3-Halocyclohexanones. Torsion angle changes after cation-carbonyl complexation dictate the facial selectivity in reactions with nucleophiles: an ab initio investigation

Abstract

The geometrical changes that take place after complexation of the carbonyl oxygen of 3-halocyclohexanones with prototypical cations such as H⁺ and Li⁺ were calculated using ab initio MO methods at 6-31G level. The torsion angle changes interpret rationally the experimentally known axial preference of the 3-eq-derivatives and the axial preference (predicted from transition state calculations) of the 3-ax-species. Stereoelectronic effects in 3-ax-halocyclohexanones and 1,3-dipolar interactions in 3-eq-halocyclohexanones after complexation of the carbonyl oxygen with cations such as H⁺ and Li⁺ account for the experimentally observed and the predicted diastereoselectivities, respectively, of these molecules. The complexation induces pyramidalization of the carbonyl carbon and controls the p orbial to adopt an energetically favorable orientation (ax or eq) for capture by a nucleophile.