| The conformational analysis of menthone (trans-2-Isopropyl-5-methylcyclohexanone), isomenthone (cis-2-Isopropyl-5-methylcyclohexanone), and several imines derived from these compounds was performed by 1H-NMR and the experimental Boltzmann distributions were compared with molecular modelling calculations. Menthone was found adopting a conformation with both methyl and isopropyl groups equatorially oriented at room temperature (21--23°C). On the other hand, isomenthone exists in an equilibrium containing ca. 79% of a conformer in which the isopropyl group is axial and the methyl equatorial. The menthone imines, in general, contain about 20% of a diaxial chair conformation and the isomenthone imines have the isopropyl group axial and the methyl equatorial. The conformational differences between these molecules is explained as a competition of pseudo allylic A(1,3) interactions between the isopropyl group and the non-bonded pair of electrons of the heteroatom, and axial-axial interactions between the alkyl chains and the protons of the ring. It was also found that semiempirical, low level ab initio, and DFT calculations poorly reproduced the experimental results due to the small energy differences (<2 kcal/mol) between conformers.; Proton-proton three bond coupling constants were recorded for the bornyl and menthyl acetate series -O2CCH3, -O 2CCCl3, and -O2CCF3, as well as for other derivatives, in order to observe the effect of group electronegativities, on 3JHH. The experimental coupling constants were found to be the same, within experimental error, between the three acetates and the corresponding alcohol. In addition, a good linear correlation was observed when the 3JHH's were plotted against first atom electronegativity (r = 0.97) instead of group electronegativity; therefore, it is suggested that atomic electronegativities might be good enough to empirically describe substituent effects on 3JHH. |
