Pericyclic reactions are a fascinating class of organic transformations that follow specific rules for stereochemistry. The TECA mnemonic helps predict outcomes, considering thermal and photochemical conditions for electrocyclic reactions and cycloadditions.

Woodward-Hoffmann rules and Frontier Molecular Orbital theory explain the stereochemical outcomes based on orbital symmetry. Reaction conditions, electron pair count, and substituents all play crucial roles in determining the final products of these concerted processes.

Pericyclic Reaction Rules and Stereochemistry

Stereochemistry prediction with TECA

• TECA mnemonic helps predict stereochemistry in pericyclic reactions (electrocyclic and cycloadditions)
  • T: Thermal conditions
    • 4n electrons lead to conrotatory motion in electrocyclic reactions and suprafacial approach in cycloadditions (Diels-Alder)
    • 4n+2 electrons result in disrotatory motion for electrocyclic reactions and suprafacial approach for cycloadditions
  • E: Photochemical (hν) conditions excite molecules to higher energy states
    • 4n electrons now follow disrotatory motion in electrocyclic reactions but maintain suprafacial approach in cycloadditions
    • 4n+2 electrons switch to conrotatory motion for electrocyclic reactions and antarafacial approach for cycloadditions
  • C: Cycloadditions involve suprafacial or antarafacial approach of reactants
  • A: Electrocyclic reactions exhibit conrotatory or disrotatory motion of substituents

Selection rules for pericyclic reactions

• Woodward-Hoffmann rules based on conservation of orbital symmetry determine stereochemical outcomes
  • Electrocyclic reactions
    • Thermal (ground state)
      • 4n electrons: Conrotatory ring opening/closing (butadiene, cyclobutene)
      • 4n+2 electrons: Disrotatory ring opening/closing (hexatriene, cyclohexadiene)
    • Photochemical (excited state)
      • 4n electrons: Disrotatory ring opening/closing
      • 4n+2 electrons: Conrotatory ring opening/closing
  • Cycloadditions
    • Thermal (ground state)
      • [4+2] cycloaddition (Diels-Alder reaction) occurs suprafacially (diene and dienophile)
      • [2+2] cycloaddition proceeds suprafacially (alkenes)
    • Photochemical (excited state)
      • [4+2] cycloaddition can occur antarafacially
      • [2+2] cycloaddition remains suprafacial
• Frontier Molecular Orbital (FMO) theory
  • Reactivity and stereochemistry arise from favorable HOMO-LUMO interactions
  • Orbital symmetry must be conserved for reactions to occur
  • Molecular orbitals play a crucial role in determining reaction feasibility and stereochemical outcomes

Reaction conditions in pericyclic outcomes

• Thermal vs. photochemical conditions dictate which set of rules apply
  • Thermal reactions follow ground state rules (4n con/dis, 4n+2 dis/con)
  • Photochemical reactions obey excited state rules (4n dis/con, 4n+2 con/dis)
• Solvent polarity influences mechanism and transition state
  • Polar solvents stabilize charged intermediates and transition states
  • Non-polar solvents promote concerted mechanisms
• Electron pair count determines stereochemical outcome
  • 4n electrons
    • Thermal: Conrotatory (electrocyclic), suprafacial (cycloadditions)
    • Photochemical: Disrotatory (electrocyclic), suprafacial (cycloadditions)
  • 4n+2 electrons
    • Thermal: Disrotatory (electrocyclic), suprafacial (cycloadditions)
    • Photochemical: Conrotatory (electrocyclic), antarafacial (cycloadditions)
• Substituents influence reactivity and stereochemistry
  • Electron-donating groups (EDGs) raise HOMO energy (methoxy, amino)
  • Electron-withdrawing groups (EWGs) lower LUMO energy (nitro, carbonyl)
  • Substituent position and electronic nature affect rate and stereochemistry

Additional Pericyclic Concepts

• Pericyclic reactions are concerted reactions, occurring in a single step without intermediates
• Aromatic transition states often characterize pericyclic reactions, contributing to their favorable energetics
• Sigmatropic rearrangements involve the migration of a σ bond across a π system
• Stereoselectivity in pericyclic reactions is governed by orbital symmetry and reaction conditions