Journal of the American Chemical Society p. 4527 - 4532 (1988)
Update date:2022-08-11
Topics:
Casarini, D.
Foresti, E.
Lunazzi, L.
Macciantelli, D.
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.
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