15N‐NMR study of activated enamines. Structural dependence of δ (15N) and nJ(N,H) in primary, secondary and tertiary enamino‐ketones, esters and amides

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Abstract
The pulse sequence INEPT was used to obtain proton‐coupled 15N‐NMR spectra in natural isotope abundance for enamines substituted in 2‐position with electron‐with‐drawing groups.The chemical shifts and coupling constants are discussed in terms of their relationship to structural features such as multiple N‐alkyl substitution, double‐bond configuration, H‐bonding, N‐lone‐pair delocalization within the conjugated system, and steric effects.It is concluded that 15N chemical shifts are a sensitive probe for local structural modifications at the N‐atom and conformational changes in a remote part of a conjugated molecule, while one‐bond N,H‐coupling essentially reflects N‐hybridization and subtle local geometric distortions. Stereospecific three‐bond N,H spin coupling to olefinic protons (4.0 ± 0.2 Hz) has been found a characteristic feature of (Z)‐isomers in all investigated compounds, whereas two‐bond coupling to olefinic protons (2J(N,H) = 0.5 to 5 Hz) is observed in (E)‐isomers. The sensitivity to solvents and steric properties of remote substituents renders geminal coupling a useful probe for studying electronic effects in the CN bond.