Pericyclic reaction

(′per·ə¦sīk·lik rē′ak·shən)

(organic chemistry) Any one of a group of reactions that involve conjugated polyenes and proceed by single-step (concerted) reaction mechanisms.

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Concerted (single-step) processes in which bond making and bond breaking occur simultaneously (but not necessarily synchronously) via a cyclic (closed-curve) transition state. Although a given reaction may appear formally to be pericyclic, it cannot be assumed to be a concerted process. In each case, the detailed mechanism of the reaction must be established experimentally. Pericyclic reactions can be promoted either by heat or by light; the stereochemistry of the reaction is determined by the mode of activation employed and the number of electrons that are delocalized in the transition state. See also Chemical bonding; Physical organic chemistry; Stereochemistry.

Four types of pericyclic reactions that are frequently encountered in organic chemistry are electrocyclic processes, cycloadditions, sigmatropic shifts, and cheletropic reactions.

Electrocyclic processes are reactions that involve their cyclization across the termini of a conjugated π-system with concomitant formation of a new σ-bond or the microscopic reverse. The sequence of steps involved in the forward reaction must be the same, in the reverse order, as that in the reverse direction when the forward and reverse reactions are carried out under identical conditions. This statement is known as the principle of microscopic reversibility.

The effect of the mode of activation upon the stereochemistry of an electrocyclic process is shown in the reaction (1),
1

where Me = methyl, for the hexatrienecyclohexadiene interconversion (a six-electron electrocyclic process). Thus, when trans,cis,trans-2,4,6-octatriene [structure (1)] is heated, disrotatory motion of the two terminal 2p orbitals occurs; that is, they rotate in opposite directions thereby resulting in exclusive formation of cis-5,6-dimethylcyclohexa-l,3-diene (2). The corresponding photochemical process results in conrotatory motion of the termini in structure (1); that is, the two terminal 2p orbitals rotate in the same direction thereby yielding trans-5,6-dimethylcyclohexa-l,3-diene (3) exclusively.

Cycloadditions occur when two (or more) π-electron systems react under the influence of heat or light to form a cyclic compound with concomitant formation of two new σ-bonds that join the termini of the original π-systems. The stereochemistry of this reaction is classified with respect to the two molecular planes of the reactants. Thus, if σ-bond formation occurs from the same face of the molecular plane across the termini of one of the component π-systems, the reaction is said to be suprafacial on that component. If instead σ-bond formation occurs from opposite faces of the molecular plane, the reaction is said to be antarafacial on that component. This distinction is illustrated in reaction (2) for two thermal processes, where the symbol ≠ indicates the structure of the transition state. Reaction (2a) shows the Diels-Alder [4 + 2] cycloaddition of butadiene (4) to ethylene (5), a six-electron pericyclic reaction in which additions across the termini of the diene (four-electron component) and dienophile (two-electron component) both occur suprafacially. Reaction (2b) shows a [14 + 2] cycloaddition in which σ-bond formation occurs suprafacially on the two-electron component [tetracyanoethylene (6)] and antarafacially on the fourteen-electron component [heptafulvalene (7)].
2a

2b

Sigmatropic shifts involve migration of a σ-bond that is flanked at either (or both) ends by conjugated π-systems. Either one or both ends of the σ-bond may migrate to a new location within the one or more flanking π-systems.

Cheletropic reactions involve extrusion of a fragment via concerted cleavage of two σ-bonds that terminate at a single atom or the reverse process. Cheletropic fragmentations may be either linear or nonlinear [reaction (3)].
3

R. B. Woodward and R. Hoffmann introduced an application of molecular orbital theory that permits prediction of rates and products of pericyclic reactions. They utilized symmetry properties of molecular orbitals to estimate relative energies of diastereoisomeric transition states for structurally similar pericyclic reactions.

In an alternative theoretical approach to understanding pericyclic reactions, the transition state is examined directly, and attempts to estimate the degree of electronic stabilization (allowedness) or destabilization (forbiddenness) inherent in that transition state are made. One such approach emphasizes the importance of frontier orbitals (highest-occupied-lowest-unoccupied molecular orbitals) in determining the course of a pericyclic reaction. See also Delocalization; Diels-Alder reaction; Electron configuration; Molecular orbital theory; Organic reaction mechanism; Woodward-Hoffmann rule.

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