Catalytic ionic hydrogenation of ketones by {[Cp*Ru(CO) 2]2(μ-H)}+

Article abstract of DOI:10.1021/om901005k

{[Cp*Ru(CO)₂]₂(μ-H)}OTf functions as a homogeneous catalyst precursor for hydrogenation of ketones to alcohols, with hydrogenations at 1 mol % catalyst loading at 90 °C under H₂ (820 psi) proceeding to completion and providing > 90% yields. Hydrogenation of methyl levulinate generates y-valerolactone, presumably by ring-closing of the initially formed alcohol with the methyl ester. Experiments in neat Et₂C=O show that the catalyst loading can be < 0.1 mol % and that at least 1200 turnovers of the catalyst can be obtained. These reactions are proposed to proceed by an ionic hydrogenation pathway, with the highly acidic dihydrogen complex [Cp*Ru(CO)₂(η²- H₂)]OTf being formed under the reaction conditions from reaction of H₂ with [[Cp*Ru(CO)₂] ₂(μ-H)⁺OTf.

Full text of DOI:10.1021/om901005k

Organometallics 2010, 29, 1045–1048 1045
DOI: 10.1021/om901005k
Catalytic Ionic Hydrogenation of Ketones by {[Cp*Ru(CO)2]2( μ-H)}þ
Paul J. Fagan,^ Mark H. Voges,† and R. Morris Bullock*,†,§
†Chemistry Department, Brookhaven National Laboratory, Upton, New York 11973-5000, §Chemical and
Materials Sciences Division, Pacific Northwest National Laboratory, P.O. Box 999, K2-57, Richland,
Washington 99352, and ^Central Research and Development, E. I. DuPont de Nemours & Co. Inc.,
Experimental Station, P.O. Box 80328, Wilmington, Delaware 19880-0328
Received November 17, 2009
Summary: {[Cp*Ru(CO)2]2(μ-H)}þOTf - functions as a homo-
These results, in which an aldehyde was produced in a
geneous catalyst precursor for hydrogenation of ketones to
alcohols, with hydrogenations at 1 mol % catalyst loading at
90 ꢀC under H2 (820 psi) proceeding to completion and providing
>90% yields. Hydrogenation of methyl levulinate generates
γ-valerolactone, presumably by ring-closing of the initially formed
alcohol with the methyl ester. Experiments in neat Et2CdO show
that the catalyst loading can be <0.1 mol % and that at least 1200
turnovers of the catalyst can be obtained. These reactions are
proposed to proceed by an ionic hydrogenation pathway, with the
highly acidic dihydrogen complex [Cp*Ru(CO)2(η2-H2)]þOTf-
being formed under the reaction conditions from reaction of H2
with {[Cp*Ru(CO)2]2(μ-H)}þOTf -.
reaction and subsequently hydrogenated, suggested that
{[Cp*Ru(CO)2]2(μ-H)}þOTf- might function as a catalyst
for ketone hydrogenations. We report here our studies that
confirm this hypothesis, with examples that show the types of
CdO bonds that can be readily hydrogenated by {[Cp*Ru-
(CO)2]2(μ-H)}þOTf-.
Results and Discussion
The bimetallic bridging hydride complex {[Cp*Ru(CO)2]2-
(μ-H)}þOTf- is easily prepared by reaction of HOTf with
Cp*Ru(CO)2H and can be isolated in excellent yield as a
crystalline solid.3 An alternate preparation involves protonation
of the metal-metal bonded dimer [Cp*Ru(CO)2]2 by HOTf.5
We found that hydrogenation of ketones is catalyzed by
{[Cp*Ru(CO)2]2(μ-H)}þOTf- in CH2Cl2 or 1,2-dichloro-
benzene, as shown in eq 3. Table 1 shows the products
resulting from catalytic hydrogenation of ketones.
Introduction
A huge variety of homogeneous catalysts for ketone hydro-
genation have been developed, with the vast majority of such
catalysts containing phosphine ligands.1 Catalysts that do not
have phosphine ligands can offer some advantages. Since
oxidation of phosphines can lead to catalyst deactivation,
catalyst durability may be improved in phosphine-free catalysts.
Our studies2-4 on new homogeneous catalysts for conver-
sion of biomass-derived materials into useful chemicals led to
the discovery that the selective deoxygenation of 1,2-propa-
nediol to produce n-propanol (eq 1) is catalyzed by
{[Cp*Ru(CO)2]2(μ-H)}þOTf- with added HOTf.
The first two entries show that simple dialkyl ketones are
hydrogenated to give alcohols. In entry 3, hydrogenation of
1-acetylnaphthalene produces 1-ethylnapthalene, the pro-
duct of a double hydrogenation. We propose that the alcohol
initially formed by hydrogenation of the CdO bond under-
goes further reaction (eq 4). Protonation of the alcohol and
loss of water would give a stabilized carbenium ion that can
then abstract hydride from Cp*Ru(CO)2H, leading to the
observed product.
Mechanistic studies of the catalytic deoxygenation reaction
showed that acid-catalyzed dehydration produced propionalde-
hyde, which was subsequently hydrogenated to n-propanol.
*Address correspondence to this author at PNNL. E-mail: morris.bullock@
pnl.gov.
(1) de Vries, J. G.; Elsevier, C. J. Handbook of Homogeneous Hydro-
genation; Wiley-VCH: Weinheim, Germany, 2007.
(2) Schlaf, M.; Ghosh, P.; Fagan, P. J.; Hauptman, E.; Bullock, R. M.
Angew. Chem., Int. Ed. 2001, 40, 3887–3890.
Entry 4 shows that the reaction conditions provide selective
hydrogenation of the CdO bond of a ketone in the presence
(3) Schlaf, M.; Ghosh, P.; Fagan, P. J.; Hauptman, E.; Bullock, R. M.
Adv. Synth. Catal. 2009, 351, 789–800.
(4) Ghosh, P.; Fagan, P. J.; Marshall, W. J.; Hauptman, E.; Bullock,
R. M. Inorg. Chem. 2009, 48, 6490–6500.
(5) (a) Stasunik, A.; Malisch, W. J. Organomet. Chem. 1984, 270,
C56–C62. (b) Nataro, C.; Thomas, L. M.; Angelici, R. J. Inorg. Chem. 1997,
36, 6000–6008.
r
2010 American Chemical Society
Published on Web 01/15/2010
pubs.acs.org/Organometallics

1046 Organometallics, Vol. 29, No. 4, 2010
Fagan et al.
Table 1. Catalytic Hydrogenation of CdO Bonds by{[Cp*Ru-
(CO)2]2(μ-H)}þOTf-a
arising from the condensation of Et2CHOH. Small amounts
of this ether product were previously observed in ionic
hydrogenations of Et2CdO catalyzed by Mo complexes.7,8
The yellow solution still contained some {[Cp*Ru(CO)2]2(μ-
H)}þ. In addition, an orange precipitate had formed, which
was identified as [Cp*Ru(CO)2]2 by its characteristic ν(CO)
IR bands at 1927 and 1745 cm-1 in CH2Cl2. The neutral
dimer results from the deprotonation of {[Cp*Ru(CO)2]2-
(μ-H)}þ (eq 7).
We previously observed decomposition of the catalyst to the
neutral dimer in our catalytic deoxygenation reactions, and
we showed that water deprotonates {[Cp*Ru(CO)2]2(μ-H)}þ
to generate [Cp*Ru(CO)2]2.2,3 This deprotonation may be
caused by the small amount of water produced from con-
densation of the alcohol to deprotonate {[Cp*Ru(CO)2]2(μ-
H)}þ in these ketone hydrogenations, though other uniden-
tified pathway(s) might also lead to the same type of catalyst
decomposition.
a Conditions: 1.0 M ketone in 1,2-dichlorobenzene with 1 mol %
{[Cp*Ru(CO)2]2(μ-H)}þOTf- at 90 ꢀC at 820 psi H2 for 12 h. Yields
>90% for each reaction.
of an ester. In contrast, entry 5 shows that the hydrogenation
of methyl levulinate generates γ-valerolactone, presumably
by ring-closing of the initially formed alcohol with the
methyl ester (eq 5) with expulsion of methanol.
Most of our studies were carried out on the {[Cp*Ru-
(CO)2]2(μ-H)}þ system, but catalytic reactions are also ob-
served in an experiment using the analogous complex with a
Cp rather than Cp* ligand. When a CD2Cl2 solution of
acetophenone (0.19 M) and {[CpRu(CO)2]2(μ-H)}þOTf-
(28 mM) was heated at 85 ꢀC under H2 (4 atm), a 91% yield
of ethylbenzene was detected after 4 days.
Good conversion to hydrogenated products can also
be obtained at significantly lower pressures and lower
temperatures than those used in Table 1. A solution of
Et2CdO (0.14 M) in CD2Cl2 containing {[Cp*Ru(CO)2]2-
(μ-H)}þOTf- (0.028 M) was heated at 65 ꢀC under H2 (4 atm;
We propose that the catalytic hydrogenation of CdO
bonds occurs from the dihydrogen complex [Cp*Ru(CO)2-
(η2-H2)]þOTf- through an ionic hydrogenation mechanism9
similar to that proposed in a series of Mo and W complexes
that we previously reported.7,8 (Scheme 1). Heinekey and co-
workers reported10 that protonation of the neutral Ru
eq 6). After 3 days, the yield of Et2CHOH was 89%, and
complete conversion was observed after 5 days.
hydride Cp*Ru(CO)2H by HBF4 Et2O at low temperature
3
generates [Cp*Ru(CO)2(η2-H2)]þBF4-. They found that this
cationic dihydrogen complex is so acidic that it can be
deprotonated by excess Et2O. Consistent with this result,
Hydrogenation of Et2CdO can also be carried out under
“solvent-free” conditions.6 A solution of {[Cp*Ru(CO)2]2(μ-
H)}þOTf- (7 mM) in Et2CdO (9.5 M as a neat liquid) was
heated at 88 ꢀC under H2 (790 psi). After 63 h, 88% of
Et2CHOH had formed, indicating a turnover number of
about 1200 for the catalyst. At this time, 7% of the Et2CdO
remained, and 5% of the product was the ether, (Et2CH)2O,
(8) (a) Kimmich, B. F. M.; Fagan, P. J.; Hauptman, E.; Bullock,
R. M. Chem. Commun. 2004, 1014–1015. (b) Wu, F.; Dioumaev, V. K.; Szalda,
D. J.; Hanson, J.; Bullock, R. M. Organometallics 2007, 26, 5079–5090.
(9) For reviews of ionic hydrogenations, see: (a) Bullock, R. M.
Chem.;Eur. J. 2004, 10, 2366–2374. (b) Bullock, R. M. Ionic Hydrogena-
tions. In Handbook of Homogeneous Hydrogenation; de Vries, J. G.,
Elsevier, C. J., Eds.; Wiley-VCH: Weinheim, Germany, 2007; Chapter 7;
Vol. 1, pp 153-197.
(10) Chinn, M. S.; Heinekey, D. M.; Payne, N. G.; Sofield, C. D.
Organometallics 1989, 8, 1824–1826.
(11) (a) Cheng, T.-Y.; Brunschwig, B. S.; Bullock, R. M. J. Am.
Chem. Soc. 1998, 120, 13121–13137. (b) Cheng, T.-Y.; Bullock, R. M. J.
Am. Chem. Soc. 1999, 121, 3150–3155.
(6) (a) Tanaka, K. Solvent-free Organic Synthesis; Wiley-VCH:
Weinheim, 2003. (b) Tanaka, K.; Toda, F. Chem. Rev. 2000, 100, 1025–
1074. (c) Cave, G. W. V.; Raston, C. L.; Scott, J. L. J. Chem. Soc., Chem.
Commun. 2001, 2159–2169. (d) Metzger, J. O. Angew. Chem., Int. Ed.
1998, 37, 2975–2978.
(7) (a) Bullock, R. M.; Voges, M. H. J. Am. Chem. Soc. 2000, 122,
12594–12595. (b) Voges, M. H.; Bullock, R. M. J. Chem. Soc., Dalton
Trans. 2002, 759–770. (c) Kimmich, B. F. M.; Fagan, P. J.; Hauptman, E.;
Marshall, W. J.; Bullock, R. M. Organometallics 2005, 24, 6220–6229.
(12) Cheng, T.-Y.; Bullock, R. M. Organometallics 2002, 21, 2325–
2331.

Note
Organometallics, Vol. 29, No. 4, 2010 1047
Scheme 1
{[Cp*Ru(CO)2]2(μ-H)}þ was placed under D2.3 All the
evidence presented above supports the mechanism
shown in Scheme 1, though we cannot rigorously exclude an
alternate mechanism involving hydride transfer from
Cp*Ru(CO)2H to the cationic ketone complex [Cp*Ru-
(CO)2(OdCR2)]þ. This alternate mechanism is shown in
the Supporting Information.
As mentioned in the Introduction, the hydrogenation
catalysts reported here differ from many known catalysts,
since {[Cp*Ru(CO)2]2(μ-H)}þ does not contain phosphines.
Chaudret and co-workers reported a phosphine-free Ru
catalyst for ketone hydrogenation.15 They found that
Tp*RuH(COD) (Tp* = hydridotris(3,5-dimethylpyrazolyl)-
borate; COD = 1,5-cyclooctadiene) was a catalyst precursor
for the hydrogenation of the CdO bond of ketones at 80 ꢀC
under 3 bar of H2 pressure. They observed much lower activity
at higher pressures and ascribed that unusual observation to the
precipitation of Tp*RuH(H2)2 under high-pressure conditions.
Shvo and co-workers found that the bimetallic Ru complex
shown in eq 10 is a catalyst precursor for ketone hydrogenation
under 500 psi of H2 at 145 ꢀC.16 At this temperature the
bimetallic complex cleaves into the 18-electron complex that
performs the hydrogenation and an unsaturated 16-electron
complex that reacts with H2 to form the complex with an acidic
OH and a hydridic RuH bond. Extensive kinetic and mechan-
istic studies by Casey and co-workers revealed that the reaction
proceeds by concerted transfer of a proton from the OH and a
hydride from the RuH bond.17
proton transfer to a ketone as shown in Scheme 1 should be
both thermodynamically favorable and kinetically facile.
Hydride transfer from Cp*Ru(CO)2H would complete
the hydrogenation of the CdO bond, giving an alcohol.
We have reported11,12 the kinetics of hydride transfer of a
series of metal hydrides, and we found that the second-
order rate constant of hydride transfer from Cp*Ru-
(CO)2H to Ph3Cþ is high (k > 106 M-1 s-1 at 25 ꢀC
in CH2Cl2).12 Hydride transfer from Cp*Ru(CO)2H
would result in a 16-electron cation shown as [Ru]þ
in Scheme 1. This cation is unlikely to have a long life-
time and will quickly bind a ketone or alcohol. It
is possible that hydride transfer generates [Cp*Ru-
(CO)2(ROH)]þ, with the alcohol initially bound to the
metal. We previously isolated alcohol complexes of tung-
sten, [Cp(CO)3W(ROH)]þOTf-, from ionic hydrogena-
tions of ketones by CpW(CO)3H and HOTf.13 These
tungsten alcohol complexes were kinetically stabilized,
and the alcohol ligands were displaced by triflate counter-
ion. No direct evidence for such an analogous species
was found in the present studies. Presumably, the alcohol
ligand would be readily displaced under the reac-
tion conditions. Casey and co-workers have observed
and characterized alcohol complexes of ruthenium
with a hydroxycyclopentadienyl ligand, {[2,3,4,5-Ph4(C4-
OH)]Ru(CO)2(ROH)}þBF4-, and have shown that they
exchange with other alcohols at low temperature.14
Alternatively, the complex shown as [Ru]þ in Scheme 1
may bind OTf-. We recently reported3 evidence that
Cp*Ru(CO)2OTf reacts under H2 to produce [Cp*Ru-
(CO)2(η2-H2)]þOTf-, so Cp*Ru(CO)2OTf should also be
an effective catalyst precursor for these hydrogenations. In
fact, in our studies of the catalytic deoxygenation of diols
catalyzed by {[Cp*Ru(CO)2]2(μ-H)}þOTf- with added
HOTf, we found that use of Cp*Ru(CO)2OTf as a catalyst
precursor gave results indistinguishable from those ob-
tained using {[Cp*Ru(CO)2]2(μ-H)}þ.3
Casey and Guan recently reported a mononuclear iron
catalyst for ketone hydrogenation that is closely related to
Evidence for the formation of the acidic dihydrogen com-
plex [Cp*Ru(CO)2(η2-H2)]þOTf
-
from {[Cp*Ru(CO)2]2-
(μ-H)}þ under H2 (eq 9) is provided by experiments in
which HD and {[Cp*Ru(CO)2]2(μ-D)}þ were detected when
(15) Vicente, C.; Shul’pin, G. B.; Moreno, B.; Sabo-Etienne, S.;
Chaudret, B. J. Mol. Catal. A: Chem. 1995, 98, L5–L8.
(16) (a) Blum, Y.; Czarkie, D.; Rahamim, Y.; Shvo, Y. Organome-
tallics 1985, 4, 1459–1461. (b) Shvo, Y.; Czarkie, D.; Rahamim, Y.;
Chodosh, D. F. J. Am. Chem. Soc. 1986, 108, 7400–7402.
(13) (a) 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. (b) Song, J.-S.; Szalda, D. J.; Bullock, R. M. Organometallics
2001, 20, 3337–3346.
(14) Casey, C. P.; Vos, T. E.; Bikzhanova, G. A. Organometallics
2003, 22, 901–903.
(17) (a) Casey, C. P.; Singer, S. W.; Powell, D. R.; Hayashi, R. K.;
Kavana, M. J. Am. Chem. Soc. 2001, 123, 1090–1100. (b) Casey, C. P.;
Johnson, J. B.; Singer, S. W.; Cui, Q. J. Am. Chem. Soc. 2005, 127, 3100–
3109. (c) Casey, C. P.; Johnson, J. B. Can. J. Chem. 2005, 83, 1339–1346. (d)
Casey, C. P.; Beetner, S. E.; Johnson, J. B. J. Am. Chem. Soc. 2008, 130,
2285–2295.

1048 Organometallics, Vol. 29, No. 4, 2010
Fagan et al.
the Shvo-type catalysts.18 Their Fe complex is also thought
to proceed through a metal-ligand bifunctional catalysis
mechanism, and it hydrogenates ketones at 25 ꢀC under
3 atm of H2.
Procedure for Catalytic Hydrogenations. The procedure used
for catalytic hydrogenations is given here, using the hydro-
genation of benzylacetone (4-phenyl-2-butanone) as an exam-
ple. A 2 mL glass vial was charged with 0.150 mL of a 1.00 M
solution of benzylacetone and 0.050 mL of a 0.030 M solution
of {[Cp*Ru(CO)2]2(μ-H)}þOTf- in 1,2-dichlorobenzene. The
reaction was heated at 90 ꢀC for 12 h at 820 psi (5.7 MPa) of
H2 gas in a sealed pressure vessel. The reaction was cooled
and analyzed using gas chromatography/mass spectrometry.
Analysis indicated 100% conversion of the benzylacetone,
with a greater than 90% GC yield of 4-phenyl-2-butanol.
Other hydrogenations reported in Table 1 were carried out
using the same procedure, and the products were identified by
GC/MS.
In summary, {[Cp*Ru(CO)2]2(μ-H)}þOTf- is a readily
prepared homogeneous catalyst for hydrogenation of ke-
tones. These reactions are proposed to proceed by an ionic
hydrogention pathway, through the formation of the catio-
nic dihydrogen complex [Cp*Ru(CO)2(η2-H2)]þOTf-. The
catalyst can be used for solvent-free hydrogenation of ke-
tones, has been used at catalyst loadings as low as 0.1%, and
gives turnover numbers greater than 1200.
Experimental Section
Acknowledgment. Research at Brookhaven National
Laboratory was carried out under contract DE-AC02-
98CH10886 with the U.S. Department of Energy. We
thank the U.S. Department of Energy, Office of Science,
Office of Basic Energy Sciences, Division of Chemical
Sciences, Geosciences, and Biosciences, and the Labora-
tory Technology Research Program, for support. Pacific
Northwest National Laboratory is operated by Battelle
for the U.S. Department of Energy.
General Procedures. All manipulations were carried out under
an atmosphere of nitrogen using Schlenk or vacuum-line techniques
or in a Vacuum Atmospheres drybox. NMR spectra were recorded
1
on Bruker AM-300 (300 MHz for H) and Bruker Avance (400
MHz for 1H) spectrometers. Products were identified by GC/MS
and/or NMR. {[Cp*Ru(CO)2]2(μ-H)}þOTf- was synthesized as
previously reported.3 Ketones were distilled (or sublimed), de-
gassed, and dried. 1,2-Dichlorobenzene was dried over P2O5 and
distilled. Hydrogenations of neat Et2CdO were carried out in a
Parr (model 4561) minireactor. Hydrogenations in NMR tubes
were carried out as described previously.7
Supporting Information Available: Alternative mechanism for
the catalytic reaction. This material is available free of charge via
the Internet at http://pubs.acs.org.
(18) Casey, C. P.; Guan, H. J. Am. Chem. Soc. 2007, 129, 5816–5817.