 Today, we will discuss a reaction which is popularly called the olefin metathesis reaction. This reaction is almost a magical reaction where olefins can be reacted with one another in the presence of a metal complex to yield a variety of different olefins. Many of them the ratio of these olefins are thermodynamically determined. It allows for the synthesis of very complex and very interesting molecules. Metathesis purely means a change in place. If we represent this schematically an olefin as A B, we take two molecules of A B and react them together in the presence of a metal complex. The metal is our catalyst and you can get molecules of A A and B B. You can imagine the scenario when A B and C D are reacted. At least four different olefins can be generated from this reaction mixture. Now, we have the possibility of converting for example, isobutylene which is very simply written this molecule to a variety of olefins including ethylene including ethylene and two three dimethyl butene. So, you can get various molecules starting with simple a simple set of olefins. This has led to an enormous spurt in the organic synthetic field. People have been able to use it effectively for making insect pheromones or herbicides, better polymers and polymer additives, fuels and especially in the field of pharmaceuticals. It has helped in the synthesis of very complex molecules which can be used as drugs for bacterial infections, hepatitis, cancer and Alzheimer's disease etcetera. At first sight, this reaction seems to be really difficult one to handle because you can get a mixture of olefins and how it can be used for making specific molecules need to be considered. This amazing utility has been brought about by the discovery of reliable catalyst. So, not only is a reaction something which needs to be adjusted so that you get only useful products. You also need to synthesize catalyst which would be reliable and this development of catalyst made was made feasible only because one could understand the mechanism of this reaction. So, the main players in this whole game where two professors Robert Grubbs and Richard Schrock and they contributed immensely to the development of this field. As a result, complex molecules like epothyelone which is basically a drug molecule which is a drug molecule and it could be synthesized in remarkably efficient synthesis using this metathesis catalyst. The catalyst themselves some of them the standard ones are pictured here. The pentacoordinated ruthenium complex is the one which was synthesized and popularized by Grubbs and Schrock synthesized this molybdenum complex and both of them led to an enormous part in the amount of organic synthesis schemes which could be carried out using metathesis reactions. This reaction has got some surprising features as I mentioned earlier. Reaction is completely reversible and this makes it easy for one to synthesize a thermodynamically most stable products very easily. But if you want to go uphill in a thermodynamic sense, you need to adjust the reaction conditions carefully. What is interesting is also the fact that early transition metal halides were originally discovered as the ones which are suitable for carrying out this reaction in the presence of aluminum alkyls which were primarily thought to be reducing agents. Now, it turns out that both aluminum alkyls both aluminum alkyls and the transition metal halides early transition metal halides are very very water sensitive. Yet a trace of water appeared to be beneficial for the reaction. There was an initial waiting time for the catalyst to become active and this was shortened significantly when a small amount of water was added to the reaction mixture. These were some surprising features of the reaction and any mechanism that one proposed should explain some of these difficulties very well. In the beginning of this talk, I will explain to you some of the mechanisms which were proposed which seem to be very reasonable. But still they did not stand the test of time and the test of scrutiny. Let us take the example of the cyclobutane mechanism. This is one of the earliest mechanisms that was proposed and it was quite attractive. What one proposed was a fact that in the presence of the metal two molecules of olefin can react together to form a cyclobutane which although transiently could be formed in the coordination sphere of the metal. So, this cyclobutane was not a stable intermediate. It could be a transient intermediate which could be formed in the coordination sphere of the metal. Subsequently it would undergo the reverse reaction and form the two olefins again. So, this is the proposed mechanism. The reverse reaction could also happen. If it proceeded in a productive fashion, if it formed A B, two molecules of A B, then it would not be productive. If it formed two molecules of A A and B B, then we say it is a productive reaction and the reaction has carried out the metathesis. Which means the change in positions of A and B with respect to one another. Now, how can one propose such a reaction mechanism? If you remember, we said that when a olefin is coordinated to a metal, electrons are depleted from the ground, the highest occupied molecule of the olefin. They are pushed into the lowest unoccupied molecule or the lumo of the olefin. So, in a sense they behave as if they are photo excited molecules. If you recollect your organic chemistry, you will realize that photo excitation can lead to a 2 plus 2 cyclo addition reaction. That is exactly what we are proposing in the course of this reaction. So, we are suggesting the formation of a cyclobutane in the coordination sphere of the metal and its cyclo reversion. Is there any precedence for such a cyclobutane ring opening and cyclobutane formation? So, that is the question that we have to answer next. This is a reaction which was known in the literature, quadricycline ring opened in the presence of a rhodium 1 complex and it resulted in the formation of nobonodyne. If one imagines the interaction of rhodium to the quadricycline, the cyclobutane ring which is marked in red, the cyclobutane ring is marked in red and if that ring interacts with rhodium, then the ring opening would lead to nobonodyne. So, clearly you do have a possibility for a metal catalyzed ring opening of a cyclobutane ring to two olefins. Now, having said that we should quickly say that no cyclobutane has ever been observed in these reactions and so it is highly unlikely that these are intermediates, but not observing an intermediate is no reason to discount it. So, let us look at some other reasons why we discount this mechanism. One of them is a fact that after you do this reaction in the presence of carbon monoxide, it is possible to isolate a product which is clearly formed as a result of an insertion reaction. The insertion has been carried out on a species which is most likely formed due to insertion of rhodium into this carbon carbon bond. So, if this carbon carbon bond breaks, if this breaks and if the metal inserts itself between the two carbon centers, then one would end up with an intermediate which I have pictured here. This intermediate can undergo an insertion reaction if carbon monoxide is present and one can isolate a new organometallic species. So, first rhodium undergoes an insertion reaction and becomes a rhodium-3 intermediate. This rhodium-3 oxidative addition product now undergoes an insertion step. So, subsequent to the insertion, we isolate an organometallic compound which is stable and that is indicative or suggestive of the fact that you indeed have the formation of an insertion oxidative addition product to start with. That oxidative addition product can also ring open in order to form the two olefin product or the nobone adyne which we have usually. This is the product that is isolated in the absence of carbon monoxide. So, this is suggestive of the fact that it is not a cyclobutane which is coordinated to the rhodium-1 and which undergoes a cyclo reversion reaction. It is unlikely that that is in fact the mechanism of the reaction. Now, that leads us to another interesting possibility and that is a possibility which we discussed in the coupling of alkynes. So, instead of alkynes, if alkenes can be reacted with a metal in such a fashion that you have oxidative addition and carbon-carbon coupling. Let us highlight the new bonds that are being formed. So, if one electron is given here and one electron given here, one electron moved in this direction then you will have three new bonds being formed and these three new bonds we can see that they will be formed between C 2, C 3, M C 4 and M C 1. That would result in a metallocycle and this metallocycle which we will remove these lines because they seem to be confusing. So, if this metallocycle is cyclo, metallocyclo pentane is formed then this can undergo, this can be an intermediate involved in the cyclo reversion of, this can be involved in the formation of the olefins again through a cyclo reversion reaction. So, if metallocyclo pentane is formed and it undergoes a reverse reaction, you will end up with the olefins again. Now, in order to carry out metathesis however, one has to carry out an isomerization of the metallocyclo pentane. So, the metallocyclo pentane which is indicated here has to undergo a migration instead of being between C 1 and C 4, it has to come between C 4 and C 3 or C 1 and C 2. It can migrate in two different ways, but either way if it goes between C 1 and C 2, it will lead to a different metallocyclo pentane, but if it moves between C 2 and C 4 and C 3, it will give you this product. So, migration between C 4 and C 3 is what gives you this product and now, cyclo reversion will give you two different olefins. So, we have carried out the metathesis reaction after oxidative coupling of the metal. That is what we looked at oxidative coupling of the metal, where metal has given up to electrons. So, it has undergone oxidation state change and it has become plus 2 and this after reaction now gives you a metathesis reaction. So, metallocyclo pentane have to undergo this isomerization, either they have to go between C 4 and C 3 or between C 1 and C 2. One person who tested this possibility was Grubbs, the person who studied the metathesis reaction extensively and this is described in the journal of American Chemical Society paper and it is in page 1300. Now, let us take a look at how he tested out this possibility. In order to test the possibility of this isomerization, he in fact made a metallocyclopentane. So, let us take the metallocyclopentane, which is pictured here. This is a metallocyclopentane he made using m x 2 and 2 alkyl lithium dilithiobutane 1, 4 dilithiobutane. So, that gives you the metallocyclopentane. If you heat this reaction mixture, you end up with formation of two olefins and that seems to be formed by breakage of these two bonds and then you get this particular product. Now, if metallocyclopentane carries out an isomerization reaction and the metal migrates, let us mark it in a different color. Now, if the metal migrates to this position, then you will end up with this product, which is this intermediate, which is listed here. If you heat that, then you should be able to get the metathesis product, which is shown here. In fact, this reaction was not happening. We did not get these products. So, this intermediate is not being formed. This is not being formed. Grubbs showed that this reaction is possibly, the metathesis reaction is not possibly going through a metallocyclopentane. But, there exists this faint possibility that these are thermodynamically the favored olefins that are formed in this reaction. So, thermodynamically it is favorable to break this bonds in the place that I have shown with blue color. But, breaking the bonds in this particular fashion, as I shown, as I am showing here, may not be thermodynamically favorable. So, to rule out this possibility, Grubbs carried out a very interesting mechanism, very interesting reaction. He carried out a very interesting reaction, where he treated a deuterolabelled dilithiocompound with the catalyst. He used tungsten hexachloride as a catalyst in the presence of ALR3. Then, when he heated the cyclo pentane, metallocyclopentane, he observed that there was only one type of an olefin being formed predominantly. That was formed in 82 percent yield. This is clear that really indicative of the fact that this reaction system is not carrying out metathesis. If metathesis had happened, then the product should have been formed in 1 is to 1 is to 2 ratio. So, you should have in fact formed double the amount of C H D C H D. So, that was not being formed. So, it is unlikely that metathesis was in fact happening in this reaction. So, Grubbs was instrumental in discounting the metallocyclopentane pathway. So, based on these experiments, a new mechanism had to be discovered. During this time, Grubbs also showed that this reaction was stereo specific in the sense that if you took the trans deuterolabelled compound, you got only the trans labelled product. If you took the cis labelled compound, you got only the cis labelled product and that clearly showed that the metallocyclopentane mechanism was not operating. Now, during this time, there was also another mechanism which was called a carbene mechanism, but this carbene mechanism was based on a tetracharbene. If you have a di olefin complex, you could in fact generate a tetracharbene. Although this sounds unusual, we should remember that it was possible to make or break a olefin into two halves by treating it with a metal complex. This is a reaction which we looked at when the synthesis of carbene. So, a similar path could have been envisaged and the formation of a tetracharbene may not be unreasonable. If this is the case, then from the tetracharbene one can in fact isolate four different olefins based on which two olefins were reacted. Let us take a look at how this can happen. If you combine it in this fashion, you would end up with C H 2 C D 2. If you combine it in this fashion, one can form in fact C D 2 double bond C D 2. Metathesis can happen if you use a tetracharbene. It remains to be seen whether this is indeed feasible or not. So, testing this carbene mechanism or the tetracharbene mechanism, a very interesting experiment was carried out. This involved the use of a 1 7 1 6 heptadiene. So, this particular compound has got eight carbons. So, it is an octadiene. So, it is a 1 7 octadiene that was used. If you carry out metathesis under the reaction conditions, one ends up with forming cyclohexene exclusively and ethylene. So, this turns out to be a case where you have the reactance position in such a fashion that the formation of a 6 membered ring is easy. Formation of the 6 membered ring turns out to be favorable and easy. So, this is the only product that is formed in the course of this reaction. So, if you have a metathesis catalyst, you get only the cyclohexene and ethylene. Now, if you have a tetracharbene that is formed, one can understand the formation of an intermediate which is pictured here. This tetracharbene can now decompose to give you these two products. Let us imagine now, if you carry out a labeling experiment, you use CD2 and CD2 as a two terminal metathylene groups. Then, you would exclusively get tetradutorated ethylene and cyclohexene as the only products that are formed in this reaction. On the other hand, if you have the non-dutorated variety, you would get only cyclohexene and ethylene. What would happen? If you have a mixture of these two, you take a mixture of these two. If the tetracharbene intermediate is in fact the favored pathway, one should get only CD2, CD2 by the reaction of if you do the tetracharbene mechanism. It is only CD2, CD2 which will give you cyclohexene and so that should give you one mole of that. If you have one mole of each of these deuterated and non-dutorated starting materials, you would get one mole of CD2, CD2 and you would get starting with this, you would get one mole of the normal ethylene. On the other hand, if any other mechanism was operating, then you would get a mixture of products. In fact, this was indeed tried and a mixture of these olefins was used for the metathesis reaction. What one found out at the end of the reaction was that you got only 0.5 moles of CD2, CD2 and one mole of CH2, CD2 surprisingly. Now, this can be done in the same way can happen only if you interchange only if you interchange the two reactants, you mix them together and so it looks as if you go through the intermediacy, you go through the intermediacy of a compound where this is also formed in the reaction pathway. So, how can you account for the formation of species like this? So, this is in fact not acceptable for the tetracarbene. If tetracarbene intermediate was only intermediate, then this cannot have been formed. So, one mole of the fully deuterated reactant and one mole of the non-dutorated reactant should give us only the non-dutorated ethylene and the fully deuterated ethylene. It cannot result in the formation of the olefin which is 50 percent deuterated. This is clearly indicating the fact that a non pair wise exchange of olefins is being carried out. So, I have shown here what is suggested by Chauvin and Harrison and they are the ones who showed that this particular mechanism was happening. The non pair wise mechanism is the only one which will account for the fact that you can have a mixture of ethylene which is labeled and unlabeled in a statistical ratio starting with two olefins which are given here which have got let us say the labeled olefin. So, if this is a labeled olefin and the label can be C 13 or it can be deuterium and you notice that you have such labeled products which are not possible unless you have a non pair wise exchange. Now, in order to understand what a non pair wise exchange is you have to invoke a mechanism in which a carbene a metal carbene is used. This is also a metal carbene intermediate involved in the reaction, but in this case a metal carbene reacts with an olefin in order to form a metallocyclobutane. So, earlier we had proposed a metallocyclo pentane this is a four membered ring. So, this is a metallocyclobutane. Now, imagine the formation of a metallocyclobutane and this metallocyclobutane can now ring open in two different ways. If it ring opened in the way in which it formed it would end up with the formation of the same materials as a starting material which means it would end up with only the starting olefin. On the other hand it can also undergo a ring breakage in a slightly different fashion. So, let us break the ring in a slightly different fashion which means I break it like. So, then I would end up with a metal carbene which is different from what we started out with this is our starting material. This is our starting material and this is our product metal containing product. So, you can see that a new carbene can be generated and a new olefin can be generated. But, we have done so in a non pair wise manner in other words we have not taken two olefins and exchange them. We have taken one carbene and exchanged only one of the methylene groups through a metallocyclobutane. Now, how can this reaction happen? This is in fact an example where you have two neutral species one is a methylene group which is attached to the metal and the other is an olefin. So, an intermediate where you have m C H 2 it is a carbene coordinated to an olefin like this can react together and form a metallocyclobutane. How can one do that? You only have to slide this olefin in this direction. So, let us slide this olefin in this direction and then you can form a bond in such a fashion. So, that you end up with the metallocyclobutane. So, this metallocyclobutane can then ring open by breaking two different bonds which are indicated in blue here. These two bonds which are indicated in blue in order to generate a new olefin. Now, this non pair wise mechanism or this non pair wise exchange appears to have some kind of a support in several individual steps which have been observed. For example, you can think of a olefin which is a vinyl ether. I have pictured here a vinyl ether and this vinyl ether can exchange with a tungsten carbene complex which has got two phenyl groups on the carbon. If you react these two species, you end up with the final product which is diphenyl ethylene, 1 1 diphenyl ethylene which is pictured here. This is the 1 1 diphenyl ethylene and a more stable carbene tungsten complex which is pictured here. So, these two products clearly suggest that you can have a non pair wise exchange of the olefin and once you form a compound in which once you form this compound where you have a metallocyclobutane. Once you form a metallocyclobutane, then the metallocyclobutane can ring open in such a fashion that you get the more stable carbene which is what I have listed here and the diphenyl ethylene. So, the Tebe reaction in fact gives further support for this mechanism and we will take a look at the Tebe reaction. The Tebe reaction involves the reaction of bis cyclopentadienyl titanium dichloride with tri alkyl aluminum. Here is a early transition metal reacting with aluminum alkyl. So, that is typical of the metathesis reaction conditions and under those conditions it is known that the methylation of trans methylation of titanium chloride leads to a dimethyl titanium compound. So, here is a dimethyl titanium compound. This is a dimethyl titanium compound which is formed as a result of the trans methylation. Now, you can eliminate a molecule of methane from here by reacting one of these hydrogens which is present on the methyl group. One hydrogen from the methyl and one methyl group are eliminated as methane. So, you end up with a carbene complex which is a titanium carbene complex. This titanium carbene complex reacts with the aluminum alkyl compound that was formed in this reaction. So, here you would end up with Al CH3 twice Cl as a product and this reacts with your carbene intermediate and forms a stable bimetallic species. This bimetallic species has got both titanium and aluminum in the product. So, this turned out to be a very useful reagent and is called it is a carbene which is now performing a bridging function between two metals. One is a main group element aluminum and the other is titanium which is a transition element and this compound was called the Tebi reagent after its discoverer. So, the Tebi reagent which we have shown here can react with a variety of ketones to form a olefin and that happens to go through an intermediate which I have drawn here and clearly shows that a metallocyclobutane can be a possible intermediate in these reactions. So, once again this reaction is driven by the thermodynamics of this process which results which is because of the favorable formation of a titanium oxygen bond in favor of a titanium carbon bond or a titanium chlorine bond. So, aluminum Al CH3 twice Cl is formed is eliminated in this process and you can have the formation of a very stable olefin and you can see that you have exchanged now you have used this methylene group you have used this methylene group which I have pictured here. We have taken that and transferred it to this position on the ketone and you have transferred the oxygen from the carbon to the titanium. So, this turns out to be a useful way of carrying out olefination or methylene transfer reaction and this has been used extensively, but this also tells us that it is possible now to carry out such reactions in solution with organometallic compounds. So, whatever we are proposing with a metallocyclobutane mechanism must be reasonable in terms of the metathesis catalysis. So, let us get back to the reaction the metathesis reaction. There was one more reaction which was studied which also seemed to give some indication that a metallocyclobutane is possible. This was a reaction which was studied with platinum. Now, although it is not a main group element or early transition element it is an element which carries out isomerization of cyclopropane. If you take the cis labelled cyclopropane it could be converted to the trans substituted cyclopropane using a platinum 2 compound. Now, this was going through an intermediate. In this particular instance it was possible to isolate the intermediate and it was shown that the intermediate that was formed is a metallocyclobutane. The two species were actually in equilibrium and you can see it pictured on this projection here. You have both the compound where you have the phenyl group up and down and both possibilities both compounds are in equilibrium. By labeling studies it was shown that it was not the phenyl group which was migrating in the metallocyclobutane. If you label the carbon let us say with carbon 13 and that is pictured here as a highlighted ball. So, this is highlighted here as a labeled carbon. Then if the phenyl group migrates one could in fact find out that it is it should have given us this product where the labeled carbon and the phenyl group are separated. However it was found that it was not going through the it was not going through a phenyl group migration and it was completely migrating because the carbon itself was moving from this position to this position. So, that would be indicative of a migration of the whole carbons and the phenyl group to a different position. These two were in equilibrium. So, you can start with either one of the metallocyclobutanes and isomerize to the other product. So, what is a possible mechanism by which this reaction can be going? So, here is one possibility which is in support of the metathesis reaction that or the carbene mechanism for the metathesis process. So, if you have a carbene if you have a metallocyclobutane and if it breaks up to give you a carbene and an olefin. So, here is a carbene and an olefin that can be formed. If you break the bonds in this fashion let us say we break these two bonds then you would end up with an olefin which is pictured here and a methylene platinum complex. So, this methylene platinum complex can now rotate around this metal olefin bond. So, if this metal olefin bond now rotates and that rotation axis is shown here in blue color and then you would end up with a different olefin platinum complex and that can in fact be pictured here. It would result in the formation of a metal olefin complex which can reform. The metal olefin complex can in fact reform in order to generate the isomerized metallocyclobutane where the phenyl group or the R group is trans to the platinum. Initially it was cis to the platinum and now it is ended up trans to the platinum because of the way in which it rotated the olefin rotated about the platinum olefin bond. So, this kind of observation also lends support to the fact that you can in fact form a carbene and an olefin and this carbene olefin can through oxidative addition reaction. You will notice that there is this is a platinum 2 center. This is a platinum 2 center and this now is a platinum 4 center because you have 2 alkyl groups in a additionally attached to the platinum. You have a platinum 4 center here you have 2 neutral groups you have an olefin and a carbene. So, it is a platinum 2 center. So, the platinum 2 becomes a platinum 4 center through an oxidative addition and a carbon carbon bond formation. So, this is an oxidative coupling reaction which has happened. So, this oxidative coupling reaction also allows you to explain not just this isomerization but it gives support to the metathesis reaction through a metallocyclobutane intermediate. So, in the in the previous example we have shown some proof for the existence of a metallocyclobutane and its isomerization. It was not possible to show though that in the cases where you do not have an oxo ring it is possible to form the metallocyclobutane in the case of early transition metals. The first proof for that came when one could carry out metathesis reactions using the modified metallocyclobutane which involve titanium also. And here is an example it was possible to prepare a titanium cyclobutane metallocyclobutane and reacted with another olefin and it turned out that they could isolate a product in which this metallocyclobutane exchange the olefin which has present in solution and the new olefin was generated in the reaction mixture. And because they used a labeled olefin and they could show that the label was present in the final product it was very clear indication that such kind of metallocyclobutane can exchange olefins and in fact an intermediate carbene should have been involved. In this case one can write this intermediate in such a fashion it is C p T i C p coordinated to C h 2 and the olefin. So, if this is the olefin intermediate titanium olefin carbene intermediate that is formed this can very easily generate the product which is indicated here. So, this clearly indicated that all the steps involved in the metathesis reaction could be substantiated through external reactions through stoichiometric reactions. So, the catalytic cycle obtained a very strong support. Now, what this means is the mechanism suggested by Chauvin and Harrison in the early days was in fact correct and what they suggested was a non pair wise exchange. In order to understand the non pair wise exchange it is in fact possible to have a small demonstration of this non pair wise exchange. We will now show this in this movie where we have four olefins sitting around a metal catalyst. These olefins are labeled in green, blue, magenta and black. And as we have the metal exchange carbines what happens is new olefins are generated and the carbines are completely scrambled. We can see this as we proceed in this reaction the metal carbene keeps changing initially it was a gray carbene. Now, the metal carbene is green and then it becomes magenta and then in the subsequent steps it will become blue and it will also become light blue. So, a complete scrambling of the methylene groups can happen through a sequential or random scrambling of the carbines which are present in the medium. So, if you have four different olefins then all possible combinations can be observed and you can have the non pair wise exchange of olefins very easily leading to complete randomization of the carbines on the olefins. If you watch the video carefully you would have noticed that to start with we had this gray metal carbene catalyst and at the end of the reaction you ended up with a different metal carbene catalyst. The gray species or the gray carbene was in fact transferred to one of the olefins and each one of these olefins has now a different carbene or one of the carbons has been completely exchanged with some other olefins. So, this is the starting point this is your starting point and this is your ending point. So, it is possible to have complete randomization of the olefins through this particular mechanisms. So, the non pair wise mechanism proposed by Chauvin and Harrison was finally accepted and this study in the mechanistic detail allowed one to make a variety of different metal complexes. So, it is very clear that a metal carbene complex is necessary and this is the essential starting point this is your starting point for the formation of different olefins in the reaction mechanism. So, research now concentrated on the synthesis of carbines which could be stable catalyst which could be used for the metathesis reaction and this was in fact done by Schrock and Grubbs and that is the reason why all three of them in fact were honored with the noble prize. So, let us take a look now at non pair wise exchange with real molecules. So, if I have R C H C H 2 which I have pictured here I can end up with a new carbene complex metal carbene and that new metal carbene can convert a different monofunctionalized olefin having an R 1 group. Now, this is a R 1 group as opposed to an R group and you can connect this R 1 and this R. So, this R 1 and this R have now been combined to give you a new olefin starting with R 1 C H C H 2 and R C H C H 2. So, this is a non pair wise mechanism which turns out to be the right mechanism for the carbene transfer or the formation of the metathesis reaction. Now, one needs to think about this fact why did they all three of them got the noble prize that is because for a long time it has been possible to produce new compounds using metathesis. Metathesis was known long before it was fully understood, but even when they did not understand the mechanism of the reaction people were able to use it, but struggled to use proper catalysts and once the role of the catalyst was understood it was possible to move forward very rapidly and use it in reactions which led to formation of very interesting compounds. The catalyst could be designed such that in each case when there was a substrate which posed a special problem it was possible to generate a catalyst which would carry out the reaction efficiently. So, Chauvin's mechanism was extremely useful in furthering it and it was Grubbs and Schrock who made the catalyst and made it possible for this field to move forward. Here the researchers were given a new challenge to grapple with and that was to construct efficient catalysts and it was only because Yawshan Chauvin discovered the mechanism they could in fact make the catalysts and Grubbs and Schrock took up this challenge and the fundamental research in this area was furthered significantly because of the catalysts that were discovered. So, in the next lecture we would in fact be discussing how olefin metathesis can be carried out in a variety of different ways and we can use it for carrying out products that would lead to rings. So, that turns out to be ring closing metathesis we can also have ring opening metathesis where strained rings are opened and we can have several reactions where you can carry out ring opening polymerization reactions. So, new polymers could be made and cross metathesis is something that we have discussed extensively in today's lecture also that is the simplest form of the metathesis reaction and Admet is another method where we can have new types of polymerization reactions carried out. So, let me conclude by saying that today's lecture involved metathesis reactions. Metathesis is a change of place between carbene units in an olefin and this reaction ultimately leads to a large number of olefins all possible olefins if you start with two different olefins and the ratio of the products that are formed a purely thermodynamically controlled. So, by controlling the reaction conditions one can form very interesting new olefin products starting with a mixture of olefins this turns out to be a key feature for utilizing this reaction. Now, it turns out that Schrock and Grubbs were instrumental in making catalysts which were novel and also they were able to design catalysts which would suit substrates. So, that new reactions could be generated. So, it was very obvious that the combination of a fundamental mechanistic study which was based on various aspects of metal carbene in organometallic chemistry led to furtherance of the metathesis reaction and utilization of the metathesis reaction in the industry which leads to synthesis of new anti cancer drugs new molecules which are pheromones which can be used for bio activity and so many different possibilities are made because of the discovery of metathesis reactions.