Introduction 

More merely, the term "pericyclic" covers all concerted reactions including a cyclic flow of electrons through a single evolution state. While in this evolution state, a concerted rearrangement of the electrons happens that reasons sigma and π- bonds to simultaneously break and form. Pericyclic reactions can be forecasted and controlled to a huge degree that makes them extremely helpful in synthesis. Pericyclic reactions are popular by synthetic chemists since the reagents and situations are mild and the reactions are extremely "clean"... unlike so many organic chemical reactions that consequence in the formation of huge quantities of brown-black, smelly by-product of unknown composition. Pericyclic reactivity can be understood in expressions of frontier molecular orbital (FMO) theory and the outcome of reactions can be predicted using the Woodward-Hoffmann rules.

General classification of pericyclic reactions 

Pericyclic reactions are generally rearrangement reactions. The main classes of pericyclic reactions are: cycloaddition, electrolytic reaction, sigmatropic reaction, group transport reaction, cheletropic reaction and dyotropic reaction. Within each subclass, it is general that reactions can either be induced to take place under thermal situations through easy heating, or under photochemical conditions. The 2 methodologies are complementary.

Cycloaddition Reactions 

A cycloaddition is a reaction that simultaneously shapes at least 2 new bonds, and in doing so, converts two or more open-chain molecules into rings. The evolution state for such reactions characteristically engages the electrons of the molecules moving in continuous rings, making it a pericyclic reaction. A concerted amalgamation of two π-electron systems to form a ring of atoms having two new σ bonds and 2 fewer π bonds is said a cycloaddition reaction. The number of participating π- electrons in each component is specified in brackets preceding the name, and the reorganization of electrons might be illustrated via a cycle of curved arrows - each representing the movement of a pair of electrons. Ring-opening process is cycloreversion.

The most general cycloaddition reaction is the [4π+2π] cyclization recognized as the Diels-Alder reaction. In Diels-Alder terminology the two reactants are termed to as the diene and the dienophile. The subsequent diagram illustrates instances of [4π+2π], a light induced [2π+2π] and [6π+4π] cycloadditions.

Fig: Cycloaddition Reactions   

Cycloaddition reactions include the formation of a cyclic product due to addition of 2 different π bond enclosing components, which are connected via newly formed two σ bonds at their ends at the expense of two π bonds. It is generally reversible and the backward reaction is as well termed to as retro-cycloaddition or a cycloreversion. The classic instance of cycloaddition is Diels-Alder reaction between a Diene and a Dienophile to provide a cyclic adduct. Cycloaddition, and the reverse procedure retrocycloaddion, can be examined in the reaction between 1, 3-butadiene and ethene to provide cyclohexene.

Fig: cyclic adduct

The 1, 3-butadiene is a conjugated π-system through 4 π-electrons and the ethene is a conjugated π-system by 2 π-electrons. The reaction between 1, 3-butadiene and ethene to provide cyclohexene is explained as a [4+2] cycloaddition reaction. This kind of cycloaddition is as well said a Diels-Alder reaction. In a Diels-Alder reaction the 4 π-electron system is termed to as "the diene" and the 2 π-electron system as the 'dieneophile'. Such terms are utilized in related [4+2] reaction systems even when the functional groups aren't actually dienes or alkenes.

Cycloaddition is a kind of X + Y  X-Y complexation, and it follows the usual thermochemistry rules. The formation of the Diels-Alder adduct is an exothermic reaction. It follows that elevated temperatures favour retrocycloaddition and low temperatures favour adducts formation.

Though, many cycloaddition reactions need moderate heating to conquer the activation energy. So a cycloaddition may require heating to make the reaction "go", but if it is heated too much the equilibrium will favour retrocycloaddition. The compound cyclopentadiene slowly undergoes cycloaddition with itself: one molecule of cyclopentadiene acts as a 4 π-electron diene and the other as a 2 π-electron dieneophile. The product is a Diels-Alder 'adduct', frequently said dicyclopentadiene. This dimeric substance can be cracked back to cyclopentadiene via heating at 150°C for an hour and then distilling off the diene monomer.

Fig: Diels-Alder cycloaddition reactions 

Diels-Alder cycloaddition reactions proceed more professionally if the diene is electron rich and the dienophile is electron poor. Cyclopentadiene is electron wealthy. The method to build the dieneophile electron pitiable is to add electron withdrawing groups, these as carbonyl functions. Maleic anhydride is an electron poor dieneophile which reacts by cyclopentadiene to provide an endo Diels-Alder adduct. Upon heating at 190°C, the endo conformation adduct accepts the more stable exo-adduct conformation.

Fig: 1-Methoxy-1,3-butadiene

1-Methoxy-1,3-butadiene reacts through acrylonitrile to provide 3-methoxy-4-cyanocyclohexene rather than the 3-methoxy-5-cyanocyclohexene isomer. This "ortho" regioselectivity of this reaction can be rationalised using FMO theory.

Cheletropic reactions  

Such are a special class of cycloadditions or retro-cycloadditions in which the two σ-bonds are either made or broken to the same atom. For instance, the reversible addition of sulfur dioxide to 1,3-butadiene is an illustration of cheletropic reaction, in which the two new σ-bonds are made to the sulfur atom.  

Fig: Cheletropic reactions   

As we know in this reaction, a lone pair on sulfur atom is equivalent a π-bond and is reorganized. One π-bond and a lone pair are disappeared, whereas two σ-bonds are formed. As well note that sulfur atom is oxidized from +4 to +6 state. 

Electrocyclic reactions 

Electrocyclic reactions are intramolecular pericyclic reactions that include the rearrangement of π-electrons in an open conjugated system leading to formation of a cyclic product through a new σ bond at the expense of a π-bond. Though the electrocyclic reactions not only involve ring-closure but as well ring opening that are termed to as retro-electrocyclic reactions. For instance, the formation of Cyclohexa-1,3-diene via heating Hexa-1,3,5-triene is an instance of ring-closure electrocyclic reaction.

That is:

Fig: new bond 

As we know that the 3-alkene must be cis for the reaction to take place. The reverse, or retroelectrocyclic, reaction can as well happen. This is seen through the ring opening of cyclobutene to 1, 3-butadiene:

Electrocyclic reactions, like all pericyclic processes, exhibit great stereoselectivity. Consider two 1,3,5-hexatriene systems embedded into longer hydrocarbon chains.

Sigmatropic rearrangements 

Sigmatropic rearrangements are concerted unimolecular isomerization reactions characterized via the overall movement of an σ-bond from one position to one more through a convoying rearrangement of π-electrons of conjugated system so as to accommodate the new σ-bond. The total number of σ-bonds and π-bonds remain unchanged. For instance, the [3,3] Cope rearrangement. The σ-bond undergoing movement is shown as thick line.

As we know though looking as electrocyclic reactions, there is no reduction in the number of π-bonds in sigmatropic reactions. Like electrocyclic reactions, sigmatropic rearrangements are unimolecular procedures. Sigmatropic reactions include the movement of a sigma-bond through the simultaneous rearrangement of the π-system. Two instances demonstrate this:

Fig: new position of bond

The [1,5] shift of hydrogen in a 1,3-pentadiene system:

As we know that the generic "R" functions have been added so that the creation can be differentiated from the starting substances. The biosynthesis of vitamin D has photochemical step, and the reaction occurs in skin cells. A 1,3-cyclohexadiene system connected via the b ring of a steroid undergoes a photoactivated, retroelectrocyclisation to provide 1,3,5-hexatriene system. The thermal reaction isn't permitted because it would be required to form a six membered ring via a trans-alkene, a sterically not possible structure. 1,3,5-hexatriene system then undergoes a thermal [1,7] sigmatropic rearrangement to provide vitamin D3. This is extra processed in the liver.

Figure: Sigmatropic rearrangement in biosynthesis of vitamin D

The [3,3] sigmatropic rearrangement of 1,5-dienes or allyl vinyl ethers, known respectively as the Cope and Claisen rearrangements, are among the most commonly used sigmatropic reactions. Three examples of the Cope rearrangement are shown in the following diagram. Reactions 1 and 2 demonstrate the stereo specificity of this reaction. Since each allyl segment is the locus of a [1,3] shift, the overall reaction is classified as a  [3,3] rearrangement. The three curved arrows describe the redistribution of three bonding electron pairs in the course of this reversible rearrangement. The diene reactant in the 3^rd reaction is drawn in an extended conformation.

Group transfer pericyclic reaction 

The concerted transfer of a group from one molecule to another due to concomitant movement of an σ-bond (from one molecule to another) and formation of a new σ-bond (between two molecules) at the expense of a π-bond is generally referred to as group transfer pericyclic reaction. For instance, the ene reaction between propene and ethene to give 1-pentene is a classic example of group transfer reaction.

Such reactions resemble sigmatropic rearrangements, since σ-bond moves. Though sigmatropic reactions are unimolecular reactions whereas the group transfers reactions are bimolecular. They as well resemble cycloadditions, because a new σ-bond is shaped at the expenditure of a π-bond. However, in group transfer reactions, no ring is formed.

Another instance of an ene reaction is seen through the reaction of methylene cyclohexene and methyl acrylate. (An aluminium chloride catalyst is needed to "activate" the enone system.)

Fig: catalysed decarboxylation

A general reaction in organic synthesis is the acid catalysed decarboxylation of a β-ketoester. The ester is hydrolysed to the β-ketoacid via the aqueous acid this quickly loses carbon dioxide.

Fig: retroene reaction

Diimide is utilized as a reducing agent, it adds H₂ to C=C and N=N bonds and departs other functions untouched:

Fig: 3-phenylpropynoic acid

For instance, 3-phenylpropynoic acid is decreased to cis-3-phenylpropenic acid (cis-cinnamic acid) via diimide:

Dyotropic rearrangements 

The pericyclic reactions that engage concerted intramolecular migration of 2 σ-bonds simultaneously are recognized as dyotropic rearrangements. Though dyotropic reactions can as well occur stepwise. There are two types of dyotropic rearrangements:

Fig: Dyotropic rearrangements 

Type-I: Two migrating groups interchange their relative positions

Type-II: The σ-bonds are migrated to new bonding sites without any positional interchange for groups.

All kinds of pericyclic reactions are concerted and include cyclic transition state with no any intermediate shaped during the reaction. The traits that differentiate them from each other are tabulated below.

SAS 7: Define pericyclic reactions.

Table Comparison of different types of pericyclic reactions

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