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

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

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

Electrocyclic Reactions Are Reversible

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

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

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

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

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

A Molecular Orbital Description of Ethane

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

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

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

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

An electrocyclic reaction is completely stereoselective

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

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

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

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

Table 29.2

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

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

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

A [2 + 2] Cycloaddition Reaction

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

Table 29.3

Sigmatropic Rearrangements

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

Table 29.4

Pericyclic Reactions in Biological Systems