The directional changes in torsion angles alone after complexation of the carbonyl oxygen with a prototypical cation such as H⁺ predict the facial selectivity of substituted cyclohexanones. An ab initio investigation

Abstract

H⁺ was taken as a prototypical cation for complexation with the carbonyl oxygen of 3-oxa-, 3,5-dioxa-, and 3-thiacyclohexanones. The geometries of the complexes were fully optimized using ab initio MO calculations with 6-31G basis set. The complexation desymmetrizes the molecular geometry further to an extent that the torsion angle changes on the axial face can be rationally used for the prediction of the facial selectivity in reactions with nucleophiles. The torsion angle changes are sensitive to the nature, position, and orientation of the substitutents. Based on the theory of stereoelectronic control and corroborated by ab initio MO calculations, a simple approach to the prediction of facial control in reactions of selected substituted cyclohexanones with nucleophiles is described. Some evidence is also presented against the known transition state models.