Why is a racemic mixture formed in the Diels-Alder cycloaddition? I note that for both endo and exo products, either enantiomer can be formed. Why is this so? enter image description here

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    $\begingroup$ In the absence of any other external effects, why would we expect anything but a racemic mixture? You have totally achiral starting materials. $\endgroup$ – Zhe Jul 25 '17 at 15:33
  • $\begingroup$ Hi Zhe, I'm not exactly sure how the mechanism of this reaction would directly lead to a racemic mixture. $\endgroup$ – Jonathan Smith Jul 25 '17 at 16:23
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    $\begingroup$ Both enantiomers are formed because all intermediate states including the end products are energetically identical. $\endgroup$ – Karl Jul 25 '17 at 16:29
  • $\begingroup$ If you take the diene as a reference plane, the dienophile can "attack" from "above" or "below". $\endgroup$ – Martin - マーチン Jul 26 '17 at 16:09
  • $\begingroup$ This is a great time for me to point out that I didn't need to even look at what the reaction was or its mechanism. >99.999% of the time, if you start with achiral reactants, you end up with either a racemic mixture or achiral products. In the remaining <0.001% of the time, you might have some amplification like in the Soai reaction, where a small imbalance in the amount of one enantiomer is magnified because it aids in catalysis. This is not one of those cases. $\endgroup$ – Zhe Aug 7 '17 at 14:46

The concerted Diels-Alder reaction has in principle eight different possible transition states. Depending on the substrates some of them will lead to the same products.

Below you can see an overview of the (non-optimised) transition state arrangements for the reaction of isoprene and propene.

Diels-Alder Transition States

From these images you should be able to determine that the following behave like enantiomers:

  • (a) and (g)
  • (b) and (h)
  • (c) and (e)
  • (d) and (f)

When you think a little bit longer about it, you will also see that (a) and (h) will form the same product. This is also true for (b) and (g), (c) and (f), as well as (d) and (e).
In total, you will therefore obtain four products two different regio-isomers and their corresponding enantiomer each.


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