Palladium phosphine complex catalysts immobilized on silica via a tripodal linker unit for the Suzuki-Miyaura coupling reactions of aryl chlorides

Article abstract of DOI:10.1016/j.molcata.2014.01.004

Silica-immobilized palladium phosphine complex catalysts bearing a tripodal linker unit were tested for their ability to facilitate the Suzuki-Miyaura coupling reactions of aryl chlorides. The catalyst containing the tripodal linker with a trimethylsilyl capping group on the residual surface silanol groups displayed better catalytic activities and lower palladium and phosphorus leaching levels than the catalysts bearing a conventional trialkoxy-type linker.

Full text of DOI:10.1016/j.molcata.2014.01.004

Journal of Molecular Catalysis A: Chemical 385 (2014) 7–12
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Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Palladium phosphine complex catalysts immobilized on silica via a
tripodal linker unit for the Suzuki–Miyaura coupling reactions of aryl
chlorides
a,∗
a
a
b
b
Norihisa Fukaya , Syun-ya Onozawa , Masae Ueda , Takayuki Miyaji , Yukio Takagi ,
a
a,∗∗
Toshiyasu Sakakura , Hiroyuki Yasuda
National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan
N.E. CHEMCAT Corporation, 678 Ipponmatsu, Numazu, Shizuoka 410-0314, Japan
a
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 13 September 2013
Received in revised form
Silica-immobilized palladium phosphine complex catalysts bearing a tripodal linker unit were tested for
their ability to facilitate the Suzuki–Miyaura coupling reactions of aryl chlorides. The catalyst containing
the tripodal linker with a trimethylsilyl capping group on the residual surface silanol groups displayed
better catalytic activities and lower palladium and phosphorus leaching levels than the catalysts bearing
a conventional trialkoxy-type linker.
1
0 December 2013
Accepted 2 January 2014
Available online 9 January 2014
©
2014 Elsevier B.V. All rights reserved.
Keywords:
Immobilization
Mesoporous silica
Palladium
Cross-coupling
1
. Introduction
having a low susceptibility to oxidation remains an area of ongoing
interest.
The Suzuki–Miyaura coupling reaction is a powerful and versa-
Previous investigations of immobilized Suzuki–Miyaura cou-
pling catalysts for aryl chlorides have mainly focused on modifying
the ligand structures of palladium complexes, such as bulky
electron-rich phosphines, palladacycles, and N-heterocyclic car-
benes, similar to the approaches used to develop homogeneous
catalysts [12–21]. In this study, we adopted a different catalyst
design strategy for promoting aryl chloride coupling reactions.
Here, we demonstrate that the immobilized catalysts could be
remarkably improved by modifying the structure of a linker
[22–25] connecting the palladium complex to the silica sup-
port. This result is surprising in view of the use of a simple
alkyldiphenyl phosphine ligand. Unlike most electron-rich phos-
phines, alkyldiphenyl phosphines are not very air-sensitive and,
hence, they are easily handled.
tile tool for synthesizing organic compounds that can be utilized as
organic electronic materials or intermediates for the preparation
of pharmaceuticals and agricultural chemicals [1–5]. Over the last
decade, studies of the coupling reactions have focused on the use
of aryl chlorides as readily accessible and inexpensive substrates;
however, aryl chlorides are much more difficult to activate than
aryl bromides or aryl iodides. Soluble palladium complexes with
electron-rich and bulky phosphine ligands have been widely and
commonly used as catalysts for coupling reactions involving aryl
chlorides [6–11]; however, these reactions tend to suffer from cer-
tain drawbacks. Homogeneous catalysts are generally connected
with the problem of catalyst separation out from a reaction mixture.
Electron-rich phosphine ligands and their palladium complexes can
be difficult to handle because of their air-sensitive nature. They are
also difficult to reuse after performing a reaction. Thus, the devel-
opment of practical immobilized aryl chloride coupling catalysts
2. Experimental
2.1. Materials
All chemicals were reagent-grade and were used without
∗
Corresponding author. Tel.: +81 29 861 6398.
Corresponding author. Tel.: +81 29 861 9399
E-mail addresses: n.fukaya@aist.go.jp (N. Fukaya), h.yasuda@aist.go.jp
further purification. A potassium diphenylphosphide (KPPh ) solu-
∗
∗
2
tion in tetrahydrofuran (THF) (0.5 mol/L) was purchased from
Sigma–Aldrich Co. Ethoxytrimethylsilane was purchased
(
H. Yasuda).
1
381-1169/$ – see front matter © 2014 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molcata.2014.01.004

8
N. Fukaya et al. / Journal of Molecular Catalysis A: Chemical 385 (2014) 7–12
from Shin-Etsu Chemical Co., Ltd. Palladium(II) acetate was
supplied by N.E. Chemcat Co. Ordered mesoporous silica with a 2D
hexagonal structure (TMPS-4) [26], used as a silica support, was
supplied by Taiyo Kagaku Co., Ltd. The specific surface area, pore
2.3. Preparation of the palladium catalysts
Mesoporous silica-immobilized palladium phosphine complex
catalysts were prepared by reacting the diphenylphosphino-
functionalized silicas 1a–3 with palladium acetate (P/Pd molar
ratio = 6/1) in THF at room temperature for 12 h. After the reac-
tion, the resulting solid was filtered, washed with THF, and dried
2
3
volume, and average pore size were 1039 m /g, 1.46 cm /g, and
◦
3
.8 nm, respectively. Mesoporous silica was dried in vacuo at 80 C
for 3 h prior to use.
◦
in vacuo at 80 C for 3 h. ICP-MS analysis proved that the palladium
concentrations in the filtrates obtained from the catalyst prepa-
ration procedure were less than the quantitative limit, and that
all catalysts contained virtually all of the added palladium. Here-
after, the palladium catalysts will be referred to as X-Pd, where
X is the type of diphenylphosphino-functionalized silica. The pre-
pared catalysts were air- and moisture-stable and could, therefore,
be easily stored and handled. The preparation procedures of 1a-Pd
and 1b-Pd are summarized in Scheme 1.
2
.2. Preparation of diphenylphosphino-functionalized silica
The structures of the organic-functionalized silica mate-
rials used in this study are summarized in Fig. 1. The
diphenylphosphino-functionalized silica 1a, in which the
tripodal linker unit was used, was prepared by grafting
3
-bromopropyltris[3-(allyldimethylsilyl)propyl]silane onto meso-
porous silica. The resulting 3-bromopropyl-functionalized silica
was reacted with KPPh2 in THF according to a procedure described
previously (the “bottom-up” method) [24].
The trimethylsilyl-capped diphenylphosphino-functionalized
silica 1b was prepared by capping the residual surface
silanol groups of 3-bromopropyl-functionalized silica with
ethoxytrimethylsilane, followed by phosphination using KPPh2.
The procedure to prepare 1b was as follows: 3-bromopropyl-
functionalized silica was dried in vacuo at 80 C for 3 h prior to
use in the end-capping reaction. A mixture of 3-bromopropyl-
functionalized silica (1.0 g) and ethoxytrimethylsilane (20 mL) was
stirred at 60 C for 20 h. The resulting solid was filtered, washed
with methanol, and dried in vacuo at 80 C for 3 h. A THF solution of
A mesoporous silica-supported Pd(OAc)2 catalyst SiO -Pd was
2
prepared by impregnating mesoporous silica with a THF solution of
Pd(OAc) at room temperature for 12 h, followed by drying in vacuo
2
◦
at 80 C for 3 h.
2.4. Characterization
Nitrogen adsorption/desorption isotherms were measured at
◦
◦
−
196 C using a Bel Japan BELSORP-mini II analyzer. All Samples
◦
were heated under vacuum at 80 C for 3 h prior to conducting the
measurements. The specific surface areas were calculated using the
BET method. The pore size distribution was obtained by applying
the BJH method applied to the adsorption branch of the nitrogen
adsorption/desorption isotherm. An elemental analysis of carbon
was conducted using a CE Instruments EA 1112 elemental ana-
lyzer. The palladium concentrations in solution were determined
using an Agilent 7500ce inductively coupled plasma-mass spec-
trometer (ICP-MS). The phosphorous content of the catalysts was
determined using a Shimadzu EDX-800HS energy-dispersive X-ray
fluorescence spectrometer (EDXRF). Scanning transmission elec-
tron microscopy (STEM) images were taken on a Hitachi HD-2000
◦
◦
KPPh (6 mL) was then added to a suspension of the trimethylsilyl-
capped 3-bromopropyl-functionalized silica (1.0 g) in dry THF
2
(
30 mL), and the mixture was stirred for 20 h. The resulting solid
was filtered, washed successively with THF, methanol, and hexane
(
five times for each solvent (10 mL)) to eliminate unreacted KPPh2
◦
as well as any by-products, and then dried in vacuo at 80 C for 3 h
to give 1b.
For comparison, diphenylphosphino-functionalized silica bear-
ing the conventional trialkoxy-type linker 2a, its trimethylsilyl-
capped derivative 2b, or diphenylphosphino-functionalized silica
bearing the conventional monoalkoxy-type linker 3 was prepared
in a similar manner using the appropriate silane coupling reagents.
29
microscope with an acceleration voltage of 200 kV. Solid-state Si,
C, and P cross-polarization/magic angle spinning (CP/MAS)
13
31
NMR spectra were recorded on a Bruker AVANCE 400WB spec-
trometer operated at 79.5, 100.6, or 162.1 MHz for 29Si, C, or P,
respectively, and using a 4 mm CP/MAS probe head. A typical spin-
ning rate was 12.5 kHz, and CP contact times were 3.5, 2.0, and
13
31
Me SiO
3
29
13
31
3
.0 ms for Si, C, and P CP/MAS, respectively.
MeMe
MeMe
Si
O
O
O
Si
O
O
O
MeMe
2.5. Catalytic testing
MeMe
Ph P
Si
Si
2
Ph P
2
Si
Si
MeMe
MeMe
The Suzuki–Miyaura coupling of aryl chloride and phenyl-
boronic acid was carried out in a Schlenk tube (30 mL) under
a nitrogen atmosphere. A typical experimental procedure was
as follows: a mixture of aryl chloride (3.0 mmol), phenylboronic
acid (3.3 mmol), potassium phosphate (3.0 mmol), the catalyst
(0.1–0.3 mol% with respect to palladium), and 4-tert-butyltoluene
(430 mg) as an internal standard for gas chromatograph analysis
Si
Si
Me SiO
3
1
a
1b
Me SiO
3
OEt
OEt
O
O
O
O
◦
Ph P
Si
were stirred in 1,4-dioxane (3 mL) at 100 C. The reaction mixture
2
Ph P
2
Si
was periodically sampled to follow the progress of the reaction by
gas chromatography. After the reaction, the catalyst was separated
by centrifugation, and the supernatant was analyzed on a Shimadzu
GC-14B gas chromatograph (GC) equipped with a thermal conduc-
tivity detector and a column (2 mm × 3 m) packed with 3% OV-101
on Chromosorb WHP (100/120 mesh) to determine the product
yields.
Me SiO
3
2
a
2b
Me Me
Si O
Ph P
2
When the nitrogen adsorption/desorption isotherm of the cat-
alyst after the reaction was measured, the catalyst was recovered
by filtration and washed successively with 1,4-dioxane, water, and
ethanol.
3
Fig. 1. Organic-functionalized silica materials.

N. Fukaya et al. / Journal of Molecular Catalysis A: Chemical 385 (2014) 7–12
9
Scheme 1. Preparation of silica-immobilized palladium phosphine complex catalysts bearing a tripodal linker unit.
Table 1
Composition and textural data obtained from the catalysts.
2
3
Catalyst
Pd content (wt%)
Ligand (mmol/g)
Me3Si (mmol/g)
Surface area (m /g)
Pore volume (cm /g)
Pore size (nm)
1
1
2
2
3
a-Pd
b-Pd
a-Pd
b-Pd
-Pd
0.51
0.55
0.53
0.59
0.51
0.54
0.29
0.31
0.30
0.33
0.29
–
–
0.21
–
0.28
–
–
603
676
993
705
980
956
0.84
0.88
1.26
0.84
1.31
1.39
3.1
3.3
3.5
3.3
3.5
3.7
SiO2-Pd
3
. Results and discussion
investigated using the catalysts 1a-Pd, 2a-Pd, and 3-Pd. The reac-
tion was conducted using potassium phosphate as a base in
◦
3.1. Preparation and characterization of the palladium catalysts
1,4-dioxane at 100 C for 3 h. The quantity of catalyst was adjusted
to 0.1 mol% relative to the palladium content. The catalyst 1a-Pd, in
which the palladium phosphine complex was anchored via a tripo-
dal linker unit, gave a higher yield (23%) than 2a-Pd (13%) or 3-Pd
(11%), which included a conventional trialkoxy- or a monoalkoxy-
type linker, respectively (Table 2, entries 1, 3, and 5). These results
indicated that the linker structure significantly affected the cat-
alytic activity, and that the tripodal linker was more effective in
promoting the reaction than the conventional linkers.
Next, the influence of trimethylsilyl capping groups on the
residual surface silanol groups was examined in the context of
the catalytic activity. The trimethylsilyl-capped catalysts 1b-Pd
and 2b-Pd displayed better catalytic activities compared to the
corresponding uncapped catalysts 1a-Pd and 2a-Pd, respectively
(Table 2, entries 2 and 4). Fig. 2 displays the nitrogen adsorp-
tion/desorption isotherms of fresh 1a-Pd and 1b-Pd as well as
the 1a-Pd and 1b-Pd recovered after the reaction. The ordered
mesoporous structure of the uncapped catalyst 1a-Pd underwent
complete collapse after the reaction. By contrast, no significant
changes in the mesoporous structure of the trimethylsilyl-capped
catalyst 1b-Pd were observed prior or subsequent to the reaction.
Thus, the positive effects of the trimethylsilyl capping groups could
be reasonably explained in terms of their ability to improve the
durability of the silica support under the strong basic conditions
Table 1 summarizes the composition and textural data obtained
from the prepared catalysts. The phosphine ligand content in the
silica-immobilized palladium phosphine complex catalysts deter-
mined by EDXRF analysis was in the range 0.29–0.33 mmol/g,
and few differences between the tripodal linker and the con-
ventional linkers (trialkoxy- and monoalkoxy-type linkers) were
observed. The trimethylsilyl moiety contents on 1b-Pd and 2b-Pd,
which were estimated by subtracting the amount of carbon that
corresponded to the ligand moieties from the total amount of car-
bon measured by elemental analysis, were 0.21 and 0.28 mmol/g,
3
1
29
13
respectively. P, Si, and C CP/MAS NMR spectroscopic stud-
ies of 1a (and 1b) revealed that the diphenylphosphino ligand was
firmly immobilized on the mesoporous silica support via three
independent Si–O–Si bonds [24]. On the other hand, diphenylphos-
phinopropyl silane in 2a (and 2b) was predominantly bound to the
silica surface through one to two Si–O–Si bonds [25]. The nitrogen
adsorption/desorption isotherms of all of the prepared catalysts
indicated type IV sorption curves that are typical of mesoporous
structures. The incorporation of the bulky tripodal phosphine lig-
ands into the mesopores (1a-Pd and 1b-Pd) largely decreased the
specific surface area and narrowed the average pore size.
3.2. Catalysis of the Suzuki–Miyaura coupling reaction
Table 2
Results of the Suzuki–Miyaura coupling reaction between 4-chlorobenzoic acid
ethyl ester and phenylboronic acid .
The performances of the prepared catalysts were mainly
a
screened for their utility in promoting the Suzuki–Miyaura
coupling reaction of 4-chlorobenzoic acid ethyl ester and phenyl-
boronic acid to give the corresponding biaryls (Scheme 2). The
influence of the linker type on the catalytic activity was initially
Entry
Catalyst
Yield (%)b
1
2
3
4
5
1a-Pd
1b-Pd
2a-Pd
2b-Pd
3-Pd
23
74
13
23
11
catalyst
0.1-0.3 mol% Pd)
(
a
Reaction conditions: 4-chlorobenzoic acid ethyl ester (3.0 mmol), phenylboronic
R
Cl
+
(HO)2B
R
◦
base, solvent, 100°C
acid (3.3 mmol), K3PO4 (3.0 mmol), catalyst (0.1 mol% Pd), 1,4-dioxane (3 mL), 100
for 5 h.
C
b
Scheme 2. The Suzuki–Miyaura coupling reaction.
Determined by GC analysis using 4-tert-butyltoluene as an internal standard.

1
0
N. Fukaya et al. / Journal of Molecular Catalysis A: Chemical 385 (2014) 7–12
7
6
5
4
3
2
1
00
00
00
00
00
00
00
0
1
00
1
1
1
1
a-Pd fresh
Hot filtration
a-Pd recovered
b-Pd fresh
8
6
0
0
10 mim, 9% yield
b-Pd recovered
Hot filtration
40
Without fitration
2
0
0
0
1
2
3
0
0.2
0.4
0.6
0.8
1
Time (h)
p/p0
Fig. 3. Hot filtration test for the Suzuki–Miyaura coupling reaction between 4-
chlorobenzoic acid ethyl ester and phenylboronic acid in the presence of 1b-Pd.
Fig. 2. Nitrogen adsorption/desorption isotherms of 1a-Pd and 1b-Pd. Solid and
hollow marks correspond to the adsorption and desorption branches, respectively.
Table 4
of the Suzuki–Miyaura coupling reaction [27]. It should be noted
that the tripodally immobilized catalyst 1b-Pd gave a much higher
yield than 2b-Pd, which included a conventional linker, even if the
residual surface silanol groups had been capped by trimethylsilyl
groups. This difference presumably arose from differences in the
stabilities of the palladium phosphine complexes anchored to the
silica support under the reaction condition (vide infra).
The linker structure of the immobilized catalysts significantly
influenced the leaching of the catalyst components from the silica
support (Table 3). The extent of palladium leaching was quanti-
fied by determining the palladium concentration in the reaction
solution using ICP-MS analysis after the catalyst had been removed
by filtration. The palladium content in the solution after conduct-
ing the reaction in the presence of the tripodally anchored catalyst
Results of the Suzuki–Miyaura coupling reaction between 4-chlorobenzoic acid
ethyl ester and phenylboronic acid .
a
Yield (%)b
Entry
Catalyst
Ligand:Pd
1
2
3
4
5
1b-Pd
SiO2-Pd
SiO2-Pd + nPrPPh2
Pd(OAc)2 + nPrPPh2
Pd(OAc)2 + nPrPPh2
6:1
0:1
6:1
4:1
6:1
74
0
17
28
33
a
Reaction conditions: 4-chlorobenzoic acid ethyl ester (3.0 mmol), phenylboronic
◦
acid (3.3 mmol), K3PO4 (3.0 mmol), catalyst (0.1 mol% Pd), 1,4-dioxane (3 mL), 100
for 5 h.
C
b
Determined by GC analysis using 4-tert-butyltoluene as an internal standard.
1b-Pd was examined by performing hot filtration experiments.
Fig. 3 shows the experimental results. After the reaction of 4-
chlorobenzoic acid ethyl ester and phenylboronic acid had been
1
b-Pd was below the detectable limit (corresponding to less than
0.008% of the initial quantity of palladium present). These results
◦
conducted at 100 C over 10 min in the presence of potassium phos-
contrasted with the extent of palladium leaching observed from
the use of the conventional immobilized catalyst 2b-Pd (0.38% of
the initial quantity of palladium present). The extents to which
the phosphine moieties of 1a-Pd or 2a-Pd leached were compared
by determining the amount of phosphorous present on the fresh
and recovered catalysts by EDXRF analysis. The phosphine content
in the recovered 1b-Pd agreed well with the content in the fresh
phate, the catalyst, and 4-tert-butyltoluene (internal standard) in
1
,4-dioxane, the solid components, including the catalyst, were
removed by filtration under hot conditions. The yield of the cor-
responding biaryl in the filtrate was 9%, as determined by GC
analysis. Fresh reagents (phenylboronic acid and potassium phos-
phate) were subsequently added to the filtrate, and the mixture was
1
b-Pd sample, whereas the phosphine content in 2b-Pd was sig-
nificantly reduced from 0.33 to 0.24 mmol/g during catalysis. These
results clearly showed that tripodal anchoring was highly effec-
tive at minimizing palladium or phosphine leaching from the silica
support during the Suzuki–Miyaura coupling reaction.
100
8
0
Two possible mechanisms underlying the Suzuki–Miyaura cou-
pling reactions in the presence of silica- or polymer-supported
palladium catalysts have been proposed: The active species are
either (i) soluble palladium species leached into the reaction
solution (homogeneous catalysis) or (ii) tethered palladium com-
plexes or palladium nanoparticle surfaces immobilized on supports
6
0
4
0
2
0
(
heterogeneous catalysis) [28,29]. The catalysis mechanism of
0
Table 3
Leaching data obtained after conducting the catalytic reaction in the presence of
1b-Pd and 2b-Pd.
Ligand (mmol/g)
Fresh
Catalyst
Pd Leaching (%)
Recovered
Fig. 4. Results of the 1b-Pd-catalyzed Suzuki–Miyaura coupling reaction between
-chlorobenzoic acid ethyl ester and phenylboronic acid using various solvents and
bases.
1
2
b-Pd
b-Pd
<0.008
0.38
0.31
0.33
0.31
0.24
4

N. Fukaya et al. / Journal of Molecular Catalysis A: Chemical 385 (2014) 7–12
11
Fig. 5. STEM images of (a) fresh 1b-Pd and (b) used 1b-Pd after the fifth catalytic run.
◦
Table 6
stirred at 100 C in the absence of the solid-phase catalyst. The cou-
pling reaction was not observed to proceed over 3 h, suggesting that
1
Catalyst recycling experiments for the Suzuki–Miyaura coupling reaction between
a
4-chlorobenzoic acid ethyl ester and phenylboronic acid in the presence of 1b-Pd .
b-Pd acted as a heterogeneous catalyst. That is, the active species
Catalytic run
Yield (%) after 5 hb
Yield (%) after 20 hb
involved in the coupling reaction of the aryl “chloride” were palla-
dium phosphine complexes tethered to the silica support, even if a
portion of the palladium had dissolved in the reaction solution dur-
ing the reaction. These results stood in contrast with the previously
reported Suzuki–Miyaura coupling reaction of aryl “bromide” with
a-Pd, in which homogeneous catalysis substantially contributed
to the reaction due to the presence of dissolved palladium species
25] (see supplementary material, Fig. S-3).
The catalytic performances of 1b-Pd were compared with
1st
2nd
95
72
71
49
49
97
97
96
72
66
3
4
5
rd
th
th
1
a
Reaction conditions: 4-chlorobenzoic acid ethyl ester (3.0 mmol), phenylboronic
◦
acid (3.3 mmol), K3PO4 (3.0 mmol), 1b-Pd (0.25 mol% Pd), 1,4-dioxane (3 mL), 100 C.
[
The catalyst recycling procedures are described in the text.
b
Determined by GC analysis using 4-tert-butyltoluene as an internal standard.
the performances of certain heterogeneous or homogeneous cat-
alytic systems in the context of an aryl chloride coupling reaction
(
Table 4). The mesoporous silica-supported Pd(OAc) catalyst SiO -
2 2
Pd was totally inactive in the context of the coupling reaction
entry 2), whereas the addition of six equivalents of diphenyl-n-
propylphosphine to the reaction system led to a 17% yield (entry
). A mixture of Pd(OAc)2 and diphenyl-n-propylphosphine, the
electron-donating groups gave moderate to poor product yields
(entries 3–5).
(
Finally, the recyclability of the 1b-Pd catalyst was assessed in
the reaction of 4-chlorobenzoic acid ethyl ester with phenylboronic
acid. After performing the reaction in the presence of potassium
3
homogeneous counterparts to 1a-Pd and 1b-Pd, gave 28–33%
yields (entries 4 and 5). Thus, the catalytic performance of the
organo-functionalized silica-based immobilized catalyst 1b-Pd
was superior to the performances of its homogeneous counterpart
or a heterogeneous impregnated catalyst.
To further probe the scope of the catalyst 1b-Pd, we exam-
ined the yield of the Suzuki–Miyaura coupling reactions as
a function of the base, solvent, and aryl chloride substrates.
Among the bases and solvents tested, potassium phosphate and
◦
phosphate in 1,4-dioxane at 100 C for 20 h, the catalyst was sep-
arated by filtration and washed successively with 1,4-dioxane,
water, and ethanol under a nitrogen atmosphere. The catalyst was
then dried in vacuo and reused in a subsequent run. The results
are presented in Table 6. The catalyst 1b-Pd could be recycled
twice, giving good yields after 20 h, although the catalytic activ-
ity reduced slightly, judging from the yields obtained after 5 h. A
decrease in yield was observed for the fourth and fifth runs. Fig. 5
displays the STEM images of fresh and recovered 1b-Pd after the
fifth catalytic run. Aggregated palladium metal particles were not
observed in the STEM image of the fresh catalyst (Fig. 5(a)). In con-
trast, palladium nanoparticles were found in the recovered catalyst
after the catalytic runs (Fig. 5(b)). These results suggested that the
observed deactivation of 1b-Pd was attributable to the formation
of the palladium metal particles.
1
(
,4-dioxane were identified as providing the best combination
Fig. 4). Under these optimized conditions, the coupling reac-
tions involving different substrates were studied using 1b-Pd
Table 5). The coupling reaction of 4-chlorobenzoic acid ethyl
(
ester with a greater quantity of the catalyst (0.3 mol% palladium)
yielded the product in 95% yield (entry 1). The 1-chloro-4-
cyano benzene, which bears an electron-withdrawing group, also
afforded the corresponding biaryl compound in an excellent yield
(
entry 2); however, the use of aryl chlorides bearing neutral or
4. Conclusions
Table 5
We successfully applied the tripodal linker unit to the prepa-
Results of the Suzuki–Miyaura coupling reaction between aryl chlorides and phenyl-
boronic acid in the presence of 1b-Pd .
a
ration of mesoporous silica-immobilized palladium phosphine
complex catalysts that promoted the Suzuki–Miyaura coupling
of aryl chlorides. The catalysts containing the tripodal linker
showed better catalytic activities and lower palladium and phos-
phorus leaching levels than the catalysts containing conventional
trialkoxy-type linkers in the context of the Suzuki–Miyaura cou-
pling reaction between 4-chlorobenzoic acid ethyl ester and
phenylboronic acid. The present catalytic system is the subject of
further testing and application development in the context of other
coupling reactions.
Entry
R
Yield (%)b
1
2
3
4
5
COOEt
CN
95
96
76
71
54
H
CH3
CH3O
a
Reaction conditions: aryl chlorides (3.0 mmol), phenylboronic acid (3.3 mmol),
◦
K3PO4 (3.0 mmol), catalyst (0.3 mol% Pd), 1,4-dioxane (3 mL), 100 C for 5 h.
b
Determined by GC analysis using 4-tert-butyltoluene as an internal standard.

1
2
N. Fukaya et al. / Journal of Molecular Catalysis A: Chemical 385 (2014) 7–12
[9] R. Martin, S.L. Buchwald, Acc. Chem. Res. 41 (2008) 1461–1473.
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