April 21, 2019, 04:31:36 AM
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Topic: Diels-Alder Endo vs. Exo  (Read 440 times)

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Offline gfeuer

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Diels-Alder Endo vs. Exo
« on: February 09, 2019, 08:26:45 PM »
Hi everyone,

So in my second-semester organic chemistry class we have been discussing the Diels-Alder reactions. They seem so simple but there are so many seemingly minuscule changes that can affect the entire reaction. I thought I had a good grasp of it but then I got to the practice problem attached below. I understand how to form each of those products, but then my friend and I got into an argument about the question posed to the side: which of these are endo? I argued that all of them are due to the fact that the two functional groups on the dienophile are equivalent electron withdrawing groups (in fact they are exactly the same) and since they are added by a concerted mechanism that involves syn addition, they will have opposite stereochemistry. That means in all of the products formed, one of those EWG will always be on the "same side" as the outside group (the methoxy group), which is how our textbook says we should think about endo and exo (the EWG should be on the same side as the outside group - granted the book only showed us monosubstituted dienophiles). My friend says no. He believes the structures in the top left and bottom right are endo because the only relevant endo structures involve either the 1,2 and 1,4 products. Would anyone be able to shine some light on this? It would be greatly appreciated.

Thank you!

Offline pgk

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Re: Diels-Alder Endo vs. Exo
« Reply #1 on: February 11, 2019, 01:59:30 PM »
Diels-Alder reaction does not really occur via rolling electron pairs, as being 2D-represented on a piece of paper.
In reality, Diels-Alder reaction occurs via superposition of the one molecule over the other and overlapping their π-orbitals, followed by dienophile’s π-electrons attack on the double bond of the diene with simultaneous migration of the diene’s double bonds and formation of the corresponding dienophile’s carbocation, which is then attacked by the π-electrons of the end-migrated diene’s double bond.   
Does this help you to conclude which is the endo- and the exo- (Diels-Alder) products formed, respectively? 
Hint: Try to imagine this in space. It is easier than trying to understand my text.
« Last Edit: February 11, 2019, 02:28:31 PM by pgk »

Offline Vidya

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Re: Diels-Alder Endo vs. Exo
« Reply #2 on: February 11, 2019, 08:40:18 PM »
Hi everyone,

 It would be greatly appreciated.

Thank you!

In endo -inside group is pushed in different plane then the incoming group which results in outside group of the diene in the same plane of the EWG of the dienophile.

I pulled out this diagram from internet ..check it

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