 Welcome to this course on Transition Metal Organometallics in Catalysis and Biology. We have been talking about olefin metathesis in the last few lectures and in that regard we have looked into various kinds of mechanisms which have been proposed for olefin metathesis and come across names like cauldron, pated grubs, chauvin and also we had seen the mechanism gaining consensus with regard to the correct one and in that process we had seen how chauvin's mechanism of having a metal carbene intermediate being accepted as the main active species for carrying out olefin metathesis reaction. In that regard we had also discussed in the previous lecture about the seminal work by Professor Tom Katz who had correctly predicted the olefinic products using different olefin explaining their formation through metal carbene intermediates. Now in that point of time we had also looked into the active species much more details and what we had observed is that this metal carbene intermediate or metal carbene moities at that time can be of two types as it is better known as the Schrock carbene and Fischer carbene and in our previous class we had also looked into the type of bonding present in Schrock carbene as well as Fischer carbene. What we had seen that in Fischer carbene both are carbonic moiety but the way they differ is in the type of bonds they make whereas in the Fischer carbene it is a ligand to metal sigma donation followed by metal to ligand pi donation whereas in Schrock carbene both are in covalent bond both the sigma as well as the pi bonds are simple covalent bonds. One can say that this is a dative bond between metal ligand and this is covalent bond between metal ligand. In this context another important discovery was made by Ardango and this is called this Ardango's bottleable carbene. Usually a carbene can exist in the singlet and the triplet state with the triplet state being more stable whereas Ardango successfully isolated singlet bottleable carbene and that is why you know which is supposedly the excited carbene and they were Ardango could stabilize them and the interesting thing about it is that they were so stable that they could be isolated and even crystallized and that led to the discovery of this huge field of in heterocyclic carbene which were found to be excellent ligand for and have great catalytic properties. This Ardango's bottleable carbene became a big hit in the area of homogeneous catalysis and we will just take a brief moment to discuss these carbene in bit more details. Ardango synthesized this from their imidazolium chloride salt in presence of sodium hydride as a base which resulted in formation of this singlet stable carbene in 96% lild and it is kind of high stable very stable with melting point about 240 degree centigrade. This AD is a bulky adamantile group and Ardango discovered this in another JSES paper in 1991. Now, what is a key thing over here is the strategy which Ardango used in stabilizing these carbene which are singlet carbene means that these carbene can donate this lone pair to metals and make a good sigma bond. Also, the way Ardango stabilized this singlet carbene is because of the presence of two nitrogen hetero atoms which would sort of pull the carbene lone pair towards inside and then it would have its empty PG orbital partially populated by a lone pair on nitrogen giving on to the carbon. So, that is how these carbene were prevented from going back to their triplet state less stable or not accessible. So, as a result singlet carbene were formed. So, this is furthermore what Ardango did is that Ardango took help of bulky adamantile substituents which would protect this lone pair from dimerization. So, this mechanism is of protecting using sterics by the adamantile group as well as the electron donation from the nitrogen lone pair as well as presence of two hetero atoms is called push-pull-pull mechanism effect which sort of lead to the stabilization of the singlet carbene. And to the extent that this singlet carbene is so stable that it has a very high boiling point of about 240 degree centigrade it can be bottled you can even buy it this singlet carbene and then use it for complexation with the metal and they are extremely a good binder to metals and they form sigma bonds with metal. So, this is how another extremely good ligand for catalysis that is singlet enhydrocyclic carbene came into being and this also is a development which has happened in industry particularly at DuPont. We had also spoken about referred to DuPont in the context of the fact that the first observation of metathesis of observing ethylene propylene copolymer from a feed of ethylene for olefin polymerization over a maloptimum catalyst was also indeed first observed at DuPont. So, here we see that another interesting contribution coming out of industrial lab and this is this bottle level singlet carbene from Ardengo. Now, we are going to spend some more time on explaining the singlet and the triplet nature of the carbene as we go along. So, as to help understand and appreciate the different types of metal carbene moieties that are possible. In a singlet carbene moiety bound to metal the carbene lone pair is an ISP2 orbital and then there is a empty PZ orbital and that is why this is called singlet carbene. Singlet carbene is more common in fissure and then in fissure carbene it is said that the carbon moiety has a heteroatom and that has the following kind of resonance structure which is populated over here. The metal is anionic nature particularly because of electron donation for the singlet carbene which makes the carbene positive and the metal negative. So, this is the ionic structure and then there is a canonical form which is neutral with another canonical form existing as m plus c minus xr and that existing with lm minus c x plus r. So, what we see that this has several canonical forms primarily all of them are ionic in nature except for one neutral form which is carbonic in nature. Similarly, one can draw a structure for the triplet carbene bound to metal and here also one sort of c is now here also it is a SP2 orbital now the PZ orbital is not vacant and also has a single electron and hence it is called triplet carbene. So, what one can see that when this is called Schrock carbene and the singlet carbene is in fissure carbene. So, what one can see that singlet carbene fissure carbene can engage in two kind of bonds one is ligand to metal sigma dative bond because of this lone pair being donated as well as metal to ligand pi dative bonds as is shown over here whereas for the Schrock carbene it can only make ligand metal sigma bond which is covalent and similarly it can make metal ligand pi bond also covalent. So, in terms of fissure carbene both two dative bonds are made in terms of Schrock carbene two covalent bonds are made and there is a sharp distinction in the reactivity of these two types of carbene as we have seen. Now, we will sort of take a look at fissure carbene interaction with the metal in with more detail. So, in fissure carbene thus the interaction that occur of this is filled to a metal d sigma orbital which is empty whereas this is the sigma interaction and the corresponding pi interaction is depicted over here. Now, this one is empty this is a sp2 field and this is pgmt interacting with a metal d pi orbital and which is filled and this is a pi type interaction this is a metal to ligand pi and this is ligand to metal sigma interaction and this is a typical of fissure carbene type of interaction the way the carbonic moiety interacts with the metal center. Similarly, we are going to look at the interaction of Schrock carbene with the metal center in that case both this p2 orbital as well as the pg orbital will have a single unpaired electron. So, this is called half field and the metal d sigma orbital is also half field and they make a bond which is metal ligand sigma kind of bond covalent. Similarly, the corresponding pg orbital having one unpaired electron is also half field interacting metal sigma d pi orbital which is also half field this is metal ligand pi interaction and which is also covalent interaction. So, what is interesting over here is that both the metal carbene moiety would look like to have a metal carbon double bond a sigma bond and the pi bond. However, the nature of bonding in Schrock carbene and fissure carbene are completely different and which sort of results in different kind of reactivity between these carbene complexes. So, we are going to sort of take a look at some of the interesting methods what have been used in synthesizing these carbene complexes starting with the fissure carbene complex by from taxonyl hexacarbonyl. So, hexacarbonyl in reactions with lithium alkyl in diethyl ether give the fissure carbene complex as shown over here and that when reacted with CH3 whole 3 O plus BF4 minus gave the fissure carbene complex in 1964. Fissure carbene complex can also be synthesized by other method the way is shown over here CO plus CF3 SO2 H resulting in the cationic complex of rhenium and this was made by John Gladys in 1983. Another synthesis method involves reacting platinum dichloride PPH3 isocyanide C and P8 in ethanol giving Cl Pt Pt PPH3 double bond C N H P H O Et and this was reported by chat in 1969. Similarly, there have been a few extremely nice examples of method have been reported for Schrock carbene complex starting with titanium dichloride as is shown here Cp2 titanium chloride plus sodium ethoxide sodium ethoxide giving Cp2 titanium CH2 CH3 this was reported by Schrock himself in 1975 and another example was reported by Gladys in the reaction of rhenium Cp NO L CH3 in presence of P H3 C plus P F6 minus in dichloromethane giving Cp rhenium CH2 L NO plus C H P H3. So, this is an important reagent this is called trityl CH3 C plus and this trityl sort of abstracts a proton. So, this is a hydrogen abstract and reagent which sort of abstracts this hydrogen from methyl and converts into a rhenium carbene bond and results in formation of CHPH3. So, this was reported by Roper in 1983. Another interesting approach for making this Schrock carbene is shown over here. So, another started with Na2, it is a metallate C-R CO5 plus this cyclopropane dichloride at a minus 20 degree centigrade to NaCl in THF they gave the corresponding CO5 C-R carbene complex as is shown over here and this is reported by Ophelia in 1968. Another interesting example or method for synthesizing is this P P H3 whole thrice osmium chloride NO reacting with diazomethane CH2 N2 giving P P H3 and minus N2 giving this osmium carbene complex with P P H3 Cl NO by Roper in 1983. So, what we see is indeed interesting examples in which these carbene complexes were made some another interesting one is discussed here in which reaction of C5 H5 Mn CO whole 2 THF in presence of diazomethane eliminates a molecule of nitrogen to give this dimer Mn CO Mn Mn with CH2 CO CO in 8 percent yield plus M bound to olefin CH2 CH2 92 percent yield this was reported by Herman 1975 and last but not the least another interesting reaction that involves cleavage of an olefin for accessing this carbene has been reported by Lappard which took this olefin and inserted that onto an iron pentacarbonyl which loses a CO to give this tetramethylene carbene complex which is accessed from the olefin. So, this is synthesized reported by Lappard in 1977. So, with this we come to an end at of today's lecture whereby we looked at these important intermediate of olefin method this is which is a metal carbene moiety and then what we have done is we have looked into the various kinds of metal ligand interaction present in metal carbene moiety we have also looked at ardengos bottleable singlet carbene the strategy and in synthesizing it behind stabilizing it and the strategy that was successfully observed while synthesizing these bottleable carbene we have also looked at various synthetic methods which are available for synthesizing the original fissure and the stroke carbene and what comes out of all this discussion is the beauty of organometallic chemistry in general because we saw that work by Lappard, Herman, Opel, Strock and so many others Gladys led to different beautiful synthetic routes to each of these carbene complexes. So, that sort of explains the aesthetics and beauty that organometallic chemistry presents as such. So, with this I come to the conclusion of today's lecture which looked into the catalytic species of olefin methodesis which is metal carbene species in general we have looked into classifications of metal carbene species the synthesis behind each of them we have also looked into various kinds of carbene including this otherwise so famous in heterocyclic carbene and the strategy used for synthesizing that. So, this has been extremely fruitful discussion where we looked at the chemistry and the reactivity of the metal carbene species and carbene species in general and their relevance to olefin metathesis. So, more of this discussion on olefin metathesis would continue in the next class where look up the development of olefin metathesis in a bit more detail. Also, lastly I would like to note that industry as we had been saying over and over again that industry had been a key player in development of organometallic chemistry and in this context I would like to mention also here that the bottle level carbene by Ardengo was indeed industrial contribution Ardengo had reported it from DuPont and we had observed the same thing that olefin metathesis the integrity about olefin metathesis was first observed indeed at DuPont in 1956 when they were trying to explain the observation of ethylene propylene copolymer when they were trying to carry out polymerization with only propylene. So, with this I conclude today's lecture we are going to be discussing more on the developmental aspect of olefin metathesis in the next class. Till then goodbye and thank you.