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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

✍ Scribed by Douglas S. Ribeiro; Raymond J. Abraham

Publisher
John Wiley and Sons
Year
2001
Tongue
English
Weight
134 KB
Volume
40
Category
Article
ISSN
0749-1581

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✦ Synopsis

Abstract

The 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 (C~2~Cl~4~) solvents. These results gave consistent values of Δ__G__(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 ^1^H and ^13^C 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.

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