Ch. 22 - Conjugated Systems II: Pericyclic Reactions

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

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Mullins 1st Edition

Ch. 22 - Conjugated Systems II: Pericyclic Reactions

Problem 40c

Chapter 21, Problem 40c

Predict the product of the Cope reactions shown.
(c)

Verified step by step guidance

Identify the structure of the starting material: The Cope rearrangement involves a 1,5-diene system. Look for a six-membered ring transition state that can form from the starting material.

Determine the electron movement: In a Cope rearrangement, the reaction proceeds through a concerted mechanism where the π-bonds and σ-bonds are reorganized. Identify the bonds that will break and form in the transition state.

Draw the transition state: Visualize the six-membered ring transition state where the electrons are delocalized. This will help in understanding the new connections that will form.

Predict the product structure: After the rearrangement, the new product will have a different connectivity. Ensure that the new structure maintains the same number of carbon atoms and that the new π-bonds are correctly placed.

Consider stereochemistry: If the starting material has stereocenters, consider how the rearrangement might affect the stereochemistry of the product. The Cope rearrangement is typically stereospecific, so the stereochemistry of the product can often be predicted based on the starting material.

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Key Concepts

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

Cope Rearrangement

The Cope rearrangement is a [3,3]-sigmatropic reaction involving the rearrangement of 1,5-dienes. It is a pericyclic reaction where the pi bonds and sigma bonds are reorganized, resulting in a new 1,5-diene structure. Understanding the mechanism and stereochemistry of this rearrangement is crucial for predicting the product.

Sigmatropic Rearrangement

Sigmatropic rearrangements are a class of pericyclic reactions where a sigma bond migrates across a pi system. The [3,3]-sigmatropic shift in the Cope rearrangement involves the movement of a sigma bond between two allylic positions, which is essential for understanding the transformation of the starting material into the product.

Thermal Activation

The Cope rearrangement is typically thermally activated, meaning it requires heat to proceed. This is because the reaction involves overcoming an energy barrier to allow the rearrangement of bonds. Recognizing the role of heat in facilitating the reaction helps in understanding the conditions under which the Cope rearrangement occurs.

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Related Practice

Textbook Question

Predict the product of the Claisen reactions shown.

(b)

1146

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Textbook Question

Predict the product of the following reactions. [When all of the reactions from this chapter are shown together, you must first decide which type of reaction each is. Is it a Diels–Alder, an electrocyclic, or a sigmatropic rearrangement? Drawing the product will be easier once this determination is made.]

(a)

838

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Textbook Question

Draw the products you’d expect to form in Assessment 22.38.

(c)

537

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Textbook Question

Given the conditions, would you expect conrotatory or disrotatory ring closing/opening? Justify this on the basis of the molecular orbital picture.

(c)

632