Electrostatic Grafting of a Palladium N-Heterocyclic Carbene Catalyst on a Periodic Mesoporous Organosilica and its Application in the Suzuki-Miyaura Reaction

Article abstract of DOI:10.1002/cctc.201500417

A periodic mesoporous organosilica material functionalized with a Pd N-heterocyclic carbene complex was synthesized by applying an electrostatic grafting method. The resulting hybrid material was characterized by solid-state ²⁹Si and ¹³C cross-polarization magic-angle spinning NMR spectroscopy, XRD, thermogravimetric analysis, and BET measurements. The hybrid was applied as a catalyst for the Suzuki-Miyaura cross-coupling between aryl chlorides and phenylboronic acid under heterogeneous and aerobic conditions. The supported catalyst exhibited excellent activity and stability and it could be reused at least six times without any loss of activity. Atomic absorption spectroscopy detected no Pd contamination in the products, and leaching tests verified that the reaction was truly heterogeneous. Palladium plus pores: Ionic interactions allow a palladium N-heterocyclic carbene (NHC) complex to be anchored on the surface of a sulfonated periodic mesoporous organosilica material. The resulting heterogeneous Pd-NHC catalyst is highly active in the Suzuki-Miyaura coupling of aryl chlorides under mild and aerobic conditions.

Full text of DOI:10.1002/cctc.201500417

DOI: 10.1002/cctc.201500417
Full Papers
Electrostatic Grafting of a Palladium N-Heterocyclic
Carbene Catalyst on a Periodic Mesoporous Organosilica
and its Application in the Suzuki–Miyaura Reaction
Fatemeh Rajabi,*[a, b] Dirk Schaffner,[a] Sascha Follmann,[a] Christian Wilhelm,[a] Stefan Ernst,[a]
and Werner R. Thiel*[a]
Dedicated to Prof. Dr. F. Preuss on the occasion of his 80th birthday
A periodic mesoporous organosilica material functionalized
with a Pd N-heterocyclic carbene complex was synthesized by
applying an electrostatic grafting method. The resulting hybrid
material was characterized by solid-state 29Si and 13C cross-po-
larization magic-angle spinning NMR spectroscopy, XRD, ther-
mogravimetric analysis, and BET measurements. The hybrid
was applied as a catalyst for the Suzuki–Miyaura cross-coupling
between aryl chlorides and phenylboronic acid under hetero-
geneous and aerobic conditions. The supported catalyst exhib-
ited excellent activity and stability and it could be reused at
least six times without any loss of activity. Atomic absorption
spectroscopy detected no Pd contamination in the products,
and leaching tests verified that the reaction was truly hetero-
geneous.
Introduction
Pd-catalyzed cross-coupling reactions are applied widely in or-
ganic synthesis.[1] Among Pd-catalyzed coupling reactions, the
Suzuki–Miyaura coupling reaction is an extremely powerful
tool for the generation of biaryl compounds, which are impor-
tant structural units in polymers, liquid crystals, chiral ligands,
natural products, and pharmaceuticals.[2] Significant efforts
have been made to develop more efficient Pd catalysts for the
Suzuki–Miyaura reaction.[3] In particular, Pd species that bear N-
heterocyclic carbene (NHC) ligands have attracted a lot of in-
terest in this context.[4] The strong s-donor capability of the
NHC ligand facilitates the oxidative addition step in the catalyt-
ic cycle. Advancements in this field over the last 25 years have
been documented in numerous publications and reviews.[5]
Although a large number of homogeneous Pd-NHC com-
plexes are found in the literature, their separation and recovery
from the reaction mixture and their reuse are still challenges.[6]
A possible strategy to solve such problems is the immobiliza-
tion of homogeneous catalysts on well-designed organic or in-
organic supports by covalent anchoring.[7] After the catalytic
transformation is completed, such a catalyst can be separated
by simple filtration, which minimizes the cost of the work-up
and saves the noble metal and the often expensive ligands.
This strategy has been applied for the immobilization of Pd-
NHC catalysts[8] and other Pd catalysts applied for the Suzuki–
Miyaura reaction.[9] As organic supports tend to undergo swel-
ling and shrinking in the presence of organic solvents, meso-
porous silica-type materials, such as SBA-15[10] and MCM-41,[11]
have been widely applied as supports. The synthesis protocols
that lead to such materials allow their porosity and other sur-
face properties to be fine-tuned. They offer large specific sur-
face areas in combination with reactive SiÀOH groups for the
covalent attachment of organic groups on their surfaces.[12]
However, organic substrates have to undergo diffusion from
the liquid phase into the pores, and the products have to do
the reverse. Thereby, the solvation of substrates and products
will be influenced strongly by the nature of the porous materi-
al. Polar porous materials will disturb the solvation of typical
organic molecules, and surfaces that possess a more nonpolar
nature should be more compatible.[13] Recently, we were able
to show this effect by reducing the number of free SiÀOH
groups in an MCM-41-based epoxidation catalyst by trimethyl-
silylation and we found an enhancement of activity and recy-
clability.[14]
[a] Dr. F. Rajabi, D. Schaffner, S. Follmann, C. Wilhelm, Prof. Dr. S. Ernst,
Prof. Dr. W. R. Thiel
Fachbereich Chemie
In this context, so-called periodic mesoporous organosilicas
(PMOs) that are materials with ordered pores consisting of or-
ganosilica groups instead of a neat silica framework[15] have in-
creasingly been used as catalyst supports during the last few
years. Such materials allow a further fine-tuning of the pore
properties in terms of polarity and polarizability. PMOs have
found a broad variety of applications in catalysis,[16] gas adsorp-
tion,[17] energy conversion,[18] organo-optoelectronics,[19] and
drug delivery.[20] Additionally, PMOs show a largely enhanced
Technische Universität Kaiserslautern
Erwin-Schrçdinger-Str. 54, 67663 Kaiserslautern (Germany)
E-mail: thiel@chemie.uni-kl.de
[b] Dr. F. Rajabi
Department of Science
Payame Noor University
19395-4697, Tehran (Iran)
E-mail: f_rajabi@pnu.ac.ir
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201500417.
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resistance against hydrolysis compared to similarly structured
neat silica materials, a feature that is of special interest for re-
actions that are performed in aqueous media.
Recently, we published a strategy on how to introduce a ni-
trophenyl unit into the backbone of a saturated imidazolinium
salt and found that the derived PdII-NHC complex showed
a fairly high catalytic activity in the Suzuki–Miyaura cross-cou-
pling of unactivated aryl chlorides and various aryl boronic
acids.[21] Herein, we describe the synthesis and characterization
of a new heterogeneous cross-coupling catalyst by the immo-
bilization of such a functionalized Pd-NHC complex on the sur-
face of a PMO support. In contrast to many other studies on
Pd catalysts that are anchored covalently on the surface of
a support material, we chose an electrostatic grafting method,
which allows the catalyst to interact more flexibly on the sur-
face. Recently, this strategy was beneficial, for example, for the
olefin hydrogenation activity of a grafted, phosphane-coordi-
nated Pd catalyst and for the olefin epoxidation activity of
phosphomolybdic acid attached to a PMO.[22]
Results and Discussion
The ligand precursor 4-(4-nitrophenyl)-1,3-bis-(2,4,6-trimethyl-
phenyl)-4,5-dihydro-3H-imidazolium chloride (1) is accessible
from 4-nitroacetophenone in good yields in just five steps.[18a]
To allow the electrostatic immobilization, the nitro group of
1 was converted into an amino unit with Sn/HCl in yields of
70% (Scheme 1). Milder ways to hydrogenate the nitro group
such as reduction with NH4(HCOO) or H2 in the presence of
Pd/C failed.
1
The formation of 2 was confirmed clearly by H NMR spec-
troscopy. The presence of the electron-donating NH2 group re-
sults in a shift of the resonances of the aminophenyl unit by
ꢀ1.0 ppm to higher field (d=6.50 and 7.04 ppm; for the NMR
spectra see the Supporting Information) compared to 1. A
broad peak at d=5.41 ppm is assigned to the protons of the
1
NH2 group. As already found in the H NMR spectrum of 1, the
two mesityl groups of the dihydroimidazolium salt 2 are hin-
dered in rotation around the CÀN bonds, which leads to four
resonances for the aromatic mesityl protons and six resonan-
ces for the aliphatic mesityl protons. The same doubling of the
number of the mesityl resonances is observed in the 13C NMR
spectrum. The reaction of 2 with PdCl2, pyridine, and K2CO3 in
THF gave the Pd-NHC complex 3 as a yellow solid in 35% yield
Scheme 1. Synthesis of [Pd-NH3+](ÀSO3-PhPMO).
treatment of PhPMO with chlorosulfonic acid resulted in a par-
tial sulfonation of the phenylene units (HSO3-PhPMO). The
spectroscopic data of the product were in agreement with
those in the literature. To determine the specific number of
acid sites on the surface, HSO3-PhPMO was titrated with 0.01m
NaOH(aq)/phenolphthalein, which showed that there are
0.78 mmol acid groups per g of HSO3-PhPMO.
1
after work-up. In the H NMR spectrum of 3, the resonance at
d=9.65 ppm typical for the NC(H)N fragment of an imidazoli-
um salt disappeared. The three other resonances of the NHC-
ring hydrogen atoms are shifted slightly to higher field com-
pared to 2. The resonances of the coordinated pyridine ligand
are found at d=8.29, 7.82, and 7.29 ppm (o-H, p-H, m-H). In
the 13C NMR spectrum, the resonance of the carbene carbon
atom is observed at d=180.7 ppm. Again the rotation around
the CÀN bonds in the NHC ligand is hindered, which leads to
For the electrostatic grafting, compound 3 was stirred with
HSO3-PhPMO for 24 h in toluene under reflux conditions. After
washing the product with water and dichloromethane, the re-
+
sulting hybrid Pd catalyst [Pd-NH3 ](ÀSO3-PhPMO) was dried
under vacuum.
The surface properties of the materials were investigated by
N2 physisorption. The isotherms of PhPMO, HSO3-PhPMO, and
[Pd-NH3+](ÀSO3-PhPMO) are shown in Figure 1. The isotherms
are of type IV with a type H4 hysteresis according to the IUPAC
classification.[24] The corresponding parameters are summarized
1
a doubled set of resonances in the H and the 13C NMR spec-
tra.
The phenylene-bridged PMO (PhPMO) support was synthe-
sized according to a published protocol (Scheme 1).[23] The
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Figure 2. Powder XRD patterns of A) HSO3-PhPMO and B) [Pd-NH3+](ÀSO3-
PhPMO). The inset shows the (100) reflection typical of hexagonally struc-
tured mesoporous materials.
Figure 1. N2 physisorption isotherms of A) PhPMO, B) HSO3-PhPMO, and
C) [Pd-NH3+](ÀSO3-PhPMO).
Table 1. Pore parameters of PhPMO, HSO3-PhPMO, and [Pd-NH3+](ÀSO3-
PhPMO).
Sample
SBET
[m2 gÀ1
Total pore
volume [cm3 gÀ1
Mean pore
diameter [nm]
]
]
PhPMO
HSO3-PhPMO
[Pd-NH3+](ÀSO3-PhPMO)
891
954
766
0.90
1.07
0.86
2.64
2.89
2.65
in Table 1. According to these data, the total pore volume and
the mean pore diameter are increased after the sulfonation of
PhPMO. This can either be explained by the removal of some
residual template molecules from the pores or by an ongoing
condensation of the framework caused by the strong acid
HSO3Cl. The shapes of the isotherms before and after the load-
ing of 3 are quite similar. As expected, the BET surface area,
the total pore volume, and the mean pore diameter become
Figure 3. TGA plot of HSO3-PhPMO (continuous line) and [Pd-NH3+](ÀSO3-
PhPMO) (dashed line).
+
smaller after the formation of [Pd-NH3 ](ÀSO3-PhPMO) because
of the ongoing filling of the pores. Generally, it can be consid-
ered that the structural features of the support material are
not severely altered during the functionalization process.
+
higher for [Pd-NH3 ](ÀSO3-PhPMO), probably because of the
higher degree of hydration of the ion pair. The material itself
decomposes between 550 and 6308C. Here the decomposition
of [Pd-NH3+](ÀSO3-PhPMO) sets in at slightly lower tempera-
tures. For differential thermogravimetric analysis data, see the
Supporting Information.
Solid-state 29Si and 13C NMR spectroscopy was performed to
gain more structural information about the materials. In the
solid-state 29Si NMR spectrum of [Pd-NH3 ](ÀSO3-PhPMO) there
+
The mesoporous materials HSO3-PhPMO and [Pd-NH3 ](ÀSO3-
PhPMO) were further investigated by powder XRD, and he
measured XRD patterns are presented in Figure 2. For HSO3-
PhPMO, there is a well resolved peak at around 2q=1.908,
which is typical for the (100) reflection of hexagonal PMO ma-
terials. This shifts slightly (2q=1.958) after the introduction of
the Pd complex. Further sharp reflections in the range of 108<
2q<408 are characteristic of phenylene-based PMOs and can
be assigned to a periodicity in the pore walls with a spacing of
7.6 Š.[25] Evidently, the structural integrity of the materials is
maintained during the functionalization process.
+
are signals exclusively in the range of d=À60 to À80 ppm,
which can be assigned to so-called T species (Figure 4). These
are Si atoms that carry one organic group and three O atoms
(RÀSiO3). The absence of Q signals, Si atoms bound to four O
atoms, indicates that the SiÀC bonds remain unchanged
during the functionalization processes.
More information about the thermal stability of the hybrid
+
materials HSO3-PhPMO and [Pd-NH3 ](ÀSO3-PhPMO) can be ex-
tracted from the thermogravimetric analysis (TGA) data
(Figure 3). Two distinct weight losses occur during the course
of the applied temperature ramp: physisorbed water is re-
leased at around 80–1208C. The amount of water is a little
Compared to the 13C cross-polarization magic-angle spinning
(CP-MAS) NMR spectrum of HSO3-PhPMO that shows only one
intense peak at d=131.3 ppm, the number of resonances in-
+
creases significantly in the spectrum of [Pd-NH3 ](ÀSO3-PhPMO)
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Table 2. Suzuki–Miyaura cross-coupling of chlorobenzene and phenyl-
boronic acid catalyzed by [Pd-NH3+](ÀSO3-PhPMO).[a]
Entry
Solvent
Base
Catalyst
loading [mol%]
Yield
[%]
1
2
3
4
5
6
7
8
H2O
H2O
EtOH
Na2CO3
K2CO3
K2CO3
K2CO3
Na2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
0.1
0.1
0.1
0.1
0.1
0.1
0.05
0.02
0.01
0.015
0.015
0.015
0.015
0.015
60
55
48
95
92
68
95
95
83
95
95[b]
95[c]
85[d]
70[e]
iPrOH
iPrOH
iPrOH/H2O (1:1)
iPrOH
iPrOH
iPrOH
iPrOH
iPrOH
iPrOH
iPrOH
iPrOH
9
Figure 4. Solid-state 29Si NMR spectra of HSO3-PhPMO (bottom) and [Pd-
NH3+](ÀSO3-PhPMO) (top).
10
11
12
13
14
[a] Reaction conditions: solvent (5 mL), PhCl (1 mmol), PhB(OH)2,
(1.1 mmol), base (1.5 mmol), 18 h, heating to reflux, isolated yields; [b] re-
action time: 10 h; [c] reaction time: 10 h, temperature 808C; [d] reaction
time: 6 h; [e] temperature: 508C.
were performed to determine whether the reaction was truly
heterogeneous. For this purpose, the catalyst was removed
from the catalysis mixture by filtration after half of the reaction
time required for the coupling of chlorobenzene and phenyl
boronic acid under the given conditions, and the filtrate was
allowed to react under the same conditions. No further reac-
tion occurred after the filtration, which suggests that immobi-
lized Pd species are the active site. Atomic absorption spec-
troscopy (AAS) detected no traces of Pd in solution down to
the limit of the method (0.5”10À4 mmolLÀ1), which confirms
the high stability of the Pd complex and its strong binding to
the surface. In the mechanism of pyridine-enhanced precata-
lyst preparation stabilization and initiation (PEPPSI) catalysts,
a dissociation of the pyridine site is one of the accepted steps.
As it is unfavorable for this ligand to recoordinate to an immo-
bilized catalytically active site, we assume that the catalyst is
stabilized by interactions with basic surface sites as we do not
Figure 5. Solid-state 13C CP-MAS NMR spectra of HSO3-PhPMO (bottom) and
[Pd-NH3+](ÀSO3-PhPMO) (top); the resonances marked with an asterisk * are
rotational side bands, and those marked with # are caused by ethanol ad-
sorbed in the pores.
(Figure 5): in the aryl region, a series of resonances are found
between d=160 and 120 ppm. A shift to a lower field in com-
parison to the spectrum of 3 in solution can be explained by
the cationic nature of the anilinium group. A series of further
signals at higher field can be assigned to the resonances of the observe any catalyst decomposition to accompany the catalyt-
+
ÀCH, ÀCH2, and ÀCH3 groups in [Pd-NH3 ](ÀSO3-PhPMO).
ic transformation.
After full characterization of [Pd-NH3 ](ÀSO3-PhPMO), the cat-
With the optimized reaction conditions in hand, we
screened the scope of the reaction using aryl chlorides with
different functional groups (Scheme 2, Table 3). Substrates that
bear electron-withdrawing substituents provided the desired
products in perfect yields, and for deactivated substrates the
yield was ꢁ90%. The catalyst again showed excellent recover-
ability and reusability over six successive runs under the given
reaction conditions.
+
alytic performance of the Pd-containing material was studied
in the Suzuki–Miyaura cross-coupling under heterogeneous
conditions. We used chlorobenzene and phenylboronic acid as
+
substrates as the homogeneous congener of [Pd-NH3 ](ÀSO3-
PhPMO) showed high activities for aryl chlorides under mild
conditions. To optimize the cross-coupling reaction, various pa-
rameters such as catalyst concentration, bases, and solvents
were investigated. A combination of K2CO3 and iPrOH gave the
+
best results (Table 2). Generally the activity of [Pd-NH3 ](ÀSO3-
PhPMO) is comparable to that of the homogeneous system. All
catalytic experiments were performed under an ambient at-
mosphere (air), and the catalyst is stable under these condi-
tions.
Under the optimized conditions, the catalyst not only
showed excellent activity but also reusability. Leaching tests
Scheme 2. Suzuki–Miyaura cross-coupling of aryl chlorides and phenyl-
boronic acid catalyzed by [Pd-NH3+](ÀSO3-PhPMO).
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cooling the reaction mixture to RT, a solution of NaOH (3.00 g) in
water (5 mL) was added slowly with cooling. The mixture was ex-
tracted three times with CHCl3 (30 mL), and the combined organic
phase was dried over MgSO4. Evaporation of the solvent resulted
in a yellow solid (yield: 82%). Elemental analysis calcd (%) for
C27H32ClN3 (434.02) C 74.72, H 7.43, N 9.68; found C 74.34, H 7.36,
N 9.78%; 1H NMR (400 MHz, [D6]DMSO): d=9.03 (s, 1H, H1), 7.13
Table 3. Suzuki–Miyaura cross-coupling of aryl chlorides and phenylbor-
onic acid catalyzed by [Pd-NH3+](ÀSO3-PhPMO).
Entry
Aryl chloride
Product
Yield
[%][a]
1
2
3
95
92
90
3
(d, J=7.5 Hz, 2H, H5), 7.05, 7.05, 7.03, 6.84 (4 s, 4H, H10), 6.50 (d,
2H, 2H, H6), 5.84 (dd, 2”3J=11.1 Hz, 1H, H2), 5.41 (s, 2H, NH2),
2
2
3
4.81 (dd, J=3J=12.3 Hz, 1H, H3 resp. H3’), 4.62 (dd, J=12.1, J=
10.3 Hz, 1H, H3 resp. H3’), 2.50, 2.46, 2.40, 2.31, 2.22, 1.80 ppm (6 s,
18H, 6”CH3); 13C NMR (101 MHz, DMSO-d6): d=158.6 (C1), 150.1,
139.7, 139.2, 136.0, 135.7, 135.4, 135.3 131.1 (C5), 129.8, 129.6,
129.5, 129.4, 129.2, 119.3 (C4), 113.6 (C6), 66.5 (C2), 55.1 (C3), 20.6,
20.4, 18.0, 17.73 17.6, 17.3 ppm (6”CH3).
4
90
5
6
95
98
Dichlorido(pyridine)[4-(4-aminophenyl)-1,3-bis-(2,4,6-trime-
thylphenyl)-4,5-dihydro-3H-imidazol-2-ylidene]palladium(II)
(3)
[a] Isolated yields; reaction conditions as in Table 2, entry 10.
A mixture of 2 (0.512 g, 1.18 mmol), PdCl2 (0.105 g, 0.59 mmol),
and K2CO3 (0.326 g, 2.36 mmol) of was stirred in pyridine (6 mL) for
three days at 608C under an atmosphere of N2. After cooling the
reaction mixture to RT, most of the volatiles were removed under
reduced pressure. The solid residue was purified by flash chroma-
tography (SiO2, hexane/ethyl acetate, 5:1) to give 3 as a yellow
solid (yield: 35%). Elemental analysis calcd (%) for C32H37Cl2N4Pd
(654.98): C 58.68, H 5.69, N 8.55; found C 57.97, H 5.76, N 8.18;
1H NMR (400 MHz, [D6]DMSO): d=8.29 (t, 3J=4.5 Hz, 2H, Hpy-o),
7.81 (t, 3J=7.4 Hz, 1H, Hpy-p), 7.34 (dd, 2H, Hpy-m), 7.04, 7.01,
Conclusions
We have synthesized an organic–inorganic hybrid material by
the electrostatic anchoring of a Pd N-heterocyclic carbene
complex onto the surface of a partially sulfonated periodic
mesoporous organosilica material. The derived heterogeneous
Pd catalyst is highly active for the Suzuki–Miyaura coupling of
aryl chlorides under mild and aerobic conditions. The hetero-
geneity of the system was examined by a leaching test per-
formed at the reaction temperature of the catalysis, and no
leaching was detected. The material stays stable under these
conditions and could be reused at least six times without any
loss of activity.
3
6.74 (3 s, 3H, H10), 6.94–6.86 (m, 2H+1H, H5+H10), 6.46 (d, J=
2
8.3 Hz, 2H, H6), 5.28–5.20 (m, 2H+1H, NH2+H2), 4.35 (dd, J=3J=
11.5 Hz, 1H, H3 resp. H3’), 4.11 (dd, 2J=11. 0, 3J=8.5 Hz, 1H, H3
resp. H3’), 2.62, 2.58, 2.51, 2.31, 2.22, 2.00 ppm (6 s, 18H, 6”CH3);
13C NMR (101 MHz, [D6]DMSO): d=180.7 (C1), 150.6, 150.5, 149.2,
138.4, 137.7, 137.2, 137.0, 136.7, 135.2, 129.5, 129.3, 129.1, 128.7,
124.4 (C4), 113.4 (C6), 66.4 (C2), 56.6 (C3), 20.59, 20.5, 19.8, 19.6,
19.6, 19.0 ppm (6”CH3).
Experimental Section
Synthesis of [Pd-NH3+](ÀSO3-PhPMO)
General remarks
Solvents were dried by standard methods. Reagents were pur-
chased from ACROS, Alfa Aesar, or Sigma–Aldrich and used without
further purification, unless otherwise noted. NMR spectra in solu-
tion were recorded by using a Bruker DPX 400 spectrometer, and
solid-state NMR measurements were performed by using
a 500 MHz Bruker Avance III widebore NMR spectrometer. Elemen-
tal analysis was performed at the Fachbereich Chemie (TU Kaisers-
lautern). Pd leaching was determined by AAS by using a PERKIN
ELMER AAnalyst 300. N2 physisorption isotherms were measured at
77 K by using a Quantachrome Autosorb 1 sorption analyzer. The
specific surface areas were calculated using the BET equation at P/
P0 =0.05–0.5. TGA was performed by using a Setaram Setsys 16/18.
Complex 3 (0.51 g, 0.78 mmol) was dissolved in CHCl3 (5 mL) and
added to a suspension of HSO3-PhPMO (1.00 g) in dry toluene
(50 mL). After the mixture was stirred for 12 h at 908C, the solid
material was collected by filtration, extracted for 24 h with CHCl3 in
a Soxhlet apparatus, and dried under vacuum at 508C. Based on
AAS analysis of a solution obtained by washing the catalyst with
nitric acid, a loading of Pd of (0.250Æ0.001) mmolgÀ1 was deter-
mined.
General procedure for the catalytic Suzuki–Miyaura cross-
coupling
A 50 mL round-bottomed flask equipped with a magnetic stirring
bar was charged with aryl chloride (1.0 mmol), phenylboronic acid
(134 mg, 1.1 mmol), K2CO3 (207 mg, 1.5 mmol), [Pd-NH3+](ÀSO3-
PhPMO) (60 mg; 0.015 mmol Pd), and isopropyl alcohol (5 mL). The
mixture was heated to 808C for 10 h under air. After cooling to RT,
CHCl3 (20 mL) was added, and the mixture was filtered. The solid
residue was washed twice with water (10 mL) and twice with ethyl
acetate (10 mL) and then dried under vacuum at 508C. After this
procedure, the catalyst could be used for the next run. The organic
4-(4-Aminophenyl)-1,3-bis-(2,4,6-trimethylphenyl)-4,5-di-
hydro-3H-imidazolium chloride (2)
Compound 1 (4.64 g, 10 mmol) and granulated tin (1.79 g,
15 mmol) were added into
a 100 mL round-bottomed flask
equipped with a reflux condenser. The flask was placed into cold
water, and concd HCl (4 mL) was added dropwise under vigorous
stirring followed by heating of the mixture to reflux for 1 h. After
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phase was dried over MgSO4 and filtered, and the solvent was re-
moved under reduced pressure. Pure products were obtained by
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F.R. gratefully acknowledges the Alexander von Humboldt Foun-
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Received: April 16, 2015
Revised: May 29, 2015
Published online on September 25, 2015
ChemCatChem 2015, 7, 3513 – 3518
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