Page 858 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

842 dispersion forces. 13 Molecular orbital interpretations emphasize secondary orbital
interactions between the orbitals on the dienophile substituent(s) and the developing
CHAPTER 10
bond between C(2) and C(3) of the diene.
Concerted Pericyclic D-A cycloadditions are sensitive to steric effects. Bulky substituents on the
Reactions
dienophile or on the termini of the diene can hinder the approach of the two compo-
nents to each other and decrease the rate of reaction. This effect can be seen in the
relative reactivity of 1-substituted butadienes toward maleic anhydride. 14

R k (25° C)
rel
R H 1
4.2
CH 3
C(CH ) < 0.05
3 3
Substitution of hydrogen by methyl results in a slight rate increase as a result of the
electron-releasing effect of the methyl group. A t-butyl substituent produces a large
rate decrease because the steric effect is dominant.
Another type of steric effect has to do with interactions between diene substituents.
Adoption of the s-cis conformation of the diene in the TS brings the cis-oriented 1- and
3
4-substituents on diene close together. trans-1,3-Pentadiene is 10 times more reactive
than 4-methyl-1,3-pentadiene toward the very reactive dienophile tetracyanoethene,
owing to the unfavorable steric interaction between the additional methyl substituent
and the C(1) hydrogen in the s-cis conformation. 15

R k rel
CH 3 H 1
H
R CH 3 10 –3

Relatively small substituents at C(2) and C(3) of the diene exert little steric
influence on the rate of D-A addition. 2,3-Dimethylbutadiene reacts with maleic
anhydride about ten times faster than butadiene because of the electron-releasing effect
of the methyl groups. 2-t-Butyl-1,3-butadiene is 27 times more reactive than butadiene.
The t-butyl substituent favors the s-cis conformation because of the steric repulsions
in the s-trans conformation.

CH 3 CH 3 CH CH 3
H 3 H
CH 3 CH 3 H
H
H H
H H
H H

13 Y. Kobuke, T. Sugimoto, J. Furukawa, and T. Funco, J. Am. Chem. Soc., 94, 3633 (1972);
K. L. Williamson and Y.-F. L. Hsu, J. Am. Chem. Soc., 92, 7385 (1970).
14 D. Craig, J. J. Shipman, and R. B. Fowler,J. Am. Chem. Soc., 83, 2885 (1961).
15
C. A. Stewart, Jr., J. Org. Chem., 28, 3320 (1963).