 Welcome to this course on Transition Metal Organometallics in Catalysis and Biology. We have been discussing about various oligomerization of olefins and alkyne reactions, and in this context we have looked at the first and the foremost of this type of reaction, which is shell higher olefin process in great detail and look at shop from the context of olefin oligomerization giving alpha alkenes. Now this shop process was a very important process, which was practiced in industrial scale for producing detergent, and this primarily had its origin for making alpha olefins for applications in surfactant and detergent, and this initially were obtained by cracking of crude oil, however later as Ziegler-Natter catalysis evolved, nickel catalyst was found which could produce alpha olefins from two C3 oligomers, all the way to C20 and above, and the only drawback about this olefin oligomerization under the shop process was that this Ziegler-Natter type coordination insertion olefin oligomerization was non-selective in the sense that it could produce a bunch of oligomers with various distribution ranging from smaller oligomers of dimer-trimer to large oligomers having more than 18 or 20 monomer unit. Now in order to make use of this large distribution of oligomers to other process, for example olefin isomerization followed by olefin metathesis, they were put in place to make use of the low molecular weight oligomers as well as high molecular weight oligomers, which individually had not much use as detergent. This is a combined process that involves alpha olefin oligomerization, isomerization and metathesis put together to make crude material for producing detergents. We have covered this in great detail in our previous lecture, and also we have looked into the alpha olefin oligomerization catalytic cycle, and the thing that comes out is that the active species is usually a nickel hydride species, the active species also what has been observed is change of solvents gives higher molecular weight linear oligomers, and this was done in the solvent is for example, Xn, lastly for the shop process what was found that addition triphenyl phosphine gives chain termination, for example, 1 butene is obtained. Even though this was a oligomerization of olefins was a very useful process, which helped develop this shop process altogether, however criticism of this process lies in its lack of selectivity, because a large number of oligomerates of various chain lengths are obtained at different ratios during the process of alpha oligomerization using the nickel catalyst, so selectivity was itself an issue. In this context, another important example is about ethylene trimarization of ethylene. The process indicates conversion of ethylene to 1 hexane. The process is important with respect to alpha olefin production or oligomerization in shop is with regard to the selectivity of this reaction. This reaction is highly selective in terms of producing 1 hexane. This discovery has an Indian impact on it and is made by the group of Professor Ayushman Sen and has been reported by Professor Ayushman Sen of Penicillvania State University and it was reported in the Journal of American Chemical Society in 2001, very recently in that in the year 123, so one beauty about this reaction is the selectivity, highly selective as compared to the alpha olefin produced by olefin oligomerization using shell-arolefin process. Note that for this reaction, an early transition metal tantalum catalyst has been used as opposed to nickel catalyst, which are often used for producing alpha olefins from ethylene oligomerization reaction. To be more precise, tantanium pentachloride is the starting precursor in presence of some metal alkyls and these metal alkyls can be several alkylating regions like trimethyl aluminum, tetramethyl-10, dimethyl zinc and endutyl lithium, so the scope of this reaction in terms of generating the active species is very broad, where different types of alkylating is a reagent like trimethyl aluminum, tetramethyl zinc, dimethyl zinc, butyl lithium are used to alkylate this tantalum 5 compound tantanium pentachloride to get the active species, which is actually a tantalium 3 species, so let me this is a very interesting discovery, which has the mark of an Indian inventor Professor Ayushman Sen currently on role in Pennsylvania State University, who had made this fine discovery. So, let me just walk you through the catalytic cycle for this trimerization of ethylene reaction. The first is the formation of the active species, which is the active species is suggested to be a tantalium 3 species, and this active species is produced from the tantalium 5 compound precursor as is shown below. In presence of am alkylating agent gives Cl3 tantalum dimethyl, so its partial alkylation that occurs of the tantalium 3 species, so pentap5 becoming 3 chloride and 2, so this is partial alkylation, which occurs to get this tantalium 5 species that reacts with ethylene to give tantalium methyl ethylene species, where the coordination insertion of where the coordination of the ethylene occurs on one of the 5 tantalium methyl bond to produce this tantalium propyl moiety. Now this tantalium propyl moiety because of electron deficiency of tantalium has beta hydrogen, which beta eliminates to produce this tantalium hydride methyl species with coordinated propylene. Then under this ethylene bound tantalium hydride species undergoes reductive elimination as is shown over here to eliminate methane as well as propene propene to generate the active species with a which is a tantalium trichloride, so this is the active species active catalyst for ethylene trimarization. One thing to note about this catalyst is that this species is both sterically unsaturated as well as electronically unsaturated, so this is a 8 electron species and this is a sort of 3 coordinate, 3 ligand. This species is highly active in terms of having very less of steric saturation, only 3 chloride ligands bound to titanium and also electronically very unsaturated because it is a 8 valence electron 3 from chloride and 5 from tantalium, so it is a 8 valence electron species, so it is also very electronically unsaturated and this is supposed to be the active species for this catalytic ethylene trimarization reaction. One beauty about Ausman's sense work is that he has successfully demonstrated the use of an early transition metal like tantalium for producing olefin oligomers like one hexene. Usually the common wisdom had been that early transition metal produces high molecular weight polymers whereas more electron rich lead transition metals produces oligomers, but here we have a nice example from sense group where he had successfully used an early transition metal for producing trimers of ethylene in form of one hexene. The second important feature about sense work is the selectivity that in olefin oligomerization using neocl catalyst in shop process, a wide range of alpha olefins bearing different chain lengths are produced, whereas Senn has showed that highly selective trimarization can proceed with this catalyst that only gives one hexene as the final product, so these two are the key features that separate sense work from the existing catalyst or which are of show superiority of sense work from the existing catalyst, which had been known for olefin oligomerization. Now, we are going to take a look at the mechanism as to how these active species in telium tetrachloride carry out this trimarization of olefins ethylene. Tentanum trachloride, which is highly sterically unsaturated as well as electronically unsaturated, reacts with two molecules of ethylene to form this pentacyclic metacyclic metacyl, so this is quite stable and it also has been shown by DFT calculation by others that this is a stable product, which will eventually form from two olefins. Now, the species, which is formed is Cl3-Tentanum metacyl, where the oxygen state of Tentanum has again increased from Tentanum 3 to Tentanum 5, and this is a sort of an oxidative reaction, which occurs by addition of two olefins and can be seen as such, Tentanum 3 and two species with Tentanum donating electron onto the ligand to form this Tentanum metacyl, which is shown over here. Subsequently, the reaction of another olefin happens by coordination insertion pathway to give to this Tentanum 7 membered ring, and this happens by simple coordination followed by insertion of this into the bond to give that 7 membered pathway. And finally, the reductive elimination to generate back this Tentanum 5 membered compound via beta hydrogen elimination is going to this, and that undergoes reductive elimination giving one hexane as the side product. This also involves reductive elimination. What is happening over here? One can see that this catalytic cycle initiates with oxidative variation, and then Tentanum becomes 4, and then goes to reductive elimination, where again the Tentanum oxygen state changes from Tentanum 5 to Tentanum 3. So this is a very nice piece of work by Professor Sen, where one could see that a Tentanum 3 species is being generated in the course of the reaction, and that is highly electron deficient as well as highly unsaturated compound, which then successfully carries out this trimarization of ethylene to give one hexane, first by oxidative addition of two ethylene molecules to give a Tentanum 5 species, which also then further undergoes one more insertion to give a 7 membered metallocycle of Tentanum, and that finally beta hydrogen eliminates as well as reductively eliminates to give one hexane as the product, and regenerating back the titanium trichloride. So with these, I come to the conclusion of today's lecture, in which we have looked into first the various important attributes of olefin oligomerization, particularly from the perspective of its active species, which is the nickel hydride species, and the effect of solvents, for example, on going to solvents like hexane, higher molecular weight polymers obtained, or the effect of addition of phosphenes, which allows chain termination and facilitates lower oligomer formation. The second aspect that we have discussed is about interesting reaction that involves highly selective trimarization of ethylene to one hexane, and this has been reported by Prof. Ayushman Sen of the Pennsylvania State University in the JSCS article, 2002 article, which we have already referred to and request the reader to read it with more detail, and in this, Prof. Sen has very nicely demonstrated that even against the conventional wisdom that even early transition metal can be successfully used for olefin trimarization in a very selective fashion. In particular, he has started with the Tantalum pentachloride using some alkylating agent. He had generated a highly unsaturated and highly electron deficient Tantalum 3 species, which selectively produces one hexane in the mechanism as it is shown below. So, very interesting work and it also highlights the scope of organometallic chemistry, particularly in terms of carrying out very different kinds of transformation, some of which may even go beyond the wisdom of the conventional logic and nice work by Sen in this regard was discussed. So, with this, I come to the conclusion of today's lecture on ethylene oligomerization and ethylene trimarization that we have discussed. We are going to talk more on few more other examples of various kinds of olefin oligomerization reactions when we meet next in this lecture series. Till then, thank you and goodbye.