Conformational analysis: Part 37. A 13C and 1H NMR and theoretical investigation of the conformational equilibrium of 2‐methylcyclohexanone oxime and of its O‐methyl ether

Abstract

AbstractThe free energy of the axial–equatorial equilibrium of 2‐methylcyclohexanone oxime (1E) and the corresponding O‐methyl oxime (2E) was determined by a novel curve‐fitting analysis of the temperature dependence of the carbon chemical shifts and by the vicinal proton–proton coupling constants in both polar (acetone) and non‐polar (C2Cl4) solvents. These results gave consistent values of ΔG(eq–ax) of 0.88 (±0.10) kcal mol−1 for 1E and 0.75 (±0.10) kcal mol−1 for 2E for both solvents. The complete assignments of the 1H and 13C spectra of the oxime 1E and of cyclohexanone oxime 3 and the 4‐tert‐butylcyclohexanone oxime 4 are recorded. The reduced value of the conformer free energy difference relative to the parent 2‐methylcyclohexanone (1.6 kcal mol−1) was attributed to the larger steric strain in the equatorial conformation of these oximes. This was confirmed by the analysis of the molecular geometry obtained by ab initio methods (RHF/6–31G* and B3LYP/6–31G**). Copyright © 2001 John Wiley & Sons, Ltd.