Immobilization of Pd on nanosilica dendrimer as SILC: Highly active and sustainable cluster catalyst for suzuki-miyaura reaction

Article abstract of DOI:10.1055/s-0029-1219816

Palladium acetate was noncovalently immobilized as a supported ionic liquid catalyst (SILC) in a nanosilica dendrimer, PAMDMAM, with the aid of an ionic liquid to form a cluster catalyst of palladium nanoparticles. The pseudo-homogeneous heterogenized catalyst, Pd-nanoPAMDMAM-SILC, was effective for Suzuki-Miyaura reactions of ortho-substituted aryl bromides or aryl triflates without a ligand in 50% aqueous ethanol in air at room temperature. The catalyst could be re-used up to five times in 93% average yield after simple centrifugation. TON reached 176,000. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-0029-1219816

1990
CLUSTER
Immobilization of Pd on Nanosilica Dendrimer as SILC: Highly Active and
Sustainable Cluster Catalyst for Suzuki–Miyaura Reaction
N
ano-SILC for Su
i
zuki–M
s
iyaura
R
e
a
action hiro Hagiwara,*a Hirokazu Sasaki,a Norio Tsubokawa,b Takashi Hoshi,b Toshio Suzuki,b Tetsuya Tsuda,c
Susumu Kuwabatac
a
Graduate School of Science and Technology, Niigata University, 8050, 2-Nocho, Ikarashi, Niigata 950-2181, Japan
Fax +81(25)2627368; E-mail: hagiwara@gs.niigata-u.ac.jp
b
Faculty of Engineering, Niigata University, 8050, 2-Nocho, Ikarashi, Niigata 950-2181, Japan
Department of Applied Chemistry, Graduate School of Engineering, Osaka University, 2-1, Yamadaoka, Suita, Osaka 565-0871, Japan
c
Received 26 January 2010
contrast to the plethora of supported catalysts on amor-
Abstract: Palladium acetate was noncovalently immobilized as a
phous silica,1 because silica nanoparticles exist in only
micromolar concentration in solution, due to aggregation
and the formation of a gel as a result of stronger interac-
supported ionic liquid catalyst (SILC) in a nanosilica dendrimer,
PAMDMAM, with the aid of an ionic liquid to form a cluster cata-
lyst of palladium nanoparticles. The pseudo-homogeneous hetero-
genized catalyst, Pd-nanoPAMDMAM-SILC, was effective for tions between the particles. Modification of the surface of
Suzuki–Miyaura reactions of ortho-substituted aryl bromides or
silica nanoparticles by grafting organic residues might be
aryl triflates without a ligand in 50% aqueous ethanol in air at room
a method to resolve this issue.
temperature. The catalyst could be re-used up to five times in 93%
Based on this assumption, we have recently developed an
efficient protocol for grafting aminopropyl residues onto
nanosilica particles, with an average size of 12 nm to pro-
vide aminopropylated nanosilica 1 (nanoNAP, Figure 1),
in which a solution of 3-aminotriethoxysilane in ethanol is
sprayed directly onto the silica nanoparticles.2 This
protocol is environmentally benign and favorable for
large-scale preparation. The subsequent syntheses of
hyperbranched polyamidoamine-grafted nanosilica G 3
PAMAM dendrimer 2 (nanoPAMAM) and polyamido-
average yield after simple centrifugation. TON reached 176,000.
Key words: dendrimers, heterogeneous catalysis, supported cataly-
sis, palladium, cross-coupling
Nanoparticles are attracting increasing attention as a nov-
el support of organometallic and organomolecular cata-
lysts due to their fine size, wide surface area, and
miscibility in solvent. Despite much anticipation, silica in
the nanometer range is not as widely used as a support, in
O
O
SiO2
Si
NH2
NR2
O
OEt
N
H
H
N
H
O
N
N
N
NR2
nano-NAP 1
O
O
O
O
N
NR2
N
H
H
HN
HN
N
N
H
NR2
O
O
O
O
O
O
SiO2
Si
N
OEt
NR2
N
H
H
N
H
N
N
NR2
12 nm
O
N
O
2 nano-PAMAM G 3 (R = H)
3 nano-PAMDMAM G 3 (R = Me)
NR2
N
H
N
N
H
O
H
N
O
NR2
Figure 1 Nanosilica dendrimers, nanoPAMAM G 3 2 and nanoPAMDMAM G 3 3
SYNLETT 2010, No. 13, pp 1990–1996
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x
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x
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Advanced online publication: 16.04.2010
DOI: 10.1055/s-0029-1219816; Art ID: U00710ST
© Georg Thieme Verlag Stuttgart · New York

CLUSTER
Nano-SILC for Suzuki–Miyaura Reaction
1991
dimethylamine-grafted nanosilica G 3 PAMDMAM
dendrimer 3 (nanoPAMDMAM) were achieved by graft-
ing methyl acrylate and then ethylenediamine or N,N-di-
methylaminoethylamine alternately, starting from
nanoNAP 1.
We investigated the immobilization of homogeneous in-
organic and organic catalysts as a supported ionic liquid
catalyst (SILC) noncovalently, in the pores of an inorgan-
ic oxide with the aid of an ionic liquid using a simple pro-
cedure (Figure 2).3 The SILC exhibited higher catalytic
activity than the homogeneous counterpart, and was re-
peatedly used in the Pd-catalyzed Mizoroki–Heck reac-
tion,4 Suzuki–Miyaura reaction,5 Ru-catalyzed olefin
metathesis reaction,6 Cu-catalyzed Huisgen [3+2] cy-
cloaddition,7 and the MacMillan imidazolidinone-cata-
lyzed asymmetric Diels–Alder reaction.8
Figure 3 Pd-nanosilica dendrimer-SILC
MAM-SILC 5 appeared as a pale yellow powder. We
found that fine palladium nanoparticles (ca. 1 nm) were
encapsulated9 and suspended uniformly inside raspberry-
shaped cluster structures (Figure 4). The tertiary amino
groups inside and the surface of nanoPAMDMAM 3
along with the ionic liquid might be effective for the sta-
bilization of palladium nanoparticles. The formation of
large raspberry-shaped cluster structures is yet to be dis-
cussed. On the other hand, Pd-nanoPAMAM-SILC 4 ap-
peared as a pale black powder confining palladium
nanoparticles of 10–20 nm diameter (Figure 5), in which
cluster structures were not observed. The lack of structure
and relatively large palladium particle size might have
arisen from the rapid reduction of Pd(II) into Pd(0) by the
primary amine, followed by irregular aggregation due to
the lack of stabilizing effect on the primary particles in
Pd-nanoPAMAM-SILC 4.
Figure 2 Reaction catalyzed by supported ionic liquid catalyst
The nanosilica dendrimer is envisaged to be a new support
for SILC, in which its homogeneity in a variety of sol-
vents and over a wide surface area is expected to give
higher catalytic activity. Moreover, the absence of pores
in nanosilica as opposed to amorphous silica is favorable
for the efficient recovery of products.
The immobilization of palladium acetate on a nanosilica
dendrimer as SILC was carried out as previously de-
scribed,4,5 by simply stirring a solution of palladium ace-
tate, the nanosilica dendrimer and an ionic liquid in THF,
followed by evacuation. Palladium acetate in butylmeth-
ylimidazolinium hexafluorophosphate {[bmim]PF6} (10
wt% to the dendrimer, 0.35 mmol/g) was absorbed into
both nanoPAMAM 2 (content of terminal amino group:
2.4 mmol/g) and nanoPAMDMAM 3 (content of terminal
dimethylamino group: 1.8 mmol/g) dendrimers as the
SILC, and maintained the original free-flowing powdery
nature, which could be handled and stored in aerobic con-
ditions at room temperature without any precautions
(Figure 3). The amount of palladium loading was 0.65–
0.8 mmol/g by weight gain, which was 10-fold more than
that of amorphous SILC.4
Figure 4 HR-TEM images of Pd-nanoPAMDMAM-SILC 5 (acce-
leration voltage: 100 kV, copper grid)
The presence of palladium and silica in the dendrimer-
SILC was confirmed by energy dispersive X-ray spectros-
copy elemental analysis. The morphology of both Pd-
nanoPAMDMAM-SILC 5 and Pd-nanoPAMAM-SILC 4
was investigated by HR-TEM analyses. Pd-nanoPAMD-
Figure 5 HR-TEM image of Pd-nanoPAMAM-SILC 4
Synlett 2010, No. 13, 1990–1996 © Thieme Stuttgart · New York

1992
H. Hagiwara et al.
CLUSTER
Table 1 Investigation of the Reaction Conditions in the Coupling Reaction of p-Bromoacetophenone (6a) and Phenylboronic acid (7a)a
Pd-nano-silica dendrimer-SILC
Br
+
(HO)2B
H2O–EtOH (1:1)
K2CO3 (2 equiv)
O
O
6a
7a
8a
Temp (°C)
r.t.
Entry
Catalyst
Support
Ionic liquid
Time (h)
Yield (%)
1
2
3
4
5
6
Pd(OAc)2
Pd(dba)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
PAMDMAM
PAMDMAM
PAMDMAM
PAMDMAM
PAMAM
[bmim]PF6
[bmim]PF6
[mbmim]PF6
–
0.5
0.5
0.5
24
100
6
r.t.
r.t.
21
51
0
r.t.
[bmim]PF6
[bmim]PF6
r.t.
24
PAMAM
reflux
4
70
a Reactions were carried out with 0.01 equiv of Pd-nanoSILC in the presence of 1.4 equiv of phenylboronic acid (7a) and 2 equiv of potassium
carbonate in 50% aq EtOH.
Suzuki–Miyaura reaction is one of the most versatile car- the reaction of p-bromoacetophenone (6a) and phenylbo-
bon–carbon bond-coupling reactions and is widely used in ronic acid (7a).
both academia10 and industry.11 Usually, a highly sophis-
ticated phosphine ligand is required for efficient conver-
sion under controlled reaction conditions. Recent research
Pd-nanoPAMDMAM-SILC 5 catalyzed the reaction rap-
idly at room temperature in air without a ligand (Table 1,
entry 1), while nanoPAMAM-SILC 4 recovered the sub-
strates completely (Table 1, entry 5). Such a prominent
on the reaction mainly focused on the development of new
highly active ligands.12 An efficient, practical, and sus-
difference in catalytic performance between two silica
tainable protocol is required; therefore, we investigated
dendrimer supports might be attributable to the smaller
the catalytic activity of Pd-nanosilica dendrimer-SILC 4
size of palladium nanoparticles, which were stabilized in-
side the cluster structure of Pd-nanoPAMDMAM-SILC 5
and 5 in the Suzuki–Miyaura reaction (Equation 1).
(vide supra). Palladium acetate was also immobilized in
nanoPAMDMAM 3 without an ionic liquid,14 although its
X
R1
catalytic performance was moderate even after 24 hours
R2
(Table 1, entry 4). In contrast, Pd-nanoPAMDMAM-
SILC 5, immobilized with 1-butyl-2,3-dimethylimidazo-
linium hexafluorophosphate {[mbmim]PF6}, recovered a
large amount of starting materials (Table 1, entry 3).
These results indicate the importance of [bmim]PF6 and
suggest that the palladium might be activated by forma-
tion of an N-heterocyclic carbene complex. Pd-nano-
PAMDMAM-SILC 5, which immobilized palladium
bisbenzylideneacetone, was not effective in the present
reaction (Table 1, entry 2).
6
Pd-nano-silica dendrimer-SILC
+
R1
H2O–EtOH (1:1)
K2CO3 (2 equiv)
8
R2
(HO)2B
7
Equation 1 Suzuki–Miyaura reaction catalyzed by Pd-nanosilica
dendrimer-SILC
The catalytic activity of Pd-nanoPAMDMAM-SILC 5
and Pd-nanoPAMAM-SILC 4 was investigated by em-
ploying the coupling reaction of p-bromoacetophenone
(6a, X = Br, R1 = COMe) and phenylboronic acid (7a,
R2 = H) as a benchmark reaction, with two equivalents of
potassium carbonate in 50% aqueous ethanol.5,13 These
were the optimum reaction conditions developed previ-
ously for Pd-amorphous-SILC.5,13f
Next, we focused on recycling Pd-nanoPAMDAM-SILC
5 in reactions with p-bromoacetophenone (6a) and phe-
Table 2 Determination of Turnover Number and Turnover Fre-
quencya
Entry
Equiv
1·10–3
1·10–4
1·10–5
5·10–6
Time (h) Yield (%) TOF
TON
1
2
3
4
3
14
22
72
100
100
100
88
330
710
1·103
Both Pd-nanosilica dendrimer-SILC 4 and 5 dispersed ho-
mogeneously in 50% aqueous ethanol without forming a
gel to give a transparent solution.
1·104
4,500
2,400
1·105
As shown in Table 1, entry 1, the reaction proceeded rap-
idly under mild reaction conditions. The high turnover
number and frequency in Table 2 exemplifies the high
1.76·105
a Reactions of p-bromoacetophenone (6a) and phenylboronic acid
catalytic performance of Pd-nanoPAMDMAM-SILC 5 in (7a, 1.4 equiv) were carried out at 60 °C in the presence of potassium
carbonate (2 equiv) in 50% aq EtOH with Pd-nanoPAMDMAM-
SILC 5 (Pd loading: 0.65 mmol/g) using new glassware.
Synlett 2010, No. 13, 1990–1996 © Thieme Stuttgart · New York

CLUSTER
Nano-SILC for Suzuki–Miyaura Reaction
1993
nylboronic acid (7a) and found that the catalysts could be The reactions of a variety of aryl halides 6 with arylboron-
recycled intact after light centrifugation and subsequent ic acids 7 are listed in Table 4, based on the optimized re-
decantation of the organic layer in 93% average yield up action conditions (Table 1, entry 1). Activated aryl
to five times without any pretreatment (Table 3). The bromides, such as 4-bromoacetophenone (6a) or 4-bro-
leaching of palladium was minimum at only 2.4 ppm (1%) monitrobenzene (6g, Table 4, entries 1 and 16), and deac-
by ICP-AES analysis. Thus, judging from the HR-TEM tivated 2-bromotoluene (6e) or 2-bromoanisole (6f,
image of Pd-nanoPAMDAM-SILC 5 after recycling five Table 4, entries 14 and 15) provided satisfactory coupling
times (Figure 6), the decrease in catalytic activity during products. In addition, aryl triflate 6d was a reactive sub-
recycle use is presumably due to the aggregation of palla- strate. When ethanol was used as a solvent instead of 50%
dium nanoparticles by deterioration of the raspberry- aqueous ethanol, the hydrolysis of aryl triflate 6d was sup-
shaped cluster structures. These results suggest that the pressed, and the coupling products 8a–c were obtained in
cluster structure of the catalyst plays an important role in satisfactory yields (Table 4, entries 4–7). A particularly
maintaining the activity of Pd nanoparticles.
noteworthy feature of the present protocol is the mild and
facile synthesis of sterically hindered ortho-substituted
biaryls, which could not be obtained easily by amorphous
SILC5 or ligandless Suzuki–Miyaura reactions.15
Table 3 Recycle Use of Pd-nanoPMADMAM-SILC 5a
Cycle
1
Time (h)
Yield (%)
100
In summary, we have successfully immobilized non-
covalently palladium nanoparticles on a nanosilica den-
drimer with the aid of the ionic liquid as Pd-
nanoPAMDMAM-SILC 5,16 and developed a new proto-
col for the Suzuki–Miyaura reaction to afford a variety of
unsymmetrical biaryls. This was achieved at a mild reac-
tion temperature in air in 50% aqueous ethanol without
adding expensive, air sensitive, and toxic phosphine
ligands. Catalyst 5 was effective and widely applicable to
both electron-poor and electron-rich substrates as a pseu-
do-homogeneous catalyst, and could be recycled several
times without any pretreatment after light centrifugation.
Compared to Pd-SILC, which was immobilized on amor-
phous silica of micrometer size, the catalytic activity of
Pd-nanoPAMDMAM-SILC 5 apparently increased, judg-
ing from the reactivity towards aryl triflates along with the
facile synthesis of ortho-substituted biaryls under mild re-
action conditions without a ligand. The high efficiency of
Pd-nanoPAMDMAM-SILC 5 is probably attributable to
the raspberry-shaped cluster structures of fine palladium
nanoparticles inside the nanosilica dendrimers, which
were stabilized by tertiaryamines and the ionic liquid
phase. The present protocol is more sustainable and effi-
cient under milder reaction conditions than existing pro-
cedures.
1 min
2b
1
3
4
7
100
3b
88
4b
82
5b
95
a Reactions were carried out at r.t. with 0.05 equiv of Pd-nano
PAMDAM-SILC 5 in the presence of 2 equiv of potassium carbonate
in 50% aq EtOH in air.
b The catalyst in the previous reaction was used after centrifugation
and decantation of the organic layer.
Figure 6 HR-TEM image of Pd-nanoPAMAM-SILC after recycle
use
Table 4 Catalytic Performance of Pd-nanoPAMDMAM-SILC 5
Entrya
117
Aryl halide 6
Arylboronic acid 7
Biaryl 8
Temp (°C)
r.t.
Time (h) Yield (%)b
0.5
0.5
24
100
100
14
Br
(HO)2B
O
O
7a
7a
6a
8a
8a
2
r.t.
I
O
6b
7a
8a
reflux
Cl
O
6c
Synlett 2010, No. 13, 1990–1996 © Thieme Stuttgart · New York

1994
H. Hagiwara et al.
CLUSTER
Table 4 Catalytic Performance of Pd-nanoPAMDMAM-SILC 5 (continued)
Entrya
4
Aryl halide 6
Arylboronic acid 7
Biaryl 8
Temp (°C)
r.t. to 60
Time (h) Yield (%)b
7a
8a
21
47
OTf
O
6d
6d
5c
6
7a
8a
r.t. to 60
60
6
2
93
86
6d
6d
O
O
7b
8b
718
60
2
93
Ph
(HO)2B
O
7c
8c
8
9
6a
6a
6a
7b
8b
r.t.
r.t.
r.t.
1
98
98
90
7c
8c
10
1019
0.5
OMe
(HO)2B
OMe
O
7d
8d
11
12
6a
6a
r.t.
r.t.
4
6
73
95
MeO
MeO
(HO)2B
O
7e
8e
(HO)2B
O
7f
8f
13
6a
r.t.
0.5
99
(HO)2B
7g
O
8g
14
15
r.t.
5
8
62
68
(HO)2B
7a
Br
6e
8h
7a
r.t. to 60
OMe
OMe
Br
6f
8i
16
17
18
7a
7a
7d
r.t. to 60
r.t. to 80
r.t.
24
55 (80)d
48
MeO
6g
Br
Br
Br
MeO
8j
24
5
H2N
6h
H2N
8k
88
O2N
6i
O2N
8l
OMe
Synlett 2010, No. 13, 1990–1996 © Thieme Stuttgart · New York

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Nano-SILC for Suzuki–Miyaura Reaction
1995
Table 4 Catalytic Performance of Pd-nanoPAMDMAM-SILC 5 (continued)
Entrya
19
Aryl halide 6
Arylboronic acid 7
Biaryl 8
Temp (°C)
r.t.
Time (h) Yield (%)b
4 86
7a
Br
8m
6j
a Reaction was carried out in air with 1.4 equiv of arylboronic acid 7 and 2 equiv of potassium carbonate in 50% aq EtOH with 0.01 equiv of
Pd-nanoPAMDMAM-SILC 5.
b Isolated pure product based on aryl halide 6.
c Reaction was carried out in EtOH.
d Based on recovered starting material.
Lett. 2002, 2, 999. (d) Ooe, M.; Murata, M.; Takahama, A.;
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Acknowledgment
This work was partially supported by a Grant-in-Aid for Scientific
Research on Priority Areas (20031011 for H. H.) from The Ministry
of Education, Culture, Sports, Science and Technology (MEXT)
and a Grant for Promotion of Niigata University Research Projects.
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(6) (a) Hagiwara, H.; Okunaka, N.; Hoshi, T.; Suzuki, T. Synlett
2008, 1813. (b) Hagiwara, H.; Okunaka, N.; Hoshi, T.;
Suzuki, T. Synlett 2008, 1813. (c) Hagiwara, H.; Nakamura,
T.; Okunaka, N.; Hoshi, T.; Suzuki, T. Helv. Chim. Acta
2010, 93, 175.
(7) Hagiwara, H.; Sasaki, H.; Hoshi, T.; Suzuki, T. Synlett 2009,
643.
(8) Hagiwara, H.; Kuroda, T.; Hoshi, T.; Suzuki, T. unpublished
results.
(9) For representative examples of dendrimer-encapsulated
metal nanoparticle catalysts, see: (a) Crooks, R. M.; Zhao,
M.; Sun, L.; Chechik, V.; Yeung, L. K. Acc. Chem. Res.
2001, 34, 181. (b) Mizugaki, T.; Murata, M.; Ooe, M.;
Ebitani, K.; Kaneda, K. Chem. Commun. 2002, 52. (c) Ooe,
M.; Murata, M.; Mizugaki, T.; Ebitani, K.; Kaneda, K. Nano
Synlett 2010, No. 13, 1990–1996 © Thieme Stuttgart · New York

1996
H. Hagiwara et al.
CLUSTER
chromatography (EtOAc–n-hexane = 1:10) provided
(15) For recent representative examples of ligandless Suzuki–
Miyaura reaction catalyzed by heterogeneous catalyst on
solid support, see: (a) Kudo, D.; Masui, Y.; Onaka, M.
Chem. Lett. 2007, 36, 918. (b) Kazmaier, U.; Hähn, S.;
Weiss, T. D.; Kautenburger, R.; Maier, W. F. Synlett 2007,
2579. (c) Artok, L.; Bulut, H. Tetrahedron Lett. 2004, 45,
3881. (d) Daku, K. M. L.; Newton, R. F.; Pearce, S. P.;
Vileb, J.; William, J. M. J. Tetrahedron Lett. 2003, 44, 5095.
(16) Preparation of Pd-nanoPAMDMAM-SILC 5
NanoPAMDMAM 3 (200 mg) powder was added to a
solution of [bmim]PF6 (19 mg, 10 wt%) and Pd(OAc)2
(36 mg, 0.16 mmol) in THF (2 mL) in open air. The
homogeneous solution was stirred at r.t. for 4 h, and
evaporated to dryness under reduced pressure. The resulting
powder was rinsed with Et2O (2 mL) five times. Each time,
the ether solution was stirred for 10 min, centrifuged for 10
min, and decanted to leave the powder. Evaporation of ether
provided Pd-nanoPAMDMAM-SILC 5 as a pale yellow
powder (247 mg, 0.5–0.6 mmol/g of Pd).
4-phenylacetophenone (8a, 102 mg) quantitatively.
(18) 4-(2-Phenylphenyl)acetophenone (8c)
Potassium carbonate (138 mg, 1.0 mmol) and Pd-
nanoPAMDMAM-SILC 5 (8 mg, 0.005 mmol) were added
to a solution of 4-bromoacetophenone (6a, 100 mg, 0.50
mmol) and (2-phenyl)phenylboronic acid (7c, 85 mg, 0.70
mmol) in 50% aq EtOH (2 mL) in open air. The solution was
stirred at r.t. for 30 min, then centrifuged for 10 min. After
decantation of the organic layer, the powder was rinsed with
Et2O–EtOH (1:1, 5 mL) five times. Each time, the resulting
homogeneous solution was centrifuged to precipitate the
SILC. The combined organic layer was evaporated to
dryness under reduced pressure. Purification by column
chromatography (EtOAc–n-hexane = 1:10) provided 4-(2-
phenylphenyl)acetophenone (8c, 134 mg) in 98% yield.
(19) 4-(4-Methoxyphenyl)acetophenone (8d)
Potassium carbonate (138 mg, 1.0 mmol) and Pd-
nanoPAMDMAM-SILC 5 (8 mg, 0.005 mmol) were added
to a solution of 4-bromoacetophenone (6a, 101 mg, 0.50
mmol) and 4-methoxyphenylboronic acid (7d, 106 mg, 0.70
mmol) in 50% aq EtOH (2 mL) in open air. The solution was
stirred at r.t. for 30 min, then centrifuged for 10 min. After
decantation of the organic layer, the powder was rinsed with
Et2O–EtOH (1:1, 5 mL) five times. Each time, the resulting
homogeneous solution was centrifuged to precipitate the
SILC. The combined organic layer was evaporated to
dryness under reduced pressure. Purification by column
chromatography (EtOAc–n-hexane = 1:3) provided 4-(4-
methoxyphenyl)acetophenone (8d, 104 mg) in 90% yield.
(17) 4-Phenylacetophenone (8a)
Potassium carbonate (138 mg, 1.0 mmol) and Pd-
nanoPAMDMAM-SILC 5 (8 mg, 0.005 mmol) were added
to a solution of 4-bromoacetophenone (6a, 100 mg, 0.50
mmol) and phenylboronic acid (7a, 85 mg, 0.70 mmol) in
50% aq EtOH (2 mL) in open air. The solution was stirred at
r.t. for 30 min, then centrifuged for 10 min. After decantation
of the organic layer, the powder was rinsed with Et2O–EtOH
(1:1, 5 mL) five times. Each time, the resulting
homogeneous solution was centrifuged to precipitate the
SILC. The combined organic layer was evaporated to
dryness under reduced pressure. Purification by column
Synlett 2010, No. 13, 1990–1996 © Thieme Stuttgart · New York

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