reaction combines a diene (a molecule with two alternating
double bonds) and a dienophile (an alkene) to make
rings and bicyclic compounds. The three double bonds in the
two starting materials are converted into two new single bonds
and one new double bond. Since this reaction forms two new
carbon-carbon bonds in a single step, it is a very useful
and powerful reaction (one which earned Otto Diels and Kurt
Alder a Nobel prize in chemistry for discovering it).
the Diels-Alder reaction works best when either the diene
is substituted with electron donating groups (like -OR, -NR2,
etc) or when the dienophile is substituted with electron-withdrawing
groups (like -NO2, -CN, -COR, etc).
requirements of the diene
of the Diels-Alder reaction is that the diene is required
to be in the s-cis conformation in order for the Diels-Alder
reaction to work. The s-cis conformation has both of the double
bonds pointing on the same side of the carbon-carbon single
bond that connects them. In solution, the carbon-carbon single
bond in the diene that connects the two alkenes is constantly
rotating, so at equilibrium there is usually some mixture
of dienes in the s-trans conformation and some in the s-cis
conformation. The ones that are at that moment in the s-trans
conformation do not react, while the ones in the s-cis conformation
can go on to react.
of the Diels-Alder's requirement for having the diene in a
s-cis conformation, dienes in rings react particularly rapidly
because they are "locked" in the s-cis conformation.
Unlike dienes in open chains in which there is usually some
proportion of the diene in the unreactive s-trans conformation,
dienes in rings are held in the reactive conformation at all
times by the constraints of the ring, making them react faster.
of Diels-Alder reaction
the stereochemistry of the Diels-Alder reaction? If your dienophile
is disubstituted (substituted twice), there is the possibility
for stereochemistry in the product. In the Diels-Alder reaction,
you end up with the stereochemistry that you started with.
In other words, if the substituents started cis (on the same
side) on the dienophile, they end up cis in the product. If
they started trans (opposite sides) on the dienophile, they
end up trans in the product.
of bicyclo products.
diene is in a ring, the product of the Diels-Alder reaction
is a bicyclo ring system (which can be somewhat intimidating
to draw at first). A bicyclo ring system is a compound in
which two rings share more than two carbons. There are two
main bicyclo ring systems that you typically have to deal
with, and these are the ones that come from the diene being
in a five-membered ring system and the diene being in a six-membered
ring system. (The nomenclature of bicyclic alkanes can be
make bicyclic products (that is, when the diene is in a ring),
and you have a dienophile that is substituted, there are two
possible products that you can form from the Diels-Alder reaction--the
endo product, in which the substituent points down
from the top of the bicyclic molecule, and the exo product,
where the substituent points towards the top of the bicyclic
molecule. In general, you form the endo product preferentially
over the exo product in the Diels-Alder reaction.
of the Diels-Alder reaction
example, since the diene is in a six-membered ring, you make
a bicyclo product. Since the dienophile is cis disubstituted,
you get the endo stereochemistry with the two cyano (CN) substituents
pointing away from the top of the bicyclo compound.