 Welcome to this lecture on Transition Metal Organometallics in Catalysis and Biology. In the course of the last few lectures, we have been discussing a very important reaction, which is olefin metathesis and has been also a seminal discovery by its own right and merit with regard to the polymerization reactions. And in fact, this is the second Nobel Prize award winning discovery after olefin polymerization and then subsequently the one that you are discussing, which is olefin metathesis. Now, we have noted in the previous lecture that this olefin metathesis reaction has its origin from olefin polymerization reaction in terms of its discovery, because research on the area of olefin polymerization led to observations of some unexpected results, which at that point of time could not be explained and later on this unexpected discovery resulted in the field of olefin metathesis. So, this olefin metathesis has its birth in olefin polymerization reaction. Now with regard to olefin metathesis, the thing which was perplexing is this cutting and stitching of olefins. Olefins that was observed when olefins were studied for polymerization reaction. It seemed that this cutting and stitching of olefin was very clean and conveniently accomplished under the presence of the catalyst and that was what intriguing at that point of time. Olefin metathesis as we have also noted has its origin in industry when people were studying olefin polymerization and we had also seen that the name olefin metathesis was coined by researcher at Goodyear Tyres and was first traced back to the tetrahedron later publication in 1967. Subsequently, we have observed that the race was on explaining or developing an understanding on the mechanism of this cutting and stitching reactions of olefin were happening, this cutting and stitching of olefins which were deemed as olefin metathesis reaction. Now these were studied intensely particularly with respect to obtaining mechanistic insight using mass spectrometry studies as well as NMR studies using isotope labeling experiments with deteriorated olefins as well as C14 labeled olefins. Now these were important developments which were simultaneously studied in the US as well as in Europe and there were parallel publication using the results of these labeling studies on deteriorated olefin as well as C13 labeled olefin both appearing in 1968. Now once this cutting and stitching mechanism of these olefin metathesis has been established, the focus shifted on obtaining the mechanistic insight of this reaction. Today what we are going to take up is on various mechanistic hypotheses which was at that time thought to be the main pathway for performing this olefin metathesis reaction and which we would be discussing in more detail in today's discussion. Now the first of the mechanistic insight was a hypothesis was proposed by the researcher of Goodyear tires in their 1968 JSES publication. What they had proposed is the reaction of olefin resulting in cyclobutane type of intermediate. I am illustrating this with the help of an example the olefin being bound to metal then forms a kind of a transition state which can exist kind of intermediate so basically giving rise to do different kind of olefin swapping. So, this is akin to similarly to what is shown over here. So, in short, it can be written something like this for two kinds of olefin in presence of a metal it goes through cyclic intermediate and then this cyclobutane intermediate which then loses metal to give the olefins with A and C and B and D counterparts. Now, the main drawback to this theory is the fact that there were no observations of any kind of cyclobutane intermediate. So, formation of cyclobutane in the process of metathesis was never observed, which sort of led to discarding the theory, but nonetheless this was very clever attempt in explaining this unusual phenomenon of cutting and stitching of olefins and the obvious proposed pathway was suggested that it went by the formation of cyclobutane intermediate and was first published by researchers from Goodyear in 1968 J.S.C. as paper. The next attempt was in explaining the mechanism was by Pettit in 1971 Professor Ronald Pettit and this was also reported in the Journal of American Chemical Society in 1971, where the proposed catalytic species was proposed to be a tetramethylene metal complex as is shown over here. So, this resulting in a very fancy and that being in equilibrium with the other metal di olefins complex. So, this is illustrated as follows for the two olefins in presence of metal giving this compound T, which would then lose the metal to give the desired cross-couple olefins, and this is called tetramethylene complex. Even though this looks like a very nice way of explaining the formation of two different kinds of olefins, however the main drawback to this is the proposed intermediate, which is the tetramethylene complex, and then that is what did not go through finally, and at some point was predicted as a plausible mechanism for the reaction. The next proposal was also a very nice effort towards explaining this formation of cross-coupled olefins using this curtain stage mechanism, and this time too it was proposed none other than by this future Nobel winning researcher, Professor Robert Grubbs. However, the mechanism that he had proposed for metathesis actually turned out to be wrong. Nonetheless, it is interesting to see another new approach as hypothesis as proposed by Professor Grubbs in explaining the mechanism of this olefin metathesis. So, Professor Grubbs mechanism was proposed in another J.S.C. paper in 1972 in the J.S.C. paper, and the mechanism what he proposed is instead of going through the earlier four-membered tetramethylene complex or via cyclobutadiene complex, actually the one that he proposed was that of metallocyclopentene complex, which sort of rearranges to give the product. Let me illustrate this over here. So, to olefin first binding to the metal, and then this formation of the metallocyclopropene resulting in this metallocyclopropene complex, and that undergoing some sort of rearrangement to finally giving the olefin adduct of the cross-coupled metathesis olefins to the metal. This I will explain in a cartoon as A and B type of olefin reacting with C and D type in presence of the metal to give C D A B that undergoing rearrangement to give B D C A, and then finally resulting in A C B D type of the metathesis product. So, the main thing was rearranging, hypothesis was rearranging metallocyclopentene intermediate. So, what is interesting over here is that even this attempt of explaining metathesis of using this rearranging metallocyclopentene intermediate turned out to be a wrong, and that the final or the other attempt by Professor Chauvin was one which finally won over all these different proposed insight. But what is by and large overwhelmingly clear is the fact that how intricate this mechanism establishing this mechanism of metathesis had been that so many brilliant minds had worked tirelessly to come up with the mechanism unfortunately many of them could not get it right. So, finally the one which really won over the ole is a mechanism by Professor Chauvin and which was published much later in 1971 in a relatively obscured journal of macromolecule chemie written in French. So, 1971 by Professor Chauvin from French Petroleum Institute and this was published in macromolecule chemie 1971 141 volume 161 page number and this is best shown by the mechanism and what it says that it goes through the formation of a metal carbene bond. So, this is suggested to be the main active species for the reaction so that turns out to be the correct one so that reacting with the olefin ended up giving R2. So, what is to be noted here is the fact that the now the metal carbene pieces has swapped from becoming CHR1 metal to becoming CHR2 metal. So, this was an interesting proposal whereby it is suggested that this metal carbene species is the active species responsible for performing this olefin metathesis and indeed this turn out to be extremely a true later on and this is what it got the field going. Now, what is all the more interesting is the fact that the complexity in the problem is highlighted very much in these failed as well as successful attempt the number of failed and successful attempt made by imminent minds at the time in trying to get an insight on to how the metathesis reactions are progressing and what is to be appreciated over here is the depth and the power of organometallic chemistry to accommodate so many different kind of hypothesis and proposals that may lead to the formation of these metathesis products and however another interesting thing that emerges out of the study is the fact that these the results of which were finally turned out to be incorrect were published in the premium journals whereas the correct one was published in not so very popular journal as the earlier ones and also in a different language than that of English. So, the science by enlarge this highlights the importance of science and a good quality of work nonetheless that if the work is good no matter where it gets published it will see the light of the day. Now, as per Professor Chauvin he said that three papers which are kind of work by fissure in terms of the fissure carbene complex CO5 tungsten CH3 and that was published in Angucan in 1964 by Professor Fissure and also the polymerization olefin polymerization ring opening polymerization of cyclopentene by Professor Nata again in Angucan international edition 1964 3723 and the industry work by Philippe Petrolium's bank and ballet work on on propylene so inspired him to come up with this proposal. So, what Professor Nata did is the we looked at the metal carbene complex first synthesized by fissure then the ring opening polymerization of cyclopentene by Julia Nata both appeared in Angucan 1964 journal and then Philippe Petrolium bailes research on propylene getting converted to ethylene and butene so he kind of looked at all of these discovery and came up of this idea of metal carbene species being the active species for this metathesis reaction and then had proposed these kind of coupling of olefin via a metal carbene species. So, this is an interesting proposal which one can appreciate now the way the perfect solution was arrived at by looking at various other complexes and various other chemistry being reported around at that time and then explaining this mechanism for olefin metathesis which had been a remarkable breakthrough. So, with these I would like to conclude today's lecture where we have discussed about various mechanistic approaches that have been taken in trying to explain the mechanism of olefin metathesis and this started with the formations are proposed intermediates of cyclobutane bound to metal by calderon and that did not find support as there were no evidence of formation of cyclobutane during this olefin metathesis reaction. The second reaction mechanistic hypothesis that we had looked at is by Petit in another JSES 1971 paper in which Arthitra methylene metal complex was proposed to be the main intermediate in olefin metathesis and that also did not find ground in the because of the fact that there were no evidence for such Arthitra methylene complex could be seen during metathesis reaction and the third another important paper by professor Bob Grubbs in JSES 1972 paper where he had suggested or to spoken about rearranging metallapenta cyclo pentane as the main active intermediate for olefin metathesis polymerization. However, even this turned out to be wrong and the beauty is even though professor Grubbs had lost out in proposing the correct mechanism for metathesis. However, his subsequent studies were significant enough to fetch him a noble price later for the same discovery and all of these efforts were going on in US where the race was on to explain the mechanism for metathesis and around at the same time by following in Europe in France particular by following the work of professor Fisher in synthesizing carbene or that of professor Natta for carrying out ring opening polymerization of cyclo pentane and as well as that of banks and bailes observation of propylene giving disproportionate to ethylene and butane. Professor Chauvin in French petroleum institute came up with a mechanism which involved the formation of metal carbene as the active species responsible for carrying out this olefin metathesis and this fortunately was published in a little bit obscure journal macro molecule chemistry in 1971 and also written in french but however it stood the test of time and went on to become the most acceptable mechanism for this olefin metathesis reaction. So, today with this we come to an end on the discussion of various approaches that were taken in trying to explain the olefin metathesis more on olefin metathesis particularly in trying to appreciate how the reaction propagates and what are its application so on and so for a lot more detail when we meet next for this NPTEL course. So, with that I conclude today's lecture thank you once again for being with me in this lecture and I look forward to discussing with you a bit more detail on olefin metathesis when we meet next on the next lecture. Since then goodbye and thank you.