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The last important topic in this chapter is “simple ring formation.”

This involves a completely new type of reaction, developed by two Germans, Otto Diels and Kurt Alder, in the 1930’s.

The reaction has come to be known as the Diels-Alder Reaction.

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The Diels-Alder Reaction

diene + dienophile cyclohexene

NOTE: = gain of bond order= loss of bond order

This is an example of a cycloaddition.

C

CC

C

C

C

C

CC

C

CC

C

C

C

C

C

C+

heat

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The Prototype Diels-Alder Reaction

CH

CH

CH2

CH2

+CH2

CH2

CH

CHCH2

CH2

CH2

CH2

200 °C

sealed tube

Cyclohexene1,3-Butadiene Ethene(Ethylene)

Because each of the reactants is a gas, the reaction must be conducted in a sealed tube at elevated pressure and temperature.

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Another simple example:

CH

CH

CH2

CH2

C

C

H

H

CH

CHCH2

CH

CHCH2

+

1,3-Butadiene Ethyne(Acetylene)

1,3-Cyclohexadiene

200 °C

sealed tube

Notice that the product still retains the essential structure of

a cyclohexene.

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Whatever substituents may be attached to the diene or to the dienophile, the same substituents will appear in the product.

C

C

B CH

C CH

A

D

+CH

CH

X

Y

heatC

CCH

CH

CHCH

A

B

C

D

X

Y

diene dienophile

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… for example:

CH

CH

CH2

CH2

+CH2

CH

C

CH3

O

140 °C

CO

CH3

3-Buten-2-one(Methyl vinyl ketone)

4-Acetylcyclohexene

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C

C

CH3 CH2

CH3 CH2

O

O

O

O

O

O

CH3

CH3

2,3-Dimethyl-1,3-butadiene

+

Maleic anhydride

100 °C

benzene

5,6-Dimethyl-3a,4,7,7a-tetrahydro-isobenzofuran-1,3-dione

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+CH

CH2

CH3heat

CH3

or

CH3

H

endo-2-Methylbicyclo[2.2.1]heptane

1,3-Cyclopentadiene

Propene

Notice the stereospecificity -- we will explore this shortly

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O

O

O

+heat

O

O

Oor

O

H

HO

O

cis-Norbornene-5,6-endo-dicarboxylic anhydride

1,3-Cyclopentadiene

Maleic anhydride

This one comes from “PLKE-Micro-3”

Notice the stereochemistry of addition -- we will explore that later

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Classic Reaction -- The Cracking of Dicyclopentadiene

This reaction is reversible -- by heating dicyclopentadiene to 180 - 200 °C, it can be “cracked” to 1,3-cyclopentadiene

H

H

+

or

Dicyclopentadiene

m.p. = -1°b.p. = 170°

heat

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OK, so what about the stereospecificity?

We know that the diene must be in the s-cis conformation

CH

CH

CH2

CH2

CH

CH

CH2

CH2

s-cis conformation s-trans conformation

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This conformational requirement leads to a stereospecific interaction with the p-orbitals of the diene and p-orbitals of the dienophile.

The interaction of these orbitals is shown on the next slide and through the use of molecular models.

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-Elimination(Section 9.13)

R C X

H

R

+ NaOH R C

R

+ Na+ + O

H HX-

a carbene

X = Cl, Br, I

A carbene is an example of divalent carbon.

Any strong base, not just NaOH, can be used to remove the H and the X.

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Example of -Elimination

H C Cl

Cl

Cl

Cl C

Cl

NaOH

Dichlorocarbene

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Another Example

H C Cl

H

Cl

C4H9LiH C

Cl

Chlorocarbene

C4H9Li is butyllithium (see Chapter 15). It is a very strongly basic substance.

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Carbenes can also be generated by photochemical processes.

CH2N2

light

or heat:CH2 + N2

Methylene

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Now that we have carbenes, what can we do with them?

Carbenes are able to add across the double bond of an alkene to form cyclopropanes (Section 14.12)

R CH CH R + R C

RR CH CH R

C

R R

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CH3 CH CH CH3 + CH2N2

lightCH2

CH CH

CH3 CH3

1,2-Dimethylcyclopropane

Example

This is another example of a simple ring formation.

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Example from “PLKE-Micro-3”

H C Cl

Cl

Cl

+ NaOH Cl C

Cl

+ NaCl

+ Cl C

Cl

Cl

Cl

+ H2O + Cl-

7,7-Dichloronorcarane

Dichlorocarbene is never isolated; it reacts with cyclohexene as soon as it is formed.