A general depiction of olefin metathesis is shown below, where you have two olefins that literally switch partners:

Transcription

1 Class 6 This starts the second 1/3 of our semester, where we talk in detail about a few select reactions. We will spend two classes talking about OLEFIN METATHESIS, mostly because it is a fairly important transformation that I think is only briefly covered in undergraduate organic classes. A general depiction of olefin metathesis is shown below, where you have two olefins that literally switch partners: The first metathesis reaction came from Ziegler and Natta, who found that 2 pentene could be treated with tungsten hexachloride and ethylaluminum dichloride to give a mixture of alkene isomers: This is an equilibrium mixture, because the olefins keep switching partners with each other. Subsequently, two researchers developed highly active catalysts for this metathesis reaction: These researchers (as well as a third guy) won the Nobel Prize in chemistry in 2005 for this work. The active catalytic species in the metathesis reaction is a metallocarbene complex with a carbon metal double bond, which is formed from the two catalysts: After that, there is a 2+2 cycloaddition between the carbene and the alkene. The alkene can be oriented in one of two ways to generate two different 4 membered rings:

2 Now the four membered rings undergo retro 2+2 cycloadditions: The top ring regenerates the starting materials: The bottom ring, however, gives the dimethyl substituted olefin and a new carbene: This new carbene undergoes a new round of metathesis to generate the diethyl substituted alkene: Now we have accounted for all of the alkenes formed by the Ziegler Natta reaction. In practice, olefin metathesis has become remarkably useful in the context of organic synthesis. There are actually several different categories of olefin metathesis: 1. Ring closing metathesis (RCM) two terminal olefins react to give a closed ring system: This is still a case of switching partners just that the unseen partner is ethene: Also note that the net effect on the ring is that it loses two carbon atoms. 2. Ring opening metathesis here you open a ring that contains an alkene and form two new alkenes in the process. The general scheme is shown below:.

3 3. Cross metathesis You start with two alkenes that switch partners to form two new alkenes: Also note here that the two substituents on the SAME side of the double bond stick together i.e. A and B are never separated, nor are C and D just the pairs of substituents swap. 4. A whole variety of metathesis based polymerizations. We are going to focus primarily on #1 and #3 today, and spend next class talking about metathesis polymerizations. A schematic of ring closing metathesis is shown below: There are two competing reactions here the first is ring closing metathesis to generate the desired product. The second possible reaction is intermolecular metathesis, which will give long chain polymers. You can bias the reaction towards forming the desired closed ring by keeping the reactant concentration low. Why does this work? Well the ring closure is a unimolecular process, so even if the reaction is very dilute, the ring closure will proceed. The intermolecular metathesis, in contrast, requires two molecules to come together, and that process will become increasingly disfavored under highly dilute conditions. Another way to accomplish high dilution experimentally is to use slow addition that is, you add slowly add a solution of diene to a solution of the catalyst. As you add the diene, it reacts with the catalyst, so that the concentration of unreacted diene hanging around in your reaction vessel is minimal. Ring closing metathesis examples: Example 1: Terminal olefin with Grubbs catalyst. Note that the Grubbs catalyst is the reagent of choice (because it is so much more stable than the Mo complexes), and that the original Grubbs catalyst works best with terminal olefins. We are going to talk in more detail next time about variations on Grubbs catalyst that are more reactive. Example 2: Medium sized ring formation using Grubbs catalyst:

4 The first reaction works great because five membered rings are so highly favored. The reason why the third reaction works well whereas the second reaction doesn t work at all has to do with the conformation of the molecule. Medium sized rings have a large entropic penalty to overcome in order to cyclize meaning they tend to be large and flexible, and only one conformation (of many, many possible ones) will be the right one for cyclization to occur, so it becomes difficult for them to cyclize. This explains why the second reaction doesn t work. The third reaction, however, has a benzene ring that pre organizes things and removes some degree of the flexibility here. Example 3: Another illustration of the importance of reactant conformation: Here the lithium cation coordinates to the oxygens and pre organizes the substrate for cyclization. Example 4: You can also form large rings using Grubbs catalyst if you carefully control conditions (like using slow addition and/or organizing the substrates):

5 You often get a mixture of olefin isomers (cis and trans), especially for the larger rings. But, more often than not, the next step is to hydrogenate the olefin, so then the double bond isomer mixture is largely inconsequential. Now we are going to talk a little about polymerization reactions that are based on an olefin metathesis type of reaction. A lot of these reactions start with a highly strained cyclic alkene monomer, that undergoes a ring opening metathesis polymerization (ROMP) reaction to generate the polymers. Let s look at a few examples: Example 1: In this case you break the double bond that does not have the CF3 groups attached to it. That s because the CF3 groups provide electronic stabilization to the strained double bond (due to their electron withdrawing abilities) and this causes the catalyst to break the other double bond. Example 2: This reaction dates back to 1955, and relies on breaking the strained double bond in the bicyclic ring system to generate a polymer: Next time we are going to talk in some detail about metathesis based polymerization reactions.