CONFORMATIONAL STUDIES BY DYNAMIC NMR-28. STEREOMUTATIONS IN SOME DI-, TRI- AND TETRAALKYLHYDRAZINES

Article abstract of DOI:10.1016/S0040-4020(01)96566-4

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 (Me2N-NMe2) was found to have, at -150 deg C, 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 PrⁱMeN-NH2, Me2N-NPrⁱ2 and Prⁱ2N-NHMe, two different motions (inversion and rotation) were detected.In the case of Me2N-NHMe it was also possible to observe the first example of anisochronous behaviour of nitrogen-bonded methyls (Me2N) induced by an aminic nitrogen that becomes chiral at low temperature.