Molecular Structure and Internal Rotations of Tetracyclohexylmethane

The stereochemistry of tetracyclohexylmethane (2), obtained by catalytic hydrogenation of tetraphenylmethane, is analyzed. The Cahn-Ingold-Prelog conformational descriptors of the ideal conformers of tetrasubstituted methanes are reevaluated and an alternative method presented. Methane 2 exists in the crystal in a (g/a/-g) conformation (of approximate S₄ symmetry) in which all the rings are in a chair arrangement and are skewed from their perfectly staggered D₂d conformation. Molecular mechanics (MM) calculations indicate that the crystallographic conformation is the lowest energy form. Analysis of the ¹H and ¹³C NMR data indicates that, even at low temperatures, the molecule undergoes a fast rotational process which exchanges the ortho and meta groups. On the basis of the MM calculations it is concluded that the preferred rotational pathway involves libration of the four rings via a transition state of D₂d symmetry in which all the rings are perfectly staggered.