Organic Chemistry

Which of the two products will be formed (or will be formed in greater yield) de-

pends on the charge distribution in each of the reactants. To determine the charge dis-

tribution, we need to draw contributing resonance structures. In the preceding example,

the methoxy group of the diene is capable of donating electrons by resonance. As a

result, the terminal carbon atom bears a partial negative charge. The aldehyde group of

the dienophile, on the other hand, withdraws electrons by resonance, so its terminal

carbon has a partial positive charge.

The partially positively charged carbon atom of the dienophile will bond preferential-

ly to the partially negatively charged carbon of the diene. Therefore, 2-methoxy-3-

cyclohexenecarbaldehyde will be the major product.

PROBLEM 15

What would be the major product if the methoxy substituent in the preceding reaction were

bonded to C-2 of the diene rather than to C-1?

PROBLEM 16

Give the products of each of the following reactions (ignore stereoisomers):

a.

b.

Conformations of the Diene

We saw in Section 7.11 that a conjugated diene such as 1,3-butadiene is most stable

in a planar confomation. A conjugated diene can exist in two different planar

conformations: an s-cis conformationand an s-trans conformation. By “s-cis,”

we mean that the double bonds are cis about the single bond The

s-trans conformation is little more stable (2.3 kcal or 9.6 kJ) than the s-cis confor-

mation because the close proximity of the hydrogens causes some steric strain

(Section 2.10). The rotational barrier between the s-cis and s-trans conformations

or is low enough to allow them to interconvert rapidly

at room temperature.

(4.9 kcal>mol 20.5 kJ>mole)

(s=single).

CH 2 CH C CH 2 + HC C C N

∆

CH 3

CH 2 CH CH CH CH 3 + HC C C N

∆

CH 3 O

CH 2

CH CH

O

δ−

δ+ δ−

δ+

CH

O

CH 3 O

resonance contributors of the diene

CH 2 CH CH CH OCH 3 CH 2 CH CH CH OCH 3

−

O

+

resonance contributors of the dienophile

CH 2 CH CH CH 2 CH CH

O−

+

318 CHAPTER 8 Reactions of Dienes • Ultraviolet and Visible Spectroscopy

Tutorial:

Diels–Alder reaction