16. Conjugated Systems

Which dienophile in each pair is more reactive in a Diels–Alder reaction?

1.

Explain why the following compounds are not optically active:

b. the product obtained from the reaction of 1,3-butadiene with trans-1,2-dichloroethene

An important variation of the Diels–Alder reaction is intramolecular, in which the diene and the dienophile are connected. This type of Diels–Alder reaction makes two new rings. Draw the compound produced in each of these examples; try to predict stereochemistry (using models will help). In some cases, Lewis acid catalysts are used; that can be ignored for this problem.

(d)

Explain why the following compounds are not optically active:

a. the product obtained from the reaction of 1,3-butadiene with cis-1,2-dichloroethene

Which of the following concerted reactions would have a more stable transition state? Why?

Figure 22.16(a) <IMAGE> shows a unique example where the Diels–Alder reaction gives a single product (no enantiomers) regardless of whether the diene attacks the dienophile from the top or bottom.

(a) Show the product of the diene attacking from the bottom and confirm that the same product is obtained.

(b) What is special about this product that makes this true?

How would the following substituents affect the rate of a Diels–Alder reaction?

c. an electron-withdrawing substituent in the diene

Write a general rule that can be used to predict the major product of a Diels–Alder reaction between an alkene with an electron-withdrawing substituent and a diene with a substituent that can donate electrons by resonance depending on the location of the substituent on the diene.

How could the following compounds be synthesized using a Diels–Alder reaction?

a.

Predict the products of the following proposed Diels–Alder reactions.

(f)

The product of the Stille coupling reaction (A) tautomerizes in a basic solution to give compound B. B spontaneously converts to C. (a) Propose a structure for A. (b) Suggest a mechanism for the conversion of A to B.

Show the steps involved in the following reaction:

While not covered explicitly in this chapter, the ene reaction occurs similarly to the Diels–Alder reaction but replaces the electrons from one bond in the diene with the electrons in a C―H bond. Draw the mechanism for the following reaction. [Number the carbons and draw in the hydrogens of the product. And, of course, make a note of bonds formed and bonds broken.]

Predict the products of the following proposed Diels–Alder reactions. Include stereochemistry where appropriate.

(d)

How would the following substituents affect the rate of a Diels–Alder reaction?

a. an electron-donating substituent in the diene