Organic Chemistry

Section 8.8 The Diels–Alder Reaction: A 1,4-Addition Reaction 313

8.8 The Diels–Alder Reaction:

A 1,4-Addition Reaction

Reactions that create new carbon–carbon bonds are very important to synthetic organ-
ic chemists because it is only through such reactions that small carbon skeletons can
be converted into larger ones (Section 6.10). The Diels–Alder reaction is particularly
important because it creates twonew carbon–carbon bonds in a way that forms a
cyclic molecule. In recognition of the importance of this reaction to synthetic organic
chemistry, Otto Diels and Kurt Alder received the Nobel Prize in chemistry in 1950.
In a Diels–Alder reaction, a conjugated diene reacts with a compound containing a
carbon–carbon double bond. The latter compound is called a dienophilebecause it
“loves a diene.”( signifies heat.)

This reaction may not look like any reaction that you have seen before, but it is
simply a 1,4-addition of an electrophile and a nucleophile to a conjugated diene. How-
ever, unlike the other 1,4-addition reactions you have seen—where the electrophile
adds to the diene in the first step and the nucleophile adds to the carbocation in the sec-
ond step—the Diels–Alder reaction is a concerted reaction: The addition of the
electrophile and the nucleophile occurs in a single step. The reaction initially looks
odd because the electrophile and the nucleophile that add to the conjugated diene are
the adjacent carbons of a double bond. As with other 1,4-addition reactions, the
double bond in the product is between C-2 and C-3 of what was the conjugated diene.

The Diels–Alder reaction is another example of a pericyclic reaction—a reaction
that takes place in one step by a cyclic shift of electrons (Section 4.9). It is also a
cycloaddition reaction—a reaction in which two reactants form a cyclic product.
More precisely, the Diels–Alder reaction is a cycloaddition reactionbe-
cause, of the six electrons involved in the cyclic transition state,fourcome from the
conjugated diene and twocome from the dienophile. The reaction, in essence, converts
two bonds into two bonds.p s

p

[42]

sp^2

R

R

CH 2 CH CH CH 2 + CH 2 CH ∆

conjugated diene dienophile

¢

Cl CH 3

Cl

+

+

+

+

CH 2 CH 3 CH 3

HCl

Cl−

CH 3

3-chloro-1-

methylcyclohexene

3-chloro-3-

methylcyclohexene

thermodynamic

product

kinetic

product

H+ adds here

diene

four electrons

nucleophile new σ bond

electrophile

new new σ bond

double bond

dienophile

two electrons transition statesix electrons

‡

Otto Paul Hermann Diels

(1876–1954)was born in Germany,

the son of a professor of classical

philology at the University of Berlin.

He received a Ph.D. from that univer-

sity, working with Emil Fischer. Diels

was a professor of chemistry at the

University of Berlin and later at the

University of Kiel. Two of his sons

died in World War II. He retired in

1945 after his home and laboratory

were destroyed in bombing raids. He

received the 1950 Nobel Prize in

chemistry, sharing it with his former

student Kurt Alder.

The Diels–Alder reaction is a

1,4-addition of a dienophile to a

conjugated diene.