Pericyclic reaction

Pericyclic reactions are chemical reactions in which the bonding conditions are changed by a concerted shift of electrons without the occurrence of radical or ionic intermediates. The transition states passed through are cyclic in nature.

The main pericyclic reactions are

Systematics

There are a total of four concepts for clarifying the reactivity of pericyclic reactions:

• Orbital correlation diagrams according to Robert Burns Woodward and Roald Hoffmann
• Top occupied orbital method according to Robert Burns Woodward and Roald Hoffmann
• Kenichi Fukui frontier orbital method
• Concept of aromatic transition states according to MJS Dewar, H. Zimmerman, and H. Evans

The frontier orbital method is only suitable for clarifying bi- or higher molecular reactions, orbital correlation diagrams and the method of the highest occupied orbital is only suitable for monomolecular or intramolecular reactions. The concept of the aromatic transition state is general. Stabilization in the cyclic transition state usually takes place via 3 pairs of electrons. Six electron reactions can be described using a six-center or five-center model (with a non-bonding electron pair), with the σ-bonds acting as a reacting species. There are also other pericyclic systems, such as three-center two-electron systems or ten-electron systems.

Reactions

Electrocyclic reactions

In electrocyclic reactions, ring closures occur between the ends of a linear conjugated system, for example 1,3-butadiene . Long-chain or substituted conjugated systems also react in this way. There are two possibilities for ring closure: either conrotatory or disrotatory . In the first case, the substituents on the terminal carbon atoms rotate in the same direction during the formation of the new bond, in the latter case in the opposite direction. Orbital correlation diagrams are one possible explanation. For this purpose, all binding and antibonding molecular orbitals of educt and product are shown in energetic order. Then orbitals of the same symmetry of educt and product are correlated with one another. If it is possible to correlate only binding and only antibonding orbitals, the process is thermally permitted and the reaction can take place. Since the reaction should take place in a concerted manner, product molecule orbitals can then continuously arise from the reactant molecule orbitals. If there is a correlation between one or more binding and antibonding orbitals, the reaction is thermally forbidden and does not take place. However, the reaction is then photochemically possible, because an excited state has a different orbital symmetry. For the disrotatory process, the product molecular orbitals have a different symmetry than for a conrotatory process. It has been shown that, for example, the cyclization of butadiene to cyclobutene is only possible under thermal conditions by means of a conrotation. ${\ displaystyle \ pi}$

The same result can be obtained with the method of the highest occupied orbital. One considers the sign or the symmetry of the HOMO of the educt. One looks for the rotation of the orbital lobes that would lead to a constructive overlap, i.e. one rotates so that lobes with the same sign meet. Here, too, the opposite result can be achieved photochemically, because the excited state has one more nodal level in the HOMO and thus exactly the opposite symmetry.

In the cycloheptatriene-norcaradiene rearrangement, the position of the equilibrium depends on the nature of the atom or group X:

In cycloadditions, a ring closure occurs through the connection of the ends of two systems. A fundamental distinction is made between a suprafacial approach of the educts to one another and an antarafacial approach. In the first case the addition takes place by the reactant on the same side of the substrate, in the latter case a bond is formed on one side and one on the other side of the substrate. Such Antara additions are sterically very unfavorable, so that in general only supra-additions take place. A classic example of a suprafacial cycloaddition is the Diels-Alder reaction . ${\ displaystyle \ pi}$

Such reactions can be explained with the frontier orbital method. One looks at the LUMO of one reactant and the HOMO of the other. In order to be able to predict the stereochemistry and the possibility of the reaction, the reactants are approximated in such a way that there is a constructive overlap of the corresponding frontier orbitals; d. H. Orbital lobes with the same sign must overlap. If this is possible, the reaction takes place under thermal conditions, otherwise photochemistry is required.

Cheletropic reactions

They are a special case of cycloadditions, in which the newly made bonds originate from the same atom. An example of this is the addition of carbenes to double bonds. These reactions can also be traced using the frontier orbital method.

Sigmatropic rearrangements

Sigmatropic rearrangements are reactions in which a bond is broken intramolecularly and then re-established at another point. An example of a sigmatropic rearrangement is the Cope rearrangement . ${\ displaystyle \ sigma}$

These reactions are explained using the method of the topmost occupied orbital, since here only interactions between the HOMOs of the migrating group and the remainder of the framework occur. The stereochemistry is thus dependent on the symmetry of the orbitals involved. Depending on the number of nodes in the molecular orbital of the theoretically intermediate polyenyl radical, the hydrogen shift takes place suprafacial or antarafacial. Depending on the orbital halves with which the s-orbital of hydrogen can constructively overlap. Should a carbon group migrate, both paths are theoretically possible, since the p orbital on carbon has two lobes with opposite signs. Thus, carbon shifts can occur with retention of configuration when only one orbital half of the migrating group is involved, or with configuration inversion when both orbital halves are involved. In general, however, the antarafacial carbon shift is geometrically very unfavorable, so that with different signs at the end of the polyenyl radical there is an antarafacial rearrangement with configuration inversion and, with the same sign, a suprafacial rearrangement with configuration retention. However, due to the high mass of the migrating group, carbon shifts are very rare and the hydrogen shift is preferred.

Concept of the aromatic transition state

If the transition state of a pericyclic reaction comes very close to the topology of a (Hückel) aromatic and the number of electrons involved corresponds to that of an aromatic , the reaction is thermally permitted. Furthermore, Möbius aromatic transition states also lead to thermally permitted reactions. In the latter there is an odd number of phase reversals in the topmost occupied orbital. In contrast, Hückel aromatic transition states have no or an even number of phase inversions. In summary it can be stated: If the sum of the number of bonds involved in the reaction and the number of phase inversions in the transition state is an odd number, the reaction is thermally permitted. The phase inversions determine the topology of the transition state and thus the stereochemistry of the reaction.