Pericyclic reaction

A concerted reorganization of bonding through a cyclic orbital interaction, including electrocyclic reactions. The participating orbitals and their electronic occupation determine the relevant symmetry selection rule.

A reaction using the same face of a participating orbital array at both reacting termini.

A pericyclic reaction that relocates a sigma bond while reorganizing an adjacent pi system.

The reverse of a cycloaddition, splitting a ring into unsaturated fragments.

A retro hetero-Diels-Alder reaction is the reverse of a Diels-Alder reaction involving a heteroatom-containing partner. For a six-membered ring with sequence $O-A-B=C-D-E$, cleavage of $O-A$ and $D-E$ can give $A=B-C=D$ and $E=O$. Whether the cycloreversion is concerted or proceeds through ionic intermediates depends on the substrate and conditions.

Orbital-symmetry rules associated with Robert Burns Woodward and Roald Hoffmann for concerted reactions. Ground-state and photochemically excited occupations can have different allowed stereochemical modes; an allowed mode does not determine every kinetic detail or guarantee a large reaction rate.

Interconversion of a conjugated pi system and a ring by formation or cleavage of one sigma bond at the pi-system termini, accompanied by pi-bond redistribution. Terminal rotation connects starting geometry with product stereochemistry.

Formation of a ring by concerted conversion of terminal pi-system overlap into a new sigma bond. Thermal four-pi closure is conrotatory, subject to the appropriate electronic-state assumptions.

Ring closure of a seven-carbon conjugated anion with eight pi electrons; thermal orbital-symmetry selection favors disrotatory motion.

For ordinary closed-shell systems, a thermal $4n$-electron electrocyclic process is conrotatory and a thermal $4n+2$-electron process is disrotatory. The corresponding simple photochemical rules are reversed. These rules relate terminal orbital phases and occupation to allowed concerted bond formation or cleavage.

Terminal rotation in opposite senses under a common viewing convention during an electrocyclic reaction.

Terminal rotation in the same rotational sense under a common viewing convention during an electrocyclic reaction. It is distinguished from disrotatory motion; looking from opposite ends without changing conventions can reverse an apparent clockwise label.