Homogeneous catalysis with inexpensive metals: Ionic hydrogenation of ketones with molybdenum and tungsten catalysts

Full text of DOI:10.1021/ja0010599

12594
J. Am. Chem. Soc. 2000, 122, 12594-12595
carbonyl hydrides are well-documented.10 Cationic metal hydrides
and dihydrogen complexes can be especially acidic.11 Metal hy-
drides are also capable of undergoing cleavage of the M-H bond
as a hydride.12 We reported13 the kinetics of hydride transfer from
a series of neutral metal hydrides to Ph3C+ where the kinetic hy-
dricity of metal hydrides was found to span a range of rate con-
stants of over 106. Successful implementation of a catalytic hydro-
genation cycle requires proper balancing of proton transfer and
hydride transfer abilities, together with the critical closure of the
catalytic cycle by the reaction of a metal complex with H2.
The reaction of Cp(CO)2(PPh3)WH (Cp )η5-C5H5) with Ph3C+-
BAr4′- [Ar′ ) 3,5-bis(trifluoromethyl)phenyl] in CH2Cl2 solution
in the presence of 3-pentanone resulted in hydride transfer from
tungsten to carbon, followed by binding of the ketone ligand to
tungsten. The resulting product, [Cp(CO)2(PPh3)W(η1-OdCEt2)]+
BAr4′-, was isolated as an orange solid in 88% yield (eq 2).14
Homogeneous Catalysis with Inexpensive Metals:
Ionic Hydrogenation of Ketones with Molybdenum
and Tungsten Catalysts
R. Morris Bullock* and Mark H. Voges
Chemistry Department, BrookhaVen National Laboratory
Upton, New York 11973-5000
ReceiVed March 27, 2000
Hydrogenation of the CdO double bond of ketones and
aldehydes is a reaction of substantial industrial importance and
is utilized in the production of numerous chemicals in the
agricultural, pharmaceutical, and other fine chemical industries.
Detailed mechanistic studies of hydrogenation reactions played
an instrumental role in the development of our modern under-
standing of the role of transition metal complexes in activating
hydrogen and delivering it to an unsaturated organic molecule.
The accepted mechanism1 involves coordination of the ketone to
the metal, followed by insertion into a metal-hydrogen bond (eq
1). The hydrogenated product is released from the metal through
When a solution of [Cp(CO)2(PPh3)W(η1-OdCEt2)]+ BAr4′-
is treated with 3-pentanone under H2 (<4 atm) at room temper-
ature, catalytic hydrogenation of the CdO bond of the ketone
occurs, producing 3-pentanol (eq 3). Monitoring the progress of
formal reductive elimination from a second metal-hydrogen
bond. These reactions traditionally use precious metals (notably
Rh and Ru)2-4 as catalysts, so alternative methods utilizing less
expensive metals could offer substantial benefits.
We report here a new class of homogeneous hydrogenation
catalysts based on molybdenum or tungsten, unusual metals for
hydrogenation catalysis. These catalysts were specifically designed
to operate by an ionic mechanism, which requires that the metal
complex be capable of reacting with H2, then sequentially deliv-
ering H+ followed by H- to a ketone. Stoichiometric ionic hydro-
genations are used in organic synthetic procedures, with CF3CO2H
commonly used as the proton source and HSiEt3 as the hydride
donor.5 We previously reported stoichiometric metal-mediated
ionic hydrogenations of ketones,6 alkenes,7 and alkynes.8 Kinetic
and mechanistic studies6,9 on stoichiometric ionic hydrogenations
of CdO bonds indicate preequilibrium protonation of the substrate
followed by rate-determining hydride transfer from the metal.
Metal hydrides are known to function as proton donors, and
kinetic and thermodynamic studies of the acidity of neutral metal
1
the reaction by H NMR spectroscopy indicates that the ketone
complex decreases in concentration, and the analogous alcohol
complex, [Cp(CO)2(PPh3)W(Et2CHOH)]+ BAr4′-, forms. In cata-
lytic hydrogenations carried out in NMR tubes, the concentration
of the alcohol complex increased to a maximum of ∼60% of the
total tungsten-containing species. This alcohol complex has been
independently synthesized and characterized.14 Its use as a catalyst
precursor results in activity comparable to that observed with the
ketone complex. At later stages of the reaction, some of the
3-pentanol is converted to the ether (Et2CH)2O (typically <10%).
A series of hydrogenation experiments were carried out with
< 4 atm H2 at 23 °C (Figure 1). These reactions carried out in
NMR tubes are not optimized for obtaining the highest catalytic
activity, but they provide a comparison of the catalytic activity
under controlled conditions. For Mo and W complexes containing
PCy3 ligands the ketone complexes were not isolated; the
catalytically active complex was prepared in situ by hydride
transfer from Cp(CO)2(PCy3)MH (M ) Mo or W) to Ph3C+BAr4′-
(1) Chaloner, P. A.; Esteruelas, M. A.; Joo´, F.; Oro, L. A. Homogeneous
Hydrogenation; Kluwer Academic Publishers: Boston, 1994.
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567-568.
(3) (a) Ohkuma, T.; Koizumi, M.; Doucet, H.; Pham, T.; Kozawa, M.;
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Soc. 1998, 120, 13529-13530. (b) Doucet, H.; Ohkuma, T.; Murata, K.;
Yokozawa, T.; Kozawa, M.; Katayama, E.; England, A. F.; Ikariya, T.; Noyori,
R. Angew. Chem., Int. Ed. 1998, 37, 1703-1707. (c) Ohkuma, T.; Ooka, H.;
Hashiguchi, S.; Ikariya, T.; Noyori, R. J. Am. Chem. Soc. 1995, 117, 2675-
2676.
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651. (b) Kursanov, D. N.; Parnes, Z. N.; Kalinkin, M. I.; Loim, N. M. Ionic
Hydrogenation and Related Reactions; Harwood Academic Publishers: New
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(6) Song, J.-S.; Szalda, D. J.; Bullock, R. M.; Lawrie, C. J. C.; Rodkin, M.
A.; Norton, J. R. Angew. Chem., Int. Ed. Engl. 1992, 31, 1233-1235.
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7176.
(9) Smith, K.-T.; Norton, J. R.; Tilset, M. Organometallics 1996, 15, 4515-
4520.
(10) For a review of proton-transfer reactions of metal hydrides, see:
Kristja´nsdo´ttir, S. S.; Norton, J. R. In Transition Metal Hydrides; Dedieu, A.,
Ed.; VCH: New York, 1992; Chapter 9, pp 309-359.
(11) (a) Chinn, M. S.; Heinekey, D. M.; Payne, N. G.; Sofield, C. D.
Organometallics 1989, 8, 1824-1826. (b) Jia, G.; Morris, R. H. J. Am. Chem.
Soc. 1991, 113, 875-883. (c) For a review of dihydrogen complexes, see:
Heinekey, D. M.; Oldham, W. J., Jr. Chem. ReV. 1993, 93, 913-926.
(12) For a review of nucleophilic reactivity of metal hydrides, see:
Labinger, J. A. In Transition Metal Hydrides; Dedieu, A., Ed.; VCH: New
York, 1992; Chapter 10, pp 361-379.
(13) Cheng, T.-Y.; Brunschwig, B. S.; Bullock, R. M. J. Am. Chem. Soc.
1998, 120, 13121-13137.
(14) Spectral and analytical data for ketone and alcohol complexes are in
the Supporting Information.
10.1021/ja0010599 CCC: $19.00 © 2000 American Chemical Society
Published on Web 12/02/2000

Communications to the Editor
J. Am. Chem. Soc., Vol. 122, No. 50, 2000 12595
(or dihydrogen) complex of Mo, which is then deprotonated by
the base.
As the catalytic reaction proceeds, the formation of [CpW(CO)2-
(PR3)(Et2CHOH)]+ (which re-enters the catalytic cycle) is ac-
companied by other organometallic products, such as CpW(CO)2-
(PR3)H and CpW(CO)2(PR3)H2+. Most revealing is the formation
of protonated phosphine, HPR3+. The formation of this product
suggests that phosphine dissociates from the metal and is pro-
tonated by MH2+. Identification of this as a catalyst decomposition
pathway provides valuable mechanistic information that is guiding
the development of the next generation of these catalysts.
Some precedent does exist for hydrogenation by Mo complexes.
Ketones can be catalytically hydrogenated by (CO)5Mo(OAc)-,
but higher temperatures (125 °C) and pressures (600 psi) were
used.19 A related catalyst for hydrogenation of acetophenone was
produced from reaction of Mo(CO)6 with MeOH/NaOMe (70 °C,
1450 psi H2 pressure).20 In both of these cases the catalysis is
Figure 1. Catalytic activity for hydrogenation of OdCEt2 (10-12 equiv)
by [Cp(CO)2(PR3)M(η1-OdCEt2)]+ BAr4′- (30 mM) at 23 °C and <4
atm H2 in CD2Cl2.
-
thought to proceed through the anionic metal hydride HMo(CO)5 ,
in the presence of OdCEt2. Approximate initial rates (1-2 turn-
overs) for the Mo catalysts are 2 turnovers/h for Mo-PCy3, 0.2
turnovers/h for Mo-PPh3, and 0.01 turnovers/h for Mo-PMe3.
Steric effects of the phosphines apparently predominate over elec-
tronic effects, presumably by promoting dissociation of the ketone
from the metal. The effect of counterions was briefly investigated
with catalysts generated from Cp(CO)2(PCy3)MoH + Ph3C+X-.
Catalysis was observed for X ) BF4-, PF6-, and OTf - (OTf )
OSO2CF3) but the activity was generally highest with BAr4′-.
The proposed mechanism for this new homogeneous hydro-
genation catalytic cycle is shown in Scheme 1. The ketone bound
to the metal is displaced by hydrogen to produce a cationic dihy-
dride complex, containing two M-H bonds. These dihydride com-
plexes have not been directly observed for Mo but have been
isolated in some cases for W, including an X-ray diffraction study
of [Cp(CO)2(PMe3)W(H)2]+OTf -.15 Proton transfer from the acid-
ic cationic dihydride to the ketone gives a neutral metal hydride
and protonated ketone. The metal hydride then transfers hydride
to the protonated ketone, forming the C-H bond and generating
the alcohol. The 16-electron organometallic cation (“M+” in
Scheme 1) is captured by ketone, completing the catalytic cycle.
Mechanistic experiments provide support for the proposed path-
way. Stoichiometric hydrogenations of acetone by [Cp(CO)2(PMe3)-
with the mechanism apparently involving metal alkoxide inter-
mediates (cf. eq 1) rather than the proton transfer and hydride
transfer processes proposed in Scheme 1.
The Mo and W systems reported here use cheap metals,
whereas traditional ketone hydrogenation catalysts use more
expensive metals such as Rh and Ru. The advantage of inexpen-
sive metals must be balanced against the catalytic activity
obtained, since some Ru catalysts3 are orders of magnitude more
reactive than our Mo complexes. The new hydrogenation catalysts
developed here exploit the versatile reactivity patterns accessible
from transition metal hydrides, through the reactivity of acidic
and hydridic M-H bonds. Other hydrogenation catalysts based
on Ru21 or Rh22 apparently proceed through heterolytic pathways
where an M-H bond and an O-H (or N-H) bond are formed
from reaction with H2. Hidai and co-workers recently reported
heterolytic cleavage of H2 with Ru catalysts for hydrogenolysis
of silyl enol ethers,23 and for conversion of coordinated N2 to
NH3. Catalysts that rely on mechanisms involving heterolytic
cleavage of H2 offer promising opportunities for further develop-
ment, and our results provide an initial step in the development
of a new series of Mo and W catalysts based on this concept.
Acknowledgment. We thank the U.S. Dept. of Energy, Office of
Science, Laboratory Technology Research Program for support. Research
was carried out at Brookhaven National Laboratory under contract DE-
AC02-98CH10886 with the U.S. Dept. of Energy with additional support
by its Division of Chemical Sciences, Office of Basic Energy Sciences.
We thank DuPont Central Research for support of this work through a
CRADA grant. We gratefully acknowledge Drs. Paul Fagan and Elisabeth
Hauptman (DuPont) for many helpful discussions, and Dr. Jeong-Sup
Song for preliminary experiments.
-
W(H)2]+OTf in either CD3CN or CD2Cl2 are essentially com-
plete in 15 min at room temperature, giving observable 2-propanol
complexes6,9 as the initial products. Protonation of a ketone by
the dihydride [Cp(CO)2(PMe3)W(H)2]+ is thermodynamically
unfavorable (pKa ) 5.616 in CH3CN for [Cp(CO)2(PMe3)W(H)2]+;
pKa ≈ -0.19,17 for Me2COH+ in CH3CN), but the hydride transfer
to protonated ketone is fast9 so hydrogenation proceeds smoothly.
The proton-transfer step could proceed through direct metal to
oxygen proton transfer, or may be mediated by OTf- as a kineti-
cally active proton carrier.18 Further evidence for heterolytic cleav-
age of H2 comes from the reaction of [Cp(CO)2(PPh3)Mo(η1-Od
CEt2)]+BAr4′- with H2 (<4 atm) in the presence of the hindered
base 2,6-di-tert-butyl-4-methylpyridine. This reaction produced
∼80% of neutral hydride Cp(CO)2(PPh3)MoH after 5 min, pre-
sumably through the intermediacy of the unobserved dihydride
Supporting Information Available: Preparative procedures and spec-
troscopic characterization of ketone and alcohol complexes (PDF). This
material is available free of charge via the Internet at http://pubs.acs.org.
JA0010599
(15) Bullock, R. M.; Song, J.-S.; Szalda, D. J. Organometallics 1996, 15,
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(16) Papish, E. T.; Rix, F. C.; Spetseris, N.; Norton, J. R.; Williams, R. D.
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a preprint.
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Scheme 1
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