Suprafacial Sigmatropic Rearrangement

Definition

A suprafacial sigmatropic rearrangement is a pericyclic reaction in which a $\sigma$ bond migrates from one atom to another within the same molecule, with the migrating group and the new bond forming on the same side (suprafacial) of the molecular plane.

5 Must Know Facts For Your Next Test

1. Suprafacial sigmatropic rearrangements follow the $(n,m)$ notation, where $n$ is the number of atoms in the migrating group and $m$ is the number of atoms the group migrates across.
2. These rearrangements typically involve the migration of hydrogen, alkyl, or aryl groups and are often observed in the context of electrocyclic reactions and Diels-Alder cycloadditions.
3. The stereochemistry of the reactants is retained in the products of a suprafacial sigmatropic rearrangement, as the migrating group and new bond form on the same side of the molecule.
4. Suprafacial sigmatropic rearrangements are classified as allowed pericyclic reactions according to the Woodward-Hoffmann rules, which predict the stereochemical outcomes of these transformations.
5. Examples of suprafacial sigmatropic rearrangements include the Cope rearrangement, the Claisen rearrangement, and the ene reaction.

Review Questions

• Explain the key features of a suprafacial sigmatropic rearrangement and how it differs from an antarafacial rearrangement.
  • A suprafacial sigmatropic rearrangement is a pericyclic reaction where a $\sigma$ bond migrates from one atom to another within the same molecule, with the migrating group and the new bond forming on the same side (suprafacial) of the molecular plane. This is in contrast to an antarafacial rearrangement, where the migrating group and new bond form on opposite sides of the molecular plane. The key distinction is the stereochemistry of the products, as suprafacial rearrangements retain the original stereochemistry, while antarafacial rearrangements result in the inversion of stereochemistry.
• Describe the $(n,m)$ notation used to classify suprafacial sigmatropic rearrangements and provide examples of common rearrangements that fall under this category.
  • Suprafacial sigmatropic rearrangements are classified using the $(n,m)$ notation, where $n$ is the number of atoms in the migrating group and $m$ is the number of atoms the group migrates across. Common examples of suprafacial sigmatropic rearrangements include the Cope rearrangement $(3,3)$, the Claisen rearrangement $(3,3)$, and the ene reaction $(2,3)$. These rearrangements typically involve the migration of hydrogen, alkyl, or aryl groups and are often observed in the context of electrocyclic reactions and Diels-Alder cycloadditions.
• Explain the significance of the Woodward-Hoffmann rules in predicting the stereochemical outcomes of suprafacial sigmatropic rearrangements and discuss the importance of these reactions in organic synthesis.
  • The Woodward-Hoffmann rules are crucial in predicting the stereochemical outcomes of suprafacial sigmatropic rearrangements. These rules classify pericyclic reactions as either allowed or forbidden based on the number of $\pi$ electrons involved, and they predict that suprafacial sigmatropic rearrangements will retain the original stereochemistry of the reactants. This predictability makes suprafacial sigmatropic rearrangements valuable synthetic tools, as they allow for the selective formation of desired stereoisomers. These rearrangements are widely employed in the synthesis of complex organic molecules, particularly in the context of natural product synthesis and the preparation of pharmaceutically relevant compounds. The ability to control stereochemistry is a key advantage of suprafacial sigmatropic rearrangements in organic synthesis.

Related terms

Pericyclic Reaction: A pericyclic reaction is a concerted rearrangement of bonds in a cyclic transition state, often involving the movement of $\pi$ electrons.

Sigmatropic Shift: A sigmatropic shift is a type of pericyclic reaction where a $\sigma$ bond migrates from one atom to another within the same molecule.

Antarafacial Rearrangement: An antarafacial rearrangement is a pericyclic reaction where the migrating group and the new bond form on opposite sides of the molecular plane.