Download A Diels-Alder Synthesis

CHEM 334L
Organic Chemistry Laboratory
Revision 4.0
A Diels-Alder Synthesis
of
cis-Norbornene-5,6-endo-Dicarboxylic Anhydride
In this laboratory exercise we will synthesize the compound cis-Norbornene-5,6-endoDicarboxylic Anhydride; a compound which, in and of itself, is of relatively little
importance.
However, the synthetic method used to generate this compound, the Diels-Alder
Reaction, is extremely important. The Diels-Alder Reaction, named after the German
chemists Otto Diels and Kurt Alder whose experiments first demonstrated the nature this
reaction type, is considered to be one of the few truly new Organic reactions of the 20th
century.
Otto Diels
(http://www.uni-kiel.de/ps/cgi-bin/
fo-bio.php?nid=diels&lang=e)
Kurt Alder
(http://en.wikipedia.org/wiki/File:Kurt_Alder.jpg)
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In the Diels-Alder Reaction, a conjugated diene will react with a dienophile, if an
appropriate electron-withdrawing group is attached to the dienophile, to produce a
carbocycle containing two new Carbon-Carbon  bonds.
This is important because the synthetic formation of Carbon-Carbon bonds is difficult
and because the products of Diels-Alder Reactions contain cyclohexene rings that can be
used to form complex molecular structures.
The Diels-Alder reaction is a special case of the more general class of cycloaddition
reactions between  (Pi) systems, the products of which are called cycloadducts. In the
Diels-Alder reaction, an assembly of four conjugated atoms containing four  electrons
reacts with a double bond containing two  electrons. Due to this, the reaction is called a
[4 + 2] cycloaddition. The mechanism for the reaction can be thought of as a concerted
movement of three  electron pairs; illustrated below for the reaction of 1,3-Butadiene
with Ethene.
(A word of caution is in order here; the mechanism for this reaction is better described
using Molecular Orbital Theory. This is discussed in the attached Appendix.) Note the
diene is in an s-cis configuration. This is required for the cycloaddition to proceed.
Dienes in the s-trans configuration are not configured such that the concerted movement
of the  electrons can proceed easily.
Electron-withdrawing groups, denoted as X in the first example, attached to the
dienophile help activate the reaction. This is because they create an alkene which is an
electron-poor target for the concerted attack of the electron-rich diene. Groups such as
-CF3 withdraw electrons inductively, whereas groups such as the carbonyl group
withdraw electrons by resonance.
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Our desired product can be produced by a Diels-Alder reaction between Cyclopentadiene
and Maleic Anhydride:
Cyclopentadiene makes for a particularly facile diene because it is locked into the s-cis
configuration by the bridging -CH2- group. Additionally, Maleic Anhydride is strongly
activated by the two -C=O fucntionalities. Therefore, our Diels-Alder reaction will
proceed without much coaxing.
As a consequence of the concerted mechanism of the Diels-Alder reaction, the reaction is
stereospecific. For example, reaction of 1,3-Butadiene with dimethyl cis-2-butenedioate
gives dimethyl cis-4-cyclohexene-1,2-dicarboxylate. On the other hand, dimethyl trans2-butenedioate yields the trans product.
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(Question: Is this an issue for our particular reation?) Additionally, Diels-Alder reactions
are highly stereocontrolled with respect to the orientation of the starting materials relative
to each other. The reaction of Cyclopentadiene with dimethyl cis 2-butenedioate, for
instance, can form two different products. One in which the two ester substituents on the
bicyclic frame are on the same side as the methylene bridge (exo product), the other in
which they are on the opposite side (trans) of the bridge (endo product). Exo
substituents are placed cis with respect to the shorter bridge; endo substituents are
positioned trans to this bridge.
The Diels-Alder reaction usually proceeds with endo selectivity. This means that the
product in which the activating electron-withdrawing group of the dienophile is located in
the endo position is formed faster than the alternative exo isomer. This happens even
though the exo product is sometimes more stable than the corresponding endo product
and is due to a variety of steric and electronic influences on the transition state of the
reaction. Thus, we can expect the reaction between Cyclopentadiene and Maleic
Anhydride to form the endo product.
Finally, Cyclopentadiene, is obtained from a light oil derived from coal tar distillation.
Because Cyclopentadiene is such a reactive system, at Room Temperature it exists as a
stable dimer, Dicyclopentadiene. This is a Diels-Alder adduct of two molecules of the
diene. Thus, generation of Cyclopentadiene involves heating the dimer to initiate a
"retro-" or "reverse-Diels-Alder" reaction.
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So, our Diels-Alder synthesis of cis-Norbornene-5,6-endo-Dicarboxylic Anhydride will
first involve the "cracking" of the Dicyclopentadiene into Cyclopentadiene. This product
will then be immediately treated with Maleic Anhydride to carry-out our desired DielsAlder reaction.
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Pre-Lab Questions
1.
What other reactions that we have studied, in CHEM 333L, involve Carbon-Carbon
bond formation?
2.
Classify each of the following alkenes as electron poor or electron rich, relative to
ethene. Explain your assignments.
3.
Add structures of the missing products or starting materials to the following
schemes:
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Procedure
Cracking the Dicyclopentadiene
This procedure may be performed collectively by the Laboratory Instructor as the
diene is particularly noxious.
1.
Measure 20 mL of Dicyclopentadiene into a 100 mL round bottom flask and
arrange for fractional distillation into an ice-cooled receiver.
2.
Heat the dimer with a heating mantle until it refluxes briskly and at a rate such that
the monmeric diene begins to distill in about 5 minutes and soon reaches a steady
boiling between 40oC and 42oC. Do not exceed the boiling point of 42oC.
Synthesis of Norbornene
1.
In a fume hood, place 6g of Maleic Anhydride in a 125 mL Erlenmeyer flask and
dissolve it in 16 mL Ehtyl Acetate by heating on a hot plate. (Always wear gloves
and handle the Maleic Anhydride in a fume hood. If the Malein Anhydride is
supplied in briquette form, powder it using a mortar and pestle. Wear goggles,
gloves and a lab coat when doing this.)
2.
Add 16 mL of Ligroin (bp 60oC-80oC) or Hexane, cool the solution thoroughly in
an Ice-Water bath, and leave it in the bath. (Some Anhydride may crystallize.)
3.
If the freshly distilled Cylcopentadiene is slightly cloudy because of moisture
condensation in the cooled receiver, add 1g of Calcium Chloride pellets to remove
the moisture.
4.
Measure 6 mL of dry Cyclopentadiene and add it to the ice-cold solution of Maleic
Anhydride. Swirl the solution in an ice bath for a few minutes until the exothermic
reaction ceases and the adduct separates as a white solid.
5.
Heat the mixture on a hot plate until the solid is completely dissolved. Allow the
solution to stand undisturbed while crystallization proceeds.†
6.
Using a short-stem funnel, filter off the crystals.
7.
Obtain an melting point for the product.
8.
Determine your percentage yield.
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†
If moisture has gotten into the system, a little of the corresponding Diacid may remain
undissolved. If this occurs, the Diacid should be removed by filtering the hot solution.
Spectroscopy
1.
Obtain an IR spectrum of the product. (Consult with your Laboratory Instructor
about how to do this.)
2.
Compare your spectrum with that published in the Appendix. Rationalize all the
"identified" IR peaks in the published spectra of the product and the
Dicyclopentadiene.
3.
Obtain an NMR spectrum of the product. (Consult with your Laboratory Instructor
about how to do this.)
4.
Assign all the NMR peaks in the spectrum.
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Post-Lab Questions
1.
What is the purpose of adding the Ligroin or Hexane in step #2 of the Synthesis
procedure?
2.
Look-up the "12 Principles" of Green Chemistry. (wikipedia anyone?) Which of
these principles are conformed with by our synthetic method? Which of these
principles are explicitly violated?
3.
In step #6 of the Synthesis, why do we use a short stemmed funnel rather than one
that has a long stem?
4.
When cracking the Dicyclopentadiene, why do we not want to exceed a boiling
point of 42oC?
5.
The Diels-Alder reaction can also occur in an intramolecular fashion. Draw the two
transition states leading to products in the following reaction:
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Appendix - Published IR Spectra
cis-Norbornene-5,6-endo-Dicarboxylic Anhydride
Dicylcyclopentadiene
Spectra taken from: A Diels-Alder Synthesis; Organic Chemistry Laboratory Manual; Portland Community
College; 2005.
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Addendum
A Molecular Modeling Exercise
This Addendum will be delivered and performed at a later date.