Wittig Rearrangement

The Wittig rearrangement is a sigmatropic rearrangement that involves the migration of an alkyl group from an oxygen atom to a carbon atom within an ether, forming a new carbon-carbon bond and producing an alcohol.

Types of Wittig Rearrangement

1,2-Wittig Rearrangement: Involves the migration of an alkyl group from the carbon atom adjacent to an ether oxygen, leading to formation of a new alcohol. The mechanism typically involves a free radical pair intermediate, with lithium transferring from the carbon to the oxygen, followed by recombination of the radicals.

2,3-Wittig Rearrangement: Known as a -sigmatropic rearrangement, where the migration occurs over two bonds. This process often involves allyl ethers and gives homoallylic alcohols as products. It is believed to proceed via a concerted, pericyclic mechanism.

Mechanism (for 1,2-Wittig Rearrangement)

1. Formation of Alkoxy-Lithium Intermediate: An ether reacts with an alkyllithium to form an alkoxy-lithium intermediate.
2. Radical Cleavage: Homolytic cleavage yields a radical pair, with lithium migrating from the carbon to the oxygen.
3. Recombination: The alkyl (or aryl) group migrates to the oxygen center, and the radical pair recombines to produce the rearranged alcohol.

Migratory Aptitude:

Tertiary alkyl > secondary alkyl > primary alkyl > methyl, reflecting radical stabilization ability