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 35

Chapter 21, Problem 35

Provide a mechanism of the Diels–Alder reaction shown and predict the regioisomer that will form.

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Identify the diene and the dienophile in the reaction. The diene should have a conjugated system of four π-electrons, and the dienophile typically contains a double or triple bond with electron-withdrawing groups.

Draw the diene in the s-cis conformation, which is necessary for the Diels–Alder reaction to occur. This conformation allows the p orbitals to overlap effectively with those of the dienophile.

Illustrate the concerted mechanism of the Diels–Alder reaction. This involves a cyclic transition state where the π-electrons from the diene and the dienophile form new σ-bonds, resulting in a six-membered ring.

Consider the regioselectivity of the reaction. Use the electron-withdrawing and electron-donating groups on the diene and dienophile to predict the major regioisomer. The most stable product is typically formed when the electron-rich end of the diene aligns with the electron-poor end of the dienophile.

Draw the predicted regioisomer, ensuring that the stereochemistry is consistent with the endo rule, which states that substituents on the dienophile prefer to be oriented towards the electron-rich diene in the transition state, leading to the endo product.

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Key Concepts

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

Diels–Alder Reaction

The Diels–Alder reaction is a [4+2] cycloaddition reaction between a conjugated diene and a dienophile, forming a six-membered ring. It is a pericyclic reaction that proceeds through a concerted mechanism, meaning bonds are formed and broken simultaneously. This reaction is stereospecific and can produce different regioisomers depending on the substituents on the diene and dienophile.

Regioselectivity

Regioselectivity in the Diels–Alder reaction refers to the preference for the formation of one regioisomer over another. This is influenced by the electronic and steric properties of the substituents on the diene and dienophile. Typically, electron-withdrawing groups on the dienophile and electron-donating groups on the diene guide the regioselectivity, often following the 'ortho-para' rule for the most stable product.

Mechanism of Cycloaddition

The mechanism of the Diels–Alder reaction involves the overlap of the π-orbitals of the diene and dienophile, forming new σ-bonds in a cyclic transition state. This concerted process does not involve intermediates, and the reaction's stereochemistry is determined by the orientation of the reactants. Understanding the orbital interactions and transition state helps predict the stereochemistry and regioselectivity of the product.

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Related Practice

Textbook Question

Predict the product of the Diels–Alder reactions shown.

(a)

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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.]

(e)

1163

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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)

1015

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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)

1232

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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

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Textbook Question

For the following electrocyclic reactions, did the substituents move in a conrotatory or disrotatory direction? Would you use heat or light to cause movement in this direction?

(a)

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