[3,3] sigmatropic rearrangement

Definition

[3,3] sigmatropic rearrangement is a type of pericyclic reaction where a sigma bond and a π bond rearrange their connectivity through a concerted process, involving a migration of a substituent across a six-membered cyclic transition state. This rearrangement typically involves the movement of two pairs of electrons, allowing for the conversion of starting materials into new products, showcasing the fascinating interplay between molecular orbitals and bonding interactions.

5 Must Know Facts For Your Next Test

1. [3,3] sigmatropic rearrangements are characterized by the migration of groups or atoms from one part of the molecule to another through a cyclic transition state.
2. These rearrangements can be thermally allowed or photochemically induced, with different products potentially formed under each condition.
3. The Woodward-Hoffmann rules govern the stereochemistry and selectivity of [3,3] sigmatropic rearrangements, allowing predictions on the outcomes based on orbital symmetry.
4. [3,3] sigmatropic rearrangements often involve allylic systems and can lead to complex molecular architectures with distinct functional groups.
5. A classic example is the Cope rearrangement, which demonstrates how [3,3] sigmatropic processes can lead to significant structural changes in organic molecules.

Review Questions

• How does the mechanism of [3,3] sigmatropic rearrangement demonstrate the concept of orbital overlap and electron movement?
  • In a [3,3] sigmatropic rearrangement, the mechanism showcases concerted electron movement where two pairs of electrons migrate through overlapping orbitals. The sigma bond involved in the migration breaks while a new sigma bond forms simultaneously, creating a cyclic transition state. This illustrates how molecular orbitals interact during the reaction and emphasizes the importance of electron delocalization in forming stable products.
• Discuss the role of temperature in determining whether a [3,3] sigmatropic rearrangement will proceed under thermal or photochemical conditions.
  • [3,3] sigmatropic rearrangements can be influenced significantly by temperature. Under thermal conditions, these reactions tend to follow the lowest energy pathway, leading to specific products dictated by orbital symmetry according to Woodward-Hoffmann rules. In contrast, photochemical conditions can provide energy that facilitates different pathways or conformations, potentially resulting in alternative products or mechanisms that might not be accessible under thermal conditions.
• Evaluate how understanding [3,3] sigmatropic rearrangements can contribute to advancements in synthetic organic chemistry and material science.
  • Understanding [3,3] sigmatropic rearrangements opens up various pathways for designing complex molecules in synthetic organic chemistry. This knowledge allows chemists to predict reaction outcomes and strategically create compounds with desired properties. Furthermore, these reactions play a crucial role in material science by enabling the synthesis of polymers and advanced materials through controlled bonding and structural alterations. Therefore, mastery of these concepts supports innovation in developing new chemical entities with specific applications.

Related terms

Pericyclic reactions: Reactions that occur through a concerted mechanism, involving cyclic transition states and often including rearrangements or additions involving π bonds.

Conjugation: A system where alternating single and multiple bonds allow for the delocalization of π electrons across adjacent atoms, stabilizing the molecule.

Electrocyclic reactions: Reactions that involve the formation or breaking of a σ bond in conjunction with the formation or breaking of a π bond, often resulting in cyclization.