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A Study of N-Butyl-(all-trans-retinylidene)amine and Its Protonated Species by 1H- and 13C-NMR. Spectroscopy

✍ Scribed by Christian Pattaroni; Jürgen Lauterwein

Publisher
John Wiley and Sons
Year
1981
Tongue
German
Weight
846 KB
Volume
64
Category
Article
ISSN
0018-019X

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✦ Synopsis

Abstract

The Schiff base of all‐trans‐retinal was investigated in organic solution by ^1^H‐ and ^13^C‐NMR. at high field. Complete assignment of the ^1^H‐NMR. peaks of N‐butyl‐(all‐trans‐retinylidene)amine (2) and the N‐butyl‐(all‐trans‐retinylidene)‐ammonium ion (3) was achieved by INDOR (internuclear double resonance). The vicinal proton coupling constants of the polyene chain show that the π‐bond orders remain unchanged in N‐butyl‐(all‐trans‐retinylidene)amine relative to all‐trans‐retinal (1), but change towards larger π‐delocalization in the N‐butyl‐(all‐trans‐retinylidene)ammonium ion. At −‐61° only one isomer of N‐butyl‐(all‐trans‐retinylidene)ammonium was observed. This was shown to be trans at the imine linkage and independent of the solvent. The trifluoroacetic acid counter‐ion can approach the positive charge of the N‐atom in the weakly polar solvent dichloromethane but not in the leveling solvent methanol. In dichloromethane the nature of the 1:1 complex is a H‐bonded (O^−^…H‐N^+^) ion‐pair whose rate of breaking and forming is rapid at RT. Strong stabilization of the ion‐pair resulted from homo‐conjugation with a second molecule of trifluoroacetic acid. Excess of acid efficiently diminished the isomerization rate at the C,N‐bond.

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