CONFORMATIONAL STUDY OF N-NITROSO-2,6-DIPHENYLPIPERIDINES AND N-NITROSO-2,6-DIPHENYLPIPERIDIN-4-ONES BY MOLECULAR MECHANICS CALCULATIONS, X-RAY CRYSTALLOGRAPHY, AND H-1 AND C-13 NMR-SPECTROSCOPY

Molecular geometries of several N-nitroso-2,6-diphenylpiperidines and N-nitroso-2,6-diphenylpiperidin-4-ones were calculated by the molecula r mechanics (MM2) method with added parameters for the N-nitroso group and by X-ray crystallography and H-1 and C-13 NMR. We found that thes e molecules adopt either a chair conformation with two phenyl groups a t 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 interfe rence of the planar NNO moiety with the neighboring equatorial substit uents. The X-ray crystal structures showed diaxial orientation of the phenyls in two compounds, assuming the chair conformations and boat-li ke piperidine ring geometry in three other nitrosamines. The H-1 and C -13 NMR spectra exhibited similar conformational preferences in soluti on to that predicted by the MM2 calculations. Most of the compounds de rived from the symmetrically substituted amines appeared to be almost conformationally homogeneous, whereas the conformational equilibria of unsymmetrically substituted nitrosamines are rather complex.