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Electrophilic Addition Reactions of Dienes

1. Mechanism 1. The Diels-Alder reaction is a one-step concerted process between a diene and dienophile to afford a

cyclohexne:

2. Since two new sigma bonds are formed with the loss of only one pi-bond the reaction is exothermic. Entropy

decreases in the process. The reaction is reversible however at very high temperature as the entropy term in the Gibbs Free Energy equation dominates.

3. The reaction is fastest when a dienophile bearing electron withdrawing groups is utilized:

4. The reaction is not in the plane of the paper as suggested above. The diene and dieneophile are orientated one above the other in the cycloaddition reaction. Since attack can come from either face of the dieneophile the reaction often results in a racemic mixture of enantiomers. If the dieneophile is a cis-alkene, a cis-cyclohexene results, likewise if a trans-alkene is used a trans-cyclohexene product is obtained:

5. As can be seen above, the diene must attain a higher energy conformer, called s-cis, for a Diels-Alder to take

place. Dienes that are locked in the s-trans conformation will not react!

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6. On the other hand, dienes that are locked in an s-cis conformation are especially reactive. Examples of these are compounds like cyclopentadiene and furan. When these dienes are used a fused ring bicyclic system is formed:

7. Like the reaction of stereospecific alkenes two stereoisomers can result from the approach of the diene from

above or below the dieneophile.

8. Groups closest to a longer bridge are called endo and those next to a shorter bridge are called exo:

9. In an exothermic process the transition state that governs reaction rate resembles the reactants. A favorable low-energy interaction between the p-orbitals of the diene and electron-withdrawing groups on the dienophile exists resulting in endo products.

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2. Practice Problems: 1. Give the products for the following reactions; propose a mechanism to explain the formation of each. In d, f

and h an alkyne is used as the dienophile. Can you elucidate those products based on your knowledge?

2. Each of the following compounds does not react in a Diels-Alder reaction. Explain why in each case:

3. Some highly fused ring systems can be made through and intramolecular Diels-Alder reaction. Suggest a mechanism for the following reaction:

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4. Predict the products for the following reactions:

5. What were the starting materials for each of the following Diels-Alder reaction products?

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KEY 1.

2.

3.

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4.

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5.

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