Organemetallic Reactions and Catalysis (14)

8/13/2019 Organemetallic Reactions and Catalysis (14)

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Organometallic reactions


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14-1 Reactions involving gain or loss of ligands


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14-1-1 Ligand dissociation & substitution

CO dissociation


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Ligand dissociation & substitution(Dissociative mechanism)


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A more complicated case :

(I)


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(II)


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Although most CO substitution reactions proceed

primarily by adissociative mechanism, an

associative pathway is more likely for complexes oflarge metals (providing favorable sites for incoming

ligands to attack) & for reactions involving highlynucleophilic ligands.


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Dissociation of phosphine


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The larger the cone angle, the more rapidly the phosphine/phosphite is lost. The overall effect is substantial; i.e., the ratefor the most bulky ligand is more than4 orders of magnitudegreater than that for the least bulky ligand.


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For many dissociation reactions, the effect of

ligand crowdingmay be more important than

electronic effects in determining reaction rates.


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14-1-2 Oxidative addition (O.A.)

O.A. reactions involve an increase in both formal

O.S. & C.N. of the metal. They are among the

most important of organometallic reactions &essential steps in many catalytic processes. The

reverse type of reaction is designatedreductive

elimination (R.E.).

n+ (n+2)+


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Ir+ (d8) Ir3+ (d6)


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This means that a metal fragment of O.S. (of n) can

normally give an oxidative addition only if it alsohas a stable O.S. (of n+2), can tolerate an increase in

its C.N. by two, & can accept two more electrons.


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Certain MLn fragments are often consideredascarbene-like.


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The first step involves dissociation of CO to give a

4-coordinate iron(0) intermediate. In the secondstep, iron(0) is formally oxidized to iron(II) & the

C.N. expanded by the addition of two iodo ligands.


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Anionic ligands


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Cyclometallations

Oxidative addition &

reductive elimination

Oxidative addition


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14-1-3 Reductive elimination (R.E.)

RE reactions often involve elimination of molecules

such as :R-H, R-R, R-X, H-H(R, R : alkyl, aryl;X : halogen)

RE


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The rate of RE reactions are also

affected by ligand bulkiness.

bulkier


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14-1-4 Nucleophilic displacementLigand displacement reactions may be described

asnucleophilic substitutions, involving incoming

ligands as nucleophiles. Organometallic complexes,

especially those carrying negative charges, may

themselves behave as nucleophiles in displacementreactions.

nucleophile

+

displaces iodide.


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Na2Fe(CO)4 : Collmans reagent


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Co2(CO)8 + Na

Naphthalene

baseCO insertion


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14-2 Reactions involving modification of ligands

14-2-1 Insertion

1,1 insertion : both bonds to the inserted molecule

are made to the same atom in that molecule.

1,2 insertion : both bonds to the inserted molecule

are made to adjacent atoms in that molecule.


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14-2-2 Carbonyl insertion (alkyl migration)

1,1 insertion


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The insertion of CO into a M-C bond in alkyl

complexes is of particular interest in its potentialapplications to organic synthesis & catalysis.

3 plausible mechanisms have been suggested forthe reaction :

Mechanism 1 : CO insertionDirect insertion of CO into metal-carbon bond.


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Mechanism 2 : CO migration

Migration of CO to give intramolecular CO

insertion. This would yield a 5-coordinate

intermediate, with a vacant site available forattachment of an incoming CO.


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Mechanism 3 : Alkyl migration

In this case, the alkyl group would migrate,

rather than the CO, & attach itself to a COcis to

the alkyl. This would also give a 5-coordinateintermediate with a vacant site available for an

incoming CO.

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Experimental evidence that may be used toevaluate these mechanisms including the following :

1. Reaction of CH3Mn(CO)5 with 13CO gives aproduct with the labeled CO in carbonyl ligands

only;none is found in the acyl position.

This rules out mechanism 1.

13C labeled


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2. The reverse reaction

which occurs readily on heating

CH3C(=O)Mn(CO)5, when carried out with13C

in the acyl position, yields product CH3Mn(CO)5with the labeled 13CO entirelycis to CH3.

No labeled CO is lost in this reaction.

This suggests mechanisms 2 or 3, and rules out 1.


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3. The reverse reaction, when carried out with 13C

in a carbonyl ligandcis to the acyl group, gives aproduct that has a 2:1 ratio ofcis totrans product.

Some labeled CO is also lost in this reaction.

This supports mechanism 3 : alkyl insertion.

13C labeled in the acyl position


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13C labeled in the acyl position


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13C labeled in a carbonyl ligandcis to the acyl group

CO Migration


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Alkyl Insertion


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In the previous discussion of Mechanisms 2& 3, it was assumed that the intermediate

was a square pyramid & that no

rearrangement to other geometry (i.e., TBP)occurred.

Other labeling studies, involving reactions of

labeled CH3Mn(CO)5 with phosphines, have

supported a square-pyramidal intermediate.


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14-2-3 1,2 Insertion

An important application of 1,2 insertions of

alkenes into metal-alkyl bonds is in the formation

of polymers.One such process is the Cossee-Arlman

mechanism, proposed for the Ziegler-Natta

polymerization of alkenes.

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According to this mechanism, a polymer chaincan grow as a consequence of repeated 1,2

insertions into a vacant coordination site, as

follows :


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14-2-4 Hydride elimination

Hydride elimination reactions are characterized

by the transfer of a hydrogen atom from a ligand

to a metal.


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The most common type is -elimination, with a

proton in a position on an alkyl ligand beingtransferred to the metal by way of an intermediate

in which the metal, the & carbons, & the

hydride are coplanar. They are important in many

catalytic processes involving organometallic

complexes.


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Several general comments can be made about-elimination reactions.

1.Alkyl complexes that lack hydrogens tend tobe more stable thermally (although other types

of elimination reactions are also known).


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14-2-5 Abstraction

Abstraction reactions are elimination reactions

in which the C.N. of the metal does not change.

In general, they involve removal of a substituent

from a ligand, often by the action of the external

reagent, such as a Lewis acid.


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14 3 Organometallic Catalysis


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14-3 Organometallic Catalysis

14-3-1 Catalytic Deuteration

A series of reductive eliminations

and oxidative additions are involved.


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14-3-2 Hydroformylation

Terminal alkene


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Note:

higher linearand branched

selectivity,

compared toHCo(CO)4See page 538

14-3-3 Monsanto Acetic Acid Process


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Synthesis of acetic acid from methanol and CO.

14-3-4 Wacker (Smidt) Process


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Synthesis of acetaldehyde

from ethylene.

14-3-5 Hydrogenation by Wilkinsons Catalyst


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Sir Geoffrey Wilkinson (1921 1996)

Nobel Prize in Chemistry (1973)


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If the double bond is sterically hindered, it reacts slowly.

Examples of Selective Hydrogenation


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Wilkinsons catalyst is thus useful for selective

hydrogenation of C=C bonds that are not sterically hindered.

Since the selectivity is due to bulky phosphine ligands, the

selectivity can be fine-tuned by using phosphines of different

cone angles.

14-3-6 Olefin Metathesis


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CH2

CH2

CHR

CHR

CH2

CHR

CH2

CHR

+ +

H2C

H2C

CHR

CHR

Metathesis is reversible and can be catalyzed by a variety

of organometallic complexes.

Chauvin Mechanism

N b l P i i


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Richard R. Schrock

Nobel Prize in

Chemistry 2005

for contribution tometathesis reactions

Yves Chauvin Robert H. Grubbs

Ring-Closing Metathesis (RCM)


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Ring-Closing Metathesis (RCM)

Intramolecular metathesis can lead to ring-formation.

Reverse reaction:

ROM


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Grubbs metathesis catalyst

The dissociation of bulky

phosphine is the key step of

the mechanism.

New development of N-heterocyclic carbene ligand


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with excellent steric requirement and more electron donating,

more thermally stable and with low sensitivity toward oxygen

and water

Olefin metathesis can be used for polymerization of Norbornene.


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Metallocyclobutane

intermediate is

confirmed by H and13

C NMR.

R ROM


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Alkyne can also undergo metathesis reactions catalyzed by


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y g y y

transition metal carbyne complexes. The intermediate is

believed to be metallocyclobutadiene species.


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14-4-1 Ziegler-Natta Polymerization


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Karl Ziegler

Nobel Prize in Chemistry 1963

Giulio Natta

14-4-2 Water Gas Reactions


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H2O + C H2 + CO at elevated temperature and pressure

The mixture of H2 and CO (synthetic gas) can be used with

metallic heterogeneous catalysts to synthesize various

useful organic products.

Fischer-Tropsch process is a catalyzed chemical reaction in

which synthesis gas (syn gas), a mixture of carbon monoxide

and hydrogen, is converted into hydrocarbons, alcohols andalkenes of various forms.

H2 + CO Alkanes Co catalyst3H2 + CO CH4 + H2O Ni catalyst

2H2 + CO CH3OH Co or Zn/Cu catalyst

For example:

Water Gas Shift Reaction

R l f CO b


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Removal of CO2 by

chemical means can

produce H2 of > 99%

purity.

Organemetallic Reactions and Catalysis (14)

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