Dienes and MO Theory
By James Ashenhurst
Cyclic Dienes and Dienophiles in the Diels-Alder Reaction
Last updated: January 18th, 2018
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.
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:
Cyclic Dienes and Dienophiles
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?
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.
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.
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:
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:
Make sense? I hope so.
Alright. Let’s try another cyclic diene. How about cyclohexadiene?
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:
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
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.
Wait – can acetylenes be used in the Diels-Alder? Yes, especially 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.
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.
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:
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)
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.
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
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.
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!