Organic Chemistry Unit 30 Review: Pericyclic Reactions

Key Concepts and Definitions

• Pericyclic reactions involve concerted bond breaking and bond forming processes that occur in a cyclic transition state
• Characterized by the reorganization of bonding electrons in a concerted manner, meaning all bond breaking and forming occurs simultaneously
• Typically involve the formation or breaking of σ (sigma) and π (pi) bonds
• Reactions are governed by orbital symmetry and follow the principle of conservation of orbital symmetry
• Key types include electrocyclic reactions, cycloaddition reactions, and sigmatropic rearrangements
• Stereochemistry of the products is determined by the topology of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO)
• Woodward-Hoffmann rules provide a framework for predicting the stereochemical outcome and thermal/photochemical conditions required for pericyclic reactions

Types of Pericyclic Reactions

• Electrocyclic reactions involve the formation or breaking of a single σ bond between the termini of a conjugated π system (butadiene)
  • Can occur through conrotatory or disrotatory motion of the termini
• Cycloaddition reactions involve the formation of two new σ bonds between two unsaturated molecules (diene and dienophile)
  • Examples include Diels-Alder reaction and 1,3-dipolar cycloaddition
• Sigmatropic rearrangements involve the migration of a σ bond with simultaneous rearrangement of the π system
  • Characterized by the order [i,j], where i and j indicate the number of atoms between the migrating groups in the reactant and product, respectively (Cope rearrangement)
• Cheletropic reactions involve the extrusion or addition of small molecules (SO2, CO2) with the formation or breaking of two σ bonds
• Group transfer reactions involve the transfer of a group (hydrogen, alkyl) from one molecule to another through a cyclic transition state (ene reaction)

Molecular Orbital Theory Basics

• Pericyclic reactions can be explained using molecular orbital theory, which describes the behavior of electrons in molecules
• Molecular orbitals are formed by the linear combination of atomic orbitals (LCAO) and can be classified as bonding, antibonding, or nonbonding
• The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) play a crucial role in determining the reactivity and stereochemical outcome of pericyclic reactions
• The symmetry of the HOMO and LUMO determines whether a pericyclic reaction is thermally or photochemically allowed
  • Thermal reactions occur through the ground state and require a bonding interaction between the HOMO and LUMO
  • Photochemical reactions occur through an excited state and require an antibonding interaction between the HOMO and LUMO
• The phase of the molecular orbitals (constructive or destructive interference) determines the stereochemistry of the products (suprafacial or antarafacial)

Woodward-Hoffmann Rules

• Developed by Robert B. Woodward and Roald Hoffmann to predict the stereochemical outcome and thermal/photochemical conditions of pericyclic reactions
• Based on the conservation of orbital symmetry, which states that the symmetry of the HOMO and LUMO must be maintained throughout the reaction
• For thermal reactions, the total number of (4q+2) suprafacial and (4r) antarafacial components must be odd
  • q and r are integers (0, 1, 2, etc.)
• For photochemical reactions, the total number of (4q+2) suprafacial and (4r) antarafacial components must be even
• The rules can be applied to electrocyclic reactions, cycloaddition reactions, and sigmatropic rearrangements
  • In electrocyclic reactions, the rules predict the direction of ring opening or closing (conrotatory or disrotatory)
  • In cycloaddition reactions, the rules predict the orientation of the components (suprafacial or antarafacial) and the allowed number of electrons (4n+2 or 4n)
  • In sigmatropic rearrangements, the rules predict the stereochemistry of the migrating group (suprafacial or antarafacial) and the allowed order [i,j]

Electrocyclic Reactions

• Involve the formation or breaking of a single σ bond between the termini of a conjugated π system
• Can occur through a conrotatory or disrotatory motion of the termini, depending on the number of electrons in the system and the thermal/photochemical conditions
  • Conrotatory motion occurs when the termini rotate in the same direction (clockwise or counterclockwise)
  • Disrotatory motion occurs when the termini rotate in opposite directions
• Thermally allowed reactions with (4n) electrons occur through a conrotatory process, while those with (4n+2) electrons occur through a disrotatory process
• Photochemically allowed reactions follow the opposite trend: (4n) electrons through a disrotatory process and (4n+2) electrons through a conrotatory process
• Examples include the electrocyclic ring opening of cyclobutene and the electrocyclic ring closing of 1,3,5-hexatriene
• The stereochemistry of the products depends on the mode of ring opening or closing (conrotatory or disrotatory) and the substitution pattern of the reactant

Cycloaddition Reactions

• Involve the formation of two new σ bonds between two unsaturated molecules, typically a diene and a dienophile
• Most common example is the Diels-Alder reaction, a [4+2] cycloaddition between a conjugated diene and an alkene (dienophile)
  • Occurs through a concerted, suprafacial-suprafacial interaction of the HOMO of the diene and the LUMO of the dienophile
  • Thermally allowed and follows the (4n+2) electron rule
  • Stereochemistry of the product is determined by the substituents on the diene and dienophile (endo or exo)
• Other examples include 1,3-dipolar cycloadditions, [2+2] cycloadditions, and hetero-Diels-Alder reactions
• Cycloaddition reactions can be intramolecular or intermolecular
• Photochemical cycloadditions, such as the [2+2] cycloaddition of alkenes, follow different stereochemical rules than thermal cycloadditions

Sigmatropic Rearrangements

• Involve the migration of a σ bond with simultaneous rearrangement of the π system
• Characterized by the order [i,j], where i and j indicate the number of atoms between the migrating groups in the reactant and product, respectively
• Most common examples are the [3,3] Cope rearrangement and the [1,5] hydrogen shift
  • Cope rearrangement involves the concerted migration of a 1,5-diene system through a chair-like transition state
  • [1,5] hydrogen shift occurs in conjugated polyene systems and involves the suprafacial migration of a hydrogen atom
• Sigmatropic rearrangements can be thermally or photochemically allowed, depending on the number of electrons and the order [i,j]
  • Thermally allowed rearrangements have an aromatic transition state with (4n+2) electrons and follow the Hückel rule
  • Photochemically allowed rearrangements have an antiaromatic transition state with (4n) electrons and follow the Möbius rule
• The stereochemistry of the products depends on the suprafacial or antarafacial nature of the migrating group and the topology of the transition state

Applications in Organic Synthesis

• Pericyclic reactions are powerful tools for the construction of complex molecular frameworks in organic synthesis
• Diels-Alder reactions are widely used for the synthesis of six-membered rings with precise stereochemical control
  • Intramolecular Diels-Alder reactions (IMDA) allow for the formation of polycyclic systems in a single step
  • Asymmetric Diels-Alder reactions, using chiral catalysts or auxiliaries, enable the enantioselective synthesis of chiral products
• Electrocyclic reactions can be employed for the synthesis of cyclic compounds with specific stereochemistry
  • Example: the synthesis of vitamin D3 involves an electrocyclic ring opening of a triene precursor
• Sigmatropic rearrangements are useful for the isomerization of unsaturated systems and the introduction of functional groups
  • Claisen rearrangement, a [3,3]-sigmatropic rearrangement of allyl vinyl ethers, is used for the synthesis of γ,δ-unsaturated carbonyl compounds
  • [2,3]-Wittig rearrangement of allylic ethers provides access to homoallylic alcohols
• Pericyclic reactions can be combined with other synthetic methods (organometallic chemistry, heterocyclic chemistry) to access complex natural products and bioactive compounds
• Computational tools and quantum chemical calculations aid in the design and optimization of pericyclic reactions in organic synthesis