Organic Chemistry

116 CHAPTER 29 Pericyclic Reactions

thermal conditions photochemical conditions

ground-state HOMO excited-state HOMO

LUMO LUMO

Figure 29.6N
Frontier molecular orbital analysis
of a cycloaddition reaction
under thermal and photochemical
conditions.

[2+2]

overlapping orbitals have

the same color (are in-phase)

diene

dienophile

HOMO

diene

dienophile

LUMO

HOMO LUMO

Figure 29.5N
Frontier molecular orbital analysis
of a cycloaddition reaction.
The HOMO of either of the
reactants can be used with the
LUMO of the other. Both situations
require suprafacial overlap
for bond formation.

[4+2]

A cycloaddition reaction that forms a four-, five-, or six-membered ring must involve

suprafacial bond formation. The geometric constraints of these small rings make the

antarafacial approach highly unlikely even if it is symmetry-allowed. (Remember that

symmetry-allowed means the overlapping orbitals are in-phase.) Antarafacial bond

formation is more likely in cycloaddition reactions that form larger rings.

Frontier orbital analysisof a cycloaddition reaction shows that overlap of

in-phase orbitals to form the two new bonds requires suprafacial orbital overlap

(Figure 29.5). This is true whether we use the LUMO of the dienophile (a system with

one bond; Figure 29.1) and the HOMO of the diene (a system with two conjugated

bonds; Figure 29.2) or the HOMO of the dienophile and the LUMO of the diene. Now

we can understand why Diels–Alder reactions occur with relative ease (Section 8.8).

p p

s

[4+2]

Tutorial:

Cycloaddition reactions

A cycloaddition reaction does not occur under thermal conditions but does

take place under photochemical conditions.

The frontier molecular orbitals in Figure 29.6 show why this is so. Under thermal con-

ditions, suprafacial overlap is not symmetry-allowed (the overlapping orbitals are out-

of-phase). Antarafacial overlap is symmetry-allowed but is not possible because of the

small size of the ring. Under photochemical conditions, however, the reaction can take

place because the symmetry of the excited-state HOMO is opposite that of the ground-

state HOMO. Therefore, overlap of the excited-state HOMO of one alkene with the

LUMO of the second alkene involves symmetry-allowed suprafacial bond formation.

∆

h

+ no reaction

+

[2+2]