Organic Chemistry

Section 29.3 Electrocyclic Reactions 111

A symmetry-allowed pathway requires

in-phase orbital overlap.

Notice that a symmetry-allowed pathwayis one in which in-phase orbitals overlap;
a symmetry-forbidden pathwayis one in which out-of-phase orbitals would overlap. A
symmetry-allowed reaction can take place under relatively mild conditions. If a reaction
is symmetry-forbidden, it cannot take place by a concerted pathway. If a symmetry-
forbidden reaction takes place at all, it must do so by a nonconcerted mechanism.
Now we are ready to learn why the electrocyclic reactions discussed at the begin-
ning of this section form the indicated products, and why the configuration of the
product changes if the reaction is carried out under photochemical conditions.
The ground-state HOMO of a compound with three conjugated bonds, such
as (2E, 4 Z, 6 E)-octatriene, is symmetric (Figure 29.3). This means that ring closure under
thermal conditionsis disrotatory. In disrotatory ring closure of (2E, 4 Z, 6 E)-octatriene,
the methyl groups are both pushed up (or down), which results in formation of the
cis product.

In disrotatory ring closure of (2E, 4 Z, 6 Z)-octatriene, one methyl group is pushed up
and the other is pushed down, which results in formation of the trans product.

If the reaction takes place under photochemical conditions,we must consider the
excited-state HOMO rather than the ground-state HOMO. The excited-state HOMO
of a compound with three bonds is asymmetric (Figure 29.3). Therefore, under
photochemical conditions, (2E, 4 Z, 6 Z)-octatriene undergoes conrotatory ring closure,
so both methyl groups are pushed down (or up) and the cis product is formed.

conrotatory

ring closure

h

(2E,4Z,6Z)-octatriene cis-5,6-dimethyl-1,3-cyclohexadiene

H H 3 C CH 3

H H

CH 3 HCH 3

1 c 42 p

(2E,4Z,6Z)-octatriene trans-5,6-dimethyl-1,3-cyclohexadiene

CH 3 CH 3 H 3 C

H CH 3

H H H

disrotatory

ring closure

disrotatory

ring closure

(2E,4Z,6E)-octatriene cis-5,6-dimethyl-1,3-cyclohexadiene

H 3 C

H 3 C

CH 3

CH 3

HH HH

1 c 32 p

The symmetry of the HOMO of the

compound undergoing ring closure

controls the stereochemical outcome

of an electrocyclic reaction.

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