Pericyclic

Pericyclic reactions are a class of organic reactions in which bond changes occur in a concerted manner through a closed, cyclic array of interacting orbitals. They proceed without discrete ionic or radical intermediates and are governed by the symmetry of the participating molecular orbitals, as formalized by the Woodward–Hoffmann rules. These reactions are typically stereospecific and depend on orbital symmetry rather than stepwise electron transfer.

The major categories are cycloadditions, electrocyclizations, and sigmatropic rearrangements. In cycloadditions, two π systems merge to form a cyclic product, as in the Diels–Alder reaction, a [4+2] cycloaddition. A [2+2] cycloaddition is thermally forbidden by orbital symmetry but can be allowed under photochemical activation. Electrocyclizations involve ring closure or opening by a cyclic redistribution of π electrons, with the thermal mode determined by the number of electrons: a 4n-electron system tends to close conrotatorily, while a 4n+2 system tends to close disrotatorily; photochemical conditions invert these preferences. Sigmatropic rearrangements, such as [3,3]-Cope and Claisen rearrangements and [1,5]-hydrogen shifts, proceed via a concerted shift of σ and π electrons through a six-membered cyclic transition state and are also governed by orbital symmetry.

Applications of pericyclic reactions include the efficient construction of rings and complex frameworks in natural product synthesis and material science. The concept of suprafacial versus antarafacial interactions and the predictive power of orbital symmetry considerations make these processes especially valuable for stereocontrolled synthetic planning. The term pericyclic highlights the concerted, cyclic rearrangements that occur around a closed loop of orbitals in a single elementary step.