Pericyclic reaction

Pericyclic reactions are chemical reactions in which the binding ratios are changed by a concerted shift of electrons occur without radical or ionic intermediates. The case passed through transition states are cyclic in nature.

The most important pericyclic reactions are

Electrocyclic reactions
Cycloadditions
Sigmatropic rearrangements
Cheletropic reactions

System

There are a total of four concepts to clarify the reactivity of pericyclic reactions:

Orbital correlation diagrams by Robert Burns Woodward and Roald Hoffmann
Method of the highest occupied orbital by Robert Burns Woodward and Roald Hoffmann
Frontier orbital method according to Kenichi Fukui
Concept of aromatic transition states by MJS Dewar, H. Zimmerman and H. Evans

The frontier molecular orbital method is only to clarify bi -or higher- molecular reactions, orbital correlation diagrams and the method of the highest occupied orbitals only for mono- or intramolecular reactions. The concept of the aromatic transition state is generally valid. Here, the stabilization of the cyclic transition state is generally about 3 electron pairs. Six-electron reactions can be described via a six-center or five- site model ( with a nonbonding pair of electrons ), the σ - bonds act as reactive species. In addition, there are other pericyclic systems such as three-center two-electron systems or tens of electron systems.

Reactions

Electrocyclic reactions

For electrocyclic ring closures reaction occurs between the ends of a linear conjugated system, for example, 1,3 -butadiene. Longer-chain or substituted conjugated systems react in this article There are two ways of ring closure: either conrotatory or disrotatory. In the first case, the substituents rotate at the terminal carbon atoms in the same direction during the formation of the new bond, in the last case in the opposite direction. An explanation is due to orbital correlation diagrams. It makes this all the bonding and antibonding molecular orbitals of reactants and products in energetic order dar. Then correlating orbitals of the same symmetry of the reactant and product together. It is only possible to correlate binding only antibonding orbitals, the operation is allowed, and the thermal reaction can occur. Since the reaction is run in concert, can then continuous development of product molecular orbitals from the Eduktmolekülorbitalen. If a correlation of one or more binding occur with antibonding orbitals, the reaction is thermally forbidden and will 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 in a conrotatory process. It turns out that, for example, the cyclization of butadiene to cyclobutene under thermal conditions is only conrotatory possible.

The same result is obtained with the method of the highest occupied orbital. We consider the sign or the symmetry of the HOMO of the reactant. One seeks the rotation of the lobes, which would lead to a constructive overlap, so you will spin that meet lobes of the same sign. Again, that is photochemically comes to the opposite conclusion, because the excited state has a nodal plane in the HOMO and thus more exactly the opposite symmetry.

In the cycloheptatriene rearrangement norcaradiene the position of the equilibrium of the nature of the atom or group X is dependent on:

Cycloadditions

In cycloadditions there is a ring-closure by linking the ends of two systems. A basic distinction between a suprafacial approach of the reactants together and a antarafacial approach. In the first case, the addition takes place through the reactants on the same side of the substrate, in the latter case, a bond on one side and one on the other side of the substrate is formed. Such Antara additions are sterically very unfavorable, so that generally take place only Supra additions. A classic example of a suprafacial cycloaddition is the Diels- Alder reaction.

Such reactions can be explained by the frontier orbital method. We here considered the LUMO of a reactant and the HOMO of the other. In order to predict the stereochemistry and the possibility of the reaction, one approaches the reactants so that there is a constructive overlap of the corresponding frontier orbitals; ie lobes of the same sign must overlap. Is this possible, so the reaction takes place under thermal conditions, otherwise photochemistry is required.

Cheletropic reactions

They are a special case of cycloadditions in which the newly formed bonds emanating from the same atom. Illustratively, this is the addition of carbenes to double bonds. These reactions are understandable on the frontier orbital method.

Sigmatropic rearrangement

Sigmatropic rearrangements are reactions in which there is intramolecularly to the solution of a bond that is being restored to a different location. An example of a sigmatropic rearrangement is the Cope rearrangement.

These reactions are explained by the method of the highest occupied orbital, since it comes only to interactions between the HOMO of the migrating group and the rest of the framework. The stereochemistry is therefore dependent on the symmetry of the orbitals involved. Depending on the number of nodes in the molecular orbital of the Polyenylradikals theoretically occurs as an intermediate, the hydrogen shift takes place suprafacially or antarafacial. Depending on what orbital halves that can overlap s orbital of hydrogen constructive. Should a carbon group hike, so both paths are theoretically possible, since the p orbital has two lobes of opposite sign on carbon. Thus, carbon offsets can occur with retention of configuration when only one orbital half of the migrating group is involved or inversion of configuration when both orbital halves are involved. In general, the antarafacial carbon offset is but geometrically very unfavorable, so it comes with different signs at the end of Polyenylradikals to suprafacial rearrangement with inversion of configuration and the same sign for suprafacial rearrangement configuration retention. However, carbon offsets are very rare due to the high mass of the migrating group and the hydrogen shift is preferred.

Concept of the aromatic transition state

When the transient state of the topology of a pericyclic reaction ( Hückel ) aromatics is very similar, and the number of electrons involved corresponds to those of an aromatic, the reaction is allowed thermally. Furthermore, Mobius -aromatic transition states lead to thermally allowed reactions. In the latter, there is an odd number of phase reversals in the highest occupied orbital. Contrast Hückelaromatische transition states have no or an even number of phase reversals. In summary it can be stated: If the sum of the number of bonds involved in the reaction and the number of phase reversals results in an odd number in the transition state, the reaction is thermally allowed. The phase reversals determine the topology of the transition state, and thus the stereochemistry of the reaction.