Magnetically separable Pd catalyst for highly selective epoxide hydrogenolysis under mild conditions

Article abstract of DOI:10.1021/ol701456w

A magnetically separable palladium catalyst was synthesized simply through a sol-gel process incorporating palladium nanoparticles and superparamagnetic iron oxide nanoparticles in aluminum oxyhydroxide matrix, which is highly active and selective for epoxide hydrogenolysis at room temperature under 1 atm H ₂. The catalyst was recycled for 25 times without loss of the activity.

Full text of DOI:10.1021/ol701456w

ORGANIC
LETTERS
2
007
Vol. 9, No. 17
417-3419
Magnetically Separable Pd Catalyst for
Highly Selective Epoxide
3
Hydrogenolysis under Mild Conditions
†
†
‡
‡
Min Serk Kwon, In Soo Park, Jum Suk Jang, Jae Sung Lee, and
Jaiwook Park*
,
†
Department of Chemistry and Department of Chemical Engineering, Pohang
UniVersity of Science and Technology, San 31 Hyojadong, Pohang, Kyungbuk 790-784,
Republic of Korea
pjw@postech.ac.kr
Received June 20, 2007
ABSTRACT
A magnetically separable palladium catalyst was synthesized simply through a sol
superparamagnetic iron oxide nanoparticles in aluminum oxyhydroxide matrix, which is highly active and selective for epoxide hydrogenolysis
at room temperature under 1 atm H . The catalyst was recycled for 25 times without loss of the activity.
−gel process incorporating palladium nanoparticles and
2
Metal nanoparticles are extensively used in many organic
transformations because of their inherent large surface-to-
volume ratio and distinct catalytic activities. Although
that shows high activity in the highly regioselective hydro-
genolysis of epoxides at room temperature under 1 atm H
2
.
1
The recyclability of the catalyst is remarkable; it retains the
original catalytic activity even after recycling 25 times.
Recently, we have developed a simple but new method
for preparing recyclable palladium catalysts through generat-
ing palladium nanoparticles from Pd(PPh ) under aerobic
various metal nanoparticles have been developed in many
reactions, they often suffer from particle agglomeration
during the reaction as well as from difficulties in separation
of the catalyst from the reaction mixtures. To overcome these
problems, nanoparticles have been immobilized onto many
different supports such as inorganic materials, organic
3
4
4
conditions. With this method, the palladium content of the
resulting catalyst was limited up to about 3 wt % because
of the solubility of Pd(PPh ) in the reaction mixture. Then,
2
polymers, dendrimers, and ionic liquids. Recently, super-
3
4
paramagnetic nanoparticles have been employed to facilitate
catalyst separation by using external magnets. However, the
supported catalysts generally have low reactivities, metal-
leaching problems, and difficult synthetic procedures.
Herein, we describe a simple synthetic procedure for the
preparation of the magnetically separable palladium catalyst
3
(
2) (a) Corma, A.; Serna, P. Science 2006, 313, 332. (b) Oyamada, H.;
Akiyama, R.; Hagio, H.; Naito, T.; Kobayashi, S. Chem. Commun. 2006,
297. (c) Song, H.; Rioux, R. M.; Hoefelmeyer, J. D.; Komor, R.; Niesz,
K.; Grass, M.; Yang, P.; Somorjai, G. A. J. Am. Chem. Soc. 2006, 128,
027. (d) Yamada, Y. M. A.; Uozumi, Y. Org. Lett. 2006, 8, 1375. (e)
4
3
Park, C. M.; Kwon, M. S.; Park, J. Synthesis 2006, 22, 3790. (f) Wu, L.;
Li, B.-L.; Huang, Y.-Y.; Zhou, H.-F.; He, Y.-M.; Fan, Q.-H. Org. Lett.
006, 8, 3605. (g) Wang, Y.; Yang, H. Chem. Commun. 2006, 2545. (h)
2
†
Department of Chemistry.
Department of Chemical Enginneering.
Zheng, N.; Stucky, G. D. J. Am. Chem. Soc. 2006, 128, 14278.
‡
(3) (a) Jun, Y.-W.; Choi, J.-S.; Cheon, J. Chem. Commun. 2007, 1203.
(b) Tsang, S. C.; Caps, V.; Paraskevas, I.; Chadwick, D.; Thompsett, D.
Angew. Chem., Int. Ed. 2004, 43, 5645. (c) Kim, J.; Lee, J. E.; Lee, J.;
Jang, Y.; Kim, S.-W.; An, K.; Yu, J. H.; Hyeon, T. Angew. Chem., Int. Ed.
2006, 45, 4789. (d) Yi, D. K.; Lee, S. S.; Ying, J. Y. Chem. Mater. 2006,
18, 2459. (e) Jun, C.-H.; Park, Y. J.; Yeon, Y.-R.; Choi, J.-R.; Lee, W.-R.;
Ko, S.-J.; Cheon, J. Chem. Commun. 2006, 1619. (f) Stevens, P. D.; Fan,
J.; Gardimalla, H. M. R.; Yen, M.; Gao, Y. Org. Lett. 2005, 7, 2085. (g)
Guin, D.; Baruwati, B.; Manorama, S. V. Org. Lett. 2007, 9, 1419.
(1) (a) Wu, L.; Li, B.-L.; Huang, Y.-Y.; Zhou, H.-F.; He, Y.-M.; Fan,
Q.-H. Org. Lett. 2006, 8, 3605. (b) Bedford, R. B.; Betham, M.; Bruce, D.
W.; Davis, S. A.; Frost, R. M.; Hird, M. Chem. Commun. 2006, 1398. (c)
Alonso, F.; Osante, I.; Yus, M. AdV. Synth. Catal. 2006, 348, 305. (d) Burda,
C.; Chen, X.; Narayanan, R.; E-Sayed, M. A. Chem. ReV. 2005, 105, 1025.
(
7
e) Astruc, D.; Lu, F.; Aranzaes, J. R. Angew. Chem., Int. Ed. 2005, 44,
852. (f) Son, S. U.; Jang, Y.; Park, J.; Na, H. B.; Park, H. M.; Yun, H. J.;
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1
0.1021/ol701456w CCC: $37.00
© 2007 American Chemical Society
Published on Web 07/28/2007

we optimized conditions for substituting palladium acetate
for Pd(PPh , and the palladium content could be increased
We tested the catalytic activity of 1 in epoxide hydro-
genolysis to give alcohols. This reductive cleavage reaction
8
3
)
4
up to 10 wt % without losing activity per mol of Pd.
Furthermore, superparamagnetic iron oxide nanoparticles
were able to be readily mixed with the suspension of
palladium nanoparticles before gelation. Palladium nano-
particles were produced from a solution of palladium acetate,
has traditionally been carried out by using stoichiometric
metal hydrides, in which the problem of waste salts is
unavoidable. Therefore, much attention has been paid to
9
the catalytic hydrogenolysis of epoxides with molecular
1
0
hydrogen or ammonium formate. However, the previous
catalyst systems are not suitable to industrial applications
because of low yield, low selectivity, low catalyst durability,
and/or harsh and complex reaction conditions. For example,
the reaction with [Cp*RuCl(cod)] requires the addition of
an amine ligand and a strong base under 10 atm of hydrogen
3
Al(O-sec-Bu) , 2-butanol, and THF. Iron oxide nanoparticles
dispersed in ethanol were added to the resulting black
5
suspension. Then, water was added to form black gel. The
gel was isolated by filtration, washed with acetone, and dried
at 120 °C for 5 h to give 1 as dark brown powder (Scheme
10d
1).
pressure. The use of Pd/C-ethylenediamine complex shows
a good selectivity for terminal epoxides, but it suffers from
1
0e
solvolysis and requires 5 atm H
2
.
Recently, Yu and co-
workers reported that the polyurea-microencapsulated pal-
ladium catalyst (PdEnCat) is a recyclable catalyst in epoxide
Scheme 1. Preparation of Catalyst
hydrogenolysis, which needs 40 atm H
oxides. Herein, we report a highly efficient and selective
2
for aliphatic ep-
1
1
epoxide hydrogenolysis under 1 atm H
2
at room temperature
by using 1.
The catalyst 1 was characterized by transmission electron
microscopy (TEM). In the TEM images, iron oxide nano-
particles of 60∼90 nm in diameter and palladium nanopar-
ticles of 2∼3 nm entrapped in fibrous aluminum oxyhy-
Table 1. Catalytic Activity Comparisona
6
droxide are observed (Figure 1). The specific surface area
product yield (%)b
time conversion
(h)
entry
catalyst
(%)b
2
3
1
2
3
4
1
4
>99
0
0
99.8
0
0
83
0.2
0
0
c
iron oxide
AlO(OH)
5% Pd/C
20
20
4
d
e
89
6
(
4
(
4
(
4
(
20)
(>99)
70.9
(>99)
65.1
(91.3)
14.9
(70)
0
(91)
70
(9)
0.9
(1.7)
0.6
(2.2)
0.4
(1.3)
0
5
6
7
5% Pd/Al2O3e
5% Pd/CaCO3e
5% Pd/BaCO3e
20)
(98.3)
64.5
(89.1)
14.5
(68.7)
0
Figure 1. TEM images of 1: (a) iron oxide nanoparticles (60∼90
nm) and Pd nanoparticles (2∼3 nm) entrapped in AlO(OH); (b)
iron oxide nanoparticle, (c) Pd nanoparticles.
20)
20)
8
4.3% PdEnCate 20
2
-1
of 1 was determined to be 579 m g by the N
2
BET method
a
The reaction was performed on 1.0 mmol of epichlorohydrin dissolved
at 77 K. The magnetic property of 1 was studied by using a
superconducting quantum interference device (SQUID). No
hysteresis was observed at room temperature in the magne-
tization curve, which is a typical superparamagnetic behav-
in 2.0 mL of EtOAc with 2.0 mol % Pd at 23 °C under hydrogen balloon.
b
Determined by GC. c Reaction run using 20 mg. d Reaction run using 50
mg. e Commercial catalysts.
7
ior. The saturation magnetization value at room temperature
The catalytic activity of 1 was compared with the activities
of various palladium catalysts in the hydrogenolysis of
-
1
is 4.7 emu g , which is sufficient for magnetic separation.
(
4) (a) Kim, N.; Kwon, M. S.; Park, C. M.; Park, J. Tetrahedron Lett.
(8) Larock, R. C. In ComprehensiVe Organic Transformations, 2nd ed.;
Wiley-VCH: New York, 1999; pp 1019-1027.
2
004, 45, 7057. (b) Kwon, M. S.; Kim, N.; Park, C. M.; Lee, J. S.; Kang,
K. Y.; Park, J. Org. Lett. 2005, 7, 1077. (c) Kwon, M. S.; Kim, N.; Seo, S.
H.; Park, I. S.; Cheedrala, R. K.; Park, J. Angew. Chem., Int. Ed. 2005, 44,
(9) Flaherty, T. M.; Gervay, J. Tetrahedron Lett. 1996, 37, 961.
(10) (a) Cavallo, A. S.; Lupattelli, P.; Bonini, C.; Ostuni, V.; Blasio, N.
D. J. Org. Chem. 2006, 71, 9891. (b) Kirm, I.; Medina, F.; Rodr ´ı guez, X.;
Cesteros, Y.; Salagre, P.; Sueiras, J. E. J. Mol. Catal. A: Chem. 2005,
239, 1381. (c) Bergad, O.; Salagre, P.; Cesteros, Y.; Medina, F.; Sueiras,
J. E. Appl. Catal., A 2004, 272, 125. (d) Rode, C. V.; Telkar, M. M.;
Jaganathan, R.; Chaudhari, R. V. J. Mol. Catal. A: Chem. 2003, 200, 279.
(d) Ito, M.; Hirakawa, M.; Osaku, A.; Ikariya, T. Organometallics 2003,
22, 4190. (e) Sajiki, H.; Hattori, K.; Hitota, K. Chem. Commun. 1999, 1041.
(f) Dragovich, P. S.; Prins, T. J.; Zhou, R. J. Org. Chem. 1995, 60, 4922.
(11) Ley, S. V.; Mitchell, C.; Pears, D.; Ramarao, C.; Yu, J.-Q.; Zhou,
W. Org. Lett. 2003, 5, 4665.
6
913. (d) Kim, M.-J.; Kim, W.-H.; Han, K.; Choi, Y. K.; Park, J. Org.
Lett. 2007, 9, 1157.
5) Deng, H.; Li, X.; Peng, Q.; Wang, X.; Chen, J.; Li, Y. Angew. Chem.,
Int. Ed. 2005, 44, 2782.
6) (a) Kim, H. J.; Lee, H. C.; Lee, J. S. J. Phys. Chem. C 2007, 111,
579. (b) Kuiry, S. C.; Megen, E.; Patil, S. D.; Deshpande, S. A.; Seal, S.
(
(
1
J. Phys. Chem. B 2005, 109, 3868. (c) Hicks, R. W.; Pinnavaia, T. J. Chem.
Mater. 2003, 15, 78. (d) Lee, H. C.; Kim, J. J.; Chung, S. H.; Lee, K. H.;
Lee, H. C.; Lee, J. S. J. Am. Chem. Soc. 2003, 125, 2882.
(7) See Supporting Information for the characterization of 1.
3418
Org. Lett., Vol. 9, No. 17, 2007

use were measured by GC after 90 min; they were consis-
tently about 65%. Increasing the reaction time to 4 h in the
2
Table 2. Hydogenolysis of Epoxidesa
2nd-25th use increased the yield to more than 94% yield.12
The hydrogenolysis with 1 was applicable for a wide range
of epoxides (Table 2). The bond between the benzylic carbon
and oxygen was cleaved almost exclusively in the hydro-
genolysis of benzylic epoxides (entries 1-5). Notably, trans-
stilbene oxide was transformed exclusively into 1,2-
diphenylethanol in contrast to the reaction with 5% Pd/C
under the same conditions, which produced a mixture of 1,2-
diphenylethanol and bibenzyl in an 85:15 ratio (entry 1). The
stereochemistry of the other C-O bond was retained in the
ring opening of (R,R)-2-methyl-3-phenyloxirane to give (R)-
1
-phenyl-2-propanol in 99% ee (entry 2). Esters and extra
hydroxyl groups are compatible and do not disturb the
regioselectivity (entries 3 and 4). As shown in the reaction
of epichlorohydrin, 1 was also effective for the highly
selective hydrogenolysis of aliphatic epoxides (entries 6 -
12). Generally, the reactions of aliphatic epoxide were slower
than those of benzylic ones. For example, the hydrogenolysis
of 2-hexyloxirane took 8 h in spite of increasing the amount
of 1 2-fold (2.0 mol % of Pd) (entry 6). However,
exceptionally, the reaction of 1,2-epoxy-3-phenoxypropane
was completed in 30 min by using 2.0 mol % of Pd (entry
7
). The regioselectivity appeared to be governed by steric
hindrance; the less hindered C-O bonds were cleaved in
the reactions of terminal oxiranes. The selectivity was not
affected by phenyloxy or hydroxyl group (entries 6 and 7).
However, the reactions of 1,4-di(oxiran-2-yl)butane and
2
-benzyloxirane were less selective to produce 1,8-octanediol
and 3-phenyl-1-propanol as the minor products in 8% and
5% yield, respectively. The hydrogelnolysis of internal
1
oxirane such as cyclohexene oxide was also successful to
give cyclohexanol in 93% yield.
In summary, we have developed a simple process for
preparing a magnetically separable and recyclable palladium
catalyst by incorporating palladium nanoparticles and iron
oxide nanoparticles in aluminum oxyhydroxide matrix and
demonstrated its high activity in the selective hydrogenolysis
of various epoxides at room temperature with a hydrogen
balloon. Moreover, we have shown that the catalyst can be
reused at least 25 times without loss of activity.
a
The reaction was performed on 1.0 mmol of substrate dissolved in 2.0
mL of EtOAc with 1.0 mol % Pd at 23 °C under hydrogen balloon.
Determined by GC. Isolated yield. Reaction run using 2.0 mol % of
Pd. 1-Octanol was produced in 4% yield. 1,8-Octanediol was produced
in 8% yield. 3-Phenyl-1-propanol was produced in 15% yield.
b
c
d
e
f
g
Acknowledgment. We are grateful for the financial
supports from KOSEF through the Center for Integrated
Molecular System, and the Korean Ministry of Education
through the BK21 project for our graduate program.
epichlorohydrin (Table 1). AlO(OH) or iron oxide itself did
not show any activity for the hydrogenolysis. The catalyst 1
showed better activity and selectivity than all of the com-
mercial catalysts such as Pd/C, Pd/Al
2
O
3
, Pd/CaCO
3
, Pd/
, 5
BaCO , and PdEnCat. In the cases of Pd/C and Pd/Al
3
2
O
3
Supporting Information Available: Experimental pro-
cedures, the magnetization curve for 1, the photographs of
reaction mixture, the NMR data for the products of epoxide
hydrogenolysis, and the GC data for epoxides and the
products of epoxide hydrogenolysis. This material is available
free of charge via the Internet at http://pubs.acs.org.
times longer reaction time is required to complete the reaction
and 3-chloro-1-propanol was produced as a byproduct in 9%
and 2% yield, respectively. In the cases of Pd/CaCO
Pd/BaCO , even after 20 h, the reactions were not completed.
Particularly, PdEnCat did not show any activity at room
temperature under 1 atm H
3
and
3
2
.
OL701456W
It is notable that 1 can be reused at least 25 times without
activity loss through a simple recovery by an external
magnet. The yields of 1-chloro-2-propanol in the 1st-21st
(12) Palladium was not detected in the product by the inductively coupled
plasma (ICP) analysis.
Org. Lett., Vol. 9, No. 17, 2007
3419