Organic Chemistry

A Cope^2 rearrangementis a [3,3] sigmatropic rearrangement of a 1,5-diene. A

Claisen^3 rearrangementis a [3,3] sigmatropic rearrangement of an allyl vinyl ether.

Both rearrangements form six-membered-ring transition states. The reactions, therefore,

must be able to take place by a suprafacial pathway. Whether or not a suprafacial path-

way is symmetry-allowed depends on the number of pairs of electrons involved in the

rearrangement (Table 29.4). Because [3,3] sigmatropic rearrangements involve three

pairs of electrons, they occur by a suprafacial pathway under thermal conditions. There-

fore, both Cope and Claisen rearrangements readily take place under thermal conditions.

PROBLEM 13

a. Give the product of the following reaction:

b. If the terminal carbon of the substituent bonded to the benzene ring is labeled with

where will the label be in the product?

Migration of Hydrogen

When a hydrogen migrates in a sigmatropic rearrangement, the sorbital of hydrogen is

partially bonded to both the migration origin and the migration terminus in the transition

state. Therefore, a [1,3] sigmatropic migration of hydrogen involves a four-membered-

ring transition state. Because two pairs of electrons are involved, the HOMO is asym-

metric. The selection rules, therefore, require an antarafacial rearrangement for a

1,3-hydrogen shift under thermal conditions (Table 29.4). Consequently, 1,3-hydrogen

shifts do not occur under thermal conditions because the four-membered-ring transition

state does not allow the required antarafacial rearrangement.

suprafacial rearrangement

migration of hydrogen

antarafacial rearrangement

14 C,

sp^2

∆

O

CH 3

∆

a Claisen rearrangement

O O

C 6 H 5 CH 3

a Cope rearrangement

CH 3

C 6 H 5

CH 3

∆

120 CHAPTER 29 Pericyclic Reactions

Tutorial:

Sigmatropic rearrangements

Table 29.4 Woodward–Hoffmann Rules for Sigmatropic Rearrangements

Number of pairs of electrons Allowed mode

in the reacting system Reaction conditions of ring closure

Even number Thermal Antarafaciala

Photochemical Suprafacial

Odd number Thermal Suprafacial

Photochemical Antarafaciala

aAlthough antarafacial ring closure is symmetry-allowed, it can occur only with large rings.

X-Ref

(^2) Cope also discovered the Cope elimination (p. 000).
(^3) Claisen also discovered the Claisen condensation (p. 000).