2-Methyl-1,3-dioxane, conformational energies

The preferred conformations of both 1-methyl-l-phenylcyclohexane and 2-methyl-2-phenyl-l,3-dioxane have the phenyl group in the axial orientation even though the conformational flee energy of the phenyl group (2.9 kcal) is greater than that for a methyl group (1.8 kcal). Explain. 

The X-ray structures of two anthraquinone derivates of 1,3-dioxane were published. In both 1 -methoxy-4-(2-methylprop-2-enyloxy)-2-[(2i ,6i )-4,4,6-trimethyl-1,3-dioxan-2-yl]-anthraquinone and 4-hydroxy-3-(2-methyl prop-2-enyloxy)-2-[(27 ,67 )-4,4,6-trimethyl-l,3-dioxan-2-yl]anthraquinone the 1,3-dioxane ring adopts the chair conformation and the substituents in positions 2 and 6 are in equatorial conformations (99AX(C)436). Finally, Freeman et al. (02JMS(T)43), employing both ab initio theory and density function theory, calculated the energies of chair, half-chair, sofa, twist, and boat conformers of 1,3-dioxane. 

A long range intrinsic deuterium isotope shift in 2,2-bis(trideuterio-methyl-5,5-dimethyl-l,3-dioxane [61] was observed by Anet and Dekmezian (1979). This molecule does not show an equilibrium isotope effect because the two rapidly interconverting chair isomers have exactly the same energy. 2-D-5.5-Dimethyl-l,3-dioxane, however, exhibits an isotope effect on the conformational equilibrium (Anet and Kopelevich, 1986a). Deuterium is preferred in the equatorial position (AG° = —49cal mol at 25°C). 

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