Development of an amphiphilic resin-dispersion of nanopalladium catalyst: Design, preparation, and its use in aquacatalytic hydrodechlorination and aerobic oxidation

Article abstract of DOI:10.1016/j.jorganchem.2006.02.042

An amphiphilic polystyrene-poly(ethylene glycol) (PS-PEG) resin-dispersion of nanoparticles of palladium was designed and prepared with a view toward use for catalysis in water. The amphiphilic PSPEG resin-dispersion of nanoparticles of palladium exhibited high catalytic performance in the hydrodechlorination of chloroarenes under aqueous conditions. The amphiphilic resin-supported nanopalladium particle also catalyzed alcohol oxidation, which is one of the most fundamental and important yet immature processes in organic chemistry, in water under an atmospheric pressure of oxygen gas to form aldehydes, ketones, and carboxylic acids.

Full text of DOI:10.1016/j.jorganchem.2006.02.042

Journal of Organometallic Chemistry 692 (2007) 420–427
www.elsevier.com/locate/jorganchem
Development of an amphiphilic resin-dispersion of
nanopalladium catalyst: Design, preparation, and its use in
aquacatalytic hydrodechlorination and aerobic oxidation
*
Yasuhiro Uozumi , Ryu Nakao, Hakjune Rhee
Institute for Molecular Science and CREST, Higashiyama 5-1, Myodaiji, Okazaki 444-8787, Japan
Received 27 January 2006; accepted 21 February 2006
Available online 1 September 2006
Abstract
An amphiphilic polystyrene-poly(ethylene glycol) (PS-PEG) resin-dispersion of nanoparticles of palladium was designed and
prepared with a view toward use for catalysis in water. The amphiphilic PSPEG resin-dispersion of nanoparticles of palladium
exhibited high catalytic performance in the hydrodechlorination of chloroarenes under aqueous conditions. The amphiphilic resin-
supported nanopalladium particle also catalyzed alcohol oxidation, which is one of the most fundamental and important yet
immature processes in organic chemistry, in water under an atmospheric pressure of oxygen gas to form aldehydes, ketones, and
carboxylic acids.
Ó 2006 Elsevier B.V. All rights reserved.
Keyword: Aerobic oxidation
1. Introduction
On the other hand, with the advent of accessible meth-
ods for the preparation and/or handling of nanometal par-
ticles, catalytic organic transformations with nanometal
particles have been gaining popularity [5,6]. One approach
employs functional polymer-supported nanoparticles that
can be used in an appropriate reaction medium and readily
removed by filtration. Providing that an amphiphilic resin-
dispersion of palladium nanoparticle catalyst is developed,
the catalyst should exhibit following features combined in
one system; e.g. (1) high catalytic activity owing to the
large surface area of the nanoparticles, (2) water-based
reactivity provided by the amphiphilicity of the PS-PEG
matrix, and (3) good recyclability of heterogeneous cata-
lysts. We wish to report herein design and preparation of
an amphiphilic PS-PEG resin-dispersion of nanosized par-
ticles of palladium, and catalytic hydrodechlorination of
aryl chlorides [7] and aerobic oxidation of alcohols[8] with
the PS-PEG supported palladium nanoparticles under
mild, aqueous conditions.
In recent years, much attention has been focused on
aqueous- [1] and heterogeneous-switching [2,3] of organic
transformations. We have developed amphiphilic polysty-
rene-poly(ethylene glycol) (PS-PEG) resin-supported palla-
dium complexes, which catalyze various synthetic organic
reactions in water under heterogeneous conditions [4].
Thus, for examples, aquacatalytic p-allylic substitution,
Heck reaction, carbonylation, cross-coupling, and cyclo-
isomerization etc. were found to proceed smoothly in water
with the PS-PEG supported palladium–phosphine com-
plexes where organic substrates must diffuse into the
hydrophobic PS matrix to form the highly concentrated
reaction sphere with an anchored catalyst performing effi-
cient organic transformations in water.
*
Corresponding author.
E-mail address: uo@ims.ac.jp (Y. Uozumi).
0022-328X/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2006.02.042

Y. Uozumi et al. / Journal of Organometallic Chemistry 692 (2007) 420–427
421
2. Results and discussion
ride (EDCI; 3 equiv.) and 1-hydroxybenzotriazole hydrate
(HOBt; 4 equiv.) in DMF. A negative Kaiser test [10] indi-
cated that the condensation was completed to form poly-
mer-supported bis(pyridyl) ligand 2 quantitatively. The
polymeric ligand 2 was complexed to palladium by treat-
ment with an equimolar amount of Pd(OAc)2 in toluene
at 25 °C to give the stable 16-electron divalent palladium
complex 3. The complex 3 was treated with benzyl alcohol
in refluxing water for 12 h to generate palladium(0) com-
plex 4 in situ, which should readily release the neutral pal-
ladium species. The nanopalladium particles were
precipitated out in PS-PEG matrix to give the desired
amphiphilic PS-PEG resin-dispersion of palladium nano-
particles (5, ARP-Pd) (loading value: 0.37 mmol Pd/g).
Transmission electron microscopy (TEM) of the resulting
palladium-resin shows that the palladium particles have a
mean diameter of 9.1 nm with a narrow size distribution
(Fig. 1).
2.1. Preparation of an amphiphilic resin-dispersion of
palladium nanoparticles
An amphiphilic resin-dispersion of palladium nanopar-
ticles was readily prepared by reduction of a PS-PEG
resin-supported bispyridine chelating palladium(II) com-
plex (Scheme 1). Thus, a bispyridine chelating ligand
anchored on PS-PEG amino-resin was obtained via dehyd-
rative condensation of N,N-bis(pyrid-2-yl)-4-aminobenzoic
acid (1) and PS-PEG resin bearing primary amino residue
at the terminal of the PEG chain (1% DVB cross-linked,
average diameter of polymer beads = 170 lm)[9] with 1-
(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochlo-
2.2. Catalytic properties of ARP-Pd
The catalytic properties of an amphiphilic resin-disper-
sion of palladium nanoparticles (ARP-Pd) were examined
for hydrodechlorination of aryl chlorides and aerobic oxi-
dation of alcohols in water.
2.2.1. Hydrodechlorination
Hydrodehalogenation of aryl halides, especially of chlo-
roarenes, has been recognized as an important chemical
transformation in organic synthesis as well as in industrial
applications [11]. Furthermore, because of the harmful nat-
ure of aryl chlorides, dechlorination has attracted increas-
ing attention. Consequently,
a
wide variety of
hydrodehalogenation reaction systems have appeared in
the literature, among them, catalytic systems, which are
usually performed with transition metal catalysts (e.g.,
Ni, Rh, Pd) and hydrogen sources (e.g., H2, metal hydrides,
formic acid, hydrazine) [12]. If the hydrodechlorination of
aryl chlorides proceeded in aqueous media with a recycla-
ble catalyst and a mild hydrogen source, the reaction sys-
tem would be an enormous plus since it would meet most
environmental requirements. It was found that hydrode-
chlorination of aryl chlorides took place smoothly in aque-
ous reaction media with ammonium formate in the
presence of the amphiphilic resin-dispersion of palladium
nanoparticles, ARP-Pd. Representative results are shown
in Table 1. When chlorobenzene was treated with ammo-
nium formate in 10% aqueous isopropanol in the presence
of 5 mol% palladium of the ARP-Pd catalyst at 25 °C for
120 min, hydrodechlorination proceeded smoothly to give
benzene in quantitative yield (>99% GC yield) (Table 1,
entry 1). Electron-rich (entries 2–6) as well as electron-defi-
cient (entries 7–10) aromatics were readily dechlorinated
under similar conditions to give the corresponding reduced
products in high yields ranging from 89% to >99%, where
wide functional group tolerance for benzylic hydroxyl, phe-
nolic hydroxyl, amine, ketone, amide, carboxylic acid, and
Scheme 1.

422
Y. Uozumi et al. / Journal of Organometallic Chemistry 692 (2007) 420–427
carboxylic ester was noted. Naphthyl chloride and pyridyl
chloride underwent hydrodechlorination to afford naph-
thalene and pyridine in 95% and 99% yield, respectively
(entries 11 and 12). Complete dechlorination of perchlori-
nated aromatics to non-chlorinated aromatics is of great
value due to the high toxicity of the perchloroarenes. Hyd-
rodechlorination of dichloroarenes (entries 13–16) and tri-
chloroarenes (entries 17–20) took place smoothly with the
ARP-Pd catalyst and ammonium formate to give benzene,
phenol, aniline, and benzoic acid in excellent yields. Nei-
ther partially dechlorinated monochloroarenes or dichloro-
arenes were detected by GC–MS analysis. Dechlorination
of pentachloroaniline was carried out using 3 equiv. of
ammonium formate with respect to the chloroarene under
otherwise similar conditions to give aniline in 85% isolated
yield, using 1 mol% of palladium to the chloroarene (entry
21). Recycling experiments were examined for dechlorina-
tion of methyl 4-chlorobenzoate (Scheme 2). After the first
use of the polymeric nanopalladium catalyst (Table 1, entry
10) to give 94% of methyl benzoate, the recovered catalyst
ARP-Pd was taken on to ten subsequent reuses and exhib-
ited stable catalytic activity. Thus, all ten recycling runs
gave >99% GC yields of methyl benzoate, the first, third
and tenth runs of which were chosen to confirm the isolated
chemical yields of the dechlorinated product to give 95%,
93% and 94% yield of methyl benzoate, respectively.
2.2.2. Aerobic oxidation
Oxidation of alcohols forming carbonyl compounds is
one of the most fundamental and important yet immature
processes in organic chemistry [13]. Thus, although a vari-
ety of methods and reagents for the oxidation have been
developed, until recently the traditional oxidation reactions
have been performed with stoichiometric amounts of heavy
metal reagents (e.g., Cr, Mn, etc.) or moisture sensitive
expensive oxidants (e.g., DCC, oxalyl chloride, etc.) and
often in environmentally undesirable media like chlori-
nated solvents which render them impractical [14]. There
is good reason to believe that the ideal goal of alcohol oxi-
dation would be the aerobic oxidation in water promoted
by a heterogeneous catalyst under atmospheric pressure
conditions resulting in a much cheaper, safer, and more
environmentally benign oxidation protocol [15]. Recently,
much work has appeared on catalytic oxidation of allylic
or benzylic alcohols with molecular oxygen and several pal-
ladium catalyst systems have been developed for the oxida-
tion. To explore the aquacatalytic potential of the ARP-Pd,
we elected to study catalytic aerobic oxidation of alcohols
Fig. 1. A scanning electron microscopy (SEM) image (a), transmission
electron microscopy (TEM) images (b and c), and the respective histogram
of particle size distribution (d).
Scheme 2.

Y. Uozumi et al. / Journal of Organometallic Chemistry 692 (2007) 420–427
423
in water. Representative results are shown in Table 2.
Thus, a mixture of 1 mol% palladium of ARP-Pd and ben-
zyl alcohol was refluxed in water under atmospheric pres-
sure of oxygen gas for 90 min. After being cooled, the
mixture was filtered and the catalyst resin was rinsed twice
with a small portion of diethyl ether. The washings were
concentrated in vacuo to give benzaldehyde quantitatively,
the purity of which was analyzed by proton nuclear mag-
netic resonance (1H NMR) and gas chromatography
(GC) to be 97% without any chromatographic purification
(Table 2, entry 1). During the reaction, no deposit of palla-
dium black on the glass wall (the so-called palladium mir-
ror) was observed. The recovered ARP-Pd showed
essentially the same TEM image and was reused with neg-
ligible loss of catalytic activity. After the workup, the aque-
ous filtrate exhibited no catalytic activity for the oxidation.
These observations would indicate that the palladium spe-
cies does not leach into the aqueous phase under the reac-
tion conditions.
Table 1
Hydrodechlorination of ArCl using ARP-Pd and Ammonium Formatea
ARP
-Pd
Ar Cl
Ar H
NH OCOH
4
Entry
Ar–Cl/Ar–H (X = Cl/H)
Yield (%)b
X
R
Secondary alcohols gave the corresponding ketones
under essentially the same conditions. Thus, phenethyl
alcohol, benzhydrol, and 1-hydroxyindane underwent cat-
alytic oxidation in water to give acetophenone, benzophe-
none, and indanone in >99%, 85%, and 95% yield,
respectively (entries 2, 3 and 4). The catalytic oxidation
of benzoin bearing an a-ketohydroxy group and isophorol
bearing an allylic hydroxy group also proceeded smoothly
to give benzyl and isophorone in 98% and 78% yields
(entries 5 and 6). It is noteworthy that this catalyst system
was also effective on the oxidation of non-activated alka-
nols. Thus, the oxidation of cyclooctanol took place in
water under atmospheric pressure of oxygen gas to give
88% yield of cyclooctanone (entry 7). Catalytic aerobic
oxidation of primary alcohols was carried out in 0.2 M
aqueous solution of potassium carbonate under otherwise
similar conditions. Thus, 1-octanol, 1-hexanol, and 1-
butanol were oxidized by 20 mol% palladium of the
ARP-Pd catalyst under oxygen gas (1 atm) in refluxing
water in the presence of potassium carbonate. The mixture
was filtered and the filtrate was acidified with hydrochloric
acid, and extracted with diethyl ether to give excellent
yields of the corresponding carboxylic acids (entries 11,
12, and 13). Recycle experiments were examined for the
oxidation of cyclooctanol. Thus, after the first run giving
88% yield of cyclooctanone (entry 7), the ARP-Pd catalyst
was successively subjected to a second oxidation to give
86% yield of cyclooctanone (entry 8), the average of the
chemical yields for 4 continuous runs being 86%, demon-
strating the practical recyclability of this catalyst (entries
7–10).
1
2
3
4
5
6
7
8
9
R = H
> 99c
> 99c
99
89
93
91
97
99
> 99
94
R = CH3
R = CH2OH
R = OCH3
R = OH
R = NH2
R = COCH3
R = CONH2
R = COOH
R = COOCH3
X
10
11
12
95c
X
> 99c
N
X
R
X
13
14
15
16
R = H
R = OH
R = NH2
99c
97
92
99
R = COOH
X
X
R
X
17
18
19
20
21
R = H
R = OH
R = NH2
> 99c
96
92
99
R = COOH
X
85
X
X
3. Conclusion
X
H2N
In summary, a novel amphiphilic PS-PEG resin-disper-
sion of nanoparticles of palladium, ARP-Pd, was designed
and prepared with a view toward using it for palladium
catalysis in water. ARP-Pd exhibited high catalytic perfor-
mance in the hydrodechlorination of chloroarenes and the
aerobic oxidation of alcohols under aqueous conditions.
Dechlorination was found to take place smoothly using
X
a
All reactions were carried out in 10% aqueous 2-propanol in the
presence of 5 mol % palladium (with respect to substrate) of ARP-Pd and
3 equiv. of ammonium formate with respect to the chloroarene at 25 °C
for 120 min.
b
Isolated yield unless otherwise noted.
c
GC yield.

424
Y. Uozumi et al. / Journal of Organometallic Chemistry 692 (2007) 420–427
Table 2
ARP-Pd Catalyzed Oxidation of Alcohols in Watera
ARP
-Pd
O
O (1 atm)
H
OH
R
2
1
2
1
2
H O, reflux
R
R
R
2
Entry
1
Substrate
Product
Pd (mol%)
Time (h)
1.5
Yield (%)
97
1
5
CHO
OH
2
3
OH
O
20
20
> 99
OH
O
5
85
4
5
OH
O
5
1
20
95
98
2.5
OH
O
O
O
6
7
OH
O
5
20
20
78
88
20
O
OH
8
9
(second run)
(third run)
(forth run)
20
20
20
40
40
40
86
83
87
90
98
93
10
11
12
13
a
n-C8H17OH
n-C6H13OH
n-C4H9OH
n-C7H15COOH
n-C5H11COOH
n-C3H7COOH
20
20
20
All reactions were carried out in refluxing water under 1 atm of oxygen gas. Pd mol%/reaction time (h): entries 1 and 5 = 1/2, entries 2-5 = 5/20, entry
7 = 20/20, entries 11–13 = 20/40. A ratio of alcohol (mmol)/water (mL) = 1/1.3. All yields are for the pure products. Oxidation of 1-alkanols were
performed in 0.2 M aqueous solution of potassium carbonate.
aqueous ammonium formate with high recyclability, which
would provide a safe and clean detoxification protocol for
aqueous chloroarene pollutants. The catalytic alcohol oxi-
dation was achieved in water under atmospheric pressure
of molecular oxygen with high recyclability to achieve high
level of chemical greenness.

Y. Uozumi et al. / Journal of Organometallic Chemistry 692 (2007) 420–427
425
4. Experimental
the water phase was observed, which would have resulted
in the formation of palladium black). The mixture was fil-
tered and the resulting resin beads were rinsed three times
with water and three times with acetone, then dried in
vacuo to give ARP-Pd.
4.1. General
All starting materials and the authentic samples of the
products (dechlorination and oxidation) were purchased
from Aldrich Chemical Co. Inc. and Tokyo Kasei Co.
Ltd., and used without further purification. All catalytic
reactions were carried out with shaking on a wrist-action
shaker (Burrel Scientific Inc.) or a Peti-Syzer (Hi-Pep Co.
Ltd.). All reaction products were identified by comparison
of their GC-MS spectra (performed on Hewlett Packard
HP 6890 with 5973 Network Mass Detector) and 1H
NMR spectra (JEOL JNM-AL400 spectrometer
4.5. General procedure for the hydrodechlorinaion
A typical procedure for the reaction of methyl 4-chloro-
benzoate in the presence of ARP-Pd to give benzoic acid is
as follows: To a mixture of ARP-Pd (60 mg, 24 lmol Pd)
and methyl 4-chlorobenzoate (85 mg, 0.5 mmol) in 10%
aqueous isopropanol (1.0 mL) was added an aqueous solu-
tion of ammonium formate (63 mg, 1.0 mmol in 0.2 mL of
H20) at 25 °C. The mixture was stirred at 25 °C for
120 min. The reaction mixture was filtered and the catalyst
beads were extracted with ethyl acetate (5 mL, 3 times).
The combined extract was washed with brine and dried
over Na2SO4 (100% conversion; >99% GC yield). The
crude product was chromatographed on silica gel to give
94% isolated yield of methyl benzoate. GC–MS spectrum
of the product showed good similarity (>95%) with the
authentic data.
(400 MHz)
or
JEOL
JNM-AL500
spectrometer
(500 MHz)) with the authentic data. Swollen gel-phase
magic angle spinning carbon 13 NMR were recorded on
JEOL JNM-AL400 spectrometer (100 MHz for 13C).
Chemical shifts of 13C NMR are given relative to CDCl3
as an internal standard (d 77.0).
4.2. PS-PEG resin-supported bispyridine ligand 2
A mixture of 5.0 g of PS-PEG amino resin (average
diameter = 170 lm, 1% divinylbenzene cross-linked, load-
ing value of amino residue = 0.4 mmol/g), EDCI
(709 mg, 3.70 mmol), HOBt (625 g, 4.63 mmol), and 4-
(bis(pyrid-2-yl)amino)benzoic acid (808 mg, 2.78 mmol) in
60 mL of DMF was shaken at room temperature for
10 h. The mixture was filtered and the resulting resin beads
were rinsed three times with DMF and three times with
dichloromethane, then dried in vacuo to give 2. Swollen
gel-phase magic angle spinning carbon 13 NMR (charac-
teristic signals are given below. Signals of PS and PEG
moieties are omitted for simplicity): d 117.2, 118.5, 125.4,
128.2, 130.5, 137.4, 147.4, 148.3, 157.6, 166.4.
4.6. Catalytic oxidation of alcohols in water
A typical procedure is given for the oxidation of 1-hex-
anol. A mixture of 106 mg of ARP-Pd (39 lmol of palla-
dium), 1-hexanol (22 lL, 0.17 mmol), and potassium
carbonate (24 mg, 0.17 mmol) in 0.9 mL of water was
refluxed for 40 h under an atmospheric pressure of oxygen
gas. After being cooled, the mixture was filtered and the
resin beads were rinsed three times with saturated aqueous
sodium bicarbonate solution. The combined filtrate was
acidified with 5% hydrochloric acid and extracted with
diethyl ether. The extract was dried over magnesium sulfate
and concentrated in vacuo to give 20 mg of hexanoic acid
(98%) as a pure compound (>99% purity on GC and pro-
ton NMR analysis).
4.3. PS-PEG resin-supported palladium acetate-bispyridine
complex 3
A mixture of 5.5 g of 2 (loading value of ligand resi-
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for simplicity): d 23.0, 116.4, 117.2, 120.1, 129.7, 135.7,
140.0, 142.1, 150.3, 150.7, 165.5, 177.5.
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