Special Topic B

Pericyclic ReactionsPericyclic Reactions

周环反应周环反应

ORGANIC CHEMISTRY

Department of Chemistry, Xiamen University

Special Topic

B.1 Introduction

ORGANIC CHEMISTRY

Department of Chemistry, Xiamen University

Special Topic

Chemists have found that there are many reactions in which certain symmetry characteristic of molecular orbitals control overall course of the reaction. These reactions are often called pericyclic reactions because they take place through cyclic transition states.Three main classes of pericyclic reactions

Electrocyclic reactions(电环化反应)Cycloadditions(环加成反应)Sigmatropic rearrangements(σ-迁移反应)

ORGANIC CHEMISTRY

Department of Chemistry, Xiamen University

Special Topic

Three Major Class of Organic Reaction Mechanisms

Polar mechanismsRadical mechanismsConcerted reactions (协同反应)

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CH3

CH3

H

HCH3

H

H

CH3 CH3

CH3H

H

ORGANIC CHEMISTRY

Department of Chemistry, Xiamen University

Special Topic

Concerted ReactionsAll bonding changes occur at the same time and in a single step;No intermediates are involved.

Pericyclic Reactions

A pericyclic reaction is one that occurs by a concerted process through a cyclic transition state.

ORGANIC CHEMISTRY

Department of Chemistry, Xiamen University

Special Topic

Molecular Orbitals of πSystems

基态 激发态

HOMO

LUMOLUMOHOMO

Ground state Excited state

Two isolatedp orbitals

Antibonding

Bonding

HOMO: Highest occupied molecular orbital(最高已占轨道)LUMO: Lowest unoccupied molecular orbital(最低未占轨道)

ORGANIC CHEMISTRY

Department of Chemistry, Xiamen University

Special Topic

HOMO

LUMO HOMO

LUMO

Ground state Excited state

Bonding

Antibonding

Four isolatedp orbitals

ORGANIC CHEMISTRY

Department of Chemistry, Xiamen University

Special Topic

HOMO

LUMO

HOMO

LUMO

Bonding

Antibonding

Six isolatedp orbitals

ORGANIC CHEMISTRY

Department of Chemistry, Xiamen University

Special Topic

Symmetric Operations

C2C2

m2 m2

π π∗

m1 m1

ORGANIC CHEMISTRY

Department of Chemistry, Xiamen University

Special Topic

1,3-ButadieneM1M1

ψ1 ψ2 ψ3 ψ4symmetry antisymmetry symmetry antisymmetry

C2C2

ψ1 ψ2 ψ3 ψ4antisymmetry symmetry antisymmetry symmetry

ORGANIC CHEMISTRY

Department of Chemistry, Xiamen University

Special Topic

Molecular Orbitals and Pericyclic ReactionsPrinciple of conservation of orbital symmetry (分子轨道对称守恒原理) — Formulated in the mid-1960s by R. B. Woodward and Roald Hoffmann.A pericyclic reaction can take place only if the symmetries of the reactant MOs are the same as the symmetries of the product Mos.Symmetry-allowed(对称性允许): the symmetries of both reactant and product orbitals match up.Symmetry-forbidden(对称性禁阻): the symmetries of both reactant and product orbitals don’t correlate.

ORGANIC CHEMISTRY

Department of Chemistry, Xiamen University

Special Topic

The Frontier-Orbital Method — by Kenichi Fukui (福井谦一 Kyoto Imperial University).Frontier orbitals

HOMOLUMO

We need to consider only these two molecular orbitals.

ORGANIC CHEMISTRY

Department of Chemistry, Xiamen University

Special Topic

B.2 Electrocyclic Reactions

ORGANIC CHEMISTRY

Department of Chemistry, Xiamen University

Special Topic

A electrocyclic reaction involves the cyclization of a conjugated polyene (or its reverse reaction).

1,3-Butadiene Cyclobutene

1,3,5-Hexatriene 1,3-Cyclohexadiene

ORGANIC CHEMISTRY

Department of Chemistry, Xiamen University

Special Topic

Electrocyclic reactions have several characteristic features:

They require only heat or light for initiation.Their mechanism do not involve radical or ionic intermediates.Bonds are made and broken in a single concerted step involving a cyclic transition state.The reactions are highly stereospecific.

ORGANIC CHEMISTRY

Department of Chemistry, Xiamen University

Special Topic

HH

CH3

CH3

hv

CH3

CH3

HH

CH3

H

HCH3

(not formed)×

ORGANIC CHEMISTRY

Department of Chemistry, Xiamen University

Special Topic

B.2A Electrocyclic Reactions of 4n π-Electron Systems

ORGANIC CHEMISTRY

Department of Chemistry, Xiamen University

Special Topic

CH3

H

HCH3 H

H

CH3

CH3

175�

trans-3,4-Dimethylcyclobutene (2E,4E)-Hexadiene

(顺旋)Conrotatory(+)

CH3

CH3

HH CH3

H

H

CH3

175�

(2E,4Z)-Hexadienecis-3,4-Dimethylcyclobutene

(顺旋)Conrotatory

Disrotatory(对旋)

hv

hvDisrotatory

(对旋)

ORGANIC CHEMISTRY

Department of Chemistry, Xiamen University

Special Topic

Stereochemistry of Electrocyclic ReactionsAccording to frontier orbital theory, the stereochemistry of an electrocyclic reaction is determined by the symmetry of the polyene HOMO.

HOMO

LUMO HOMO

ψ2

ψ3

Ground state

Excited state

Movies: heat 1 2 3 4 5 light 1 2 3 4

ORGANIC CHEMISTRY

Department of Chemistry, Xiamen University

Special TopicH

HH

H

Conrotatory

hvDisrotatory

CH3

CH3

HH

CH3

H

HCH3

CH3

CH3

HH

Conrotatory

hvDisrotatory

ORGANIC CHEMISTRY

Department of Chemistry, Xiamen University

Special Topic

B.2B Electrocyclic Reactions of (4n + 2) π-Electron Systems

ORGANIC CHEMISTRY

Department of Chemistry, Xiamen University

Special Topic

hv

Conrotatory

CH3

CH3

HH

CH3

CH3

HH

(2E,4Z,6E)-Octatriene cis-5,6-Dimethyl-1,3-cyclohexadiene

Disrotatory (对旋)

CH3

H

HCH3

CH3

H

HCH3

(2E,4Z,6Z)-Octatriene trans-5,6-Dimethyl-1,3-cyclohexadiene

Disrotatory(对旋)

ORGANIC CHEMISTRY

Department of Chemistry, Xiamen University

Special Topic

Stereochemistry of Electrocyclic Reactions

HOMO

LUMO HOMO

ψ3

ψ4

Ground state

Excited state

Movies: heat light

ORGANIC CHEMISTRY

Department of Chemistry, Xiamen University

Special Topic

Woodward-Hoffmann rulesA thermal electrocyclic reaction involving 4n πelectrons (where n = 1,2,3,…) proceeds with conrotatory motion; the photochemical reaction proceeds with disrotatory motion.A thermal electrocyclic reaction involving (4n+2) πelectrons (where n = 1,2,3,…) proceeds with disrotatory motion; the photochemical reaction proceeds with conrotatory motion.

ConrotatoryDisrotatoryLightDisrotatoryConrotatoryHeat

4n + 24nπElectrons

ORGANIC CHEMISTRY

Department of Chemistry, Xiamen University

Special Topic

B.3 Cycloaddition Reactions

ORGANIC CHEMISTRY

Department of Chemistry, Xiamen University

Special Topic

Cycloaddition reactions — two molecules of alkene and polyene react to form a cyclic product.

+

+hv

heat

A [2+2] cycloaddition

A [2+4] cycloaddition

Alkene Alkene

AlkeneDiene

Cyclobutane

Cyclohexene

ORGANIC CHEMISTRY

Department of Chemistry, Xiamen University

Special Topic

Cycloaddition reactions resemble electrocyclic reactions in the following important ways:

Sigma and pi bond are interconverted.Cycloaddition reactions require only heat or light for initiation.Radicals and ionic intermediates are not involved in the mechanism for concerted cycloadditions.Bonds are made and broken in a single concerted step involving a cyclic transition state.Cycloaddition reactions are highly stereospecific.

ORGANIC CHEMISTRY

Department of Chemistry, Xiamen University

Special Topic

B.3A [2 + 2] Cycloadditions

ORGANIC CHEMISTRY

Department of Chemistry, Xiamen University

Special Topic

+hv

HOMO

ψ2

ψ1

LUMO HOMOLUMO

Ground state

Excited state

ORGANIC CHEMISTRY

Department of Chemistry, Xiamen University

Special Topic

The frontier orbitals are the HOMO of one reactant and the LUMO of the other

热反应

+

HOMO LUMO对称性禁阻

Thermal reactionsSymmetryforbidden

光反应

+

HOMO LUMO对称性允许

Photochemical reactions

Symmetryallowed

ORGANIC CHEMISTRY

Department of Chemistry, Xiamen University

Special Topic

hv+2

2hv

+

+hv

+ +

+

ORGANIC CHEMISTRY

Department of Chemistry, Xiamen University

Special Topic

B.3B [4 + 2] Cycloadditions

ORGANIC CHEMISTRY

Department of Chemistry, Xiamen University

Special Topic

+ Diels-Alderreaction

Thermal reactions —— Symmetry allowedPhotochemical reactions —— Symmetry forbidden

ψ2 LUMO

HOMO

ψ2

LUMOψ3

HOMO

ψ2

Thermal reactions

Symmetry allowed Symmetry allowed

ORGANIC CHEMISTRY

Department of Chemistry, Xiamen University

Special Topic

B.4 Sigmatropic Rearrangements

ORGANIC CHEMISTRY

Department of Chemistry, Xiamen University

Special Topic

HCH2 CH CH CH CD21 2 3 4 5

CH2 CH CH CH CD2

H

1 2 3 4 5

[1,5] rearrangement

A sigmatropic rearrangement is a process in which a σ-bonded substituent atom or group migrates across a πelectron system from one position to another.A σ bond is broken in the reactant, the π bonds move, and a new σ bond is formed in the product.The σ-bonded group can be either at the end or in the middle of the πsystem.

ORGANIC CHEMISTRY

Department of Chemistry, Xiamen University

Special Topic

C

CC

C

CH

1

2

3

4

5 C

CC

C

CH

1

2

3

4

5 C

CC

C

CH

1

2

3

4

5

A 1,3-diene Cyclic transition state A 1,3-diene

300�1

1

2

2

3

3

1

12

2

3

3

O O[3,3] rearrangement

1

1

2

2

3

3

1

1 2

2

3

3

Cope rearrangement

Claisen rearrangement

[3,3] rearrangement

σ bond broken σ bond formed

ORGANIC CHEMISTRY

Department of Chemistry, Xiamen University

Special Topic

O

CC

C

C1

23

O

CC

C

CC

1

2

3 CC

C

CC

1

2

3

A ally vinyl ether Cyclic transition state An unsaturated aldehyde

C2

3

1

3

2

1

O

3

2

1

σ bond formedσ bond broken

ORGANIC CHEMISTRY

Department of Chemistry, Xiamen University

Special Topic

(same side)

Suprafacial

Antarafacial(opposite side)

ORGANIC CHEMISTRY

Department of Chemistry, Xiamen University

Special Topic

HH

H

Transition state

Suprafacial [1,5] hydrogen shift

ORGANIC CHEMISTRY

Department of Chemistry, Xiamen University

Special Topic

CH3

O CH3

33

2

2

11

CH3

OCH3 CH3

OHCH3

CH3

O

CH3

11 2

2 3

3 [3,3] shift [3,3] shift

Claisen rearrangement Cope rearrangement