^(15)N Nuclear Magnetic Resonance Spectroscopy. Natural-Abundance ^(15)N Nuclear Magnetic Resonance Spectra of Enamines

The ^(15)N chemical shifts of 18 cyclic enamines have been determined at the natural-abundance level of ^(15)N using the Fourier transform technique. The shifts depend on the size of both the cycloalkene and nitrogen-containing rings. Methyl substituents on the cycloalkene ring also influence the chemical shifts of enamines. Tertiary amines formed on hydrogenation of cyclic enamines are found to have ^(15)N chemical shifts 3.9-19.7 ppm upfield of the shift for the corresponding unsaturated compound. A carbonyl group in conjugation with an enamine group results in a large down field shift of approximately 30-40 ppm for the nitrogen resonance of the enamine.