🧫Organic Chemistry II Unit 7 – Pericyclic reactions

What Are Pericyclic Reactions?

• Pericyclic reactions involve concerted reorganization of bonding electrons through a cyclic transition state
• Occur in a single step without any intermediates formed
• Characterized by the simultaneous breaking and forming of multiple bonds
• Electrons move in a circular pattern, hence the term "pericyclic" (meaning around a ring or circle)
• Examples include electrocyclic reactions, cycloadditions, sigmatropic rearrangements, and group transfer reactions
• Governed by orbital symmetry and follow specific selection rules (Woodward-Hoffmann rules)
• Play a crucial role in organic synthesis for creating complex molecular structures

Key Characteristics of Pericyclic Reactions

• Concerted mechanism where all bond breaking and forming occurs simultaneously
• No intermediates are formed during the reaction process
• Involve a cyclic transition state with a continuous overlap of orbitals
• Exhibit high stereoselectivity due to the ordered nature of the transition state
• Follow a predictable stereochemical outcome based on the Woodward-Hoffmann rules
• Require specific orbital symmetry between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO)
• Can be classified as thermally or photochemically induced depending on the energy source
• Examples include the Diels-Alder reaction, Cope rearrangement, and electrocyclic ring opening/closing

Types of Pericyclic Reactions

• Electrocyclic reactions involve the opening or closing of a single bond to form a cyclic system
  • Examples: Conrotatory and disrotatory ring opening/closing of cyclobutene and 1,3-butadiene
• Cycloadditions involve the formation of two new bonds between two unsaturated systems
  • Examples: Diels-Alder reaction (4+2 cycloaddition) and 1,3-dipolar cycloaddition
• Sigmatropic rearrangements involve the migration of a sigma bond with a simultaneous relocation of a pi bond
  • Examples: Cope rearrangement (3,3-sigmatropic) and Claisen rearrangement (3,3-sigmatropic)
• Cheletropic reactions involve the extrusion or addition of small molecules (e.g., CO, SO2) to an unsaturated system
• Group transfer reactions involve the transfer of a group (e.g., hydrogen, alkyl) between two molecules
• Ene reactions involve the transfer of a hydrogen atom and the formation of a new pi bond
  • Example: The reaction between propene and maleic anhydride

Molecular Orbital Theory in Pericyclic Reactions

• Pericyclic reactions can be explained using molecular orbital theory and orbital symmetry
• The highest occupied molecular orbital (HOMO) of one component interacts with the lowest unoccupied molecular orbital (LUMO) of the other component
• For a pericyclic reaction to occur, the HOMO and LUMO must have the correct symmetry and overlap effectively
• The Woodward-Hoffmann rules predict the stereochemical outcome based on the symmetry of the HOMO and LUMO
• Thermally allowed pericyclic reactions involve the interaction of ground state orbitals (HOMO and LUMO)
• Photochemically allowed pericyclic reactions involve the interaction of an excited state orbital (SOMO) with a ground state orbital
• The conservation of orbital symmetry determines the feasibility and stereochemistry of the reaction
• Examples: In the Diels-Alder reaction, the HOMO of the diene and the LUMO of the dienophile must have matching symmetry for a successful cycloaddition

Woodward-Hoffmann Rules

• The Woodward-Hoffmann rules predict the stereochemical outcome of pericyclic reactions based on orbital symmetry
• Developed by Robert B. Woodward and Roald Hoffmann in the 1960s
• Based on the conservation of orbital symmetry during the reaction
• Classify pericyclic reactions as thermally or photochemically allowed/forbidden
• For electrocyclic reactions, the rules predict conrotatory or disrotatory ring opening/closing depending on the number of electrons involved
  • 4n electrons: Conrotatory thermal, disrotatory photochemical
  • 4n+2 electrons: Disrotatory thermal, conrotatory photochemical
• For cycloadditions, the rules predict the stereochemistry of the product based on the number of electrons involved
  • 4n electrons: Thermally forbidden, photochemically allowed
  • 4n+2 electrons: Thermally allowed, photochemically forbidden
• Sigmatropic rearrangements follow a similar pattern based on the number of atoms and electrons involved
• The rules provide a powerful tool for predicting the stereochemical outcome of pericyclic reactions

Stereochemistry in Pericyclic Reactions

• Pericyclic reactions exhibit high stereoselectivity due to the ordered nature of the transition state
• The stereochemical outcome is determined by the Woodward-Hoffmann rules and the principle of conservation of orbital symmetry
• In electrocyclic reactions, the stereochemistry depends on the mode of ring opening/closing (conrotatory or disrotatory)
  • Example: Conrotatory ring opening of cyclobutene leads to (E,E)-1,3-butadiene
• In cycloadditions, the stereochemistry of the product is determined by the relative orientation of the components (endo or exo approach)
  • Example: The Diels-Alder reaction typically favors the endo product due to secondary orbital interactions
• Sigmatropic rearrangements can result in the formation of new stereogenic centers or the inversion of existing ones
  • Example: The Cope rearrangement of 1,5-dienes leads to a chair-like transition state with predictable stereochemistry
• The stereochemistry of pericyclic reactions can be used to control the three-dimensional structure of the products
• Stereospecific pericyclic reactions are valuable tools in asymmetric synthesis for creating chiral molecules

Applications in Organic Synthesis

• Pericyclic reactions are widely used in organic synthesis for constructing complex molecular frameworks
• The Diels-Alder reaction is a powerful tool for forming six-membered rings with high regio- and stereoselectivity
  • Examples: Synthesis of steroid frameworks, natural products (reserpine), and pharmaceuticals (oseltamivir)
• Electrocyclic reactions are used to form or break rings in a controlled manner
  • Example: Synthesis of vitamin D3 involves an electrocyclic ring opening of previtamin D3
• Sigmatropic rearrangements allow for the selective migration of functional groups and the formation of new carbon-carbon bonds
  • Example: The Claisen rearrangement is used in the synthesis of allyl phenols and other aromatic compounds
• Cheletropic reactions are employed for the extrusion or addition of small molecules (CO, SO2) in organic synthesis
• Ene reactions are useful for introducing new functional groups and forming carbon-carbon bonds
  • Example: The synthesis of menthol involves an ene reaction as a key step
• Pericyclic reactions can be combined with other synthetic methods (e.g., organometallic chemistry, heterocyclic chemistry) to access complex targets
• The stereospecificity and predictability of pericyclic reactions make them valuable tools in total synthesis and medicinal chemistry

Practice Problems and Examples

1. Predict the product and stereochemistry of the thermal electrocyclic ring opening of cis-3,4-dimethylcyclobutene.
2. Draw the transition state and product of the Diels-Alder reaction between cyclopentadiene and maleic anhydride. Explain the endo selectivity.
3. Propose a mechanism for the thermal Cope rearrangement of 1,5-hexadiene. Predict the stereochemistry of the product.
4. Identify the type of pericyclic reaction in the following transformation: (Z)-1,3-pentadiene + ethylene → (E)-1,4-heptadiene.
5. Determine whether the following electrocyclic ring closing is thermally or photochemically allowed: (E,E,E)-2,4,6-octatriene → cis-5,6-dimethyl-1,3-cyclohexadiene.
6. Suggest a pericyclic reaction that could be used to synthesize the following compound: 2-methyl-1,3-cyclohexadiene.
7. Predict the stereochemical outcome of the thermal Claisen rearrangement of (E)-1-methoxy-1,3-butadiene.
8. Design a synthetic route to obtain (E)-3-methylcyclopentene using a pericyclic reaction as the key step.