wilkinson catalyst
TRANSCRIPT
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Catalytic hydrogenation
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Wilkinsons catalyst
The complex RhCl(PPh3)3 (also known as Wilkinsons catalyst) became the first highlyactive homogeneous hydrogenation catalyst that compared in rates with heterogeneous
counterparts.
Wilkinson, J. Chem. Soc. (A) 1966, 1711.
H2 (1 atm), RTBenzene
1-octene octane
RhPh3P
Ph3P PPh3Cl
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Wilkinsons catalyst
But ethylene is not hydrogenated due to formation of a strongly bonded ethylene complex.
RhPh3P
Cl PPh3PPh3
H2C=CH2 + RhPh3P
Cl PPh3-PPh3
However, ethylene reacts with the preformed dihydride complex. This implies that the
dihydride formation precedes olefin complexation in the catalytic cycle.
RhPh3P
Cl PPh3
PPh3
2 H2C=CH2 + RhPh3P
Cl PPh3-PPh3
H
HH3C-CH3+
Wilkinsons catalyst is compatible with a range of functional groups because the mechanism
does not involve hydride ion transfer.
O O
OR
O
OH
C NO2 ORN
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Hydrogenation mechanismWilkinsons 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
Rh
PPh3
PPh3
Solv
16-e (1)
(2)(3)
(4)
Cl
Rh
PPh3
PPh3
H
H2
H Cl
Rh
PPh3
PPh3
H
H Cl
Rh
PPh3
PPh3
Cl
H
H2C
R
R
CHR H
R C C
HH
H
H
H
16-e
18-e
16-e
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.
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Wilkinsons catalyst selectivity
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.
Increasing
rate
A polar functional group may
accelerate catalysis by assistingolefin 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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Wilkinsons catalyst selectivity
Hydrogenation is stereoselective:
RhPh3P
Cl PPh3PPh3
H H
HO2C CO2H
D D
HO2C CO2HD2H H
meso compound,major product
benzene, rt
RhPh3P
Cl PPh3PPh3
C3H7 CH3D2
cis: trans > 20:1benzene, rt
D D
C3H7 CH3
+ hexane
Rh preferentially binds to the least sterically hindered face of the olefin:
RhH
Ph3P Cl
PPh3
H
RhPh3P
Cl PPh3PPh3
H2
R=H : 73% endoR=Me : 92% endo
benzene/EtOH, rtR
RRh
H
Ph3P Cl
PPh3
H
R
+
R
H
CH2H
R
CH2H
H
endo exo
less hindered
more hindered
Wilkinson, J. Chem. Soc. (A) 1966, 1711
Rousseau, J. Mol. Cat. 1979, 5, 163.
Jardine, Prog. Inorg. Chem. 1981, 28, 63.
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Wilkinsons 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
RhPh3P
Cl PPh3PPh3
Pd/C
acetone, H2 (1 atm)rt, 75%
C6H6/EtOH, H2 (1 atm)rt, 95%
O
O
O
O
O
O
cis-disubstituted
tetrasubstituted
H
RhPh3P
Cl PPh3PPh3
H2 (1atm), benzene/EtOH,rt, 80%
O
HO
CO2Me
OAc
cis-disubstituted
trans-disubstituted
O
HO OAc
CO2Me
Cis-disubstituted C=C react faster than trans-disubstituted C=C:
Schneider, JOC1973, 38, 951.
Pedro, JOC1996, 61, 3815.
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Wilkinsons catalyst selectivity
Site selectivity Directing group effects:
RhPh3P
Cl PPh3PPh3
KOR, H2 (6.8 atm), benzene,50 C, 68%
OH
MeO
OH
MeO
cis-isomer (exclusive)note: a mixture ofcis andtrans isomers resulted with Pd/C
H
OK
MeO
Rh
PPh3
O PPh3HH
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
RhPPh3PPh3
IrPPh3
NRh
Ph3P
Cl PPh3PPh3
Schrock-Osborn
catalyst
Crabtrees
catalyst
Wilkinsons
catalyst
Substrates TOF
4000
10
6400
4500
3800
4000
700
650
13
Cationic catalysts are the most active homogeneous hydrogenation catalysts developed so
far:
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Catalytically active species
RhPPh3
PPh3
H2Rh
S
S PPh3
Ph3Psolvent = S
Catalyst precursor
Rh
S
S PPh3
Ph3P
H
H
Only speciesobservable by NMR
unobservableintermediate
H2
Rh
Ph2P
PPh2
H2
solvent = S
RhS
S
Ph2P
PPh2
Rh
S
S
Ph2P
PPh2
H
H
unobservableonly speciesobserved by NMRin the absence of olefin
With bidentate ligands, olefin coordination can precede oxidative addition of H2 (S =
methanol, ethanol, acetone).
With monodenate ligands, the hydrogenation may involve formation of a dihydride
intermediate:
The difference is due to the strong trans-influence of hydride and phosphine ligands, whichmake unfavorable a trans H-M-PR3 structural arrangement.
Halpern, JACS 1977, 99, 8055; Schrock & Osborn, JACS 1976, 98, 2134.
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Halperns mechanism of hydrogenation for cationic Rh
catalysts with bidentate phosphines
RhS
S
Ph2P
PPh2
observed by NMR
Rh
O
Ph2P
PPh2
Ph R
NHAc
Ph
R = CO2Me
NHR
Rh
O
Ph2P
PPh2
Ph
HN R
H
H
H2
observed by NMR
RhO
Ph2P
PPh2
Ph
HNR
H
S
R
NHAc
Ph
rate-detrminingstep
(E)-methyl 2-acetamido-3-phenylacrylate
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
IrPCy3
NOH
Me
2.5 mol%
CH2Cl2, H2(1atm), rt
OH
MeH
OH
HMe
64 : 16-isopropyl-3-methylcyclohex-2-enol
2-isopropyl-5-methylcyclohexanol
IrH
Cy3P PyOH
H
Me iPr
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:
Intermediate:
Other functionalities also direct:
OH
97%
H2 /Ir cat.
Me
OH
Me H
56 : 1
>99%
H2 /Ir cat.
Me Me H
124 : 1
O Me O MeO
>99%
H2 /Ir cat.
Me
O
Me H
999 : 1
Me Me
Hoveida, Chem. Rev. 1993, 93, 1307.
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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 firstdemonstration that a chiral metal complex could effectively transfer chirality to a non-chiral
substrate.
RhPP
H2(1 atm), rt
i-PrOH, >99% yield
DIPAMP - chiral (C2)diphosphine
MeO
OMe
Ph
NHAc
CO2H
NHAc
CO2HPh
93 % ee(E)-2-acetamido-3-phenylacrylic acid
(S)-2-acetamido-3-phenylpropanoic acid
Knowles, JACS 1975, 97, 2567.
A variety of bidentate chiral
diphosphines have been
synthesized and used to make
amino acids by hydrogenation
of enamides:
PPh2
PPh2
Chiraphos
PPh2
PPh2
NORPHOS SKEWPHOS
PPh2
PPh2
O
O
PPh2
PPh2
DIOP
PPh2
PPh2
BINAP
P
P
R
R
R
R
DuPHOS
PPh2
PPh2HH Fe PPh2
NMe2
PPh2
BICP JOSIPHOSFor review on DuPhos:Burk,Acc. Chem. Res 2000, 33, 363.
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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.
Morris, Coord. Chem. Rev. 2004, 248, 2201-2237.
Steps: (I) substrate addition (outer sphere), (II) simultaneous hydride and proton transfer,
(III) H2 addition, (IV) regeneration of the catalyst.
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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
Morris, Coord. Chem. Rev. 2004, 248, 2201-2237.
Note: Only trans-RuH2 are active catalysts, because of the strongly hydridicnature oftrans-
dihydrides.
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Structures of the intermediate species
18-e trans-dihydride 16-e amido-hydride
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Noyoris transfer hydrogenation catalysts
Very efficient for enantioselective transfer hydrogenation.
Noyori,Acc. Chem. Res. 1997, 30, 97; JACS 2000, 122, 1466; JOC2001, 66, 7931
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Intermediates in Noyoris transfer hydrogenation
18-e hydride 16-e amido complex