Conformational Study of N-Nitroso-2,6-diphenylpiperidines and N-Nitroso-2,6-diphenylpiperidin-4-ones by Molecular Mechanics Calculations, X-ray Crystallography, and 1H and 13C NMR Spectroscopy

Article abstract of DOI:10.1021/jo00128a011

Molecular geometries of several N-nitroso-2,6-diphenylpiperidines and N-nitroso-2,6-diphenylpiperidin-4-ones were calculated by the molecular mechanics (MM2) method with added parameters for the N-nitroso group and by X-ray crystallography and 1H and 13C NMR.We found that these molecules adopt either a chair conformation with two phenyl groups at the axial positions or a distorted boat-like conformation.Contrary to the earlier literature suggestions, a diequatorial chair conformer is destabilized by the A^(1,3) strain resulting from a steric interference of the planar NNO moiety with the neighboring equatorial substituents.The X-ray crystal structures showed diaxial orientation of the phenyls in two compounds, assuming the chair conformations and boat-like piperidine ring geometry in three other nitrosamines.The 1H and 13C NMR spectra exhibited similar conformational preferences in solution to that predicted by the MM2 calculations.Most of the compounds derived from the symmetrically substituted amines appeared to be almost conformationally homogeneous, whereas the conformational equilibria of unsymmetrically substituted nitrosamines are rather complex.