hey everyone, it's professor Dave. I want to tell you some things about the Diels-Alder reaction.
every Diels-Alder reaction is going to have two key players here.
there's going to be a diene and a dienophile.
a diene, as you might expect, is an alkene with two pi bonds, so this is the diene
there's a pi bond there and a pi bond there
and the dienophile is the thing that reacts with the diene.
what is common of every Diels-Alder reaction is that we're going to be forming a six-membered ring
this is very useful in synthetic chemistry because there a lot of naturally occurring compounds
that we wish to synthesize that have six-membered rings. this is a very stable ring
it is utilized a lot in nature
so what happens is, this is a concerted reaction
where the pi electron density is shuffled around
this is how we're gonna draw these arrows
it is arbitrary which direction around the ring we draw the arrows because this is a concerted reaction
it's not the case that these pi bonds are moving one at a time, what ends up happening
whereas we had three pi bonds to begin with, shuffling around, what we're going to get is
one pi bond remaining and two new sigma bonds, that's these here.
so if we look at the transition state
we basically have a six-membered ring of partial pi electron density
that's illustrating how all of this is shuffling around at once, but what we end up with is always going to be
a six-membered ring with one pi bond and here are our two new sigma bonds
so there's an element of stereospecificity to examine here, let's say we are looking at
a cis dienophile, a cis dienophile will add in syn fashion to the diene, and that will result in these R groups
being cis to one another on the product
whereas if we have a trans dienophile, that is going to add in anti fashion to the diene
and so that is going to give us the groups trans to one another in the product
however, whereas this is certainly stereospecific, because cis on this side or the other would
yield the same product, here there is still an element of variability, depending on which face
of the alkene is approached, we could have a wedge and a dash here or a dash and a wedge
and those would in fact be two different molecules
once again, cis alkenes add syn, trans alkenes add anti.
the situation with butadiene and ethene was a simplified situation, let's take a look at a more complicated one
where both the diene and the dienophile are already rings themselves.
the way to draw the correct product is to always recall that we are forming a six-membered ring
between the four carbons that are doubly bound carbons in the diene and the two carbons in the dienophile.
let's number these just to be sure what's going on
always be aware that we are forming a six-membered ring between these six carbons
and then everything else is just sort of where it already is. so this is what we're going to get
the reason this looks a little funny is that we have these carbons participating both in this
five-membered ring and the new six-membered ring
so if we go ahead and draw these arrows we can see where that new six-membered ring is going to be.
let's draw them in, but the point is that this carbon here that was in the five-membered ring
that carbon is not participating in the new six-membered ring, but it didn't go anywhere, it's still there.
basically we have this situation where we have an additional carbon jutting up, out of the board a little bit
and so that's going to still be there, and the rest of the maleic anhydride will be projecting downwards
so every Diels-Alder product will have a six-membered ring, and it will be formed from specifically
those six carbons that we just discussed.
that's the easy part, the hard part is trying to figure out where all the other stuff is.
so we want to always make sure that we are not adding or subtracting any carbons or any material
we want to make sure that all of the bonds are where they're supposed to be.
it is the case that we are always just going to be moving these three pi bonds, one of them will remain a pi bond
but then the other two will go to form these new sigma bonds, so those are the new bonds there
and that's the new six-membered ring.
let's take a look at it from a different viewpoint
imagine that we are looking at these molecules edge-on
this is the case with the Diels-Alder reaction, in order for the orbitals to line up, remember that these are p orbitals
extending perpendicular from the plane of the molecule
the diene must be lowered onto the dienophile
so that's how the chemistry occurs. once again, this is the same product from a side view
here's that bridge head carbon, that's the 5th carbon on the diene that was not participating in the reaction
and then here are these two new bonds, that are forming the new six-membered ring
let's go ahead and number this one
those are the carbons that correspond to these, which are the ones that correspond to these.
in any given reaction, it is always the case that one molecule is dumping electron density
from its highest occupied molecular orbital into the lowest unoccupied molecular orbital
of some other molecule. with the Diels-Alder that's very important to look at because
it may or may not be the case that a Diels-Alder reaction is possible depending on the configuration of the molecular orbitals
let's take a look at butadiene here, if you look at the orbital diagram, there are four pi electrons
that need to be accounted for. two fit in each orbital
so here are the first two in the first orbital, and the second orbital contains one node
so that is what is going to put us in this configuration with the lobes oriented as they are
so that's the highest occupied molecular orbital for butadiene, and then the dienophile
electron density is being dumped into those so we need to examine the lowest unoccupied molecular orbital
that the electron density is going to go into
so over here we have one orbital full with the two pi electrons, and then the lowest unoccupied one
is this one with one node, we can see here that everything is in phase and ready to undergo
the Diels-Alder reaction, these are going to overlap nicely and these are going to overlap nicely
however, if there were more pi electrons or this molecule was different in some way
such that these were not in phase, then the Diels-Alder would not be able to occur
so that's something to keep in mind, we always have to check the orbital diagram
to make sure that a Diels-Alder reaction is possible.
there's one other point that we want to discuss
the diene can approach the dienophile in either exo or endo fashion, that's not necessarily
implying exothermic or endothermic, they're just terms that are describing the geometry of what's going on.
it is an exo Diels-Alder reaction if the groups projecting from the dienophile are extending away
from the diene, so this is going to lower down, and this is more probably more sterically favorable
there's not as much steric hindrance associated with an exo Diels-Alder reaction
with the endo, see how the R groups are projecting directly under the plane of the diene as it approaches
this can have something to do with the transition state of the Diels-Alder reaction
if there are functional groups present such that some kind of an interaction can occur
a slight stabilizing effect between maybe some electron deficient groups where the pi electrons
can interact with those groups in the transition state of the molecule
depending on what's going on, exo or endo may be favored for varying kinetic or thermodynamic reasons
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and as always feel free to email me with questions