Activity and Recyclability Improvement Through Adjusting the Tethering Strategy for Pd-Catalyzed Suzuki-Miyaura Coupling Reaction of Aryl Chlorides

Article abstract of DOI:10.1007/s10562-015-1609-1

Four mesoporous SBA-15-immobilized N-heterocyclic carbene (NHC) palladium catalysts with different nitrogen ligands were synthesized. The activity and recyclability of the catalysts were investigated in Suzuki-Miyaura cross-coupling reaction of typical ar

Full text of DOI:10.1007/s10562-015-1609-1

Catal Lett (2015) 145:2001–2009
DOI 10.1007/s10562-015-1609-1
Activity and Recyclability Improvement Through Adjusting
the Tethering Strategy for Pd-Catalyzed Suzuki–Miyaura
Coupling Reaction of Aryl Chlorides
Jing-Fan Zhang1 Mingda Wang1 Xu-Qing Guo1 Shu Liu2 Hui Cao1
•
•
•
•
•
Yuan-Long Li1 Quan Zhou1 Xiu-Feng Hou1
•
•
Received: 31 May 2015 / Accepted: 15 August 2015 / Published online: 4 September 2015
Ó Springer Science+Business Media New York 2015
Abstract Four mesoporous SBA-15-immobilized N-
heterocyclic carbene (NHC) palladium catalysts with dif-
ferent nitrogen ligands were synthesized. The activity and
recyclability of the catalysts were investigated in Suzuki–
Miyaura cross-coupling reaction of typical aryl chlorides
and arylboronic acids. Bulky NHC moiety was essential for
efficiency, while tethering of nitrogen ligands on both
support and NHC group could dramatically improve
recyclability of the catalysts. The strategy offers an alter-
native strategy for designing highly efficient and recyclable
immobilized catalysts.
Graphical Abstract Mesoporous silica-immobilized pal-
ladium catalysts with the bulky N-heterocyclic carbene and
tethered ‘‘throw-away’’nitrogen ligands were found to be
highly efficient and recyclable for Suzuki–Miyaura cou-
pling reaction of inactive aryl chlorides.
& Shu Liu
liushu@shciq.gov.cn
Keywords N-heterocyclic carbene Á Palladium Á
Immobilization Á Suzuki–Miyaura reaction Á Aryl
chlorides Á Catalyst recycling
& Xiu-Feng Hou
xfhou@fudan.edu.cn
1
Department of Chemistry, Fudan University,
Shanghai 200433, China
2
Technical Center for Industrial Product and Raw Material
Inspection and Testing, Shanghai Entry-Exit Inspection and
Quarantine Bureau, Shanghai 200135, China
123

2002
J.-F. Zhang et al.
(the recovery and reuse of catalysts), attracted much atten-
tion and showed great potentials [30–32].
1 Introduction
Several materials such as silica [33–35], polymers [36–
40] have been developed for the immobilization of NHC-
Pd catalysts. Jin’s group reported a silica gel supported
NHC-Pd catalyst in 2008 [33]. Lee and co-workers
depicted a series of polystyrene-supported NHC-Pd cata-
lysts [38–40]. These catalysts provided good yields in the
Suzuki–Miyaura coupling of aryl bromides and aryl
iodides. To the best of our knowledge, successful examples
of highly active coupling of aryl chlorides with arylboronic
acids were rare [41]. Based on our previous work [42–47],
we designed four immobilized catalysts with different Pd-
NHC moieties using SBA-15 as support. Our result
demonstrated that both catalytic activities and recycling
performance were improved through adjusting the tether-
ing strategy of ligands to supports.
Palladium-catalyzed cross-coupling reactions, discovered
in the 1970s, have witnessed a meteoritic development and
continued to be the focus of current organometallic
research [1, 2]. As a consequence of their versatility for the
formation of C–C bond, synthetic chemists poured much
attention into this area and created multitudinous methods
[3, 4]. Among these reactions, Suzuki–Miyaura cross-
coupling reaction [5, 6], where the organoboron reagents
are treated as coupling partner with aryl halides to form C–
C bonds, is one of the most synthetically valuable tools for
the construction of biaryl derivatives [7–9]. For aryl
halides, the use of aryl chlorides is typically more attractive
due to their low cost and wide availability. However, the
C–Cl bond strength is relatively high (bond dissociation
energies for Ph-X: Cl: 96 kcal mol-1; Br: 81 kcal mol-1
;
I: 65 kcal mol-1), which makes their activation difficult
[10]. The emergence of highly active catalysts provides
new opportunities for the Suzuki–Miyaura coupling of
these substrates.
2 Experimental
2.1 Reagents and Methods
The chemistry of N-heterocyclic carbenes (NHCs) has
attracted increasing attention, since the discovery of the first
stable free imidazole-2-ylidene in 1991 by Arduengo et al.
[11, 12]. NHCs have been applied in various catalytic reac-
tions due to their strong r-donating ability and low p-ac-
cepting ability [13–15]. Moreover, the NHCs show relatively
gratifying air and moisture stability and have higher disso-
ciation energies [16]. Herrmann et al. applied this family of
ligands to palladium-catalyzed coupling reactions first and
designed a first-generation catalyst that can achieve the
cross-coupling of unactivated aryl chloride with phenyl-
boronic acid [17]. Previous studies by Navarro’s group
verified that the transition of stable Pd (II) complex to active
Pd (0) species before entering catalytic cycle is crucial.
During this process the Pd (II) complex must lose some of its
ligands (‘‘throw-away’’ ligands) [18]. They chose triethy-
lamine (TEA) as a ‘‘throw-away’’ ligand to synthesize NHC-
Pd complex, (IPr)PdCl2(TEA) (IPr = N,N0-bis(2,6-diiso-
propylphenyl)imidazol-2-ylidene), which was highly effi-
cient for Suzuki coupling reactions. Although these catalysts
performed excellent catalytic activity [19–22], they have
some limitations in terms of separation, recycling and con-
tamination of ligand residues to products [23]. In the past few
years, heterogeneous catalysts such as Pd/C [24], Pd/SiO2
[25, 26], Pd/KF-Al2O3 [27], Pd/SWNT [28] and Chitosan-
based Pd catalysts [29], were developed to overcome these
limitations. Nonetheless, their catalytic activities were
unsatisfactory for unreactive aryl chlorides. Therefore,
immobilization of catalysts, a method which combines the
advantages of both homogeneous control (single-site
molecularly-defined catalysts) and heterogeneous catalysis
All manipulations werecarried out under nitrogenatmosphere
using standard Schlenk techniques unless otherwise stated.
Solvents were distilled under nitrogen from sodium ben-
zophenone (hexane, diethyl ether and toluene) or calcium
hydride (dichloromethane). Methanol and isopropanol was
distilled over Mg/I2. The starting materials [Pd(COD)Cl2]
[48], [Pd(PhCN)2Cl2] [49], 2-(1H-imidazol-1-yl)pyrimidine
[50], 1-(2,6-diisopropylphenyl)-1H-imidazole [51] and SBA-
15 [52] were synthesized according to literature procedures.
Other chemical reagents were obtained from commercial
sources and used without further purification. NMR spectra
were recorded using Bruker spectrometers operating at
400 MHz (1H) and 100 MHz (13C) in CDCl3. Solid state 13
C
and 29Si CP MAS NMR spectra were performed at
100.6 MHz using a Bruker AV-400 spectrometer. FT-IR
spectra were analyzed on a Nicolet AVATAR-360 IR spec-
trometer using KBr discs in the range of 4000–400 cm-1. The
SAXS patterns were measured with a Bruker D4 powder
X-ray diffractometer using Cu Ka radiation (45 kV,
650 mA). TEM images were observed on a JEOL JEM 2010
electron microscope, operated at an acceleration voltage of
200 kV. The loading amount of palladium in catalysts was
analyzed using an inductively coupled plasma optical emis-
sion spectrometer (ICP, Varian VISTA-MPX).
2.2 Preparation of Compound 3
To a solution of 1,2-bis(bromomethyl)benzene(10 mL) in
toluene, 1-(2,6-diisopropylphenyl)-1H-imidazole (12 mmol)
123

Activity and Recyclability Improvement Through Adjusting the Tethering Strategy…
2003
was added slowly. The mixture was stirred at room tempera-
turefor 48 h. After filtration, the filtrate was washedsequently
with toluene and diethyl ether, and then vacuum-dried to
afford the crude product. The residue was purified by column
chromatography using dichloromethane:methanol = 20: 1 as
eluent to gain the compound 3 (white solid, 89 %).
(0.3 mmol) was added. After stirring for 24 h, the reaction
mixture was filtered. The filtrate was added into the sus-
pension of SBA-15 (1.5 g) in toluene (10 mL). After the
mixture was stirred at 80 °C for 48 h, the solid was filtered
and washed sequently with dichloromethane, ethanol and
ether, and then vacuum-dried to afford catalyst 5.
1H NMR (CDCl3, 400 MHz, ppm) d 10.39 (s, 1H, NCHN),
7.81 (s, 1H, Ph), 7.59 (d, J = 6.4 Hz, 1H, NCHCHN), 7.52
(m, 1H, NCHCHN), 7.41 (m, 3H, Ph), 7.29 (d, J = 7.6 Hz,
2H, Ph), 7.20(s, 1H), 6.27(s, 2H, CH2N), 4.90(s, 2H, CH2Br),
2.38 (m, 2H, CH(CH3)2), 1.21 (d, J = 6.4 Hz, 6H, CH
(CH3)2), 1.11 (d, J = 6.4 Hz, 6H, CH(CH3)2).13C NMR
(CDCl3, 100 MHz, ppm) d 145.3 (NCN), 137.9 (Ph), 137.6
(Ph), 131.3 (Ph), 131.2 (Ph), 130.3 (NCCN), 130.0 (Ph), 129.7
(NCCN), 124.5 (Ph), 123.9 (Ph), 123.0 (Ph), 50.8 (CBr), 31.3
(CN), 28.5 (CH(CH3)2), 24.2 (CH(CH3)2), 24.1 (CH(CH3)2).
2.5 Preparation of SBA-15 Immobilized Pd-NHC
Catalysts 6 and 7
To a solution of compound 3 (0.36 mmol) in dichlor-
omethane (10 mL), sodium carbonate (0.36 mmol) and N-
methyl-3-(trimethoxysilyl)propan-1-amine
(0.36 mmol)
were added. The mixture was refluxed for 48 h and then
evaporated in vacuum. The residue was washed with die-
thyl ether for three times to remove excess silane. It was
then dissolved in dichloromethane (10 mL) and added to a
˚
Schlenk vessel containing activated 4 A molecular sieves
2.3 Preparation of SBA-15 Immobilized Pd-NHC
Catalyst 4
and Ag2O (0.3 mmol). This suspension was stirred at room
temperature in the absence of light for about 3 h, and then
Pd(COD)Cl2 (0.3 mmol) was added. After stirring for 24 h,
the reaction mixture was filtered. The filtrate was added
into the suspension of SBA-15 (1.5 g) in toluene (10 mL).
After stirring at 80 °C for 48 h, the solid was filtered and
washed sequently with CH2Cl2, ethanol and ether before
vacuum-dried to afford catalyst 6.
To a 50-mL Schlenk tube containing 10 mL toluene, added
1-(2,6-diisopropylphenyl)-1H-imidazole (0.3 mmol) and
3-iodopropyltrimethoxysilane (0.36 mmol). The mixture
was refluxed under nitrogen for 48 h and evaporated in
vacuum. The residue was washed with diethyl ether for
three times. Then the white solid was dissolved in
dichloromethane (10 mL) and added to a Schlenk vessel
Catalyst 7 was prepared in the same method using N-
methyl-3-(trimethoxysilyl)propan-1-amine instead of (3-
aminopropyl)trimethoxysilane.
˚
containing activated 4 A molecular sieves (50 mg) and
Ag2O (0.3 mmol). This suspension was stirred at room
temperature in the absence of light for about 3 h, and then
Pd(PhCN)2Cl2 (0.3 mmol) and triethylamine (0.5 mmol)
were added. After stirring for 24 h, the mixture was fil-
tered, added into the suspension of SBA-15 (1.5 g) in
toluene (10 mL) and then stirred overnight at 80 °C. The
mixture was centrifuged, washed with CH2Cl2 and water
and dried to afford catalyst 4.
2.6 The Suzuki–Miyaura Cross-Coupling Reaction
In a typical reaction, aryl halide (1.0 mmol), arylboronic
acid (1.5 mmol), base (2 mmol) and isopropyl alcohol
(3 mL) were added into a 25 mL Schlenk tube before
palladium catalyst (Pd: 1 mol %) was added. The mixture
was stirred at 82 °C for 24 h and then extracted with
ethyl acetate twice. The combined ethyl acetate phase
was dried with anhydrous MgSO4, filtered and evapo-
rated under reduced pressure before purified by column
chromatography.
2.4 Preparation of SBA-15 Immobilized Pd-NHC
Catalyst 5
2-(1H-imidazol-1-yl)pyrimidine (0.36 mmol) was added to
3-iodopropyltrimethoxysilane (0.36 mmol) in toluene
(10 mL), the mixture was stirred under nitrogen at 110 °C
for 2 days and then evaporated in vacuum to remove
toluene. The residue was washed with diethyl ether for
three times to afford white solid. The solid was dissolved in
dichloromethane (10 mL) and added to a Schlenk vessel
2.7 Catalyst Recovery and Reuse
In view of catalyst 4–7 were solid catalysts, they could be
readily separated from the reaction mixture through simple
filtration, and reused for Suzuki coupling reaction. After
each cycle the catalyst was filtered off, washed with
dichloromethane and water, and then vacuum-dried. The
recovered catalyst was used for consecutive reactions
under the same conditions.
˚
containing activated 4A molecular sieves (50 mg) and
Ag2O (0.3 mmol). This suspension was stirred for about
3 h in the absence of light, and then Pd(COD)Cl2
123

2004
J.-F. Zhang et al.
form the imidazolium salts with trimethoxysilyl group.
Then, the catalysts 6 and 7 were prepared in the similar
method [53].
3 Results and Discussion
3.1 Synthesis and Characterization of Immobilized
Catalysts
The FT-IR spectra of catalyst 4–7 were shown in Fig. 1.
The characteristic peaks of SBA-15 were maintained for all
four catalysts. The broad band at 1642 cm-1 was due to O–
H bending vibration. The absorption bands at 460 and
1088 cm-1 were attributed to d(Si–O) and m(Si–O),
Immobilized catalysts (4–7) with different palladium NHC
moieties were synthesized according to the pathway shown
in Scheme 1. The reaction of 1-(2,6-diisopropylphenyl)-
1H-imidazole 1 and 3-iodopropyltrimethoxysilane afforded
the imidazolium salt. The catalyst 4 was obtained by in situ
transmetalation from the silver carbene complex of the
corresponding imidazolium salt with Pd(PhCN)2Cl2 and
triethylamine, followed by the addition of SBA-15. 2-(1H-
imidazol-1-yl)pyrimidine 2 as functionalized imidazole,
Pd(COD)Cl2 as palladium precursor, catalyst 5 was pre-
pared according to similar procedures. The compound 3
was synthesized through the reaction of 1-(2,6-diiso-
respectively.
Some
new
bands
appeared
at
2930–2970 cm-1 probably assigned to the saturated C–H
stretching vibrations of palladium NHC moieties.
The solid-state 13C CP/MAS NMR spectra of catalysts
4–7 provided further evidence for the presence of the
palladium NHC moiety on the silica support (Fig. 2). The
weak signals at 150 ppm were attributed to the Pd-carbene
carbons. In Fig. 2a, the resonances near 10 and 45 ppm
were attributed to the saturated carbon of methyl and
methylene group in the triethylamine, while the strong
signals at 20–30 ppm were assigned to the saturated car-
bons of the 2,6-diisopropylphenyl group and the linker to
SBA-15. The signal of the saturated carbon tethered to the
propylphenyl)-1H-imidazole
1
with
1,2-bis(bro-
momethyl)benzene [51]. In the presence of Na2CO3, 3
reacted with N-methyl-3-(trimethoxysilyl)propan-1-amine
and 3-(trimethoxysilyl)propan-1-amine, respectively, to
O
I
Si
O
N
iPr
O
O
Ag2
Pd(PhCN)2
Cl Pd Cl
N
N
Cl NEt3
,
2
N
N
SBA-15
Si
O
O
iPr
O
1
4
SBA-15
Cl
Pd Cl
O
O
I
Si
N
O
O
N
Ag2
N
N
N
Pd(COD)Cl2
Si
N
N
N
O
SBA-15
O
O
5
2
SBA-15
SBA-15
O
Si
O
O
H
O
N
Si
O
R
O
Br
N
O
Ag2
Pd(COD)Cl2
N
R
iPr
N
Pd
Br
Cl
SBA-15
Na2CO3
Cl
N
3
N
iPr
6 R=CH3
7 R=H
Scheme 1 Synthesis of immobilized catalysts 4–7
123

Activity and Recyclability Improvement Through Adjusting the Tethering Strategy…
2005
nitrogen atom in imidazole ring was at 45 ppm, while the
resonance peaks at 120–140 ppm were assigned to the
unsaturated carbons of aromatic ring and the imidazole,
which was consistent with those of Pd-NHC structures
[54]. In the spectra of catalyst 5, the resonance peak near
48 ppm was attributed to the saturated carbon tethered to
the nitrogen atom in the imidazole, while the resonance of
linker carbon to SBA-15 was at around 15 ppm. In addition
to these, the signals near 120 ppm were attributed to the
unsaturated carbons of imidazole ring, and the resonances
of the pyrimidine were at 120–160 ppm (Fig. 2b), which
was in good agreement with the Palladium complexes in
Thomas’s work [55]. The spectra of catalysts 6 and 7 both
showed the signals at 15 ppm due to carbons linked to
SBA-15, and the resonances at 20–30 ppm were attributed
to the methyl and methylene group unlinked to the nitrogen
atom. The signals at 40–70 ppm were assigned to the
Fig. 1 FT-IR spectra of immobilized catalysts 4–7
Fig. 2 13C CP/MAS NMR spectra of immobilized catalysts 4–7
123

2006
J.-F. Zhang et al.
techniques. In the SAXS patterns of these palladium
modified silica catalysts (Fig. 4), all four catalysts dis-
played the characteristic peaks of SBA-15, which means
the basic structure of SBA-15 remained during the immo-
bilization process [54]. The TEM characterization further
confirmed that catalyst
6
possesses well-ordered
mesostructure with two-dimensional-hexagonal arrange-
ment of mesopores (Fig. 5) [52, 57].
The loading amounts of grafted palladium are 0.80,
0.56, 2.33, 5.66 wt% for catalyst 4, 5, 6 and 7 respectively,
as determined by ICP.
3.2 Catalytic Performance of Immobilized Catalysts
4–7
Fig. 3 29Si CP/MAS NMR spectra of immobilized catalysts 4–7
The catalytic activity of catalysts 4–7 was tested in the
Suzuki reactions of 4-chloroacetophenone and phenyl-
boronic acid. Firstly, we carried out the reactions at 82 °C
using KOH (2.0 mmol) as base, i-PrOH (3 mL) as solvent
(Table 1, entries 1–4). Catalysts 4, 6 and 7 with bulky
substituent 2,6-diisopropylphenyl connected to Pd-NHC
moieties were found to be highly efficient. Almost quan-
titative yields (98, 99 and 99 % yield) were achieved.
These results indicate catalysts 4, 6 and 7 are more efficient
than most of the immobilized catalysts reported in the lit-
erature [15]. For catalyst 5 with less sterically demanding
pyrimidine group [55], the yield was only 35 %, and no by-
products were observed. These results are consistent with
the previous findings that the steric bulk of the substituents
surrounding the metal centre enhances catalytic activity
[15]. The recycling of the catalysts was also tested
(Table 1, entries 1–4). The catalyst 5 continued to be
inactive as in the first cycle [58]. Interestingly, catalyst 6
and 7 could be reused for five times with promising yield,
whereas, catalyst 4 only gave trace coupling product in the
second cycle [59]. This result may be attributed to the loss
of the N-containing throw-away ligand (triethylamine)
coordinated to the palladium centre for 4 during catalyst
recovering process. For catalysts 6 and 7, where the amine
moieties (as N-containing throw-away ligands) were linked
with NHC moieties via an ortho-dimethylphenyl group and
also tethered to support, the throw-away ligand could
coordinate back to palladium centre and the catalytic
activity was thus enhanced (Scheme 2). In addition, the
intramolecular hydrogen bonding interaction in catalyst 7,
which is between the dialkylamine group and the adjacent
chlorine attached to the palladium, results in the activity of
the recycled catalysts slightly decreases [60]. In order to
further investigate the catalytic property and recycle per-
formance of catalyst 6, 4-chloroacetophenone 8a and more
Fig. 4 SAXS patterns of immobilized catalysts 4–7 and SBA-15
saturated carbons tethered to the nitrogen atom, while the
signals near 130 ppm were attributed to the unsaturated
carbons of imidazole ring [53]. The resonance peaks at
120–140 ppm were ascribed to the benzene rings in the
2,6-diisopropylphenyl group and the ortho-dimethylphenyl
group (Fig. 2c, d).
As displayed in Fig. 3, the 29Si CP-MAS NMR spectra
of 4–7 showed strong ‘‘Q’’ bands at -100 ppm originating
from the network structures of SBA-15. The weak ‘‘T’’
signals at -60 ppm were derived from silylether groups of
palladium complex moiety [56], confirming that palladium
NHC moieties were tethered to the supports.
Furthermore, the structural elucidation of these catalysts
was performed in some details using SAXS and TEM
123

Activity and Recyclability Improvement Through Adjusting the Tethering Strategy…
2007
Fig. 5 TEM images of
immobilized catalyst 6
Table 1 Suzuki–Miyaura reaction of aryl chlorides with arylboronic acids with catalysts 4–7
Entrya
Catalyst
Substrate
Product
Yields per cycle (%)b
1
2
3
4
5
6
7
a
4
5
6
7
6
6
6
8a
8a
8a
8a
8a
8b
8b
9a
9a
9a
9a
9b
9a
9b
10a
10a
10a
10a
10b
10c
10d
98/trace
35/11/11
99/97/92/90/84c
99/85/91/72
99/99/87/87/50c
84/50/48/49d
77/23/20d
Reaction conditions: aryl chloride (1.0 mmol), arylboronic acid (1.5 mmol), KOH (2.0 mmol), catalyst (Pd: 1 mol%), isopropanol (3 mL),
82 °C, reaction time 24 h except specially pointed out
b
Determined by the 1H NMR
c
Reaction time was 72 h
d
Reaction time was 48 h
sterically hindered 4-methylphenylboronic acid 9b were
used as substrates, the catalyst could be reused five times
with yields of 99, 99, 87, 87 and 50 % (Table 1, entry 5).
The leaching of the Pd components was evaluated in the
reaction, the ICP spectroscopy of the each crude product
revealed that 0.15, 0.25 and 0.58 % of Pd was leached in
the first, fourth and fifth run, respectively. The product
yield decrease gradually for the recycling experiments is
most likely due to the decomposition and cleavage of a
trace amount of tethered metal-NHC complex under reac-
tion conditions as well as the loss of the catalyst during
recovery [61]. For the reaction of more unreactive electron-
rich 4-methoxychlorobenzene 8b with the phenylboronic
acid 9a or 4-methylphenylboronic acid 9b, it also per-
formed good yield in first run and fine yields in three or
four cycles (Table 1, entries 6–7). These results are com-
parable to the reported work with electron-rich aryl chlo-
rides as substrates [62–65].
123

2008
J.-F. Zhang et al.
Ar-Ar'
SBA-15
O
O
O
Si
N
dissociation
coordination
L
Pd0
iPr
reductive elimination
N
N
iPr
SBA-15
SBA-15
O
O
O
O
O
O
Si
oxidative
addition
Ar-Cl
Si
N
iPr
Ar
L
PdII
N
iPr
Pd
Ar
L
SBA-15
N
N
N
N
iPr
O
O
O
iPr
Si
N
iPr
L
PdII
ClB-
(OH)3
Ar
transmetalation
Cl
N
N
OH-
Ar'-B-
(OH)3
iPr
Ar'-B(OH)2
Scheme 2 Proposed mechanism of Suzuki coupling reaction with catalyst 6
method for designing highly efficient and recyclable
immobilized catalysts.
4 Conclusions
We successfully designed and prepared four immobilized
palladium NHC complexes with different throw-away
nitrogen ligands using SBA-15 as support. In the probe
coupling reaction of 4-chloroacetophenone with phenyl-
boronic acid, catalysts 4, 6 and 7 with sterically hindered
NHC-Pd cores were highly effective. Catalyst 4 with a
throw-away triethylamine ligand was found to be ineffi-
cient after the first cycle. The catalyst 6 or 7 whose
nitrogen ligand was covalently connected with support and
NHC moiety, however, could be reused for at least three
cycles. These results revealed that catalytic activity and
recyclability can be remarkably improved through adjust-
ing the tethering method for throw-away ligands. Catalyst 6
performed well for typical aryl chlorides and arylboronic
acids. Our strategy offers an alternative and attractive
Acknowledgments Financal support from the National Natural
Science Foundation of China (Nos. 20871032, 20971026 and
21271047), ShanXi Science and Technology Department of China
(Project No. MH2014-07), and General Administration of Quality
Supervision, Inspection and Quarantine of China (Nos. 2015IK217)is
gratefully acknowledged.
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