is shown to correlate well with the free enthalpy of activation ΔG# for restricted rotation about the NC(α) bond in enamines with extended conjugation. Δδ(N) values of substituted anilinostyrenes correlate also with 13C-chemical shifts of the β-carbon in the enamine system and with Hammett σ-constants of the aniline substituents. The experimental results suggest that differential 15N shifts are a useful probe to study n, π-interaction in enamines.