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Ch. 22 - Conjugated Systems II: Pericyclic Reactions
Mullins - Organic Chemistry: A Learner Centered Approach 1st Edition
Mullins1st EditionOrganic Chemistry: A Learner Centered ApproachISBN: 9780137566471Not the one you use?Change textbook
All textbooksMullins 1st EditionCh. 22 - Conjugated Systems II: Pericyclic ReactionsProblem 50
Chapter 21, Problem 50

The [2 + 2] cyclization is especially useful when done intramolecularly. 
(a) What makes intramolecular reactions more favorable? 
(b) Predict the product of the following cyclization (slight modification of a reaction from Angew. Chem. 2011, 50, 5149)
Chemical structure illustrating a [2 + 2] cyclization reaction with hydroxyl and methoxy groups, indicating photochemical conditions.

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1
Intramolecular reactions are more favorable because they involve the reaction of functional groups within the same molecule, which reduces the entropy loss compared to intermolecular reactions. This proximity increases the effective concentration of the reacting groups, leading to a higher reaction rate.
In the given structure, identify the two alkenes that will participate in the [2 + 2] cycloaddition. The reaction is photochemically induced, as indicated by the 'hv' symbol, which suggests that the reaction will proceed via a photochemical pathway.
The [2 + 2] cycloaddition involves the formation of a four-membered ring by the interaction of two π-bonds. In this case, the reaction is intramolecular, meaning the two alkenes are part of the same molecule.
Consider the stereochemistry of the starting material. The stereochemistry of the product will be influenced by the stereochemistry of the starting alkenes and the mechanism of the cycloaddition. Photochemical [2 + 2] cycloadditions often proceed with retention of configuration.
Draw the product of the cyclization, ensuring that the new four-membered ring is formed between the two alkenes. Pay attention to the stereochemistry of the newly formed ring and any existing stereocenters in the molecule.

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Key Concepts

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

Intramolecular Reactions

Intramolecular reactions occur within a single molecule, allowing reactive groups to come into close proximity. This proximity increases the likelihood of successful collisions between reactants, leading to a higher reaction rate. Additionally, intramolecular reactions often benefit from reduced entropic penalties compared to intermolecular reactions, making them thermodynamically more favorable.
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Cyclization

Cyclization refers to the process of forming a cyclic compound from a linear precursor. In organic chemistry, this often involves the formation of rings through the reaction of functional groups within the same molecule. The stability of the resulting cyclic structure can be influenced by factors such as ring strain and the presence of substituents, which can affect the overall reactivity and product distribution.
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Reaction Mechanism

A reaction mechanism outlines the step-by-step sequence of elementary reactions that occur during a chemical transformation. Understanding the mechanism is crucial for predicting the products of a reaction, as it reveals how bonds are broken and formed. In the context of cyclization, knowing the mechanism helps in anticipating the stereochemistry and regioselectivity of the final product.
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Related Practice
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.]

(b)

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Textbook Question

Suggest a diene and a dienophile that would give the following products.

(c)

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Textbook Question

The Claisen rearrangement can be used to attach allyl groups to benzene via initial alkylation of a phenol.

(a) Predict the identity of A and B

(b) provide an arrow-pushing mechanism for each step. [The phenol alkylation is simply a Williamson ether synthesis reaction.]


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Textbook Question

By forming a Lewis acid–Lewis base complex with the dienophile, Lewis acids are able to increase the rate of Diels–Alder reactions. Why might this be true?

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Textbook Question

Only after working Assessment 22.53, predict the product of the following reactions.

(a)

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Textbook Question

The Ireland–Claisen reaction is a variant of the Claisen reaction studied in this chapter.

(a) Suggest a mechanism for the first step.

(b) Predict the product (B) that would result from heating intermediate A.

(c) Hydrolysis of B gives a carboxylic acid.

Suggest a mechanism for this reaction.

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