Topic: "Introduction of Pericyclic Reactions"

 

Topic:  "Introduction of Pericyclic Reactions"

Pericyclic reactions are a class of organic reactions in which the bond-forming and bond-breaking events occur in a concerted manner through a cyclic transition state. The important feature is neither ions nor radicals are formed as intermediates. These reactions have been studied extensively in organic chemistry and play an important role in the synthesis of complex organic molecules.

An example is the reaction between butadiene and propenal which occurs on heating.

One of the most important classes of pericyclic reactions involves the conjugated polyene system. A conjugated polyene is a molecule that contains alternating double and single bonds in a linear arrangement. The delocalized π-electrons in the conjugated system give rise to unique chemical properties that can be exploited in pericyclic reactions.

Transition state:

A state corresponding to the highest potential energy along this reaction coordinate. It is often marked with the double dagger ‡ symbol.

Background:

Several pericyclic reactions, including the Diels-Alder reaction (1928), the Cope rearrangement (1940), and the Claisen rearrangement (1912), were independently known.

The key characteristics of ionic and radical reactions were fully understood by the end of the 1950s. Pericyclic reactions, however, were not acknowledged as a distinct class.

Doering referred to them as "No-mechanic reaction" in the 1960s.

Several reactions' riddles remained unsolved.

• ü  Comparing the ratio of ethylene molecules to cyclobutane with that of acetylenes/arenes to four-membered rings
• ü  changing cis, trans-butadiene from cis-3,4-dimethylcyclobutene
• ü  pentadienyl cation cyclization

R. B. Woodward came across disrotatory electrocylization, commonly known as "counter thermodynamic stereochemistry," in 1963.

A set of guidelines defining the stereochemistry of distinct groups of pericyclic reactions were first introduced in 1965 by R. B. Woodward and R. Hoffmann.

Based on the symmetry of the molecular orbitals, Abraham and Longuet-Higgins provided an explanation of the correlation diagrams.

For this specific group of reactions, the term "pericyclic" was first used in 1969, setting the guidelines.

Characteristics:

• Cyclic transition phase
• There is little to no solvent impact.
• Certainly stereospecific
• Neither electrophiles nor nucleophiles are involved.
• No cationic, anionic, or radical intermediates are used.
• Photochemical promotion is used more frequently while reaction can also be heat catalyzed.

Classification:

Generally, classified into following classes

Ø  Cycloaddition

Ø  Electrocyclization

Ø  Sigmatropic rearrangements

Ø  Group transfer reactions

Cycloadditions:

Concerted cycloaddition reactions are one type of pericyclic reaction that is commonly used in organic synthesis. In a cycloaddition reaction, “two or more unsaturated molecules react to form a cyclic product in which there is a net reduction of the bond multiplicity”. This is a cyclization reaction. This is designated as [A+B] where A and B refers to number of atoms containing π-electrons.

Three important classes of cycloaddition reactions

ü            Diels-Alder reaction
ü            [2+2] Cycloaddition
ü            [1,3]-Dipolar cycloaddition

Diels Alder reaction:

In this reaction, conjugated diene and a dienophile react to form a cyclic product with a six-membered ring. The reaction occurs in a concerted manner, with the new bonds forming simultaneously.

                                

1,3-Dipolar cycloaddition:

[2+2] Cycloaddition:

Electrocyclic reactions:

Another type of pericyclic reaction is the electrocyclic addition reaction. In an electrocyclic addition, a cyclic molecule undergoes a bond-breaking and bond-forming event to form a new cyclic product. The reaction is initiated by the absorption of light or heat. The reaction can be either thermally or photochemically driven, and the stereochemistry of the product is dependent on the conformation of the starting material.

“Reversible reaction involving ring closure of a conjugated polyene to a cycloalkene, or ring opening of cycloalkene to a conjugated polyene.”

Example of ring closure of 1,3,5-hexatriene forms 1,3-cyclohexadiene, a product with one more bond and one fewer bond than the reactant.

Example of ring opening of cyclobutene forms 1,3-butadiene, a product with one fewer bond and one more " bond than the reactant.

Sigmatropic rearrangements:

In a sigmatropic addition reaction, a group is transferred from one part of the molecule to another. The reaction occurs in a concerted manner, and the stereochemistry of the product is dependent on the conformation of the starting material.

“Molecular rearrangements in which σ-bonded atom or group, flanked by one or more π-electron systems, shifts to a new location with a corresponding reorganization of the π-bonds are called sigmatropic reactions.” The total number of σ-bonds and π-bonds remain unchanged.

§  (3,3)-sigmatropic rearrangement

§  (2,3)-sigmatropic rearrangement

§  (1,5)-sigmatropic rearrangement

Claisen Rearrangement or 3,3-Sigmatropic rearrangement:

ü  This is example of (3,3) sigmatropic rearrangement having one step mechanism without ionic intermediate or any charge.

ü  Similar like a cycloaddition reaction.

  

The π-system in the conjugated polyene is crucial in these reactions. The π-system allows for the delocalization of electrons, which in turn leads to increased stability of the reaction intermediates. The π-system also provides a pathway for the electrons to move through the molecule, facilitating the formation of new bonds and the breaking of old ones.

Group transfer reactions:

These reactions are characterized by the transfer of a group from one  molecule or from one part of the molecule to another.

Example of group tarnsfer reaction is ene or alder-ene reaction.

Conclusion:

 In conclusion, pericyclic reactions are an important class of organic reactions that involve the concerted formation and breaking of bonds through a cyclic transition state. The conjugated polyene system is an important component of pericyclic reactions, allowing for the delocalization of electrons and facilitating the formation of new bonds. The three types of pericyclic reactions discussed above - concerted cycloadditions, electrocyclic additions, and sigmatropic additions and group transfer - all rely on the unique properties of the π-system in the conjugated polyene. Understanding the mechanisms and applications of pericyclic reactions is crucial in the synthesis of complex organic molecules.