Conformational Analysis of (+)- (3R)-Methylcyclohexanone from Temperature-Dependent Circular Dichroism Measurements

Conformational analysis of 3-methylcyclohexanone assumed a special importance some time ago in connection with the so-named Klyne and Robins and Walker noted that in the axial conformation of 3-methylcyclohexanone one of the 1,3-diaxial H:CH3 interactions of methylcyclohexane was missing and concluded that the conformational energy difference between axial and equatorial methyl forms should be lowered from the value of 7.11 kJ/mol (for methylcyclohexane) to one-half the value, or 3.55 kJ/mol. The difference (3.55 kJ/mol) between methyl A-values in methylcyclohexane and 3-methylcyclohexanone is called the 3-alkylketone effect. In order to generate an experimental value, thermochemical equilibration studies of cis-3,5-dimethylcyclohexanone on Pd(C) were carried out by Allinger and Freiberg to yield a conformational energy of ΔH° = 5.69 kJ/mol for the trans to cis epimerization. The authors concluded in favor of a greatly reduced 3-alkylketone effect of ca. 1.7 kJ/mol. The value of ΔH° differs from that of Rickborn (7.5 kJ/mol) derived from isomenthone and menthone equilibrations, but falls near to the data derived by Cotterill and Robinson in their base-catalyzed equilibration studies of 2,5-dimethylcyclohexanone [ΔH(e → a) = +5.4 kJ/mol and ΔS(e → a) = -0.2 eu] and 2-t-butyl-5-methylcyclohexanone [ΔH(e → a) = 6.4 kJ/mol and ΔS(e → a) = -0.5 eu]. Subsequently, Allinger et al. indicated conformational energies associated with the equatorial (E), axial (A) and twist (T) conformers (Fig. 1) of 3-methylcyclohexanone as 0, 5.9, and 13.8 kJ/mol, respectively, with a room temperature conformational mixture of 94, 5, and 1%, respectively. More recently Nakamura and I'Haya calculated rotational strengths using CNDO/2 and INDO methods for various probable conformers of 3-methyl-cyclohexanone and suggested a mixture of 83% E, 9% A, 7% T and 1% twist chair at 300°K, and 100% E at 81°K. Surprisingly, there has been no experimentally derived conformational analysis of 3-methylcyclohexanone itself; so, we report herein our conformational analysis of (+)-(3R)-methylcyclohexanone using variable temperature circular dichroism (CD) spectroscopy.