text: In an effort to explain the surprising stereoselectivities in the systems above, alternative explanations to the Cieplak effect have been proposed. In substituted cyclohexanones, the tendency of small reducing agents to add hydride axially is proposed to be caused by torsional strain instead of hyperconjugation. In an equatorial attack, the nucleophile approaches by eclipsing a neighboring hydrogen atom and subsequently pushes the carbonyl substituents into eclipsing positions as it pyramidalizes the carbonyl carbon. In an axial approach, the nucleophile approaches gauche to neighboring hydrogen atoms and so does not cause eclipsing interactions while pyramidalizing the carbonyl carbon. It is this torsional strain—the energy cost of rotating the bonds out of an eclipsed position—that favors axial over equatorial approach.
  In the case of substituted norbornones, stereoselectivity may be explained by electrostatic interactions between substituents and nucleophiles. Electron-withdrawing groups create a partial positive charge on the alpha carbon, which interacts favorably with the partial negative charge on the incoming nucleophile. This interaction may guide attack syn to the electron-withdrawing substituent, and anti to electron-donating substituents. This conclusion is supported by computations, where modeling the partial charges predicts product distribution without including orbital interactions. The same explanation has been made to justify similar results in the case of substituted adamantones.
  Similarly, in Houk's trans-decalone system, the nucleophile with its partial negative charge prefers to attack away from the partial negative charge of the acyl ester. When this substituent is axial, the equatorial pathway brings the nucleophile into closer proximity and is therefore disfavored. This is less pronounced for equatorially substituted ester because the group is now positioned further away from the carbonyl.