Conformational Studies by Dynamic NMR. 35. Structure, Conformation, and Stereodynamics of Hindered Naphthylamines

Article abstract of DOI:10.1021/ja00222a007

The low-temperature 1H and 13C NMR spectra of a number of N-ethyl- and of N-isopropyl-N-alkyl-1-naphthylamines allowed the measurement of the interconversion barriers between the two twisted enantiomeric conformers.The free energyes of activation increased substantially with the dimension of the N-alkyl groups (e.g., δG^**₁₇₃ is 8.3 Kcal mol^-1 when N-alkyl is a methyl group, and δG^**₃₇₃ is 19.8₅ Kcal mol^-1 when N-alkyl is a tert-butyl group in N-ethyl-N-alkyl-1-naphthylamines).Nuclear Overhauser enhancement (NOE) experiments confirmed that these molecules adopt a twist conformation and indicated that in N-methyl as well as in N-ethyl-N-isopropyl-1-naphthylamines the N-isopropyl moiety is closer to H-8 than to H-2.Thes results, obtained in solution, were confirmed by the X-ray investigation of an analogous derivative in the solid state.On the other hand, the less hindered secundary 1-naphthylamines turned out to be planar in solution (NOE experiments) as well as in the solid state (X-ray diffraction).Finally it was found that electron-releasing (e.g., -NH2) and electron attracting (e.g., -NO2) substitunets in position 4 increase and reduce, respectively, the interconversion barriers of the twisted tertiary 1-naphthylamines.As expected this trend is opposite to that reported in the case of planar N-methylanilines containing analogous substituents in position 4.