Sigmatropic reaction

Sigmatropic reaction is a type of chemical reaction that involves a rearrangement of sigma bonds (σ bonds) within a molecule, accompanied by the migration of one or more pi bonds (π bonds). These reactions are a subset of pericyclic reactions, which are characterized by their concerted nature, meaning that bonds are broken and formed in a single step without the formation of intermediates. Sigmatropic reactions are governed by the Woodward-Hoffmann rules, which predict the stereochemistry and feasibility of pericyclic reactions based on the conservation of orbital symmetry.

Overview

Sigmatropic reactions involve the shift of a σ bond adjacent to one or more π systems, leading to a new arrangement of atoms within the molecule. The most common types of sigmatropic reactions are classified by the [i,j] notation, where i and j denote the positions of the atoms involved in the σ bond migration relative to the π system. For example, a [1,5] sigmatropic shift involves the migration of a σ bond over five atoms.

Types of Sigmatropic Reactions

Several types of sigmatropic reactions are recognized, including but not limited to:

• Cope Rearrangement: A [3,3] sigmatropic rearrangement involving the migration of an alkylidene group over a diene system. This reaction is thermally allowed and can proceed under mild conditions.
• Claisen Rearrangement: A [3,3] sigmatropic shift that occurs in allyl vinyl ethers, leading to the formation of γ,δ-unsaturated carbonyl compounds. The Claisen rearrangement is an important method for the synthesis of complex molecular structures in organic chemistry.
• Carroll Rearrangement: A less common [1,5] sigmatropic shift that involves the migration of a σ bond over five atoms, typically resulting in the formation of new chiral centers.
• Heteroatom Sigmatropic Reactions: These include shifts involving heteroatoms such as oxygen, nitrogen, or sulfur. An example is the Oxy-Cope Rearrangement, a variant of the Cope rearrangement that involves oxygen-containing substrates.

Mechanism

The mechanism of sigmatropic reactions involves the concerted migration of a σ bond along with the reorganization of π electrons. This process is facilitated by the cyclic overlap of molecular orbitals, which allows for the simultaneous making and breaking of bonds. The specific details of the mechanism depend on the type of sigmatropic reaction and the nature of the substrates involved.

Applications

Sigmatropic reactions are widely used in the synthesis of complex organic molecules, including natural products and pharmaceuticals. Their ability to efficiently rearrange molecular frameworks while forming new chiral centers makes them valuable tools in synthetic organic chemistry.

Woodward-Hoffmann Rules

The Woodward-Hoffmann rules play a crucial role in predicting the outcome of sigmatropic reactions. These rules, based on the conservation of orbital symmetry, help determine whether a pericyclic reaction will proceed under thermal or photochemical conditions and predict the stereochemistry of the product.

See Also

• Pericyclic reaction
• Cope Rearrangement
• Claisen Rearrangement
• Woodward-Hoffmann rules