Origin of facial diastereoselection. Evidence for negative role of antiperiplanar hyperconjugation effects in the transition state of carbene insertion.

[reaction: see text] Quantitative analysis of the transition state (TS) structures of the insertion reactions of 5-X-2-adamantanylidenes (X = H, OH, NH(2), SiMe(3)) into MeOH or cyclohexane revealed that the magnitude of antiperiplanar hyperconjugative stabilization involving the incipient bond effects (the AP effect) decrease at TS, strongly suggesting that the AP effect contributes "net destabilization" 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 carbene bridge-flipping proximal-distal conformers was found to be the origin of facial diastereoselection of these carbenes.