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
- Formation of Alkoxy-Lithium Intermediate: An ether reacts with an alkyllithium to form an alkoxy-lithium intermediate.
- Radical Cleavage: Homolytic cleavage yields a radical pair, with lithium migrating from the carbon to the oxygen.
- Recombination: The alkyl (or aryl) group migrates to the oxygen center, and the radical pair recombines to produce the rearranged alcohol.
Mechanism for 2,3-Wittig Rearrangement
- Carbanion Formation: Similar to the [1,2] type, a strong base generates a carbanion on the carbon adjacent to the oxygen.
- Concerted Shift: The key step is a concerted sigmatropic shift, where three pairs of electrons reorganize simultaneously through a five-membered cyclic transition state.
Migratory Aptitude:
Tertiary alkyl > secondary alkyl > primary alkyl > methyl.
| Comparision: [1,2] Wittig Rearrangement vs. [2,3] Wittig Rearrangement | ||
|---|---|---|
| Feature | [1,2] Wittig Rearrangement | [2,3] Wittig Rearrangement |
| Mechanism Type | Radical-pair dissociation-recombination (non-concerted). | Concerted sigmatropic shift (via a five-membered cyclic transition state). |
| Atoms Involved in Shift | 2-atom shift (group moves from O to the adjacent α-carbon). | 3-atom shift (rearrangement across three atoms). |
| Temperature | High temperatures (>100°C) | Low temperatures (-78°C to 0°C) |
| Product Selectivity | Low selectivity, often yielding complex mixtures (thermodynamic control). | High regioselectivity and stereoselectivity (kinetic control). |