Transcript Slide 1

Lecture 37
Organometallic reactions and catalysis
1) Catalytic olefin hydrogenation
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A thermodynamically favorable reaction may be slow at modest temperatures and thus of little
value for synthesis. Increased temperatures may help making it faster, but competing
reactions may be accelerated as well.
A more attractive approach to increase the rate of a reaction is to use catalysis.
We will discuss the most well studied and/or useful examples of homogeneous catalysis:
olefin hydrogenation (addition of hydrogen across C=C bond)
olefin hydroformylation (addition of hydrogen and formyl HC(O) across C=C bond)
olefin metathesis
olefin polymerization
carbonylation of methanol (Monsanto acetic acid process)
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Olefin hydrogenation. The following reaction is favorable but does not proceed at all at room
temperature:
0
H2C
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CH2 + H2
H3C
CH3
G = -24 kcal/mol
In the beginning of the 20th century Paul Sabatier discovered that hydrogenation of olefins
can be accelerated with finely dispersed Ni, Pt or Pd (heterogeneous catalysis).
If hydrogenation of substrates with many functional groups in them is needed, heterogeneous
catalysis is not a good choice because of the lack of selectivity:
H2/Pd
PhCH2O
HO
2) Catalysts for homogeneous olefin hydrogenation
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The first effective and selective homogeneous catalyst for olefin hydrogenation is RhCl(PPh3)3
known as Wilkinson’s complex.
It allows for fast selective hydrogenation of variety of olefins at room temperature and 1 atm
of H2 gas:
H2/RhCl(PPh3)3
H
C6H6/EtOH; r.t
H
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Turnover frequency
(TOF) =
700 turnovers/h
H2/RhCl(PPh3)3
C6H6/EtOH; r.t
H
H
O
carvone O
Wilkinson’s complex is unique in the sense that its analogues derived from more electron-rich
and less bulky phosphines such as PEt3, RhCl(PEt3)3, or iridium, IrCl(PPh3)3, which forms
stronger bonds with ligands, are not active in hydrogenation.
More active cationic complexes were found later such as [Rh(PPh3)2(cod)]+PF6-,
[Ir(cod)(PCy3)(py)]+PF6- etc (cod = 1,5-cyclooctadiene). Metal atoms here are more electron
deficient and bind olefins more strongly which is an important step of the catalysis.
Rh
PPh3
PPh3
TOF = 4000 for 1-hexene
Ir
PCy3
TOF = 6400 for 1-hexene
N
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Homogeneous catalysts are used mostly in the lab and in pharmaceutical industry when
selectivity is important and if separation of homogeneous catalyst is not a big problem.
• At times, proving the real nature of a catalyst
cat
type % yield
(heterogeneous/homogeneous) may be not a simple task.
RhCl(PPh3)3
hom
100
Polymer-bond olefin hydrogenation test can be used:
H
Pd/C
het
0
H2/cat
polymer backbone
H
Ni(OAc)2/NaBH4
het
0
3) Mechanism of olefin hydrogenation by Wilkinson catalyst
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Wilkinson complex is not only highly active, it is also tolerant of a number of functional groups which may be
present in the olefin substrate: C=O, OAc, COOH, OH, NO2, C≡N, C(OR)2, OR etc.
Its selectivity with respect to the type of C=C bond to be hydrogenated:
CN
R
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700
R
650
>
>
>
,
Bu
>
>>
R
R
R
CH
R
R
>
>
R
13
0
Detailed kinetic studies allowed to establish the mechanism of olefin hydrogenation by RhClL3. The
Wilkinson complex A is actually a pre-catalyst which form catalytically active 14 electron species B (K = 10-5
M). Bulkiness of L helps accelerate this step. Then B reacts fast with H2 to produce dihydride C.
C is capable of coordinating olefin to form D.
H
H2
Bulky olefins will be less prone to form D,
L Rh H
L Rh L
Cl
L
Cl
L
consistent with the activity of olefins (see above).
C C
L
A 16e
-L
The rate limiting step of the reaction is D  E.
H
Fast reductive elimination of alkane to
K
L Rh H
-L
regenerate B is the last and fast step of
Cl
L
C C
the reaction catalytic cycle. The catalytic cycle
H
C C
of the reaction includes intermediates B-E.
H2
L
L
H
D 18e
Empty coordination sites in the intermediates
Cl Rh L
Cl Rh L
B, C, E may be occupied by solvent molecules
B 14e
C 16e
(benzene – ethanol).
slow
Ethylene cannot be hydrogenated by RhClL3
H
fast
since if forms very strong complex with Rh.
L Rh C C
C C
Cl
L
Alkynes are even more reactive than alkenes
H
H
H
because of the stronger binding to Rh. They
E 16e
can be selectively hydrogenated to olefins.
4) Similar catalytic reaction of olefins
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Very important type of catalytic hydrogenation reactions is asymmetric hydrogenation. Here is an
example of such hydrogenation leading to enantiomerically pure L-Dopa, (Parkinson’s disease drug). All
you need is to use chiral ligands attached to Rh atom:
chiral diphosphine
Ar'
Rh
Ph P
AcO
P Ph
MeO
CO2Me
H2
NHAc
AcO
CO2Me
NHAc
MeO
L-Dopa
Ar'
Ar' = o-MeOC6H4
There is a number of catalytic addition reactions which proceed via mechanisms similar to those for olefin
hydrogenation.
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Hydrosilylation. One of the simplest and efficient (pre)catalysts is H2PtCl6 or PdL4 (L = phosphine).
H
SiR3
SiR3
H2PtCl6
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H
Hydrocyanation. The most known catalysts for synthesis of adiponitrile are NiL4 complexes with L =
P(OAr)3.
H
CN
CN
NC
NiL4
H
H