Fast N-CO rotational equilibria in twist-boat conformations of N -ethoxycarbonyl- r- 2 , c -6-diphenylpiperidin-4-ones and N -ethoxycarbonyl- r -2, c -6-diphenylpiperidines

Abstract

The effects of N-COOEt and alkyl substituents on the preferred conformations of six N-ethoxycarbonyl-r-2, c-6-diphenylpiperidin-4-ones 11-16 and six corresponding piperidines 23-28 have been determined. The spectral data and the calculated dihedral angles indicated that the N-ethoxycarbonyl derivatives 11, 16, 23 and 24 adopt flattened-boat conformations while the X-ray crystal structure of 23 showed a distorted boat conformation. The other unsymmetrically substituted N-ethoxycarbonyl derivatives 12-15 and 25-28 prefer twist-boat conformations in solution. The preferred conformation of 15 obtained through X-ray crystallography is also a twist-boat. The barriers for the N-CO rotation determined using variable temparature 1 H NMR studies for the N-ethoxycarbonyl derivatives 11 and 14 are 47.9 and 50.5 kJ mol - 1 , respectively. The N-CO rotation barriers are lower than those observed for N-NO rotation in N-nitroso-r-2, c-6-dimethylpiperidine and N-nitroso-r-2, c-6-diphenylpiperidines. The conformational preferences of the N-ethoxycarbonyl derivatives 11-16 and 23-28 has also been determined by the semiempirical molecular orbital calculations (AM 1 of MOPAC 6).