A palladium catalyst supported on carbon-coated cobalt nanoparticles-preparation of palladium-free biaryls by suzuki-miyaura reactions in ethanol

Article abstract of DOI:10.1002/ejoc.201403038

The preparation of a heterogeneous Pd catalyst supported on carbon-coated magnetic nanoparticles (NPs) bearing 4-(diphenylphosphinomethyl)phenyl groups is discussed. These NPs were initially prepared by chemical vapour deposition (CVD) of carbon on prefor

Full text of DOI:10.1002/ejoc.201403038

FULL PAPER
DOI: 10.1002/ejoc.201403038
A Palladium Catalyst Supported on Carbon-Coated Cobalt Nanoparticles –
Preparation of Palladium-Free Biaryls by Suzuki–Miyaura Reactions in
Ethanol
Antoine Derible,[a] Carine Diebold,[a] Joseph Dentzer,[a] Roger Gadiou,[a]
Jean-Michel Becht,*[a] and Claude Le Drian*[a]
Keywords: Heterogeneous catalysis / Nanoparticles / Reusable catalysts / Cross-coupling / Chemical vapor deposition /
Cobalt / Palladium
The preparation of a heterogeneous Pd catalyst supported
on carbon-coated magnetic nanoparticles (NPs) bearing 4-
(diphenylphosphinomethyl)phenyl groups is discussed.
50 μequiv. Finally, the Pd leaching was very low: in most
cases only 1 μequiv. of Pd is found in the reaction medium
(which amounts to less than 1 ppm of the product). Com-
These NPs were initially prepared by chemical vapour depo- pared to other catalysts supported on magnetic particles, this
sition (CVD) of carbon on preformed Co NPs; however, the catalyst is very simple to prepare even in large quantities
particles prepared in one step by reducing flame synthesis and is easy to recover completely owing to the high magnetic
were more appropriate for functionalization. The Pd catalyst
was then prepared by three simple steps. We show that
Suzuki–Miyaura couplings can be efficiently performed in
the presence of very low amounts of supported Pd, usually
saturation of the Co nucleus. To the best of our knowledge,
it is the simplest and the most-active Pd catalyst supported
on carbon-coated Co nanoparticles for Suzuki–Miyaura reac-
tions. The reusability of the catalyst was also ascertained.
have developed highly active soluble catalysts that allow
couplings in the presence of extremely low amounts of Pd
catalyst.[10] Another strategy is the replacement of the solu-
ble catalysts by heterogeneous catalysts that can be reco-
vered from the reaction mixtures by filtration and reused.[11]
For this purpose, the Pd catalyst is generally immobilized
on an organic[12] or inorganic[13] support. For example, we
have developed various efficient Pd catalysts supported on
a Merrifield resin bearing diaryl- or (aryl-tert-butyl)phos-
phanyl groups.[14] Some of these catalysts are particularly
reactive for the Suzuki–Miyaura reactions of arylboronic
acids with aryl bromides or even aryl chlorides.[14c]
In recent years, the use of magnetic nanoparticles (NPs)
as support for different reagents has elicited a huge interest,
especially in biology-related areas.[15] Their major advan-
tages are their intrinsically high surface area, the excellent
accessibility of the surface-bound reagents and the possibil-
ity to recover them from any reaction mixture by simple
application of an external magnetic field.[16] Many catalysts
have been prepared from magnetic NPs,[17] which have
almost exclusively been iron oxide (maghemite or magne-
tite)[18,19] or NiFe2O4 particles.[20,21] In the field of palla-
dium catalysis, the Pd is either adsorbed on the iron oxide
itself[22] or grafted onto the surface of NPs formed of an
iron oxide core surrounded by a silica[23] or polymer[24,25]
shell bearing carbene, phosphanyl or amino ligands. To im-
prove magnetic recovery (vide infra), iron oxides could be
replaced by metallic NPs. For example, Jang et al. have pre-
Introduction
Pd-catalyzed cross-couplings are fundamental transfor-
mations in modern organic synthesis and offer a powerful
route for the construction of carbon–carbon bonds under
mild reaction conditions with high yields.[1] In particular,
the Suzuki–Miyaura reaction is the most powerful and the
most widely used coupling for the formation of aryl–aryl
bonds. The resulting biaryls find numerous important ap-
plications in the fields of pharmaceuticals,[2] organic light-
emitting device (OLED) materials,[3] liquid crystals,[4] metal
ligands for catalysis[5] and molecular recognition.[5]
The Suzuki–Miyaura coupling is traditionally performed
by reacting aryl halides with arylboronic acids in the pres-
ence of ca. 10–50 mequiv. of a soluble Pd catalyst.[6–9] Pd is
a scarce natural resource found only in small quantities in
different ores and, therefore, it is obtained only as a byprod-
uct of other metals (nickel, copper, chromium, platinum
group metals, etc.). These soluble Pd catalysts are usually
very expensive and cannot be recovered after reactions for
direct reuse. To overcome these drawbacks, several groups
[a] Institut de Science des Matériaux de Mulhouse (UMR-CNRS
7361), Université de Haute Alsace,
15 rue Jean Starcky, 68057 Mulhouse cedex, France
E-mail: jean-michel.becht@uha.fr
claude.le-drian@uha.fr
http://www.is2m.uha.fr
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201403038.
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pared a magnetic catalyst bearing very small Pd NPs sup- Al(C8H17)3 afforded Co NPs A with a homogeneous size of
ported on carbon NPs containing iron. This catalyst was 12 nm. These Co NPs were air-stable and could be used
used for the Suzuki–Miyaura coupling of a heteroaryl iod- without particular precautions.
ide in the presence of 12 mequiv. of supported Pd.[26]
Recently, carbon-coated cobalt (Co/C) NPs have emerged
Preparation of Co/C NPs by Chemical Vapour Deposition
as a promising class of magnetic support. Compared to iron
oxide, these NPs present an improved magnetic saturation
(MS,Co = 158 emug–1 vs. MS,magnetite Ͻ92 emug–1) and ex-
cellent chemical and thermal stabilities.[27] However, to the
best of our knowledge, their use as a magnetic support for
catalysts for the formation of C–C bonds has only been
very sparsely studied.[28] For example, the group of Stark
has reported a Pd catalyst immobilized on Co/C NPs to
which polymeric chains bearing diphenylphosphanyl groups
were covalently attached.[27] In the presence of 11 mequiv.
of supported Pd, the Suzuki–Miyaura reactions of aryl iod-
ides afforded biaryls in good yields. Later, the same group
described a Pd catalyst grafted on a Co/C metal core to
All Co/C NPs were characterized by transmission elec-
tron microscopy (TEM, Figure 2). Initially, the Co NPs
were placed in a furnace, which was heated from 20 to
750 °C over 70 min under argon; then, a 2.5% propylene/
argon mixture was injected for 2 h at 750 °C, and Co/C NPs
A1 were obtained. The size of the Co core increased to at
least 200 nm owing to Ostwald ripening,[33] and the carbon
shell surrounding them was ca. 35 nm thick. A reduced
chemical vapour deposition (CVD) reaction time of 1 h
gave similar results. Therefore, the CVD conditions were
reoptimized to minimize the coalescence of the Co NPs. For
this purpose, the furnace was preheated to 700 °C under
argon, and the introduction of the Co NPs A was immedi-
ately followed by the introduction of the 2.5% propylene/
argon mixture. After 15 min, NPs A2 were obtained with a
Co core of 20–100 nm and a carbon shell 10–20 nm thick.
It is important to note that a sizeable amount of free carbon
(a sort of soot that is not fixed to the Co nuclei) was also
present. This soot will not be recovered by application of
an external magnetic field and, therefore, had to be re-
moved.[34] According to previous literature reports, the
Co/C NPs A2 were placed in N-methylpyrrolidone (NMP),
which is known to spread the graphite planes.[35] After mag-
netic separation, the obtained NPs A3 had lost almost all of
which
amphiphilic
N-isopropylacrylamide
polymer
branches bearing diphenylphosphanyl groups were cova-
lently attached.[29] This catalyst was successfully used up to
ten times for the Suzuki–Miyaura reactions of iodobenzene
and phenylboronic acid in the presence of 30 mequiv. of
supported Pd. Very recently, the group of Majoral and Ca-
minade developed the first reusable pyrene-tagged dendritic
Pd catalysts grafted onto Co/C nanoparticles through π–π
stacking for the Suzuki–Miyaura reactions of aryl bromides
with 5 mequiv. of supported Pd.[30,31]
Herein, we would like to report the preparation of a sim-
ple Pd catalyst supported on magnetic Co/C NPs bearing
4-(diphenylphosphinomethyl)phenyl groups covalently at-
tached to the carbon shell and their use for the Suzuki–
Miyaura reactions of aryl bromides under environmentally
friendly reaction conditions in the presence of low to ex-
tremely low amounts of supported Pd (50–500 μequiv.).
Results and Discussion
Preparation of Co NPs A
According to the report by Bönnemann et al., Co NPs
have been prepared by thermolysis of Co2(CO)8 in toluene
under reflux in the presence of Al(C8H17)3 followed by
smooth air oxidation (Figure 1).[32] A 10:1 ratio of Co2(CO)8/
Figure 2. TEM images of Co/C NPs obtained by CVD: (a) A1
(CVD 750 °C, 2 h), (b) A2 (CVD 700 °C, 15 min), (c) A3 (A2
washed successively with NMP, H2O, cyclohexane, AcOEt and
Et2O), (d) A4 (CVD 600 °C, 15 min) and (e) A5 (CVD 750 °C,
15 min).
Figure 1. TEM image of Co NPs A.
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Supported Palladium Catalyst on Nanoparticles
the unfixed carbon (soot). Brunauer–Emmett–Teller (BET)
Inspired by a report by the group of Stark,[27] we then
analysis proved the absence of porosity, and the specific sur- activated Co/C NPs A3 by sonication in 0.1 m aqueous HCl
face ranged from 9 to 30 m2 g–1. We then tried to modify for 30 min. TEM analysis of the resulting Co/C NPs A6
the CVD temperature. The procedure in which the furnace showed the presence of numerous empty carbon shells (Fig-
is heated before the introduction of the reagents was used; ure 3). The replacement of 0.1 m HCl by 8 m HNO3 or 0.1 m
after 15 min at 600 °C, numerous Co NPs were uncoated HNO3 at 20 °C or even at reflux gave similar results. More-
and formed agglomerates (A4), whereas Ostwald ripening over, attempts to react these NPs with 3 under the condi-
was important after 15 min at 750 °C, and carbon shells of tions of Table 1, Entry 2 led to no improvement of the
various thicknesses of up to 40 nm formed (A5). Attempts amount of Br fixed. Therefore, we conclude that the oxi-
to form the carbon shell of the Co/C NPs by polymerization dation of the Co core is easier than the activation of the
and carbonization steps from furfuryl alcohol were unsuc- carbon shell. A likely explanation could be that the carbon
cessful.
shell formed by CVD is constituted of graphene layers that
both present only few defects and are not resistant enough
to efficiently protect the Co nucleus.
Functionalization of Co/C NPs A3 Obtained by CVD
4-(Bromomethyl)phenyl groups were then grafted onto
the Co/C NPs A3. For this purpose, NPs A3 were treated
with the aryldiazonium salt 3, which was prepared in four
steps from p-toluidine (Scheme 1).[36,37]
Figure 3. TEM image of Co/C NPs A6 (obtained by activation of
NPs A3 with 0.1 m aqueous HCl).
Functionalization of Co/C NPs A7 Obtained by Reducing
Flame Synthesis
We then focused our attention on the use of Co/C NPs
A7, which were prepared in one step by the group of Stark
by a completely different method: the reducing flame syn-
thesis.[40] We intended to determine if their carbon shell had
a reactivity different to that of the A3 shells. TEM images
showed that the Co cores of these NPs A7 were coated by
thin carbon shells (Figure 4). By the procedure described
for the NPs A3 (see Table 1), the NPs A7 were treated with
3 at 20 °C for 45 min under sonication to yield the function-
alized NPs B2 containing 2.1% of Br (i.e., 0.26 mmol of Br
per g).[38] This value is sufficiently high since it is of the
same order of magnitude as the amount of chloride present
in the Merrifield-type resins that we had used formerly.[14]
Scheme 1. Synthesis of aryldiazonium tetrafluoroborate 3.
The conditions for the reaction of Co/C NPs A3 with 3
to give functionalized NPs B1 were optimized (Table 1). Af-
ter sonication at 20 °C for 15 min in the presence of 3, only
0.15% of Br (i.e., 0.02 mmol of Br per g) was introduced.
Longer reaction times, higher temperatures or even heating
the reaction mixture under reflux brought only insufficient
improvements, and the Br content remained below 1%
(Table 1, Entries 2–4).
Table 1. Optimization of the reaction conditions.
Entry[a] Conditions Reaction time
T [°C]
Br [mmolg–1][b]
1
2
3
4
sonication
sonication
sonication
heating
15 min
12 h
12 h
20
20
50
0.02
0.10
0.07
0.05
12 h
100
[a] Reactions performed with Co/C NPs A3 (200 mg) and 3
(2.25 mmol) in H2O (30 mL). [b] The amount of Br present on the
functionalized Co/C NPs was determined by colorimetric titration
(experimental error of ca. Ϯ0.02 mmolg–1; for further details, see
Supporting Information).
Figure 4. TEM image of Co/C NPs A7.
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J.-M. Becht, C. Le Drian et al.
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This enhanced reactivity of the carbon shell can be ex-
The Suzuki–Miyaura coupling with K3PO4·3H2O and
plained by the method of synthesis, which produces a 1 mequiv. of supported Pd in toluene afforded 4a in 52%
higher number of structural defects than CVD. Therefore, yield (Table 2, Entry 1). In the presence of Na2CO3 as base
we chose NPs A7 prepared by reducing flame synthesis as and either toluene, dimethoxyethane (DME) or N,N-di-
the starting material and used B2 to prepare a hetero- methylacetamide (DMA) as solvent, 4a was obtained in
geneous Pd catalyst.
moderate yields of 15–57% (Table 2, Entries 2–4). An im-
provement was achieved in N,N-dimethylformamide
(DMF), as 4a was obtained in 72% yield (Table 2, Entry 5).
We then focused our attention on the use of 96% EtOH, as
4a was obtained in 98% yield even in the presence of only
50 μequiv. of supported Pd (Table 2, Entries 6 and 7) and
still in 82% yield with only 20 μequiv. of supported Pd
(Table 2, Entry 8). Different bases (Table 2, Entries 9–13) or
the replacement of 96% EtOH by a 1:1 mixture of EtOH/
H2O or by H2O (Table 2, Entries 14 and 15) gave 4a in only
lower yields even with up to 1 mequiv. of supported Pd.
Notably, a blank test reaction of 4-bromoacetophenone and
phenylboronic acid with unfunctionalized Co/C NPs A7
gave no reaction. The replacement of 4-bromoacetophen-
one by 4-chloroacetophenone or 4-methoxyphenyl tri-
fluoromethanesulfonate was unsuccessful even in the pres-
ence of 0.5 mequiv. of supported Pd, and the starting mate-
rials were recovered unchanged. Finally, the reaction of 4-
iodoacetophenone instead of 4-bromoacetophenone af-
forded 4a quantitatively (conditions of Table 2, Entry 7).
The homogeneous or heterogeneous nature of the cataly-
sis was then determined. For this purpose, the catalyst was
magnetically removed after only 20 min of reaction under
the conditions of Table 2, Entry 7. The yield was then 14%
and remained almost identical (15%) after the reaction mix-
ture was heated under reflux for a further 20 h. Identical
results were obtained when the magnetic separation was re-
placed by a hot filtration through a 0.2 μm polytetrafluoro-
ethylene (PTFE) membrane. Therefore, we concluded that
soluble Pd entities played no significant role in the reaction.
We determined that only ca. 1.9% of the initial amount of
Pd was lost from the catalyst under the conditions of
Table 2, Entry 7 after the normal end of the reaction and
the magnetic removal of C1 (see Supporting Information).
We checked that the removal of the catalyst by filtration
through a 0.2 μm PTFE membrane instead of magnetic re-
covery afforded the same value for the Pd loss. Therefore,
we can conclude that the magnetic recovery of the catalyst
is completely efficient.
Preparation of Pd Catalyst Supported on Co/C NPs
Bearing (4-Bromomethyl)phenyl Groups
By the procedure previously used in our laboratory,[39]
the NPs B2 were treated with lithium diphenylphosphide,
and then Pd was introduced by an exchange reaction with
a soluble Pd complex to form catalyst C1 (Scheme 2).
Scheme 2. Preparation of Pd catalyst C1.
Suzuki–Miyaura Couplings with Pd Catalyst C1
The reaction conditions were optimized with 4-bromo-
acetophenone and phenylboronic acid as model substrates
(Table 2).
Table 2. Optimization of the reaction conditions for the Suzuki–
Miyaura coupling.
Entry Solvent
Base
Pd [mequiv.] Yield [%][c]
1[a]
toluene
toluene
DME
DMA
DMF
EtOH/H2O 96:4 Na2CO3
EtOH/H2O 96:4 Na2CO3
EtOH/H2O 96:4 Na2CO3
EtOH/H2O 96:4 Li2CO3
EtOH/H2O 96:4 K2CO3
EtOH/H2O 96:4 Cs2CO3
EtOH/H2O 96:4 NaOH
EtOH/H2O 96:4 Et3N
K3PO4·3H2O
Na2CO3
Na2CO3
Na2CO3
Na2CO3
1
1
52
15
33
57
72
99
98
82
61
70
74
71
65
69
7
2[a]
3[a]
1
1
1
1
4[a]
Various biaryls were then prepared under the conditions
of Table 2, Entry 7 (Table 3). The couplings of phenylbor-
onic acid with various aryl bromides bearing an electron-
withdrawing or an electron-donating group afforded the bi-
aryls. Good-to-excellent yields in the presence of only
50 μequiv. of supported Pd were obtained (Table 3, En-
tries 1, 2 and 4–9). Notably, higher amounts of supported
Pd (500 μequiv.) are sometimes required to obtain the bi-
aryls in good yields (Table 3, Entries 3 and 8). Similar
trends were observed for the reactions of 4-bromoaceto-
phenone with various substituted phenylboronic acids
(Table 3, Entries 10–12).
5[a]
6[b]
7[b]
0.05
0.02
1
8[b]
9[b]
10[b]
11[b]
12[b]
13[b]
14[b]
15[a]
1
1
1
1
1
1
EtOH/H2O 1:1 Na2CO3
H2O
Na2CO3
[a] Reactions performed with 4-bromoacetophenone (1 equiv.,
0.5 mmol), phenylboronic acid (1.1 equiv.), base (1.2 equiv.) and
catalyst C1 in various solvents (2 mL) containing 1% of H2O at
100 °C. [b] Same conditions as [a] but at reflux. [c] Yields calculated
The reaction of 4-bromoacetophenone with (thiophen-3-
yl)boronic acid afforded 4m in 28% yield in the presence of
1
by H NMR spectroscopy of the crude reaction mixture.
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Supported Palladium Catalyst on Nanoparticles
Table 3. Suzuki–Miyaura reactions in the presence of catalyst C1.
no significant decrease in the yield (Table 4). We also found
that the amount of Pd present in the reaction mixture after
magnetic separation of the catalyst was ca. 1.1% of the ini-
tial amount under these conditions.
Table 4. Reusability of catalyst C1.
Run[a]
1
2
3
4
5
6
Yield (%)[b]
99
98
93
95
92
90
[a] Reactions performed with 4-bromoacetophenone (1 equiv.,
0.5 mmol), (thiophen-3-yl)boronic acid (1.1 equiv.), Na2CO3
(1.2 equiv.) and C1 (500 μequiv. of supported Pd, 27 mg) in 96%
EtOH (2 mL) under reflux. [b] Isolated yields.
Entry[a]
R1
R2
Product
Yield [%][b]
1
2
3
4
5
6
7
8
4-Ac
4-CN
4-NO2
4-Cl
H
H
H
H
H
H
H
H
4a
4b
4c
4d
4e
4f
4g
4h
4i
98
90
15 (80)[c]
89
Finally, freshly prepared C1 was examined by TEM be-
fore use and again after six uses, and the TEM images
showed that no significant changes occurred. Function-
alized Co/C NPs of ca. 10–80 nm were observed in each
case (Figure 5).
H
77
99
90
4-Me
3-Me
2-Me
4-OMe
4-Ac
4-Ac
4-Ac
59 (86)[c]
86
9
H
10
11
12
4-Me
2-Me
4-OMe
4j
4k
4l
90
49 (85)[c]
98
[a] Reactions performed with aryl bromide (1 equiv., 1.0 mmol),
arylboronic acid (1.1 equiv.), Na2CO3 (1.2 equiv.) and catalyst C1
(50 μequiv. of supported Pd, 2.7 mg) in 96% EtOH (2 mL) under
reflux. [b] Isolated yields. [c] Reactions performed in the presence
of catalyst C1 (500 μequiv. of supported Pd, 27 mg).
Figure 5. TEM images of catalyst C1 (a) before use and (b) after
50 μequiv. of supported Pd but in 99% yield in the presence
of 500 μequiv. of precious metal. Similar results were ob-
served for the coupling of 2-bromopyridine with phenylbor-
onic acid (Scheme 3).
six uses.
Conclusions
In this report, we have described the preparation of a Pd
catalyst supported on Co/C NPs. We found that the carbon
shell formed by CVD was too unreactive to be function-
alized by reaction with an aryl radical[40] and too water-
permeable to retain the Co core when oxidized by an acid.
In contrast, Co/C NPs formed by reducing flame synthesis
allowed us to obtain the brominated B2, from which cata-
lyst C1 was formed in three simple steps. It was successfully
used for various Suzuki–Miyaura couplings of aryl
bromides with arylboronic acids, usually in the presence of
50 μequiv. of supported Pd. We have shown that C1 can be
reused up to six times with no significant loss of efficiency.
To the best of our knowledge, C1 is the most efficient and
simple catalyst supported on Co/C NPs for Suzuki–Mi-
yaura reactions of aryl bromides. Notably, only 1 μequiv. of
Pd is lost in the reaction medium; therefore, the tedious and
costly purification steps often required to free the product
from Pd traces can be avoided. Further studies are un-
derway to expand the use of C1 for Suzuki–Miyaura reac-
tions of other halides, especially chlorides,[14c,14d] and to
other C–C bond-forming reactions.
Scheme 3. Suzuki–Miyaura reactions of heterocyclic substrates
(conditions of Table 3).
We determined if it was possible to reuse C1. For practi-
cal reasons, we chose a reaction requiring 500 μequiv. of
supported Pd, namely, the cross-coupling of 4-bromoaceto-
phenone and (thiophen-3-yl)boronic acid. After the reac-
tion, C1 was magnetically recovered from the reaction mix-
ture, washed, dried and reused in another Suzuki–Miyaura
Experimental Section
General: Anhydrous solvents [tetrahydrofuran (THF) and toluene]
coupling. Catalyst C1 can be used more than six times with were distilled from sodium/benzophenone. The reagents were ob-
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J.-M. Becht, C. Le Drian et al.
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tained from commercial sources (Acros Organics, Sigma–Aldrich
and Alfa Aesar) and were used without further purifications. The
Co/C NPs A7 were obtained from Turbobeads, Zürich, Switzer-
land. They were freed of the unfixed carbon by sonication in NMP
(see Supporting Information). The silica gel was purchased from
The NPs (450 mg) were then dried for 4 h under vacuum
(0.1 mbar).
Preparation of Pd Catalyst C1. Step 1: The reaction was performed
under an atmosphere of argon. Lithium wire (358 mg, 51.6 mmol,
3 equiv.) was added to a solution of chlorodiphenylphosphine
(3.09 mL, 17.2 mmol, 1 equiv.) in dry THF (40 mL). The reaction
mixture was stirred at 20 °C for 20 h. The resulting dark red solu-
tion was added in small portions to a suspension of NPs B2 (1 g)
in dry THF (40 mL). The reaction mixture was stirred at 20 °C for
72 h and then quenched by the addition of a 2:1 mixture of acetone/
H2O (30 mL). The Co/C NPs bearing diphenylphosphanyl groups
were recovered magnetically, washed successively with H2O (3ϫ
30 mL), acetone (3ϫ 30 mL), EtOH (3ϫ 30 mL) and Et2O (3ϫ
30 mL) and then dried under vacuum (0.1 mbar) for 20 h.
Step 2: The reaction was performed under an atmosphere of argon.
Pd2dba3 (dba = dibenzylideneacetone; 6.5 mg, 7.1 μmol) was added
at 20 °C to a suspension of Co/C NPs bearing diphenylphosphanyl
groups (500 mg) in dry toluene (10 mL). The reaction mixture was
stirred at 20 °C for 20 h. The Pd catalyst C1 was recovered magneti-
cally, washed successively with toluene (3ϫ 10 mL) and Et2O (3ϫ
10 mL) and then dried under vacuum (0.1 mbar) for 20 h to afford
C1 as a black solid (498 mg). Elemental analysis (wt.-%): P 0.44,
Pd 0.2.
Merck and had
a 0.063–0.2 mm granulometry (70–230 mesh
ASTM). Transmission electron microscopy was performed with a
Philips CM200 microscope. The external magnet used for magnetic
separation was an S-25-07-N magnet (Supermagnete, Uster, Swit-
zerland).
tert-Butyl p-Tolylcarbamate (1): A solution of p-toluidine (17.7 g,
165 mmol, 1 equiv.), di-tert-butyl dicarbonate (43.2 g, 198 mmol,
1.2 equiv.) and Na2CO3 (22.0 g, 207 mmol, 1.25 equiv.) in methanol
(160 mL) was stirred for 1 h at 20 °C. The reaction mixture was
filtered under vacuum, and the solid residue was washed with Et2O
(3ϫ 30 mL). The filtrate was then concentrated under reduced
pressure. The precipitate was recrystallized in cyclohexane and
dried under vacuum (0.1 mbar). Compound 1 was obtained as a
1
white solid (34.0 g, 99% yield), m.p. 86 °C (ref.[41] m.p. 86 °C). H
NMR (400 MHz, CDCl3): δ = 1.56 (s, 9 H), 2.32 (s, 3 H), 6.79 (s,
3
3
1 H), 7.11 (d, JH,H = 8.1 Hz, 2 H), 7.30 (d, JH,H = 8.1 Hz, 2 H)
ppm. 13C NMR (100 MHz, CDCl3): δ = 20.5, 28.2, 80.0, 118.6,
129.2, 132.2, 135.7, 152.9 ppm.
Synthesis of Biaryls 4a–4n: Catalyst C1 (50 or 500 μequiv. of sup-
ported Pd) was added to a solution of aryl bromide (1 mmol,
1 equiv.), arylboronic acid (1.1 mmol, 1.1 equiv.) and Na2CO3
(1.2 mmol, 127.2 mg, 1.2 equiv.) in 96% EtOH (2 mL). The reac-
tion mixture was sonicated for 10 s and then heated at reflux for
20 h. After the reaction mixture cooled to 20 °C, C1 was recovered
magnetically and then washed successively with AcOEt (3ϫ 3 mL)
and EtOH (3ϫ 3 mL); each wash was performed under sonication
(15 s). The combined organic phases were washed with H2O
(20 mL), dried with MgSO4, filtered and concentrated under vac-
uum. The residue was purified by flash chromatography on silica
gel with AcOEt/cyclohexane as eluant. After the residue was dried
under vacuum (0.1 mbar), pure biaryls were obtained.
tert-Butyl 4-(Bromomethyl)phenylcarbamate (2): A solution of 1
(3.36 g, 16.2 mmol, 1 equiv.) and N-bromosuccinimide (2.88 g,
16.2 mmol, 1 equiv.) in CCl4 (100 mL) was prepared in a Pyrex re-
actor. The reaction mixture was stirred and irradiated with a mer-
cury lamp for 1 h and then filtered under reduced pressure. The
filtrate was concentrated under reduced pressure. The orange solid
was recrystallized in cyclohexane and dried under vacuum
(0.1 mbar). Compound 2 was obtained as an orange solid (3.7 g,
80% yield). The determination of the melting point was not pos-
sible because the decomposition of the product was observed at ca.
110–120 °C (2 was described in the literature as a brown oil).[42] 1
H
NMR (400 MHz, CDCl3): δ = 1.53 (s, 9 H), 4.49 (s, 2 H), 6.55 (s,
1 H), 7.33 (m, 4 H) ppm. 13C NMR (100 MHz, CDCl3): δ = 28.3,
33.6, 80.8, 118.5, 129.8, 132.2, 138.5, 152.5 ppm. FTIR: ν = 640,
˜
Supporting Information (see footnote on the first page of this arti-
cle): Experimental procedures for the preparations of NPs A, A1,
A2, A4, A5 and for the determinations of the Br and Pd contents
775, 827, 1054, 1153, 1234, 1317, 1411, 1508, 1594, 1698, 2971,
3372 cm–1. HRMS: calcd. for [12C12 H1681Br14N16O223Na]+
1
310.033; found 310.020.
1
and H and 13C NMR spectra of 1, 2, 3 and 4a–4n.
4-(Bromomethyl)benzenediazonium Tetrafluoroborate (3): A solu-
tion of 2 (16.9 g, 59 mmol, 1 equiv.) and 50% tetrafluoroboric acid
(22 mL, 0.17 mol, 2.9 equiv.) in acetonitrile (375 mL) was stirred
for 45 min at 20 °C. After the solution cooled to 0 °C, tert-butyl
nitrite (7.7 mL, 65 mmol, 1.1 equiv.) was added dropwise, and the
reaction mixture was stirred for 2 h at 20 °C. The diazonium salt 3
was precipitated by the addition of Et2O. The precipitate was col-
lected by filtration and washed with Et2O to afford 3 as a yellow
powder (11.3 g, 67% yield), m.p. 93–94 °C.[43] 1H NMR (400 MHz,
Acknowledgments
The authors are grateful to the Centre National de la Recherche
Scientifique (CNRS) for financial support and for a grant to C. D.,
to the Région Alsace for a grant to C. D., to the Université de
Haute Alsace for a grant to A. D., to Prof. Dr. W. J. Stark, Dr.
R. N. Grass and Dr. A. Schätz (ETH Zürich, Turbobeads Zürich)
for generous gifts of the carbon-coated cobalt NPs and for helpful
discussions, to Dr. L. Vidal (UMR-CNRS 7361) for TEM images,
to Dr. Didier Le Nouën (EA 4566) for H NMR spectra, to M.-P.
Hirn for BET experiments and to Dr. P. Brender (UMR-CNRS
7361) for helpful discussions.
3
[D6]acetone):[32] δ = 4.92 (s, 2 H), 8.17 (d, JH,H = 8.8 Hz, 2 H),
3
8.86 (d, JH,H = 8.8 Hz, 2 H) ppm. 13C NMR ([D6]acetone): δ =
1
31.5, 120.4, 133.2, 134.5, 154.0 ppm. FTIR: ν = 686, 715, 848,
˜
1031, 1317, 1427, 1583, 1695, 2287, 3104, 3565 cm–1.
Preparation of Co/C NPs Bearing (4-Bromomethyl)phenyl Groups
(B2): Co/C NPs A7 (500 mg) were suspended for three minutes in
deionized H2O (20 mL) in an ultrasonic bath. The diazonium salt
3 (641 mg, 2.25 mmol) and H2O (10 mL) were then added to the
suspension. The mixture was sonicated for 45 min. NPs B2 were
recovered magnetically. The washing steps were performed by sus-
pending B2 successively in H2O (3ϫ 30 mL), cyclohexane (3ϫ
30 mL), AcOEt (3ϫ 30 mL) and Et2O (1ϫ 30 mL) followed at
each step by 5 min of sonication and magnetic recovery of the NPs.
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Received: August 5, 2014
Published Online: October 21, 2014
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© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2014, 7699–7706