Conformational Analysis. XXX. Conformational Equilibrium of the N-Methyl Group in N-Methyl-trans-decahydroquinoline. The N-Methylpiperidine Problem

Abstract

The 13C NMR signals of the methyl groups in N-methyl-trans-decahydroquinoline and some of its analogs with “mobile” N-methyl groups are compared with corresponding signals of N-methyl groups which are constrained, sterically, to remain in either the equatorial or the axial position. From these data, the conformational equilibrium N-methyl (axial)⇌N-methyl (equatorial) is evaluated; the conformational energy -ΔG° is between 1.8 and 2.45 kcal/mol in chloroform and possibly somewhat larger in benzene. The appropriateness of the conformationally biassed (anancomeric) models is discussed. A less reliable value of -ΔG° = 1.35-1.77 kcal/mol in chloroform was determined for the N-methylpiperidine equilibrium (Me-a ⇌ Me-e), for which no suitable model for the Me-axial conformation is available. © 1975, American Chemical Society. All rights reserved.