Organic Chemistry

Section 29.4 Cycloaddition Reactions 115

number of electrons that interact in the reaction. The Diels–Alder reaction is a
cycloaddition reaction because one reactant has four interacting electrons
and the other reactant has two interacting electrons. Only the electrons partici-
pating in electron rearrangement are counted.

In a cycloaddition reaction, the orbitals of one molecule must overlap with the
orbitals of the second molecule. Therefore, the frontier molecular orbitals of both
reactants must be evaluated to determine the outcome of the reaction. Because the new
bonds in the product are formed by donation of electron density from one reactant to
the other reactant, we must consider the HOMO of one of the molecules and the
LUMO of the other because only an empty orbital can accept electrons. It does not
matter which reacting molecule’s HOMO is used. It is required only that we use the
HOMO of one and the LUMO of the other.
There are two modes of orbital overlap for the simultaneous formation of two
bonds—suprafacialand antarafacial. Bond formation is suprafacialif both
bonds form on the same side of the system. Bond formation is antarafacialif the
two bonds form on opposite sides of the system. Suprafacial bond formation is
similar to syn addition, whereas antarafacial bond formation resembles anti addi-
tion (Section 5.19).

suprafacial

bond formation

antarafacial

bond formation

s p

p

s s

s

CH 2 CH 3 OCC CCOCH 3

OO

[8 + 2] cycloaddition

+

CO 2 CH 3

CO 2 CH 3

H 3 C

H 3 C

+

O

O

O

H 3 C

H 3 C

O

O

O

[4 + 2] cycloaddition (a Diels–Alder reaction)

[2 + 2] cycloaddition

H 3 C

C

CH 3

CH 2

+

O

H 3 C

H 3 C

O

h

p p

[4+2] p

p