Organic Chemistry

The endo product is formed faster when the dienophile has a substituent with

electrons. It has been suggested that this is due to stabilization of the transition state by

the interaction of the porbitals of the dienophile’s substituent with the porbitals of the

new double bond’s being formed in what was the diene. This interaction, called

secondary orbital overlap, can occur only if the substituent with the porbitals lies un-

derneath (endo) rather than away from (exo) the six-membered ring.

PROBLEM 17

Which of the following conjugated dienes would not react with a dienophile in a

Diels–Alder reaction?

a. c. e.

b. d. f.

PROBLEM 18 SOLVED

List the following dienes in order of decreasing reactivity in a Diels–Alder reaction:

SOLUTION The most reactive diene has the double bonds locked in an s-cis conforma-

tion, whereas the least reactive diene cannot achieve the required s-cis conformation be-

cause it is locked in an s-trans conformation.

2,3-Dimethyl-1,3-butadiene and 2,4-hexadiene are of intermediate reactivity because they

can exist in both s-cis and s-trans conformations. 2,4-Hexadiene is less apt to be in the

required s-cis conformation because of steric interference between the terminal methyl

groups. Consequently, 2,4-hexadiene is less reactive than 2,3-dimethyl-1,3-butadiene.

PROBLEM-SOLVING STRATEGY

What diene and what dienophile were used to synthesize the following compound?

The diene that was used to form the cyclic product had double bonds on either side of the

double bond in the product, so put in the bonds, then remove the bond between them.p p

O

COCH 3

CH 3

s-cis s-trans s-cis s-trans

>>>

CH 2

O

CH 2

CH 2

CH 2

p

320 CHAPTER 8 Reactions of Dienes • Ultraviolet and Visible Spectroscopy

When the dienophile has electrons
(other than the electrons of its
carbon–carbon double bond), more of
the endo product is formed.

P

P