Ch. 22 - Conjugated Systems II: Pericyclic Reactions

Mullins - Organic Chemistry: A Learner Centered Approach 1st Edition

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Ch. 22 - Conjugated Systems II: Pericyclic Reactions

Problem 14

Chapter 21, Problem 14

Figure 22.16(a) <IMAGE> shows a unique example where the Diels–Alder reaction gives a single product (no enantiomers) regardless of whether the diene attacks the dienophile from the top or bottom.
(a) Show the product of the diene attacking from the bottom and confirm that the same product is obtained.
(b) What is special about this product that makes this true?

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Identify the diene and dienophile involved in the Diels–Alder reaction. The diene is a conjugated system with two double bonds, and the dienophile is typically an alkene or alkyne with electron-withdrawing groups.

Draw the diene and dienophile in their correct orientations. For the diene, ensure that the p orbitals are aligned to allow for the formation of new sigma bonds. For the dienophile, position it so that it can interact with the diene.

Consider the approach of the diene to the dienophile from the bottom. In a Diels–Alder reaction, the diene and dienophile undergo a concerted cycloaddition to form a six-membered ring. Draw the transition state where the new sigma bonds are forming.

Complete the cycloaddition by forming the new sigma bonds between the diene and dienophile. This will result in a cyclohexene ring. Ensure that the stereochemistry is consistent with the approach from the bottom.

Analyze the product to determine why it is unique. The product is likely symmetrical or has a plane of symmetry, which results in the same product regardless of the approach direction. This symmetry prevents the formation of enantiomers, leading to a single product.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Diels–Alder Reaction

The Diels–Alder reaction is a [4+2] cycloaddition between a conjugated diene and a dienophile, forming a six-membered ring. It is a pericyclic reaction that proceeds via a concerted mechanism, often used to synthesize complex cyclic structures. Understanding the stereochemistry and regiochemistry of the reactants is crucial for predicting the product.

Stereochemistry

Stereochemistry involves the study of the spatial arrangement of atoms in molecules and its impact on chemical reactions. In the Diels–Alder reaction, the orientation of the diene and dienophile can lead to different stereoisomers. However, certain symmetrical or constrained systems can lead to a single stereochemical outcome, as seen in the given example.

Symmetry in Organic Molecules

Symmetry in organic molecules can lead to unique chemical behavior, such as producing a single product in reactions that typically yield multiple stereoisomers. In the context of the Diels–Alder reaction, symmetry in the diene or dienophile can result in identical products regardless of the approach direction, due to equivalent spatial arrangements.

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Related Practice

Textbook Question

In each Diels–Alder reaction shown, predict the product that will result.(a)

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Textbook Question

Reduction of an alkyne using the Lindlar catalyst, a reaction presented in Section 10.6.2, produces only the cis-alkene. Why?

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Textbook Question

Predict the product of the following Diels–Alder reactions. Where a racemic mixture is produced, show both enantiomers and explain how each is formed.

(d)

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Textbook Question

In each Diels–Alder reaction shown, predict the product that will result.

(c)

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Textbook Question

Assuming the diene approaches the dienophile from the top, predict the product of the following Diels–Alder reactions.

(c)

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Textbook Question

In each Diels–Alder reaction shown, predict the product that will result.

(a)

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