Solvent‐Induced Deuterium Isotope Effects in 13C‐ and 15N‐NMR. Spectra of Enaminones

The effect of deuterium on the 13C and 15N chemical shifts of enaminones has been investigated. D/H isotope shifts are reported for neutral and protonated species, i.e., when the isotope is exchanged on the C(2)‐, N‐, or O‐atoms. In cases of slow exchange the isotope shifts were obtained from solutions containing both isotopomers, whereas for fast exchange (acidic solutions) either separate NMR. sample tubes (15N‐NMR.) or coaxial tubes (13C‐NMR.) were used. In neutral molecules the isotope effects δC(D, H) are intrinsic in nature. In acidic solutions, the enaminocarbonyl cations formed exhibit δC(D, H)‐ and δN(D, H)‐values which are discussed in terms of the proton transfer. The mesomeric character of the cations is reflected by characteristic features in the δC(D, H)‐ and δN(D, H)‐values, which can be ascribed to isotopic perturbation of resonance. O‐Protonation shifts in the 15N‐resonance, observed for the first time, are large and positive (+60 to +76 ppm), in contrast to amides, where the effects are of the same sign but an order of magnitude smaller. Both protonation shifts and solvent‐induced isotope effects are discussed in connection with the nucleophilic character of the reactive centers in the enaminone synthon.