 In this lecture, we will discuss some of the important organometallic reactions which deal with carbon monoxide. Carbon monoxide is probably one of the most important raw materials that is available to the chemist. It is important that we have some means of converting it into useful chemicals. Because of the small polarity of carbon monoxide, it is not possible to carry out reactions with carbon monoxide, which you can do with carbon dioxide. So, how is carbon monoxide, which is unreactive activated? If you look at the industrial scene, it is clear that there are several reactions such as the water gas shift reaction, the synthesis of acetic acid or even dimethyl carbonate, which is extensively used in CDs in the manufacture of CDs. For example, all of these utilize carbon monoxide as a raw material. So, how exactly does organometallic chemistry succeed in deactivating or deactivating? How exactly does organometallic chemistry succeed in activating this unreactive molecule? If you look at the chemistry that has been developed, you will realize that carbon monoxide has an extensive chemistry that includes alkyl group, aryl groups, alkynes, alkenes and a combination of all of these groups to form useful materials. So, we will consider each one of these initially separately and then we will look at some complicated reactions where multiple groups are involved. So, in all of these reactions, carbon monoxide is activated and it is made possible for a reaction between the unreactive carbon monoxide which is now coordinated to the metal and an alkyl group or an aryl group or a vinyl or allyl group. So, let us take up the first reaction which is probably one of the simplest reactions that is available in this series of chemistry and that belongs to the carbonylation of methanol. This chemistry is that of the carbonylation of methanol where methanol is reacted with carbon monoxide to generate acetic acid. Now, it turns out that if you heat the two under very high pressures, you will not get acetic acid by any means, but the presence of rhodium iodide and methyl iodide as a catalyst. These two are used in catalytic quantities and that is the molar ratios in which you have to do the reaction. So, it is extremely small amounts of rhodium iodide and methyl iodide that are necessary to push the reaction from the left to the right. Why use rhodium iodide? Can we use rhodium chloride or bromide? The answer is no. Actually, it has been shown that H i is essential for driving the reaction from the left to the right. Now, this is strange until you consider the fact that H i actually converts methanol into methyl iodide and that is this reaction that we are talking about right here, which undergoes a simple insertion reaction of methyl iodide with carbon monoxide to give you acetyl iodide. If you add up all three reactions, the hydrolysis of acetyl iodide to give you acetic acid and the two reactions that we just talked about, you will come to the conclusion that you just need to add methanol and carbon monoxide to get acetic acid. So, it turns out that although you need methyl iodide in catalytic quantities and rhodium iodide also, this reaction is an extremely profitable reaction where you have taken two rather unreactive species and converted them into acetic acid, which is such a useful raw material in the industry. Now, let us take a look at the science behind this whole process. Why do we need methyl iodide? Methyl iodide is the one, which will undergo this first step in this reaction where the rhodium diido rhodium carbonate species, which I am marking for you right here, the one that undergoes oxidative addition. You will notice that this is a d h species and it has got a nice filled d orbital that is in the d z squared along the z axis in the d z squared orbital and that is perpendicular to the plane in which the ligands are kept. Now that carries out a nucleophilic substitution reaction on methyl iodide. So, you need a fairly good leaving group for the iodo group to leave from the methyl species. So, that you can achieve this six coordinated compound, which is now d six and it is octahedral in nature and you have rhodium one compound being converted to rhodium three, because you have undergone oxidative addition. This is an oxidative addition reaction and it is now set up to carry out the next step, which is the insertion of carbon monoxide. Now, as we mentioned in the studies in insertion reactions, it is the methyl group, which migrates to the carbon monoxide. So, the methyl group migrates to the carbon monoxide, so that you have an acetyl group coordinated to the rhodium. Now, this whole step is in fact an equilibrium process. This keeps oscillating back and forth, but if you have excess pressure of carbon monoxide, you can drive the reaction to the left side of this catalytic cycle, so much so that you will end up with this compound in solution, which now reductively eliminates. So, it undergoes a reductive elimination. This is rhodium three species and that undergoes reductive elimination to give you the catalytically active R H I 2 C O 2 minus species, which is the resting state of the catalyst. This is the resting state of the catalyst and acetyl iodide is eliminated in the reductive elimination step. So, as we saw earlier, acetyl iodide will be now converted to hydroidic acid and acetic acid. The hydroidic acid is capable of converting the methanol to methyl iodide, which we have pictured here. So, essentially we have converted methanol to acetic acid. So, this step that we have just considered, this catalytic cycle that we have just considered is the simplest of the carbonylation reactions that we would consider today. There is only one oxidative addition and there is an insertion process and a reductive elimination. So, this is an extremely simple catalytic cycle. Now, we will consider a more complex situation, where there are two key steps in the whole reaction and that is the hydro formulation reaction. You will realize that the previous reaction that we discussed was primarily developed by this company Monsanto. Now, we are talking about another chemical company called Rourkemi, which developed this hydro formulation reaction. The person who was primarily responsible for this famous discovery was Otto Roland and he did this as early as 1938. So, you can see that this is long before the renaissance that took place in organometallic chemistry. The discovery of ferrocene, which happened in 1956, but this reaction is an extremely useful reaction, because it converts alkenes in the presence of hydrogen and carbon monoxide to very useful aldehydes. Now, it is possible for this reaction to give you an isomer and we have illustrated this in the second equation that we have written. We have written the hydro formulation of propylene with hydrogen and carbon monoxide. That gives you the normal aldehyde, that is the butary aldehyde and the isobutary aldehyde, which is the isomer, which would be formed if the hydrogen adds on to the terminal carbon and that becomes C H 3. If C H 2, this terminal C H 2 becomes C H 3 and the C H O group, the carbon monoxide is added to the middle carbon in the propene. So, this turns out to be a complication and in fact, the main advantage of the main improvements that have been done in this reaction is the improvement in the selectivity of this whole process. Now, Rowland used dichobalt octacarbonyl as a catalyst. It turns out to be a very efficient catalyst, if you use it at these high temperatures 120 and 170 degrees and with a fairly high pressure of carbon monoxide and hydrogen. So, you will realize that in many of these chemistries that we are going to talk about today, you have to use high pressure of carbon monoxide in order to drive the equilibrium to the right and make the whole situation very favorable and fast. Now, it turns out that this olefin hydro formulation reaction underwent a significant improvement in 1976 and that is the time when Union carbide introduced a catalyst which was based on rhodium. The catalyst that was used by Otto Rowland was on cobalt and that would have given him H C O C O 4. So, the catalyst that was used by him was identical to this species except for the ligands and the metal have been changed. The oxidation state and the electron count are the same for these two complexes. So, H R H C O 2 P P H 3 twice was a catalyst that was used by Union carbide. In fact, they used molten P P H 3 P P H 3 above the melting point of the compound was used in the reaction medium. They found that they could convert the propene to normal butyraldehyde in very good yield. Now, you know that cobalt is 1000 times cheaper and so one has to ask the question why would you use rhodium which is so much more expensive. The answer of course is a fact that rhodium is 1000 times more effective and so it turns out to be more economical in the long run to use rhodium rather than cobalt. The temperatures and pressures are again very high and they have to be kept high in order to drive the reaction to the right. I will write a catalytic cycle now which is based on rhodium but analogous reaction with cobalt can be very easily written. The resting state is again a rhodium 1 compound. Here, we have written a rhodium 1 species as the resting state of the catalyst and the reaction starts with coordination of an olefin to this D H species. Now, we form a co-ordinatively saturated D 10 alkene complex and that is a D 8 alkene complex. Now, you have a hydrogen cis right next to an alkene and the alkene would co-ordinate in such a way that the bulky group will indicate that with. So, the bulky group in fact coordinates closer to the hydrogen. So, that would be the preferred way of coordinating the alkene and the alkene now undergoes insertion reaction. The negatively charged or the formally negatively charged species migrates on to the neutral ligand in such a way that you have now an alkyl rhodium 1 compound. So, you had a hydro alkene complex and after the migration of the hydride you now have an alkyl complex. This again is a rhodium 1 species. So, we have just carried out during the course of this one step a migration reaction which does not change the oxidation state of rhodium. Now, you can in the next step carry out a carbonolation reaction. So, we can add a carbon monoxide to this species to this rhodium 1 alkyl molecule. We can add a carbon monoxide and that gives us a penta coordinate rhodium 1 species which can undergo migratory insertion reaction again. Now, the anionic group is this alkyl group here. So, this alkyl group migrates on to the carbon monoxide and as a result of this migration. So, now you have an acyl complex. So, you have an acyl complex with a vacant coordination site right here vacant coordination site right here. We also have as a result of this migration we have this acyl group and this can add on hydrogen to have an oxidative addition reaction. So, this is the oxidative addition reaction and that happens right here and with oxidative addition of hydrogen you would end up adding two groups in a cis position and this results in the formation of a dihydride that is the final step in this catalytic cycle. You have a dihydride and an acyl moiety and these the acyl moiety is cis to the hydrogen. So, now, you can eliminate this aldehyde. This aldehyde can be eliminated and that is what is coming out in the catalytic cycle right there to regenerate the catalytically active species which is a rhodium one complex. So, this is a reductive elimination step. So, very often a reductive elimination step is the final step in the reaction where your catalytically active species is regenerated. After the regeneration of the catalyst you might have either an oxidative addition or you might have a simple substitution reaction. In this catalytic cycle the oxidative addition and the reductive elimination have happened in the very last two steps. So, a similar catalytic cycle can be written for cobalt only difference would be the fact that H C O C O 4 would be the catalyst instead of the hydridorodium species that we have written right here. Otherwise, all the steps would be extremely similar. Now, the hydro formulation reaction is something that is being actively pursued because it gives us a convenient entry into aldehydes and this has to be done very selectively and also one should note that rhodium is a very expensive metal. So, one normally recovers the rhodium tries to recover the rhodium after the after the reaction and this turns out to be an expensive process also. So, we should also avoid contamination of the product with the catalyst in this case rhodium. It has been said that in some of the expensive catalysts that have been used early processes were so inefficient that the concentration of the catalyst in the product was greater than the concentration of the metal in the ore from which it was initially synthesized. So, this has to be prevented and one has to have efficient means of recovering the catalyst. So, hydro formulation in water was one possible solution to this whole difficulty and company Rhone Poulenc, Rhone Poulenc was a French company which discovered a water soluble rhodium catalyst. Once again the catalyst was the rhodium one species which is the D 8 species which has been described earlier. So, the mechanism of the reaction would be identical except that by introduction of the SO 3 H group the SO 3 H group makes the phosphine water soluble and in turn the phosphine coordinated to the rhodium makes the catalyst water soluble. This is a catalyst which is quite popular and has been abbreviated as TPPTS and it is pictured here. It has got 3 SO 3 H groups which make the compound extremely water soluble and it is usually kept as a sodium salt. The separation of the catalyst is easy because one only has to remove the aldehyde which is usually organic compound soluble and organic solvents. So, it can be extracted and the water containing the catalyst can be recycled. So, hydro formulation in water is one of the means by which one can carry out the reaction efficiently and at the same time save the catalyst and use it for the next step of the reaction. So, in all these cases you notice that the anionic substrate is the one that adds on to the neutral substrate. In this case the neutral substrate is carbon monoxide. It is also possible to carry out the reaction in such a way that the second neutral substrate is also added and then subsequently a reductive elimination is carried out. So, multiple insertions can happen in the catalytic cycle. Example of such a reaction is the Repay reaction. Walter Repay again working in a German company called BASF which is still around and is an extremely old company which used to manufacture unline and soda ash. These chemicals are bulk chemicals which are used in the chemical industry in very large amounts. They discovered some very interesting chemistry which went along with acetylene carbon monoxide and water. You will notice that essentially a molecule of water and carbon monoxide have been added to acetylene in order to generate this very useful acid. So, the reaction was carried out again under high pressure of carbon monoxide. But notice because you are using acetylene this reaction was a great technical challenge to compress acetylene without an explosion was in fact a difficult task to accomplish. It is to the credit of Walter Repay who discovered ways and means of handling acetylene so that it would not explode even when it was pressurized and used in this particular reaction. So, he used it at high temperature at 180 degrees in the presence of nickel bromide and cupricirid as catalyst. He was able to convert acetylene to acrylic acid and in the presence of an alcohol instead of water you can directly convert it into the ester. So, you can either make the ester or the acid depending on whether you use water or an alcohol. So, these reactions were carried out primarily with nickel as a catalyst and cupricirid was just a promoter that was added. If you do the reaction at a higher pressure of carbon monoxide slightly higher pressure of carbon monoxide then the same reaction would undergo a second addition of carbon monoxide and water to this acrylic acid and generate this saturated diacid. So, you can see that the reaction can be an extremely versatile and extremely useful reaction where you can convert acetylene which is again available in bulk quantities to useful organic compounds which are required in the industry for the synthesis of drugs and pharmaceuticals and di-stuffs and so on. So, how exactly does this reaction work? Here is a possible scheme that we have written with nickel and a same reaction scheme can be written for the second step also only the first step is shown here and that too with nickel tetra carbonyl. Nickel tetra carbonyl can lose two molecules of carbon monoxide and undergo oxidative addition to give you this nickel 2 complex. Here nickel is an oxidation state of plus 2 and you have a hydridonical compound which has got this x group which is a group that has to be added. In this particular case you would have to have o r x is o r or it could be o h if you are using water in the reaction medium. So, if you end up having this nickel 2 plus complex which can then coordinate to acetylene. So, the next step is of course coordination of an acetylene molecule and that is what we have here. So, you have an acetylene molecule coordinated to the nickel and you have a hydrogen next to the on the nickel next to the acetylene you can have an insertion reaction. This insertion reaction would give you a vinyl nickel species and that is shown here. A vinyl nickel species where the hydrogen in the cis position has migrated on to the acetylene and that gives you again a nickel 2 species, but now you have to have the nickel 2 species can has carbon monoxide which to which it can migrate to. So, you have a migration of this vinyl group. The vinyl group migrates on to the carbon monoxide which is in the cis position and that gives you the acyl moiety which is pictured here. The acyl moiety can now do a reductive elimination of the two groups the x group and the acyl moiety to give you the product. So, this is your product and this step is your reductive elimination that gives you back nickel 0 which has got only two carbon monoxide and if it oxidatively adds H x which in this particular case it would be the R OH. So, if you start with nickel bromide you can also think of adding the B R and the final step you would have a hydrolysis reaction or an alcohol is reaction. So, because you would have nickel tetra carbonyl is generated from nickel bromide in the in the catalytic cycle. You need only nickel carbonyl, but you can generate it in situ from nickel dibromide and carbon monoxide. So, the rapid catalytic cycle involves an oxidative addition and insertion to insertions in fact an insertion of carbon monoxide insertion of an acetylene and that gives you the required acrylic acid at the end if water is used in the reaction medium. Now, let us take a small detour to heterogeneous catalysis and just mention two important industrially important reactions which have got a lot of relevance to the organometallic chemistry that we are studying. This reaction is actually the water gas shift reaction where water and carbon monoxide is converted to carbon dioxide and the extremely useful energy rich species which is hydrogen. Now, this reaction is normally carried out under heterogeneous conditions. You have a catalyst which is mostly iron supported on copper carbon monoxide and copper and these reactions are surprisingly are difficult to carry out in a homogenous conditions. So, there are practically no homogenous analogs for these heterogeneous reactions. At catalyst like the species that I have indicated here have been used to show that these reactions can be done at least on a small scale or even if they are inefficient they show that such chemistry is in fact feasible. So, the reason why I have chosen these examples is because even though these heterogeneous reactions are really like a black box you do not know what is going on, but still sufficient organometallic chemistry must be going on. There is sufficient organometallic chemistry that is available which shows that these reactions can be mimicked in the homogenous scale also. So, let me give you a catalytic cycle that is possible with the possible catalytic cycle written with iron pentacarbonyl and here if you have iron pentacarbonyl and you have water reacting with it. Let us consider it in two steps with OH minus attacking the carbon monoxide. You can have this OH minus attacking this carbon monoxide to give you COOH coordinated it is an SL a carboxylato anion which is coordinated to the iron and that will have a negative charge because we have added a negatively charged species. We can eliminate from this species the CO2 because you have the right groups with the right connectivity if you eliminate CO2 and if the hydrogen moves on to the iron. So, that would be exactly like a beta hydride elimination. So, you would get an iron hydride species. This iron hydride species that we have written here can react with water and that can generate if it donates a proton. If it donates a proton here essentially this water is for donating a proton. Then you would get a di hydride this is a di hydride that we have here with the iron which can eliminate di hydrogen which can eliminate di hydrogen to give you iron pentacarbonyl back. So, this is a simple catalytic cycle that demonstrates the water gas shift reaction. Although on an industrial scale this is not efficient. So, you still people still use the heterogeneous catalyst for this reaction although there is a homogeneous analog which is available although inefficient. So, what we have done is we have done the reverse of the you have done in insertion on the carbon monoxide which has given as a hydride species and that hydride species gets protonated in this step with water and di hydrogen is eliminated in order to get you the iron pentacarbonyl back. So, the next reaction that I want to discuss is a fissure tropes process which is again an extremely important process which is in fact the key energy source for some countries especially South Africa where there is a plant which is called the sassol plant which converts carbon monoxide and hydrogen to alkenes to products which are like the petroleum products which are alkenes and alkenes and sometimes some oxygenated products are also there like alcohols and aldehydes. Now, the question is how does this reaction work? Now, carbon monoxide is reacted in the presence of a metal surface and so it is easy to imagine the metal surface as reacting with carbon monoxide to generate metal carbonyl species which can be reacted with hydrogen and it might get converted to a methylene species and these species can couple. So, this is shown here in a pictorial fashion, but essentially what we are saying is that the metal carbonyl can undergo hydrogenation reactions or the activated hydrogen which is like a hydride can undergo migration reactions and eventually generate metal carbines. These carbines can couple as we have shown here if two carbines are close together then they can couple and the couple product can undergo an insertion reaction. Here is an insertion reaction and here is a coupling reaction reaction. These are steps which we are repeatedly encountering in carbon monoxide chemistry and this is the kind of chemistry must that must be going on on the surface of the heterogeneous catalyst and this heterogeneous catalyst is a one that is efficient in generating the petroleum products. So, suffice it to say that heterogeneous catalysis is very efficient and it cannot be supplanted by organometallic chemistry completely, but there are sufficient enlightenment on what is going on inside the heterogeneous catalyst by studying organometallic chemistry. Now, let us go back to the carbon relation reactions that we were discussing. Now, we will talk about a palladium promoted carbon relation reaction. Now, these are extremely useful in the laboratory and I have shown for you a reaction which can generate a kumarine and kumarines and isocumarines are synthesized very efficiently in the laboratory using this palladium promoted. This is catalyzed by palladium and usually palladium in the zero oxidation state is what is the resting state of the catalyst, but it is generated in situ in the presence of carbon monoxide. Carbon monoxide efficiently reduces palladium 2 to palladium 0. So, palladium 0 is an actual resting state of the catalyst. It is interesting that a variety of nucleophiles can be used what we have written as N U H can be anything from alcohols or amines and this R X is could be a variety of groups. It could be vinyl, aryl and the insertion reactions can happen very efficiently with as even complicated molecules as the one that is shown here. Let us write the catalytic cycle now to see how exactly this reaction can proceed. So, here is the reaction where you have palladium 0 as the resting state of the catalyst and that can readily do an oxidative addition reaction. So, palladium goes to palladium plus 2. This is palladium plus 2 the formal oxidation state and it has got the R and X groups attached to the palladium which can now undergo an insertion reaction. So, for the insertion reaction migratory insertion reaction to happen we initially add a carbon monoxide ligand and generate this species active species where the R group migrates on to the carbon monoxide. Once the R group migrates to the carbon monoxide you have an acyl moiety. This acyl moiety is attached to the attached to the X group which came from the R X. Now instead of doing a reductive elimination we can have an attack by the nucleophile which we have which we want to react R X with. So, if the nucleophile attacks the carbon monoxide the CO group of the acyl moiety. So, that is the CO group if the nucleophile attacks here you can end up with R CO N U and the N U H will give the proton to the palladium. So, this ends up generating a species which has got a hydrido palladium X group. This can eliminate H X and you regenerate palladium 0. So, this reaction is a simple reaction also. It involves an oxidative addition. So, that is this step an oxidative addition and a reductive elimination in the last step. So, the oxidative addition in fact happens between R and X. So, X has to be a good leaving group. This has to be a good leaving group and the nucleophile has to be a better nucleophile than the X group which is coming out as H X. So, as long as these conditions are satisfied and this condition is satisfied for a variety of nucleophiles. If you have a halide and an amine which is coming in then you have no problem and the triethylamine is basically a base which mops up the H X that is generated. So, at the end of the reaction E T 3 N H plus is liberated or rather the H plus that is liberated is taken up as a is converted into this N E T 3 H plus salt X minus salt and that is removed from the reaction sphere. So, you could have a direct attack of the A cell group with a nucleophile. The nucleophile can also coordinate prior to the reaction to the palladium before the attack on the A cell group. So, these two possibilities are there. Another reagent which is useful in the laboratory is called Coleman's reagent because Coleman developed it to a great extent and this is nothing but the reaction of iron pentacarbonyl with sodium, but sodium by itself does not react with iron pentacarbonyl. But if you react it with in the presence of benzophenone, benzophenone is like a catalyst because it generates the ketyl radical anion and this radical anion is capable of transferring the electron to iron and that results in the tetracarbonylate di anion iron tetracarbonylate di anion which is called Coleman's reagent. So, this is the Coleman's reagent that we are talking about and its chemistry has been extensively studied because it is such a nucleophilic species. It has got a lot of electron density on the iron, lot of electron density on the iron. It can carry out nucleophilic attacks on many, many electrophilic organic compounds which are electrophiles. Here, we have shown two possibilities. One is R X and that will give you an alkyl ferrate, carbonylate species. Alkyl iron is a very high level carbonylate species or an acyl carbonylate species. So, it is also possible to convert this alkyl carbonylate species with addition of either a ligand or P P H 3 or carbon monoxide itself to the acyl carbonylate species. So, you can generate this acyl carbonylate species very efficiently. This carbonylate species which is nothing but the R group with an added carbon monoxide can undergo another reaction with another electrophile. Here, we have shown the reaction with R X. That gives you a ketone. This gives you a ketone. So, starting with R X, you have converted it into a ketone or to an acid by reaction with oxygen. In that case, the iron carbonylate anion is decomposed directly. Now, you can also treat it with an acid. If you treat it with an acid, you generate an aldehyde. As in the previous case, you can also react it with a good nucleophile to generate R C O N U. So, you will notice that along with addition of a nucleophile, you have managed to introduce a carbon monoxide into the molecule. So, Coleman's reagent turns out to be a useful reagent for synthesizing laboratory scale chemicals easily. I emphasize the fact that it is a laboratory reagent, not an industrial reagent because one has to use a very expensive material in order to generate the Coleman's reagent. So, you have to use sodium and refluxing dioxin in order to generate the Coleman's reagent. Now, let us take a look at an example where we have converted an organic bromide into a cyclic molecule using Coleman's reagent. Just to illustrate the chemistry that is behind this whole process, suppose you take this homo-alylic bromide, you have if the bromine was here, then we would have called it an allylic bromide, but this homo-alylic bromide you have an extra C H 2 group. If you treat that with the iron carbonylate species, you end up with this molecule, which is identical to this compound, although we have written arrow here. So, these two things are identical. I have written the carbon monoxide separately. I have indicated the carbon monoxide separately because we are now going to do a migratory insertion reaction. Now, because it is a super nucleophile, it did a simple SN 2 substitution on this carbon. So, first we did an SN 2 substitution on this carbon, got the iron attached to the organic moiety. Now, this is an anionic alkyl group, which is going to do a migration on to the carbon monoxide. So, if it does a migration to the carbon monoxide, you end up with an acyl species. This acyl species is also an anion. Remember, we started out with the di anion, F E C O 4 2 minus and we eliminated bromide. So, we are left with the mono anion that can still undergo another addition of carbon monoxide. This gives us back iron tetra carbonyl acyl species. Now, notice that this species now has got an acyl group and this is the negatively charged group that is there, which I have colored in red. That can undergo a migratory insertion. This time the migratory insertion happens on to an olefin. Earlier, we are doing just insertion reactions on carbon monoxides or in some cases, we have of course, done it on an alkyne. Now, here we are doing it on an alkene and as a result, the iron will be attached to the second carbon atom. So, this is carbon atom 1, this is carbon atom 2 and the iron will be found on the second carbon atom and the migrating group is found on the first carbon atom of the alkene. So, this species, which is a carbonate anion can in fact get get attached to a proton and then undergo a reductive elimination step. So, this is the last step is in fact a reductive elimination which will give you in this particular case a fairly simple molecule, but you can imagine the synthesis of a more complex structure, where pentanone is needed and that can be constructed with this chemistry with the Coleman's reagent. So, you have a series of migrations and insertion reaction of a carbon monoxide in order to generate this cyclic structure. That reaction can also be done with instead of having an alkyl bromide, it can also be done on an epoxide. Now, here I have shown you another metal, which is capable of carrying out this kind of chemistry and instead of the cobalt instead of the iron dianion. Now, we have a sodium cobaltate tetra carbonyl cobaltate, which is going to carry out this nucleophilic attack at this position and that opens up the ring as an anion, as an O minus species here. In this reaction, you have this neutral group carbon monoxide and an alkyl species, which I have marked and read for you. This alkyl species can migrate to one of the carbon monoxide, which is present on the cobaltate. If it does that and adds on a carbon monoxide, we have put we have included two steps here. One is the migration of the alkyl group and the other is addition of carbon monoxide. So, addition of carbon monoxide and migration gives you this new compound, which has got O minus attached to an acyl moiety, which is attached to the cobaltate. Now, you can do an alcoholysis using R O H, which means you will have the R O H attacking in this position and liberation of C O C O 4 minus and this of course, gives you gives you the ester directly, an ester and an alcohol directly. So, this tells you how useful this reaction can be. You have attack of nucleophilic organometallic agent on epoxides, halides and esters and so on. So, up to now we have not considered the coupling of two neutral ligands. In the subsequent reaction, we are going to look at how two neutral ligands can be coupled in an oxidative fashion. Now, just to remind you, this reaction is not something new. We have discussed the coupling of two acetylene fragments on a cobalt cobalt system, cobalt 1 to give us a cobalt 3 oxidatively coupled cobalt 3 species. So, this is going to be a very similar reaction. So, what we have is an oxidative coupling reaction and here is a reaction, which is called the Poisson-Kahn reaction, where an acetylene, an alkene and carbon monoxide are mixed together in order to generate a cyclopentenone. The previous reaction in one of the previous reactions, we considered the generation of a pentanone and here the saturated form, here it is a cyclopentenone that is generated in this reaction. The important thing is that it is a single step, single step one pot reaction that is carried out with carbon monoxide, dichobold octacarbonyl and two organic compounds. One is an alkene, the other is an alkyne and you end up converting all three species in one step into a cyclic pentenone and that is what we have here. In order to just show you how the reaction can be done with more complex molecules, I have shown you norbonine here as an alkene. So, because it is a dinuclear species, we can imagine the dinuclear species CO 2, CO 8 losing two carbon monoxides and complexing with the alkene and the alkyne, one cobalt complexing to an alkene, the other complexing to an alkyne and if you do an oxidative coupling now, now you have a cobalt zero species to start with and after the oxidative coupling, each of these metal centers would increase the oxidation state by one unit and you would get a cobalt one cluster where two cobalt one centers are adjacent to one another and this dimetallacyclohexene, which is what we have here can introduce or can insert a carbon monoxide. So, this can insert a carbon monoxide plus CO that can give you new species, which is actually a heptanone, a dimetalloheptanone, which can reductively eliminate. Now, reductive elimination gives us this complex structure, which has been stitched together in the coordination sphere of the metal. So, you can see how fairly complex reactions can be carried out in a one part reaction. Now, it is been possible to carry out this reaction because only because the metal is capable of changing its oxidation state to plus one and also because the two metal centers are close enough together to stitch together these two dissimilar fragments. Now, it has been shown for in the case of acetylenes, which has substituted with only one in one one side that they can be combined with simple acetylene to give you an aromatic ring. So, this aromatic ring can also be generated in the presence of cobalt di carbonyl and in this case it people have been fortunate to isolate and characterize the intermediate that is being formed and this is called a flyover complex. This flyover complex is basically having the three acetylene moieties attached together on the di cobalt species. The di cobalt species is coordinated to three allylic fragments, two allylic fragments with three carbons each and you can see that this R group and this R group are coming from the two acetylenes, which we have used and the central acetylene is we will mark it here with a different color. So, that you can see it. So, this is the central acetylene that has been added and that is the central acetylene that we have added and the two other substituted acetylenes are bearing the R groups and they are attached in such a way that they are as far away as possible from the central cobalt cluster. So, they will not be sterically congested, but now reductive elimination from this di allyl unit generates the arene selectively. So, you can combine alkenes and alkynes or two alkynes together along with carbon monoxide. In fact, if you treat iron pentacarbonyl with acetylene, simple acetylene if you treat it with simple acetylene and carbon monoxide it is possible to achieve the synthesis of a quinone, which is now coordinated to iron tricarbonyl. So, you can generate this molecule in the coordination sphere simply by stitching together two acetylenes and carbon monoxide, two insertions oxidative coupling of two acetylenes together and sequential insertions of carbon monoxide will give you this quinone, which is coordinated to iron tricarbonyl. So, we will end with this last example, which is the Dutz reaction. The Dutz reaction involves the combination of vinyl carbene and an acetylene. Vinyl carbene acetylene can be combined together along with carbon monoxide to give you this fairly complicated ring structure. This is a good example, which tells you that very complex chemistries can be carried out in the coordination sphere of the metal atom. So, we have a mechanism of the Dutz reaction, which is described here. We have acetylene which is coordinated to the chromium in such a fashion that you end up with a reaction, which is very similar to what happened in the metal carbene metathesis chemistry. So, you have the formation of cyclobutene, where there is a chromium on the cyclobutene ring. This now inserts carbon monoxide. That is a key step and that is indicated in green here. The inserted carbon monoxide comes from the coordination sphere of the metal. The added carbon monoxide just fills in vacant coordination sphere. Subsequently, we can do an allylic shift of this group, which is attached here because it is in an allylic position. So, you can see this 1, 2 and 3. That will give you this chromium tetra carbonyl compound, which will result in the formation of this di phenol. On one side, you would have ether because you started out with the ether here. So, that is the group, which is present here. So, these are extremely useful reactions. I will end with a di carbonylation, which was discovered by Periasami. That is again with iron. That can also lead to very interesting molecules, which are pictured here, cyclobutene, diones and so on. The double carbonylation is a reaction, which has rarely been observed. It is a very unique reaction that has been discovered, which use of simple iron pentacarbonyl and cuprous chloride as a catalyst. The reaction can be modified to give a variety of substrates, which are pictured here. One example, one possible intermediate is what I have shown you on this particular screen, where iron, because it can dimerize, it can give you these very interesting species, where two iron centers can be coordinated to the acetylene and sequentially insert carbon monoxide. So, the possible insertion mechanism for the double carbonylation is shown here in the last slide. So, the possibility is endless and anionic species like H, R and A R can insert carbon monoxide. You can have a huge library of reactions built on the insertion of carbon monoxide.