Imines and derivatives. Part 23. Anomalous 1H NMR spectrum of N-[1-(1-naphthyl)ethylidene]-1-phenyl-2-propylamine: conformation in solution, atropisomerism and an X-ray crystal structure

Abstract

¹ H NMR spectra of the title imine in solution indicate the presence of three stereoisomers due to E/Z isomerism about the imino bond and atropisomerism about the 1-naphthyl imino bond in the Z-isomer. The most abundant Z-isomer, which was isolated in crystalline form, exhibits a highly unusual NMR signal at δ 5.9 assigned to a naphthyl proton α to the imino group. The X-ray crystal structure reveals that this hydrogen atom is situated only 2.7 Å above the face of the phenyl ring of the N-1-phenyl-2-propyl moiety and hence experiences a large diamagnetic ring current shielding effect. An attractive edge-to-face ring interaction involving this naphthyl proton and the π-electrons of the proximate phenyl group could account for this observation. The marked temperature dependence of this NMR signal is quantitatively analysed in terms of a fast equilibrium in solution between the conformation favoured in the solid state and a second conformation which lacks the ring current effect.