Conformational analysis. 39. Carbon-13 NMR spectra of saturated heterocycles. 9. Piperidine and N-methylpiperidine

Computational methods for conformational analysis of unsymmetrical 1,3-diamines: 3-aminotropanes

“…Additionally, Hartree᎐Fock and AM1 methods predict equatorial preference of 1.9 and 2.1 kcalrmol. Although these values are close to the reported Avalue of N-methylpiperidinium ion, 6 they may be too high, considering the additional steric strain in Ž . the tropane skeleton see above .…”

“…2 On the other hand, ab initio and MM3 methods are in agreement with each other and with experimental results. 2,6 Specifically, chair conformation ␣1a is the minimum energy structure, 1.0᎐1.4 kcalrmol lower than the chair conformation with an axial N-methyl group ␣1c, and 2.0᎐6.4 kcalrmol lower than either boat conformation. These are in reasonable agree-Ž ment with the equatorialraxial preference A-.…”

Influence of calculation level and effect of methylation on axial/equatorial equilibria in piperidines

“…The equilibria of the 2-Me group have been the object of particular experimental attention. Eliel et al 24 used 13 C NMR spectroscopy to find an increase in ⌬ E for are Ž . 2 Ear2Ee with respect to 0m 2.5 vs. 1.7 kcalrmol , while a ''normal'' value was determined for Ž .…”

“…On the one hand, ⌬ E is larger for 2 because 2Ea is destabilized are due to the shortening of the C-N bond, which reduces the nonbonded distances between the axial 2-Me and the axial hydrogen at C6. 24,56 On the other hand, ⌬ E is reduced when passing from 2 are to 2m due to the destabilization of 2mEe, because due to the puckering of the ring in the nitrogen region the equatorial N-Me and 2-Me groups are closer than the equatorial N-Me and the axial 2-Me groups of 2mEa. 24,56 Our calculations for 2Ear2Ee and 2mEar2mEe reproduce the experimental trends and ⌬ E are decreases when passing from 2 to 2m.…”

“…24,56 On the other hand, ⌬ E is reduced when passing from 2 are to 2m due to the destabilization of 2mEe, because due to the puckering of the ring in the nitrogen region the equatorial N-Me and 2-Me groups are closer than the equatorial N-Me and the axial 2-Me groups of 2mEa. 24,56 Our calculations for 2Ear2Ee and 2mEar2mEe reproduce the experimental trends and ⌬ E are decreases when passing from 2 to 2m. The NBO analysis suggests that ⌬ E for 2Ear2Ee are is larger than in 0m due to the balance between the increase of ⌬ E and the reduction of ⌬ E .…”