Ch. 16 - Metals in Organic Chemistry

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

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Mullins 1st Edition

Ch. 16 - Metals in Organic Chemistry

Problem 21

Chapter 15, Problem 21

Suggest a mechanism for the reaction shown in Figure 16.30 using IZnCH₂I as the cyclopropanating reagent.

Verified step by step guidance

Identify the starting materials and the reagent: The reaction involves a substrate with a double bond and the reagent IZnCH₂I, which is a cyclopropanating agent.

Understand the role of IZnCH₂I: This reagent is used to form cyclopropane rings by inserting a CH₂ group across a double bond. The zinc (Zn) acts as a stabilizing metal center that facilitates the transfer of the CH₂ group.

Initiate the mechanism by forming a carbenoid: The IZnCH₂I reagent can be thought of as generating a carbenoid species, which is a reactive intermediate similar to a carbene but stabilized by the metal (Zn).

Propose the interaction with the double bond: The carbenoid species will interact with the π electrons of the double bond in the substrate. This interaction leads to the formation of a three-membered cyclopropane ring.

Conclude the mechanism: The cyclopropanation is completed when the CH₂ group is inserted across the double bond, resulting in the formation of a cyclopropane ring. The zinc iodide (ZnI₂) is released as a byproduct.

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Key Concepts

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

Cyclopropanation Reaction

Cyclopropanation is a chemical reaction that introduces a cyclopropane ring into a molecule. It typically involves the addition of a carbene or carbenoid to an alkene, forming a three-membered ring. Understanding the nature of the cyclopropanating reagent, such as IZnCH₂I, is crucial for predicting the reaction pathway and the stereochemistry of the product.

Carbenoid Reagents

Carbenoid reagents, like IZnCH₂I, are compounds that mimic the behavior of carbenes, which are highly reactive species with a divalent carbon atom. These reagents facilitate the formation of cyclopropane rings by transferring a CH₂ group to an alkene. Recognizing the properties and reactivity of carbenoids helps in understanding their role in cyclopropanation reactions.

Reaction Mechanism

A reaction mechanism outlines the step-by-step process by which reactants transform into products, detailing the movement of electrons and the formation and breaking of bonds. For cyclopropanation using IZnCH₂I, the mechanism involves the generation of a carbenoid intermediate, its interaction with the alkene, and the subsequent formation of the cyclopropane ring. Understanding mechanisms is essential for predicting reaction outcomes and designing synthetic pathways.

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Related Practice

Textbook Question

Predict the product of the following reactions.

(a)

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Predict the product of the following Negishi coupling reactions.

(b)

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Predict the product of the following reactions.

(b)

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Textbook Question

Work backward to show how the cyclopropane would be synthesized from the chloroalkane shown.

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Textbook Question

To determine the stereochemistry of curacin A by synthesis, it would have been necessary to prepare all stereoisomers of the C₁₇―C₂₀ cyclopropane fragment. How would the reaction in Figure 16.30 be modified to produce the other stereoisomers shown here?

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Textbook Question

Predict the product of the following Negishi coupling reactions.

(a)

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