Diels-Alder Cycloaddition reaction.pdf · The Diels-Alder reaction is stereospeciﬁc, i.e. the reaction is a syn addition, and the conﬁguration of the dienophile is retained in

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Dr. Zerong Wang at UHCL

Diels-Alder CycloadditionA lab practice for the

reaction betweencyclopentadiene and

maleic anhydride


The Diels-Alder Cycloaddition

Conjugated diene Dienophile Diels-Alder reaction:

* Stereospecific* Prefer Endo product to Exo product

O

O

O

+

O

O

O

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Diels-Alder reaction

C

C

C

CC

C

CH

H

H

H

H

H

O

H

H H

CH3 CCH3

O

+ Benzene

Heat

Conjugated diene Dienophile


The general Diels-Alder reaction forms acylohexene product

Two new σ bonds are formed at the expense oftwo π bonds

The conjugated diene is a 4π-electron system The dienophile (“diene lover”) is a 2π-electron

system The product is called an adduct

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Factors Favoring the DA Reaction

The simplest possible example of aDiels-Alder reaction goes at very low

yield and requires hightemperatures


To proceed in good yield and at lowtemperature the dienophile should haveelectron-withdrawing groups It also helps if the diene has electron-releasing

groups Dienes with electron-donating groups and

dienophiles with electron-withdrawing group canalso react well together

Factors Favoring the DA Reaction

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Dienes

The dienes must be in the s-cisconformation to react

S-Trans conformation would lead toformation of a highly unstable transbond in a 6-membered ring

Cyclic dienes which must be in the s-cis conformation are highly reactive


Conjugated diene Contain alternating double and single bond:

Adopt S-cis conformation :

More stable than non-conjugated diens.

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S-cis conformation of diens

H

H

S-cis

S-trans


H

CC

C

CH3

CH3C

H

C

C

H

H

H

H

H3CCH3

Severe steric strain in s-cis form

S-trans

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• Cyclopentadiene is so reactive it spontaneouslyundergoes Diels-Alder reaction with itself at roomtemperature

• This dimer can be “cracked” (undergo retro-Diels-Alderreaction) by heating and the cyclopentadiene productisolated by distillation.


Dienophile Has carbon carbon double or triple bond

that is next to the positively polarizedcarbon of a electron-withdrawingsubstituent group

Reactive and uncreative

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C

CC

O

H

HH

H

Propenal(Acrolein)

Reactive

δ+

δ-


C

C

C

O

HH

H OCH2CH

3

Ethyl propenoate (Ethyl acrylate)

Reactive

δ-

δ+

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Maleic Anhydride

Reactive

H

C

O

O

C

C

C

H

O

δ-

δ-

δ+

δ+


Benzoquinone

Reactive

CC

CC

C

C

H

H

O

O

H

H

δ-

δ-

δ+

δ+

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Propenenitrile (Acrylonitrile)

Reactive

C

C

C

HH

H

Nδ-

δ+


O

C

C

C

H

OCH3

Methyl Propynoate

Reactive

δ-

δ+

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C

C

C

CH2

CH3

O

HH

H

Unreactive


N

Unreactive

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O

Unreactive


Heating 1,3-butadiene and maleic anhydridegives a 6-membered ring product in 100%yield

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StereochemistryThe Diels-Alder reaction is

stereospecific, i.e. the reaction is asyn addition, and the configuration of

the dienophile is retained in theproduct


Stereospecificity The stereochemistry of the starting dienophile is

maintained during the reaction, and a single productstereoisomer results.

Example:

C

C

C

C

H CH2

CH2

CH3

H

H

H

+

CO2CH

3 H

HCH

3

CO2CH

3

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CH3H

HCO2CH3

C

C

C

C

H CH2

CH2

HH

HH3C

+

CO2CH

3


HighestOccupiedMolecularOrbital

LowestUnoccupiedMolecularOrbital

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HC CH

suprafacial

HC CH

antarafacial


oror

suprafacial (s)process

antarafacial (a)process

π4s + π2s(π2s + π2s + π2s)(π2a + π2a + π2s)

π4a + π2s(π2s + π2a + π2s)(π2a + π2s + π2s)

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Woodward-Hoffmann Rules

In any concerted process, the startingmaterial orbitals must be transformed intoproduct orbitals of the same symmetry.


!4A

"2A

!3S

!2A

"1S

!1S

#4A#3S

"2A

"1S

#2A#1S

thermallyallowed

photochemicallyforbidden

avoided crossings-orbitals of the same symmetry do not cross

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Endo & Exo product


• When the molecules approach each other there arefavorable interactions between the LUMO of maleicanhydride and the HOMO of cyclopentadiene

• In the endo orientation favorable secondary oribtalinteractions between the LUMO of the carbonyl groups andthe HOMO of the cyclopentadiene carbons at the C2 andC3 positions of the diene can also occur

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Molecular Orbital Considerations that Favor an EndoTransition State

• When maleic anhydride and cyclopentadiene reactthe major product is the endo product

• The major product has the anhydride linkage endo


Secondary orbital overlap favors the endoproduct formation

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Documents

Diels-Alder Cycloaddition reaction.pdf · The Diels-Alder reaction is stereospeciﬁc, i.e. the reaction is a syn addition, and the conﬁguration of the dienophile is retained in