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

5AG'H NMR spectra of the title imine in solution indicate the presence of three stereoisomers due to € / Z isomerism about the imino bond and atropisomerism about the 1 -naphthyl imino bond in the Zisomer. The most abundant Z-isomer, which was isolated in crystalline form, exhibits a highly unusual NMR signal at 6 5.9 assigned to a naphthyl proton cc to the imino group. The X-ray crystal structure reveals that this hydrogen atom is situated only 2.7 a above the face of the phenyl ring of the N-I -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 n-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.

Stereodynamics and edge-to-face CH–π aromatic interactions in imino compounds containing heterocyclic rings

“…−1.5 to −2.0 kcal mol −1 ). 10,11 The face tilted-T (c) which has been observed in our imino [12][13][14] and biaryl 15 model systems and in the cyclophanes reported by Kim et al, 16 Fukazawa et al 17 and Schladetsky Q4 et al 18 is a distortion of the T-structure (b) produced by limitations of the linker groups. It is probably not quite an energy minimum for the benzene dimer and therefore probably has a reduced interaction energy.…”

“…These distances are longer than most of those previously reported for molecules with face tilted-T geometry. 9,[12][13][14][15] The inter ring tilt angle A-C (36°) is also unusually low, hence the geometry lies slightly closer to a parallel displaced structure (tilt angle 0°) than a T-shaped structure (tilt angle 90°). The distance of Ho above the ring C plane is likely to increase as the tilt angle decreases from 90°in the T-structure to 0°in the parallel displaced structure.…”

“…0.2-0.6 kcal mol −1 ) to the free energy of activation ΔG ‡ based on previous estimates of the CH-π interaction free energy (ΔG) in related compounds. 12,13 However the contribution to raising the activation enthalpy (ΔH ‡ ) is likely to be much larger (ca. 1.3-1.8 kcal mol −1 ) based on reported estimates of the CH-π interaction enthalpy 13,14 in related compounds.…”

“…−1.4 kcal mol −1 for the face tilted-T structure (c) which may reflect some strain in the linker. 12,14 In geometries b, c, d, and e there is a potentially stabilising CH-π interaction 19 (two in the case of structures d and e). However MO calculations indicate that less specific dispersive forces are involved and may well make a large contribution to the binding energy.…”

Through-Space Spin–Spin Coupling in van der Waals Dimers and CH/π Interacting Systems. An Ab Initio and DFT Study

“…Hamor et al [45] found that the E/Z exchange by rotation about the imino bond is slow on the NMR time scale. The most abundant Z isomer has been isolated, and the X-ray structure reveals that the hydrogen atom H1 is only 2.7 ä above the face of the phenyl ring C1 ± C6.…”