A highly active heterogeneous palladium catalyst supported on a synthetic adsorbent

Article abstract of DOI:10.1002/chem.200801989

A method to prepare heterogeneous palladium catalyst highly dispersed in a synthetic adsorbent DIAION HP20, has been demonstrated. Heterogeneous palladium catalyst are suitable for organic synthetic chemistry due to their high air-stability, reconverability, reusability, and non-residual property. The catalyst was obtained by adding DIAION HP20 into a rust-colored solution of Pd(OAc)₂ in MeOH at room temperature. The resulting products was collected by filtration, washed with H₂O and MeOH, and dried in vacuo to give a palladium-DIAION HP20 complex as a grey powder. The products was made from a physical interaction between the orbitals of the palladium species and π-electrons of the benzene rings of the adsorbents. The thermogravimetry- differential thermal analysis of the complex showed that the complex has no volatiles.

Full text of DOI:10.1002/chem.200801989

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DOI: 10.1002/chem.200801989
A Highly Active Heterogeneous Palladium Catalyst Supported on a
Synthetic Adsorbent
Yasunari Monguchi,[a] Yuki Fujita,[a] Koichi Endo,[a] Shinobu Takao,[b]
Masatoshi Yoshimura,[b] Yukio Takagi,[b] Tomohiro Maegawa,[a] and Hironao Sajiki*[a]
Heterogeneous palladium catalysts have recently drawn
much attention in organic synthetic chemistry based on sus-
tainable and industrial standpoints, as they have many ad-
vantages over homogeneous catalysts, owing to their air-sta-
bility, recoverability, reusability, non-residual property, etc.[1]
Many palladium catalysts embedded on various supports,
such as mesoporous silica,[2] zeolite,[3] clay,[4] hydroxyapa-
tite,[5] and organic polymers,[6] etc[7] have been reported as a
characteristic heterogeneous catalyst for a wide range of re-
actions. Palladium on charcoal (Pd/C), which has been com-
monly used as a hydrogenation catalyst,[8] has found wide
application in various types of reactions including cross-cou-
pling.[9,10] However, the catalyst activity depends on the sup-
pliers or even manufacturing lot numbers of the same prod-
uct number (brand), as the charcoal is obtained from natural
resources, such as peat or sawdust and contaminated with
ultratrace amounts of impurities, as typified by metals de-
rived from the soil, air, and/or water from the natural envi-
ronment. On the other hand, synthetic adsorbents are indus-
trially produced under the strictly controlled conditions to
guarantee their quality. DIAION HP20, a commercially
available (Mitsubishi Chemical Corporation) polystyrene-
polyvinylbenzene-based synthetic adsorbent used for chro-
matographic separation, possess a very high surface area
(ꢀ590 m2gꢁ1) and fine pore structures (30 nm of radius) on
the particle (440 mm of average diameter) that can effective-
ly adsorb organic compounds.[11] DIAION HP20 is also
stable in organic solvents and in both acidic and basic solu-
tions, and these features would meet the requirements as a
good support of the heterogeneous palladium catalysts. In
this paper, we describe the preparation of palladium on
DIAION HP20, its characterization and application for the
hydrogenation and ligand-free Suzuki–Miyaura coupling re-
action.
The catalyst was prepared as follows. DIAION HP20
(5.00 g) was added to a rust-colored solution of PdACHTUNGTRENNUNG(OAc)2
(527 mg) in MeOH (50 mL), and the suspension was stirred
at room temperature under an argon atmosphere for 4 days.
The supernatant solution gradually turned colorless-clear
and the originally colorless adsorbent was tarnished to grey,
indicating that the PdII species was captured by the adsorb-
ents and reduced to Pd0 species.[12,13] The resulting grey solid
was collected by filtration, washed with H2O and MeOH,
and dried in vacuo to give a palladium-DIAION HP20 com-
plex (10% Pd/HP20) as a grey powder.[14] The luminescence
measurement suggests that the complex forms through a
physical interaction between the orbitals of the palladium
species and p-electrons of the benzene rings of the adsor-
bents,[6b,15] since the phosphorescence (t=1 ms, T1!S0) was
observed at l=375 nm together with the fluorescence at
around l=300 nm (t=5 ns, S1!S0) derived from polysty-
AHCTUNGTREGrNNUN ene. The thermogravimetry-differential thermal analysis
(TG/DTA) of the complex indicated that no volatiles were
contained in the complex.[16] Scanning transmission electron
microscopy (STEM) of the 10% Pd/HP20 showed that the
palladium particles (white dots in Figure 1) have a 3.1 nm
average size (Figure 1), in good agreement with the particle
size (4 nm) obtained from the X-ray diffraction (XRD) anal-
ysis. Electron probe microanalysis (EPMA) of the 10% Pd/
HP20 clarified that the palladium metal is uniformly distrib-
uted on the adsorbent (Figure 2). These data indicate that
the fine palladium particles are highly dispersed on the
DIAION HP20 and strongly suggest the potential catalyst
activity of 10% Pd/HP20 toward the palladium-catalyzed re-
actions.
[a] Dr. Y. Monguchi, Y. Fujita, K. Endo, Dr. T. Maegawa,
Prof. Dr. H. Sajiki
Department of Medicinal Chemistry, Gifu Pharmaceutical University
5-6-1 Mitahora-higashi, Gifu 502-8585 (Japan)
Fax : (+81)58-237-5979
E-mail: sajiki@gifu-pu.ac.jp
[b] Dr. S. Takao, M. Yoshimura, Dr. Y. Takagi
N.E. Chemcat Corporation
678 Ipponmatsu, Numazu, Shizuoka 410-0314 (Japan)
The hydrogenation of various substrates possessing a vari-
ety of reducible functionalities was first investigated, and
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200801989.
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COMMUNICATION
Table 1. 10% Pd/HP20-catalyzed hydrogenation.
10% Pd/ 10% Pd/C[a]
HP20
Entry Substrate
Product
Time Yield Time Yield
[h]
[%][b] [h]
[%][b]
1
2
2
100
1
100
3.5
100 3.5
95
3
4
2
85
2
94
91
Figure 1. STEM image of 10% Pd/HP20.
24[c]
100 24[c]
5
6
4
7
97
5
2
94
87
100
7
8
24[c]
2
100 24[c]
100
93
100
100
1
4
9
3
99
91
24[c]
100 24[c]
Figure 2. EPMA of 10% Pd/HP20
10
[a] Obtained from the N.E. Chemcat Corporation. [b] Isolated yield. [c] Due to
the difficulty in monitoring by TLC, the reaction was continued for 24 h. Cbz=
benzyloxycarbonyl. Bn=benzyl.
the results are summarized in Table 1 together with those of
the 10% Pd/C-catalyzed reaction for comparison. The N-
Cbz groups were readily deprotected (Entries 1 and 2) and
both the alkyne and alkene functionalities were hydrogenat-
ed to the corresponding alkanes (Entries 2–5). Hydrogenoly-
sis of the benzyl esters as well as the benzyl ethers was effi-
ciently achieved (Entries 3, 5, and 6), and the cyano, nitro,
and azido functionalities were smoothly reduced to give the
corresponding amines (Entries 7, 8, and 9). The Pd/HP20
also catalyzed the hydrodebromonation (Entry 10). The cat-
alyst efficiencies between 10% Pd/HP20 and 10% Pd/C for
the hydrogenation are demonstrated to be nearly identical.
The Suzuki–Miyaura coupling reaction between the aryl-
boronic acids and aryl halides is very useful and efficient for
the preparation of biaryl units, which are often included as a
partial structure for a wide variety of functional materials
such as pharmaceuticals, agrochemicals, liquid crystals,
etc.[17] We have recently reported the ligand-free, heteroge-
neous Pd/C-catalyzed Suzuki–Miyaura reaction in aqueous
media, and the conditions were widely applicable to the
cross-coupling between a variety of boronic acids and aryl-
bromides or aryltriflates.[10a–e] We next investigated the ap-
plication of the present 10% Pd/HP20 to the ligand-free
Suzuki–Miyaura reaction as a heterogeneous catalyst. The
reaction was performed using various aryl bromides,
1.1 equiv of an arylboronic acid, 0.5 mol% of 10% Pd/
HP20, and 3.5 equiv of Na3PO4·12H2O in a 50% iPrOH so-
lution at room temperature. As a result, the 10% Pd/HP20
efficiently catalyzed the ligand-free Suzuki–Miyaura reac-
tion of arylbromides bearing either an electron-withdrawing
or electron-donating group on the aromatic ring (Table 2).
Furthermore, the 10% Pd/HP20 also catalyzed the cross-
coupling reaction between the heteroarylboronic acid and
heteroaryl bromide in the ligand-free fashion.[18] As shown
in Scheme 1, the coupling between 3-bromoquinoline and
dibenzofranboronic acid was completed within 6 h to give
the desired heterobiaryl in 90% yield, which exceeds the
result of the 10% Pd/C-catalyzed coupling reaction (12 h,
75%).[10e]
The reusability of 10% Pd/HP20 would enhance the utili-
ty from the perspective of cost- and waste-reductions in in-
dustrial applications. We next investigated the reusability of
the 10% Pd/HP20 for the hydrogenation of 4-nitroanisole.
The 10% Pd/HP20 could be recovered and reused until at
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H. Sajiki et al.
Table 2. Ligand-free 10% Pd/HP20-catalyzed Suzuki–Miyaura coupling
reaction.[a]
Experimental Section
Preparation of 10% Pd/HP20: In a 300 mL-round bottom flask contain-
ing a stirrer bar were added Pd(OAc)2 (527 mg) and MeOH (50 mL),
AHCTUNGTRENNUNG
and the mixture was stirred at room temperature under an argon atmos-
phere. To the resulting solution was added DIAION HP20 (Mitsubishi
Chemical Corporation, 5.00 g), and the mixture was stirred at room tem-
perature until the MeOH layer turned colorless (4 days). The resulting
gray solid was collected on a Kiriyama funnel (Kiriyama Glass Co., Ltd.,
Tokyo, Japan), successively washed with H2O (15 mL) and MeOH
(15 mL) , and then dried under reduced pressure for 2 days to produce
the 10% Pd/HP20 (2.31 g).
Entry
R1
R2
Time [h][b]
Yield [%][b]
1
2
3
4
5
4-NO2
4-CHO
2-Me
4-OMe
4-OMe
H
H
H
2-OMe
3-OMe
4 (3.5)
2.5 (3)
3 (4)
7 (6)
24 (10)
97 (93)
100 (91)
90 (90)
100 (90)
96(96)
Typical hydrogenation procedure using 10% Pd/HP20 (Table 1): To a
test tube containing a stir bar were added the substrate (1 mmol), 10%
Pd/HP20 (10 wt% of the substrate), and MeOH (2 mL), then the system
was sealed with a septum. The air inside was replaced with H2 (balloon)
by five vacuum/hydrogen cycles. The mixture was stirred at room temper-
ature for a given time and then passed through a membrane filter (Milli-
pore Corporation, Billerica, MA; Millex-LH, 0.45 mm) to remove the cat-
alyst. The filtrate was concentrated in vacuo to give the pure product.
[a] 0.5 mol% of 10% Pd/HP20 and 3.5 equiv of Na3PO4·12H2O were
used. [b] Results using the 10% Pd/C are indicated in the parentheses.[10c]
Typical procedure of 10% Pd/HP20-catalyzed Suzuki–Miyaura reaction:
To a test tube containing a stir bar were added the aryl bromide
(500 mmol), arylboronic acid (550 mmol), 10% Pd/HP20 (2.7 mg,
2.5 mmol), Na3PO4·12H2O (665 mg, 1.75 mmol), H2O (1 mL), and iPrOH
(1 mL), then the system was sealed with a septum. The air inside was re-
placed with Ar (balloon) by five vacuum/argon cycles. The mixture was
stirred at room temperature for a given time and then passed through a
membrane filter (Millipore Corporation, Billerica, MA; Millex-LH,
0.45 mm) to remove the catalyst. To the filtrate were added EtOAc
(15 mL) and H2O (15 mL), and the layers were separated. The aqueous
layer was extracted with EtOAc (15 mL ꢁ2) and the combined organic
layers were washed with brine (10 mL), dried over Na2SO4, filtered, and
concentrated in vacuo. The residue was purified by flash column chroma-
tography on silica gel (hexane/EtOAc) to give the corresponding biphen-
yl (Table 2), the structure of which was confirmed by comparison to the
literature structure.[10c]
Scheme 1. Ligand-free 10% Pd/HP20 or 10% Pd/C-catalyzed hetero-
Suzuki–Miyaura coupling. 1.0 mol% of 10% Pd/HP20 or 10% Pd/C was
used with 1.0 equiv of Na3PO4·12H2O.
least the 4th run without any significant loss of the catalytic
activity (Table 3). Furthermore, no leached Pd was detected
in the filtrate after the removal of the catalyst by the induc-
tively coupled plasma (ICP) assay [<1 ppm (the detection
limit)].
Table 3. Reuse test of 10% Pd/HP20.[a]
Acknowledgements
We thank Mitsubishi Chemical Corporation for a gift of DIAION HP20.
Run
1st
2nd
3rd
4th
Yield [%] of 4-Methoxyaniline
Yield [%] of Recovered 10% Pd/HP20
100
98
96
95
100
97
100
91
Keywords: heterogeneous catalysis
·
hydrogenation
·
palladium · Suzuki–Miyaura reaction · synthetic adsorbents
[a] All amounts of recovered catalyst were used for the next run.
In conclusion, we developed a novel and heterogeneous
palladium catalyst (10% Pd/HP20) highly dispersed on/in a
synthetic adsorbent, DIAION HP20, which has a high sur-
face area. The 10% Pd/HP20 could be efficiently prepared
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Heterogeneous Palladium Catalyst
COMMUNICATION
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Received: September 26, 2008
Published online: December 15, 2008
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