Because the reaction is concerted, it exhibits a high degree of stereocontrol, and can be employed early in a synthetic route to establish stereochemistry.
Stereoselective methods employing chiral starting materials have been used to effect either asymmetric induction or simple diastereoselection[3] (1)After carbanion formation, the [2,3]-Wittig rearrangement is rapid and selective at low temperatures.
Large substituents on the other side of the ether oxygen prefer to occupy the exo position (RE) to avoid A1,3 strain.
[7] (4)The asymmetric induction approach relies on stereocenters already set in the starting material that are unaffected by the reaction (chiral auxiliaries).
Enantioselectivity in these reactions is often low, suggesting that the association between the conjugate acid of the base and the rearranging carbanion is likely weak.
(6)A variety of allylic ethers undergo the Wittig rearrangement—the fundamental requirement is the ability to generate the appropriate carbanion in the substrate.
[14] (9)A highly enantioselective method employing chromium carbonyl complexes involves the use of the acidified phenyl ring as an anion-stabilizing group.
Thus, the development of transmetallation methods that allowed the selective generation of carbanions from carbon-tin bonds represented a profound methodological advance.