15N NMR spectroscopy in the investigation of the structure of enaminones

Abstract

Geometry investigation of compounds which can exist in the form of different, mutually interconverting stereoisomers/rotamers is still the subject of interest. NMR spectroscopy is very helpful in such structural studies. Especially ¹⁵N NMR spectroscopy, which is known to be sensitive to structural changes in remote parts of molecules, is expected to be useful for the examination of n,π-conjugated systems as enaminones. In the present work, ¹⁵N NMR spectroscopy was applied to the study series of enaminones of type R¹R²N-CH=CH-C(O)H and R¹R²N-CH=CH-C(O)Me. Measurements at low temperatures (-60 and -90°C) in various solvents such as methanol-d₄, acetone-d₆, methylene chloride-d₂, and chloroform-d, using various compound concentrations, enabled the observation of signals of individual rotamers and allowed us to find relationships between the ¹⁵N chemical shifts and the structure of compounds. The experimental data was supplemented by DFT/6-311++G(2d,p) calculations of the shielding constants and the indirect spin-spin coupling constants. Topological analysis using Atom-In-Molecule approach revealed, in addition to the presence of intramolecular NH^...O hydrogen bonds in some rotamers, also non-covalent interactions of NH^...HCH₂, =O^...H₃C, and H₂CH^...HCH₂ in Z rotamers. Theoretical spin-spin indirect coupling constants including Ramsey paramagnetic spin-orbit and diamagnetic spin-orbit contributions turned out to be a convenient descriptor of noncovalent interactions.