Lecture Catalytic Organometallics 2 - 09.04.14
CC Attribution 3.0 Germany:
You are free to use, adapt and copy, distribute and transmit the work or content in adapted or unchanged form for any legal purpose as long as the work is attributed to the author in the manner specified by the author or licensor.
Dyker, Gerald
Basics of Transition Metal Catalysis
 Welcome to the second part of catalytic organometallics. Subject of today are the basics of transition metal catalysis. And we will start with the electron configuration of, well, generally, catalytically active complexes. In the main group chemistry, we have the 8 electron rule, which means we have the electron configuration of, we try to adopt the electron configuration of inert noble gas. Similarly, in transition metal catalysis, we have the 18 electron rule. So, let's start with an example. Palladium in the oxidation state 0. Well, palladium is the element number 46. And as the electron configuration, we have the electron configuration of krypton, and additionally, fully occupied d orbitals, and 4 d orbitals, and mt5s orbital, and the 3 5p orbitals also empty. To get to the electron configuration of xenon, we of course need two electrons for filling up the 5s orbital, and another 6 electrons for filling up the 3p orbitals. Well, then we have the electron configuration of xenon, and well, this must not necessarily be very stable, but it is clearly saturated. Well, okay, so for palladium, this means for palladium 0, this means, well, there is a rather common complex with phosphines, triphenylphosphines, well, with four equivalents of triphenylphosphine, we get tetraquist triphenylphosphine palladium, which is regarded as a very useful catalyst in palladium-catalyzed reactions. Well, although it is a saturated complex, co-ordinatively saturated, so how do we have to count to notice that this is co-ordinatively saturated? Well, let's start with a table. A ligand, a phosphine ligand, our three substituents has the charge of 0, but with the three electron pair at the phosphine, we have two donor electrons, which are donated to the formation of the complex. Oxidation state 0 for phosphine units makes overall eight electrons, that means this complex has, then the configuration, electron configuration of xenon is co-ordinatively saturated, and since this is then saturated, well, it can't interact, for instance, with all of them, because it is saturated. Therefore, this means this is certainly not the catalytically active species. Well, but no problem. Of course, equilibrium is in solution with the palladium with only three phosphine ligands, a 16 electron complex, and the one with just only two phosphine ligands, a 14 electron complex, which is indeed regarded as the active catalytic species, for instance, in the famous Heck reaction. So, let's have a look at other ligands, an olefin, where the metal is coordinated to the central electron density. This would be called as an etter 2, alkyne complex. You can also call it de-hapto, it's another word for it, de-hapto, di-hapto complex. The charge, again, is zero, and it also delivers two electrons. The same is, of course, for the similar alkyne complex, etter 2 alkyne. And now compare it with a complex like this. Here we have an ethanol substituent, or a vinyl substituent, or even alkyne or aryl metal. Well, in all these cases, the ligand is regarded to have the charge minus one, and it delivers two electrons. Other interesting examples, well, just to add, of course, the same is true for halides, like chloride, bromide, iodide, or other substituent acetate, for instance. Also minus one, and here, delivering two donor electrons. Let's go on with this table. What about this one here? A cyclopentadienyl ligand. This is etter 5 cyclopentadienyl. The charge minus one, and it delivers, overall, six electrons. We also know complexes. Aryl, benzoic, aryl ring, is coordinated by a metal, for instance, chromium complexes, we got to know in stoichiometric organometallics. In this case, well, we have a charge of zero, and again, six electrons. Another interesting case, one of allyl complexes, this here is the etter 3 allyl complex, charge minus one, and overall, four electrons. Well, I should add that this is the general scheme for an etter 6 aryl complex. Well, let's have a look at this complex, this triphenylphosphine palladium dichloride. While the tetra-chis palladium, the tetra-chis triphenylphosphine palladium complex has a tetrahedral coordination sphere. In this case, it is a square planar. We have an oxidation state of plus 2, since we have the 2 minus 1 chloride substituents. Well, since this is a palladium 2 complex, and with the palladium 0 complex, we need 8 electrons to get the noble gas configuration. With palladium 2, we need 2 electrons more. That means we are in need of 10 electrons. All we have, overall, 4 substituents, all delivering 2 donor electrons. That means overall, this is a 16 electron complex, which is coordinatively unsaturated. That means that, for instance, this palladium complex could coordinate to an olefin, for instance. That then reaction could start. It could be already catalytically active. Another nice example. Such an intermediary complex we will discuss later on, maybe already next week. This intermediary complex has been trapped and stabilized by a phenylen ligand, the presence of phenylen. This can be isolated. This has been done by Professor Alan Kenty from the University of Tasmania in Hobart. He did a nice experiment with that. Let's see what kind of complex is that. It's also a 16 electron complex, coordinatively unsaturated. Minus 1, minus 1, that means this is the oxidation state of plus 2 of the palladium. Since we have four ligands delivering two electrons each, this is then a 16 electron complex. Coordinatively unsaturated, that means it can undergo an additional reaction. It reacts indeed with, for instance, methyl iodide. It adds methyl iodide. This complex is also in moderate conditions to be isolated. What kind of complex do we have here? Now we have four substituents, which are regarded as charged. Minus 1, that means this is the palladium 4 complex. Starting from palladium 0, it's in need of four electrons. Palladium 0 is in need of eight electrons. Palladium 4 in need of 12 electrons. We now have six substituents, all with two donor electrons. That means 12 will find. This is then co-ordinatively situated. That's how you can count that quite easily. Let's go on with key steps of transition metal catalysis. First of all, there is, of course, the ligand exchange. One transition metal plus a ligand, with a ligand there, and a ligand prime, another ligand is offered. Overall, in some kind of equilibrium, we will get to the other situation with the X-changed ligands. There are some similarities to, for instance, nucleophilic substitution. We have a co-ordinatively unsaturated complex of this. This is a general scheme. So, square planar, then here we have a trans ligand. This one cis, and this one also cis. This is a nucleophilic other ligand to be exchanged. This should be exchanged by the Y-ligand. It should be nucleophilic. This is co-ordinatively unsaturated, and then it just adds in. From a geometry, which is square planar, we will get to a situation of a square pyramidal geometry. Now, just a little change. The X-ligand moves down to a position that is located trans to the Y-ligand. This ligand just moves a little bit to this position. Then the geometry has changed to bi pyramidal. Now you can imagine that again a change could take place, that this substituent moves into the plane of those three L-ligands. Then we are again in a square pyramidal geometry. Finally, X leaves a complex, and it is clear that we will have then the initial situation, but with X, X changed with Y. You see that these ligand exchange reactions or substitution reactions, some kind of substitution reactions could proceed much more complicated as compared to what we are used to in chemistry substitution at carbon centres. You can imagine that those ligands, of course, have a significant influence on the reaction rate, especially the trans ligand is the one who is determining the reaction rate. By applying the right ligand, you can indeed trigger the reaction rate of such processes. In the case of a ligand exchange with a co-ordinateively saturated complex, for instance with this tetrahedral, nickel-tetra-carbonile complex, well, before you can exchange something, the one carbon monoxide has to leave the complex, and this is generally the slow reaction step, then you have a co-ordination site where another ligand can co-ordinate, and well, that second step is generally the fast reaction step. Alternatively, with certain ligands present, co-ordination sites can be opened by a reaction you sometimes call a slippage, for instance a ring slippage. Well, let's imagine we have an etter-3 allyl complex as we said already here in that table. We have a charge of minus 1 for the ligand and for donor electrons, and here we can sometimes observe an equilibrium with the etter-1 allyl complex. So again, still minus 1, but clearly only 2 donor electrons. Imagine that this one is with other ligands co-ordinatively saturated as maybe an 18-electron complex, then you have here the situation of an unsaturated 16-electron complex. Or cyclopentadilule minus 1, 6 electrons. From etter-5 cyclopentadilule complex to an etter-3 cyclopentadilule complex, minus 1, 4 electrons. This is the so-called ring slippage. Of course, thermodynamically disfavored since it loses its aromaticity through this step. Nevertheless, it can go on in equilibrium to the etter-1 complex. So here, etter-5, etter-3, etter-1, and here we have again minus 1 and 2 electrons. Another example, not a cyclopentadilule complex, but just a pentadilule complex of a manganese carbonyl. So here manganese has the oxidation state plus 1. It's a D6 complex, 18 electrons saturated. There's an equilibrium possible from etter-5 to etter-3. And now it is an unsaturated, co-inventively unsaturated manganese complex. If another ligand is offered, it can coordinate to the manganese. So, and if now in equilibrium a carbon monoxide leaves the complex, then it can return to the etter-5 situation. Now with only two carbon monoxide ligands, but in addition with the ligand, which has been introduced by that ligand exchange. So after ligand exchange reactions, let's discuss reactions which are called oxidative addition and its reverse reaction, retro reaction, reductive elimination. Generally scheme, a metal in the oxidation state N, reacts with a molecule AB and the oxidative addition will lead to a complex with A and B as ligands or substitutes at the metal and the metal has changed to the oxidation state N plus 2. So and the retro reaction is the other way around and it's called reductive elimination. So generally rather electron rich transition metal complexes, those for instance with phosphine ligands, react quite nicely with carbon halide bonds under oxidative addition reaction. If you have carbon monoxide or cyanide as ligands, then this diminishes the electron density and hinders oxidative addition reactions of that kind. On the other hand, electron deficient complexes might react with more electron rich single bonds, for instance carbon hydrogen bonds. Well, let's have a look for some examples and the mechanisms which can occur for oxidative addition reactions. For instance, the SN2 type oxidative addition. Let's assume such as square planar complex with methyl iodide. It should be the slow reaction step just adding the methyl iodide via a nucleophilic substitution SN2 type process. So and the fast reaction step should be the next one. So where the iodide then also coordinates to the metal. Well actually we already have discussed an example for that. Well it is the oxidative addition from a palladium 2 complex to a palladium 4 complex as reported by Alan Canty. So this is SN2 type. Another type of oxidative addition once again with the general scheme. So these brackets here symbolize that there is of course not a naked metal. There are already ligands and substitutes. So it reacts with the reagent AB and in equilibrium that unsaturated complex might coordinate to the electron density of the single bond between A and B. This is called an agostic interaction. And it often leads then via a 2 electron 3 center bonded system to the oxidative addition product. So that is the next one. One example which has been studied in detail is a tungsten complex. So what kind of geometry does this drawing symbolize? Well those 4 substituents, well ligands are in a plane and with this one it is square pyramidal. If molecular hydrogen is offered then that molecular hydrogen can be coordinated to the tungsten. And indeed this dihydrogen. Complex dihydrogen complex was isolated at low temperatures and was analyzed by X-ray diffraction. And it was found that we have a bond length between those two hydrogen atoms of 0.82 angstroms compared to 0.74 for the molecular hydrogen. There is a further equilibrium to the complex where the hydrogens are clearly separated. Well I didn't find the distance for that case. However this is then not a dihydrogen complex anymore but a dihydride complex. And it should be clear for everyone that these equilibriums are important for hydrogenation reactions under homogeneous catalytic conditions. Similarly the oxidative addition of palladium 0 to carbon halide bonds where the carbon is sp2 hybridized. In these cases we cannot assume some SN2 kind process. As you know we don't have SN2 reactions also in the normal carbon chemistry when a carbon is sp2 hybridized. We have always addition elimination processes. Palladium 0 with these four ligands could be for phosphine ligands as already discussed earlier. 14 electron complex L2 palladium which is regarded to be the one that is catalytically active for the oxidative addition step. So an arrow halide then it is assumed that also in this case we have three centered system. And finally from palladium 0 we have changed. Well 14 electron palladium 0 complex to a palladium 2 complex still co-ordinatively unsaturated since this is a 16 electron complex. So these will be important as intermediates and we will discuss the head reaction in the next lecture. Other mechanisms which might occur for oxidative addition processes are ionic pathways for instance in cases where you have cat ionic transition metal complexes which are very often highly reactive because they are so electron deficient. And one should keep in mind that also radical reaction pathways can occur and have been indeed proven for some cases. So next subject are insertion reactions. For instance insertion of carbon monoxide. Carbon monoxide is offered to a co-ordinatively unsaturated complex with a substituent R, maybe an alkyl or aro substituent. So it will co-ordinate to the metal. It could insert into the carbon metal bond forming such an assail complex. And indeed this reaction it is an equilibrium since it is reversible. Similarly the insertion of an olefin and that's almost the same as the insertion of an alkyl. An olefin plus again such a complex with a substituent R. You see the mechanism is principle all the same. The olefin is co-ordinated at the metal and the insertion reaction takes place. So in this case that insertion reaction we would call a carbometallation if R is indeed a carbon-centered substituent. And this is also a reversible process. Well if R just is a hydride then it's a hydro-metallation and this as you know is also reversible process as you have learned for the hydro-peration. And this type of process well generally is a concerted one which proceeds in a Zern fashion. Let's assume we have hydrogens at the better position. So then for a better hydride elimination this carbon-carbon bond should rotate until a hydrogen is in a Zern-Perry planar position. And then a concerted better hydrogen elimination, a better hydride elimination. And occur. The first step with the metal still coordinated to the olefin while the re-addition could occur and so on. There's also a typical process which we will discuss again in the next session with the HEC reaction of course. So what remains to be mentioned that is the trans-metallation. Of course you already know the trans-metallation as a typical step of in stoichiometric organometallics. So we have some organometallic compound and a transition metal complex with a substituent X. Most of the time halide. And this would be then the trans-metallation where M' in brackets symbolizes a transition metal complex. A complexated transition metal and M' can be lithium, magnesium, zinc, boron, tin, zirconium, silicon and some more. So with this overview we have discussed today. We have all the instruments we need for discussing transition metal catalyzed reactions in details. And as I said we will start then with the HEC reaction on next Tuesday. Thank you for listening. See you next week.
 Welcome to the second part of catalytic organometallics.  Subject of today are the basics of transition metal catalysis.  And we will start with the electron configuration of,  well, generally, catalytically active complexes.  In the main group chemistry, we have the 8 electron rule,  which means we have the electron configuration of,  we try to adopt the electron configuration of inert noble gas.  Similarly, in transition metal catalysis, we have the 18 electron rule.  So, let's start with an example.  Palladium in the oxidation state 0.  Well, palladium is the element number 46.  And as the electron configuration,  we have the electron configuration of krypton,  and additionally, fully occupied d orbitals,  and 4 d orbitals, and mt5s orbital,  and the 3 5p orbitals also empty.  To get to the electron configuration of xenon,  we of course need two electrons for filling up the 5s orbital,  and another 6 electrons for filling up the 3p orbitals.  Well, then we have the electron configuration of xenon,  and well, this must not necessarily be very stable,  but it is clearly saturated.  Well, okay, so for palladium,  this means for palladium 0, this means,  well, there is a rather common complex with phosphines,  triphenylphosphines,  well, with four equivalents of triphenylphosphine,  we get tetraquist triphenylphosphine palladium,  which is regarded as a very useful catalyst  in palladium-catalyzed reactions.  Well, although it is a saturated complex,  co-ordinatively saturated,  so how do we have to count to notice that this is co-ordinatively saturated?  Well, let's start with a table.  A ligand, a phosphine ligand,  our three substituents has the charge of 0,  but with the three electron pair at the phosphine,  we have two donor electrons,  which are donated to the formation of the complex.  Oxidation state 0 for phosphine units  makes overall eight electrons,  that means this complex has,  then the configuration, electron configuration of xenon  is co-ordinatively saturated,  and since this is then saturated,  well, it can't interact, for instance, with all of them,  because it is saturated.  Therefore, this means this is certainly not  the catalytically active species.  Well, but no problem.  Of course, equilibrium is in solution  with the palladium  with only three phosphine ligands,  a 16 electron complex,  and the one with just only two phosphine ligands,  a 14 electron complex,  which is indeed regarded as the active catalytic species,  for instance, in the famous Heck reaction.  So, let's have a look at other ligands,  an olefin,  where the metal is coordinated to the central electron density.  This would be called as an etter 2, alkyne complex.  You can also call it de-hapto,  it's another word for it, de-hapto, di-hapto complex.  The charge, again, is zero,  and it also delivers two electrons.  The same is, of course, for the similar alkyne complex,  etter 2 alkyne.  And now compare it with a complex like this.  Here we have an ethanol substituent,  or a vinyl substituent,  or even alkyne or aryl metal.  Well, in all these cases,  the ligand is regarded to have the charge minus one,  and it delivers two electrons.  Other interesting examples,  well, just to add, of course,  the same is true for halides,  like chloride, bromide, iodide,  or other substituent acetate, for instance.  Also minus one,  and here, delivering two donor electrons.  Let's go on with this table.  What about this one here?  A cyclopentadienyl ligand.  This is etter 5 cyclopentadienyl.  The charge minus one,  and it delivers, overall, six electrons.  We also know complexes.  Aryl, benzoic, aryl ring,  is coordinated by a metal,  for instance, chromium complexes,  we got to know in stoichiometric organometallics.  In this case,  well, we have a charge of zero,  and again, six electrons.  Another interesting case,  one of allyl complexes,  this here is the etter 3 allyl complex,  charge minus one,  and overall, four electrons.  Well, I should add that this is the general scheme  for an etter 6 aryl complex.  Well, let's have a look at this complex,  this triphenylphosphine palladium dichloride.  While the tetra-chis palladium,  the tetra-chis triphenylphosphine palladium complex  has a tetrahedral coordination sphere.  In this case, it is a square planar.  We have an oxidation state of plus 2,  since we have the 2 minus 1 chloride substituents.  Well, since this is a palladium 2 complex,  and with the palladium 0 complex,  we need 8 electrons to get the noble gas configuration.  With palladium 2, we need 2 electrons more.  That means we are in need of 10 electrons.  All we have, overall, 4 substituents,  all delivering 2 donor electrons.  That means overall, this is a 16 electron complex,  which is coordinatively unsaturated.  That means that, for instance,  this palladium complex could coordinate to an olefin, for instance.  That then reaction could start.  It could be already catalytically active.  Another nice example.  Such an intermediary complex we will discuss later on,  maybe already next week.  This intermediary complex has been trapped and stabilized  by a phenylen ligand, the presence of phenylen.  This can be isolated.  This has been done by Professor Alan Kenty  from the University of Tasmania in Hobart.  He did a nice experiment with that.  Let's see what kind of complex is that.  It's also a 16 electron complex,  coordinatively unsaturated.  Minus 1, minus 1, that means this is the oxidation state  of plus 2 of the palladium.  Since we have four ligands delivering two electrons each,  this is then a 16 electron complex.  Coordinatively unsaturated, that means it can undergo  an additional reaction.  It reacts indeed with, for instance, methyl iodide.  It adds methyl iodide.  This complex is also in moderate conditions to be isolated.  What kind of complex do we have here?  Now we have four substituents, which are regarded as charged.  Minus 1, that means this is the palladium 4 complex.  Starting from palladium 0, it's in need of four electrons.  Palladium 0 is in need of eight electrons.  Palladium 4 in need of 12 electrons.  We now have six substituents, all with two donor electrons.  That means 12 will find.  This is then co-ordinatively situated.  That's how you can count that quite easily.  Let's go on with key steps of transition metal catalysis.  First of all, there is, of course, the ligand exchange.  One transition metal plus a ligand, with a ligand there,  and a ligand prime, another ligand is offered.  Overall, in some kind of equilibrium,  we will get to the other situation with the X-changed ligands.  There are some similarities to, for instance, nucleophilic substitution.  We have a co-ordinatively unsaturated complex of this.  This is a general scheme.  So, square planar, then here we have a trans ligand.  This one cis, and this one also cis.  This is a nucleophilic other ligand to be exchanged.  This should be exchanged by the Y-ligand.  It should be nucleophilic.  This is co-ordinatively unsaturated, and then it just adds in.  From a geometry, which is square planar,  we will get to a situation of a square pyramidal geometry.  Now, just a little change.  The X-ligand moves down to a position  that is located trans to the Y-ligand.  This ligand just moves a little bit to this position.  Then the geometry has changed to bi pyramidal.  Now you can imagine that again a change could take place, that this substituent moves into the plane of those three L-ligands.  Then we are again in a square pyramidal geometry.  Finally, X leaves a complex, and it is clear that we will have then the initial situation, but with X, X changed with Y.  You see that these ligand exchange reactions or substitution reactions,  some kind of substitution reactions could proceed much more complicated  as compared to what we are used to in chemistry substitution at carbon centres.  You can imagine that those ligands, of course, have a significant influence on the reaction rate,  especially the trans ligand is the one who is determining the reaction rate.  By applying the right ligand, you can indeed trigger the reaction rate of such processes.  In the case of a ligand exchange with a co-ordinateively saturated complex,  for instance with this tetrahedral, nickel-tetra-carbonile complex,  well, before you can exchange something, the one carbon monoxide has to leave the complex,  and this is generally the slow reaction step,  then you have a co-ordination site where another ligand can co-ordinate,  and well, that second step is generally the fast reaction step.  Alternatively, with certain ligands present,  co-ordination sites can be opened by a reaction you sometimes call a slippage,  for instance a ring slippage.  Well, let's imagine we have an etter-3 allyl complex  as we said already here in that table.  We have a charge of minus 1 for the ligand and for donor electrons,  and here we can sometimes observe an equilibrium with the etter-1 allyl complex.  So again, still minus 1, but clearly only 2 donor electrons.  Imagine that this one is with other ligands co-ordinatively saturated  as maybe an 18-electron complex, then you have here the situation of an unsaturated 16-electron complex.  Or cyclopentadilule minus 1, 6 electrons.  From etter-5 cyclopentadilule complex to an etter-3 cyclopentadilule complex,  minus 1, 4 electrons. This is the so-called ring slippage.  Of course, thermodynamically disfavored since it loses its aromaticity through this step.  Nevertheless, it can go on in equilibrium to the etter-1 complex.  So here, etter-5, etter-3, etter-1, and here we have again minus 1 and 2 electrons.  Another example, not a cyclopentadilule complex, but just a pentadilule complex  of a manganese carbonyl.  So here manganese has the oxidation state plus 1. It's a D6 complex, 18 electrons saturated.  There's an equilibrium possible from etter-5 to etter-3.  And now it is an unsaturated, co-inventively unsaturated manganese complex.  If another ligand is offered, it can coordinate to the manganese.  So, and if now in equilibrium a carbon monoxide leaves the complex,  then it can return to the etter-5 situation.  Now with only two carbon monoxide ligands, but in addition with the ligand,  which has been introduced by that ligand exchange.  So after ligand exchange reactions, let's discuss reactions which are called oxidative addition  and its reverse reaction, retro reaction, reductive elimination.  Generally scheme, a metal in the oxidation state N,  reacts with a molecule AB and the oxidative addition will lead to a complex with A and B as ligands  or substitutes at the metal and the metal has changed to the oxidation state N plus 2.  So and the retro reaction is the other way around and it's called reductive elimination.  So generally rather electron rich transition metal complexes,  those for instance with phosphine ligands, react quite nicely with carbon halide bonds under oxidative addition reaction.  If you have carbon monoxide or cyanide as ligands, then this diminishes the electron density  and hinders oxidative addition reactions of that kind.  On the other hand, electron deficient complexes might react with more electron rich single bonds,  for instance carbon hydrogen bonds.  Well, let's have a look for some examples and the mechanisms which can occur for oxidative addition reactions.  For instance, the SN2 type oxidative addition.  Let's assume such as square planar complex with methyl iodide.  It should be the slow reaction step just adding the methyl iodide  via a nucleophilic substitution SN2 type process.  So and the fast reaction step should be the next one.  So where the iodide then also coordinates to the metal.  Well actually we already have discussed an example for that.  Well it is the oxidative addition from a palladium 2 complex to a palladium 4 complex as reported by Alan Canty.  So this is SN2 type.  Another type of oxidative addition once again with the general scheme.  So these brackets here symbolize that there is of course not a naked metal.  There are already ligands and substitutes.  So it reacts with the reagent AB and in equilibrium that unsaturated complex might coordinate to the electron density of the single bond between A and B.  This is called an agostic interaction.  And it often leads then via a 2 electron 3 center bonded system to the oxidative addition product.  So that is the next one.  One example which has been studied in detail is a tungsten complex.  So what kind of geometry does this drawing symbolize?  Well those 4 substituents, well ligands are in a plane and with this one it is square pyramidal.  If molecular hydrogen is offered then that molecular hydrogen can be coordinated to the tungsten.  And indeed this dihydrogen.  Complex dihydrogen complex was isolated at low temperatures and was analyzed by X-ray diffraction.  And it was found that we have a bond length between those two hydrogen atoms of 0.82 angstroms compared to 0.74 for the molecular hydrogen.  There is a further equilibrium to the complex where the hydrogens are clearly separated.  Well I didn't find the distance for that case.  However this is then not a dihydrogen complex anymore but a dihydride complex.  And it should be clear for everyone that these equilibriums are important for hydrogenation reactions under homogeneous catalytic conditions.  Similarly the oxidative addition of palladium 0 to carbon halide bonds where the carbon is sp2 hybridized.  In these cases we cannot assume some SN2 kind process.  As you know we don't have SN2 reactions also in the normal carbon chemistry when a carbon is sp2 hybridized.  We have always addition elimination processes.  Palladium 0 with these four ligands could be for phosphine ligands as already discussed earlier.  14 electron complex L2 palladium which is regarded to be the one that is catalytically active for the oxidative addition step.  So an arrow halide then it is assumed that also in this case we have three centered system.  And finally from palladium 0 we have changed.  Well 14 electron palladium 0 complex to a palladium 2 complex still co-ordinatively unsaturated since this is a 16 electron complex.  So these will be important as intermediates and we will discuss the head reaction in the next lecture.  Other mechanisms which might occur for oxidative addition processes are ionic pathways for instance in cases where you have cat ionic transition metal complexes which are very often highly reactive because they are so electron deficient.  And one should keep in mind that also radical reaction pathways can occur and have been indeed proven for some cases.  So next subject are insertion reactions.  For instance insertion of carbon monoxide.  Carbon monoxide is offered to a co-ordinatively unsaturated complex with a substituent R, maybe an alkyl or aro substituent.  So it will co-ordinate to the metal.  It could insert into the carbon metal bond forming such an assail complex.  And indeed this reaction it is an equilibrium since it is reversible.  Similarly the insertion of an olefin and that's almost the same as the insertion of an alkyl.  An olefin plus again such a complex with a substituent R.  You see the mechanism is principle all the same.  The olefin is co-ordinated at the metal and the insertion reaction takes place.  So in this case that insertion reaction we would call a carbometallation if R is indeed a carbon-centered substituent.  And this is also a reversible process.  Well if R just is a hydride then it's a hydro-metallation and this as you know is also reversible process as you have learned for the hydro-peration.  And this type of process well generally is a concerted one which proceeds in a Zern fashion.  Let's assume we have hydrogens at the better position.  So then for a better hydride elimination this carbon-carbon bond should rotate until a hydrogen is in a Zern-Perry planar position.  And then a concerted better hydrogen elimination, a better hydride elimination.  And occur.  The first step with the metal still coordinated to the olefin while the re-addition could occur and so on.  There's also a typical process which we will discuss again in the next session with the HEC reaction of course.  So what remains to be mentioned that is the trans-metallation.  Of course you already know the trans-metallation as a typical step of in stoichiometric organometallics.  So we have some organometallic compound and a transition metal complex with a substituent X.  Most of the time halide.  And this would be then the trans-metallation where M' in brackets symbolizes a transition metal complex.  A complexated transition metal and M' can be lithium, magnesium, zinc, boron, tin, zirconium, silicon and some more.  So with this overview we have discussed today.  We have all the instruments we need for discussing transition metal catalyzed reactions in details.  And as I said we will start then with the HEC reaction on next Tuesday.  Thank you for listening. See you next week.
Lecture_Catalytic_Organometallics_2_Prof_G_Dyker_090414_wUvU090jHAI_0_5339.png Lecture_Catalytic_Organometallics_2_Prof_G_Dyker_090414_wUvU090jHAI_5340_5843.png Lecture_Catalytic_Organometallics_2_Prof_G_Dyker_090414_wUvU090jHAI_5844_5939.png Lecture_Catalytic_Organometallics_2_Prof_G_Dyker_090414_wUvU090jHAI_5940_6335.png Lecture_Catalytic_Organometallics_2_Prof_G_Dyker_090414_wUvU090jHAI_6336_6431.png Lecture_Catalytic_Organometallics_2_Prof_G_Dyker_090414_wUvU090jHAI_6432_6575.png Lecture_Catalytic_Organometallics_2_Prof_G_Dyker_090414_wUvU090jHAI_6576_6995.png Lecture_Catalytic_Organometallics_2_Prof_G_Dyker_090414_wUvU090jHAI_6996_8267.png Lecture_Catalytic_Organometallics_2_Prof_G_Dyker_090414_wUvU090jHAI_8268_11135.png Lecture_Catalytic_Organometallics_2_Prof_G_Dyker_090414_wUvU090jHAI_11136_11339.png Lecture_Catalytic_Organometallics_2_Prof_G_Dyker_090414_wUvU090jHAI_11340_12923.png Lecture_Catalytic_Organometallics_2_Prof_G_Dyker_090414_wUvU090jHAI_12924_14075.png Lecture_Catalytic_Organometallics_2_Prof_G_Dyker_090414_wUvU090jHAI_14076_14375.png Lecture_Catalytic_Organometallics_2_Prof_G_Dyker_090414_wUvU090jHAI_14376_15251.png Lecture_Catalytic_Organometallics_2_Prof_G_Dyker_090414_wUvU090jHAI_15252_15899.png Lecture_Catalytic_Organometallics_2_Prof_G_Dyker_090414_wUvU090jHAI_15900_17543.png Lecture_Catalytic_Organometallics_2_Prof_G_Dyker_090414_wUvU090jHAI_17544_26939.png Lecture_Catalytic_Organometallics_2_Prof_G_Dyker_090414_wUvU090jHAI_26940_27503.png Lecture_Catalytic_Organometallics_2_Prof_G_Dyker_090414_wUvU090jHAI_27504_27683.png Lecture_Catalytic_Organometallics_2_Prof_G_Dyker_090414_wUvU090jHAI_27684_27743.png Lecture_Catalytic_Organometallics_2_Prof_G_Dyker_090414_wUvU090jHAI_27744_29351.png Lecture_Catalytic_Organometallics_2_Prof_G_Dyker_090414_wUvU090jHAI_29352_32939.png Lecture_Catalytic_Organometallics_2_Prof_G_Dyker_090414_wUvU090jHAI_32940_34187.png Lecture_Catalytic_Organometallics_2_Prof_G_Dyker_090414_wUvU090jHAI_34188_35903.png Lecture_Catalytic_Organometallics_2_Prof_G_Dyker_090414_wUvU090jHAI_35904_36863.png Lecture_Catalytic_Organometallics_2_Prof_G_Dyker_090414_wUvU090jHAI_36864_37667.png Lecture_Catalytic_Organometallics_2_Prof_G_Dyker_090414_wUvU090jHAI_37668_38783.png Lecture_Catalytic_Organometallics_2_Prof_G_Dyker_090414_wUvU090jHAI_38784_38903.png Lecture_Catalytic_Organometallics_2_Prof_G_Dyker_090414_wUvU090jHAI_38904_39515.png Lecture_Catalytic_Organometallics_2_Prof_G_Dyker_090414_wUvU090jHAI_39516_39887.png Lecture_Catalytic_Organometallics_2_Prof_G_Dyker_090414_wUvU090jHAI_39888_40055.png Lecture_Catalytic_Organometallics_2_Prof_G_Dyker_090414_wUvU090jHAI_40056_41567.png Lecture_Catalytic_Organometallics_2_Prof_G_Dyker_090414_wUvU090jHAI_41568_41627.png Lecture_Catalytic_Organometallics_2_Prof_G_Dyker_090414_wUvU090jHAI_41628_43103.png Lecture_Catalytic_Organometallics_2_Prof_G_Dyker_090414_wUvU090jHAI_43104_44783.png Lecture_Catalytic_Organometallics_2_Prof_G_Dyker_090414_wUvU090jHAI_44784_47627.png Lecture_Catalytic_Organometallics_2_Prof_G_Dyker_090414_wUvU090jHAI_47628_49043.png Lecture_Catalytic_Organometallics_2_Prof_G_Dyker_090414_wUvU090jHAI_49044_49103.png Lecture_Catalytic_Organometallics_2_Prof_G_Dyker_090414_wUvU090jHAI_49104_49499.png Lecture_Catalytic_Organometallics_2_Prof_G_Dyker_090414_wUvU090jHAI_49500_49979.png Lecture_Catalytic_Organometallics_2_Prof_G_Dyker_090414_wUvU090jHAI_49980_50327.png Lecture_Catalytic_Organometallics_2_Prof_G_Dyker_090414_wUvU090jHAI_50328_59303.png Lecture_Catalytic_Organometallics_2_Prof_G_Dyker_090414_wUvU090jHAI_59304_62255.png Lecture_Catalytic_Organometallics_2_Prof_G_Dyker_090414_wUvU090jHAI_62256_64007.png Lecture_Catalytic_Organometallics_2_Prof_G_Dyker_090414_wUvU090jHAI_64008_64523.png Lecture_Catalytic_Organometallics_2_Prof_G_Dyker_090414_wUvU090jHAI_64524_66407.png Lecture_Catalytic_Organometallics_2_Prof_G_Dyker_090414_wUvU090jHAI_66408_66467.png Lecture_Catalytic_Organometallics_2_Prof_G_Dyker_090414_wUvU090jHAI_66468_68135.png Lecture_Catalytic_Organometallics_2_Prof_G_Dyker_090414_wUvU090jHAI_68136_70523.png Lecture_Catalytic_Organometallics_2_Prof_G_Dyker_090414_wUvU090jHAI_70524_71975.png Lecture_Catalytic_Organometallics_2_Prof_G_Dyker_090414_wUvU090jHAI_71976_72839.png Lecture_Catalytic_Organometallics_2_Prof_G_Dyker_090414_wUvU090jHAI_72840_73511.png Lecture_Catalytic_Organometallics_2_Prof_G_Dyker_090414_wUvU090jHAI_73512_74963.png Lecture_Catalytic_Organometallics_2_Prof_G_Dyker_090414_wUvU090jHAI_74964_75575.png Lecture_Catalytic_Organometallics_2_Prof_G_Dyker_090414_wUvU090jHAI_75576_76295.png Lecture_Catalytic_Organometallics_2_Prof_G_Dyker_090414_wUvU090jHAI_76296_80243.png Lecture_Catalytic_Organometallics_2_Prof_G_Dyker_090414_wUvU090jHAI_80244_80315.png Lecture_Catalytic_Organometallics_2_Prof_G_Dyker_090414_wUvU090jHAI_80316_80831.png Lecture_Catalytic_Organometallics_2_Prof_G_Dyker_090414_wUvU090jHAI_80832_81347.png Lecture_Catalytic_Organometallics_2_Prof_G_Dyker_090414_wUvU090jHAI_81348_81479.png Lecture_Catalytic_Organometallics_2_Prof_G_Dyker_090414_wUvU090jHAI_81480_81983.png Lecture_Catalytic_Organometallics_2_Prof_G_Dyker_090414_wUvU090jHAI_81984_83411.png