Antarafacial Sigmatropic Rearrangement

Definition

An antarafacial sigmatropic rearrangement is a type of pericyclic reaction where a substituent migrates from one position to another within a molecule, with the new substituent position being on the opposite face of the molecule relative to the original position.

5 Must Know Facts For Your Next Test

1. Antarafacial sigmatropic rearrangements involve the migration of a substituent to the opposite face of the molecule compared to its original position.
2. These rearrangements are characterized by a cyclic transition state where the migrating substituent moves in the opposite direction to the new substituent position.
3. Antarafacial sigmatropic rearrangements often occur in cyclic systems and can lead to the formation of new stereoisomers.
4. The stereochemistry of the product is determined by the orientation of the migrating substituent relative to the rest of the molecule.
5. Antarafacial sigmatropic rearrangements are less common than their suprafacial counterparts due to the higher energy barrier required to achieve the opposite-face migration.

Review Questions

• Explain the key features that distinguish an antarafacial sigmatropic rearrangement from other types of sigmatropic rearrangements.
  • The defining feature of an antarafacial sigmatropic rearrangement is that the migrating substituent moves to the opposite face of the molecule compared to its original position. This is in contrast to a suprafacial sigmatropic rearrangement, where the substituent migrates to the same face. Antarafacial rearrangements require a higher energy transition state due to the opposite-face migration, making them less common than their suprafacial counterparts. Additionally, antarafacial rearrangements can lead to the formation of new stereoisomers, as the orientation of the migrating substituent relative to the rest of the molecule is altered.
• Describe the role of the cyclic transition state in an antarafacial sigmatropic rearrangement and how it influences the stereochemistry of the product.
  • In an antarafacial sigmatropic rearrangement, the migration of the substituent occurs through a cyclic transition state. This transition state is characterized by the migrating substituent moving in the opposite direction to the new substituent position, resulting in the substituent ending up on the opposite face of the molecule. The orientation of the migrating substituent within this cyclic transition state ultimately determines the stereochemistry of the final product, as the new substituent position will be opposite to the original position. This allows for the potential formation of new stereoisomers that would not be accessible through a suprafacial rearrangement.
• Analyze the factors that contribute to the relative rarity of antarafacial sigmatropic rearrangements compared to suprafacial rearrangements, and explain the significance of this difference in organic chemistry.
  • Antarafacial sigmatropic rearrangements are less common than their suprafacial counterparts due to the higher energy barrier required to achieve the opposite-face migration of the substituent. The cyclic transition state necessary for an antarafacial rearrangement is more strained and energetically unfavorable compared to the transition state of a suprafacial rearrangement, where the substituent migrates to the same face of the molecule. This increased energy requirement makes antarafacial rearrangements less kinetically favorable, leading to their relative scarcity in organic reactions. The rarity of antarafacial rearrangements is significant because they represent a unique class of pericyclic reactions that can provide access to different stereochemical outcomes, which is crucial for the synthesis of complex organic molecules with specific spatial arrangements. Understanding the factors that govern the prevalence of these rearrangements is essential for predicting and controlling the stereochemical outcomes of organic transformations.

Related terms

Sigmatropic Rearrangement: A sigmatropic rearrangement is a pericyclic reaction where a substituent migrates from one position to another within a molecule.

Suprafacial Sigmatropic Rearrangement: A suprafacial sigmatropic rearrangement is a type of pericyclic reaction where a substituent migrates from one position to another within a molecule, with the new substituent position being on the same face of the molecule relative to the original position.

Pericyclic Reaction: A pericyclic reaction is a type of organic reaction that involves the cyclic movement of electrons.