A number of N-ketiminyl-2,2,6,6-tetramethylpiperidines (TMP-N=CR2) have been synthesized and investigated by 1H NMR spectroscopy. The piperidyl methyls are either nonequivalent in pairs at room temperature or become nonequivalent in pairs at low temperatures. This is shown to be due to restricted rotation about the N-N bond, a phenomenon that is not observed in less hindered hydrazones even at very low temperatures. The free energies of activation for rotation in TMP-N=CR2 were determined by line shape analysis. They increase from a low of 7.6 kcal/mol for R2 = H2 through 11.6, 14.35, and 18.0 kcal/mol for R2 = (CH2)4, (CH2)5, and (CH2)6, respectively. This trend is rationalized in terms of a conformation in which the plane of the N=CR2 moiety is perpendicular to the dynamically averaged plane of the tetramethylpiperidine ring. This conclusion is supported by an x-ray diffraction determination of the structure and conformation of TMP-N=C-(CH3)C6H5. The rotational barriers of some n-nitrosoamines are also reported.
Journal of the American Chemical Society98, no. 24 (1976): 7484–7488.