hey guys for today's practice problem we have these two dihalides we have these
two vicinal dihalides and we want to notice that they differ in their
stereochemistry and what we want to do is we want to start from an alkyne and
we want to show the pathway to get to these two different vicinal dihalides so
think about the reactions you know that can do this kind of chemistry and give this a shot.
okay we know we have to start from an alkyne that is our
designated starting material and we know that we're trying to get these de these
dihalides and so certainly a reaction that ideally is going to come to mind is
is dihalogenation right that is definitely one way we know to get di
halides and specifically vicinal because we know in a dihalogenation we're
gonna get two halogens and they're gonna be on adjacent carbons that's just how
that reaction works, so dihalogenation is something we want to be able to do
but we need to start from an alkyne we know how to get from an alkyne to one of
these so let's see what we can do we're looking at this here this is a very
straightforward situation because we know that dihalogenation we know that
the halogens are added in anti fashion and if you want to go back to that
tutorial you can see why the stereochemistry works like that but this
is exactly the molecule that is gonna come from just the alkyne as drawn
right we know that we can get we know that we can get that from this and
remember this is a retrosynthetic arrow it means back in time so we're not
saying we're gonna do this reaction we're saying that we from this with we
back in time to this because from this we can get that that is just a regular
dihalogenation with Br2 and that's how that's going to work and so
but we want to start from an alkyne so we've got an alkyne do we know ways to
get from alkynes to alkenes we actually do remember we had that tutorial where
we talked about different ways to go selectively from the alkyne to the
alkene so we know that this is a valid retrosynthetic step so we're gonna be
able to go from the from the butene we're gonna be able to do this so that's
our two-step situation so what the thing is now we need to say what reagents
we're gonna use to do this so starting with the butene how can we selectively
go from the from this alkyne to the E alkene remember this is a this is an E
alkene because these methyls are going on in opposite directions with respect
to the plane of the pi bond and so what was the reagent there we knew that we
could run this it was sodium and ammonia there was an analogous situation using
lithium it was always one of these sodium or lithium situations and
depending on what you're doing in class you may prefer one over the other we
know that that is going to allow us to get to the E alkene right so that we had
to remember because there's stereospecificity involved there and
then just regular dihalogenation so we might do that in CCl4 but certainly Br2
that is going to get us to this one so we are all set with that this is our
pathway to get to that we're gonna select them we're going to go from the
alkyne we're gonna selectively go to the E alkyne and then we're gonna do di
halogenation so well let's look over here it's not going to work as well for
this because if we did dihalogenation to this guy we're gonna get this not
this these actually have different stereochemistry and that is relevant so
what if we just do one thing how about we just do this let's just rotate this
let's rotate this 180 degrees so that means let's draw that a little better
so let's say we keep that there and then what we've done is we've flipped this
like this so this methyl is still in the plane there but if this is still in the
plane that wedge bond right that got rotated all the way back to the dash so
that is now on the dash right so this looks a little bit better because from
here since we know that dihalogenation that halogens are added in anti fashion
now this isn't a conformation that looks like it's it's more readily going
backwards to get to the alkene that we want right this is now a valid
retrosynthetic step because if we start from this alkene and we do die
halogenation we are going to get this stereochemistry and once again if we if
we if we you know rotate this bond we are going to get that in that
conformation that looks a little more familiar so we can go backwards to this
and then from here again we do know that we have a way to get there from the
alkyne right so what was our way to do that so let's say we're starting with
our alkyne how do we go forwards now right we made our
retrosynthetic pathway now we want to go forward starting from the alkyne what
are we gonna do to get from here to the Z alkene what was our way of doing that
that was our lindlar's catalyst and so you can you can memorize those those
conditions or you can maybe even just write lindlar and so lindlar we know is
going to get us to the Z alkene and then from there now we can just perform di
halogenation so we can do regular dihalogenation and that
is going to get us to here right so that was our situation here and we know that
this is just another confirmation of this and so we have shown the two ways
of getting the desired stereochemistry on the day the on the dihalide product
and the key centered around these stereospecific transformations from the
alkyne to the alkene going preferentially to E or Z respectively.