Palladium-Schiff-base-silica framework as a robust and recyclable catalyst for Suzuki-Miyaura cross-coupling in aqueous media

Article abstract of DOI:10.1039/c5ra01574j

A silica supported palladium catalyst, Pd@imine-SiO₂ was prepared by immobilizing Pd(OAc)₂ onto silica gel through coordination of imine, generated via Schiff-base condensation between 3-aminopropyltriethoxysilane (APTES) functionalized silica gel and salicylaldehyde. The prepared catalyst was characterized by FT-IR, BET surface area measurements, XRD, SEM-EDX, EDS-mapping and ICP-AES analysis. The imine-based catalyst exhibited excellent activity in the Suzuki-Miyaura cross-coupling reactions of aryl bromides with arylboronic acids in iPrOH/H₂O (1:1) at room temperature. The reaction proceeds under mild reaction conditions and the catalyst is recyclable, thus offering an environmentally benign alternative to the existing protocols.

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Palladium-Schiff-base-Silica Framework as a Robust
and Recyclable Catalyst for the Suzuki-Miyaura Cross-
Coupling in Aqueous Media
Cite this: DOI: 10.1039/x0xx00000x
Tahshina Beguma, Manoj Mondala, Pradip K. Gogoi*a and Utpal Bora*ab
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
www.rsc.org/
Nowadays, many effective solid supports are available that are
easily accessible and often provides effective platform for
Abstract
Silica supported palladium catalyst, Pd@imineꢀSiO2 was
prepared by immobilizing Pd(OAc)2 onto silica gel through
coordination of imine, generated via Schiffꢀbase condensation
between 3ꢀaminopropyltriethoxysilane (APTES) functionalized
silica gel and salicylaldehyde. The prepared catalyst was
characterized by FTꢀIR, BET surface area measurements, XRD,
SEMꢀEDX, EDSꢀmapping and ICPꢀAES analysis. The imineꢀ
based catalyst exhibited excellent activity in the SuzukiꢀMiyaura
crossꢀcoupling reactions of aryl bromides with arylboronic acids
heterogeneous catalysis. Silica, due to its low cost, easy
accessibility, high thermal stability and excellent porosity14 and large
surface area received utmost attention compared to other effective
supports like alumina, poly (vinyl chloride), MCMꢀ41,15 SBAꢀ15,16
polymeric ionic liquid17 etc. Moreover, the highly active surfaceꢀ
silanol groups present in the silica promotes the effective anchoring
of organic ligands onto its surface,18 and subsequently the
immobilization of the metal via covalent bond on the support. Till
date, noticeable advances have been carried out in synthesizing
novel and efficient silicaꢀsupported heterogeneous palladium catalyst
to perform SuzukiꢀMiyaura reaction.19 However, many of these
methods still uses toxic organic or biphasic coꢀsolvents, elevated
reaction temperature and/or require high catalyst loading with large
excess of boronic acids.20 In addition, many of these heterogeneous
catalysts suffer from the problem concerning poor recyclability due
to certain unwanted inactivation processes, such as aggregation and
growth of less reactive large palladium particles. Therefore, there is
a need to develop novel recoverable catalytic systems that can be
efficiently reused under mild reaction condition while retaining the
original activity of the homogeneous catalytic centers.
In coordination chemistry and catalysis, Schiffꢀbases are
considered as an important class of Nꢀbased ligands, as they possess
high selectivity and sensitivity towards metal ions and are well
regarded as chelating multidentate ligands. However, in
homogeneous catalysis the palladium Schiffꢀbase complexes often
gets deactivated due to the formation dimeric peroxoꢀ and ꢁꢀoxo
species.21 To overcome these drawbacks, researchers have devised
various immobilized palladium based catalyst as efficient and
recyclable catalyst for the SuzukiꢀMiyaura coupling of a variety of
electronically diverse aryl halides under ambient reaction
condition.22
in iPrOH/H2O (1:1) at room temperature. The reaction proceeds
under mild reaction conditions and the catalyst is recyclable, thus
offering an environmentally benign alternative to the existing
protocols.
Introduction
Compounds containing carbonꢀcarbon bonds have found extensive
applications in the synthesis of natural products, agrochemicals,
pharmaceuticals, and polymers.1 Among the various methods the
palladiumꢀcatalyzed SuzukiꢀMiyaura reaction developed for the
linkage of two similar/dissimilar aryl fragments via the crossꢀ
coupling of aryl halide and aryl boronic acid has became the most
dominant method for synthesizing homoꢀaryl and biaryl derivatives.2
Various palladium based catalysts ligated with simple tertiary
phosphines, hemilabileꢀtype phosphines, sterically crowded
biphenylꢀtype phosphines,2d,3 imine,4 NHC,5 amine,6 bisamides,7
triazole,8 Schiffꢀbase,9 acetanilide,10 and oximeꢀether11 have been
commonly used for SuzukiꢀMiyaura cross coupling reaction.
However, the practical application of homogeneous catalyst in
pharmaceutical industry remains challenging because the expensive
catalysts are not easily reusable, hard to separate from reaction
mixture (a frequent industrial concern) and often contaminates the
product.12 Therefore, in order to meet regulatory standards, the
methods for removal of these toxic metals from active
pharmaceutical ingredients need to be highly efficient.13 Therefore,
the use of heterogeneous catalyst seems promising and advantageous
over homogenous catalysts terms of minimizing contamination of
the products, easy isolation, reusability and longꢀterm stability of the
solid catalyst.
Through this communication, we wish to report the synthesis of
a
novel salicylaldehydeꢀbased Schiffꢀbaseꢀpalladium complex
immobilization on silica support. The catalyst was found to be
highly efficient in the SuzukiꢀMiyaura cross coupling reaction in
aqueous solution and at room temperature and could be recovered by
simple filtration and reused up to eight consecutive cycles without
significant loss in activity.
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OHC
HO
OH
OH +
OH
EtO
O
O
O
Toluene
reflux, N2 atm.
8 h
+
Si(CH2)3NH2
EtO Si(CH2)3NH2
EtO
SiO2
APTES
APTES@SiO2
ethanol, reflux,
6 h
HO
O
O
O
O
O
O
O
O
O
O
N(CH2)3Si
Pd
Pd(OAc)2
acetone, r.t., 24 h
Si(H2C)3N
Si(CH2)3N C
O
H
imine@SiO2
Pd@imineꢀSiO2
Scheme 1. Preparation of the supported Schiff base Pd(II) complex
vOꢀH stretching and bending vibrations of the adsorbed water in the
samples. The absorption peaks at 1050ꢀ1150 cmꢀ1 are due to the
stretching vibrations of the framework and SiꢀOꢀSi groups. The
presence of several bands with medium intensity in 2830ꢀ2930 cmꢀ1
region is ascribed to vCꢀH stretching of methylene groups. Apart from
the case of I, the vNꢀH in the region 1500ꢀ1600 cmꢀ1 were visible in all
samples (Figure 1). This indicates the attachments of APTES onto
the mesoporous silica (I) matrix. The bands observed at 1670 cmꢀ1
and 1460 cmꢀ1 for III could be attributed to the vC=N and vCꢀO
stretching frequency, and at 540 cmꢀ1 in IV indicates vPdꢀN stretching
frequency.24
Results and Discussion
Synthesis and Characterization of the materials
The synthetic procedure for the preparation of the palladium
complex was synthesized in three steps following a previous
reported procedure and is summarized in Scheme 1.23
The first step involves the modification of silica surface by
treating APTES and solid silica gel (60ꢀ120 mesh) in dry toluene
under N2 atmosphere at reflux condition. In the second step, a
Schiffꢀbase ligand was synthesized by adding APTES@SiO2 and
salicylaldehyde in ethanol at reflux to give the corresponding
imine@SiO2 ligand, which is yellow colored solid after final
purification. Finally the ligand precursor was used for the synthesis
of Pd@imineꢀSiO2 complex by treating with Pd(OAc)2 in acetone at
room temperature for 24 h.
XRD, ICP-AES, SEM-EDX and EDS mapping analysis
Xꢀray diffraction patterns of amorphous silica and palladium
immobilized silica gel are shown in Figure 2. Free SiO2 exhibits a
peak at 2θ value of 21.10. On immobilization of palladium acetate,
two characteristic peaks of Pd at 2θ= 38.12 and 47.23, corresponding
to the (111) and (200) lattice planes of the face centered cubic phase
of palladium were observed, which is in consistent with reported
literature.23,25 Considering the results of the ICPꢀAES and SEMꢀ
EDX observations, we were able to determine the metal attachment
on the surface of solid supports. The metal content of Pd@imineꢀ
SiO2 was determined by ICPꢀAES, which suggests 2.465 wt% or
0.023 mmol palladium loading in the immobilized complex.
Figure 1: FTIR spectra; (I) Silica gel (SiO2), (II) APTES@SiO2,
(III) imine@SiO2 and (IV) Pd@imineꢀSiO2
Due to insolubility of the silica supported Pd(II) complex in almost
all organic solvents, its structural investigations were limited only to
its physicochemical properties, such as FTIR, SEMꢀEDX, EDSꢀ
mapping, ICPꢀAES, N2 adsorptionꢀdesorption spectral data etc. The
FTIR spectra of SiO2, APTES@SiO2, imine@SiO2 and Pd@imineꢀ
SiO2 are presented in Figure 1. The FTIR spectra of the entire
samples shows absorption bands at around 3400 and 1638 cmꢀ1 for
Figure 2: XRD pattern of (a) free silica, and (b) Pd@imineꢀSiO2
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Figure 3: SEMꢀEDX spectra of (a) silica gel (SiO2) (b) Pd@imine–SiO2.
(a)
(b)
Figure 4: SEM image of (a) silica gel (SiO2), (b) Pd@imineꢀSiO2.
According to the results of the SEMꢀEDX spectra shown in Figure 3,
the metal complex shows metal content along with Oꢀ and Siꢀ
proportions, which confirms the formation of metal complex with
the anchored ligand at various sites. The SEM image of the free SiO2
and supported Pdꢀcatalyst clearly showed the morphological change
which occurred on the surface of silica after being loaded with
palladium. As intensely, observed in Figure 4 that, the presence of
palladium causes a significant decrease in the silica particle size.
To further confirm this Pd@imineꢀSiO2 structure, we take the
elemental mapping by energy dispersive Xꢀray absorption
spectroscopy (EDS). Figure 5 shows that the distribution of active
palladium sites and silica is very uniform throughout the whole
particle.
considerable decrease in the BET surface area and pore volume
compared to the solid supported SiO2 suggests the possibility of the
organic moieties anchored successfully on the surface of SiO2.
N2 adsorption-desorption isotherm
N2 adsorptionꢀ desorption isotherm analysis provides information on
the specific surface area and porosity of the prepared samples. All of
them exhibited the type IV isotherm according to the IUPAC
classification with a typical hysteresis loop, featuring mesoporous
material with highly uniform pore size distribution. Figure 6 displays
the N2 adsorptionꢀdesorption isotherm and pore size distribution of
the silica based samples under study. The structural parameters are
summarized in Table 1. According to BET measurement, the surface
area for SiO2 was 421.9 m2gꢀ1 which upon functionalized with
APTES and then treatment of salicylaldehyde changed from 333.2
m2gꢀ1 to 293.5 m2gꢀ1 respectively with corresponding pore volume
0.449 cm3gꢀ1, 0.415 cm3gꢀ1 and 0.374 cm3gꢀ1 respectively. While a
decrease in the surface area (210.1 m2gꢀ1) and pore volume (0.274
cm3gꢀ1) was observed when palladium was immobilized onto
imine@SiO2.
Figure 5: (a) SEM image; EDS mapping of elements: (b) Oxygen,
(c) Si, (d) Pd and (e) Si+O+Pd.
We also observed an increase in pore diameter in the case of
Pd@imineꢀSiO2 (3.26 nm) than the parent SiO2 (2.05 nm). A
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Table
2
Optimization of the SuzukiꢀMiyaura reaction of 4ꢀ
Table 1 Surface area and pore size distribution of the silica based
samples
bromonitrobenzene with phenylboronic acid in the presence of Pd@imineꢀ
SiO2
Entry catalyst
Surface area
(m2 gꢀ1)
421.9
Pore volume
(cm3 gꢀ1)
0.449
Pore radius
(nm)
2.05
1
2
SiO2
APTES@
SiO2
Entry
1
2
3
4
5
6
7
8
Solvent
Base
Time (h) Yield b(%)
333.2
0.415
2.20
Ethanol
H2O
PEG400
iPrOH
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
Na2CO3
Na3PO4.12 H2O
NaOH
KOH
6
5
5
7
9
7
4
8
5
8
3
8
6
6.5
8
8
24
9
30
42
58
63
55
65
85
40
78
90
99
40
52
56
48
3
4
imine@ SiO2 293.5
0.374
0.274
2.29
3.26
Pd@imineꢀ
SiO2
210.1
THF
Ethanol/H2O (1:1)
PEG400/H2O (1:1)
PEG400/H2O (1:3)
PEG400/H2O (3:1)
iPrOH/H2O (1:1)
iPrOH/H2O (1:1)
iPrOH/H2O (1:1)
iPrOH/H2O (1:1)
iPrOH/H2O (1:1)
iPrOH/H2O (1:1)
iPrOH/H2O (1:1)
iPrOH/H2O (1:1)
iPrOH/H2O (1:1)
iPrOH/H2O (1:1)
9
10
11
12
13
14
15
16
17
18
19
Et3N
Cs2CO3
―
Na2CO3
Na2CO3
trace
No reaction
82c
98d
8
aReaction conditions: 4ꢀbromonitrobenzene (0.5 mmol), phenylboronic acid
(0.6 mmol), base (1.5 mmol), solvent (4 mL), Pd@imineꢀSiO2 (20 mg, 0.463
b
mol% Pd), base (1.5 mmol) ca. 27 ºC in air unless otherwise noted. Isolated
yield. cPd@imineꢀSiO2 (15 mg), dPd@imineꢀSiO2 (30 mg).
Figure 6: N2 adsorptionꢀdesorption isotherm of the prepared samples
Pd@imine-SiO2 catalyzed Suzuki-Miyaura reaction
Catalyst screening and base-solvent optimization
Further investigation was carried out to study the influence of the
bases on the same reaction condition. We have found a dramatic
effect of bases in the progress of the reaction. K2CO3 as a base gave
moderate results in most of the organic solvents (Table 2, entries 1ꢀ
9) and then we obtained the good results under the same conditions
in iPrOH/H2O (Table 2, entry 10). It was remarkable that, during this
study, Na2CO3 was found to be the most effective base with 99%
yield of the isolated product (Table 2, entry 11) under similar
conditions. However, some inorganic bases like Na3PO4.12H2O,
NaOH, KOH, Cs2O3 or organic bases such as Et3N resulted in lower
yields (Table 2, entries 12ꢀ16). However, the reaction did not
proceed in the absence of base (Table 2, entry 17). In our next move,
we then decided to study the influence of catalyst amount in the
reaction. Decreasing the amount of catalyst resulted in lower yields
under the same conditions (Table 2, entry 18) where higher amount
of catalyst did not improve the yield or reaction time (Table 3, entry
19). Then, we successfully applied the optimized condition for the
SuzukiꢀMiyaura crossꢀcoupling reaction of structurally different aryl
and heteroaryl bromide with arylboronic acid to produce the
corresponding biaryls (Table 3). The aryl bromides with electron
withdrawing and electron donating substituents underwent coupling
reaction with arylboronic acid effectively to give products in high
yields in iPrOH/H2O. The catalytic system is equally effective for
electronically diverse arylboronic acids in the present case. These
results are quite significant as the desired biaryls could be achieved
at room temperature using iPrOH/H2O as a solvent and with
relatively low catalyst loading. To study the scope limitation of the
present catalyst, we have also studied the crossꢀcoupling using
chlorine based substrate, but found no conversion under present
catalyst loading (Table 3, entries 21 and 22).
To optimize the reaction conditions, a series of experiments under
varied conditions in terms of solvents, bases and the amount of
catalyst for a model Suzuki coupling reaction was performed by
reacting 4ꢀbromonitrobenzene 1 with phenylboronic acid 2 as
illustrated in Table 2. The reaction was carried out successfully at
room temperature under open air without any special precautions.
Screening of solvents using K2CO3 as a base showed that the
reaction was proceed in both protic and aprotic solvents although
significant variation in yields were noticed. Generally, the presence
of water facilitates the solubility of the bases, which activates the
boronic acid, resulting in the enhancement of the course of the
reaction. This is maybe the reason for low yield in EtOH, PEG
(PEG400), iPrOH and THF (Table 2, entries 1, 3, 4, and 5). Although,
these solvents are usually good in conventional Pdꢀcatalyzed crossꢀ
coupling reactions,26 they exhibits lower efficiency under present
reaction conditions. When pure water was used, the reaction gave
poor results after 8 h (Table 2, entry 2). However, when water was
used as a coꢀsolvent with iPrOH or PEG400 quantitative yield was
achieved (Table 2, entries 6, 7, 8 and 9). It could be seen that the
yield of the product was promising and only a 58% yield was
obtained in PEG400 in the absence of H2O (Table 2, entry 3). With
the adding of H2O in PEG400, the yield increased (Table 2, entry 7)
then decreased when the amount of water increases in the solvent
system (Table 2, entry 8). The high yield of 85% was achieved when
the volume ratio of PEG400 to H2O was 1:1 (Table 2, entry 7). Thus
keeping H2O as coꢀsolvent, our study revealed that iPrOH/H2O (1:1)
combination is the best solvent system for our catalytic system.
Since the SuzukiꢀMiyaura reactions are implemented on a
regular basis in large scale for the preparation of pharmaceuticals,
agrochemicals, polymers, natural products and advanced materials in
chemical industries, we were keen to investigate the scope for scaleꢀ
up to prepare grams instead of milligrams of biaryls. With the
optimized reaction conditions and workup method on small scale, we
next evaluated their performance upon scaleꢀup. Thus, the reaction
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of excess 4ꢀbromonitrobenzene (5 mmol), phenylboronic acid (6 7). Being a solid catalyst, it could be easily recovered by
mmol), Na2CO3 (7.5 mmol), Pd@imineꢀSiO2 (100 mg, Pd 2.315 centrifugation after the reaction, and after thoroughly washing with
mol% Pd) and iPrOH/H2O (20 mL) was performed at room iPrOH/H2O followed by EtOAc and drying in oven, it can be used
temperature for 3 h. To our delight, we were able to isolate 0.9885 again for fresh coupling reaction. However, in successive cycle the
grams (99.3%) of 4ꢀnitroꢀ1,1'ꢀbiphenyl.
reaction requires additional time for completion. The palladium
content after 8th run was determined by ICPꢀAES analysis, and found
to be 0.24 ppm. Since, it is confirmed that no significant leaching of
palladium occurs during the catalysis and recovery, the deactivation
of the catalyst after successive cycles may influence the suitable
catalytic conversion (gradual decrease in yield) after successive
reuse. To compare these results with conventional catalyst, we have
examined the efficiency of Pd/C in the SuzukiꢀMiyaura reaction
under identical conditions (except the catalyst). Almost negligible or
trace amount of crossꢀcouple product was obtained when the
reaction of 4ꢀbromonitrobenzene and phenylboronic acid in the
presence of 0.46 mol% of 10 wt% Pd/C, Na2CO3 and isopropanolꢀ
water (1:1) was run for 3 h (compared 99% yield with Pd@imineꢀ
SiO2 complex within 3 h).
Table 3 SuzukiꢀMiyaura reaction of aryl bromides with different arylboronic
acidsa
Entry
1
2
3
4
5
6
7
8
X
R1
NO2
NO2
NO2
OMe
OMe
OMe
CHO
CHO
CHO
COMe
COMe
COMe
H
R2
H
4ꢀF
4ꢀOMe
H
4ꢀF
4ꢀOMe
H
4ꢀOMe
4ꢀF
Time (h)
Yieldb (%)
99
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
3
4
5.5
2
3
3.5
5
3.5
5
4
5
6
95
94
94
90
92
98
97
95
95
93
90
100
95
100
75
50
25
0
9
10
11
12
13
14
15
H
4ꢀOMe
4ꢀF
H
4ꢀMe
4ꢀF
1
2.5
3.5
H
H
96
16
17
18
Br
Br
Br
H
6
5.5
7
95
96
97
1
2
3
4
5
6
7
8
No. of cycle
gure 7: Recycling of the catalyst for the Suzukiꢀ Miyaura reac
4ꢀOMe
Though, the negative leaching test strongly suggests that the
Pd@imineꢀSiO2ꢀcatalyzed SuzukiꢀMiyaura reaction proceeds via
heterogeneous mechanism, the process of “release and redepositing”
back of the soluble palladium on the insoluble support during the
reaction could not be ruled out.25 To clarify the catalytic nature of
the palladium catalyst we designed catalyst poisoning test using
Hg(0). Mercury is recognized as the most powerful in poisoning
metals that are capable of forming an amalgam with Pt, Pd and
Ni.27,29 Hg(0)ꢀpoisoning test is performed by adding excess Hg(0) to
the reaction mixture prior to the addition of the catalyst. If the
reaction gets suppressed by Hg(0), it evident the presence of a Pd(0)
state; however, if the catalyst retains its activity, the presence of
palladium in high oxidation state can be assumed. For the mercury
poisoning test, Hg(0) (molar ratio to [Pd] ~ 400) was taken in the
reaction flask containing iPrOH/H2O before the addition of reactants
and stirred for 0.5 h at room temperature. To this suspension 4ꢀ
bromonitrobenzene (0.5 mmol), phenylboronic acid (0.6 mmol) and
K2CO3 were added, and the reaction mixture was stirred for required
time at room temperature. We observed that the catalyst retains its
activity in the presence of large excess of Hg(0) under vigorous
stirring, providing the corresponding biaryl in excellent yield
without any noticeable induction time (99%, 3 h). This infers that
this SuzukiꢀMiyaura reaction is promoted by the intact Pd@imineꢀ
SiO2 catalyst, and it is expected that the palladium remains in higher
oxidation state during the reaction course.
4ꢀF
19
20
21
22
Br
Br
Cl
Cl
OMe
OMe
Me
2ꢀMe
3ꢀCN
H
4
3
24
24
96
97
No reaction
No reaction
COMe
H
aReaction conditions: aryl bromide (0.5 mmol), arylboronic acid (0.6 mmol),
Pd@imineꢀSiO2 (20 mg, 0.463 mol% Pd), Na2CO3 (1.5 mmol), iPrOH/H2O (4
mL), ca. 27 ºC in air unless otherwise noted. bIsolated yield.
Distinguishing between homogeneous or heterogeneous pathway
Solid catalysts are often consider to follow heterogeneous pathway,
however, in certain condition they are found to function as
precatalyst for more active soluble palladium species or
nanoparticles. To evaluate actually catalytic species various control
experiments have been performed.17a, 24 First, we pretreated 20 mg
(0.463 mol% Pd) of Pd@imineꢀSiO2 with Na2CO3 in 50% iPrOH for
3 h and passed through a filter paper, and the filtrate was used for the
coupling reaction between 4ꢀbromonitrobenzene and phenylboronic
acid. However, negligible progress (2% by GCꢀMS analysis) was
observed upon further stirring of the catalystꢀfree solution for 3 h.
The leach palladium in the filtrate was found to be 0.18 ppm by
analysis with inductively coupled plasma atomic emission
spectrometry (ICPꢀAES). While this results support the strong
coordination and stability of palladium ions in the catalyst which
prohibits metal leaching during the reaction, it should not be
consider solely for the heterogeneity of the supported catalysts.
To confirm the heterogeneity, we performed reusability test for
the Pd@imineꢀSiO2ꢀcatalyzed SuzukiꢀMiyaura reaction. On the
basis of our recyclability test, we found that the Pd@imineꢀSiO2
catalyst could be consistently used up to eight consecutive coupling
reactions of phenylboronic acid with 4ꢀbromonitrobenzene (Figure
Conclusions
In summary, we have developed a novel palladiumꢀbased
heterogeneous catalyst by immobilizing Pd(OAc)2 onto imineꢀ
functionalized silica gel via coordination. The synthesized
catalyst exhibited excellent activity for the SuzukiꢀMiyaura
crossꢀcoupling reactions of aryl bromides with arylboronic
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acids at low catalyst loading (20 mg, 0.463 mol% Pd). This acetone. The mixture was stirred at room temperature for 24 h. The
method proceeds under aqueous media and with a recyclable solid powder became charcoalꢀgrey in color during the stirring. The
catalytic system, offering an environmentally benign alternative resulting solid was washed with acetone in Soxhlet and then dried.
to the existing protocols.
Experimental Section
General Information
The prepared catalyst is designated as Pd@imineꢀSiO2 and stored for
further applications.
General Information about Catalytic Experiments
SuzukiꢀMiyaura crossꢀcoupling reactions were carried out under
aerobic condition at room temperature (ca. 28 °C). Progress of the
reactions was monitored by using aluminumꢀcoated TLC plates
(Merck silica gel 60F254) under UV lamp. The products were isolated
by column chromatographic technique using silica gel (60ꢀ120
mesh). The isolated products were characterized by comparing their
1H NMR, mass spectral data and melting point with the authentic
samples.
All chemicals were obtained commercially and used as received
without further drying or purification. FTꢀIR spectra (4000ꢀ250 cmꢀ
1) were recorded in KBr or CHCl3 on a Shimadzu Prestigeꢀ21 FTꢀIR
spectrophotometer. For the N2 adsorption experiments, the samples
were first degassed under vacuum for 2 h at 150 °C. Subsequently,
the nitrogen adsorptionꢀdesorption isotherms were determined using
liq. N2 at ꢀ78 °C. The surface area of the samples was calculated
according to the BET equation. Pore size distribution was evaluated
using BJH algorithm. 1H spectra were recorded in CDCl3 using
TMS as an internal standard on a JEOL, JNM ECS NMR
spectrometer operating at 400 MHz and on Advance DPX 300 MHz
FTꢀNMR spectrometer operating at 300 MHz. Gas chromatographic
analysis coupled with mass spectrometry (GCꢀMS) was performed
on an Agilent Technologies GC system 7820A coupled with a mass
detector 5975 and SHRXIꢀ5MS column (15 m length, 0.25 mm inner
diameter, 0.25 micron film thickness). The melting points of the
products were determined by using BUCHI B450 melting point
apparatus. SEM analyses were carried out with “JEOL, JSM Model
6390 LV” scanning electron microscope, operating at an accelerating
voltage of 15 kV. EDX spectra as well as element mapping analyses
were also performed in the same instrument attached to the scanning
electron microscope. Palladium loading in the catalyst and its
possible leaching after catalytic reaction was determined using
inductively coupled plasmaꢀatomic emission spectrometry (ICPꢀ
AES) on a Thermo Electron IRIS Intrepid II XSP DUO. The course
of the reaction was followed by TLC on silica gel plates (Merck,
silica gel 60F254), using nꢀhexaneꢀethyl acetate as eluent. Reaction
Typical Procedure for Suzuki-Miyaura Reactions of Aryl
Halides Using Palladium Complex
A 50 ml roundꢀbottom flask was charged with a mixture of aryl
halide (0.5 mmol), aryl boronic acid (0.6 mmol), Na2CO3 (1.5
mmol), catalyst (20 mg, Pd 0.463 mol% Pd) and solvent (4 mL). The
mixture was stirred at room temperature for the required time. After
completion, the reaction mixture was subjected to centrifugation and
the residual solid after filtration was washed with the same solvent
(4 mL) for three times. The filtrate was diluted with water (10 mL)
and extracted with ether (3×10 mL). The resultant organic phases
was washed with brine (2×10 ml) and dried over anhydrous Na2SO4,
filtered and evaporated under reduced pressure. The residue was
subjected to silica gel column chromatography (ethyl acetateꢀhexane,
1:9) to obtain the desired products. For recycling experiments,
residue catalyst after filtration was washed with excess water (3×5
ml) and diethyl ether (3×5 ml) in sequence. After drying the
recovered catalyst at 100 °C overnight in an oven, the residual
catalyst was subjected to consequent run of the crossꢀcoupling by
charging with the required amount of substrates (aryl halide,
arylboronic acid), Na2CO3 and isopropanolꢀwater, without further
addition of catalyst. The isolated products were confirmed by
comparing their 1H NMR, mass spectral data and melting point with
that of reported samples.
1
products were confirmed by comparing the H spectra with those
reported in the literature. Silica gel with particle size 60ꢀ120 mesh
was purchased from SRL Chemicals, India. 3ꢀ(triethoxysilyl)propanꢀ
1ꢀamine or 3ꢀaminopropyltriethoxysilane (APTES) was purchased
from Sigma Aldrich. Pd(OAc)2 was purchased from Spectrochem
Pvt. Ltd., India.
Representative example of a Suzuki-Miyaura coupling on a 5
mmol Scale: Reaction between 4-bromonitrobenzene and
phenylboronic acid
Catalyst preparation
APTES functionalized Silica gel: Modification of Silica Surface
A 100 ml roundꢀbottom flask was charged with a mixture of 4ꢀ
bromonitrobenzene (5 mmol), phenylboronic acid (6 mmol), Na2CO3
(7.5 mmol), Pd@imineꢀSiO2 (100 mg, Pd 2.315 mol% Pd) and
iPrOH/H2O (20 mL). The mixture was stirred at room temperature
for 3 h. The completion of the reaction was monitored by using TLC.
After completion, the reaction mixture was subjected to
centrifugation and the filtrate was diluted with water (25 mL) and
extracted with ether (3×25 mL). The resultant organic phase was
washed with brine (3×25 ml) and dried over anhydrous Na2SO4,
filtered and evaporated under reduced pressure. The residue was
subjected to silica gel column chromatography (ethyl acetateꢀhexane,
1:9) to obtain the desired products. The analytically pure product 4ꢀ
nitroꢀ1,1'ꢀbiphenyl was obtained in 0.9885 grams (99.3%).
The solid silica gel (60ꢀ120 mesh) was dehydrated by heating at 100
°C for 2 h prior to use. Silica (5.0 g) was refluxed with 2 mmol
(442.7 mg) of APTES for 6ꢀ7 h in 50 mL of dry toluene under N2
atmosphere. After cooling, the solids were separated out by filtration
and washed repeatedly through Soxhlet extraction with toluene. The
solid product was dried overnight in an oven at 100 °C and the
resulting material is denoted as APTES@SiO2.
Synthesis of Schiff-base functionalized Silica gel
The Schiffꢀbase ligand was synthesized by adding 1.25 g of
APTES@SiO2 and 1.06 mmol (129.4 mg) of salicylaldehyde in the
ethanol solution and stirring for 6 h under reflux. The resulting
yellow colored solid was filtered and washed repeatedly through
Soxhlet extraction by ethanol followed by acetone and dried in oven
at 100 °C for overnight and denoted as imine@SiO2.
Acknowledgements
We gratefully thank UGC, New Delhi (India) for financial support
(No.41ꢀ254/2012 (SR)). The authors are also thankful to UGC, New
Delhi for Special Assistance Program (UGCꢀSAP) and the DST,
New Delhi, for financial assistance under the DSTꢀFIST program to
the Department of Chemistry, Dibrugarh University. T. Begum and
Immobilization of Pd(OAc)2 onto imine@SiO2
Finally, palladium derived catalyst was prepared by adding 1 g of
yellowish imine@SiO2 solid with 50 mg of Pd(OAc)2 in 20 mL of
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M. Mondal are thankful to UGC, New Delhi for financial assistance
in the form of UGCꢀBSR (RFSMS) Fellowship. SAIF (STIC),
Cochin is acknowledged for ICPꢀAES analytical service.
Org. Process Res. Dev., 2008, 12, 1209ꢀ1217; (c) J. Brown, A.
Chighine, M. A. Colucci, N. Galaffu, S. C. Hirst, H. M. Seymour, J.
J. Shiers, R. D. Wilkes, J. G. Williams and J. Wilson, Tetrahedron
Lett., 2008, 49, 4968ꢀ4971.
Notes and references
14. V. Polshettiwar, C. Len and A. Fihri, Coord. Chem. Rev., 2009, 253
,
a
Department of Chemistry, Dibrugarh University, Dibrugarh 786004,
2599ꢀ2626.
Assam, India
15. (
a
) H. Zhao, J. Peng, R. Xiao and M. Cai, J. Mol. Catal. A.
: Chem.,
b Department of Chemical Sciences, Tezpur University, Napaam, Tezpur
2011, 337, 56ꢀ60; (
b
) S. Jana, S. Haldar and S. Koner, Tetrahedron
784028, Assam, India
Lett., 2009, 50, 4820ꢀ4823.
16. Q. Yang, S. Ma, J. Li, F. Xiao and H. Xiong, Chem
2495ꢀ2497.
E-mail:dr.pradip54@gmail.com, utbora@yahoo.co.in,
ubora@tezu.ernet.in
.
Commun., 2006,
1.
(
a
) J. Hassan, M. Sevignon, C. Gozzi, E. Schultz and M. Lemaire, 17. F. Liu, G. Feng, M. Lin, C. Wang, B. Hu and C. Qi, Journal of
Chem. Rev., 2002, 102, 1359ꢀ1469; ( ) K. C. Nicolaou, P. G. Bulger Colloid and Interface Science, 2014, 435, 83ꢀ90.
and D. Sarlah, Angew. Chem., Int. Ed., 2005, 44, 4442ꢀ4489; ( ) X. F. 18. ( ) C. M. Crudden, M. Sateesh and R. Lewis, J. Am. Chem. Soc.,
2005, 127, 10045ꢀ10050; ( ) L. Yin and J. Liebscher, Chem. Rev.,
) W. Chen, P. Li and L. Wang, Tetrahedron
b
c
a
Wu, P. Anbarasan, H. Neumann and M. Beller, Angew. Chem., Int.
Ed., 2010, 49, 9047ꢀ9050.
b
2007, 107, 133ꢀ173; (
c
,
2.
(
a
) N. Miyaura and A. Suzuki, Chem. Rev., 1995, 95, 2457ꢀ2483; (
A. F. Littke and G. C. Fu, Angew. Chem., Int. Ed., 2002, 41, 4176ꢀ 19. (
4211; ( ) S. Kotha, K. Lahiri and D. Kashinath, Tetrahedron, 2002,
58, 9633ꢀ9695; (
6722ꢀ6737; ( ) A. Suzuki and Y. Yamamoto, Chem. Lett., 2011, 40
894ꢀ901; (
and V. Polshettiwar, Chem. Soc. Rev., 2011, 40, 5181ꢀ5203; (
b
)
2011, 67, 318ꢀ325.
a
) V. Polshettiwar, C. Len and A. Fihri, Coord. Chem. Rev., 2009,
c
253, 2599ꢀ2626; (
b
) P. Veerakumar, M. Velayudham, K.ꢀL. Lu and
) A.
Martínez, J. L. Krinsky, I. Peñafiel, S. Castillón, K. Loponov, A.
d
) A. Suzuki, Angew. Chem., Int. Ed., 2011, 50
,
S. Rajagopal, Appl. Catal. A: Gen. 2013, 455, 247ꢀ260; (c
e
,
f
) A. Fihri, M. Bouhrara, B. Nekoueishahraki, J.ꢀM. Basset
Lapkin, C. Godard and C. Claver, Catal. Sci. Technol., 2015, 5, 310ꢀ
g
) A.
319.
Balanta, C. Godard and C. Claver, Chem. Soc. Rev., 2011, 40, 4973ꢀ 20. (
a
) N. T. S. Phan, M. V. D. Sluys and C. W. Jones, Adv. Synth.
4985; ( ) J. D. Sellars and P. G. Steel, Chem. Soc. Rev., 2011, 40
h
,
Catal., 2006, 348, 609ꢀ679; (b) L. Yin and J. Liebscher, Chem. Rev.,
5170ꢀ5180; (
1873ꢀ1876.
i
) M. Mondal and U. Bora, Green Chem., 2012, 14
,
2007, 107, 133ꢀ173; (
Chem. Rev., 2009, 253, 2599ꢀ2626; (
111, 2251ꢀ2320; ( ) N. Fukaya, M. Ueda, S. Onozawa, K. K. Bando,
T. Miyaji, Y. Takagi, T. Sakakura and H. Yasuda, J. Mol. Catal. A.
Chem., 2011, 342-343, 58ꢀ66; ( ) S. Paul and J. H. Clark, Green
Chem., 2003, , 635ꢀ638.
21. ( ) J. P. Collman, V. J. Lee, C. J. KellenꢀYuen, X. Zhang, J. A. Ibers
and J. I. Brauman, J. Am. Chem. Soc., 1995, 117, 692ꢀ703; ( ) K. J.
Ballus Jr., A. K. Khanmamedova, K. M. Dixon and F. Bedioui, Appl.
c
) V. Polshettiwar, C. Len and A. Fihri, Coord.
d
) A. Molnar, Chem. Rev., 2011,
3.
(a
) C. A. Fleckenstein and H. Plenio, Chem. Soc. Rev., 2010, 39, 694ꢀ
e
711; (
b
) F. Bellina, A. Carpita and R. Rossi, Synthesis, 2004, 15
,
:
2419ꢀ2440.
f
4. J. Zhou, X. Guo, C. Tu, X. Li and H. J. Sun, Organomet
.
Chem.,
5
2009, 694, 697ꢀ702.
a
5.
(a
) W. A. Herrmann, C. P. Reisinger and M. J. Spiegler, Organomet.
Chem., 1998, 557, 93ꢀ96; ( ) O. Navarro, H. Kaur, P. Mahjoor and S.
P. Nolan, J. Org. Chem., 2004, 69, 3173ꢀ3180; ( ) Z. Xiong, N.
Wang, M. Dai, A. Li, J. Chen and Z. Yang, Org. Lett., 2004, , 3337ꢀ
3340; ( ) N. Marion and S. P. Nolan, Acc. Chem. Res., 2008, 41
1440ꢀ1449.
b
b
c
Catal. A Gen., 1996, 143,159ꢀ173; (c) C. Heinrichs and W. F.
6
Holderich, Catal. Lett., 1999, 58, 75ꢀ80.
d
,
22. (
Molnar, Chem. Rev., 2011, 111, 2251ꢀ2320; (
Niu, R. Li and J. Ma, Catal. Sci. Technol., 2014,
W. R. Reynolds, P. Plucinskibc and C. G. Frost, Catal. Sci. Technol.,
2014, , 948ꢀ954; ( ) A. Martínez, J. L. Krinsky, I Peñafiel, S.
Castillón, K. Loponov, A. Lapkin, C. Godard and C. Claver, Catal.
Sci. Technol., 2015, , 310ꢀ319.
a
) L. Yin and J. Liebscher, Chem. Rev., 2007, 107, 133ꢀ173; (
) P. Wang, H. Liu, J.
, 1333ꢀ1339; (
b) A.
c
6.
(
a
b
) B. Tao and D. W. Boykin, J. Org. Chem., 2004, 69, 4330ꢀ4335;
) J. H. Li and W. J. Liu, Org. Lett., 2004, , 2809ꢀ2811.
4
d)
(
6
7. D. P. Costa and S. M. Nobre, Tetrahedron Lett., 2013, 54, 4582ꢀ
4584.
4
e
8. E. Amadio, A. Scrivanti, V. Beghetto, M. Bertoldini, M. M. Alam
5
and U. Matteoli, RSC Adv., 2013,
3, 21636ꢀ21640.
23. C. Sarmah, D. Sahu and P. Das, Catal. Today, 2012, 198, 197ꢀ203.
9.
(a) B. Banik, A. Tairai, N. Shahnaz and P. Das, Tetrahedron Lett., 24. K. Dhara, K. Sarkar, D. Srimani, S. K. Saha, P. Chattopadhyay and
2012, 53, 5627ꢀ5630; (b) A. Dewan, U. Bora and G. Borah,
A. Bhaumik, Dalton Trans., 2010, 39, 6395ꢀ6402.
Tetrahedron Lett., 2014, 55, 1689ꢀ1692.
25. (
2014, 126, 85ꢀ93; (
Liew, Appl. Surf. Sci., 2009, 255, 9463ꢀ9468.
26. ( ) M. V. Khedkar, T. Sasaki and B. M. Bhanage, RSC Adv., 2013,
7791ꢀ7797; ( ) X. Xie and S. Ma, Chem. Commun., 2013, 49, 5693ꢀ
5695; ( ) S. G.Modha, V. P.Mehta and E. Van der Eycken, Chem.
a
) A. R. Hajipour, Z. Shirdashtzade and G. Azizi, J. Chem. Sci.,
10. A. Dewan, Z. Buragohain, M. Mondal, G. Sarmah, G. Borah and U.
Bora, Appl. Organomet. Chem., 2014, 28, 230ꢀ233.
11. M. Mondal and U. Bora, Tetrahedron Lett., 2014, 55, 3038ꢀ3040.
12. R. Martin and S. L. Buchwald, Acc. Chem. Res., 2008, 41, 1461ꢀ
1473.
b
) X. Tong, Y. Zhao, T. Huang, H. Liu and K. Y.
a
3,
b
c
13. (
a
) D. Barbaras, J. Brozio, I. Johannsen and T. Allmendinger, Org.
Soc. Rev., 2013, 42, 5042ꢀ5055
Process Res. Dev., 2009, 13, 1068ꢀ1079; (
b
) M. J.Girgis, L. E. 27. J. A. Widegren and R. G. Finke, J. Mol. Catal. A: Chem., 2003, 198,
Kuczynski, S. M. Berberena, C. A. Boyd, P. L. Kubinski, M. L.
Scherholz, D. E. Drinkwater, X. Shen, S. Babiak and B. G. Lefebvre,
317ꢀ341.
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28. Y. Kitamura, S. Sako, A. Tsutsui, Y. Monguchi, T. Maegawa, Y.
Kitade and H. Sajikia, Adv. Synth. Catal., 2010, 352, 718ꢀ730.
29. (
Lavalleye, A. F. Sowinski, A. N. Izumi, S. S. Moore, D. W. Brown
and E. M. Staudt, Organometallics, 1985, , 1819ꢀ1830; ( ) C. Paal
and W. Hartmann, Chem. Ber., 1918, 51, 711ꢀ737.
a) G. M. Whitesides, M. Hackett, R. L. Brainard, J. P. P. M.
4
b
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Graphical Abstract
Palladium-Schiff-base-Silica Framework as a Robust and Recyclable Catalyst for the
Suzuki-Miyaura Cross-Coupling in Aqueous Media
Tahshina Beguma, Manoj Mondala, Pradip K. Gogoia and Utpal Bora*ab
Br
X
Na2CO3,
iPrOH-H2O
R1
R2
O
O
O
X
+
R1
R2
O
O
O
O
N(CH2)3Si
Pd
B(OH)2 r.t., 1-7 h
Si(H2C)3N
O
90-100%
18 examples
R1= H, NO2, OMe, CHO, Ac
R2= H, F, OMe, X= C, N
Pd@imine-SiO2