Origin of facial diastereoselection in 2-adamantyl cations. Theoretical evidence against the Felkin–Anh and the Cieplak models

Quantitative analysis of the transition state (TS) structures for the reactions between 5-Cl- or 5-SiMe3-2-adamantyl cations and methanol revealed that the magnitude of antiperiplanar hyperconjugative stabilization involving the incipient bond (the AP effect) decrease at TS, strongly suggesting that the AP effect contributes to `net destabilizationʹ of TS (i.e., increase in activation energy), in sharp contrast to the proposals of the Felkin–Anh and the Cieplak models. The equilibrium population between two bridge(C1–C2–C3)-flipping E–Z cation conformers was found to be the origin of facial diastereoselection of these carbocations.