Conformational Preference of the 2-Diphenylphosphinoyl Group in 1,3-Dioxane. Is There an O-C-P(O) Anomeric Effect?

The use of counterpoise substituents permitted the quantitative determination of the large equatorial preference of the 2-diphenylphosphinoyl group in 1,3-dioxane, ΔG°_307K[P(O)Ph₂] = -3.23 kcal/mol. This result contrasts strongly with the significant axial preference of the same group in the 1,3-dithiane ring (+1.0 kcal/mol). Nevertheless, evaluation of the different steric requirements in these heterocycles reveals that the magnitude of the OC-P(O) and S-C-P(O) anomeric effects in the sulfur and oxygen heterocycles is quite similar, close to 3 kcal/ mol.