The Effect of a Para Substituent on the Conformational Preference of 2,2-Diphenyl-1,3-dioxanes: Evidence for the Anomeric Effect from X-ray Crystal Structure Analysis(1).

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The molecular structures of 2,2-di(para-substituted phenyl)-1,3-dioxanes were elucidated for the first time by X-ray crystallographic analysis, which revealed two important structural features: (1) These compounds have the chair conformation in which electron-withdrawing aryl groups [viz. p-nitro- or p-(trifluormethyl)phenyl] are always axial and electron-donating aryl groups (viz. p-methoxyphenyl) are always equatorial. (2) In these compounds as well as in symmetrically substituted 2,2-diphenyl-1,3-dioxane the axial C(2)-aryl bond is longer than the equatorial C(2)-aryl bond. The axial preference of the electron-withdrawing aryl group was also demonstrated in solution by (1)H and (13)C NMR spectroscopy. The anomeric carbon substituted with an electron-withdrawing aryl group resonates at an unusually high field, as does the aromatic carbon bearing the electron-withdrawing substituent. The observed (13)C NMR data clearly indicate enhanced electron density at these carbons due to the anomeric effect. Semiempirical molecular orbital calculations by the MOPAK PM3 method reproduced the bond lengthening for axial C(2)-aryl, while the heat of formation derived from this calculation failed to support the axial preference of electron-withdrawing aryl groups. The X-ray crystallographic data on the conformational preference and bond lengths at the anomeric carbon, as well as the solution NMR spectroscopic data, clearly indicate the anomeric effect that is best rationalized in terms of stabilizing interaction between the lone-pair electrons on the ring oxygens and the antibonding orbital of the axial C(2)-aryl bond.