Dienes and MO Theory

By James Ashenhurst

Cyclic Dienes and Dienophiles in the Diels-Alder Reaction

Last updated: January 27th, 2020 |

Cyclic Dienes (e.g. Cyclopentadiene) and Cyclic Dienophiles In The Diels-Alder Reaction

Last post in this series we introduced the Diels-Alder reaction. We saw that no matter how complex the diene or dienophile, or no matter what substituents are attached, the pattern of bonds that form and break are always the same.

That also goes for reactions of cyclic dienes and dienophiles, although it takes practice to get used to it!

Table of Contents

  1. The Bond-Forming / Bond Breaking Pattern Followed By All Diels-Alder Reactions
  2. Cyclic Dienes and Dienophiles In The Diels-Alder
  3. Applying The Key Diels-Alder Bond-Forming/Bond-Breaking Pattern To A Cyclic Diene (Cyclopentadiene)
  4. Putting Diels-Alder Products In Perspective
  5. Example:  1,3-Cyclohexadiene In The Diels-Alder Reaction
  6. A Third Example Of A Cyclic Diene: Pyrone
  7. Cyclic Dienophiles In The Diels-Alder Reaction
  8. Trust The Pattern, Even With Weird-Looking Dienophiles
  9. Notes

1. The Bond-Forming / Bond Breaking Pattern In All Diels-Alder Reactions

In the Diels Alder reaction, these three things always occur:

  • 3 pi bonds break
  • 2 sigma bonds and one new pi bond are formed
  • A new six-membered ring is formed

Here’s the basic pattern:

reprise of key pattern of diels alder reaction diene plus dienophile giving new six membered ring 3 carbon carbon pi bonds broken 3 new carbon carbon bonds formed

2. Cyclic Dienes and Dienophiles In The Diels-Alder

OK. Let’s up the complexity a notch.

It’s clear that the Diels-Alder forms a new, six-membered ring.

But what if your diene is already part of a ring?

For instance, what would be the product of the following Diels-Alder?

diels alder cyclic example between cyclopentadiene and alkene what does the product look like
If I had a duck for every time I’ve seen a student struggle with this example, there would be a hell of a lot of quacking in my backyard. I would also be in gross violation of local municipal bylaws on livestock. Hence, today’s post is devoted to cyclic dienes and dienophiles.

3. Applying The Key Diels-Alder Bond-Forming/Bond-Breaking Pattern To A Cyclic Diene (Cyclopentadiene)

My first suggestion with examples like this is that it might help to block out all the non-essential bits so you can focus on the core reaction. So let’s call the carbon that’s attached to both C-1 and C-4 , “C-7” and give it a color (grey) that lets us focus on the diene itself. 
to discover what the reaction of cyclopentadiene with alkenes gives what product it might be helpful to greyscale the key carbon in cyclopentadiene
Next, let’s apply the pattern.

Does the pattern of bonds that form and break (above) say anything about breaking or forming a bond to C-7? NO. 

So don’t touch C-7!  : – )

Following the pattern gives us the following molecule:

diels alder reaction of cyclopentadiene with dienophile giving diels alder product with cyclopentadiene carbon greyscaled

4. Putting Diels-Alder Products In Perspective

Note that we’ve drawn the product molecule [2.2.1] bicyclo-2-heptene from a “birds’ eye view” , i.e. from above. That’s the simplest way to do it. The trouble is that it doesn’t really convey the stereochemistry of the product.

Hence, when a bicyclic product is formed, what you’ll often see is that it will be drawn in “perspective”, i.e. from the side.

This throws a lot of students off.

Molecules are three dimensional objects just like many other items in our everyday lives. The difference is that you likely have much less familiarity with what molecules look like from various perspectives than you do with, say, cars.

So here’s a quick analogy:

diels alder products in perspective are not unlike looking at cars from either the top or the side its the same car same with molecules top view side view of bicyclic product

Make sense? I hope so.

5.  Example:  1,3-Cyclohexadiene In The Diels-Alder Reaction

Alright. Let’s try another cyclic diene. How about 1,3-cyclohexadiene?

what about example of 1 3 cyclohexadiene with dienophile what does product look like

Again, if you focus on the diene itself without getting psyched out by the fact that it’s cyclic,  you should get the following product:

diels alder reaction of cyclohexadiene with dienophile gives bicyclic product

As the size of the cyclic diene goes up, it gets progressively more difficult to draw the bridge in a way such that it fits inside the new six-membered ring. Two carbons is about the limit, and even that looks weird.

So if you find that drawing the “side view” molecule throws you off, my (unpopular) advice is the following: find a molecular model kit and build the darn thing.   #chemnagging

6. A Third Example Of A Cyclic Diene: Pyrone

OK. Let’s finish up on dienes with a really wild-looking example. And this time we’ll use an a dienophile that is actually practical to use (dimethylacetylenedicarboxylate, or DMAD) due to its electron withdrawing groups.

what about diels alder reaction of pyrone 2 pyrone with dienophile alkyne with pyrone

Wait – can acetylenes be used in the Diels-Alder? Yesespecially if they are attached to electron withdrawing groups. Just treat them the way you’d treat a “normal” dienophile; break a C-C pi bond and leave the other one intact.

Chances are high that you’ve never seen a pyrone before. It doesn’t matter. Just trust in the pattern of bonds that form and break in the key pattern!

Here’s what the product looks like.

diels alder reaction between pyrone and alkyne gives the same pattern of bonds formed and bonds broken with alkyne and pyrone drawn in perspective

Notice how using an alkyne in the Diels-Alder results in an additional alkene in the final product, since only one of its pi-bonds is broken.

The “top-down” perspective looks really ugly. It’s difficult to fit the carbonyl into the six-membered ring (I had to cheat and increase the ring size). The “perspective” view looks much better, in my opinion. Your instructor will likely agree with me.  Therefore, get comfortable with interpreting the “perspective” view, especially for bridged bicyclic compounds.

7. Cyclic Dienophiles In The Diels-Alder Reaction

Dealing with cyclic dienophiles is slightly easier since they don’t lead to bridged bicyclic products. But they are still worth looking at.

One of the most common cyclic dienophiles you’ll encounter is maleic anhydride:

diels alder reaction with butadiene and a cyclic dienophile giving diels alder product can be shown with stereochemistry

We left out the stereochemistry here since today we’re just focused on getting the pattern of bonds that form and break correct. (although we did put the stereochemistry of the product in greyscale, below)

8. Trust The Pattern – Even With Weird-Looking Dienophiles!

Seem easy? Then let’s throw something weird at you. This is a super-hot dienophile called MTAD.

Don’t let it psyche you out. Work through the pattern.

diels alder reaction of cyclic dienophile with diene butadiene dienophile is mtad hetero diels alder reaction

This is an example where the pi bond that is being broken is not a carbon-carbon pi bond. We call these reactions, “hetero Diels-Alder reactions” and you will likely not see them in an introductory course. However, the key pattern of breaking three pi bonds and forming two single bonds and a pi bond still holds. The only difference is that we’re breaking an N-N pi bond instead of a C-C pi bond.

The reactions above might look like gnarly, really weird looking examples, but wouldn’t you rather see them now rather than, say,  on an exam? And isn’t it good to know that no matter how weird, all of these reactions follow the same general pattern?

TRUST THE PATTERN!

Next Post: Stereochemistry of The Diels Alder Reaction


Notes

As noted, the stereochemistry of the products was mostly left out so that we could focus on the bond-forming / bond-breaking pattern.

Let’s fill them in.

products of cyclic dienes and dienophiles diels alder products shown with stereochemistry depicted

Those “wedged” bonds drawn to the C-7 carbon indicate that the one-carbon “bridge” is pointing out of the page. Likewise the two-carbon bridge in the reaction with cyclohexadiene. This might seem like applied common sense, since a bridged bicyclic molecule where the C-1 bond is a wedge and the C-4 bond is a dash is geometrically impossible (try building a model).

In the next post we’ll explore the rules for stereochemistry in the Diels-Alder by examining examples such as the following (below). See if you can discern the key pattern!

challenge see if you can detect stereochemistry pattern of diels alder reaction diene stereochemistry dienopile stereochemistry

 

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