Page 912 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

896 Figure 10.23 illustrates the classification of the MOs of butadiene and cyclobutene.
There are two elements of symmetry that are common to both s-cis-butadiene and
CHAPTER 10 cyclobutene: a plane of symmetry and a twofold axis of rotation. The plane of symmetry
Concerted Pericyclic is maintained during a disrotatory transformation of butadiene to cyclobutene. In the
Reactions
conrotatory transformation, the axis of rotation is maintained throughout the process.
Therefore to analyze the disrotatory process, the orbitals must be classified with
respect to the plane of symmetry, and to analyze the conrotatory process, they must
be classified with respect to the axis of rotation.

H H H
H H H
H H H H H
H H H H
H H H H H

plane of symmetry is axis of symmetry is
maintained during disrotation maintained during conrotation

Both the disrotatory and the conrotatory process can be analyzed by comparing
the symmetry classification of reactant and product orbitals given in Figure 10.23.
The orbitals are arranged according to energy in Figure 10.24, and the states of like
symmetry for the disrotatory process are connected. It is seen that in the disrotatory
process, not all of the ground state orbitals of cyclobutene correlate with ground
state orbitals of butadiene. The bonding orbital of cyclobutene is transformed into
an antibonding orbital (
of butadiene. In the reverse process,
of butadiene is
3
2
∗
transformed into the antibonding orbital of cyclobutene. Because of the failure of
the orbitals of the ground state molecules to correlate, the transformation would lead
to a high-energy TS, and the disrotatory reaction is said to be symmetry forbidden.
Analysis of the conrotatory process is carried out in exactly the same way. In this
case the element of symmetry that is maintained throughout the reaction process is
the twofold rotation axis. The resulting correlation diagram is shown in Figure 10.24.
The conrotatory reaction is symmetry allowed, since the bonding orbitals of butadiene
correlate with the bonding orbitals of cyclobutene and vice versa. Figure 10.25
is a pictorial representation of the orbital in the reactant, transition structure, and
product.

Classification with ∗ ∗
Respect to plane S S A A
Respect to axis S A S A

Classification with ∗ ∗
Respect to plane S S A A
Respect to axis S A S A

Fig. 10.23. Elements of symmetry for and classification of orbitals for disrotatory and conrotatory inter-
conversion of 1,3-butadiene and cyclobutene.