So, let's start, welcome to part 22 of stoichiometric organometallics, subject of today are transition metal carbonyl complexes in organic synthesis.

And we will start with so-called Fischer-Carbin complexes of chromium, they are easy synthesized,

nice method and the original method from Fischer is starting with chromium hexacarbonyl

and letting this react with a nucleophilic organometallic species, for instance here there's an example phenyl lithium, then already a carbene complex is formed and after an

alkylation very often this strange looking reagent with a three valent oxygen,

oxonium is applied the trimethyl oxonium tetrafluoro borate, better known as mer vine salt,

of course a very strong alkylating agent. Oh alkylation occurs and this is

a carbene complex of chromium with an rather electrophilic carbon center and carbene complexes of transition metals with electrophilic carbon center are called Fischer-Carbin complexes.

So as I said with an electrophilic carbon center, those with those carbene complexes

with a nucleophilic carbon center are called Schrock-Carbin complexes and well you know

already a few of those, for instance we saw that the discussion with olefination reagents like Tebbe reagent and Pitazis reagent, for instance here we have a more nucleophilic

carbon center and these are called then Schrock-Carbin complexes. But now back to the Fischer-Carbin complexes of chromium. They show a lot of interesting reactivities

and one especially marvelous process was introduced by Karl-Heinz Dutz

and it is since then called the Dutz reaction. You start the Dutz reaction

with this type of Fischer-Carbin complexes complex and let this react with an alkyne

with a small and a large substituent. One carbon monoxide unit is exchanged

for that alkyne and the next step is nicely explained by a two plus two cycloaddition

reaction. I know this is this is wrong. We've lost of course what one called carbon monoxide.

Next step you can understand as an electrocyclic ring opening. As you see this is a

multi-step process. The next step an insertion of one carbon monoxide unit occurs

forming a ketene which is still

complexated by the chrome carbonyl now with three carbon monoxide units at the chromium.

Next step can be explained by

an electrocyclization. A tautomerization here

leads to a re-aromatization and as you will notice the Dutz reaction is some kind of

annihilation reaction. Here in this case well if you have to start with a vinyl compound

um well you can form a single ring but here it is an acceleration reaction at at um yeah benzene ring system. Well you can get rid of the chromium here

de-complexation by um applying air or iron chloride or just carbon monoxide under pressure also de-complexates. Just one example of the

numerous applications which the Dutz reaction found is the synthesis of vitamin k. Here is the example vitamin k2. Well as an alkyne this still rather simple

alkyne was applied. With the Dutz reaction following this scheme we should get

to this type of product with a large substituent of the alkyne here and the smaller the CH3 group here on that side. Well just let us draw that.

And de-complexation with an oxidative work up for instance with silver oxide as

oxidizing agent there's of course also cheaper possibilities. You can oxidize until the oxidation state of the paraquinone and this is vitamin k2. And there are

other vitamins in the k family with for instance longer chains, um

um most of them have been synthesized by the Dutz group with yields better than 50 percent.

Well on the other hand chrome carbonyl complexes of aromatic systems have found the interest because of lots of synthetic applications these type of complexes offer.

Let's have a closer look at what is possible. So the chromium carbonyl complex for aromatic

system with well some substituent R and the second substituent here with hydrogen benzylic position and leaving group here at this position. So what opportunities does it offer? First interesting information is this is a chiral molecule a case of planar chirality.

The chromium sitting here is conservating chiral information can deliver the chiral information

for reactions we would then try to achieve at that system. This hydrogen has an enhanced acidity because these chromium aromatic complexes

stabilize especially anions in benzylic position and also radicals in benzylic position and also

cations. Therefore if you have a leaving group here it reacts must much faster in solvolysis reactions. Our next interesting fact is that these hydrogens sitting at the

chromium carbonyl benzene ring are more acidic can be much more easy deprotonate instance in DOM type reactions and very important is also that even

while electron rich looking systems with alkoxy groups become electron deficient by the complexation with the chromium carbonyl and well this reacts then under some kind of

nucleophilic attack. Okay so nucleophilic aromatic substitution becomes possible possible or nucleophilic addition reaction at such systems. Well let's have a look

for examples. So a chiral acetal has been formed and here we have chiral information

in the side chain and it is possible then forming a chromium complex starting with

a chromium hexacarbonyl. Typical conditions are 140 degrees bis-en-butyo-ether THF while in the pressure tube 62 percent of the chromium carbonyl complex was isolated

and this is formed highly diastereoselectively that means from the chiral center

chiral information is translated into planar chirality since if we then would hydrolyze the acetal ah sorry this is not a good example then

in this case we would then have an achiral complex again but imagine that we don't have you're missing one the foxy group then after hydrolysis of the acetal

we would get a chiral still a chiral chromium complex nevertheless here in this case and this has been done it's for instance easy to deprotonate either here or there

and please have in mind that both hydrogens are different we can selectively deprotonate one of them well if we lithiate here and then that this react for instance with

an aldehyde then a chiral center at basilic position would be formed and of course we could do that with high diastereoselectivity and after hydrolyzing of the acetal and

complexating we would have then the product with high enuncioselectivity here an example for that type of nucleophilic attack i was talking about

of course with the tms group and within the foxy group this has a high electron density density and nucleophiles wouldn't have a chance to attack that aromatic system

transforming that under the usual reaction conditions transforms this this to an electron

deficient system which readily reacts with nucleophiles for instance generated

uh from this propagolic tertiary butyl ester with lda in thf at uh minus 78 degrees we have the ester enolate this is then the nucleophile and the nucleophilic enolate will

attack from the side which is opposite to the chromium carbonyl so conjugated negative charge

is then in the ring system and after protonation and

de-complexating we should end up with this product well i hope it is clear that

it is synthesized with high diastereoselectivity but in this case racemic

because we didn't put in chiral information itself into this reaction sequence and of course this

is an interesting system highly functionalized chiral centers highly diastereoselective here in this system this diene system substituted from the foxy group and the tms group you could think about a lot of chemical transformations yeah well and

the yield 92 percent and the diastereoselectivity is better than 99.21

two more examples looking rather simple first with lda the isobutyl nitrile is deprotonated

reacts as the nucleophile it will attack that olefin cc bond formation to this position since the benzylic anion is stabilized by the aromatic system complexated by the chromium

carbonyl so and then in benzylic position we trap the anion with methyl iodide de-complexation

then we'll give this product with 65 percent yield and if we can provide this

as one enantiomer then we would get this enantioselectively and highly diastereoselectively

turning these two chiral centers deprotonating this ketone with sodium hydride

then calculating the methyl iodide will lead to this product with the

generated chiral center well this has been done with an 80 percent yield you could now

de-complexate hoping for moderate reaction conditions since in basic and in

uh acidic solution this chiral center of course would epimerize well you could for instance use this one for reducing the carbonyl group to the alcohol and then this is configurationally stable there and then you can de-complexate again lots of

opportunities which these type of systems offer let's have a look at iron atta5 complexes

these are cationic complexes of this type also interesting for introducing further

substituents by a nucleophilic attack therefore a bit reminding of

what we have seen with the chromium carbonyl complexes so one example for an application and at the same time for the synthetic value of this type of

iron carbonyl complexes and demonstrating how they are synthesized now from a group of Hans-Joachim Knolke so how do you get to this type of cationic complexes well

first step aromatic system was treated with lithium in liquid ammonia and in the presence

of an alcohol in that case it was tertiary butanol well as you know this

are typical conditions for the birch reduction well more often you use sodium but it also works with lithium secondly with dimethyl sulfate under basic reaction conditions the acid was

esterified birch reduction regularly is giving the cyclohexadiene with the olefins

not conjugated but actually this is not important in that case so the ester was

isolated with about 80 percent yield after these two steps now the iron pentacarbonyl in the

this n-butyl ether adds a higher boiling solvent 140 degrees 40 hours the olefins are summarized into conjugation and this iron complex is

iron carbonyl complex is isolated again in about 80 percent yield few steps more which are not

that interesting reducing the ester to the alcohol functionality and then

transformation of the alcohol functionality into a leaving group canonical group tested a lot in that case and finally for their purpose best was an esterification

with para-nitrobenzoic acid moiety well to get to that cationic pentadienyl

system we have to oxidize here and this is achieved with

our triphenyl carbenium tetrafluoro borate this is the electrophile

induces a one electron oxidation so grabs one electron radical cation which is subsequently deprotonated and gives rise for the type of reactive complex

we were looking for now we have with that benzoate para-nitrobenzoate a leaving group here that means an electrophilic center there

nucleophilic substitution could occur on the other hand this here this carbon is another electrophilic carbon i forgot to mention this works very well with 92 percent yield so treating

this with para-anisidine the nucleophilic aniline functionality reacts first

here now we have intermolecular this electrophilic carbon and one two three four five six

and one six relationship to this nucleophilic carbon simply an electrophilic aromatic substitution takes place connecting this carbon to that one so and the result

of this reaction step is this spirocyclic system this reaction works with 80 percent

yield and subsequently one can de-complexate this is easily possible with mild oxidizing agents for instance with anoxides like that last type of reaction new type of

reaction for today so imagine a reaction where you have an alkyne an olefin and carbon

monoxide the two center plus two center plus one center cyclization could give a cyclopentanone

would be a rather interesting reaction this type of reaction was found to be

possible if you don't apply just carbon monoxide well you could apply it with the correct catalyst but it was invented first stoichiometrically by introducing the carbon monoxide units as complex with this cobalt octa carbonyl one example

as alkyne this homopropagolic alcohol was applied the first reaction step the

this cobalt carbonyl loses two of the carbon monoxide units and adds to the alkyne forming a nice little cluster type molecule of this type you can isolate this type of

complexes doing chemistry with that and it was observed by porcelain and can't in

1973 than reported and since then called the porcelain can't reaction but these complexes

then react with olefins norbornene as an olefin with enhanced reactivity since this is a somewhat strained system resulting in this functionalized cyclopentanone

and this was done with 60 percent yield this type of reaction works very well intramolecularly for instance with this alkyl propagyl ether 75 percent yield of this

product was achieved so this type of substrate was later on also a test reagent for trying to get the porcelain can't reaction catalytically you can imagine you should

have some pressurized carbon monoxide present in principle it should work catalytically and after finding out that for instance triphenylphosphite as additional ligand

than with three atmospheres of carbon monoxide in dmf at 120 degrees then just three percent of

the bis cobalt octacarbonyl is sufficient for letting this run catalytically later on

all this was found in 90 93 may 94 so about 20 years after the initial porcelain can't report and well subsequently it was found out that not only cobalt is

able to induce this type of reaction later on catalyze this type of reaction but also titanium complexes ruthenium and rhodium complexes and meanwhile especially with

the possibility to have further chiral ligands at the metal catalyst there are also numerous examples of the enansio selective catalytic porcelain can't reaction

but as you see this is more our subject of next semester's lecture about catalytic organometallics

and therefore this is the end of this semester lecture thank you for listening see you next semester