 Welcome to this lecture on Transition Metal Organometallics in Catalysis and Biology. We have been talking about olefin metathesis reactions in the last few lectures and in particular we have been discussing the various mechanistic pathways that have been proposed with regard to explaining olefin metathesis reaction and also we have noted in the previous lecture that how even though different mechanisms have been proposed for olefin metathesis how the one which is the correct one gain the credence gradually over period of time. Now, what we had discussed in the last lecture is that there are two schools of thoughts which were initially proposed for olefin metathesis reaction. The first one was this pair wise mechanism which was indicating that both the olefins form a pair and then the reaction proceeds. However, and there were three pathways or three hypothesis that were put forward based on pair wise mechanism by Calderon, Petit and Grubbs. However, in the end by more detail isotope leveling studies confirmed the reaction proceeding by non pair wise mechanism and the one which has been proposed by Chauvin. Here the important criteria is that the active species is a metal carbene species as shown over here is the active species that carry out this metathesis reaction. Another important fallout of the Chauvin mechanism is that it is a chain reaction in the sense that there are several types of metal carbene species which are generated and they carry out this metathesis reaction in a chain fashion. We are going to be looking at it in bit more detail particularly this chain reaction aspect of metathesis when we in today's lecture when we take up this topic in much more detail. So, what this slide says is that two olefins with different substitutes A and B, C and D come together to give olefins which are now A and C, B and D as well as A and B, B and C. So, this is that famous cutting and stitching definition as if somebody had cut these olefins in fragment and then stitched them up needlessly to give all the combinations possible through this chain reaction. Proceeding further, we have also discussed this non-pair wise mechanism and what we had observed that how it goes via formation of this metallocyclobutane intermediate and then that results in the different kinds of olefins as well as different kinds of metal carbene active species. So, what is interesting to see is that there is one the starting initiating process metal carbene active species but however during the course of the metathesis many other metal carbene species are formed and these two which is shown over here and they are also equally capable of carrying out these olefin metathesis reaction and this is why this is called a chain process so that a set of chain reactions take place by this formation of many species. The main support for this non-pair wise mechanism was provided by Grubbs in this JACS paper where he had synthesized this deuterated and the terminal deuterated octane and which according to him would give these cyclohexene which is unreactive and this would give d4 ethylene whereas that when this cyclohexene that can be knocked off by this protonated octane to give this complex now that can undergo first metathesis exchanging these groups CD2 to CH2. So, what we have here is CH2 and this side is CD2 that can fast exchange to give the product which is CD2 and then CH2 CH2 whereas these can first exchange to give CD2 CH2 which is this and then CD2 CH2. So, what Grubbs did is Grubb proposed these various products that may be obtained particularly with regard to the formation of d4 and this is CD2 CH2 is d2 and CH2 CH2 is d0 which would also be formed this should be d0 and what he tried to do in this paper is that he estimated the statistical distribution of these two types of ethylene d4 d2 and d0 and then observed how closely the statistical distribution was matched in terms of pair wise or non pair wise product. So, let me illustrate this in bit more detail. So, for these two type of olefin which is deuterated versus protonated the Grubb estimated that according to pair wise mechanism the ratio would be 1 to 0 to 1.6 to 1 for these three products whereas for Chauvin it will be 1 to 2 to 1 and this is what was experimentally observed giving prediction in favor of the pair wise mechanism. So, this was a fantastic set of experiments performed by Grubbs that finally settled the issue put all the confusion to rest and gave a verdict in favor of Chauvin's pair wise mechanism. Now, what to give a perspective of how the reaction proceeds I am going to sort of just show the chain wise propagation of olefin metathesis in the following few steps over here. For example, these two olefin in presence of a metal carbene species gave olefin with A, C, B, D plus A, D plus B, C. Now, the reaction proceeded as follows. So, the initiating species is a metal carbene species and that reacting with olefin A and B giving B, A that would give X, A plus M, B whereas this would give X, B plus M, A. Then another possibility would be this species which is M, A, then plus C, D the other olefin giving A, C plus M, D, H, C plus M, D plus A, D plus M, C. Now, again which is formed over here can react with C, D giving B, C plus M, D plus M, D plus B, D plus M, C. Again this M, C can react with C, D to give A, C plus M, B, M, B plus C plus M, A and lastly again M, D plus A, B can give D, A plus D, B plus. Now, what is interesting over here is the fact that from one carbene initiating species so many different carbene species. So, from M, X which was the starting species all are the species M, B, M, A, A, M, C, M, D, A, A and so on and so forth M, D, M, C, M, B, M, A, M, B, M, A. So many different species are being generated as the codes of the reaction. So, that is why this sort of acts as a chain reaction and that is why these mass isotopic experiments were performed in order to know the statistical distribution of these various products, which sort of finally helped to explain these Chauvin's non-pair wise mechanism in explaining the metathesis reaction. So, we have seen how complex this metathesis reaction in and one can see that by just looking at different active species that are being produced by this initiating metal carbene species and how that helped in propagating the reaction in a chain fashion all giving and stretching different forms of olefin. Now, we will sort of explain these olefin metathesis in bit more detail and these olefin metathesis not only can be expected for alkene, but can also be observed for alkynes. So, alkene metathesis as well as alkene metathesis. We have to see that this reaction is not only limited to alkene, but also known for alkynes. A very nice example of this is what we had just discussed earlier is that of propene in presence of catalyst giving ethylene plus butene. So, another interesting thing is that the versatility of the metathesis reaction is very versatile and very versatile and can be catalyzed by homogeneous and heterogeneous catalyst, catalyzed under both homogeneous and heterogeneous conditions. And the first observation of conversion of linear olefins into longer and shorter chain moieties at a molybdenum catalyst. So, this was the first observation of conversion of linear olefins into longer and shorter chain moieties at a molybdenum catalyst. And this was sort of seen with the first observation of conversion of linear olefins into longer and shorter chain moieties at a molybdenum catalyst. So, this is very much illustrated by the above example where propylene got converted to shorter ethylene and longer butene. So, this was first sort of observed by conversion of olefins to the longer and the shorter moieties. Now, olefin metathesis was intriguing phenomenon and failed to begin with and then once the mechanism was sort of understood, the next biggest challenge was in trying to earmark the boundary of the reaction or the capability of the reaction. And what came into being that there are a wide range of different types of reaction, all of which belong to the family of olefin metathesis. So, in that context, a large number of olefin metathesis reaction came into being. And all of them are slightly different in type, but they all fell in this subclass of olefin metathesis reaction. And we are going to take a look at some of the classifications or different types of olefin metathesis reactions that are being. Now, as mentioned earlier that as we had said that the olefin metathesis is very versatile and is not only restricted to olefin, but also is equally observed for alkyne, which also undergo similar kind of metathesis reaction. Now, one thing to note here is the fact that alkyne metathesis is less explored than alkyne metathesis. Alkyne metathesis is less explored compared to alkyne metathesis. Now, with these, we come to the conclusion of today's lecture, where we have looked into the different mechanistic aspects of this non-pairwise mechanism proposed by Chauvin, where one can see that this non-pairwise mechanism was established through isotope labeling experiments in terms of the ratios of different isotopically labeled olefins were produced in a particular experiment. What came to the fore during this course of experiment is that the active species or the initiating species is a metal carbene species, which is different from that of the two olefins involved. And this metal carbene species during the course of metathesis reaction transforms or changes to many other types of metal carbene species. So, a singular active species gives rise to several types of many more metal carbene species, all of which are equally active in olefin metathesis reaction, as a result as chain reaction set in place, leading to formation of different combinations of olefins, which can possibly be formed. Another interesting point that we had observed is that the transition state for this metathesis reaction is a metal cyclo butane kind of intermediate, which leads to the formation of olefin as well as an active species, which is the metal carbene species. So, these isotope experiments were reported by Grubbs, which favored Chauvin's proposal of non-pairwise mechanism towards the formation of olefin metathesis products. So, with this, we come to the end of our discussion of various kinds of mechanisms, which have been proposed in explaining the metathesis reaction. And we are going to be looking at taking up different varieties of metathesis reactions or different types of metathesis reactions that have been reported so far, and also the subsequent applications of metathesis reactions in the coming lecture. So, once again I thank you for being with me in this lecture, and we are going to be looking at different types of metathesis reactions in more detail when we cover the topic in the next lecture. Till then, goodbye and thank you.