Pericyclic reaction

By definition, pericyclic reactions proceed through a concerted mechanism involving a single, cyclic transition state.

Because of this, prior to a systematic understanding of pericyclic processes through the principle of orbital symmetry conservation, they were facetiously referred to as 'no-mechanism reactions'.

These are consistent with experimental data, supporting an ordered, concerted transition state for the former and a multistep radical process for the latter.

For reactions involving (4n + 2)-electron systems (2, 6, 10, ... electrons; odd number of electron pairs), Hückel topology transition states are proposed, in which the reactive portion of the reacting molecule or molecules have orbitals interacting in a continuous cycle with an even number of nodes.

Equivalently, pericyclic reactions have been analyzed with correlation diagrams, which track the evolution of the molecular orbitals (known as 'correlating' the molecular orbitals) of the reacting molecules as they progress from reactants to products via a transition state, based on their symmetry properties.

In the case of the Diels-Alder reaction shown below, resonance arguments make clear the direction of polarization.

Although a pseudopericyclic reaction proceeds through a cyclic transition state, two of the orbitals involved are constrained to be orthogonal and cannot interact.

Although this appears to be a 4-electron Hückel topology forbidden group transfer process, the empty p orbital and sp2 hybridized B–H bond are orthogonal and do not interact.