 Welcome to this course on Transition Metal Organometallics in Catalysis and Biology. We have been discussing olefin metathesis in the context of catalyst development perspective, and in that regard what we had discussed is the fact that the strategies which were put in place in making the catalyst effective one, with that effectivity I mean that the catalyst should be highly active in terms of being able to convert substrate to product, and towards this goal several designing changes have been adopted in the course of making catalyst for olefin metathesis. We had seen the strategies which have been employed by Grubbs, Schrock, Lappert and others for designing Gladys for designing catalyst for olefin metathesis reaction. Now in this discussion I would like to draw the attention towards the discussion about co-ordinative and electronic saturation and unsaturation aspects that plays a crucial role in the design of catalyst co-ordinative and electronic saturation and saturation of catalyst. Now by these one refers to the sterics as well as electronics sterics and electronics. So, one plays with the sterics and electronics of the catalyst complex by changing the coordination and making the metal center more crowded when it is saturated as whereas when if something is co-ordinatively unsaturated that means that there are vacant sites accessible to the metal center. Same is the case of electronic saturation or unsaturation. Now this follows from the fact that there are ligands which either donate electron to the metal center or ligands which pull away or suck electron density from the metal center making the metal more electron rich or more electron deficient as is the case, and thereby modulating the reactivity of the catalyst with refer to particular substrates. So, here is the discussion about the modulation of sterics as well as electronics in designing the catalyst aspect. Now in this discussion in our previous class we have spoken about how these steric and electronic factors were put in place for designing Grubbs catalyst and what we had observed is that for the ruthenium carbene Grubbs catalyst the Grubbs catalyst the measures were taken to make this metal center more favorable for the metathesis reaction and in this case what we had observed that the people have played with the sterics in terms of putting bulky triphenyl phosphine substituent and then also played with electronics in terms of making more sigma donating ligand and so that the metal center is more electron rich and hence can help perform metathesis reaction in a better way. So, we had also seen that these catalyst the Grubbs catalyst you know carries out metathesis but the initiation takes place by two important step one is phosphine dissociation and the second one is olefin binding. So, what to be discussed over here is the point that the sterics have been employed the steric factors have been used for taking care of phosphine dissociation whereas this olefin binding is successfully taken care of by modulating the electronics of the metal center. So, I am going to illustrate this in another set of example particularly with Grubbs second generation catalyst in which the cyclohexylphosphine is replaced with NHC or N-heterocyclic carbines and these are ligands of the type shown over here and they are very good sigma donors. So, when the this cyclohexylphosphine one of the phosphine is replaced by NH-heterocyclic carbine or NHC then the Grubbs second generation catalyst which is supposedly more active than the first generation catalyst in metathesis reaction can be synthesized and and that have been successfully employed by controlling enhancement in the steric and electronics at this ruthenium metal center. Let me illustrate this with this example. So, this is the Grubbs second generation catalyst which has a cyclohexylphosphine bound to ruthenium as well as a carbine with bulky mesoidal substituents bound to the metal center. So, these the sterics allows the first step which is phosphine dissociation to give this complex which is to give this complex which is co-ordinatively unsaturated because this is co-ordinatively unsaturated because there is a vacant site which is generated. So, this is the active species which is co-ordinatively. Now, to this the olefin comes and binds to give the to this this olefin comes and binds olefin comes and binds in the vacant site and this is called olefin coordination step olefin binding step. Now, this is facilitated because of the first step which is dissociation of the phosphine and that is promoted by the steric factor promotes this phosphine dissociation which helps in term allows the olefin binding. Now, once the olefin binds then that results in the complex. Now, this is a olefin bound to ruthenium complex. Now, what we had observed that both sterics and the electronics are important in modulating the metathesis reaction and this is the instance where the electronics comes into play as NHCs are good sigma donors. So, they make the metal center more electron rich and as a result olefin binding binding is facilitated by metal to ligand pi back donation. So, this is a important contribution what we see that electronics kicks in or helps the second step where the good sigma donating ability of the NHC ligand makes the metal ruthenium metal very electron rich and as a result the metal to olefin pi back donation is favored and which results in the good binding of this olefin otherwise olefins are not very good binding ligands to the metal. Now, once that successful binding of olefin happens the next is the metathesis reaction which gives this four-membered ruthenium metallocyclobutane intermediate and that eventually gets converted to product with the formation of a different kind of carbene species and a different kind of olefin bound to the metal. So, what is to be seen over here is that a different olefin bound species is formed where instead of ruthenium benzyl species carbene species over here there is a new kind of ruthenium carbene species and instead of the olefin over here a new kind of olefin metathesis product olefin is formed. So, here what is observed that proper choice of sterics and electronics help facilitate the first steps first is the phosphine dissociation and the next is the olefin binding resulting in favoring the metathesis reaction. So, this is a very important demonstration of the catalyst development strategies which often involve modulation of the sterics and electronics and what has been observed that this strategy has been put in place in a very efficient manner while designing Grubbs second generation catalyst in which one of the phosphine has been replaced by a bulky anhydrocyclic carbene ligand which because of its bulky mesetylene substituents help facilitate phosphine dissociation giving rise to coordinatively unsaturated species and now that also allows olefin binding and then simultaneously with that the presence of very good sigma donating ligand further reinforces the olefin binding to ruthenium because of metal to olefin pi back donation and then the stabilization of the ruthenium olefinic complex because of the sterics and electronics leads to the formation of the ruthenium metallocyclobutane intermediate resulting information of another ruthenium carbene species with the metathesis catalyst. So, overall Grubbs second generation is better than the phosphine complex and presence of bulky and electron donating group the presence of bulky and electron team group carbene carbon increases the catalyst activity. So, this was a very nice demonstration of the modulation of sterics and electronics in enhancing the activity of the metathesis catalyst. Now, in this context it is important to mention a beautiful work done by a Herman in which he had indeed shown superior activity of the second generation catalyst over the first generation catalyst. Herman's work demonstration that the second generation of catalyst is better than the first generation and this has beautiful work is published in Anguicam 98372490. Now, what Herman did, Herman actually compared the activity of Grubbs first generation second generation catalyst in ring opening polymerization of cyclo octane compared activity of Grubbs first and second generation catalyst in ring opening metathesis polymerization of cyclo octane. This is best illustrated by the equation and this was carried out at 25 degree centigrade in cyclo octane to catalyst ratio of 250 is to 1 and the struggling result of superior activity of the second generation catalyst could be seen very prominently in Herman's experiment and which sort of once for all establish the superiority of the second generation catalyst over the first generation once. So, what Herman observed is the superiority of the second generation over the first and when he and he observed that by plotting yield with time that of the conversion and for the catalyst which is a first generation for the catalyst shown over here Grubbs first generation he observed sort of a steady growth in the product whereas when he took the second generation catalyst which was in isopropyl he observed a drastic increase in the reaction yield and then plotting. So, this is generation 2. So, the result distinctly showed that the second generation catalyst is better than the first generation catalyst. Now, these led to the development of the Grubbs second generation catalyst in much more detail and then the next ongoing thing over here was in development of chiral catalyst and in this regard significant development in chiral metathesis reaction was also observed with the following complexes metathesis catalyst synthesized of molybdenum this is the answer which kind of complex another is the derivative of similar type with t butyl t butyl the third one is another type with the dichloro imido binol kind of setup and the last but not the least is the carbene enythrocyclic carbene with one side isopropyl group. So, this shows this sort of shows that the extent of creativity in terms of people coming up with so many different modifications of chiral metathesis catalyst that not only were synthesized but also their applicability in the asymmetric modifications reaction performed and some of representative examples of such range of complexes are also shown over here. So, with and these the molybdenum ones are imido carbene complexes. So, there is a metal ligand multiple bonding two types one is metal carbene and metal nitrogen these are all imido carbene complexes and the last fourth one is the ruthenium complex where it is bound to NHCs as well as cyclohexyl phosphine. So, with these I come to conclusion to today of today's lecture where we have looked into the catalyst development aspect particularly from the use of sterics and electronics in facilitating the binding of olefin and subsequently favoring the metathesis reaction. And we have also in that process looked into Harman's experiment which sort of distinguished grads generation to catalyst from generation one and successfully showed that these second complexes better than the first one. So, with that we come to an end of today's lecture and we are going to be talking about this catalyst development particularly from the application point of view in more details when we meet next in this series of course lectures. So, with that once again I would like to thank you for being with me and I look forward to being with you in your next lecture where we take up this topic in more details. Thank you.