Immobilization of dipyridyl complex to magnetic nanoparticle via click chemistry as a recyclable catalyst for Suzuki cross-coupling reactions

Article abstract of DOI:10.1055/s-2008-1072597

A magnetic nanoparticle (MNP)-supported di(2-pyridyl)methanol palladium dichloride complex was prepared via click chemistry. The MNP-supported catalyst was evaluated in Suzuki coupling reaction in term of activity and recyclability in DMF. It was found to be highly efficient for Suzuki coupling reaction using aryl bromides as substrates and could be easily separated by an external magnet and reused in five consecutive runs without obvious loss of activity. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-2008-1072597

1418
LETTER
Immobilization of Dipyridyl Complex to Magnetic Nanoparticle via Click
Chemistry as a Recyclable Catalyst for Suzuki Cross-Coupling Reactions
Immobilization of
u
D
ipyridyl Co
a
mplex to
M
n
agnetic Na
n
g
oparticle hua Lv, Wenpeng Mai, Rizhe Jin, Lianxun Gao*
State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences,
Graduate School of Chinese Academy of Sciences, Changchun 130022, P. R. of China
Fax +86(431)85685653; E-mail: lxgao@ciac.jl.cn
Received 20 December 2007
conjugation between appropriately functionalized part-
Abstract: A magnetic nanoparticle (MNP)-supported di(2-pyr-
ners of azides and alkynes. It fulfills the advantages of
modularity, almost quantative yields, moderate reaction
conditions, ease of introduction of both the required azide
idyl)methanol palladium dichloride complex was prepared via click
chemistry. The MNP-supported catalyst was evaluated in Suzuki
coupling reaction in term of activity and recyclability in DMF. It
was found to be highly efficient for Suzuki coupling reaction using and alkyne groups, which are compatible with a broad
aryl bromides as substrates and could be easily separated by an ex-
range of functional groups and reaction conditions. These
ternal magnet and reused in five consecutive runs without obvious
appealing advantages have led CuAAC to wide applica-
loss of activity.
tions in fields such as drug design,16 polymer science,17
biochemistry,18 molecular biology and solid-phase organ-
Key words: Suzuki coupling, supported catalysis, palladium, di(2-
pyridyl)methanol, magnetic nanoparticles
ic transformations.19
Palladium-catalyzed cross-coupling reactions are power-
ful methods for C–C bond formation. Many supported-
The immobilization of homogeneous catalysts to facilitate
catalyst-promoted cross-coupling reactions have been re-
catalysts separation and recycling is of great importance
ported to fulfill catalyst separation and recycling using
in catalyst science.1 Various supports have been explored
PS, PEG, silica gel as supports with different kinds of
in the past decades, such as polymers,2 inorganic solids,3
ligands such as NHCs,20 phosphine ligands,21 and dipy-
fluorous tags,4 and ionic liquids.5 However, despite the
ridyl ligands.22 But only few reports have been seen in lit-
advantages mentioned above, substantial decrease of ac-
erature using magnetic nanoparticles as catalyst supports.9
tivity and selectivity often appeared due to the low solu-
Our group has dedicated to the preparation of high effi-
bility of the supported catalysts in the reaction media
cient catalysts for cross-coupling reactions and their fur-
which resulted in a slow diffusion of reactants and cata-
ther reuse.23 Recently, we have reported a PEG-supported
lysts.6 In the case of fluorous phase and ionic-liquid-sup-
dipyridyl catalyst and its application in Suzuki coupling
ported catalysts, the preparation of the catalysts was
reaction.23a Herein we would like to present a simple and
difficult and time-consuming. Nanoparticles have
efficient preparation of a novel MNP-supported dipyr-
emerged as attractive supports for solid-phase organic
idyl–Pd complex by using click chemistry as a practical
transformations7 due to their high surface areas brought
tagging method and its application in Suzuki coupling re-
by their nanometer scale. Consequently, the activity of the
actions. High activity was observed and the catalyst could
supported catalysts was greatly improved as compared to
be easily isolated from the reaction mixture by simple
catalysts supported on traditional support matrix. But it
magnetic decantation and reused several runs without sig-
still came up with the problem of catalyst separation and
nificant loss of activity.
further recycling due to the colloidal property of the cata-
The magnetic nanoparticles were readily prepared by con-
ventional coprecipitation method with an average diame-
ter of 10 nm. The particles were then coated with a thin
layer of silica according to a literature method24 in order
to prevent the aggregation of the nanoparticles. As shown
in Scheme 1, further modification of the surface of the sil-
ica-capped MNP with excess azide-functionalized silane
3 in refluxed THF afforded 4.25 Direct propargylation of 1
give rise to 2 in high yield.26 Following a typical process
developed by Gmeiner,19a 6 mol% CuI was found to be
optimal for the cycloaddition of 2 and 4 to achieve 5,27 and
the reaction process was judged by the complete disap-
pearance of the FT-IR absorption of the azide group (2104
cm–1, Figure 1). Then, the MNP-supported dipyridyl–Pd
lyst. This could be solved by using magnetic
nanoparticles8 as catalyst supports, thus the catalyst could
be easily separated from the reaction media by the appli-
cation of an external magnetic field. During the past years,
a lot of magnetic-supported catalysts have been reported
in organic transformations such as cross-coupling reac-
tions,9 hydroformylation,10 olefin hydrogenation,11 asym-
metric hydrogenation,12 nucleophilic substitution,13 and
esterification.14
Recently, the copper-catalyzed azide–alkyne cycloaddi-
tion (CuAAC) reaction,15 which was coined as ‘click reac-
tion’, has been proved to be a powerful tool for
SYNLETT 2008, No. 9, pp 1418–1422
x
x.
x
x.
2
0
0
8
complex
6
was obtained by refluxing
5
with
PdCl2(MeCN)2 in toluene.28 In each step, the isolation and
Advanced online publication: 07.05.2008
DOI: 10.1055/s-2008-1072597; Art ID: W22007ST
© Georg Thieme Verlag Stuttgart · New York

LETTER
Immobilization of Dipyridyl Complex to Magnetic Nanoparticle
1419
OH
O
Br
c
a,b
N
N
N
N
N
1
2
Fe3O4
SiO2
EtO
EtO
EtO
d
Si
N3
Si
OEt
Cl
EtO
OEt
e
3
O
O
O
O
O
O
f
N
N
Fe3O4
SiO2
Si
N
Si
N3
Fe3O4
SiO2
O
N
N
4
5
g
O
O
O
Cl
Pd
N
N
Fe3O4
SiO2
Si
N
O
N
Cl
N
6
Scheme 1 Preparation of MNP-supported di(2-pyridyl)methanol–Pd complex 6. Reaction conditions: a) –78 °C, n-BuLi, then 2-pyridylcar-
boxaldehyde, –78 °C, 75%; c) NaH, 0 °C, DMF, propargyl bromide, 60%; d) butanone, NaN3, reflux, 72h, 90%; e) THF, reflux, 24 h; f) CuI,
DIPEA, DMF–THF (1:1); g) PdCl2(MeCN)2, toluene, reflux, 24 h.
Figure 1 FT-IR Spectra of (a) di(2-pyridyl)methanol propargyl
Figure 2 TEM image of 5 (scale bar 50 nm)
ether (2); (b) magnetic nanoparticles 4, and (c) 5
vealed 6 containing 0.58 mmol/g Pd, which was quite near
to the value of the ligand suggesting the almost complete
coordination of the ligand with Pd.
purification of the MNP was achieved via an external
magnetic field. The TEM image of 5 is illustrated in
Figure 2, which showed the core-shell structure of the sil-
ica-caped MNP (the long strips in Figure 2 were consid-
ered as the margin of the copper grids). The loading
capacity of 4, 5, and 6, calculated from elemental analysis
data of N are 0.684 mmol/g, 0.628 mmol/g, and 0.60
mmol/g, further showed the high efficiency of the click
chemistry for the immobilization of ligand. ICP-OES re-
Initial studies of reaction conditions for Suzuki coupling
were performed with bromobenzene and phenylboronic
acid as the model reaction with different solvents, bases,
and catalyst loadings (Table 1). Using K2CO3 as base and
6 as catalyst, the reaction proceeded much slower in tolu-
ene, water, and DMF–water than in DMF (Table 1, entries
Synlett 2008, No. 9, 1418–1422 © Thieme Stuttgart · New York

1420
G. Lv et al.
LETTER
1–4). When Cs2CO3, Na2CO3, or NaOAc was used as
base, an obvious decrease in yield was observed (Table 1,
entries 5–7). Reducing the amount of 6 resulted in a direct
decrease in yield even with longer reaction time (Table 1,
entry 8).
Table 1 Suzuki–Miyaura Coupling of PhB(OH)2 with Bromoben-
zenea
Br
B(OH)2
+
Entry Pd
(%)
Base
Solvent
Temp
(°C)
Time Yield
With the optimized reaction conditions in hand, cross-
coupling reactions of different aryl halides with aryl bo-
ronic acids were performed and the results are summa-
rized in Table 2. Under standard conditions, which use
DMF as solvent, K2CO3 as base, and with 0.2% catalyst
loading, good to excellent yields were obtained with bro-
moarenes bearing both electron-withdrawing groups and
releasing groups (Table 2, entries 1–4). But for the sub-
strate with an ortho substituent, the yield of the coupling
products dropped slightly even using more catalyst load-
ing (Table 2, entry 5). Two other kinds of aryl boronic ac-
ids were also tested and they all reacted smoothly with
aryl bromides with high product yields (Table 2, entries
6–10). Unfortunately, the catalytic system was less effec-
tive with substrates such as aryl chloride and heterocyclic
bromide even using 1% of 6 and prolonged reaction time
(Table 2, entries 11 and 12). It was worthy to note that
magnetic nanoparticle 6 was air and moisture stable, and
the coupling reactions could be carried out under air di-
rectly.
(h)
(%)b
60
83
92
86
85
75
60
85
1
2
3
4
5
6
7
8
0.2
K2CO3
K2CO3
K2CO3
K2CO3
Cs2CO3
Na2CO3
NaOAc
K2CO3
toluene
H2O
110
100
100
5
0.2
0.2
0.2
0.2
0.2
0.2
0.1
5
DMF
5
DMF–H2Oc 100
5
DMF
DMF
DMF
DMF
100
100
100
100
5
5
5
8
a Reaction conditions: bromobenzene (2 mmol), PhB(OH)2 (2.6
mmol), base (4 mmol), solvent (5 mL).
b Isolated yield.
c DMF–H2O = 3:2.
Table 2 MNP-Supported Catalyst 6 in Suzuki Coupling of Aryl Halides and Aryl Boronic Acida,29
Entry
1
Aryl halides
Aryl boronic acids
Time (h)
9
Product
Yield (%)b
92
B(OH)2
MeO
Br
MeO
7a
2
3
5
3
92
99
B(OH)2
B(OH)2
Br
7b
O
O
Br
7c
4
5
3
99
83
B(OH)2
B(OH)2
OHC
7d
OHC
Br
12
Br
7e
6
O
3
99
O
B(OH)2
Br
7f
7
8
3
5
97
92
OHC
Br
B(OH)2
B(OH)2
OHC
7g
Br
7h
Synlett 2008, No. 9, 1418–1422 © Thieme Stuttgart · New York

LETTER
Immobilization of Dipyridyl Complex to Magnetic Nanoparticle
1421
Table 2 MNP-Supported Catalyst 6 in Suzuki Coupling of Aryl Halides and Aryl Boronic Acida,29 (continued)
Entry
9
Aryl halides
Aryl boronic acids
Time (h)
6
Product
Yield (%)b
91
O
O
F3C
B(OH)2
Br
CF3
7i
10
11
12
6
20
20
90
<20
46
MeO
Br
F3C
B(OH)2
MeO
7j
CF3
Cl
B(OH)2
7b
Br
N
B(OH)2
N
7k
a Reaction conditions: aryl halide (2 mmol), aryl boronic acid (2.6 mmol), 6 (0.004 mmol), K2CO3 (4 mmol), DMF (5 mL), 100 °C.
b Isolated yield.
c The amount of 0.4 mol% of Pd were used.
d The amount of 1 mol% of Pd was used.
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Having established the scope of the new reaction system,
we then turned our attention to the recovery and reuse of
the catalyst 6. We chose the cross-coupling of 4-bromo-
acetophenone and phenylboronic acid as the model reac-
tion. After the completion of the reaction, the reaction
mixture was diluted with diethyl ether and the catalyst
was easily separated by an external magnet, washed with
diethyl ether, water, dried under vacuum, and then used
directly in the next run. No obvious loss in catalytic activ-
ity was observed in five recycling runs under the same re-
action conditions (Table 3).
Table 3 Reuse of 6 in Suzuki Coupling of 4-Bromoacetophenone
and Phenylboronic Acid
Run
1
2
3
4
5
a Yield (%)
a 99
a 99
a 99
a 96
a 95
In summary, we have described a novel MNP-supported
di(2-pyridyl)methanol-derived palladium chloride 6 pre-
pared via click chemistry. The catalyst was proved to be
highly efficient in Suzuki reactions with a variety of aryl
bromoarenes as substrates. Furthermore, the MNP-sup-
ported catalyst could be easily separated from the reaction
system by magnetic decantation and reused for several
runs with little loss in activity. Due to the wide application
of dipyridyl ligand, further effort to extend the use of 6 in
other reactions is still in progress.
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Synlett 2008, No. 9, 1418–1422 © Thieme Stuttgart · New York

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LETTER
2006, 71, 537. (d) Zhu, Y. H.; Peng, S. C.; Emi, A.; Su, Z.;
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literature method30) and 3-azidopropyltriethoxysilane 3 (0.5
g, 2.02 mmol) was refluxed in anhyd toluene for 24 h, then
the mixture was submitted to external magnetic field, and
washed sequentially with toluene (3 × 10 mL), Et2O (3 × 10
mL). The loading was determined to be 0.684 mmol g–1 by
elemental analysis. FT-IR: 2104, 1635, 1487, 1101, 969,
798, 585 cm–1.
(26) Synthesis of Di(2-pyridyl)methyl Propargyl Ether (2)
To a stirred solution of 1 (0.93 g, 5 mmol) in anhyd DMF (10
mL) at 0 °C under Ar was added NaH (0.24 g, 6 mmol) in
batches, the mixture was stirred at 0 °C for 1 h. After the
dropwise addition of propargyl bromide (0.65 mL, 6 mmol)
over 10 min, the reaction mixture was raised to r.t., and
stirred for another 10 h. Then the system was quenched with
MeOH (10 mL) and submitted to flash chromatography on
silica gel (PE–EtOAc, 1:1) to afford 2 as dark brown solid
(0.6 g, 60%). 1H NMR (600 MHz, CDCl3): d = 8.57 (d, 2 H,
J = 4.8 Hz), 7.72 (t, 2 H), 7.62 (d, 2 H, J = 7.8), 7.19 (t, 2 H),
5.93 (s, 1 H), 4.28 (d, 2 H, J = 2.4), 2.45 (t, 1 H). 13C NMR
(100 MHz, CDCl3): d = 159.4, 149.3, 136.8, 122.8, 121.9,
83.8, 75.1, 56.6, 29.7. Anal. Calcd for C14H12N2O: C, 74.98;
H, 5.39; N, 12.49. Found: C, 74.96; H, 5.36; N, 12.39.
(27) Preparation of Di(2-Pyridyl)methanol-Functionalized
MNP 5 via Click Reaction
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Compounds 2 (0.25 g, 1.12 mmol) and 4 (1 g) were mixed
with CuI (0.008 g, 3.77 × 10–2 mmol) in DMF–THF (1:1, 10
mL) under Ar atmosphere, to this was injected DIPEA (1
mL), and the reaction mixture was stirred with a mechanic
stirring at r.t. The reaction was monitored by FT-IR as was
indicated by the almost complete disappearance of IR signal
of 2104 cm–1 which stands for the azide group. Then the
reaction mixture was submitted to magnetic separation, and
the MNP were washed sequentially with Et2O (3 × 20 mL),
H2O (3 × 20 mL), then acetone (3 × 20 mL), finally dried
under vacuum. FT-IR: 1646, 1597, 1475, 1434, 1091, 794,
588, 463 cm–1.
(28) Preparation of 6
To the freshly prepared PdCl2 (MeCN)2 (0.16 g, 0.62 mmol)
solution in toluene (10 mL) was added 5 (1 g), the mixture
was mechanically stirred and refluxed for 12 h, the reaction
system was magnetically separated, the MNP were washed
with H2O (3 × 20 mL) and MeOH (5 × 20 mL) then dried in
vacuo.
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(29) Typical Procedure for Suzuki Coupling
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A mixture of aryl halide (2 mmol), arylboronic acid (2.6
mmol), 6 (0.004 mmol), K2CO3 (4 mmol), and DMF (5 mL)
was added to a flask and stirred at 100 °C, open to the air, for
the desired time until complete consumption of the starting
substrates as judged by TLC. After completion of the
reaction, the reaction mixture was cooled to r.t., and to the
solution was added Et2O (30 mL), then a magnet was used to
separate the catalyst, washed with H2O (3 × 10 mL), then the
organic phase was evaporated and the residue was purified
by flash chromatography to afford the desired coupling
products. The MNP-supported catalyst 6 was washed with
Et2O, H2O, dried, and reused directly in the next run. All the
coupling products could be found in literature.
(24) Philipse, A. P.; van Bruggen, M. P. B.; Pathmamanoharan,
C. Langmuir 1994, 10, 92.
(25) Preparation of 4
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Synlett 2008, No. 9, 1418–1422 © Thieme Stuttgart · New York

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