hey it's professor Dave let's talk about Baeyer Villiger oxidation.
so in the previous tutorial we looked at Beckmann rearrangement and we're gonna see a lot
of similarities here with the Baeyer Villiger oxidation so let's refresh our
memories remember with the Beckmann rearrangement if we were looking at a
cyclic substrate we saw that we were going from a six membered ring ketone to
a seven membered lactam so essentially what the Beckmann rearrangement did if
we just zoom all the way out is we just took a nitrogen atom and we stuck it
right in the middle of this carbon-carbon bond right there and so
now we've got a nitrogen and we know that a cyclic amat is called a lactam
that's what that functional group is called so Baeyer Villiger oxidation is
actually very similar we're just inserting an oxygen atom instead so this
was 1899 two fellows Adolph Baeyer and Victor Villiger so again some pretty old
chemistry here but it's very similar we know that by by doing bayer billig or
oxidation this is just a generic symbol for oxidation we'll look at the
mechanism in a moment we're going from that same substrate cyclohexanone to a
seven membered ring which is a cyclic ester and a cyclic ester is called a
lactone right so we've got this lactone so this is very useful if you want to go
from a ketone to an ester or from a cyclic ketone to a lactone and in
particular if we want to form a seven membered ring because that's a little
bit harder to make than six membered rings six-membered rings form very
readily because they're the most stable kind of ring we can get so this is an
interesting way to make a seven membered ring and so let's take a look at the
mechanism here the the key reagent is going to be a peracid and so when we
say peracid and this may be familiar we remember mCPBA is how we achieved
epoxidation remember when we looked at epoxides so we've seen peracids
before and the key thing here is it looks like a carboxylic acid but you'll
notice we've got some alkyl and then a carbonyl and then an oxygen and then
another oxygen and then the proton so we have an oxygen oxygen covalent
bond which is a little bit atypical right we usually don't see that we
usually see oxygen carbon bonds oxygen hydrogen bonds but here we've got this
oxygen oxygen bond so what we're gonna do is first it is an acid right and so
if it's an acid we're gonna do an acid-base reaction the only thing that's
going to act as a base is this oxygen so let's go ahead and protonate use this
peracid to protonate the ketone and so that's protonated there now that that's
protonated this carbon is highly susceptible to nucleophilic attack we've
got a negative charge here so let's let's have this come over here and we're
gonna kick that up here so we're going to neutralize that oxygen so now we've
got our hydroxyl and we've got this whole thing on there the rest of that
the rest of that peracid and so this is the part that not only does the product
look similar to the Beckmann rearrangement but also we're gonna see a
rearrangement step here that's a bit similar mechanistically to what we saw
earlier and so let's scramble this around let's have this come back down
and instead of kicking this off what if we break this carbon-carbon bond and
have this carbon coordinate instead to this oxygen and we're gonna kick that
off and so we can see this oxygen forms this pi bond here and then we're
breaking open the ring momentarily so that that bond can go and coordinator
this oxygen and kick this off so you might think it's a little weird we're
going from a six membered ring to a seven membered ring that's not so
favorable but what is favorable is where where where where this oxygen oxygen
bond is breaking an oxygen oxygen bond oxygen oxygen bonds are very weak they
they have very low bond enthalpies so it's very favorable to be breaking this
oxygen oxygen covalent bond right that's why when we look at peroxides they're
really good candidates for initiation steps to to initiate radical reactions
because it's very easy to home allies an oxygen oxygen bond because it's so weak
and so this is an easy bond to break and so what happens is we get that
rearrangement and we get our seven seven membered ring here right if this carbon
is now attached to that oxygen instead that oxygen has now essentially inserted
itself into the ring and we end up right if this now we don't have a peracid
anymore you just have that part that's just now
a regular carboxylic acid so that's not going to promote any any more reactions
really so we did that and we've got our we've got our lactone so that's the
basic mechanism here we can see this is the key step here right this
rearrangement is where you go from the six membered ring to the seven membered
ring where the oxygen inserts to two very to two points that are a little bit
minor but may be important to note if we were doing this not on a cyclic
substrate but actually on a linear substrate which means one of the alkyl
groups would formerly migrate instead of a ring just getting larger we would have
retention of stereochemistry so whatever stereochemistry we would
have on an alkyl fragment when that migrates and coordinates to the oxygen
that stereochemistry will be retained so that's just one thing to keep in mind
and then the other thing to keep in mind is that peracids are highly reactive so
you have to be careful what substrate you're using because if there are other
functional groups that are easily oxidized like pi bonds or some other
functional groups that is going to kind of mess up your reaction a little bit so
but this is great with with cyclohexanone and that's no problem
this is going to go wonderfully to the seven membered ring to the lactone and
so that's a little bit about Bayer Villiger oxidation.