Allylic rearrangement Definition:
- Allylic rearrangement (or allylic shift) involves the migration of a substituent or functional group from one allylic position to another within a molecule.
- Allylic positions are carbon atoms adjacent to a carbon-carbon double bond (C=C).
Mechanism of Allylic rearrangement:
-
Formation of Allylic Intermediates
- The reaction typically involves the formation of allylic carbocations or allylic radicals.
- These intermediates are stabilized by resonance, allowing electron delocalization.
-
Migration (1,2-Shift)
- A functional group (e.g., alkyl or hydroxyl group) shifts from one allylic carbon to another.
- This step allows for greater stability due to resonance effects.
-
Rearrangement and Product Formation
- After migration, the molecule rearranges into a more stable product, often with enhanced regio- or stereoselectivity.
Example: Wagner-Meerwein Rearrangement
- A common allylic rearrangement that occurs when an allylic alcohol is treated with a strong acid.
-
Steps of the Reaction:
-
Protonation of Hydroxyl Group
- The -OH group is protonated by an acid, forming an oxonium ion.
-
1,2-Alkyl Shift (Rearrangement Step)
- A methyl or alkyl group shifts to the adjacent carbon.
-
Deprotonation and Product Formation
- The molecule rearranges, forming a more stable product.
-
-
Example Reaction:
Starting Material: 3-Methyl-2-Butene-1-Ol
$CH3-CH=CH-CH2OH$
Significance of Allylic rearrangement:
- Allylic rearrangements are important in organic chemistry:
- They lead to the formation of different structural or regioisomers.
- They are encountered in the synthesis of complex organic compounds.
- They can be useful for controlling the regioselectivity and stereoselectivity of reactions.
Thank you for reading from Firsthope's notes, don't forget to check YouTube videos!