Download Catalytic hydrogenation

Catalytic hydrogenation
1
Wilkinson’s catalyst
The complex RhCl(PPh3)3 (also known as Wilkinson’s catalyst) became the first highly
active homogeneous hydrogenation catalyst that compared in rates with heterogeneous
counterparts.
Wilkinson, J. Chem. Soc. (A) 1966, 1711.
Ph3P
PPh3
Rh
Cl
Ph3P
1-octene
H2 (1 atm), RT
Benzene
octane
2
Wilkinson’s catalyst
Wilkinson’s catalyst is compatible with a range of functional groups because the mechanism
does not involve hydride ion transfer.
O
C N
O
O
OR
OH
NO2
OR
But ethylene is not hydrogenated due to formation of a strongly bonded ethylene complex.
+
H2C=CH2
Cl
PPh3
Rh
PPh3
Ph3P
-PPh3
Cl
PPh3
Rh
Ph3P
However, ethylene reacts with the preformed dihydride complex. This implies that the
dihydride formation precedes olefin complexation in the catalytic cycle.
2 H2C=CH2
+
H
Cl
PPh3
Rh
Ph3P
H
PPh3
-PPh3
Cl
PPh3
Rh
Ph3P
+
H3C-CH3
3
Hydrogenation mechanism
Wilkinson’s catalyst, RhCl(PPh3)3 is used in benzene/ethanol solution in which it
dissociates to some extent; a solvent molecule (Solv) fills the vacant site:
RhCl(PPh3)3 + Solv ' RhCl(Solv)(PPh3)2 + PPh3
PPh3
HH
H
C C
R
HH
Cl
Rh
Solv
PPh3
16-e
(4)
H2
(1)
PPh3
H
H2C
R
Rh
PPh3
H
Cl
H
PPh3
CH
H 16-e
(3)
(2)
H
H
PPh3
Cl
Rh
Rh
Cl
PPh3
16-e
R
PPh3
18-e
R
Steps: (1) H2 addition, (2) alkene addition, (3) migratory insertion, (4) reductive elimination
of the alkane, regeneration of the catalyst.
Halpern, Chem. Com. 1973, 629; J. Mol. Cat. 1976, 2, 65; Inorg. Chim. Acta. 1981, 50, 11.4
Wilkinson’s catalyst selectivity
Increasing rate
The rate of hydrogenation depends on (a) presence of a functional group in the vicinity of
the C=C bond and (b) degree of substitution of the C=C fragment.
A polar functional group may
accelerate catalysis by assisting
olefin coordination to Ru
Terminal C6-C12 alkenes are
hydrogenated at the same rate
Conjugated dienes react slower
Hydrogenation of internal and
branched alkenes is the slowest
(note: cis is faster than trans!)
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Wilkinson’s catalyst selectivity
Hydrogenation is stereoselective:
H
HO2C
H
CO2H
Cl
PPh3
Rh
PPh3
Ph3P
D2
benzene, rt
D
H
HO2C
Cl
PPh3
Rh
Ph3P
PPh3
D
H
CO2H
C3H7
D
D
+ hexane
CH3
C3H7
D2
benzene, rt
meso compound,
major product
CH3
cis: trans > 20:1
Rh preferentially binds to the least sterically hindered face of the olefin:
less hindered
Cl
PPh3
Rh
PPh3
Ph3P
R
H2
benzene/EtOH, rt
H
Ph3P
Cl
Rh
H
PPh3
R
H
Ph3P
Cl
Rh
H
PPh3
+
R
more hindered
CH2H
H
CH2H
Wilkinson, J. Chem. Soc. (A) 1966, 1711
Rousseau, J. Mol. Cat. 1979, 5, 163.
Jardine, Prog. Inorg. Chem. 1981, 28, 63.
R
endo
R=H : 73% endo
R=Me : 92% endo
H
R
exo
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Wilkinson’s catalyst selectivity
Site selectivity: Preferential hydrogenation of the least sterically hindered C=C bonds (note
that heterogeneous hydrogenation catalysts are often not selective):
O
O
O
O
Pd/C
cis-disubstituted
acetone, H2 (1 atm)
rt, 75%
O
O
H
Cl
PPh3
Rh
Ph3P
PPh3
tetrasubstituted
C6H6/EtOH, H2 (1 atm)
rt, 95%
O
O
O
Pedro, JOC 1996, 61, 3815.
Cis-disubstituted C=C react faster than trans-disubstituted C=C:
cis-disubstituted
O
CO2Me
HO
OAc
Cl
PPh3
Rh
PPh3
Ph3P
H2 (1atm), benzene/EtOH,
rt, 80%
O
CO2Me
HO
OAc
trans-disubstituted
Schneider, JOC 1973, 38, 951.
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Wilkinson’s catalyst selectivity
Site selectivity – Directing group effects:
OH
OH
Cl
PPh3
Rh
Ph3P
PPh3
H
KOR, H2 (6.8 atm), benzene,
50 °C, 68%
MeO
OK
MeO
MeO
PPh3
O
PPh3
Rh
H
H
cis-isomer (exclusive)
note: a mixture of cis and
trans isomers resulted with Pd/C
MeO
Base-assisted formation of the alkoxide resulted in displacement of the chloride ligand and
directed olefin complexation.
Thompson, JACS 1974, 96, 6232.
Jardine, Prog. Inorg. Chem. 1981, 28, 63
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Cationic catalysts
Cationic catalysts are the most active homogeneous hydrogenation catalysts developed so
far:
PPh3
Rh
PPh3
Ir
Schrock-Osborn
catalyst
PPh3
Cl
PPh3
Rh
Ph3P
PPh3
N
Crabtree’s
catalyst
Substrates
Wilkinson’s
catalyst
TOF
4000
6400
700
10
4500
650
3800
13
4000
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Catalytically active species
With bidentate ligands, olefin coordination can precede oxidative addition of H2 (S =
methanol, ethanol, acetone).
Rh
Ph2
P
P
Ph2
H2
solvent = S
Ph2
P
S
Rh
S
P
Ph2
H Ph2
P
S
Rh
H
P
S Ph2
only species
observed by NMR
in the absence of olefin
unobservable
With monodenate ligands, the hydrogenation may involve formation of a dihydride
intermediate:
Rh
PPh3
PPh3
Catalyst precursor
H2
solvent = S
PPh3
S
Rh
S
Ph3P
unobservable
intermediate
H2
H
PPh3
S
Rh
Ph3P
H
S
Only species
observable by NMR
The difference is due to the strong trans-influence of hydride and phosphine ligands, which
make unfavorable a trans H-M-PR3 structural arrangement.
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Halpern, JACS 1977, 99, 8055; Schrock & Osborn, JACS 1976, 98, 2134.
Halpern’s mechanism of hydrogenation for cationic Rh
catalysts with bidentate phosphines
Ph2
P
Ph
S
Rh
S
P
Ph2
R
R = CO2Me
Ph
NHAc
(E)-methyl 2-acetamido3-phenylacrylate
NHAc
Ph
HN
R
Ph
R H Ph
2
P
Rh
P
O
S Ph2
Ph2
RP
NH Rh
P
O
Ph2
observed by NMR
HN
observed by NMR
Ph
H Ph2
P
R Rh
P
H
O Ph2
H2
rate-detrmining
step
Steps: (1) alkene addition, (2) H2 addition, (3) migratory insertion, (4) reductive elimination
of the alkane, regeneration of the catalyst.
Halpern, Science 1982, 217, 401.
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Cationic catalysts: substrate-directed hydrogenation
The unsaturated cationic catalysts can bind a ligating group of the substrate in addition to
the olefin. This bidentate coordination determines the selectivity of hydrogenation:
Ir
OH
PCy3
N
OH
OH
2.5 mol%
Me
H
Me
Me
CH2Cl2, H2 (1atm), rt
H
64 : 1
6-isopropyl-3methylcyclohex-2-enol
2-isopropyl-5-methylcyclohexanol
H
Intermediate:
Cy3P
Py
Ir
OH
H
Hoveida, Chem. Rev. 1993, 93, 1307.
i
Me
Pr
Other functionalities also direct:
OH
OH
O
Me
H2 / Ir cat.
97%
Me
Me H
56 : 1
Me
O
Me
O
Me
O
H2 / Ir cat.
H2 / Ir cat.
>99%
>99%
Me H
124 : 1
Me
Me
Me H
999 : 1
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Asymmetric hydrogenation
A bidentate, C2 symmetric version of the cationic Schrock-Osborn catalyst affords high
levels of enantioselectivity in the hydrogenation of achiral enamides. This was the first
demonstration that a chiral metal complex could effectively transfer chirality to a non-chiral
DIPAMP - chiral (C2)
substrate.
Knowles, JACS 1975, 97, 2567.
Ph
diphosphine
MeO
P
Rh
P
(S)-2-acetamido-3-phenylpropanoic acid
OMe
CO2H
NHAc
(E)-2-acetamido-3phenylacrylic acid
A variety of bidentate chiral
diphosphines have been
synthesized and used to make
amino acids by hydrogenation
of enamides:
NHAc
H2 (1 atm), rt
i-PrOH, >99% yield
93 % ee
PPh2
PPh2
PPh2
O
PPh2
PPh2
PPh2
PPh2
O
PPh2
Chiraphos
NORPHOS
SKEWPHOS
DIOP
R
PPh2
PPh2
For review on DuPhos:
Burk, Acc. Chem. Res 2000, 33, 363.
CO2H
Ph
P
R
P
R
PPh2
H
H
PPh2
Fe
NMe2
PPh2
PPh2
R
BINAP
DuPHOS
BICP
JOSIPHOS
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H2-hydrogenation and transfer hydrogenation of C=O
(ketones, aldehydes) and C=N (imines) bond
The catalytic hydrogenation of polar C=O and C=N bonds are key reactions in fine chemical
and pharmaceutical synthesis. A very important group of catalysts operate by hydride
transfer to the substrate in the outer coordination sphere of the complex. Hydrogen can
come from H2 or from an organic donor, such as 2-propanol.
H2 hydrogenation:
R1R2C=Q + H2 → R1R2CH-QH
Transfer hydrogenation:
R1R2C=Q + DH2 → R1R2CH-QH + D
e.g. DH2 = (CH3)2CH-OH and D = (CH3)2C=O
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Metal-ligand bifunctional catalysts.
Noyori has coined the term “metal-ligand bifunctional catalysts, describing systems
containing an ancillary ligand cis to the hydride that assists in the hydride transfer step and
this ligand must have an NH or OH (protic) group.
Steps: (I) substrate addition (outer sphere), (II) simultaneous hydride and proton transfer,
(III) H2 addition, (IV) regeneration of the catalyst.
Morris, Coord. Chem. Rev. 2004, 248, 2201-2237.
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Enantioselective hydrogenation of polar bonds
Ruthenium complexes containing chiral diphosphine (e.g. (R)-binap) and diamine (e.g.
(R,R)-diamine) ligands are very efficient enantioselective hydrogenation catalysts:
Only the S-form of the
alcohol is produced
Note: Only trans-RuH2 are active catalysts, because of the strongly hydridic nature of transdihydrides.
Morris, Coord. Chem. Rev. 2004, 248, 2201-2237.
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Structures of the intermediate species
18-e trans-dihydride
16-e amido-hydride
17
Noyori’s transfer hydrogenation catalysts
Very efficient for enantioselective transfer hydrogenation.
Noyori, Acc. Chem. Res. 1997, 30, 97; JACS 2000, 122, 1466; JOC 2001, 66, 7931
18
Intermediates in Noyori’s transfer hydrogenation
18-e hydride
16-e amido complex
19