In this leah4sci video we're going to look at the Stereochemistry of the Diels Alder
Reaction.
In previous videos, we looked at the Diels Alder Reaction Mechanism and a shortcut for
quickly identifying products.
The problem is, everything was flat.
If you take a look at this reaction, we have a Diene with two substituents, a dienophile
with 2 substituents and the product drawn with no stereochemistry.
This is acceptable for the starting molecules because this carbon and this carbon on the
Diene are both Sp2 hybridized that means they're trigonal planar or flat as well as the dienophile.
Again, sp2 and sp2 hybridized, trigonal planar and flat.
Let's quickly number the molecule, we have 1, 2, 3, 4, A and B.
This is the trick I teach in the previous videos which you can find on my website linked
below or by visiting Leah4sci.com/DielsAlder.
If we now look at the product with the same numbering scheme.
You'll notice that the substituent on carbon 1, carbon 4, A and B are all sitting on SP3
hybridized carbons.
Sp3 is tetrahedral or 3-dimensional and that means the substituents have to go up and out
of the page or down and into the page and the question is which one?
Let's start with the stereochemistry of the Diene.
If I have a substituent coming out of Carbon 2 or Carbon 3, we don't have to worry about
it.
Why not?
Both the starting Diene and the Cyclohexene product, have the substituents sitting on
an sp2 carbon.
It started out flat, it ended flat and so the substituent is still in the plane of the
page, no stereochemistry.
This changes if the substituent sits on carbon 1 or 4.
Since this carbon is sp2 hybridized, one substituent will point straight up and on carbon 4 straight
down.
The second substituent would point inward down on carbon 1, up on carbon 4.
Let's call the outside substituents A and the inside substituents B.
We'll react with a simple Dienophile to give a product that looks like this.
If the substituent is outside, that means pointing straight up or straight down, then
the substituent goes on dashes into the page.
If the substituent is inside, our product will be on a wedge coming up and out of the
page.
This used to confuse me and so I like to think of it as follows.
The inside substituents are close together and so we want to bring them closer.
How do we bring them closer?
By drawing them up and out of the page they're coming directly at me, closer to me.
The outside substituents are further apart and so we push them away, how? by pushing
them down and into the page.
This works even if your starting molecule is a ring such as the cyclopentadiene.
Notice that the ring cancels my inside substituents and hydrogen will count as the outside and
so for our product, we start with our cyclohexene.
The starting molecule is 1, 2, 3, 4 with carbon 5 as the tip that we're going to draw upward
the way I taught you in the previous video.
Upward, because it's the inside substituent and so they're close together, we bring them
closer up and out of the page.
And while you don't have to show hydrogens for the sake of stereochemistry I'm going
to draw them down because they're further away and so we push them further down into
the page.
You can also draw your cyclohexene as a bread basket with carbon 5 coming up and out of
the page and the hydrogen which is typically invisible down and into the page.
Now let's take a look at the dienophile stereochemistry, specifically Endo and Exo.
Starting with the simple molecule that has an aldehyde substituent, to give me the following
product.
If I draw cyclohexene.
I have an option of putting the aldehyde group coming up and out of the page, or going down
and into the page.
The convention for this comes from having ring substituents but we'll simplify it here
to say that if it's coming up and out of the page, this is the Exo substituent, and the
way I think of it is that it's exiting up out of the page.
If it's going down and into the page, I think of it as the Endo substituent because it enters
into the page.
Endo going IN and Exo coming UP and OUT.
Now it happens if I have two substituents on my dienophile such as 2 aldehydes in opposite
or trans configuration, or 2 aldehydes in the same or Cis configuration.
The Cis and Trans stereochemistry will be retained.
And once again I have the option of drawing it two different ways.
For the Trans option, I can put the upper aldehyde out and the lower one in, or the
opposite giving me two different products that in this case are enantiomers of each
other.
Be careful though they're not always enantiomers.
If I add a methyl group here, now the two products become diastereomers and I challenge
you to figure out why that changes.
Same thing for the Cis configuration, the product will still be Cis and so I have the
option of bringing both of the groups up and out of the page or both of the groups down
and into or into the page.
Since both substituents go in the same direction, when they come up and out of the page or exit
the page, we get the Exo and when they go down and into or into the page we get Endo.
For even more on the Diels Alder Reaction including the tricks and shortcuts that I
mentioned earlier, visit my website linked below or go to leah4sci.com/DielsAlder.
Again that's leah4sci.com/DielsAlder.
For even more on organic chemistry, including practice quizzes, cheat sheets, videos and
more, follow along with your syllabus on my website at Leah4sci.com/Syllabus