How does a Diels Alder reaction work?
It's pretty cool, actually. It is probably the first ring formation reaction you will have learned.
What you have in a Diels-Alder reaction is an s-cis conjugated diene and a dienophile.
A conjugated diene, has two double bonds that are separated by a single bond. A dienophile is literally a "lover of dienes", and can be a regular alkene if you wish.
A basic Diels-Alder mechanism looks like this:
where the conjugated diene must be s-cis for a reaction to occur. Otherwise, carbons 1 and 4 on the diene are too far apart to react in a concerted fashion.
And they're pretty much all like that (with heat), when the reactants are symmetrical or don't have any electron-withdrawing/donating groups.
ASYMMETRICAL SUBSTITUENTS GIVE RISE TO REGIOSELECTIVITY
When you have an asymmetrically substituted conjugated diene and/or dienophile... that's when things get more complicated, because you'll get a major and a minor product.
The identity of the substituents
For instance, let's say
RESONANCE STRUCTURES ALLOW PREDICTION OF PREFERRED ORIENTATIONS
When you draw out the resonance structures for each participant, you should notice that,
We can assert that the partially positive end of one molecule will preferentially line up with the partially negative end of the other molecule when one or both molecules are asymmetrical.
STEREOSELECTIVITY OF DIELS-ALDER REACTIONS
Lastly, if both substituents on the dienophile are related to each other in a cis or trans conformation, the product will retain that conformation.
DIELS-ALDER PRACTICE PROBLEMS
You may recognize that, aside from preferred orientations, this mechanism is very straightforward and patterned. That means you should expect some very visual variations on the Diels-Alder reactants. Here are some "fun" examples for you to figure out.
Note that some may have no reaction, if you have an s-trans conjugated diene.