Definition:
Allylic rearrangement, also known as allylic shift or allylic 1,3-shift, involves the migration of a substituent or functional group from one allylic position to another in a molecule.
Allylic positions are carbon atoms adjacent to a carbon-carbon double bond.
Mechanism:
1.Formation of Allylic Intermediates:
Allylic rearrangements often involve the formation of allylic carbocations or allylic radicals.
These intermediates are stabilized by the delocalization of electrons via resonance.
2.Migration (1,2-Shift):
The migrating group moves from one allylic position to another, typically involving a 1,2-shift.
3.Rearrangement and Product Formation:
After the migration, the rearranged product is formed, often with increased stability due to resonance or other factors.
Example: Wagner-Meerwein Rearrangement
Reaction: Allylic alcohol treated with a strong acid.
Steps:
Protonation of the hydroxyl group to form an allylic oxonium ion.
1,2-Alkyl shift (Wagner-Meerwein rearrangement).
Deprotonation to yield the rearranged product.
Example Reaction:
Starting Material: 3-methyl-2-butene-1-ol.
Steps:
Protonation of the hydroxyl group.
1,2-Alkyl shift of the methyl group.
Deprotonation to yield the rearranged product, 3-methyl-2-buten-1-ol.
Significance:
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.