hey guys for today I want to do a very straightforward synthesis problem
here we've got a starting material and a desired product and we need to get there
in three steps so take a look at what we're working with and see what you can do.
so let's see if we can put this puzzle together take a look at the
desired product we've we see that we're trying to get an alcohol here and we're
starting out with some with some alkene functionality so we're probably thinking
that we're working with addition reactions
maybe elimination reactions we're trying to add things to pi bonds maybe generate
new pi bonds and so what we're seeing is we could probably get this as an
addition reaction if we had a pi bond here
right if we had a pi bond there then then we'd be in business but we don't we
have a pi bond over here so let's see if we can figure out a way to slowly shift
that functionality up towards that portion of the molecule so if we want to
do let's say an addition reaction we know we can do some different addition
reactions with alkenes and we want to try to get the action up there we want
to try to get functionality up in that part of the molecule so let's say we do
a hydrohalogenation now what kind of hydrohalogenation do we want to do we
could do a markovnikov hydrohalogenation or an anti markovnikov one and if we want
to have things going up in that direction
we might want to do a markovnikov hydrohalogenation because if we do that
that's going to set us up for continuing to move the functionality in that
direction so let's say we do that and we get this situation here okay so we've
done a markovnikov hydrohalogenation and we've added HBr
and the Br went to the more substituted carbon participating in that double bond
again because of the the carbocation intermediate we're going to proceed via
the more stable carbocation intermediate so if you want to go back
and check out that tutorial that will help explain that so we've done our
hydrohalogenation and now what do we want to do we're looking over here we've
identified that maybe if we get pi bond functionality there we're gonna be able
to do some kind of hydration but how are we gonna do that we want to we want to
get a pi bond so clearly we're gonna do some kind of elimination so if we want
to do an elimination though we want to think about the regiochemistry here we
want to think about what kind of elimination because depending on what
kind of base we use we could get some different products and so as it happens
we are gonna want to use a very sterically hindered base we're gonna
want to use a bulky base so let's say we use tert butoxide and maybe we would run
that in tert-butanol and so the reason that we're gonna want to do that is this
right we've got some protons here we've got beta protons there we've got beta
protons there we've got beta protons here but if we're trying to get that pi
bond there then we're gonna need to you we're gonna need to this has got to be
kinetically controlled right not thermodynamically controlled because a
small base is gonna get one of these guys and just go back to where we
started and that's that way we want the pi bond we want the pi bond up here so
that we can do some addition and get that hydroxyl there so we're gonna need
a big bulky base like tert-butoxide tert-butoxide is preferentially going to
grab one of these guys and do E2 that way and so that is what is going to get
us to here which is a much better situation because now we have pi bond
functionality where we want it to be able to do some to do some desired
chemistry so once again bulky base that's how we did that and now we know
we want to do some kind of hydration because we are primed
we've got this pi bond if we add an H and an OH then we are going to be able
to get this the product that we want but do we want to do a markovnikov hydration
or an anti markovnikov hydration? now of these two carbons this is clearly the
more substituted than this one and the OH ends up on the less substituted
carbon. so what method do we know to do anti
markovnikov hydration? well we've got our hydroboration oxidation right that is
that is the way that we know to do that so we could put in these conditions here
right that would be our hydroboration and then we could do our oxidation and
we did go over that reaction and that mechanism in another tutorial and so
that would be our anti markovnikov right that's our anti markovnikov hydration
and that is indeed going to get us to this desired product so this is a really
great problem because it's assuming it's a very simple transformation but it
makes you think critically right when we know we're gonna do some eliminations
and some additions but each step along the way we're thinking about
regiochemistry right we we started here and we knew we knew we needed to do some
markovnikov chemistry because we wanted to get the functionality here and not
here if we got the functionality here then we'd be looking at these
possibilities for pi bonds upon elimination and then we're going the
wrong direction around the molecule we want to get up going get going that way
so we did markovnikov hydrohalogenation to get the bromine in there and the
reason we did that is that gave us the option to do an elimination to get a pi
bond there we ended up doing a Hofmann elimination right this gave us our
Hofmann elimination product and that was the regio chemistry we wanted because if
we had used say sodium hydroxide we would have gotten one of these protons
instead and just been right back where we started but we did tert-butoxide we
went hoffman and that got us to this intermediate right here and then from
there we knew that we wanted to do hydration and we understood that we
needed to do an anti markovnikov hydration so we did our hydroboration
oxidation and we've got our desired product so that's a nice little
three-step synthesis.