Organic Chemistry

Section 8.8 The Diels–Alder Reaction: A 1,4-Addition Reaction 315

To construct the HOMO and LUMO needed to illustrate the transfer of electrons
in a Diels–Alder reaction, we need to look at Figures 7.8 and 7.9 on pages 287 and

288. We see that the HOMO of the diene and the LUMO of the dienophile are
     asymmetric (Figure 8.4a) and that the LUMO of the diene and the HOMO of the
     dienophile are symmetric (Figure 8.4b).
     A pericyclic reaction such as the Diels–Alder reaction can be described by a the-
     ory called the conservation of orbital symmetry. This simple theory says that peri-
     cyclic reactions occur as a result of the overlap of in-phase orbitals. The phase of the
     orbitals in Figure 8.4 is indicated by their color. Thus, each new bond formed in a
     Diels–Alder reaction must be created by the overlap of orbitals of the same color.
     Because two new bonds are formed, we need to have four orbitals in the correct
     place and with the correct color (symmetry). The figure shows that, regardless of
     which pair of HOMO and LUMO we choose, the overlapping orbitals have the same
     color. In other words, a Diels–Alder reaction occurs with relative ease. The
     Diels–Alder reaction and other cycloaddition reactions are discussed in greater de-
     tail in Section 29.4.

PROBLEM 12

Explain why a cycloaddition reaction will not occur at room temperature. Recall

that at room temperature all molecules have a ground-state electronic configuration; that

is, the electrons are in the available orbitals with the lowest energy.

Stereochemistry of the Diels–Alder Reaction
If a Diels–Alder reaction creates an aymmetric carbon in the product, identical
amounts of the Rand Senantiomers will be formed. In other words, the product will be
a racemic mixture (Section 5.19).

The Diels–Alder reaction is a syn addition reaction with respect to both the diene
and the dienophile: One face of the diene adds to one face of the dienophile. Therefore,

[2+2]

s

s

overlapping orbitals have

the same color (are in-phase)

diene

dienophile

HOMO

diene

a. b.

dienophile

LUMO

HOMO LUMO

>Figure 8.4

The new bonds formed in a

Diels–Alder reaction result from

overlap of in-phase orbitals.

(a) Overlap of the HOMO of the

diene and the LUMO of the

dienophile. (b) Overlap of the

HOMO of the dienophile and the

LUMO of the diene.

s

N

N

CH 2 CH CH CH 2 CH 2 CH C N C

H

+ +

C

H

∆

asymmetric carbon