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

The molecular structures of 2,2-di(para-substituted phenyl)-1,3-dioxanes we re elucidated for the first time by X-ray crystallographic analysis, which revealed two important structural features: (1) These compounds have the ch air conformation in which electron-withdrawing aryl groups [viz. p-nitro- o r p-(trifluormethyl)phenyl] are always axial and electron-donating aryl gro ups (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 preferen ce of the electron-withdrawing aryl group was also demonstrated in solution by H-1 and C-13 NMR spectroscopy. The anomeric carbon substituted with an electron-withdrawing aryl group resonates at an unusually high geld, as doe s the aromatic carbon bearing the electron-withdrawing substituent. The obs erved C-13 NMR data clearly indicate enhanced electron density at these car bons due to the anomeric effect. Semiempirical molecular orbital calculatio ns by the MOPAK PM3 method reproduced the bond lengthening for axial C-2-ar yl, while the heat of formation derived from this calculation failed to sup port the axial preference of electron-withdrawing aryl groups. The X-ray cr ystallographic data on the conformational preference and bond lengths at th e anomeric carbon, as well as the solution NMR spectroscopic data, clearly indicate the anomeric effect that is best rationalized in terms of stabiliz ing interaction between the lone-pair electrons on the ring oxygens and the antibonding orbital of the axial C-2-aryl bond.