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.
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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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