Let's have a look at alkene and alkyne metathesis.
So in general, a metathesis reaction is going to exchange two partners, like we see in this
reaction, right, you might remember metathesis reactions from precipitation reactions, from
general chemistry and so you might have two aqueous salts that swap partners, one of them
would form a solid and one would remain spectator ions in the product.
We might have called those double displacement reactions, but metathesis reaction is a general
term well with Grubbs metathesis we're going to do the same type of thing, except we're
going to have reaction partners that are alkenes and so if we're talking about a, b, c and
d, we're gonna have different parts of our reaction that are gonna be exchanged with
one another and so we might have two identical alkenes, like we have here, and this is the
best scenario because when we're partnering these up, we are going to be able to exchange
partners in such a way that we form a gas from one of them, okay so if we have a, two
terminal alkenes we would end up forming from from those two metathesis partners, a gas
ethylene which would bubble out of our solution, driving the reaction to a mixture of E and
Z, here we drew the Z form, but E and Z alkene products and so this is very much an equilibrium
system.
Okay so we're gonna form an equilibrium that's going back and forth, back and forth and so
anything we can get to drive the reaction to the product that we wanted, it's gonna
be good for Grubbs metathesis so we're gonna use a Grubbs catalyst, we can either write
Grubbs or Grubbs catalyst there on the line reaction, not too worried about the form I
go to the chemical supplier, I can buy one of several generations of Grubbs catalyst
that can do the job depending on what my substituents are, we'll just stick with the general Grubbs
catalyst here, okay.
Now, if we want we can do cross metathesis and so we can have two different terminal
alkenes, but there is a little bit of an issue with this.
So if I have two different alkene partners, what I'm going to end up with is an exchange
that's going to form a couple of by-products and so what I
could end up with the Grubbs catalyst, is yes I could end up with the cross metathesis
product, that I want again a mixture of E and Z for this by coupling together and eliminating
our gas, but the problem is we can also react each of them with themselves and so we're
gonna end up with themselves and so we're gonna end up with a by-product, E and Z variety
because there's a way to mix these two together and get the product that we see here, this
by-product or the other alkene we could end up with a catalyst form that in or excuse
me, a final product alkene that mixes these two together.
So if we're going for the cross metathesis product, we would end up having a significant
amount of our equilibrium form with the two by-products that we see, it's still going
to be an efficient reaction because we are driving off again ethylene gas in order to
do this reaction and so this is manageable, but it's not the best in the world and if
we're very poor at our planning, we might try some internal alkenes, which are not only
going to be very slow to react with our, some probably we have an R3 and R4 even here, will
be very slow to react, very difficult to bind with the catalysts and we're going to end
up with too many products and so I don't want to see these unsymmetrical internal alkenes
recommended as part of the synthesis for Grubbs metathesis, okay so yeah they might go they
probably won't, in my experience the terminal alkenes are best for this, but we could end
up with a mixture of products, either way we're not going to get much of what we're
looking for with this particular, with this particular set of reactants.
One thing that is nice that we can do with a Grubbs catalyst is we could take a system
in which we have multiple alkenes here that we could then do a ring closing metathesis
in which we still bubble off our alkene ethylene here as a gas to help drive this, but we would
then make a ring with what's left over, one, two, three, four, five and six and so we would
end up closing this ring and a form of ring closing metathesis that works very much the
same way that a normal Grubbs catalysis would so it's a very, it's a very neat reaction
and very useful in some synthesis applications and very similar to alkene metathesis, the
metathesis, is Schrock metathesis, so we could take two terminal alkynes under the same conditions,
best if those terminal alkynes are identical in nature, that way when we put the Schrock
catalyst in, we end up with ethyne or ethylene gas or excuse me, settling gas, bubbling off
to help drive our reaction and then we get our product which is the alkyne metathesis
product.
We end up exchanging the two hydrogen and carbon SP carbons in the middle here, they
these pair up and then our reactants pair up on the end to give us our final product
here, okay same deal, terminal alkynes are our best, identical terminal alkynes are best.