Pericyclic reactions reshape molecules through concerted electron movements, creating new bonds without intermediates. These reactions follow strict rules based on orbital symmetry, leading to predictable and stereospecific outcomes in cycloadditions, sigmatropic shifts, and electrocyclic processes.

Woodward-Hoffmann rules guide chemists in predicting allowed reactions and product stereochemistry. By considering orbital symmetry and electron count, we can determine if a reaction will proceed thermally or photochemically, opening up diverse synthetic possibilities.

Pericyclic Reactions Fundamentals

Characteristics of pericyclic reactions

• Pericyclic reactions involve concerted electron redistribution through cyclic orbital arrays without intermediates
• Stereospecific outcomes result from orbital symmetry-governed processes
• Cycloadditions form two new σ bonds (Diels-Alder reaction, [2+2] cycloaddition)
• Sigmatropic rearrangements migrate σ bonds across π systems (Cope rearrangement, Claisen rearrangement)
• Electrocyclic reactions interconvert π and σ bonds (ring opening/closing of cyclobutene)

Application of Woodward-Hoffmann rules

• Conservation of orbital symmetry predicts allowed and forbidden reactions
• Thermal reactions: [4n+2] electrons allowed conrotatory, [4n] electrons allowed disrotatory
• Photochemical reactions: [4n+2] electrons allowed disrotatory, [4n] electrons allowed conrotatory
• Determine reaction feasibility and product stereochemistry
• Guide electrocyclic ring closures and cycloaddition reactions

Orbital Symmetry and Reaction Mechanisms

Orbital factors in pericyclic mechanisms

• Orbital symmetry elements classify symmetric and antisymmetric orbitals
• Frontier Molecular Orbitals (HOMO and LUMO) drive pericyclic reactions
• Correlation diagrams visualize orbital energy changes and identify symmetry-allowed pathways
• Suprafacial and antarafacial interactions determine orbital phase matching
• Symmetry-allowed processes differentiated from symmetry-forbidden reactions

Photochemical vs thermal pericyclic pathways

• Thermal reactions use ground state reactants (thermal [4+2] cycloaddition, 6π electrocyclization)
• Photochemical reactions involve excited state reactants (photochemical [2+2] cycloaddition, 4π electrocyclization)
• Thermal reactions limited by activation energy barriers, photochemical reactions overcome barriers through excitation
• Thermal reactions often yield thermodynamically favored products, photochemical reactions access higher energy intermediates
• Regioselectivity in cycloadditions and stereoselectivity in electrocyclic reactions influenced by reaction conditions
• Complementary thermal and photochemical pathways enable diverse synthetic route design