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1 Case Western Reserve University
Chapter 29 Pericyclic Reactions Organic Chemistry 4th Edition Paula Yurkanis Bruice Irene Lee Case Western Reserve University Cleveland, OH ©2004, Prentice Hall

2 Reactions of Organic Compounds
A pericyclic reaction, a reaction that occurs as a result of reorganizing the electrons in the reactant(s)

3 Pericyclic Reaction I An intramolecular reaction in which a new s bond is formed between the ends of a conjugated p system

4 Electrocyclic Reactions Are Reversible

5 Pericyclic Reaction II
Two different p bond-containing molecules react to form a cyclic compound

6 Pericyclic Reaction III
A s bond is broken in the reactant, a new s bond is formed in the product, and the p bonds rearrange


8 Note The electrocyclic reactions and sigmatropic
rearrangements are intramolecular reactions The cycloaddition reactions are usually intermolecular reactions Common features among the three pericyclic reactions are concerted reactions are highly stereoselective are not affected by catalysts

9 The configuration of the product formed depends on:
the configuration of the reactant the number of conjugated double bonds or pairs of electrons in the reacting system whether the reaction is a thermal or a photochemical reaction A photochemical reaction takes place when a reactant absorbs light A thermal reaction takes place without the absorption of light

10 Conservation of Orbital Symmetry Theory
Explains the relationship among the structure and configuration of the reactant, the conditions (thermal or photochemical) under which the reaction takes place, and the configuration of the products States that in-phase orbitals overlap during the course of a pericyclic reaction

11 A Molecular Orbital Description of Ethane

12 Four p atomic orbitals interact to give the four p MOs of
1,3-butadiene (LUMO) (LUMO) (HOMO) (HOMO)

13 From the MO diagram of 1,3-butadiene …
y1 and y3 are symmetric MOs y2 and y4 are asymmetric MOs The ground state HOMO and the excited HOMO have opposite symmetry

14 Note A MO is bonding if the number of bonding interactions is
greater than the number of nodes A MO is antibonding if the number of bonding interactions is fewer than the number of nodes The normal electronic state of a molecule is known as its ground state The ground state electron can be promoted from its HOMO to its LUMO by absorption of light (excited state) In a thermal reaction the reactant is in its ground state; in a photochemical reaction, the reactant is in its excited state

15 A Molecular Orbital Description of 1,3,5-hexatriene

16 An electrocyclic reaction is completely stereoselective




20 To form the new s bond in the electrocyclic reaction, the p orbitals
at the end of the conjugated system must overlap head-to-head

21 Only the symmetry of the HOMO is important in determining the
course of the reaction

22 A symmetry-allowed pathway is one in which in-phase
orbitals overlap If a reaction is symmetry-forbidden, it cannot take place by a concerted pathway The symmetry of the HOMO of the compound undergoing ring closure controls the stereochemical outcome of an electrocyclic reaction



25 Determining the stereochemistry of the product of an
electrocyclic reaction disrotatory ring closure conrotatory ring closure disrotatory ring closure

26 Table 29.2

27 Cycloadditions are classified according to the number of
p electrons that interact in the reaction

28 The frontier molecular orbitals of both reactants must be
considered: the HOMO and LUMO

29 Frontier Orbital Analysis of a [4 + 2] Cycloaddition Reaction

30 A [2 + 2] Cycloaddition Reaction

31 Frontier MO Analysis of the [2 + 2] Cycloaddition Reaction

32 Table 29.3

33 Sigmatropic Rearrangements


35 Consider the transition state of the reaction,
Sigmatropic rearrangements have cyclic transition states Rearrangement must be suprafacial if the transition state has six or fewer atoms in the ring

36 Table 29.4









45 Pericyclic Reactions in Biological Systems



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