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PERICYCLIC
REACTIONS
Presenter,
Vikas R. Mathad
I - M. Pharm.
SJM College of Pharmacy.

Contents
 Pericyclic reactions
 Cycloadditions
 Stereochemistry
 Mode of addition
 Examples
2

Pericyclic reactions
 A pericyclic reaction is one involving a concerted bond reorganization in which the essential
bonding changes occur within a cyclic array of the participating atomic centers.
 An example is the reaction between butadiene and propenal which occurs on heating:
 The important feature is neither ions nor radicals are formed as intermediates.
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Classification
a) Cycloadditions
b) Electro-cyclic reactions
c) Cheletropic reactions
d) Sigma tropic reactions
e) Ene - reactions
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Cycloaddition
 A cycloaddition is a pericyclic chemical reaction, in which "two or more unsaturated molecules
(or parts of the same molecule) combine with the formation of a cyclic adduct in which there is
a net reduction of the bond multiplicity.
 The reaction between butadiene and ethylene is the simplest example of a general procedure for
forming six-membered rings, developed by Diels and Alder, in which a conjugated diene reacts
with a compound containing a C=C or C≡C bond (a dienophile).
 The reaction are described as [𝜋4 + 𝜋2] cycloadditions, denoting the numbers of interacting 𝜋-
electrons in each component.
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Stereochemistry
 The cycloadditions are stereospecific. For example, dimethyl maleate and dimethyl fumarate
react with butadiene to give, respectively, cis- and trans- dimethyl cyclohexane-4,5-
dicarboxylates.
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Stereochemistry
 In the dimerization of cyclopentadiene, where one molecule acts as diene and the other as
dienophile, two orientations in the product are possible, each of which arises from suprafacial -
suprafacial interaction of the reactants.
 In most of the cases the thermodynamically less stable endo product predominates. This has
been accounted for in terms of a secondary interaction of the HO and LU orbitals in the
transition state for endo addition which is sterically impossible for exo addition:
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Stereochemistry
 When both diene and dienophile contain substituents, more than one product can be formed.
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Mode of addition
 Suprafacial Mode: The suprafacial mode of interaction involves the face-to-face addition i.e.,
the addition occurs at same face of the pi-system.
 Antarafacial Mode: The antarafacial mode of interaction involves the opposite face of the
pi-system.
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Suprafacial-Suprafacial interaction
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Suprafacial-Antarafacial interaction
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Woodward – Hoffmann Rule
Total number of
(4q+2)s and (4r)a
Reaction conditions Allowed mode of ring
closure
Odd number Thermal Suprafacial
Odd number Photochemical Antarafacial
Even number Thermal Suprafacial
Even number Photochemical Antarafacial
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Huisgen cycloadditions
 The Huisgen cycloaddition reaction is a (2+3) cycloaddition.
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Nitrone-olefin cycloaddition
 The Nitrone-olefin cycloaddition is a (3+2)cycloaddition.
 The nitrone-olefin [3+2] cycloaddition reaction is the combination of a nitrone with
an alkene or alkyne to generate an isoxazoline or isoxazolidine via a [3+2] cycloaddition
process.[1] This reaction is a 1,3-dipolar cycloaddition, in which the nitrone acts as the 1,3-
dipole, and the alkene or alkyne as the dipolarophile.
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Cheletropic reactions
 Cheletropic reactions are a subclass of cycloadditions. The key distinguishing feature of
cheletropic reactions is that on one of the reagents, both new bonds are being made to the same
atom. The classic example is the reaction of sulfur dioxide with a diene.
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Reference
 Principles of organic synthesis, Third edition, R. O. C. Norman and J. M. Coxon, Chapter 9,
Page No. 270-283.
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Pericyclic reactions

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Editor's Notes