Lack of manifestation of an anomeric effect in 2‐diphenylphosphinoyl‐1, 3‐dioxane and 2‐diphenylphosphinoyl‐1, 3‐oxathiane¹

The spectroscopic evidence for the predominance of the equatorial conformers in the title compounds was confirmed by the study of derivatives containing counterpoise substituents, and by chemical equilibration of anancomeric models. ΔG 27o_°C [P(O)Ph₂] ≳ 3·2 kcal/mol was determined in the dioxane, and ΔG 55o_°C [P(O)Ph₂] = 1·42 ± 0·12 kcal/mol in the oxathiane. It follows then that the strong anomeric interaction observed previously in SCP segments does not show up in the six‐membered heterocycles 2 and 7, which contain OCP moieties. This may be due to an inherent inability of oxygen to act as an electron donor to the axial P(O)Ph₂ substituent, or to a dominant repulsive steric interaction in the axial conformers.