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 30c

Chapter 21, Problem 30c

Predict the product of the following electrocyclic reactions.
(c)

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Identify the type of electrocyclic reaction: The given reaction involves a cyclohexene derivative undergoing an electrocyclic reaction under photochemical conditions (indicated by 'hv').

Determine the number of π-electrons involved: The cyclohexene derivative has a conjugated π-system with 4 π-electrons.

Apply Woodward-Hoffmann rules: For a 4 π-electron system under photochemical conditions, the reaction proceeds via a conrotatory mechanism.

Predict the stereochemistry of the product: In a conrotatory process, the substituents at the termini of the π-system rotate in the same direction. This will affect the stereochemistry of the product.

Draw the product structure: Based on the conrotatory motion, draw the resulting cyclobutene product, ensuring the correct stereochemistry is depicted as a result of the rotation.

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Key Concepts

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

Electrocyclic Reactions

Electrocyclic reactions are a type of pericyclic reaction where a conjugated pi-electron system undergoes a ring closure or opening. These reactions are characterized by the concerted movement of electrons, leading to the formation or breaking of a sigma bond. The stereochemistry of the product is determined by the conrotatory or disrotatory motion of the substituents, which is influenced by the number of pi electrons and whether the reaction is thermally or photochemically induced.

Photochemical Activation

Photochemical activation involves the absorption of light energy (hv) to promote electrons to higher energy states, facilitating reactions that might not occur under thermal conditions. In electrocyclic reactions, light can induce a change in the stereochemical pathway, often leading to a disrotatory mechanism for systems with 4n pi electrons. This is crucial for determining the stereochemistry of the product in the given reaction.

Conrotatory and Disrotatory Mechanisms

Conrotatory and disrotatory mechanisms describe the stereochemical pathways in electrocyclic reactions. In a conrotatory process, substituents rotate in the same direction, while in a disrotatory process, they rotate in opposite directions. The choice between these mechanisms depends on the number of pi electrons and whether the reaction is thermally or photochemically driven, affecting the final stereochemistry of the product.

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Related Practice

Textbook Question

Predict the product of the following electrocyclic reactions.

(a)

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Textbook Question

Predict the product of the following sigmatropic rearrangements. Be sure to rationalize the stereochemical outcome with a chair-like transition state.

(b)

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Textbook Question

Predict the product of the following sigmatropic rearrangements. Be sure to rationalize the stereochemical outcome with a chair-like transition state.

(d)

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Textbook Question

Identify the following dienes as being in the s-cis or s-trans conformation. If they are in the s-trans conformation, draw them in the s-cis conformation. [It may not always be possible.]

(c)

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Textbook Question

Classify the following conjugated systems as having 4n or 4n + 2 π electrons.

(c)

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Textbook Question

Predict the product of the following electrocyclic reactions.

(b)

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