Electrocyclic reaction

An electrocyclic reaction converts a conjugated π system into a ring by forming a σ bond between its ends, or opens that ring by breaking the bond. Its stereochemical course depends on the number of π electrons and whether the reaction is thermal or photochemical.

Woodward–Hoffmann rules: Rules that predict pericyclic reaction outcomes from orbital symmetry and electron count. They determine which terminal rotations are allowed for thermal and photochemical electrocyclizations.
Conjugated system: A sequence of adjacent atoms with overlapping p orbitals that allows electrons to be delocalized. Electrocyclic reactions use this continuous orbital array as the chain that closes or opens.
Nazarov cyclization: A reaction that converts a divinyl ketone into a cyclopentenone through electrocyclic ring closure. Its key ring-forming step is a 4π electrocyclization.
Cycloaddition: A reaction in which two or more unsaturated components combine to form a ring. Unlike electrocyclic closure, it joins separate π systems rather than connecting the ends of one.
Conrotatory motion: A terminal rotation in which both ends of a conjugated system rotate in the same direction. This rotation mode occurs for electrocyclic systems with 4n π electrons under thermal conditions.
Molecular orbital: A mathematical description of an electron’s allowed state across a molecule. The occupied orbitals of the conjugated chain determine its terminal phases.
Bergman cyclization: A cycloaromatization in which an enediyne forms a reactive 1,4-dehydrobenzene intermediate. Its ring-forming rearrangement includes a thermal 6π electrocyclization.
Sigmatropic rearrangement: A pericyclic reaction in which a σ bond migrates across a conjugated π system. It rearranges a σ bond instead of forming or breaking one at the system’s termini.
Disrotatory motion: A terminal rotation in which the two ends of a conjugated system rotate in opposite directions. This mode is thermally favored for systems with 4n+2 π electrons.
π bond: A covalent bond formed by sideways overlap of orbitals, with electron density above and below the internuclear axis. The reaction reorganizes π bonding as a new σ bond forms or disappears.

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