How to Predict the Formation of Racemic Mixtures or Single Products from a Diels Alder Reaction?

Question

So far, I understand the mechanism of the Diels Alder reaction and am able to correctly predict the product of some slightly complex examples.

However, it seems that in some examples, a racemic mixture is formed, whereas in others that's not the case.

I have basic understanding of the Endo Rule (endo product favored over exo product), but have trouble with the stereo chemistry involved.

I also know that the symmetry of the diene and dienophile has to do with it, but can't seem to find a consistent pattern.

Any help would be appreciated.

Answer 1

It is very simple: if you do not have any enantiopure compound present at the beginning of the reaction, the end product will either be inherently symmetric (contain a plane of centre of symmetry) or racemic. This will be because there are two symmetry-equivalent paths of attacks that will lead to the two enantiomeric products (if enantiomers are even generated) and neither of the two paths is inherently more probable. It is a great exercise to try getting both enantiomers for a stereouncontrolled reaction!

If you do have any enantiomeric excess in your reactants, your products will also have some enantiomeric excess. If the chiral centre is far from the reactive site, the ee will be low; if it is very close or part of the reactive site the ee will typically be much higher. This is because the transition states will now be diastereomeric rather than enantiomeric, meaning they differ in energy and one will be more likely.

While it is still a great exercise to draw the two different transition states leading to different products it can be much harder to distinguish the more likely major product; especially if it is a chiral catalyst giving enantioselectivity.