(E,Z)-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 N-arylimines is possible with a single compound (13b) under the proviso of slow rotation about the N-aryl single bond. Labelling by H-1 and C-13 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 Lime-averaged C(s) 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) contrarotation about the N-aryl single bond. However, the latter combination is concluded to be predominantly inversion-like by comparisons with related imines.