Textbook Question
Predict the product of the following sigmatropic rearrangements. Be sure to rationalize the stereochemical outcome with a chair-like transition state.
(d)
16. Conjugated Systems
Cope Rearrangement
Problem 40c
Textbook Question
Predict the product of the Cope reactions shown.
(c) 
Verified step by step guidance1
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.
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Definition of Cope Rearrangement
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.
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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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