The role of hyperconjugation in the conformational analysis of methylcyclohexane and methylheterocyclohexanes.

Full geometry optimizations were carried out at the HF/6-31G** and B3LYP/6-31G** levels for methylcyclohexane, 2-, 3-, and 4-methyltetrahydropyran, 2-, 3-, and 4-methylpiperidine, 2-, 3-, and 4-methylthiane, 2-, 4-, and 5-methyl-1,3-dioxane, and 2-, 4-, and 5-methyl-1,3-dithiane and also for S-methyl thianium. Constrained geometry optimizations were carried out for methylcyclohexane, 2-methyl-1,3-dioxane, and the axial conformers of 2- and 3-methyltetrahydropyran and 2- and 3-methylpiperidine. The steric repulsion model, which is believed to account for the conformational energies of the cited compounds, was tested by stretching bonds and bending angles so that the axial methyl group is either forced to approach the ring gamma methylenes or get farther away from them. The calculated energies show that the energy costs of these perturbations are not dependent on the distances between the axial methyl group and the ring gamma methylenes and are not dependent on whether the methyl is axial or equatorial. It is shown that, besides the steric repulsion model, the conformational energies of the compounds studied are dictated by hyperconjugative interactions involving mainly the methine hydrogen. The C[bond]C lengths of the axial and equatorial conformers of methylcyclohexane are shown to be related to hyperconjugation.