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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.