orgo question

[echoyjeff222]

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Passage XIII, TBR Ochem section III ...

So the question was

The answer was supposed to be A

However, doesn't the Diels Alder reaction create different enantiomers preferentially based on endo vs. exo stability? Or does this reaction not display that?

 

[The Brown Knight]

The question itself can be rephrased as: which reaction makes a racemic mixture or achiral product?
The beginning of the passage itself states that Rxn 1 produces a "mixture of enantiomers", so that's how I arrived at A without using an O chem when I previously did this passage.

In the reaction, in the transition state you could have either the diene at the top and anhydride at the bottom or vice versa - and this WILL produce enantiomers but at an equal rate for both. Visualize the situation, do you see any difference in the stability of the transition state in whether you have the diene at the bottom or the top?
The planarity of both reactants makes each enantiomer equally likely to form, which is why you have the racemic mixture.

Sorry, I don't entirely remember the endo/exo terminology, but the above is how I was thinking it.

 

[plage noire]

I'm wondering the same thing as the OP as well because I also learned that the endo product is favored due to increased transition state stability. Is this only applicable for "bridged" products - i.e. starting with a cyclic diene (such as 1,3-cyclopentadiene).

Also, compound 2 is supposed to have a double bond on its left side, right? Otherwise there wouldn't be enough "available" electrons to even be considered a dienophile...?

 

[amy_k]

Yes that should be maleic anhydride with a double bond on the left, you are right.

The kinetic endo product should be favored unless there is a catalyst. The addition of heat causes an increase in exo though (the thermodynamic product) and if the reaction is heated and left for a long time exo is favored. So I am not sure if they are getting at that adding heat (the delta sign) increases the amount of exo to cause a 50:50 mixture which would be far-fetched? The fact that there is a mistake in the reagent structure makes me doubt this question..