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Organic Chemistry
Table of Contents
Unit 30 Overview: Organic Chemistry: Pericyclic Reactions
30.1 Molecular Orbitals of Conjugated Pi Systems
30.2 Electrocyclic Reactions
30.3 Stereochemistry of Thermal Electrocyclic Reactions
30.4 Photochemical Electrocyclic Reactions
30.5 Cycloaddition Reactions
30.6 Stereochemistry of Cycloadditions
30.7 Sigmatropic Rearrangements
30.8 Some Examples of Sigmatropic Rearrangements
30.9 A Summary of Rules for Pericyclic Reactions

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30.8 Some Examples of Sigmatropic Rearrangements

Citation:

Sigmatropic rearrangements are fascinating reactions where bonds shift within molecules. These transformations involve the migration of sigma bonds through cyclic transition states, leading to new molecular structures. They're key in organic synthesis and occur in various systems.

Understanding the mechanisms and orbital considerations of sigmatropic rearrangements is crucial. From [1,5] shifts in cyclic systems to Claisen and Cope rearrangements, these reactions follow specific rules and offer powerful tools for creating complex molecules in chemistry.

Sigmatropic Rearrangements

Sigmatropic rearrangements in cyclic systems

  • [1,5] sigmatropic rearrangement involves migration of a sigma bond between atoms separated by a $\pi$ system through a concerted, cyclic transition state
    • Proceeds through a six-membered cyclic transition state allowing for optimal orbital overlap
  • In cyclopentadiene systems, [1,5] hydrogen shift occurs between positions 1 and 5 interconverting the two double bonds
    • Results in resonance-stabilized cyclopentadienyl anion or cation depending on the direction of the shift
  • In pentadiene systems, [1,5] hydrogen shift occurs between terminal carbons leading to formation of a conjugated diene
    • Increases stability due to extended $\pi$ conjugation
  • Rearrangement is thermally induced and reversible following Woodward-Hoffmann rules for pericyclic reactions
    • Thermally allowed process when total number of $(4q+2)$ $\pi$ electrons is involved in cyclic transition state ($q$ is an integer)
    • Follows a suprafacial-suprafacial pathway conserving stereochemistry
  • Orbital symmetry considerations determine the allowed pathways for these rearrangements

Mechanism of Claisen rearrangement

  • Claisen rearrangement is a [3,3] sigmatropic rearrangement involving migration of an allyl group from an oxygen to a carbon atom
    • Occurs in allyl vinyl ethers or allyl phenyl ethers converting them to $\gamma,\delta$-unsaturated carbonyl compounds
  • Proceeds through a highly ordered, six-membered cyclic transition state with a chair-like conformation
    • Minimizes steric interactions and allows for optimal orbital overlap between the allyl group and the vinyl ether
  • Reaction is initiated by heating, typically around 200°C, and is stereospecific retaining stereochemistry of the allyl group
    • Can be accelerated by electron-donating substituents on the aromatic ring stabilizing the transition state
  • Mechanism involves:
    1. Thermal excitation of the allyl vinyl ether to reach the transition state
    2. Concerted bond breaking and bond formation in the six-membered cyclic transition state
    3. Migration of the allyl group to the $\alpha$-carbon of the newly formed carbonyl compound
    4. Restoration of the carbonyl double bond and formation of the $\gamma,\delta$-unsaturated product
  • Widely used in organic synthesis for generating $\gamma,\delta$-unsaturated aldehydes and ketones
  • The concerted mechanism ensures stereochemical control throughout the rearrangement

Cope vs oxy-Cope rearrangements

  • Cope rearrangement is a [3,3] sigmatropic rearrangement involving migration of a $\sigma$ bond in 1,5-dienes
    • Proceeds through a six-membered cyclic transition state resulting in formation of a new 1,5-diene
    • Requires high temperatures (200-300°C) and is reversible
  • Oxy-Cope rearrangement is a variation of the Cope rearrangement occurring in 1,5-dien-3-ols (allyl vinyl alcohols)
    • Involves migration of an alkoxide anion instead of a carbon-carbon double bond
    • Proceeds through an enolate intermediate resulting in formation of a $\beta,\gamma$-unsaturated carbonyl compound
  • Key differences between Cope and oxy-Cope rearrangements:
    • Oxy-Cope occurs under milder conditions (lower temperatures) due to increased acidity of the allylic alcohol
    • Oxy-Cope results in carbonyl compounds while Cope results in 1,5-dienes
    • Oxy-Cope proceeds through an enolate intermediate while Cope proceeds through a concerted mechanism
  • Applications:
    • Cope rearrangement is used in synthesis of complex organic molecules (natural products, pharmaceuticals)
    • Oxy-Cope rearrangement is employed in synthesis of $\beta,\gamma$-unsaturated carbonyl compounds as valuable synthetic intermediates

Molecular Orbital Considerations in Sigmatropic Rearrangements

  • Thermal activation provides the energy required to overcome the activation barrier and reach the transition state
  • Molecular orbitals play a crucial role in determining the allowed pathways for sigmatropic rearrangements
  • The transition state geometry is influenced by the symmetry and overlap of the participating molecular orbitals

Key Terms to Review (34)

Anti stereochemistry: Anti stereochemistry describes the spatial arrangement in a chemical reaction where two substituents are positioned on opposite sides of a double bond or ring structure after the reaction. It is particularly relevant in the halogenation of alkenes, resulting in products where the added atoms are located across from each other.
Claisen rearrangement: The Claisen rearrangement is a chemical reaction where an allyl vinyl ether is transformed into a γ,δ-unsaturated carbonyl compound through a [3,3]-sigmatropic shift. This reaction involves the movement of sigma bonds and the reorganization of electrons without the formation of free intermediates.
Sigmatropic reaction: A sigmatropic reaction is a type of organic chemical reaction where a sigma bond (σ-bond) migrates across a conjugated pi-electron system in a single, concerted step. These reactions are characterized by their regio- and stereospecificity, occurring without the formation of intermediates.
Cope rearrangement: The Cope rearrangement is a thermal, pericyclic reaction involving the 3,3-sigmatropic rearrangement of 1,5-dienes to form isomeric 1,5-dienes. It occurs without the aid of catalysts and involves a concerted movement of electrons and atomic positions within the molecule.
Transition state: In organic chemistry, the transition state is a high-energy, temporary condition where reactants are transformed into products during a chemical reaction. It represents the point of maximum energy on the energy diagram before the formation of products.
Stereochemistry: Stereochemistry is the study of the three-dimensional arrangement of atoms in molecules and how this arrangement affects the chemical and physical properties of the substance. It examines the spatial orientation of atoms and their relationship to one another, which is crucial in understanding many organic chemistry concepts.
Molecular Orbitals: Molecular orbitals are the wave functions that describe the behavior of electrons in a molecule. They are formed by the combination of atomic orbitals and play a crucial role in understanding the structure, bonding, and reactivity of chemical compounds.
Enolate: An enolate is a negatively charged oxygen-containing species that arises from the removal of a proton from the α-carbon of a carbonyl compound. Enolates are important reactive intermediates in various organic reactions, including aldol condensations, Claisen condensations, and α-substitution reactions.
Claisen Rearrangement: The Claisen rearrangement is a sigmatropic rearrangement reaction in organic chemistry where an allyl vinyl ether is converted into a substituted cyclohexenone. It is a powerful tool for forming carbon-carbon bonds and creating complex cyclic structures.
Transition State: The transition state is a key concept in organic chemistry that describes the highest-energy intermediate along the reaction pathway. It represents the point where the reactants are being converted into products, with the system at its most unstable and energetically unfavorable configuration.
Concerted Mechanism: A concerted mechanism refers to a reaction that occurs in a single, continuous step without the formation of any discrete intermediates. In a concerted mechanism, the bonds that are being formed and broken happen simultaneously, leading to the product in a single, coordinated process.
Conjugated Diene: A conjugated diene is a type of organic compound that contains two carbon-carbon double bonds separated by a single carbon-carbon bond. This structural arrangement allows for the delocalization of electrons, leading to unique chemical properties and reactivity.
Pericyclic Reactions: Pericyclic reactions are a class of organic reactions that involve the concerted rearrangement of pi-electrons within a cyclic transition state. These reactions are characterized by their unique mechanism, which allows for the formation or cleavage of cyclic structures through the simultaneous breaking and forming of chemical bonds.
Cyclopentadiene: Cyclopentadiene is a cyclic, unsaturated hydrocarbon with the molecular formula C₅H₆. It is a key component in the Diels-Alder cycloaddition reaction and is also involved in the formation of aromatic ions and sigmatropic rearrangements.
Woodward-Hoffmann Rules: The Woodward-Hoffmann rules are a set of principles that describe the stereochemical outcomes of pericyclic reactions, such as electrocyclic reactions, cycloadditions, and sigmatropic rearrangements. These rules provide a framework for predicting the feasibility and stereochemistry of these types of organic reactions based on the topology of the molecular orbitals involved.
Cyclopentadienyl cation: The cyclopentadienyl cation is a planar, aromatic carbocation with a five-membered ring structure. It is a key intermediate in various organic reactions, particularly those involving sigmatropic rearrangements.
Cyclopentadienyl Anion: The cyclopentadienyl anion is a planar, aromatic, and highly stabilized negatively charged species consisting of a five-membered carbon ring with one delocalized electron. It is a key intermediate in various organic chemistry reactions and concepts.
Cope Rearrangement: The Cope rearrangement is a sigmatropic rearrangement reaction in organic chemistry, where a [3,3]-shift of substituents occurs in a cyclic or acyclic system containing a conjugated diene. This rearrangement is a powerful tool for the synthesis of complex organic molecules.
Sigmatropic Rearrangements: Sigmatropic rearrangements are a class of pericyclic reactions in organic chemistry where atoms or groups migrate from one position in a molecule to another position within the same molecule. These rearrangements involve the simultaneous breaking and forming of sigma bonds, leading to the formation of a new product with a different connectivity compared to the starting material.
Allyl Vinyl Ethers: Allyl vinyl ethers are a class of organic compounds that contain both an allyl group and a vinyl ether functional group. These compounds are of interest in the context of sigmatropic rearrangements, as they can undergo a thermal [3,3]-sigmatropic rearrangement known as the Claisen rearrangement.
Thermal Activation: Thermal activation refers to the process by which a chemical reaction is initiated or accelerated by the input of thermal energy, such as heat. This concept is particularly relevant in the context of understanding the stereochemistry of thermal electrocyclic reactions and the mechanisms of sigmatropic rearrangements.
Oxy-Cope Rearrangement: The oxy-Cope rearrangement is a sigmatropic rearrangement reaction that involves the migration of a hydrogen atom and a substituent group between the termini of a conjugated triene system. It is a key example of the broader class of sigmatropic rearrangements discussed in Section 30.8.
[1,5] Sigmatropic Rearrangement: A [1,5] sigmatropic rearrangement is a type of pericyclic reaction in organic chemistry where a substituent migrates from one position to another position five atoms away, resulting in the formation of a new carbon-carbon bond.
Allyl Vinyl Alcohols: Allyl vinyl alcohols are a class of organic compounds that contain both an allyl group (a three-carbon chain with a terminal double bond) and a vinyl alcohol functional group (a carbon-carbon double bond adjacent to a hydroxyl group). These molecules are important intermediates in various sigmatropic rearrangement reactions.
[1,5] Hydrogen Shift: The [1,5] hydrogen shift is a type of sigmatropic rearrangement, which involves the migration of a hydrogen atom from one position to another within a molecule. This process is characterized by the movement of the hydrogen atom from the 1-position to the 5-position, or vice versa, in the molecular framework.
Pentadiene: Pentadiene is a conjugated diene compound with the molecular formula C5H8. It consists of a five-carbon chain with two carbon-carbon double bonds separated by a single carbon-carbon bond. Pentadienes are important intermediates in sigmatropic rearrangements, a class of pericyclic reactions that involve the migration of a substituent group within a molecule.
1,5-dienes: 1,5-dienes are a class of organic compounds that contain two carbon-carbon double bonds separated by three single-bonded carbon atoms. These types of dienes are of particular interest in the context of sigmatropic rearrangements, a class of pericyclic reactions involving the migration of a substituent from one position to another within a molecule.
Orbital Symmetry: Orbital symmetry is a fundamental concept in organic chemistry that describes the spatial arrangement and interaction of molecular orbitals involved in pericyclic reactions, such as electrocyclic reactions, cycloadditions, and sigmatropic rearrangements. It helps predict the stereochemical outcomes and feasibility of these concerted reactions.
[3,3] Sigmatropic Rearrangement: 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.
Allyl Phenyl Ethers: Allyl phenyl ethers are a class of organic compounds that consist of an allyl group (a propene-derived substituent) bonded to a phenyl ether functional group. These compounds are of interest in the context of sigmatropic rearrangements, a type of pericyclic reaction involving the migration of a substituent within a molecule.
Suprafacial-Suprafacial Pathway: The suprafacial-suprafacial pathway is a specific type of sigmatropic rearrangement where the migration of a substituent occurs on the same side of the molecule, preserving the stereochemistry of the reactants. This pericyclic reaction is an important concept in the context of organic chemistry reactions.
γ,δ-unsaturated carbonyl compounds: γ,δ-unsaturated carbonyl compounds are organic molecules that contain a carbonyl group (C=O) with two adjacent carbon-carbon double bonds, where one of the double bonds is in the γ (gamma) position and the other is in the δ (delta) position relative to the carbonyl group. These compounds are important in the context of sigmatropic rearrangements, a class of pericyclic reactions that involve the migration of a σ-bond to a new position within the molecule.
β,γ-Unsaturated Carbonyl Compound: A β,γ-unsaturated carbonyl compound is an organic compound that contains a carbonyl group (C=O) conjugated with a carbon-carbon double bond. This structural feature allows for unique reactivity and is an important consideration in the context of sigmatropic rearrangements.
1,5-dien-3-ols: 1,5-dien-3-ols are a class of organic compounds that contain a hydroxyl group (-OH) at the 3-position and two carbon-carbon double bonds at the 1- and 5-positions within the molecule. These compounds are of interest in the context of sigmatropic rearrangements, a type of pericyclic reaction that involves the migration of a substituent to a new position within the molecule.