(E, Z)1‐Equilibria, 17 Demonstration of the Nitrogen Inversion Mechanism of Imines in a Schiff Base Model

Experimental differentiation between pure CN double bond rotation and nitrogen inversion in N1‐arylimines is possible with a single compound (13b) under the proviso of slow rotation about the N–aryl single bond. Labelling by 1H and 13C nuclei at the diastereotopic faces of the CN moiety as well as of the N–aryl group is the clue to a successful stereodynamic analysis, as performed by variable‐temperature NMR spectroscopy of 13b, a sterically congested and chiral model compound. Interpretation of similar measurements on a second model (13d) is less straightforward. The experimental observation of time‐averaged Cs symmetry by NMR coalescences is only compatible with a mechanism of (E/Z) stereomutation either by pure inversion at sp2 nitrogen or by a contribution from CN rotation together with a synchronized (geared) controtation about the N–aryl single bond. However, the latter combination is concluded to be predominantly inversion‐like by comparisons with related imines.