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Lecture 38
Organometallic reactions and catalysis
1) Catalytic olefin hydroformylation. Monsanto acetic acid process
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Olefin hydroformylation and Monsanto acetic acid process belong to the class of catalytic
carbonylation reactions in which a carbonyl functionality is introduced into a molecule.
Olefin Hydroformylation. In the hydroformylation reaction hydrogen atom and formyl formed
from H2 and CO are added across olefinic C=C bond to form linear and branched aldehydes:
H
Me
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C
H
CH2
CO, H2
cat
Me
H
C
H
H2
C
C
H
linear
H2C
O formyl
+
O
Me
C
C
H
H
branched
RhH(CO)L 3 (Union Carbide, Hoechst, BASF)
HCo(CO) 4 (Exxon, BASF)
HCo(CO) 4-n(L)n (Exxon, BASF)
Olefin hydroformylation is used in industry for multimillion ton annual production of aldehydes
(butanal, in particular).
The HCo(CO)4 – catalyzed hydroformylation was discovered in 1938 (Otto Roelen). Its
mechanism was suggested in 1960-s.
O carboxyl
H H2
Aldehydes formed in
Me C C
C
hydrocarboxylation
O
hydroformylation reaction
OH
, H2
H
O
C
can react with H2 to produce
O alkoxycarbonyl
H H2
primary alcohols.
Me C CH2 CO, HOR Me C C
C
hydroalkoxycarbonylation
H
C
OR
O
H
Later a number of other
,H
N
R
carbonylation reactions was
2
O
H H2
amidocarbonyl
hydroamidocarbonylation
discovered (see scheme).
Me C C
C
H
NR2
2) Mechanism of the catalytic olefin hydroformylation by HCo(CO)4
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2
+H
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The currently accepted mechanism of olefin hydroformylation by HCo(CO)4 is given below.
The catalyst, A, forms readily from Co2(CO)8 and H2 (100oC, up to 100 atm of CO and H2).
Subsequent ligand exchange leads to olefin complex B where C=C bond is “activated” due to
coordination to cobalt atom.
H
Insertion
-CO
OC
O
H
of olefin into Co-H
Co CO
OC
bond leads to
OC
H
A
H
Co
CO
alkylcobalt C.
OC
O
+C
Subsequent insertion linear / branched 3 : 1
+H2
CO
B
of CO into Co-C bond
CO
O
produces acylcobalt D.
H
H
+CO
CO
OC Co
O
The rate limiting step is
CO
H
OC
H
oxidative addition of H2 to
Co CO
OC
C
H
OC
acylcobalt D to produce
Co
E
OC
H
CO
dihydride E and to transform
OC
CO
Co CO
strong H-H bond into two
OC
weaker Co-H bonds.
D
CO
-C
O
O
Subsequent reductive
slow
O
+C
branched isomer
elimination of aldehyde
also forms
H
H
OC
and coordination of CO
Co CO
OC
regenerates the active
OC Co CO
OC
species A.
CO
CO
3) Modified catalysts for olefin hydroformylation
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Co – phosphine modified catalysts. Studies performed at Shell showed that addition of trialkylphosphine
ligands changes dramatically the reaction rate and selectivity. When HCo(CO)3(PR3) forms, Co-CO bonds
become much stronger and it becomes possible to decrease CO pressure without causing catalyst
decomposition.
becomes stronger
Bulkier PR3 favor greater linear / branched
becomes more hydride-like
H
aldehyde ratios ( up to 8 : 1).
CO
Finally, more hydride-like Co-H promotes aldehyde
OC Co
hydrogenation to alcohols:
CO
O
R
H2
PR3
H2
C OH
R
strong -donor
H
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Further studies allowed to establish that Rh analogues such as HRh(CO)(PPh3)3 are 103-104 times more
active in olefin hydroformylation so that it can be performed at ambient pressure of H2 and CO and room
temperature. This catalyst favors also greater linear to branched aldehydes ratio (up to 20 :1).
The reaction mechanism is similar to that with HCo(CO)4. HRh(CO)(PPh3)2 is considered as catalytically
active species.
H
OC
PPh3
Rh
PPh3
- PPh3
OC
Ph3P
Rh
PPh3
H
PPh3
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Currently a number of more sophisticated modifications of the olefin hydroformylation catalysts are known
which also allow to perform olefin hydroformylation stereospecifically.
4) Monsanto acetic acid process: MeOH + CO  MeCOOH
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This is another carbonylation reaction introduced in early 70-s by Monsanto. This reaction involves dual
catalysis with HI and with salts of [RhI2(CO)2]- anionic complex.
Hydrogen iodide is responsible for conversion of MeOH into MeI. Rhodium catalyst is responsible for
carbonylation of MeI into MeCO-I. Finally, HI is regenerated in the end of Rh-catalytic cycle as a result of
hydrolysis of acetyliodide.
Me
O
C
OH
OC
I
MeOH
Rh
CO
I
Me-I
Me
OC
I
Rh
CO
I
I
Me
O
C
reductive
elimination
migratory
insertion
Me
Me
I
CO
OC
I
Rh
I
C
I O
CO
OC
I
Rh
I
C
I O