Allylic rearrangement

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:

1. 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.

2. 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.

3. 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:

    1. Protonation of Hydroxyl Group

       • The -OH group is protonated by an acid, forming an oxonium ion.

    2. 1,2-Alkyl Shift (Rearrangement Step)

       • A methyl or alkyl group shifts to the adjacent carbon.

    3. 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.

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