[3,3] Sigmatropic Rearrangement

Definition

A [3,3] sigmatropic rearrangement is a type of pericyclic reaction in organic chemistry where a substituent migrates from one atom to a neighboring atom through a cyclic transition state involving a 3-membered ring. This rearrangement is a key concept in understanding the reactivity and mechanisms of certain organic transformations.

5 Must Know Facts For Your Next Test

1. The [3,3] designation in the term refers to the number of atoms involved in the cyclic transition state of the rearrangement, with 3 atoms in the migrating group and 3 atoms in the receiving group.
2. These rearrangements are typically thermally allowed and proceed through a concerted, pericyclic mechanism with a cyclic, six-membered transition state.
3. The [3,3] sigmatropic rearrangement is a key step in many important organic transformations, such as the Claisen rearrangement and the Cope rearrangement.
4. The stereochemistry of the starting material is often retained in the product of a [3,3] sigmatropic rearrangement due to the concerted, pericyclic nature of the reaction.
5. The migratory aptitude of the substituents involved in the [3,3] sigmatropic rearrangement can be predicted using the Woodward-Hoffmann rules for pericyclic reactions.

Review Questions

• Describe the key features of a [3,3] sigmatropic rearrangement and explain how it differs from other types of sigmatropic shifts.
  • A [3,3] sigmatropic rearrangement is a pericyclic reaction in which a substituent migrates from one atom to a neighboring atom through a cyclic transition state involving a 3-membered ring. This is in contrast to other sigmatropic shifts, such as [1,3] or [1,5] shifts, which involve different-sized cyclic transition states. The [3,3] designation refers to the number of atoms in the migrating group and the receiving group, respectively. These rearrangements typically proceed through a concerted, thermally allowed mechanism and often retain the stereochemistry of the starting material.
• Explain the role of the Woodward-Hoffmann rules in predicting the feasibility and stereochemical outcome of a [3,3] sigmatropic rearrangement.
  • The Woodward-Hoffmann rules for pericyclic reactions can be used to predict the feasibility and stereochemical outcome of a [3,3] sigmatropic rearrangement. These rules consider the symmetry of the molecular orbitals involved in the cyclic transition state to determine whether the reaction is thermally allowed or photochemically allowed. For a [3,3] sigmatropic rearrangement, the reaction is typically thermally allowed and proceeds through a concerted, pericyclic mechanism. The Woodward-Hoffmann rules can also be used to predict the stereochemistry of the product, as the rearrangement often retains the stereochemistry of the starting material due to the concerted nature of the transformation.
• Discuss the importance of [3,3] sigmatropic rearrangements in organic synthesis and provide examples of key reactions that involve this type of transformation.
  • The [3,3] sigmatropic rearrangement is a crucial concept in organic chemistry, as it is a key step in many important organic transformations. Two prominent examples are the Claisen rearrangement and the Cope rearrangement. The Claisen rearrangement involves the migration of an allyl or benzyl group from an ether or ester to an adjacent carbon, forming a new carbon-carbon bond. The Cope rearrangement is a [3,3] sigmatropic shift of a substituent from one carbon to another in a cyclic transition state. These rearrangements are widely used in organic synthesis for the construction of complex molecules and the formation of new carbon-carbon bonds. Understanding the mechanism and stereochemical outcomes of [3,3] sigmatropic rearrangements is essential for designing efficient synthetic routes and predicting the reactivity of organic compounds.

Related terms

Pericyclic Reaction: A pericyclic reaction is a type of organic reaction that involves the concerted reorganization of bonds within a cyclic transition state.

Sigmatropic Shift: A sigmatropic shift is a pericyclic reaction in which a substituent migrates from one position in a molecule to a new position, usually adjacent, with the migration path forming a cyclic transition state.

Cope Rearrangement: The Cope rearrangement is a [3,3] sigmatropic rearrangement involving the migration of a substituent from one carbon to another in a cyclic transition state.