The Cation Complexation Model Predicts the Experimental π-Facial Selectivity of 2-ax- and 2-eq-Substituted Cyclohexanones. A Detailed Ab Initio MO Investigation

The geometrical changes on complexation of the carbonyl oxygen with prototypical cations such as H+ and Li+ are in tune with the polarity features of the C–X bonds in 2-ax-X-cyclohexanones (X=Cl, F, SR, OR; R=H, Me); the stereoelectronic effects apply. While the 2-ax-Cl- and 2-ax-SR-cyclohexanones are predicted to favor axial attacks, the 2-ax-OR- and 2-ax-F-cyclohexanones must capture a nucleophile predominantly from the eq-direction. The eq-selectivity of 2-ax-OR-cyclohexanone is in contradiction with the torsional model of Anh and Felkin. The Houk model will also fail due to the eminent steric interactions arising from the 2-ax-OMe function in the eq-TS. Neither the Anh–Felkin model nor the Cieplak model could be applied to 2-eq-MeO-cyclohexanone. The complexation model, however, predicts eq-selectivity in full agreement with experimental results. Second order perturbation theory analysis of the Fock matrix in NBO basis indicates that the role of the antiperiplanar effects is not as significant as perceived earlier by Cieplak and by Anh and Felkin.