Palladium nanoclusters supported on propylurea-modified siliceous mesocellular foam for coupling and hydrogenation reactions

Article abstract of DOI:10.1002/chem.200701361

This paper describes the synthesis, characterization and applications of palladium (Pd) nanoparticles supported on siliceous mesocellular foam (MCF). Pd nanoparticles of 2-3 nm and 4-6 nm were used in reactions involving molecular hydrogen (such as hydrogenation of double bonds and reductive amination), transfer hydrogenation of ketones and epoxides, and coupling reactions (such as Heck and Suzuki reactions). They successfully catalyzed all these reactions with excellent yield and selectivity. This heterogeneous catalyst was easily recovered by filtration, and recycled several times without any significant loss in activity and selectivity. The palladium leaching in the reactions was determined to be much less than the FDA-approved limit of 5 ppm. Furthermore, the catalyst can be stored and handled under normal atmospheric conditions. This immobilized catalyst allows for ease of recovery/reuse and minimization of waste generation, which are of great interest in the development of green chemical processes.

Full text of DOI:10.1002/chem.200701361

DOI: 10.1002/chem.200701361
Palladium Nanoclusters Supported on Propylurea-Modified Siliceous
Mesocellular Foam for Coupling and Hydrogenation Reactions
Nandanan Erathodiyil, Samuel Ooi, Abdul M. Seayad, Yu Han, Su Seong Lee, and
Jackie Y. Ying*[a]
Abstract: This paper describes the syn-
thesis, characterization and applications
of palladium (Pd)nanoparticles sup-
ported on siliceous mesocellular foam
(MCF). Pd nanoparticles of 2–3 nm
and 4–6 nm were used in reactions in-
volving molecular hydrogen (such as
hydrogenation of double bonds and re-
ductive amination), transfer hydroge-
nation of ketones and epoxides, and
coupling reactions (such as Heck and
Suzuki reactions). They successfully
catalyzed all these reactions with excel-
lent yield and selectivity. This hetero-
geneous catalyst was easily recovered
by filtration, and recycled several times
without any significant loss in activity
and selectivity. The palladium leaching
in the reactions was determined to be
much less than the FDA-approved
limit of 5 ppm. Furthermore, the cata-
lyst can be stored and handled under
normal atmospheric conditions. This
immobilized catalyst allows for ease of
recovery/reuse and minimization of
waste generation, which are of great in-
terest in the development of green
chemical processes.
À
Keywords: C C coupling reactions ·
heterogeneous
hydrogenation
catalysis
·
·
mesoporous
materials · palladium nanoparticles
Introduction
more efficient heterogeneous catalysts.[6,10] Immobilized ho-
mogeneous catalysts would provide for ease of catalyst re-
covery and reuse, and would minimize the waste generation
and use of toxic chemicals, which is of great interest in the
development of green chemical processes.[7]
The pharmaceutical industry produces 25–100 kg or more of
waste for every kg of active pharmaceutical ingredient
[1]
(API)manufactured.
According to a leading practitioner
of the industry, the potential waste co-produced with APIs
is in the range of 500 million to 2 billion kg per year.[2] Thus,
the potential savings associated with waste reduction would
be significant compared to pharmaceutical industryꢀs annual
sales (US$500 billion in 2003).[2] Most pharmaceuticals syn-
theses involve the use of homogeneous catalysts, which are
difficult to separate from the products. The resulting metal
contamination of the products poses a serious concern in
the pharmaceutical industry.[3] Heterogeneous catalysts may
be more stable, less expensive, and more easily separated
from the products.[4] However, their activity and selectivity
are often lower than that of homogeneous catalysts.[5] Thus,
increasing efforts have been devoted towards developing
This work is focused on designing supported palladium
catalysts. Palladium-catalyzed reactions have become an im-
portant tool in organic synthesis, owing to their high effi-
ciency, selectivity, and their ability to perform a range of
possible transformations.[8] Palladium-based catalysts have
shown remarkable performance in coupling and hydrogena-
tion reactions.[9] However, despite their high activity, the ho-
mogeneous palladium-based catalysts suffer from low stabil-
ity and high costs, which prevent their application in indus-
trial processes. As a heavy metal, palladium is also highly
undesirable as a contaminant of pharmaceutical products.
To overcome these challenges associated with conventional
palladium-based catalysts, heterogeneous and heterogenized
catalysts have been developed.[10] Palladium has been sup-
ported on materials such as carbon, zeolites, silicates and
polymers for catalytic applications.[11] Although these sup-
ported palladium catalysts allowed for ease of recovery, pal-
ladium leaching remained a significant problem.[12]
[a] Dr. N. Erathodiyil, S. Ooi, Dr. A. M. Seayad, Dr. Y. Han,
Dr. S. S. Lee, Prof. J. Y. Ying
Institute of Bioengineering and Nanotechnology, 31 Biopolis Way,
The Nanos, Singapore 138669 (Singapore)
Fax : (+65)6478-9020
E-mail: jyying@ibn.a-star.edu.sg
Supporting information for this article is available on the WWW
under http://www.chemeurj.org/ or from the author.
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Chem. Eur. J. 2008, 14, 3118 – 3125

FULL PAPER
Results and Discussion
tration during the synthesis. For example, if the amount of
PdACHTRE(UNG OAc)2 was reduced by half, a similar procedure would
In this study, palladium was immobilized on siliceous meso-
cellular foam (MCF)through the use of urea ligands (see
Scheme 1). MCF was selected as the support, as it possesses
result in the formation of 2–3 nm Pd particles (Figure 2).
Scanning transmission electron microscopy (STEM)(Fig-
ure S3 in the Supporting Information)and Si and Pd map-
ping by transmission electron
microscopy (TEM)(see the
Movie S1 in the Supporting In-
formation)illustrated the uni-
formity of the Pd particles dis-
persed within the mesopores of
MCF. Scanning electron micro-
Scheme 1. Immobilization of Pd nanoclusters on Urea-MCF.
scopy (SEM)(Figure 3)and ni-
a three-dimensional, interconnected pore structure with ul-
tralarge cell-like pores (24–42 nm)connected by windows of
9–22 nm.[13] Templated by oil-in-water microemulsions, MCF
also has a high surface area of 500–800 m2gÀ1 and a high sur-
face concentration of silanols. The urea groups were intro-
duced onto the pore surface of spherical MCF microparti-
cles[14] by reacting (CH3O)3Si
(CH3O)3Si(CH2)3NHCONH(CH2)3Si(OCH3)3 with 5508C-
ACHTRE(UNG CH2)3NHCONH2 or
A
N
ACHTREUNG
calcined MCF in toluene at 808C for 24 h. The urea ligand
loading in the resulting Urea-MCF was determined by ele-
mental analysis to be 1.8 mmolgÀ1. Pd
ACHTRE(UNG OAc)2 (1.5 mmol)
was dissolved in toluene/ethyl acetate, and introduced to
Urea-MCF (1 g). The mixture was stirred at room tempera-
ture for 30 min, and then heated at 608C for 24 h. The ini-
tially dark brown palladium acetate solution was reduced to
elemental palladium, and was deposited onto the MCF sup-
port via the urea ligands. The Pd/Urea-MCF was filtered,
washed and dried. The supernatant was clear after the reac-
tion, indicating the complete formation and immobilization
of Pd on MCF after the heating and drying processes.
Figure 2. TEM micrograph of Pd/Urea-MCF with Pd nanoclusters of 2–
3 nm.
The Pd nanoclusters deposited on Urea-MCF have a size
distribution in the range of 4–6 nm (Figure 1), and were dis-
persed uniformly on the support (see Figures S1 and S2 in
the Supporting Information). The size of the nanoclusters
could also be controlled by changing the PdCAHTRE(UNG OAc)2 concen-
Figure 3. SEM micrographs of Pd/Urea-MCF microparticles.
trogen adsorption-desorption isotherms (Figures S4 and S5
in the Supporting Information)illustrated that the uniform,
ultralarge mesopores of the MCF support were retained in
the Pd/Urea-MCF catalysts. X-ray diffraction (XRD)studies
confirmed the ultrafine grain size and high dispersion of Pd
nanoclusters in the Pd/Urea-MCF catalyst (see Figures S6
and S7a in the Supporting Information).
The catalytic activity of Pd/Urea-MCF was examined for
the Suzuki coupling reaction of iodoanisole and phenyl bor-
onic acid in an ethanol/water mixture (9:1 volume ratio), by
using Na2CO3 as the base (Table 1). After 24 h, the coupling
product was isolated in 99% yield (Table 1, entry 1). Pd/
Figure 1. TEM micrograph of Pd/Urea-MCF with Pd nanoclusters of 4–
6 nm.
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Y. J. Ying et al.
Table 1. Suzuki coupling of aryl halides with aryl boronic acids.[a]
Pd/Urea-MCF catalyst also
provided excellent yields for
the Heck coupling of iodoar-
enes (Table 2). Coupling of
iodo and bromoarenes with
acrylates resulted in the corre-
sponding cinnamate esters in
good to excellent yields. The
catalyst was recycled without
any significant loss in activity
and selectivity in the coupling
reaction of iodobenzene with n-
butyl acrylate (Table 2, entry 5).
Excellent yields were ach-
ieved for the transfer hydroge-
nation of various ketones over
Pd/Urea-MCF. Typically, trans-
fer hydrogenation of ketones
was carried out using 10 mol%
of the 5 wt% Pd/Urea-MCF
catalyst and 5 equivalents of
formic acid/triethylamine mix-
ture as the hydrogen source in
ethyl acetate at room tempera-
ture. The heterogeneous cata-
lyst was easily recovered and
reused several times without
any loss in reactivity and selec-
tivity (Table 3). After 10 runs of
transfer hydrogenation of ace-
tophenone (Table 3, entry 1),
the Pd nanoclusters remained
highly dispersed on Urea-MCF
(Figure 5), and the catalyst
demonstrated negligible loss in
activity. The X-ray diffraction
(XRD)pattern of Pd/Urea-
MCF consisted of Pd0 peaks.
Peak broadening analysis by
Scherrerꢀs method indicated
that the average Pd crystallite
Entry
Aryl halide
Aryl boronic acid
Product
t [h]
Yield [%][b]
1
6
99
0.167[c]
99[c]
89
2
3
12
12
97
94
91
4
12
12
12
5
6[d]
99
98
96
6[e]
7
8
20
12
92
[a] Reaction conditions: 1 mol% of 5 wt% Pd/Urea-MCF catalyst with Pd nanoclusters of 2–3 nm, 1 mmol of
aryl halide, 1.25 mmol of aryl boronic acid, 1.5 mmol of sodium carbonate, 5 mL of ethanol/water mixture
(volume ratio=9:1), 808C, argon atmosphere. [b] Isolated yield. [c] Under microwave conditions. [d] Recycled
five times without any loss in the product yield. [e] Volume ratio of ethanol/water mixture used=1:1.
Urea-MCF catalysts with 2–3 nm and 4–6 nm Pd nanoclus-
ters were compared to commercial 10 wt% Pd/C catalyst
(Sigma-Aldrich)with 2–3 nm Pd nanoclusters for the Suzuki
coupling of iodobenzene and phenylboronic acid. Pd/Urea-
MCF with 2–3 nm Pd nanoclusters outperformed the com-
mercial 10 wt% Pd/C catalyst and the Pd/Urea-MCF cata-
lyst with 4–6 nm Pd nanoclusters (Figure 4). Pd leaching was
examined by analyzing the supernatant of the reaction. No
Pd was found to have leached from the Urea-MCF support
even after 5 days of a blank experiment under similar reac-
tion conditions. The Suzuki reaction proceeded faster when
the solvent was changed to an ethanol/water mixture of 1:1
volume ratio at 808C (Table 1, entry 6), giving a 98% yield
Figure 4. Suzuki coupling of iodobenzene and phenylboronic acid (first
in 6 h instead of 24 h under the typical homogeneous reac-
~
&
^
run)over ( )10% Pd/C, and Pd/Urea-MCF with ( )2–3 nm and ( )4–
6 nm Pd nanoclusters.
tion conditions.[15]
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Supported Palladium Catalysis
FULL PAPER
Table 2. Heck coupling of aryl halides and alkenes.[a]
in 99.9% yield, and the catalyst
was successfully recycled and
reused for 10 runs without any
loss in activity. Hydrogenation
also proceeded with excellent
yield and catalyst recyclability
Entry
Aryl halide
Alkene
Product
t [h]
Yield [%][b]
1
18
96
for
a
variety of olefins
(Table 4).
Next, the catalytic activity of
Pd/Urea-MCF for the reductive
amination of aldehydes and
amines was examined. This re-
action is very important for
producing secondary amines in
the pharmaceuticals and spe-
cialty chemicals industry. Excel-
lent conversions and yields of
the corresponding amines were
achieved under mild conditions,
and the catalyst was recycled 10
times without any significant
loss in reactivity and selectivity
(Table 5).
The reductive ring opening of
epoxides to the corresponding
alcohols has emerged as a pow-
erful tool in organic synthesis.
It is of great interest to develop
a practical, economical, and en-
vironmentally friendly process
for this reaction. The efficiency
and stability of Pd/Urea-MCF
catalyst were examined with
trans-stilbene oxide as a sub-
strate (Table 6, entry 4). Nota-
bly, the hydrogenolysis reaction
reached completion with excel-
lent yields in 10 successive runs.
2
3
4
24
18
24
92
95
89
5[c]
20
24
20
24
92
88
93
84
6
7
8
[a] Reaction conditions: 1 mol% of 5 wt% Pd/Urea-MCF catalyst with Pd nanoclusters of 2–3 nm, 1 mmol of
aryl halide, 1.25 mmol of olefin, 2 mmol of triethylamine, 5 mL of toluene, 1008C, argon atmosphere. [b] Iso-
lated yield. [c] Recycled five times without loss in activity and selectivity.
size grew slightly from 2–3 nm to 4–5 nm after 10 runs (see
Figure S7a,b in the Supporting Information). The filtrate of
the reaction system was clear (see Figure S8b in the Sup-
porting Information); inductively coupled plasma mass spec-
trometry (ICP-MS)confirmed that only a total of 1.25% of
the Pd loaded in 5 wt% Pd/Urea-MCF was leached after 10
runs. In contrast, the filtrate of the reaction system contain-
ing 10 wt% Pd/C was brown in color (see Figure S8a in the
Supporting Information), owing to almost 90% leaching of
the Pd loaded after just one run. Interestingly, when the
transfer hydrogenation of various ketones was performed in
water at 608C with ammonium formate as the hydrogen
source instead of formic acid/triethylamine mixture, the re-
action was completed more rapidly (i.e., in 2 h instead of
24 h), giving an excellent yield of the corresponding alcohol.
Pd/Urea-MCF was also examined for the hydrogenation
of activated olefins (Table 4)such as dimethylitaconate
(Table 4, entry 1)under a low pressure (40 psi.) The hydro-
genated product, 1-methyl-dimethylsuccinate, was obtained
The catalyst could be recovered by simple filtration, and be
reused without loss of activity. A variety of benzylic epox-
ides were then subjected to the same hydrogenolysis condi-
tions, and good yields of homobenzylic alcohols were ob-
tained consistently. To further extend the scope of this cata-
lytic system, we turned our attention to the hydrogenolysis
of chiral epoxides, which are used for the synthesis of anti-
inflammatory arylpropionic acids. Hydrogenolysis of chiral
a-methylstyrene oxide generated the corresponding terminal
alcohol with the retention of configuration in excellent yield
(Table 6, entry 8).
In the cases of transfer hydrogenation, hydrogenation, hy-
drogenolysis, and reductive amination, the total Pd leached
after 10 recycles was determined by ICP-MS to be <1.5%
of the total Pd loaded in the catalyst. In the cases of Heck
and Suzuki coupling reactions, the total Pd leached after 5
recycles was <5% as determined by ICP-MS. The superna-
tant of the heterogeneous reaction systems containing Pd/
Urea-MCF was very clear (Figures S8 and S9 in the Sup-
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3121

Y. J. Ying et al.
Table 3. Transfer hydrogenation of ketones.[a]
iodobenzene and p-methoxy-
phenyl boronic acid as well as
the transfer hydrogenation of
Entry
Ketone
Product
t [h]
Yield [%][b]
87
acetophenone
demonstrated
1[c]
24
the advantage of urea ligand in
the formation and stabililization
of nanoparticles (Table S1 in
the Supporting Information).
Pd/Urea-MCF was found to
provide even faster reaction
rates for coupling reactions and
transfer hydrogenations with
the aid of microwave. Excellent
0.25[d]
2[e]
85
82
2
24
2[e]
88
84
88
3
4
24
conversions
were
achieved
under low frequency and short
reaction period. For example,
99% conversion was attained in
10 min in the Suzuki coupling
of iodobenzene and phenylbor-
onic acid (Table 1, entry 1).
85% yield of the corresponding
alcohol was achieved in 15 min
for the transfer hydrogenation
of acetophenone under micro-
wave (Table 3, entry 1). In con-
trast, 87% yield would be ob-
tained in 24 h under normal re-
action conditions at room tem-
perature.
24
2[e]
84
80
96
5
18
18
24
6[c]
99
7
92
94
99
2[e]
8
9
48
24
Conclusion
95
99
In conclusion, active Pd nano-
clusters could be easily pre-
pared with high yields on urea-
modified MCF. They were
formed by the reduction of Pd-
10[c]
12
AHCTREU(NG OAc)2, and were stabilized by
the urea ligands on the MCF
surface. The ultralarge meso-
pores of MCF facilitated reac-
tions involving bulky substrates.
The resulting Pd/Urea-MCF
catalyst demonstrated excellent
activity for transfer hydrogena-
11
12
12
15
99
99
À
tion, hydrogenation and C C
[a] Reaction conditions: 10 mol% of 5 wt% Pd/Urea-MCF catalyst with Pd nanoclusters of 2–3 nm, 1 mmol of
ketone, 5 mmol of formic acid/triethylamine (volume ratio=1:1), 5 mL of ethyl acetate, 258C. [b] Isolated
yield. [c] Recycled 10 times without any loss in activity and selectivity. [d] Under microwave conditions.
[e] Used ammonium formate as hydrogen source and water as solvent at 608C.
coupling reactions, and were su-
perior to commercially avail-
able 10 wt% Pd/C or polymer-
supported Pd-Encat.[16] The
facile synthesis and novel
design of supported metal cata-
porting Information). Negligible leaching of Pd was ob-
served, and this novel catalyst could be easily isolated and
recycled. A comparative study of Pd/Urea-MCF with Pd/
silica gel and Pd/unmodified MCF in the Suzuki coupling of
lyst described herein might be widely applied to derive met-
allic nanoclusters supported on modified MCF for a wide
variety of catalytic reactions in chemical syntheses. We are
currently examining other Pd-catalyzed reactions, and im-
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Supported Palladium Catalysis
Table 4. Hydrogenation of olefins.[a]
FULL PAPER
Experimental Section
Entry
Olefin
Product
t [h]
Yield [%][b]
99.9
All chemicals were purchased from
Aldrich, and used as received without
further purification. 1H and 13C nucle-
ar magnetic resonance (NMR)spec-
troscopy (Bruker 400 MHz Spectrome-
1[c]
6
ter),
Fourier-transform
infrared
2
3
6
6
99.9
99.9
(FTIR)spectroscopy
(Digilab
FTS7000 FTIR spectrometer equipped
with MTEC 300 photoacoustic detec-
tor), XRD (Philips XPert PRO X-ray
diffraction system), N2 adsorption-de-
sorption
analysis
(Micromeritics
ASAP 2020m system), elemental anal-
ysis (Exeter Analytical CE440 CHN
analyzer), TEM and STEM (FEI
Tecnai G20, 200 kV), SEM (JEOL
JSM-6700F)and ICP-MS (Elan
DRCII, PerkinElmer SCIEX)were
employed in catalyst characterization.
Products from catalytic reactions were
analyzed by GC (Agilent 6890N).
4
6
99.9
5
8
8
99.9
99.9
Synthesis of Urea-MCF: Spherical
MCF microparticles (1 g)synthesized
according to the method reported[14]
were dried for 24 h at 1008C, and
cooled to room temperature under
argon. Dry toluene (20 mL)was added
to the MCF. Next, a solution of trime-
thoxysilylpropyl urea (2.2 mmol)in
toluene (2 mL)was introduced. The
mixture was stirred under argon for
10 min, and then heated at 808C for
24 h. It was cooled to room tempera-
ture, filtered, and washed several
times with toluene, ethanol, acetone
and dichloromethane to remove any
unreacted precursor. The resulting ma-
terial was suspended in ethanol, and
6[c]
7
8
18
18
99
99
[a] Reaction conditions: 1 mol% of 5 wt% Pd/Urea-MCF catalyst with Pd nanoclusters of 2–3 nm, 1 mmol of
olefin, 40 psi or 100 psi of hydrogen (for entries 1–6 and 7–8, respectively), 5 mL of methanol/ethanol, 258C.
[b] Isolated yield. [c] Recycled 10 times without any loss in product yield.
heated at 608C overnight, filtered, washed and dried. Elemental analysis
showed a loading of 1.80 mmol of urea per gram of MCF.
Synthesis of Pd/Urea-MCF: Urea-MCF (1 g)was suspended in dry tolu-
ene (20 mL), and a solution of palladium acetate (0.55 or 1.10 mmol de-
pending on the requirement)in CH 2Cl2 (2 mL)was added dropwise. The
mixture was heated at 608C until the supernatant became colorless under
argon (24 h). It was then cooled to room temperature, filtered, washed
and dried to obtain a dark brownish black solid, Pd/Urea-MCF. Elemen-
tal analysis showed 0.55 or 1.10 mmol of Pd loading per gram of Urea-
MCF, corresponding to 5 wt% Pd/Urea-MCF or 10 wt% Pd/Urea-MCF,
respectively.
General Procedure for Suzuki Coupling Reaction: An oven-dried reac-
tion vial was charged with aryl halide (1 mmol), aryl boronic acid
(1.25 mmol), sodium carbonate (1.5 mmol), catalyst (1 mol% of 5 wt%
Pd/Urea-MCF,) ethanol (4.5 mL)and water (0.5 mL)under argon. The
resulting reaction mixture was stirred at room temperature for 10 min,
and then heated at 808C for a desired period. The reaction was moni-
tored by GC. After completion of the reaction, the mixture was cooled
to room temperature, and was filtered through a sintered glass funnel,
washed with water (5”5 mL)and ethanol (5”5 mL,) and dried under
vacuum. The catalyst was recycled five times.
Figure 5. TEM micrographs of Pd/Urea-MCF catalysts with Pd nanoclus-
ters of a)4–6 nm and b)2–4 nm after 10 runs of transfer hydrogenation
of acetophenone at room temperature.
mobilizing other metals (e.g. Rh, Ru, Cu, Ir, and In)on
MCF for various catalytic applications.
General Procedure for Heck Coupling Reaction: An oven-dried reaction
vial was charged with aryl halide (1 mmol), olefin (1.25 mmol), triethyla-
mine (2 mmol), catalyst (1 mol% of 5 wt% Pd/Urea-MCF), and toluene
(5 mL)under argon. The resulting reaction mixture was stirred at room
temperature for 10 min, and then heated at 1008C for a desired period.
The reaction was monitored by GC. After completion of the reaction,
the mixture was cooled to room temperature, and was filtered through a
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3123

Y. J. Ying et al.
Table 5. Reductive amination of aldehydes.[a]
funnel, washed with methanol (5”
5 mL), and dried under vacuum. The
catalyst was recycled 10 times.
Entry
Amine
Aldehyde
Product
t [h]
Yield [%][b]
General Procedure for Hydrogenolysis
of Epoxides and Diols: An oven-dried
reaction vial was charged with epoxide
or diol (1 mmol), ammonium formate
(5 mmol), catalyst (10 mol% of 5 wt%
Pd/Urea-MCF)and ethyl acetate
(5 mL)under argon. The resulting re-
action mixture was stirred at room
temperature for 24 h, and the progress
of the reaction was monitored by GC.
After completion of the reaction, the
mixture was filtered through a sintered
glass funnel, washed with ethyl acetate
(5”5 mL), and dried under vacuum.
1[c]
6
99.9
2[c]
8
99.9
3
4
6
8
99.9
99.9
Acknowledgements
We thank Dr. Santhiagu Ezhilvalavan
for his assistance with XRD studies.
This work was supported by the Insti-
tute of Bioengineering and Nanotech-
nology (Biomedical Research Council,
Agency for Science, Technology and
Research, Singapore).
5
6
6
8
99.9
99.9
[a] Reaction conditions: 1 mol% of 5 wt% Pd/Urea-MCF catalyst with Pd nanoclusters of 2–3 nm, 1.01 mmol
of amine, 1.0 mmol of aldehyde, 40 psi of hydrogen, 5 mL of methanol, 258C. [b] Gas chromatography (GC)
yield. [c] Recycled 10 times without any loss in activity and selectivity.
sintered glass funnel, washed with toluene (5”5 mL), and dried under
vacuum. The catalyst was recycled five times.
[1] This refers to the concept of atom efficiency applied to chemical
synthesis presented by B. Trost and R. Sheldon. See for example, T.
Collins, Science 2001, 291, 48–49.
[2] a)B. W. Cue, Jr., Chem. Eng. News 2005, 83(39), 46–47; b) Chem.
Eng. News 2005, 83(36), 40–48; c) Chem. Eng. News 2005, 83(23),
21–29.
General Procedure for Transfer Hydrogenation of Ketones: An oven-
dried reaction vial was charged with ketone (1 mmol), formic acid/trie-
thylamine mixture (volume ratio=1:1)or ammonium formate (5 mmol),
catalyst (10 mol% of 5 wt% Pd/Urea-MCF)and ethyl acetate or water
(5 mL)under argon. The resulting reaction mixture was stirred at room
temperature for 24 h, and the progress of the reaction was monitored by
GC. After completion of the reaction, the mixture was filtered through a
sintered glass funnel, washed with ethyl acetate (5”5 mL), and dried
under vacuum. The catalyst was recycled 10 times.
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Received: August 30, 2007
Published online: February 7, 2008
Chem. Eur. J. 2008, 14, 3118 – 3125
ꢁ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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