Biogenous iron oxide-immobilized palladium catalyst for the solvent-free Suzuki-Miyaura coupling reaction

Article abstract of DOI:10.1016/j.tetlet.2011.11.044

Iron oxide produced by iron-oxidizing bacteria, Leptothrix ochracea, (biogenous iron oxide: BIO) was used as a support for immobilized palladium catalysts with organic cross-linkers. Palladium immobilized on BIO bearing imidazolium chloride delivered the desired biaryl products in sufficient yields in the Suzuki-Miyaura coupling reactions under solvent-free conditions and could be reused several times without significant loss of catalytic activity. It is shown that BIO can be exploited as a useful support for immobilization of palladium and the BIO-immobilized palladium catalyst effectively promotes the solvent-free Suzuki-Miyaura coupling reactions.

Full text of DOI:10.1016/j.tetlet.2011.11.044

Tetrahedron Letters 53 (2012) 329–332
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Tetrahedron Letters
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Biogenous iron oxide-immobilized palladium catalyst for the solvent-free
Suzuki–Miyaura coupling reaction
⇑
Kyoko Mandai, Toshinobu Korenaga, Tadashi Ema, Takashi Sakai , Mitsuaki Furutani, Hideki Hashimoto,
Jun Takada
⇑
Division of Chemistry and Biochemistry, Graduate School of Natural Science and Technology, Okayama University, Tsushima, Kita-ku, Okayama 700-8530, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Iron oxide produced by iron-oxidizing bacteria, Leptothrix ochracea, (biogenous iron oxide: BIO) was used
as a support for immobilized palladium catalysts with organic cross-linkers. Palladium immobilized on
BIO bearing imidazolium chloride delivered the desired biaryl products in sufficient yields in the
Suzuki–Miyaura coupling reactions under solvent-free conditions and could be reused several times
without significant loss of catalytic activity. It is shown that BIO can be exploited as a useful support
for immobilization of palladium and the BIO-immobilized palladium catalyst effectively promotes the
solvent-free Suzuki–Miyaura coupling reactions.
Received 7 September 2011
Revised 6 November 2011
Accepted 9 November 2011
Available online 15 November 2011
Keywords:
Biogenous iron oxide
Immobilized catalyst
Palladium catalyst
Ó 2011 Elsevier Ltd. All rights reserved.
Suzuki–Miyaura coupling reaction
Solvent-free reaction
Introduction
O
(RO)3Si
L
O Si
O
L
Microbially-produced iron oxide, particularly produced by iron-
oxidizing bacteria, Leptothrix ochracea, is referred to as biogenous
iron oxide (BIO) and has an unique amorphous and porous hollow
microtube structure that cannot be constructed artificially.1 The
fascinating structure of BIO was elucidated by Takada and his co-
workers,1 and its application to new functional materials is the
subject of active investigations. We have recently reported the
preparation of lipase immobilized on BIO through organic cross-
linkers and its use in the kinetic resolution of secondary alcohols.2
The BIO-immobilized lipase showed high catalytic activity proba-
bly because of the specific shape, surface, and nanostructure of
BIO suitable for the dispersion of each enzyme molecule on the
surface of BIO. The structure of the organic cross-linker was also
found to have a critical role for the enzymatic activity. These re-
sults encouraged us to investigate the utilization of BIO as a sup-
port for transition metal catalysts such as palladium catalysts.3,4
In particular, the Suzuki–Miyaura coupling reaction, one of the
well-known palladium-catalyzed reactions, has been widely used
for practical syntheses of various biphenyl-type organic com-
pounds.5 Although most reactions are performed using homoge-
neous catalytic systems, many immobilized palladium catalysts
on a variety of supports such as polymer, carbon, mesoporous sil-
ica, zeolite, or metal oxides have been studied to improve the cat-
toluene, 100 ºC, 24 h
BIO
modified BIO
MA: L = NH2, R = Et
DA
: L = HN(CH2)2NH2, R = Me
TA : L = HN(CH2)2NH(CH2)2NH2, R = Me
PS : L = N=C(C5H4N), R = Et
IM
: L =
, R = Et
Cl
N
N
O
Pd(OAc)2
toluene, 100 ºC, 4 or 5 h
O Si
O
L
Pd
BIO-immobilized Pd
Scheme 1. Schematic representation of the preparation of BIO-immobilized Pd
catalysts.
alytic activity and recyclability.6 In this study, we report the
development of a BIO-immobilized palladium catalyst for the Su-
zuki–Miyaura coupling reactions. The unique structure of BIO
was found to enhance the catalytic activity in the coupling reac-
tions, either in water solvent or under solvent-free conditions.
Results and discussion
⇑
Corresponding authors. Tel.: +81 86 251 8090; fax: +81 86 251 8092.
E-mail addresses: tsakai@cc.okayama-u.ac.jp (T. Sakai), jtakada@cc.okayama-
We attempted to modify the surface of BIO with substituted tri-
alkoxysilanes bearing various functional groups at their termini,
u.ac.jp (J. Takada).
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.11.044

330
K. Mandai et al. / Tetrahedron Letters 53 (2012) 329–332
Table 1
Analytical data of modified and BIO-immobilized Pd and results of the Suzuki–Miyaura coupling reactions with BIO-immobilized Pd under various reaction conditions
BIO-immobilized Pd
K2CO3, 50 ºC, 16 h
MeO
Br
+
B(OH)2
2a
MeO
1a
3a
Modified BIO
BIO-immobilized Pd
Conversion to 3a (%)c
Entry
L
Coveragea
Pd contentb
Reaction conditions
mmol/g (wt %)
mmol/g (wt %)
DMFd
H2O with
TBABe
No solventf
NH2
1
2
BIO–MA
BIO–DA
1.48 (12.76)
1.39 (17.43)
BIO–MA–Pd
BIO–DA–Pd
0.62 (6.6)
0.64 (6.8)
34
40
88
91
87
88
NH2
N
H
H
N
3
BIO–TA
0.97 (16.62)
BIO–TA–Pd
0.57 (6.1)
39
65
84
N
H
NH2
N
4
5
BIO–PS
BIO–IM
0.77 (13.44)
1.58 (29.68)
BIO–PS–Pd
BIO–IM–Pd
0.48 (5.1)
0.89 (9.4)
26
55
34
92
89
90
N
N
N
Me
Cl
a
b
c
Determined by elemental analysis.
Determined by ICP-OES analysis.
Reactions were performed with 0.30 mmol of 4-bromoanisole (1a), 0.33 mmol of phenylboronic acid (2a), and 0.60 mmol of K2CO3 with 5 mg of BIO-immobilized Pd
under the indicated conditions at 50 °C for 16 h. Conversions were determined by analysis of 1H NMR spectra.
d
DMF (1 mL) was added to the mixture of BIO-immobilized Pd, 1a, 2a, and K2CO3 before stirring in an argon atmosphere.
H2O (1 mL) was added to the mixture of BIO-immobilized Pd, 1a, 2a, K2CO3, and 0.15 mmol of TBAB before stirring in air.
BIO-immobilized Pd, 2a, and K2CO3 were mixed until homogeneous before addition of 1a and the mixture was heated in air.
e
f
which are capable of coordinating palladium for the attachment of
palladium on BIO (Scheme 1). For L groups of trialkoxysilanes, we
chose nitrogen-containing functional groups such as monoamine
(MA), diamine (DA), triamine (TA), and pyridinyl Schiff base (PS).
Imidazolium chloride (IM), which is a precursor of N-heterocyclic
carbene that is well-known as a powerful ligand for metal cata-
lysts,7 was also employed. The general preparation procedure for
modified BIO and BIO-immobilized Pd is shown in Scheme 1. Com-
mercial (MA, DA, TA) or prepared (PS, IM) trialkoxysilanes and BIO
were mixed in dry toluene at 100 °C for 24 h in an argon atmo-
sphere. Subsequently, the modified BIO was treated with Pd(OAc)2
in dry toluene at 100 °C for 4–5 h. BIO-immobilized Pd was ob-
tained after washing and drying under vacuum. Analytical data
of the modified BIO and the BIO-immobilized Pd as well as the cat-
alytic activity of the palladium catalysts in the Suzuki–Miyaura
coupling reactions are listed in Table 1. The coverage of organic
cross-linkers on BIO is 0.77–1.58 mmol/g (12.76–29.68 wt %), as
determined by elemental analysis, and the palladium content of
BIO-immobilized Pd catalysts is in a range of 0.48–0.89 mmol/g
(5.1–9.4 wt %), as determined by ICP–OES analysis.
The catalytic activity of BIO-immobilized Pd was investigated in
the Suzuki–Miyaura coupling reaction of 4-bromoanisole (1a) and
phenylboronic acid (2a) as representative substrates and K2CO3 as
a base. The reactions were performed in the presence of 5 mg of
BIO-immobilized Pd in N,N-dimethylformamide (DMF) at 50 °C
for 16 h. Incidentally, the reaction did not proceed in the presence
of 5 mg of unmodified BIO. As shown in Table 1, conversions to the
desired 4-methoxybiphenyl (3a) were from 26 to 55%, and BIO–
IM–Pd exhibited the best catalytic activity (55% conv.) among
them. Instead of DMF, water was used as a solvent in combination
with tetra-n-butylammonium bromide (TBAB) for the reaction.8 All
palladium catalysts gave improved conversions in the reactions in
water as compared to the reactions in DMF. Among them, BIO–IM–
Pd also yielded the best conversion (92% conv.). At this point, it
was assumed that the reaction would actually proceed in liquid
4-bromoanisole (1a), and not in the water layer. Thus, the reactions
of 1a and 2a in the presence of BIO-immobilized Pd and K2CO3
were performed without solvent at 50 °C for 16 h.9 When BIO–
MA–Pd, BIO–DA–Pd, and BIO–IM–Pd were used, conversions to
3a were comparable to those obtained in the reactions in water.
However, BIO–TA–Pd and BIO–PS–Pd showed their best catalytic
activities in the reaction without solvent (84% and 89% conv.,
respectively). The investigation revealed that BIO–IM–Pd gave bet-
ter overall results involving coverage of organic cross-linkers, Pd
content as well as conversions obtained in the Suzuki–Miyaura
coupling reactions under three reaction conditions. When 1a and
2a were allowed to react with Pd(OAc)2 and K2CO3 as a reference
reaction under solvent-free conditions, the product 3a was ob-
tained with only 56% conversion (Eq. 1). In addition, addition of
1.6 mol % 1-butyl-3-methylimidazolium chloride to the reaction
with palladium acetate yielded 3a in lower conversion (67% conv.).
Accordingly, it is unambiguous that the obtained higher reactivity
of our catalyst arises from BIO as a support for the immobilized
palladium catalyst.
Br
Pd(OAc)2
(1 mol %)
+
PhB(OH)2
MeO
K2CO3
100 ºC, 1 h
ð1Þ
OMe
1a
2a
3a
56% conversion
We further investigated the reaction condition to optimize the
solvent-free Suzuki–Miyaura coupling reactions of 1a and 2a with
BIO–IM–Pd (Table 2). Various inorganic bases other than K2CO3
and triethylamine that are used as a representative organic base
were tested in the reaction with the catalyst (1 mol % Pd) at
100 °C for 1 h in air. K2CO3 was found to be the best choice to give
the highest conversion to the product 3a (Table 2, entries 1 and 7–
13). The reaction with K2CO3 at 120 °C gave 90% conversion to 3a
(entry 3). As the reaction temperature decreased, the conversions
to 3a diminished (82% conv. at 80 °C, entry 4, and 63% conv. at
50 °C, entry 5). The reaction can be sufficiently facilitated with
0.1 mol % of the palladium catalyst at 100 °C to give the product
3a in 84% yield (entry 6). Silica is often used as a support for palla-
dium-immobilized catalysts in the Suzuki–Miyaura coupling reac-
tions.6,10 We prepared silica gel-immobilized Pd (silica gel–IM–Pd)

K. Mandai et al. / Tetrahedron Letters 53 (2012) 329–332
331
Table 2
Table 4
Screening of reaction conditions of the solvent-free Suzuki–Miyaura coupling
Recycling test of BIO–IM–Pd in the reaction of 4-bromonitrobenzene and phenylbo-
ronic acid
reactiona
Br
Br
BIO-IM-Pd
BIO-IM-Pd
(0.5 mol % Pd)
+
+
PhB(OH)2
PhB(OH)2
MeO
O2N
K2CO3
K2CO3
120 ºC, 1 h
temp, 1 h
OMe
1a
NO2
1e
2a
3a
2a
3g
Entry Base
Pd loading (mol %) Temp (°C) Convb. (%)
Yieldc (%)
Run number
Conv.a (%)
1st
97
2nd
98
3rd
98
4th
94
5th
94
1
2d
3
4
5
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
1.0
1.0
1.0
1.0
1.0
0.1
100
100
120
80
90
53
90
82
63
86
45
74
87
16
55
60
5
90
—
—
—
—
84
45
74
87
—
56
60
—
Determined by analysis of 1H NMR spectra.
a
50
3d in 84% yield, respectively (entries 7 and 8). Interestingly, the
reaction between 4-bromonitrobenzene (mp 125 °C) (1e) and 2a
(mp 217 °C) is a solid-state reaction; however, it provided the
desired product 3g in high yield (95% yield, entry 9). Among the re-
ports on Suzuki–Miyaura coupling reactions without solvent,12,13
the solid-state reactions are few.14 Thus, it is notable that BIO–
IM–Pd is an effective immobilized palladium catalyst for the sol-
vent-free Suzuki–Miyaura coupling reactions while the reaction
was not applicable to the corresponding aryl chlorides such as
chlorobenzene and 4-chloroanisole.
6
100
100
100
100
100
100
100
100
7
Na2CO3 1.0
8
9
10
11
12
13
Cs2CO3
K3PO4
KOt-Bu
KF
KOH
NEt3
1.0
1.0
1.0
1.0
1.0
1.0
a
The reaction was performed with 0.50 mmol of 1a, 0.55 mmol of 2a, and
1.00 mmol of base.
Determined by analysis of 1H NMR spectra.
b
c
Isolated yield after purification by silica gel column chromatography with
Recyclability of BIO–IM–Pd was examined in the reaction of 1e
and 2a in the presence of 0.5 mol % of BIO–IM–Pd and K2CO3 at
120 °C for 1 h. After the extraction of the coupling product 3g with
ethyl acetate, the BIO–IM–Pd was purified by centrifugation and
reused in the next reaction with freshly added 1e, 2a, and K2CO3.
This procedure was repeated four times until the 5th run. As
shown in Table 4, it was found that the reactivity of the catalyst
was maintained over five runs without significant loss of its reac-
tivity. A similar recyclability was also confirmed by using a liquid
substrate such as 4-bromoanisole.
To obtain further information on BIO–IM–Pd, scanning electron
microscopy (SEM) was performed. SEM images of unmodified BIO,
BIO–IM, and BIO–IM–Pd are shown in Figure 1. No significant
change of the surface structure was observed on BIO–IM in the
course of surface modification ((a) and (b)). In the case of BIO–
IM–Pd, evenly dispersed nanosized particles were detected on
the slightly deformed surface of BIO–IM (c), although the surface
deformation is seen. Average particle size of these particles is
13.2 nm, which was determined by the analysis of SEM images of
BIO–IM–Pd (the size distribution histogram is in Supplementary
data). It is known that palladium nanoparticles15 or palladium
clusters16 have high catalytic activity. Hence, the observation indi-
cates that the reactivity of our immobilized palladium catalyst is
ascribed to palladium nanoparticles that are attached and
hexane/EtOAc = 50:1.
Silica gel–IM–Pd, which was prepared from commercially available silica gel
(BW-127ZH, Fuji Silysia Chemical Ltd., Japan), was used.
d
similar to BIO–IM–Pd according to our preparation procedure and
tested it in the reaction with 1 mol % Pd loading at 100 °C for 1 h
under solvent-free conditions. As a result, silica gel–IM–Pd gave
lower conversion as compared to BIO–IM–Pd (53% conv., entry 2,
vs 90% conv., entry 1), indicating that the catalytic activity was
much improved by immobilization on BIO.
The coupling reactions of various aryl halides and arylboronic
acids were performed under solvent-free conditions (Table 3).11
The reactions of bromobenzene (1b) and arylboronic acids bearing
electron-donating and electron-withdrawing groups (2b–e) fur-
nished the desired products 3b–d in 77–91% yields (entries 1–4).
The reaction of 1b and 4-(trifluoromethyl)phenylboronic acid
(2f), which has a low reactivity due to its electron-withdrawing
p-CF3 group, was facilitated by 3 mol % Pd catalyst and a longer
reaction time to give the product 3e in good yield (84% yield, entry
5). The coupling between non-substituted 1b and 2a delivered the
desired product 3f in 85% yield with 0.5 mol % catalyst at 120 °C for
4 h (entry 6). The reaction of 4-methyl- and 4-fluoro-substituted
aryl bromides (1c and 1d) and 2a afforded 3b in 84% yield and
Table 3
Solvent-free Suzuki–Miyaura coupling reactions of aryl halides and arylboronic acidsa
BIO-IM-Pd
Ar-Ar'
+
Ar'B(OH)2
ArX
1
K2CO3, temp, time
2
3
Entry
ArX
Ar’
Pd loading (mol %)
Temp (°C)
Time (h)
Product
Conv.b (%)
Yieldc (%)
1
2
3
4
5
6
7
8
9
Bromobenzene
Bromobenzene
Bromobenzene
Bromobenzene
Bromobenzene
Bromobenzene
4-Bromotoluene
4-Bromofluorobenzene
4-Bromonitrobenzene
1b
1b
1b
1b
1b
1b
1c
1d
1e
4-Methylphenyl
4-Methoxyphenyl
3-Fluorophenyl
4-Fluorophenyl
2b
2c
2d
2e
1.0
0.5
0.5
0.5
3.0
0.5
0.5
0.5
0.5
120
100
120
120
120
120
120
100
120
4
1
1
1
16
4
1
1
1
3b
3a
3c
3d
3e
3f
3b
3d
3g
83
91
88
93
94
90
92
95
98
77
91
83
82
84
85
84
84
95
4-(Trifluoromethyl)phenyl
2f
Phenyl
Phenyl
Phenyl
Phenyl
2a
2a
2a
2a
a
b
c
Reactions were performed with 0.50 mmol of ArX, 0.55 mmol of Ar0B(OH)2, and 1.00 mmol of K2CO3 with BIO–IM–Pd in air.
Determined by analysis of 1H NMR spectra.
Isolated yield after purification by silica gel column chromatography.

332
K. Mandai et al. / Tetrahedron Letters 53 (2012) 329–332
Figure 1. SEM images of (a) BIO, (b) BIO–IM, and (c) BIO–IM–Pd.
7. For recent reviews, see: (a) Marion, N.; Nolan, S. P. Acc. Chem. Res. 2008, 41,
1440; (b) Kantchev, E. A. B.; O’Brien, C. J.; Organ, M. G. Angew. Chem., Int. Ed.
2007, 46, 2768; (c) Kantchev, E. A. B.; O’Brien, C. J.; Organ, M. G. Aldrichim. Acta
2006, 39, 97.
8. Without TBAB, the reaction did not proceed sufficiently. For the enhancement
of Suzuki-Miaura cross coupling reaction in water by TBAB, see: (a) Schmöger,
C.; Szuppa, T.; Tied, A.; Schneider, F.; Stolle, A.; Ondruschka, B. ChemSusChem
2008, 1, 339; (b) Badone, D.; Baroni, M.; Cardamone, R.; Ielmini, A.; Guzzi, U. J.
Org. Chem. 1997, 62, 7170.
9. Typical procedure for the Suzuki–Miyaura coupling reactions with BIO-immobilized
palladium under solvent-free condition: In a test tube with a screw cap, BIO-
immobilized Pd, arylboronic acid, and K2CO3 were mixed well until
homogeneous. After the addition of aryl bromide, the mixture was heated
under the conditions of the temperature and reaction time that were indicated
without stirring. Organic materials were extracted with ethyl acetate (four
times) by using centrifugation and concentrated under reduced pressure.
10. For recent examples on silica-supported palladium catalysts for the Suzuki–
Miyaura coupling reactions, see: (a) Fukuya, N.; Ueda, M.; Onozawa, S.; Bando,
K. K.; Miyaji, T.; Takagi, Y.; Sakakura, T.; Yasuda, H. J. Mol. Catal. A: Chem. 2011,
342–343, 58; (b) Chen, W.; Li, P.; Wang, L. Tetrahedron 2011, 67, 318; (c) Yang,
H.; Han, X.; Li, G.; Ma, Z.; Hao, Y. J. Phys. Chem. C 2010, 114, 22221; (d) Feng, X.;
Yan, M.; Zhang, T.; Liu, Y.; Bao, M. Green Chem. 2010, 12, 1758.
dispersed well on the surface of modified BIO, and these palladium
nanoparticles promote the Suzuki–Miyaura coupling reactions. De-
tailed analysis of the catalyst and its role in the solvent-free Suzu-
ki–Miyaura coupling reactions will be further investigated.
In conclusion, we have prepared new types of BIO-immobilized
palladium catalysts and successfully applied them to the Suzuki–
Miyaura coupling reactions. Combination of the porous nanostruc-
ture of BIO and choice of suitable organic cross-linker notably en-
hanced the catalytic activity to promote the reaction even in a non-
solvent system as well as in a solid-state system. The catalyst could
be reused at least five times and was useful. Developing the func-
tional utility of naturally produced and ubiquitous materials such
as BIO will be an urgent subject for environmentally benign
synthesis.
Acknowledgments
11. The products 3a–h were identified by comparison with the spectroscopic data
in the literatures: For 3a, 3b, 3e, 3f, 3g, see: (a) Bandari, R.; Höche, T.; Prager,
A.; Dirnberger, K.; Buchmeiser, M. R. Chem. Eur. J. 2010, 16, 4650; For 3c, see:
(b) Hartmann, C. E.; Nolan, S. P.; Cazin, C. S. J. Organometallics 2009, 28, 2915;
For 3d, see: (c) Döbele, M.; Vanderheiden, S.; Jung, N.; Bräse, S. Angew. Chem.,
Int. Ed. 2010, 49, 5986.
12. For the solvent-free Suzuki–Miyaura coupling reactions with solid-supported
palladium catalysts, see: (a) Braga, D.; D’Addario, D.; Polito, M. Organometallics
2004, 23, 2810; (b) Basu, B.; Das, P.; Bhuiyan, Md. M. H.; Jha, S. Tetrahedron Lett.
2003, 44, 3817; (c) Kabalka, G. W.; Wang, L.; Pagni, R. M.; Hair, C. M.;
Namboodri, V. Synthesis 2003, 217; (d) Melucci, M.; Barbarella, G.; Sotgiu, G. J.
Org. Chem. 2002, 67, 8877; (e) Villemin, D.; Caillot, F. Tetrahedron Lett. 2001, 42,
639; (f) Kabalka, G. W.; Pagni, R. M.; Wang, L.; Namboodiri, V.; Hair, C. M. Green
Chem. 2000, 2, 120; (g) Kabalka, G. W.; Pagni, R. M.; Hair, C. M. Org. Lett. 1999, 1,
1423.
This study is financially supported by a Special Grant for Educa-
tion and Research from the Ministry of Education, Culture, Sports,
Science, and Technology, Japan. We are grateful to the SC-NMR
Laboratory of Okayama University for the measurement of NMR
spectra.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2011.11.044.
13. For the solvent-free Suzuki–Miyaura coupling reactions with nonimmobilized
palladium catalysts, see: (a) Li, J.-H.; Deng, C.-L.; Xie, Y.-X. Synth. Commun.
2007, 37, 2433; (b) Klingensmith, L. M.; Leadbeater, N. E. Tetrahedron Lett.
2003, 44, 765.
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