key term - [1,3] sigmatropic rearrangement

Definition

A [1,3] sigmatropic rearrangement is a type of molecular rearrangement in which a sigma bond and a pi bond are involved in a migration process, leading to the reorganization of atoms or groups in a molecule. This rearrangement specifically involves the shifting of a substituent from one atom to another that is separated by one single bond, effectively transforming the molecular structure. The concept is essential in understanding how certain reactions can occur through concerted mechanisms and plays a significant role in various synthetic pathways.

5 Must Know Facts For Your Next Test

1. [1,3] sigmatropic rearrangements are commonly seen in reactions such as the Cope and Claisen rearrangements, showcasing their importance in organic synthesis.
2. The rearrangement can occur via different pathways depending on whether it is thermal or photochemical, with specific stereochemical outcomes.
3. During the rearrangement, electrons move in a concerted manner, which means there are no stable intermediates formed during the process.
4. The concept of Woodward-Hoffmann rules can help predict the feasibility and stereochemistry of [1,3] sigmatropic rearrangements based on orbital symmetry.
5. Products formed from [1,3] sigmatropic rearrangements can be influenced by factors like sterics and electronics, affecting the reaction pathway and selectivity.

Review Questions

• What are the characteristics of [1,3] sigmatropic rearrangements, and how do they differ from other types of rearrangements?
  • [1,3] sigmatropic rearrangements are characterized by the migration of a substituent from one atom to another across a single bond, involving a concerted mechanism where bonds are broken and formed simultaneously. This contrasts with other types of rearrangements that may proceed through stable intermediates or involve different bond migrations. The specific feature of being '1,3' indicates the direct relationship between the atoms involved in the migration process, which influences both reaction kinetics and mechanisms.
• Discuss the significance of [1,3] sigmatropic rearrangements in organic synthesis, particularly focusing on the Cope and Claisen rearrangements.
  • [1,3] sigmatropic rearrangements play a crucial role in organic synthesis by providing efficient pathways to generate complex molecules from simpler precursors. The Cope rearrangement allows for the conversion of 1,5-hexadiene into various isomeric products under thermal conditions, while the Claisen rearrangement offers access to allylic alcohols through the reaction of allyl vinyl ethers. Both reactions highlight how these rearrangements can be utilized strategically to construct functionalized organic compounds while exhibiting predictable regio- and stereochemistry.
• Evaluate how Woodward-Hoffmann rules apply to [1,3] sigmatropic rearrangements and their implications for reaction outcomes.
  • Woodward-Hoffmann rules are essential for predicting the outcomes of [1,3] sigmatropic rearrangements by analyzing the symmetry properties of molecular orbitals involved in the reaction. When assessing whether a particular [1,3] rearrangement will proceed thermally or photochemically, these rules help identify if the transition state is allowed or forbidden based on orbital symmetry considerations. This application can provide insight into stereoselectivity and reactivity trends for different substrates undergoing this type of rearrangement, ultimately guiding synthetic strategies in organic chemistry.

Related terms

Sigmatropic Rearrangement: A class of reactions where a sigma bond breaks and reforms as part of the movement of atoms or groups within a molecule.

Concerted Reaction: A reaction mechanism where bonds are formed and broken simultaneously in a single step, without intermediates.

Pericyclic Reaction:

A class of reactions that involves the concerted reorganization of bonding electrons through cyclic transition states.