The cation complexation model predicts the experimental pi-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 prototy pical 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 ster eoelectronic effects apply. While the 2-ax-Cl- and 2-ax-SR-cyclohexanones a re 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-sel ectivity of 2-ax-OR-cyclohexanone is in contradiction with the torsional mo del of Anh and Felkin. The Houk model will also fail due to the eminent ste ric interactions arising from the 2-ax-OMe function in the eq-TS. Neither t he Anh-Felkin model nor the Cieplak model could be applied to 2-eq-MeO-cycl ohexanone. The complexation model, however, predicts eq-selectivity in full agreement with experimental results. Second order perturbation theory anal ysis of the Fock matrix in NBO basis indicates that the role of the antiper iplanar effects is not as significant as perceived earlier by Cieplak and b y Anh and Felkin. (C) 2000 Elsevier Science Ltd. All rights reserved.