 Welcome to this course on Transition Metal Organometallics in Catalysis and Biology. We have been discussing metathesis reaction in details over the last few lectures. As we had observed in a previous lecture that olefin metathesis is a very important polymerization reaction, which involves cutting and stitching of olefins. Now this reaction has wide applicability, which we are going to cover in subsequent lectures, but what is more intriguing about this reaction is its complexity, its difficulty in comprehending the mechanistic pathway, how the reaction was proceeding or how the polymerization was proceeding in its earlier stages of development. olefin metathesis actually has its origin from olefin polymerization, this also we had discussed at length in earlier lecture. And what we had also observed that there were about four different mechanisms which have been proposed for explaining olefin metathesis and three of them proved to be not true, however that does not take away any sheen from the mechanisms which have been proposed, because those are the ones that gives the glimpses of various pathways that are feasible for explaining such complex mechanism. So let me just recapitulate what we had discussed in the earlier class. In particular, we had spoken about a mechanism by Calderon that was published in JSC in 1968 that speaks about the presence of cyclobutane intermediate, however the reaction or this pathway was deemed not true, because there was no evidence of formation of any kind of cyclobutane during the olefin metathesis reaction as has been suggested in the mechanism. So what it says is that two different olefins A and B, for example, reacts in presence of M to give this metallocyclobutane coordinated to M, and then the metathesis reaction happens, finally the M is released to give the cross metathesis product. We had also observed in our previous class that this mechanism was not found to be true. The next mechanism was proposed by Roland Petit in JSC in 1971, which spoke about the presence of tetramethylene intermediate bound to the metal, which is the four methylene groups at these four corners formation of a tetramethylene metal bound intermediate resulting in the desired metathesis product, however this later on in the stage of the game, this evidence also this mechanism also lost support, because of lack of evidence and hence proven to be not true. Then there was another evidence, which was proposed by Grubbs that was this metallocyclobutane intermediate and which was also proven to be not correct, and we had discussed this in detail in our earlier class, where we have two olefins A and B reacting with the other one C and D in presence of metal gives a metallocyclobutane intermediate and which sort of undergoes spontaneous rearrangement and that would eventually result in formation of intermediate and these are called the rearranging metallocyclo propane intermediate and this was proposed by Bob Grubbs himself in 1972, 538 followed by an inner cam paper in 1973, 166. However, this particular rearrangement also did not find support and was proven to be wrong subsequently and the correct mechanism as we had observed was proposed by Professor Chauvin, correct mechanism has been proposed by Professor Chauvin in macromolecal chemistry 1971, 141, 161. Now, this was a completely different mechanism and he has proposed this mechanism on the basis of three evidences or three observation as we had also mentioned earlier. First is the formation of fissure carbene complex that was published that time in Angiocam and then by Professor Fischer and the second paper that he sort of used or at least taken notice of before proposing this mechanism was this ring opening polymerization of cyclopentane and third was this observation from Philip Petroleum about disproportion of propylene. So, all of these led him to propose that the active species indeed a metal carbene species of the type this which sort of helps in propagating these olefin metathesis reaction. Now, this at that time was a radical belief because at that instance of time there were two popular beliefs in the sense that these olefin cross metathesis reactions are olefin metathesis reaction proceeds by two pathways or two proposed pathways. One was ion pair pathways and the other one was pair wise pathway and non pair wise pathway. So, let me elaborate that in bit more detail the most commonly proposed mechanism was first of pair wise pathway. Now, pair wise pathway it seems very simple from the name in the sense that two olefins they form a pair and undergo metathesis polymerization as brought about by a metal. So, for pair wise pathway an example of it is shown over here. For example, if there are two types of olefin they react to each other in presence of a metal to make a pair and that sort of proceeds by formation of some kind of cyclobutane templated with the aid of the metal resulting in the formation of the olefinic product. So, this conventionally is called the pair wise pathway where the two olefins they come and form a pair. So, these two olefins and they make a pair as shown over here and then cyclobutane intermediate with the aid of the metal to give the corresponding product. This has been very nicely covered by a review by Professor Bob Grubbs in Tetrahedron 2004-67172-7140. I request our interested student of this course to look up this review which will explain the contemporary arguments that were put forth at that time in elaborating the mechanism. Now, in this context it is also worth mentioning that at that point of time even though the pair wise pathway later on proved on to be wrong, but much of the mechanisms were on the basis of pair wise pathway. The three mechanisms which I had discussed earlier as proposed by Calderon, Petite and Grubbs were based on the different versions of a pair wise mechanism. Whereas, Chauvin proposed the correct mechanism and this turned out to be a non-pair wise mechanism. This non-pair wise pathway was an outcome of a metal carbene complex species being the active species of this reaction. This shows how radical had been Chauvin's proposal of a non-pair wise pathway. This mechanism was later known as the carbene mechanism in which this is illustrated as shown below. Chauvin's non-pair wise mechanism are also popularly called carbene mechanism. This is illustrated by the equation CHR and the corresponding product is MCHR2 plus CH2 CHR1 and the reaction also could go the other way CHR1 plus CH2 CHR2 giving MCH2 plus CHR1 CHR2. Basically, this carbene non-pair wise mechanism, what it gave is it gave a range of different kinds of olefinic products obtained by the combination of these two olefins, which is this and this. Basically, these two kinds of olefins led to two different active species, which is over here and they participated in it and gave a wide range of products. Now, these sort of led to a race in trying to understand how mechanism really is proceeding and to understand this a lot of substituted isotopic metathesis reactions were performed based on which support in favor of this non-ion pair mechanism. Now, along Chauvin's pathway, Schrock has also made the Schrock carbene and Kc has demonstrated a similar pathway similar to that of Chauvin and that mechanism is also highlighted over here. This is nothing but Schrock carbene giving MCHR8 plus CHR8. Now, this MCRH reacting with an olefin R1, R1 H H would give this metallocyclobutane R1, R1 and that would give another carbene species C R R1 plus olefin RCH double bond CHR1. This can react with another olefin R2 CH double bond CHR2 to give another species, which can be C R2 H and so on and so forth. This was proposed by Kc and is nicely elaborated in a JCS paper by Professor Bob Grubbs 1975-1997, 3, 2, 6, 5, 3, 2, 6, 7. What is observed over here is that the reaction seems to generate several kind of carbene species in a for example RH, then that participates to give C R R R1, then that gives C R2 H and so on and so forth. Basically, what is interesting about it is that there is not a single active species, there is a single type metal carbene, but during the process of metathesis several metal carbene species are simultaneously formed that participates in propagating in this reaction. What is very interesting over here is that one type of metal carbene species is coming more effective in carrying out these catalysis. This was explained by isotope experiment carried out by Professor Bob Grubbs in that JCS paper 1975-1997, 3, 2, 6, 5, 2, 3, 2, 6, 7. In that paper, what they did was they synthesized these special olefin with deuteropel CD2 and then the corresponding non-deuterated counterpart CH2 and then had a metathesis catalyst. What they were looking at is the formation of this wood result in cyclohexane and plus formation of three kinds of ethylenes, for example, CD2, CD2, which they call D4 species or CD2 CH2, which they said D2 species and then CH2 CH2, which said D0 species and then they looked at the distribution of these three species D4, D2 and D0 for non-pairwise mechanism as well as for pairwise mechanism. Now, the values for pairwise and these values of the distribution were estimated values for D4, D2 and D0 olefins ethylenes were calculated and what they came up with is that for pairwise mechanism, if they were just simple pair, these values should be 1 to 1.6 to 1 and for non-pairwise mechanism, the value should be 1 to 2 to 1. Now, these values were calculated using various equations fed into the computer and their estimate was for a pairwise mechanism that this mixed D2 should be in the ratio of 1 is to 1.6 into 2, but what really was observed is that the D2 ethylene was about 2 and they closely matched the pairwise mechanism and these provided support for Chauvin's non-pairwise mechanism for olefin metathesis. Now, another interesting thing notable over here is the fact that even though Grubbs whose own proposal was for a rearranging cyclopentane type of intermediate, his own proposal did not turn out to be true, but he did this very good famous isotope experiment by looking at the distribution of which he could establish Chauvin's mechanism of non-pairwise olefin metathesis. With this, I come to an end of today's lecture, where we looked into how the Chauvin's mechanism of non-pairwise pathway for olefin metathesis got established because of isotope label studies performed by Grubbs. Another important hallmark of Chauvin's mechanism we had observed is that the active species though are of one type, metal carbene type, but during the course of metathesis reaction several active species or several metal carbene species are simultaneously formed which individually propagate this metathesis reaction. That is an interesting observation which has come out of Chauvin's non-pairwise proposed mechanism. Another observation we have noted is that the Chauvin's mechanism was later supported by Kc for Schrock type complexes and part of it which we have also discussed today. So, with this we come to end of today's lecture, we are going to be looking a bit more details of Chauvin's mechanism as we take up in the next class, particularly with regard to the various kinds of active species or metal carbene species that are formed during the course of this reaction. So, with this we come to an end of today's lecture. Thank you for being with me in this lecture. Are we going to be taking up this olefin metathesis mechanism in particular in a bit more detail and looking into various aspects of Chauvin's non-pairwise mechanism in the next class? I look forward to being with you that time. Till then, goodbye. Thank you.