The allylic position is the atom bound to a double bonded atom. The substituents on the allylic carbon and the doubly bonded atoms can result in allylic strain. Contributed by: Sophia Robinson, (Undergraduate), Physical Organic Chemistry I, CHEM 7240 (Sigman), University of Utah, 2015

1. Organic Pedagogical Electronic
Network
Allylic Strain
Created by Sophia Robinson
Physical Organic Chemistry I
CHEM 7240 (Sigman), 2015

2. Allylic Strain
http://isites.harvard.edu/fs/docs/icb.topic93502.files/Lectures_and_Handouts/05-Conformational_Anal-2.pdf
Hoffman, R.W. Chem Rev. 1989, 89, 1841
Allylic strain arises from eclipsed conformations
when Z allylic substituents and Z substituents at the
2- or 3-positions are large enough to create an
unfavorable nonbonding repulsion. Strain between
the allylic and 2-position substituents is called A1,2-
strain (2.7 kcal/mol). Strain between the allylic and
3-position substituents is called A1,3-strain (3.9
kcal/mol). A1,2-strain and A1,3-strain affect the
diastereoselectivity of reactions.
The allylic position is the atom bound to a double
bonded atom. The substituents on the allylic carbon
and the doubly bonded atoms can result in allylic
strain.

3. Allylic Strain
http://isites.harvard.edu/fs/docs/icb.topic93502.files/Lectures_and_Handouts/05-Conformational_Anal-2.pdf
Hoffman, R.W. Chem Rev. 1989, 89, 1841
Consider the general structure where X and Y are
C, N, or O:
The allylic center and its substituents often lead to
a significant steric bias toward reactions occurring
at the double bond.

4. Allylic Strain
Hoffman, R.W. Chem Rev. 1989, 89, 1841
Consider rotation about the bond of the two substituted allylic systems shown:
These values are from ab intio calculations performed by Houk. A and B are both
minima in conformational energy while C is not an energy minimum.
When the C(3) substituent is a methyl rather than hydrogen, conformational
equilibrium strongly favors D. With the methyl present, E is destabilized by allylic 1,3-
strain and is an energy maximum. F is an energy minimum but is far less energetically
favorable than D.

5. Examples of Reactions influenced by Allylic Strain
Diels-Alder
Hoffman, R.W. Chem Rev. 1989, 89, 1841
The diastereoselectivity of a
Diels-Alder reaction is
increased by presence of an
additional substituent on
carbon 2 as the substituent
destabilizes transition state B
through A1,3-strain.
Two transition states for the Diels-Alder
reaction are shown in which opposite
faces of the diene are shielded:

6. Examples of Reactions influenced by Allylic Strain
Hydroboration Oxidation
The alkene aligns itself as shown such that the smallest of the
substituents is staggered with the alkene double bond to prevent
A1,3-strain. The borane then approaches from the side of the
medium-sized methyl substituent rather than the larger R1
substituent:
Houk, K.N.; Rondan, N.G.; Wu, Y.D.; Metz, JT; Paddon-Row, M.N.; Tetrahedron, 1984, 40, 2257

7. Problems
Hoffman, R.W. Chem Rev. 1989, 89, 1841
Fleming, I and coworkers, Chem. Commun. 1985, 318;

8. Solutions
1.) R = Me, A is major diastereomer (87 A : 13 B)
R = Et, A is major diastereomer (80 A : 20 B)
R = CHMe2, B is major diastereomer (40 A : 60 B)
2.) B. B is the most stable, C is the least stable
3.) B (> 95% ds) (A ds = 50 %)
4.) B. B is major diastereomer. If SiMe3 substituent is not present, no
diastereoselectivity is observed
5.) C (ds = 90%)

9. This work is licensed under a
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License.
Contributed by:
Sophia Robinson, (Undergraduate)
Physical Organic Chemistry I
CHEM 7240 (Sigman), University of Utah, 2015