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

Abstract

Abstract Experimental differentiation between pure CN double bond rotation and nitrogen inversion in N 1‐arylimines is possible with a single compound ( 13b ) under the proviso of slow rotation about the N –aryl single bond. Labelling by 1 H and 13 C 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 C s symmetry by NMR coalescences is only compatible with a mechanism of ( E/Z ) stereomutation either by pure inversion at sp 2 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.