 Welcome to this course on Transition Metal Organometallics in Catalysis and Biology. We have been discussing alkene alkyne metathesis in the last lecture and we have been talking about it in the context of the individual metathesis reactions that is the alkene metathesis and alkyne metathesis. What we have also learned is the fact that these alkene alkyne metathesis indeed reaction in which the alkene undergoes metathesis first followed by the alkene alkyne and in principle these are in short they are called in-ine metathesis. In this context in our previous lecture we had looked into the mechanism which is relevant for in-first pathway or which simply means that the alkene undergoes metathesis first followed by the metathesis of the alkyne and what is the implication of this mechanism is that a metal carbene species is the active species for performing this metathesis reaction. In the previous class we have also looked into the full catalytic pathway for this in-first mechanism and in today's class we are going to look at different factors which are put in place to carry out this in-ine metathesis in a in-first pathway and in the process also to avoid cross metathesis or other side metathesis reactions which might as well also be carried out under similar conditions. One of the safeguard in place to carry out this in-ine metathesis reaction is that these reactions in-ine metathesis are conducted under high dilution conditions to avoid competing cross alkyne metathesis reactions and these in-ine metathesis in particular is more applicable for ring closing in-ine metathesis. These reactions are nothing but intramolecular metathesis reactions. It seems very intuitive to think that these intramolecular in-ine metathesis are carried out under high dilution conditions in order to suppress all other kinds of cross metathesis which are intermolecular in nature. Other cross metathesis which have been suppressed by these methods are for example, cross alkyne metathesis and cross alkyne metathesis and both of these intermolecular metathesis reactions. It is no surprise that these in order to suppress intermolecular metathesis reactions like cross alkyne metathesis and cross alkyne metathesis, these ring closing in-ine metathesis or R-C-E-Y-M in short what is called which is an intramolecular reaction that is carried out under high dilute conditions. Now a lot of investigation has been performed with respect to establishing the mechanism of this in-ine metathesis reaction as we are overseeing and in one of a seminal example by Mori et all who had showed that by taking suitable substrates presence of ethylene as a gas provides better opportunity for catalyst regeneration. So, example by Mori et all in J.O.C. General of Organic Chemistry, 1968, 63, 608, 2, 6, 0, 83 showed the presence of excess ethylene provides better opportunity for catalyst regeneration. This provides better opportunity for catalyst regeneration and this has been developed for the system shown over here one more percent ethylene one atmosphere gave the cyclic compound. Now the advantage of ethylene was ethylene maintains a higher concentration of the active catalyst maintains higher concentration of active catalyst active catalyst and reduces and reduces catalyst in resting state. Now this is shown by the equations for example this substrate and the catalyst they would react and in the process which was discussed earlier would generate this active species which is the ruthenium carbene moiety attached to this cyclo pentane ring giving aplicarcer which is conjugated or this is properly called as the vinyl carbene. So that in presence of ethylene would give one atmosphere of ethylene would give this metallocyclobutane species and that would undergo rearrangement as is shown over here to give the product plus the active species. Now this is a different active species compared to the active species which is shown over here and again reacts with the substrate as is shown here to again give a metallocyclobutane ruthenium compound. So this active species comes again and then reacts with the substrate to give this metallocyclobutane compound which eliminates ethylene to give the active species. So which is another active species as is formed over here. So in terms of active species what we see is that there are about three active species the first one being this, second one being this, third one being this, there are being formed. So if you look at the earlier statement that what has been achieved by ethylene that ethylene maintains higher concentration of the active catalyst. So all of these can now come into the cycle and carry out this metathesis and the second point it says that reduces catalyst resting state because all of these active species can further participate in the reaction, so they reduces the active state. Now another take home message is that the catalysis species can also undergo self-metathesis as is shown over here metallocyclobutane ruthenium compound and that can undergo rearrangement as is shown to give these two species. Now this is an interesting improvement on enoying metathesis and what it does is that by carrying the reaction under high dilute condition also in presence of gases like ethylene the catalyst resting state time is reduced and the effectivity of the reaction is increased. Now this leads, this is building up more on this in first mechanism where the alkene is formed first undergoes metathesis first. Now there is also another line of investigation which was on the iron first mechanism in which the alkene would undergo metathesis first, but this from the experimental evidence experiment studies it has shown that an alkene first pathway would lead to various lidsgear isomers, the selectivity would not be as good as for the enine first pathway. So an alkene first in first pathway would lead to a mixture of lidsgear isomers and this is illustrated by this beautiful example for example for the substrate that reacting with the active species. What we have over here is this alkene reacting with the active species carbene in this iron first pathway resulting in this metallocyclobutene, metallocyclobutene in compound. Now the mixture of lidsgear isomers, the selective issue comes from the fact that there are two kinds of metallocyclobutene can be formed, the other being, so the catalyst can bind in both ways, one is this, the other is this and this would give a product as is shown over here, whereas the other mode of binding would give a product which is shown over here and the interesting thing is that these two products are different kind of active species and they can undergo exchange or interchange from one form to another depending on isomerization rate, which depends on dilution, so these two species which are formed by the differential binding of this ruthenium carbene species to the alkene would result in two different active species which can also interchange depending on the dilution and as a result the products that emerge from there are also a mixture of products as shown here. This is formed from the cyclization of this carbene species with this olefin, which gives a six-membered ring, whereas the cyclization of this carbene species with this would give a five-membered ring as is shown here, what we see is that the mixture of two different this is a five-membered ring, that two different mixture is formed if this was going to be a iron first mechanism and that arises because of formation of two different active species and that of because of the binding of this active species in two different pathways, so now even because of the complexity in the number of different products obtained in terms of the loss of regioisomers formed at the reaction where to proceed by alkyne first pathway, the evidence is based on NMR and other mechanistic study does favor this in first pathway where such a problem of regioisomerism does not appear. With this we come to the end of today's discussion on the various mechanistic aspects of alkyne or iron first pathway, which has been discarded given the fact that this mechanism if at all would happen would show the formation of various kinds of regioisomers because of differential binding of the active species to the alkyne, however such a possibility does not is observed in the real E9 metathesis reaction, which supports the earlier proposed mechanism that this is an in first pathway, which is prevalent in the E9 metathesis pathway. Now with this let me just sum up what we have been discussing in today's class, in today's lecture we have looked at the method involved in improving this catalyst of E9 metathesis and this has been done by performing this E9 metathesis in presence of one atmosphere ethylene. What ethylene does actually is that it provides a better opportunity for catalyst regeneration by maintaining a higher concentration of the active catalyst and also decreases the resting states of the catalyst. Basically a presence of ethylene helps in carrying out these E9 metathesis reactions better and also these reactions we have noticed is that they are carried out under high dilution conditions in order to avoid cross alkene metathesis and cross alkyne metathesis reactions occurring under concentrated conditions. This is quite intuitive given the fact that cross alkene metathesis and cross alkyne metathesis are both intermolecular phenomenon, whereas this ring closing E9 metathesis is an intramolecular phenomenon and they are better done under dilute conditions. We have also looked in the possible pathway of ion first mechanism and what is the main drawback of ion first mechanism is that it produces a large number of reduisomers and there is a lack of product selectivity. However, given the fact that these E9 metathesis reactions are highly selective and exclusively gives one product that also provides a circumstantial evidence in favor of E9 first mechanism. With this, I come to the conclusion of today's lecture. We are going to take a look at some more examples of E9 metathesis or alkene alkyne metathesis when we meet next in the course. Till then, goodbye and thank you.