CHEMISTRY OF N-NITROSO COMPOUNDS .1. SYNTHESIS AND STEREODYNAMICS OF N-NITROSOPIPERIDINES AND N-NITROSOPIPERIDIN-4-ONES

The stereochemistry of a series of N-nitroso-r-2,c-6-diphenylpiperidin-4-ones (10-15) and N-nitroso-r-2,c-6-diphenylpiperidines (16-18) in solution has been investigated by H-1 NMR, C-13 NMR, and dynamic H-1 NMR spectroscopic studies. Unlike the r-2,c-6-dialkyl-N-nitrosopiperidines, which have diaxial alkyl groups, these N-nitroso-r-2,c-6-diphenylpiperidines (16-18) and N-nitroso-r-2,c-6-diphenylpiperidin-4-ones (10-15) have diequatorial phenyl groups even though the nitroso group is coplanar to the dynamically averaged plane of the piperidine ring. The rotamer populations of the N-NO orientations in the nitrosamines were derived from continuous wave H-1 NMR spectral data and are correlated with the steric bulk of the substituents present at the C-3 and C-5 positions. N-Nitroso-r-2,c-6-diphenylpiperidine-4-one (10) was found to have a higher rotational barrier than N-nitroso-r-2,c-6-diphenylpiperidine (16). We attribute this difference to a greater degree of coplanarity (C3-C2-N1-C6-C5) in 10 than in 16. The rotational barriers in 10 were found to decrease as the bulk of the substituents at C3 and C5 increased. The rotational barrier in N-nitroso-r-2,c-6-diphenylpiperidine (16) was found to be lower than the reported rotational barriers of N-nitroso-r-2,c-6-dimethylpiperidine and N-nitroso-2,2,6,6-tetramethylpiperidine.