Dienes and

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Chapter 16 Dienes and Diels Alder Reaction
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Conjugated Dienes

• Conjugated dienes are compounds having two double
  bonds joined by one ? bond.
• Conjugated dienes are also called 1,3-dienes.
• Three stereoisomers are possible for 1,3-dienes
  with alkyl groups bonded to each end carbon of
  the diene.

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• Two possible conformations result from rotation
  around the CC bond that joins the two double
  bonds.

• Note that stereoisomers are discrete molecules,
  whereas conformations interconvert.

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The Diels-Alder Reaction

• The Diels-Alder reaction is an addition reaction
  between a 1,3-diene and an alkene (called a
  dienophile), to form a new six-membered ring.

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Some examples of Diels-Alder reactions are shown
below
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Predict the missing products.
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All Diels-Alder reactions have the following
features in common

• They are initiated by heat that is, the
  Diels-Alder reaction is a thermal reaction.
• They form new six-membered rings.
• Three ? bonds break, and two new CC ? bonds and
  one new CC ? bond forms.
• They are concerted that is, all old bonds are
  broken and all new bonds are formed in a single
  step.

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Synthesis of a natural product using the
Diels-Alder reaction
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Several rules govern the course of the
Diels-Alder reaction.

• The diene can react only when it adopts the s-cis
  conformation.

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2. Electron-withdrawing substituents in the
dienophile increase the reaction rate.

• In a Diels-Alder reaction, the conjugated diene
  acts as a nucleophile and the dienophile acts as
  an electrophile.
• Electron-withdrawing groups make the dienophile
  more electrophilic (and thus more reactive) by
  withdrawing electron density from the
  carbon-carbon double bond.
• If Z is an electron-withdrawing group, then the
  reactivity of the dienophile increases as follows

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A carbonyl group is an effective
electron-withdrawing group because it bears a
partial positive charge (?), which withdraws
electron density from the carbon- carbon double
bond of the dienophile.
Common dienophiles in the Diels-Alder reaction
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3. The stereochemistry of the dienophile is
retained.
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Predict the missing product.
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• When endo and exo products are possible, the endo
  product is preferred.
• The terms endo and exo are used to indicate the
  position of Z.

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• In a Diels-Alder reaction, the endo product is
  preferred, as shown in the two examples below.

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A new term A bicyclic ring system in which the
two rings share non-adjacent carbon atoms is
called a bridged ring system.
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Propose synthetic pathways for each of the
following compounds. Use Diels Alder reaction as
a key step in the synthesis.
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Propose a mechanism for the following reaction.
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Identify the structure for the product.