Conformational studies by dynamic NMR—28 : Stereomutations in some di-, tri- and tetraalkylhydrazines

Abstract

Low temperature NMR spectra allowed us to “freeze” some of the internal motions in a number of di-, tri- and tetraalkylhydrazines and to measure the corresponding free energies of activation. In particular, tetramethylhydrazine (Me 2N-NMe 2) was found to have, at - 150°, two pairs of diastereotopic methyls : this is due to the fact that both N-inversion and N,N-rotation are slow at this temperature and that a gauche conformation is adopted. The observed barrier (6.0 kcal mol -1) has been attributed to N,N-rotation, the barrier due to N-inversion being higher and not measurable via NMR in the presence of a concomitant fast rotation. In other cases, notably Pr iMeN-NH 2, Me 2N-NPr i 2 and Pr i 2N-NHMe, two different motions (inversion and rotation) were detected. In the case of Me 2N-NHMe it was also possible to observe the first example of anisochronous behaviour of nitrogen-bonded methyls (Me 2N) induced by an aminic nitrogen that becomes chiral at low temperature.