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.