PERICYCLIC

Introduction

Pericyclic reactions are concerted processes that occur by way of a cyclic transition state in which more than one bond is formed or broken within the cycle.

The classic example of such a process is the Diels–Alder cycloaddition reaction, one of the most common and useful synthetic reactions in organic chemistry.

Cycloaddition reactions, sigmatropic rearrangements and electrocyclic reactions all fall into the category of pericyclic processes.

a) Concerted reaction that proceed via a cyclic transition state

b) No distinct intermediates in the reaction

c) Bond forming and bond breaking steps are simultaneous but not necessarily Synchronous.

1) Classification:

a) Electrocyclic ring closing and ring opening reaction

b) Cycloaddition and Cycloreversion reaction

c) Sigmatropic Rearrangements

d) Chelotropic Reaction

e) Group transfer Reaction

2) Methods of Analyzing Pericyclic Reaction :

a) Orbital symmetry correlation method : (Woodward, Hoffmann, Longuet-Higgins and Abrahamson)

b) The frontier orbital method : (Woodward, Hoffmann and Fukui)

c) Transition state aromaticity method : (Dewar and Zimmerman)

Woodward-Hoffmann Rules:

Predicts the allowedness or otherwise of pericyclic reactions under thermal and photo- chemical conditions using the above methods.

Therefore a basic understanding of molecular orbitals of conjugated polyene systems and their symmetry properties is essential to apply the above methods.

3) ELECTROCYCLIC REACTIONS :

a) Cyclization of an acyclic conjugated polyene system

b) The terminal carbons interact to form a sigma bond

c) Cyclic transition state involving either 4n electrons or 4n+2 electrons.

Electrocyclization of butadiene (4n) and hexatriene (4n+2)

4) CYCLOADDITION REACTIONS :

a) Reaction of two components to form a cyclic compound

b) Ring forming reactions

c) Pericyclic type – both components are π systems

d) Intramolecular and intermolecular versions

Classification :

Based on the number of π electrons involved in each component. The numbers are written within a square bracket e.g. [2π + 2π], [2π + 4π] etc

a) Electrocyclic ring closing and ring opening reaction (6pi), (4pi),

b) Cycloaddition and Cycloreversion reaction (6pi), (4pi),

c) Sigmatropic Rearrangements (3,3), (1,5), (1,3), (1,7),(2,3) Chelotropic Reaction

d) Group transfer Reaction (related to sigmatropic)

In the reactions :

See below :

5) The Diels–Alder Cycloaddition Reaction :

Diels–Alder reactions occur between a conjugated diene and an alkene, usually called the dienophile.

first an open-chain diene with a simple unsaturated aldehyde or any unsaturated electron withdrawing group as the dienophile.

5.1) Stereochemistry :

The Diels–Alder reaction is stereospecific.

If there is stereochemistry in the dienophile, then it is Retained in the product.

Thus cis and trans dienophiles give different diastereoisomers of the product.

Esters of maleic and fumaric acids provide a simple example.

5.2) Stereochemistry of Diene :

With a trans, trans-diene we simply exchange the two sets of substituents, in this example putting Ph where H was and putting H where the bridging CH group was.

Below is the reaction :

The remaining case—the cis, trans-diene—is rarer than the first two, but is met sometimes.

This is the unsymmetrical case and the two substituents clearly end up on opposite sides of the new sixmembered ring.

5.3) The endo rule for the Diels–Alder reaction :

The two hydrogen atoms must be cis in the product but there are two possible products in which these Hydrogens are cis. They are called exo and endo.

The product is, in fact, the endo compound.

This is impressive not only because only one diastereoisomer is formed but also because it is the less stable one.

How do we know this?

Well, if the Diels–Alder reaction is reversible and therefore under thermodynamic control, the exo product is formed instead.

The best known example results from the replacement of cyclopentadiene with furan in reaction with the same dienophile.

5.4) The solvent in the Diels–Alder reaction :

Water, a most unlikely solvent for most organic reactions, has a large accelerating effect on the Diels–Alder reaction.

Even some water added to an organic solvent accelerates the reaction. And that is not all.

The endo selectivity of these reactions is often superior to those in no solvent or in a hydrocarbon solvent.

5.5) Reactivity of Dienophile :

In electron-demand Diels-Alder reactions, dienes are activated by electron-donating substituents, such as alkyl, -NR2 and -OR.

Electron-rich dienes accelerate the reaction with electron-deficient dienophiles, as illustrated by the relative reactivity trend.

5.6) Intramolecular Diels–Alder reactions :

When the diene and the dienophile are already part of the same molecule it is not so important or them to be held together by bonding interactions across space and the exo product is often preferred.

If carbonyl group conjugated with the dienophile.

The less stable cis ring junction is formed because the molecule can fold so that the carbonyl group can enjoy a bonding overlap with the back of the diene.

This time the linking chain has to adopt a boat-like conformation.

On the other hand, we give the dienophile a conjugating group at the other end of the double bond, stereoselectivity is lost.

The trans-alkene, two products are formed and both retain the trans geometry of the dienophile.

But once again a nearly 50:50 mixture of endo and exo products is formed.

5.7) Regeoselectivity in Diels alder reaction :

A diene with an electron-donating group (X) at one end or in the middle and a dienophile with an electron-withdrawing group (Z) at one end.

Below are the products formed :

5.8) To make the reactive imtermediate Dinophile ( Benzyne):

By using the 1,2 dihalides in presence of the metal.

6) Heterodienophiles :

The Diels–Alder reaction is by no means restricted to the all-carbon variant.

No significant loss of reactivity is encountered when one or both of the atoms of the dienophile multiple bond with a heteroatom.

Carbonyl groups in aldehydes and ketones add to 1,3-dienes and the reaction has been used to prepare derivatives of 5,6-dihydropyrans.

Formaldehyde reacts only slowly but reactivity increases with reactive carbonyl compounds bearing electron-withdrawing groups, such as glyoxylate derivatives.

7) Cycloaddition reactions with allyl anions and allyl cations :

The possibility of analogous six π-electron cycloadditions involving allyl anions and allyl cations to give five- and seven-membered rings respectively is predicted by the Woodward– Hoffmann rules.

These species can be produced from α,α’-dibromoketones, from α-halo-trialkylsilyl enol ethers or from allyl sulfones and a Lewis acid.

For example, the 2-oxyallyl cation can be prepared from the dibromide and its cycloaddition with furan derivative.

8) (1,3-Dipolar) Cycloaddition Reactions :

The 1,3-dipolar cycloaddition reaction, like the Diels–Alder reaction, is a 6π electron pericyclic reaction, but it differs from the Diels–Alder reaction in that the 4 π component, called the 1,3-dipole, is a three-atom unit containing at least one heteroatom and which is represented by a zwitterionic octet structure.

The 2 π component, here called the dipolarophile (rather than the dienophile), is a compound containing a double or triple bond.

The product of the reaction is five-membered heterocyclic compound.

Some Transformations :

Some starting materials for this reaction :

Reaction Examples:

Stereospecific Reaction :

With Raney Nickel in Aqueous Acid :

With Zn AcOH :

With LAH :

9) [2+2] Cycloaddition Reactions :

The [2+2] cycloaddition reaction has found considerable use in synthesis, particularly for the formation of compounds containing a four-membered ring.

The combination of two alkenes leads to a cyclobutane ring, although most alkenes do not undergo a thermal [2+2] cycloaddition reaction with another alkene.

Tetrafluoroethene is unusual in that it is able to form (tetrafluoro)cyclobutanes with many alkenes under thermal conditions.

Ketenes (R2=C=C= O) react with alkenes under thermal conditions to give cyclobutanones.

However, many [2+2] cycloaddition reactions are carried out under photochemical conditions.

The reaction is stereospecific within each component but there is no endo rule—there is a conjugating group but no ‘back of the diene’.

The least hindered transition state usually results.

9.1) Regioselectivity in photochemical [2 + 2] Cycloadditions :

HOMO/HOMO and LUMO/LUMO interactions in this type of reaction

9.2) Thermal [2 + 2] Cycloadditions :

There are some thermal [2 + 2] cycloadditions giving four-membered rings.

These feature a simple alkene reacting with an electrophilic alkene of a peculiar type.

It must have two double bonds to the same carbon atom.

The most important examples are ketenes and isocyanates.

The structures have two p bonds at right angles

Preparation of Ketene