 to this course on transition metal organometallics in catalysis and biology. We have been talking about various aspects of olefin metathesis over a series of lecture and in the past lecture we have focused on a new aspect which is the aspect of catalyst development. Now in this what we have observed is that this is a key area of which sort of led to the explosion of the field olefin metathesis. This is an area which showcased the capability of olefin metathesis as a reaction in terms of transformations that it can undertake in terms of the amounts to the product that it can churn out. So, basically the depth and the breadth of this olefin metathesis reaction was built on the back of catalyst development aspect. So, in this regard we have spoken about the various types of metal carbene complexes which were synthesized. One thing for sure to note is that carbene is a kind of very react sensitive moiety to be stabilized on metal and they are extremely reactive not only to various substrates but also to the presence of air and moisture in the reaction vessel. So, to make a catalyst out of these extremely active species is of a challenge just for stabilizing them is itself a challenge and in this process of catalyst development among the various metals lead transition metal early transition metal that have been used what we had observed is Graves contribution in making ruthenium emerging out as victorious for developing olefin metathesis catalyst ruthenium as and this is because is easy to handle also easy to prepare and functional group tolerant. So, this is what makes a ruthenium carbene complexes as extremely a good catalyst for among other metal carbene catalyst for olefin metathesis reaction. Now, in this in this race for developing a good ruthenium carbene complexes for olefin metathesis several ruthenium carbene complexes were synthesized and in this regard prominent are those which have been which are made by Graves and actually several versions of improved versions of graph catalyst were subsequently turned out by Graves himself. So, these are known as Graves catalyst then second generation Graves catalyst third generation graph catalyst so on and so forth. So, we are going to take a look at some of the existing catalysts which have been developed for olefin metathesis centered around ruthenium. So, this is what is prominently called as Graves catalyst. The next comes Graves second generation catalyst which is with the enetocyclic carbene with mesatyl substituents. So, this is second generation Graves catalyst actually there are two versions. So, this one is a saturated version and also there is a unsaturated version of the same type is just the backbone unsaturated version and then there are several other. So, what is to be noted over here is that this cyclohexyl one of the cyclohexyl phosphine has been replaced with enetocyclic carbene. This is a imidazole based where it is there is a unsaturation unsaturated backbone and then this is a five membered where this is a completely CH2 CH2. So, these when cyclohexyl phosphine are replaced with enetocyclic carbene they become even better in terms of olefin metathesis activity and hence these two are called second generation Graves catalyst where one of the phosphine has been replaced by NHCs. Now, there are other variants of carbene as well one such prominent variant is Schrock's catalyst which we will see and this is a molybdenum based 2,6 isopropyl substituents phenyl Cf3 Cf3 Cf3 CH3. So, this is Schrock's molybdenum catalyst and then comes a tethered alkoxy compound of ruthenium which is this another variant of this tethered alkoxy complex with carbene is also reported and so this slide sort of provide a glimpse of the glimpse of the kind of imagination that goes into developing these so many different variants of the catalyst based centered around ruthenium and molybdenum. So, not only the richness of the slide in terms of the catalyst development where so many different catalysts of similar types have been synthesized and the other thing which also talks about is the ability to stabilize so many different varieties of carbene complexes this also highlights the synthetic capability of organometallic chemistry as such in which the successful synthetic route to so many variants could be established. Now, the next what we are going to be talking about is in trying to understand the developmental aspect of catalyst with regard to the mechanism of this reaction. So, what is important to note over here is all of these catalysts actually are pre-catalysts because the real catalyst is slightly different from these pre-catalysts in the sense that it undergoes phosphine dissociation to give a vacant coordination site and that sort of then reacts with olefin to give the product. So, this is illustrated in more detail in the subsequent slide. For example, so the first step what happens is this phosphine dissociation resulting in these active species of ruthenium having a vacant site and the next step thus involves reacting these species with ruthenium to give these alkene bound ruthenium species as is shown over here. Now, what is to note is that once this species is formed then the reaction sort of can move towards metathesis in the sense that it can provide the required metallocyclobutane intermediate and that can then finally give the corresponding product. So, if one were to follow this metathesis using the ruthene carbene in four simple step of reaction what is important in catalyst development is the first two reaction which is step one involves phosphine dissociation and then step two involves olefin binding and that is determined represented by step two usually a olefin binding is slower and also that is why what sort of is needed to enhance the metathesis reaction. So, now the effort is more in trying to convert this olefin make this olefin binding from slower to faster and this can be achieved by following strategies. The reason the olefin binding is slower because the ruthenium binds to olefin in two way fashion first is the metal to olefin sigma donation and then is metal pi back donation. So, this is important in understanding this metal olefin binding this forward sigma donation as well as pi back donation and this is dependent on electron richness of the metal. So, what is important is that to design catalyst which will not only make phosphine dissociation faster pcy3 dissociation faster as a result that will promote better or improve olefin binding which is so this is step one the step one becomes faster than step two improves now if the step two improves then subsequently the metathesis also would improve now for binding of olefin the two factors are required which is metal to ligand to metal sigma donation and metal to ligand pi back donation and for which the metal has to be electron rich. So, in the strategy for developing the catalyst what is required is to make step one faster that is phosphine dissociation faster and how is that done we are going to take a look that is done by putting more sterically demanding ligands on the ruthenium so that it becomes crowded and the phosphine dissociation becomes faster secondly to if the phosphine dissociation becomes faster then there the phosphine dissociates and there is a vacant site there is a vacant site where the olefin would come and bind so that would indirectly help binding of olefin now binding of olefin is also facilitated further by making the metal center more electron rich electron rich in terms of putting ligands which are more better sigma donor so there are like both sterics and as well as electronics both are modified such that these two reactions reaction one for of phosphine dissociation and reaction two of olefin binding are optimized so as to get better olefin polymerization catalyst and this is achieved by the following strategy for this ruthenium complex as is discussed here and we are going to now talk about how to improve this increase the catalyst activity and the first is in this regard the first strategy is to increase phosphine dissociation and this can be brought about by replacement of pph3 by pcy3 bulky ligand so by putting more bulky ligand it facilitates phosphine dissociation as well as by replacement of chlorine by iodine on the catalyst so the idea is more bulky phosphine ligands with larger core triangles will more bulky phosphine ligands with larger cone angle would be a better for the dissociation of the phosphines similarly what is to be noted over here is the fact that that only one of the phosphine gets dissociated and the other gets dissociated whereas the other stays on metal now this dissociation is facilitated by sterics and the electronics are modulated by this phosphine which stays relation by this second phosphine that stays onto the metal and for these what is required is more electron rich phosphines donating phosphines this is actually applicable for the phosphine one that particular one which stays on the metal for better binding of olefins so what one sees that the first the first strategy of going for bulkier phosphines that improves improves step one which is phosphine dissociation and the second strategy of more electron rich phosphines this one improves two that is olefin binding so and this is electronics so what we see this is electronics modulation so steric modulation is more appropriate for step one that which was phosphine dissociation and electronics modification is more appropriate for the step two which is electronic olefin binding so in this class what we had done is we have looked into the various kinds of phosphine ruthenium catalyst which have been synthesized for olefin metathesis we have looked at how various improvements have been done in terms of replacing one of the phosphines with more electron donating and bulkier nitrocyclic carbene variants ligands we have also seen how tethering of an alkoxy a group in the ruthenium carbene carbene complexes have been undertaken we have also seen molybdenum catalyst has developed by Schrock then in the course of further discussion in this lecture what we have also done is we have looked at the elementary steps in which the catalyst ruthenium carbene complexes undertake olefin metathesis and then looked upon the strategies which are put in place in order to improve the activity of the ruthenium catalyst what we have observed is that the most many all of the ruthenium complexes which are used for metathesis are actually pre catalyst that means that they go on to form a different species by a more unsaturated electronically as well as a coordinated unsaturated species by dissociation of phosphine in the first step to form the active species to which the olefin then binds to give the olefin coordinated adduct and then that subsequently undergoes metathesis reaction. So, what the general affinity shows that olefin binding is very slow which happens in the step 2. So, what we had seen that proper modulation of the interplay of the sterics as well as electronics can help facilitate the metathesis reaction by improving or increasing phosphine dissociation this has been achieved by putting more bulkier ligands around the metal center for example on moving from triphenylphosphine to tricycloacylphosphine or on going from chlorine to iodine the more bulkier the ligands around ruthenium is the first step is facilitated in terms of phosphine dissociation is affected and second thing is about increasing the binding of the olefin which by inherent nature is a slow binder to the ruthenium and this has been achieved by by improving the through electronics by improving the electron density of the metal center and this has been done by putting more electron rich phosphine or in heterocyclic carbines on the metal. Now, it is to be noted that Grubbs first generation catalyst of the Grubbs original catalyst has two cyclohexylphosphine of which one dissociates to give the active species whereas the second phosphine stays on to the metal and that phosphine then modulates the electron density around metal for it to effectively bind to olefin so that the metathesis reaction becomes faster. So, what we see is successful implementation of sterics implementation of strategies based on sterics and electronics in enhancing or jacking up the activity of the olefin metathesis catalyst. So, with this we come to conclusion of today's lecture where we have looked at different types of ruthenium metathesis catalyst that have been synthesized all the variants of it and also we have looked at the different strategies that have been put in place in order to improve on the activity of the ruthenium catalyst. What we had noted is that there are two key steps which are in the beginning of this metathesis reaction. First is the formation of the catalyst through phosphine dissociation, this has been improved upon by putting more bulkier ligand around the metal and the second step is of course the olefin binding and this has been improved upon by putting using electronics as a tool by putting more electron rich or sigma donating ligands on the metal, so that olefin binding is facilitated and as a result the metathesis goes up. So, with this we come to conclusion of today's lecture, we are going to look at the strategy of catalyst development in much more detail as we take up this topic of olefin metathesis in next class. Till then, goodbye and thank you.