 Welcome to this course on Transition Metal Organometallics in Catalysis and Biology. Today we will be discussing ring opening metathesis. This topic of today's discussion is other subclasses of this olefin metathesis reaction particularly cross metathesis as well as admet or acyclic dien metathesis reaction that we had discussed in our earlier class. One of the major drawback of olefin metathesis reaction in general is the lack of selectivity. We had observed this issue propping up in both the earlier two discussed examples particularly in cross metathesis as well as in admet reactions that we have spoken about. So today we are going to be talking about ring opening metathesis or which is popularly called as ROM. Now ROM this ring opening metathesis is nothing but reversal of ring closing metathesis which is exactly opposite of ROM and one of the major purpose of this ROM ring opening metathesis is for making terminal dienes. The factor which guides ring opening metathesis is the presence of ring strain in the molecule. So ring strain favours ROM, hence strain substrates like norbonynes and cyclobutanes are good for ROM and as is the case with other metathesis reaction here too selectivity is an issue because of the formation of different cross metathesis and self metathesis products. So the point to note here is the following that ROM or ring opening metathesis is kind of opposite of ring closing metathesis. Now one important thing or one characteristic thing about metathesis reactions olefin metathesis reaction is that these reactions are mainly thermonutrile in the sense that they are more or less the reactant and the product are more or less of equal energy because if a double bond is broken that is a new double bond, C-C double bond is formed so there is not much of thermodynamic gain. So the driving force behind ring opening metathesis thus arises from release of ring strains in the olefinic substrates. So if the olefinic substrates containing the double bond is under strain due to constrained ring so substrates are extremely good for ring opening metathesis and hence norbonynes cyclobutanes are good for ring opening metathesis reaction. This reaction is also ROM is also suitable for synthesizing terminal dienes. So this is a method in which cyclic olefins are opened up to give terminal dienes and here too selectivity is an issue because different olefins arising from cross metathesis as well as self metathesis arise and then each of these cross metathesis and self metathesis products they also exhibit as a mixture of E and G isomers and which further complicates the process. Now there are two types of ring opening metathesis reaction known. The first one is cometathesis with an acyclic alkene and this is called R-O-C-M and the second type is ring openings metathesis polymerization or R-O-MP. And these are thermodynamically favorable if there is a release of ring strain. So here we see that R-O-M can be of two types in which cometathesis with acyclic diene which is called R-O-C-M or ring opening metathesis polymerization which is R-O-MP and these are favorable in case there is a release of ring strain. Now this being the case it is thus important to notice that not all substrates are suitable for R-O-M reaction and hence there are substrates which are specific to R-O-M metathesis reactions and these are the olefinic substrates which definitely should have ring strain. So the first that we spoke about was of norbanene, then cyclobutene or another type as is shown over here. So these are cyclobutene, then dinorbanene, then benzvalene, so the way they are dinorbanene there are tris-norbanene known as well. So tris-norbanene and then norbanene with alkyne, so these are norbanene with alkyne. So what we see is that primarily these are mainly strain rings and also the strain rings are of two varieties or three varieties. The common one is norbanene, they appear as mono-norbanene, dinorbanene, trinorbanene, norbanene with alkyne substrates as it is shown over here, the other type that we had observed is cyclobutene of the stuff over here as well as benzvalene which is the ring substrates like that over the one drawn over here. So these are different types of strain ring substrates which undergo ring opening metathesis and that is because of opening of the ring chain. In this context, this is a nice example of the first one which is called ring opening cross metathesis commonly called ROCM. It has been said that one of the drawbacks of metathesis reaction is the lack of selectivity and the selectivity usually arises due to the formation of cross metathesis product in E and Z mixture ratios as well as due to formation of homo olefin metathesis products which also appear as E and Z ratio. This homo olefin metathesis products are more commonly called as self metathesis. So in order to overcome this lack of selectivity with regard to self metathesis reactions or to suppress self metathesis reaction, the strategy of an involves carrying out this reaction in high dilution with excess of alpha olefin which suppresses the self metathesis reaction. The reaction is performed in high dilution in excess of alpha olefin suppressing the self metathesis reaction. This is best illustrated by the reaction shown below. This 7 membered diene in presence of light gives this bicyclic compound by this 2 plus 2 cyclo addition giving rise to this cyclo butane derivative which is highly strained. Now when this cyclo butane derivative is treated with alpha olefin in excess in presence of Grubbs catalyst, then following cross metathesis product is obtained in quite a good selectivity as is shown here. The yield is quite good 89 % and even the selectivity E Z ratio is about 1 is to 3. So this is a reaction with high selectivity and high yield. So this is an example of ROM with high selectivity and this is demonstrated by snapper in a journal of chemical society article in 1995. So this is a strategy which was consciously employed in enhancing the yield of the ring opening cross metathesis product and the two prong strategy involved performing the reaction under high dilution conditions along with the presence of excess amount of the alpha olefin which did the trick. Now the idea of doing this reaction under excess dilution led to separation of the self metathesis products and then the subsequent coupling with alpha olefin gave the desired cross metathesis product. Another example of ROCM is with functionalized mono substituted olefin selective ROM. Here also bit enhanced selectivity of ring opening metathesis was observed. This is given by this following equation. This is the highly strained normal mean like ring and that when reacted with this olefin in presence of graph catalyst then the following functionalized mono substituted olefinic products are obtained. As is shown over here the yield is quite high about 83 percent and easy ratio is also in favor of 2 is to 1 is to 1 and this is worked by Blakert 1997. So with this we come to the conclusion of today's lecture in which we have looked into ring opening metathesis particularly the challenges associated with ring opening metathesis with regard to the lack of selectivity arising from self metathesis as well as cross metathesis and also because of the thermonutrality of the olefin metathesis reaction in general that all this varied mixture of different metathesis products be formed. Now we have also noted that the driving force for olefin metathesis ring opening metathesis arises because of the release of the ring strain. So one of the primary criteria for becoming a substrate for ring opening metathesis is that the substrate should have considerable ring strain and because of which substrates like an orborn in or its derivatives or cyclobutane or benzfulbene they are extremely good substrates for ring opening metathesis reactions. We have also seen that ring opening metathesis are of two types one is ring opening cross metathesis and another one is ring opening metathesis polymerization in ring opening cross metathesis ring containing olefin opens up and then the metathesis happens with another olefin whereas in the ring opening metathesis polymerization the strain ring opening olefin opens up and undergoes self metathesis with another ring to give a polymer chain. We have also discussed about how the yields of the cross metathesis products can be increased enhanced in ring opening cross metathesis reaction and this was achieved successfully achieved by carrying out the reaction under dilute condition in presence of excess amount of alpha olefin. We have also seen the reaction with alpha olefin with a functionalized monosubstituted olefin using graph catalyst. So, this sort of shows that this is a useful method for making olefins with terminal dienes because that is what is the product that comes out because of ring opening polymerization of strain olefinic substrates. So, we are going to look into more of the applications of ring opening metathesis polymerization as we continue to discuss this in the next lecture. I once again thank you for being with me in this lecture and we are going to be discussing ring opening metathesis in a greater great more detail when we take this topic up in the next lecture. Thank you.