Nuclear Magnetic Resonance Spectra. Nitrogen Inversion Rates of N-Substituted Aziridines (Ethylenimines)

The nuclear magnetic resonance spectra of various cyclic imines ranging in ring size from three to six have been examined. The spectra of N-substituted aziridine (ethplenimine) derivatives were found to be strongly temperature dependent as would be expected if the nitrogen atoms and attached groups do not lie in a plane and inversion occurs rather slowly. It has been possible to evaluate some factors which affect the inversion rates of non-planar nitrogen atoms in cyclic imines. As mould be expected, attachment of unsaturated groups to non-planar nitrogen increases the inversion rate as the result of conjugation with the nitrogen unshared electron pairs. The rates are also increased by bulky groups whether attached to nitrogen or to the carbons of the imine ring. Substitution of alkyl groups for one hydrogen or for two cis-hydrogens attached to carbon appears to make the molecules assume preferred Configurations with the N-substituent trans to the ring substituent (s). The inversion rates most probably are decreased in hydroxylic solvents because of stabilization of the separate configurations by hydrogen bonding between the solvent and the imino nitrogen. The data so far obtained indicate that substituted aziridines with molecular asymmetry due to trivalent nitrogen are likely only to be resolvable into reasonably stable optical antipodes at temperatures below - 50º. The nitrogen inversion rates of N-substituted azetidines (trimethylenimines) and larger-ring imines appear to be too great to be measurable by nuclear magnetic resonance techniques at temperatures above-77º.