wilkinson catalyst

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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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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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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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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

wilkinson catalyst

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