 Welcome to this lecture on Transition Metal Organometallics in Catalysis and Biology. We have been talking about olefin metathesis, particularly catalysis development aspects in the last few lectures and we have focused on alkene metathesis to begin with and what we had seen in the last few lectures that both sterics and electronics they play a powerful role in modulating the catalyst activity particularly in making catalyst more efficient in their catalytic processes. So, in that regard we had seen how putting more bulky ligands on the metal help facilitated better activity for grubs catalyst by allowing phosphine dissociation and further incorporation of very good sigma donating ligands on the metal facilitated olefin binding which both together help facilitated this metathesis reaction. So, this quite a bit of which organometallic chemistry particularly with regard to the metal ligand binding as well as metal ligand pi bactonation all of those interaction played a crucial role in the evolution of olefin metathesis catalyst which went on to become better with the discovery of second generation catalyst as opposed to the grubs first generation catalyst. We had also discussed about Harman's work on the comparison of grubs first generation and second generation catalyst in the ring opening metathesis polymerization of cycloctene and that definitely proved that indeed the second generation catalyst bearing NH2C carbene were more superior in terms of fastness to produce the product than the corresponding grubs first generation catalyst. So, with this behind us today we are going to sort of look at the key points of this catalyst as we proceed in today's lecture and these are particularly with respect to alkene metathesis that we are talking about. So, this metal carbene moiety acts as the active center in catalyst. Second thing that of the various metals which could stabilize a metal carbene moiety the metals of choice for metathesis reactions where molybdenum, tungsten, rhenium and ruthenium and ruthenium are the metal of choice for metathesis reaction. The earlier times the catalyst were derived from transient metal halide as is shown over here and carbon ion donors, tungsten hexachloride plus dimethyl aluminium chloride would give tetrachlorotimethyl that in presence of ethanol would eliminate water to give tungsten oxo dichloride with this intermediate transition state giving ClWCl oxo carbene plus methane. So, this is another way of generating tungsten oxo carbene complexes. Now, in this regard we should mention that some of the earlier molybdenum version that we had seen where of molybdenum imido carbene of similar or related formulation. So, the way this particular complex which was prepared from the halide and this dimethyl aluminium chloride resulting in the tungsten oxo carbene complex. So, along the same line the Schrock had synthesized the corresponding imido carbene complex as is shown over here. So, oxo complex was synthesized much later in 1990 and when this is imido molybdenum complex methyl CF3. So, this is a very complicated complex which has both the imido and the carbene complex and also has this CF3 methyl moieties around the alko oxides. So, this is kind of a complex however this was found to be highly active catalyst but poor functional group tolerant but poor then the next important thing is while increasing the electron withdrawing nature of R the catalyst catalytic activity increases the catalytic activity increased with increasing electron withdrawing nature of R and this can be seen why the CF3 groups were put in place. Another important feature of this complex is the sterically demanding olefins even sterically demanding olefin tri and tetrasubstituted tri and tetrasubstituted olefins can undergo matrices. So, this is a very interesting development where this molybdenum imido carbene as prepared by Schrock went on to become an extremely high active catalyst then the catalytic activity could be increased by putting electron withdrawing groups and also can be because of higher activity it can be used for slow reacting tri and tetrasubstituted olefins. The only drawback to this highly active molybdenum catalyst is that they are not very functional group tolerant or the catalyst would easily be poisoned by different functional groups in the olefin. So, these are supposed to be the limitation of this catalyst molybdenum catalyst. Now, we had seen this tungsten oxo carbene complex which was synthesized from tungsten oxalide then we have seen Schrock's molybdenum imido carbene complex and now we are going to go and look at Grubb's phosphine carbene complex Grubb's complex and this was synthesized much later 1995 about 5 years later than that of the Schrock's which was synthesized in 1990 and this angle between these two halide was 167 degrees. Now the main feature of this ruthenium compound is that these are functional group tolerant like and it could tolerate groups like CO, OH, NH2, NH2, Mido and that the catalyst would not get poisoned by the presence of this group in the olefin and hence this was supposedly a very big attribute for this ruthenium carbene complex. The other important features included selectivity towards sterically unhindered olefins and strained olefins and the third thing is that tri and tetra substituted olefins cannot undergo metathesis tri tetra substituted olefins did not undergo metathesis. The point to note is the following now when one compares Grubb's complex with that of the Schrock's complex as discussed earlier one major difference is that this is exactly opposite to that of the Grubb's complex in terms of being functional group tolerant whereas Grubb's compound which was a molybdenum was highly reactive towards functional group and intolerant and hence the catalyst would get poisoned in presence of various functional group like CO, OH, NH2 whereas the Grubb's ruthenium complex is exactly the opposite and that it does not get affected by the presence of other functional group and hence the catalyst would survive the presence of other functionality and would effectively carry out the catalyst. Another important difference is that the Schrock's catalyst being highly reactive could carry out metathesis of tri and tetra substituted olefins whereas the Grubb's one is exactly the opposite and it did not facilitate or did not allow the metathesis reactions with tri and tetra substituted olefins. Third thing the third difference is obviously the ruthenium carbene is more air stable whereas the Schrock's molybdenum carbene is more sensitive to air and moisture. Lastly, the fourth difference is that ruthenium carbene has only one kind of multiple bond whereas the Schrock's complex has two kind of multiple bonding one between molybdenum and carbene and the other between molybdenum and immoidomoity. Another interesting thing is that even though they are completely complementary to each other together they made a very good set of catalysts combination where every type of metathesis reactions could be facilitated. One important thing as I was talking about this co-ordinative and electronic saturation and saturation is a very important factor in guiding catalyst activity and in this regard both the Schrock Grubb's catalysts have a low co-ordination number and these primarily we had discussed by arising from phosphine dissociation in Grubbs catalyst in Grubbs catalyst low co-ordination number. This is very important in order to facilitate alkene attack on central metal atom or this is called alkene binding. So the point to note is this low co-ordination number very is important for alkene binding and that has been achieved by phosphine dissociation in presence of Grubbs catalyst and another advantage of olefin metathesis reaction is the fact that olefin metathesis can be carried out both under homogeneous conditions as well as heterogeneous catalysis conditions and as preferred by industry most of the industrial processes utilizes heterogeneous catalyst till date industrial processes utilizes heterogeneous catalyst so whereas the laboratory applications are mostly used laboratory applications and towards this development a lot of different kinds of catalysts using ruthenium molybdenum and other metals were synthesized and as we had seen that the activity of Grubbs catalyst was improved by replacing phosphines with NHCs and so on and so forth and several types of or several variants of Grubbs catalyst were developed and they were later characterized as first generation and second generation type of catalyst and in this regard the notable few catalysts which are by far the best in terms of the activity are shown over here the variants of Grubbs catalyst that are that show great activity to start with is the this is the first Grubbs catalyst then can the carbene substituted Grubbs catalyst followed by liminosol based Grubbs catalyst followed by the B-scardene based Grubbs catalyst so what we see over here is that a significant improvement of Grubbs catalyst happens as phosphines get replaced by NHCs to becoming that with 2 NHCs where both phosphines have been replaced with NHCs and the activity keeps getting better so with this we come to the conclusion of today's lecture in which we have looked into various types of catalysts that have been developed starting with the tungsten based carbene complexes to that of molybdenum to that of ruthenium and what we saw that between Stokes molybdenum and Grubbs ruthenium catalyst there is a presence of complementarity in terms of the Stokes molybdenum catalyst being extremely sensitive to air and moisture whereas Grubbs ruthenium catalyst is stable to air and moisture Stokes molybdenum catalyst is highly reactive and hence functional group intolerant whereas Grubbs ruthenium catalyst is functional group tolerant Stokes molybdenum catalyst is good for the reactions of tri and tetrasubstituted bulky olefinic substrates whereas the Grubbs ruthenium catalyst extremely actually does not undergo any reaction with the sterically demanding tri and tetrasubstituted olefins so there exists huge complementarity between these two catalysts but the end the appreciation of for metathesis reaction is more possible because of the synthesis of Grubbs catalyst which is sort of virtually took out this metathesis chemistry out of the glove box to be able to successfully practiced in open air now one thing which is important over here that for this metathesis reaction olefin binding is a crucial step and that is facilitated by a low coordination number of the catalyst and this is achieved by phosphine dissociation giving rise to low coordinated metal species for both Schrock and Grubbs catalyst which can then finally bind olefin giving metathesis reaction so apart from this industrial processes prefer heterogeneous catalyst whereas homogeneous catalysis are still preferred under laboratory applications reactions with this we conclude this our discussion on olefin metathesis reaction and we are going to alkene metathesis reaction and we are going to take up a cross metathesis another variant of olefin metathesis reaction in more detail when they meet in the next class till then goodbye and thank you you