The next reaction that we'll look at is
the Claisen reaction. Now this one is named
after a person and it's really analogous
to the aldol in a lot of ways. So I would
challenge you again to pause the video
and see if you can draw at least the
first part of the Claisen reaction and
see what's going to happen. Of course
again you've got a base. You've got a
carbonyl that has an alpha proton that
can be deprotonated to make an
enolate and the enolate can react with
another molecule the starting material.
So try and see if you canto draw that out first and
then come back and see how you did.
So we have our enolate, the electron pair
comes back in to reform the carbonyl,
this electron pair comes out, looks for
somebody that wants electrons, this of course
has electrons. Its a nucleophile, it's looking for
an electrophile, which is the carbonyl
carbon pushing electron pair out onto
oxygen.
So once again I drew these two pieces of
it in different colors so you can see.
This electron pair comes back to reform
the carbonyl here. It still has its
alkoxide next to it making it an ester. Here's
the carbon that attacked the carbonyl.
Here's what used to be the carbonyl
carbon of the other molecule. Electron
pair went out on to oxygen, which is up here
now. And there's a methyl group
attached to that carbon and there's also
methoxide attached to that carbon.
Now here's there's a small difference in
the mechanism, because we've already
learned nucleophilic addition vs.
nucleophilic acyl substitution
mechanisms and if we have a good leaving
group, this electron pair from oxygen
instead of just being reprotonated
can come back down and push off our
leaving group and alkoxide is a good
enough leaving group to have that happen.
Here's our methyl out here and our methyl is still
there. Our alkoxide is gone. This electron
pair came back down to form a new
carbonyl. And then this other part of the
molecule is still the same over here.
So now what do we get? I'll redraw the
molecule just because I've kind of made
a mess of it a little bit now.
Now, instead of a beta hydroxy carbonyl, what we
get is a beta dicarbonyl or in this case
of a beta keto ester. But the broader
category would be a beta dicarbonyl.
So we have our one carbonyl here. Here's the
alpha position, here's the beta position.
So instead of a beta hydroxy carbonyl we
have a beta dicarbonyl or like a beta
keto ester. Now I assume that you could
imagine that a crossed Claisen can be just as
complicated as a crossed aldol, getting
a whole bunch of different products.
So, we're not going to go through that one
again because synthetically that's not
really useful. We don't actually do those
kind of reaction. But, what we can do is
once again we can do a crossed Claisen
where one of the molecules has no alpha
protons and the other one does have an
alpha proton. So in this case we can
react methyl benzoate with methyl
acetate. And now if we treat that with
some type of base, example sodium
ethoxide again. Now only one of the
molecules has an alpha proton that can be
deprotonated, so only one enolate will
form.
That enolate then can be forced to
react with the other ester, in ways that
we've already talked about, usually by
concentration type effects. So then when
this electron pair comes back down,
here's our nucleophile, here's who has
electrons, who wants them is a carbonyl
carbon, pushing electron pair up onto
oxygen.
Now I think the hardest part of all this
really is drawing the structures of
each point. So it's this carbon here
that's next to the carbonyl carbon that
attacked the carbonyl on the benzoate.
So here's the carbonyl carbon
from the benzoate and here is the
carbon that attacked. Next to it is a
carbonyl and next that is an alkoxide.
So, I think the hardest part about these
reactions is actually not starting to
draw the mechanism, it's trying to draw
your appropriate intermediates and
products. I think that's the challenge.
And so one of the keys here is that this
carbon is going to be next to the carbonyl
that reformed and this carbon
here is the one that adds to the
carbonyl carbon from the other molecule.
So it's this carbon here that added. It has a
carbonyl oxygen next to it with an
O-methyl after that. So now to finish off
the mechanism, the electron pair comes
back in, alkoxide leaves,
and again we get our beta keto ester.
So, just like with aldol reactions, we
can do an intramolecular Claisen
reaction just like we did an
intramolecular aldol reaction. It has a
special name it's called a Dieckmann
cyclization or Dieckmann reaction. I'm not
particularly concerned that you know
that. But we treat it with a base. Base
is going to deprotonate alpha to the
carbonyl. This molecule is symmetrical so it
doesn't matter which proton you take
off that's alpha to the carbonyl. It forms
the enblate.
This electron pair can come out, try to
find a new carbonyl to attack, pushing the
electron pair up. And now we have to
count the members of our ring. We have a one
two three four five membered ring. Ok so
I start with one of the top here, this has
an O minus and it's got an O methyl group on it.
Two three and four don't have anything.
Five is the one that just attached. So if this
is 1 this would be 5 and 5 has a
carbonyl with an ester on it. So once
again electron pair comes back down
because it does have a good leaving group, the
alkoxide. The alkoxide leaves.
And now we have a cyclic beta keto ester.
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