Magnetically recoverable osmium catalysts for dihydroxylation of olefins

Article abstract of DOI:10.1016/j.tetlet.2011.04.030

We prepared magnetically recoverable osmium catalysts by use of magnetite, quaternary ammonium salts, and potassium osmate(VI), and applied them to the dihydroxylation of olefins. By employing 2 mol% of the magnetic osmium catalyst, the dihydroxylation reaction proceeded smoothly to provide the corresponding vicinal diol in a good chemical yield. The osmium catalyst was readily recovered by use of an external magnet, and was reused repeatedly.

Full text of DOI:10.1016/j.tetlet.2011.04.030

Tetrahedron Letters 52 (2011) 3137–3140
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Tetrahedron Letters
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Magnetically recoverable osmium catalysts for dihydroxylation of olefins
Ken-ichi Fujita a, , Satoshi Umeki a, Manabu Yamazaki a, Taku Ainoya b, Teruhisa Tsuchimoto b,
⇑
Hiroyuki Yasuda a
a National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan
b Department of Applied Chemistry, School of Science and Technology, Meiji University, Higashimita, Tama-ku, Kawasaki, Kanagawa 214-8571, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
We prepared magnetically recoverable osmium catalysts by use of magnetite, quaternary ammonium
salts, and potassium osmate(VI), and applied them to the dihydroxylation of olefins. By employing
2 mol% of the magnetic osmium catalyst, the dihydroxylation reaction proceeded smoothly to provide
the corresponding vicinal diol in a good chemical yield. The osmium catalyst was readily recovered by
use of an external magnet, and was reused repeatedly.
Received 7 March 2011
Revised 4 April 2011
Accepted 6 April 2011
Available online 12 April 2011
Ó 2011 Elsevier Ltd. All rights reserved.
Osmium-catalyzed dihydroxylation of olefins is one of the most
useful transformations for the preparation of vicinal diols.1
Although these reactions have had widespread application in
organic synthesis, several obstacles stand in the way of their
large-scale application to the drugs and fine chemicals indus-
tries—namely, their high cost, toxicity, volatility and possible con-
tamination with toxic osmium in the final product.2 One of the
most promising solutions to these problems is the immobilization
of the catalytic osmium to an insoluble matrix. Previously, heterog-
enization of catalytic osmium by immobilization on various organ-
ic and inorganic supports has been successful, so that the
immobilized osmium catalyst could be recovered by filtration of
the reaction mixture.3–5 But the lower activity of the immobilized
catalyst remains a major problem. Recently, the immobilized
homogeneous osmium catalysts, which have soluble backbones
such as dendrimer and poly(ethylene glycol), were reported by
us6 and by other groups.7 In these cases, the activity of the immo-
bilized homogeneous catalysts were as high as those of the
non-supported ones, and the former could be recovered by
reprecipitation of the reaction mixture.
Magnetic nanoparticles have emerged as effective new supports
for immobilization, because magnetic nanoparticle-supported cat-
alysts can be separated from the reaction medium by an external
permanent magnet.8 The magnetic separation circumvents time-
consuming and laborious separation steps, and allows for practical
continuous catalysis. Very recently, magnetic nanoparticle-
supported and magnetically recoverable transition metal catalysts
such as palladium,9 ruthenium,10 nickel,11 and copper12 have been
increasingly reported.13 Most magnetic catalysts have excellent
activity, because the catalytic active sites are immobilized on
nanoparticles having high surface areas. We report herein the
synthesis of magnetically recoverable osmium catalysts by use of
quaternary ammonium salts immobilized on magnetite and
potassium osmate(VI) and their application to the dihydroxylation
reaction of olefins. We found that the introduction of a dendritic
skeleton to the quaternary nitrogen immobilized on a magnetite
backbone caused an efficient recovery of the magnetic osmium cat-
alyst and a decrease in the leaching of osmium during the dihydr-
oxylation reaction. At the outset of this study, no example of a
magnetically recoverable osmium catalyst had been reported.14
A magnetic nanoparticle-supported osmium catalyst 4 was syn-
thesized as follows (Scheme 1). Magnetite (Fe3O4) nanoparticles
were chosen for use as magnetic supports since they could be read-
ily prepared by the conventional coprecipitation method.15 The
silane coupling agent 2 was synthesized by stirring of the acetoni-
trile solution of [(chloromethyl)phenylethyl] trimethoxysilane 1
and triethyl amine at 70 °C for 5 h under an argon atmosphere.
The reaction mixture was evaporated to dryness and the thus ob-
tained 2 was used for the next step without further purification.
Next, magnetite and 2 were refluxed in ethanol for 20 h under an
argon atmosphere to afford magnetite-supported ammonium
chloride 3, which was separated by magnetic decantation using
an external magnet. The loading of 3 was 0.18 mmol/g, which
was determined by elemental analysis of chlorine. The magne-
tite-supported osmium catalyst
4 was prepared by an ion-
exchange procedure. Namely, 3 and K2OsO4 were vigorously
stirred in water at room temperature, followed by magnetic decan-
2À
tation. It was found that the ion-exchange for OsO4 was com-
plete, because the osmium species were not detected by ICP-AES
in the decanted solution (Os content of 4: 0.083 mmol/g).
We then examined the utility of 4 as an osmium catalyst
by performing cis-dihydroxylation of olefins (Table 1). By
employing 2 mol% of the catalyst 4, cis-dihydroxylation reaction of
trans-b-methylstyrene was carried out by the use of N-methylmor-
pholine N-oxide (NMO) as a re-oxidant in aqueous acetone (ace-
tone:H2O = 2:1 (v/v)) at room temperature. The dihydroxylation
reaction smoothly proceeded to completion in 5 h. The magnetic
⇑
Corresponding author.
E-mail address: k.fujita@aist.go.jp (K. Fujita).
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.04.030

3138
K. Fujita et al. / Tetrahedron Letters 52 (2011) 3137–3140
Cl
N+Et3Cl-
H3C-O
NEt3
H3C-O
H3C-O Si
H3C-O
H3C-O Si
H3C-O
CH3CN
70 °C, 5 h
1
2
Fe3O4
N+Et3Cl-
O
O
Fe3O4
Si
C2H5OH
reflux
20 h
OCH3
3
N+Et3
1/2 [OsO4 ]
_
O
O
K2OsO4
2
Fe3O4
Si
H2O
rt, 5 h
OCH3
4
Scheme 1. Preparation of 4.
Table 1
G0:
G1:
CH2
CH2
Dihydroxylation of trans-b-methylstyrene catalysed by 4a
O
O
4 (2 mol% Os)
OH
HO
O
O
O
O
NMO
Ph
−
G2: CH2
Acetone H2O
Ph
rt, 5 h
5
O
O
First
85 (17.7)
Second
Yieldb (%) (Os leaching (%))
71 (7.2)
a
Reaction conditions: 4 (2 mol%), olefin (1 equiv), NMO (1.3 equiv), acetone–H2O
Figure 1. Structural formulas of Gn dendrons (n = 0–2).
(2:1, v/v, 0.33 M based on olefin), carried out at room temperature for 5 h.
b
Determined by integration of 1H NMR absorptions referring to an internal
standard.
and thus we observed a positive dendritic effect on the recyclabil-
ity of the catalyst. In consideration of these results, we next de-
signed 9[Gn], which has a dendritic skeleton attached to the
quaternary nitrogen immobilized on a magnetite backbone.
The silane coupling agent 7[Gn], the magnetite-supported den-
dritic quaternary ammonium bromide 8[Gn] (the structural formu-
las of Gn are shown in Figure 1), and the corresponding osmate(VI)
9[Gn] were prepared according to Scheme 2, which was similar to
Scheme 1.17 Also, in the preparation of 9[Gn], the ion-exchange for
osmium catalyst 4 was collected using an external magnet, and the
reaction mixture was then transferred out of the reaction vessel.16
As a result, the corresponding diol was obtained in a good chemical
yield (85%), but the leaching of osmium off the catalyst, which was
determined by ICP-AES, was large (17.7%), contrary to our expecta-
tion (see the column labeled ‘First’ in Table 1). In addition, the re-
use of the catalyst 4 did not afford sufficient activity (see the
column labeled ‘Second’ in Table 1).
2À
OsO4 was complete. The Os contents of 9[Gn] are also shown in
Scheme 2.
2À
Recently, we have found that an OsO4 core dendrimer that
We then examined the catalytic activity and recyclability of the
various generations of 9[Gn] by performing the cis-dihydroxylation
reactions of trans-b-methylstyrene (Table 2). As a result, the
consists of poly(benzyl ether) dendron is very effective for the
dihydroxylation reaction.6 The higher generation-derived dendritic
osmium catalyst was more efficiently recycled by reprecipitation,
Br-
N+
H3C-O
H3C-O
H3C-O Si
H3C-O
Gn
Gn Br
H3C-O Si
N(CH3)2
H3C-O
CH3
DMF
70 ºC, 2 h
CH3
6
7
[Gn]
_
Fe3O4
2
Br-
N+
1/2 [OsO4
]
O
O
Gn
O
O
Gn
K2OsO4
Fe3O4
Fe3O4
Si
Si
N+
CH3
C2H5OH
reflux
20 h
H2O
rt, 5 h
CH3
OCH3
OCH3
CH3
CH3
8
[Gn]
9
[Gn]
Os content
Br content
G0: 0.086 mmol/g
G1: 0.061 mmol/g
G2: 0.056 mmol/g
G0: 0.185 mmol/g
G1: 0.131 mmol/g
G2: 0.120 mmol/g
Scheme 2. Preparation of 9[Gn].

K. Fujita et al. / Tetrahedron Letters 52 (2011) 3137–3140
3139
Table 2
Table 3
Dihydroxylation of trans-b-methylstyrene catalysed by 9[Gn]a
Dihydroxylation of various olefins catalysed by 9[G2]a
9[G2] (2 mol% Os)
9[Gn] (2 mol% Os)
R3
R4
R1
R2
HO
R3
OH
OH
HO
NMO
NMO
R1
Ph
–
Acetone H2O
–
Acetone H2O
R4
R2
Ph
rt
rt
5
Entry
1
Olefin
Product
Time
(h)
Yieldb
(%)
Gn
Yieldb (%) (Reaction time, Os leaching)
First
Second
OH
Ph
G0
G1
G2
91 (3 h, 14.3%)
87 (3 h, 5.4%)
91 (2 h, 3.3%)
94 (7 h, 9.4%)
88 (4 h, 3.7%)
95 (2 h, 3.1%)
OH
2
96
Ph
OH
a
Reaction conditions: 9[Gn] (2 mol%), olefin (1 equiv), NMO (1.3 equiv), acetone–
2
3
3
3
87
95
Ph
OH
OH
OH
H2O (2:1, v/v, 0.33 M based on olefin), carried out at room temperature for indicated
time.
Ph
OH
b
Determined by integration of 1H NMR absorptions referring to an internal
Ph
Ph
standard.
Ph
OH
4
5
3
4
90
93
Ph
HO
HO
Ph
HO
HO
6
7
8
9
2
3
8
6
86
97
91
89
OH
Ph
OH
Ph
Ph
OH
OH
O
O
OH
Ph
C8H17
OH
C8H17
C4H9
OH
Figure 2. Dihydroxylation catalyzed by 9[G2].
OH
C4H9
10
9
88
C4H9
C4H9
a
The reaction conditions were the same as in Table 2.
Isolated yield.
b
Table 4
Catalyst recycling in dihydroxylations by use of 9[G2]a
Entry Olefin
Yieldb, Reaction time
First
Second
Third
Fourth
Fifth
1
96%, 2 h 98%, 2 h 95%, 3 h 97%, 3 h 96%, 3 h
95%, 3 h 96%, 3 h 95%, 3 h 94%, 4 h 97%, 5 h
Ph
2
Ph
Ph
3
4
90%, 2 h 99%, 2 h 88%, 2 h 93%, 3 h 92%, 4 h
97%, 3 h 99%, 3 h 99%, 3 h 96%, 4 h 95%, 7 h
Figure 3. Recovery of 9[G2].
Ph
a
The reaction conditions were the same as in Table 2.
Isolated yield.
b
dihydroxylation proceeded smoothly in all generations. Figure 2
shows the reaction mixture of the dihydroxylation catalyzed by
9[G2]. In the magnetic separation after the dihydroxylation, the
second-generation magnetic catalyst 9[G2] was most clearly
collected from the reaction mixture, as shown in Figure 3. Further-
more, the leaching of osmium in the use of 9[G2] was the least
among all generations (see the column labeled ‘First’ in Table 2).
Also in the case of recycling of the catalyst 9[Gn], G2 afforded
the least osmium leaching (see the column labeled ‘Second’ in
Table 2). Although we have no definitive explanation at the mo-
ment for the finding that the osmium leaching was lowest in G2,
the surrounding of catalytic osmium by the dendron having a
hydrophobic periphery could be responsible. The relationship be-
tween the generation number of 9[Gn] and the osmium leaching
is one of the positive dendritic effects.6,18,19 Our present results
also indicate the dendritic effect on the recyclability of a catalyst,
2À
similar to those in the case of our recently reported OsO4 core
dendrimer.6
Encouraged by these results, we subsequently performed
cis-dihydroxylation of various olefins by employing 2 mol% of the
second-generation dendritic osmium catalyst 9[G2], which was

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K. Fujita et al. / Tetrahedron Letters 52 (2011) 3137–3140
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14. A magnetically recoverable chiral ligand for the asymmetric dihydroxylation
has been reported. In this case, each recycling round was performed by use of a
new batch of OsO4 and a re-oxidant: Lee, D.; Lee, J.; Lee, H.; Jin, S.; Hyeon, T.;
Kim, B. M. Adv. Synth. Catal. 2006, 348, 41–46.
Finally, the reusability of the catalyst 9[G2] was examined again
using various olefins (Table 4). In these experiments, it was found
that the catalyst 9[G2] could be efficiently recycled up to five times
by magnetic separation, and the corresponding diols were consis-
tently obtained in good chemical yields in 2–7 h in all cases.
In summary, by employing a novel magnetic osmium catalyst
having a dendritic skeleton, the dihydroxylation reaction pro-
ceeded smoothly and the osmium catalyst was efficiently recycled
up to five times. Furthermore, a positive dendritic effect on the os-
mium leaching was observed. It would be expected that the design
of the magnetically recoverable catalyst, in which a dendron is
introduced to the active site, could be applied to various transition
metal catalysts.
Supplementary data
X-ray diffraction patterns of magnetite, 8[G2], and 9[G2] and
typical procedures for the preparations of 7[Gn], 8[Gn], and 9[Gn]
are available. Supplementary data associated with this article can
be found, in the online version, at doi:10.1016/j.tetlet.2011.04.030.
15. (a) Liu, X.; Ma, Z.; Xing, J.; Liu, H. J. Magn. Magn. Mater. 2004, 270, 1–6; (b)
Massart, R. IEEE Trans. Magn. 1981, 17, 1247–1248.
16. General procedure: To an acetone–H2O (2:1, v/v) solution (3 mL) of olefin
(1 mmol) was added
a magnetic osmium catalyst (0.02 mmol) and N-
methylmorpholine N-oxide (NMO; 1.3 mmol) successively at room
temperature under an argon atmosphere. After stirring the resulting mixture,
the dihydroxylation reaction was completed (monitored by TLC). After the
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2À
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