Silica tethered Pd-DABCO complex: An efficient and reusable catalyst for suzuki-miyaura reaction

Article abstract of DOI:10.1007/s10562-012-0912-3

A palladium-based catalyst supported on DABCO-functionalized silica was successfully prepared by a facile procedure. The prepared heterogeneous catalyst showed a high activity for the Suzuki-Miyaura reaction of aryl bromides, affording excellent yield in all the cases investigated. Interestingly, the catalyst could be quantitatively recovered from the reaction mixture and recycled for five times without any significant loss in activity. Furthermore, this protocol could be extended to the palladium catalyzed synthesis of annulated pyrazines. Springer Science+Business Media, LLC 2012.

Full text of DOI:10.1007/s10562-012-0912-3

Catal Lett (2012) 142:1388–1396
DOI 10.1007/s10562-012-0912-3
Silica Tethered Pd–DABCO Complex: An Efficient and Reusable
Catalyst for Suzuki–Miyaura Reaction
•
Arjun Kumbhar Santosh Kamble
•
•
•
Sanjay Jadhav Gajanan Rashinkar
Rajashri Salunkhe
Received: 13 July 2012 / Accepted: 21 September 2012 / Published online: 9 October 2012
Ó Springer Science+Business Media New York 2012
Abstract
A
palladium-based catalyst supported on
mediated by palladium species in the presence of phos-
phine ligands in homogeneous conditions which result in
the difficulty in product separation and reuse of expensive
catalysts continuously. One means to circumvent this
problem is the design and synthesis of new improved
heterogeneous catalysts with superior activity, easy sepa-
ration and facile reusability. In the last few years, numer-
ous methods have been developed to immobilize palladium
on a large variety of solid supports such as mesoporous and
amorphous silica [12–15], colloidal supported nanoparti-
cles [16], polymers [17–20], sepiolite clay [21], carbon
[22], hydrotalcite [23], biopolymers [24–26], zeolite [27],
and perovskite [28]. More recently, new smart materials,
such as magnetic nanoparticles have emerged with great
potential for catalyst recovery [29–31].
DABCO-functionalized silica was successfully prepared by
a facile procedure. The prepared heterogeneous catalyst
showed a high activity for the Suzuki–Miyaura reaction of
aryl bromides, affording excellent yield in all the cases
investigated. Interestingly, the catalyst could be quantita-
tively recovered from the reaction mixture and recycled for
five times without any significant loss in activity. Fur-
thermore, this protocol could be extended to the palladium
catalyzed synthesis of annulated pyrazines.
Keywords DABCO ꢀ Suzuki reaction ꢀ Heterogeneous
catalysis ꢀ Catalyst recycling
1 Introduction
As a consequence of the need for robust heterogeneous
catalysts, the development of organic modified silica has
gained tremendous attention in recent years and the advent
of mesoporous silicas has spurred this further. The organic
groups can be robustly anchored on the surface of silica to
generate catalytic sites [32]. For useful activity, modified
silicas generally possess a balance of high active site
loading, together with a high surface area, large pore sizes
and customized particle size. This ensures access to sub-
strates and solvents if needed and, coupled with good
chemical, mechanical and thermal stabilities, covers some
of the broad spectrum of performance requirements.
1,4-Diazabicyclo[2.2.2]octane (DABCO), is a cage-like,
small diazabicyclic molecule with medium-hindrance and
received considerable attention as an inexpensive, eco-
friendly, highly reactive and non-toxic catalyst for various
organic transformations. The first use of DABCO as a
ligand in Pd catalyzed phosphine-free cross-coupling
reaction was reported in 2004 [33]. In continuation of our
work related to green chemistry [34, 35], we report herein
Palladium catalyzed Suzuki–Miyaura reaction is one of the
most effective synthetic method used for the construction
of biaryl units [1–6] which are molecular components in
pharmaceuticals. They are also present in many herbicides,
alkaloids like ancistrocladine as well as in engineering
materials such as conducting polymers, molecular wires
and liquid crystals [7–11]. Tremendous amounts of results
have been accumulated for this reaction and new findings
are still increasing. One of the current interests in this
reaction is the development of a catalyst of high perfor-
mance involving sustainable and environmentally benign
reaction conditions. The Suzuki–Miyaura reaction is often
A. Kumbhar ꢀ S. Kamble ꢀ S. Jadhav ꢀ G. Rashinkar ꢀ
R. Salunkhe (&)
Department of Chemistry, Shivaji University, Kolhapur 416004,
Maharashtra, India
e-mail: rss234@rediffmail.com
123

Silica Tethered Pd–DABCO Complex
1389
2.3 General Procedure for the Suzuki–Miyaura
Reaction
silica tethered DABCO–Pd complex that constitute a
highly effective system for the Suzuki–Miyaura reaction of
aryl halides with aryl boronic acids.
An oven-dried Schlenk flask, equipped with a magnetic stir
bar, septum and a condenser was charged with aryl halide
(1.0 mmol), arylboronic acid (1.2 mmol), K2CO3 (2 mmol),
catalyst 3 (0.130 g, 1 mol%) and 5 mL of 95 % ethanol. The
flask was immersed and stirred in an oil bath at 80 °C. Upon
complete consumption of starting materials as determined by
TLC analysis, catalyst was separated out by filtration and the
water (20 mL) was added. The filtrate was extracted thrice
with diethyl ether (10 mL). The combined organic layers
were collected, dried over anhydrous Na2SO4 and concen-
trated in vacuum to afford product having sufficient purity
for further characterization.
2 Experimental
2.1 General Remarks
1H NMR and 13C NMR spectra were recorded on a Brucker
AC (300 MHz for H NMR and 75 MHz for 13C NMR)
1
spectrometer using CDCl3 and DMSO-d6 as solvent and
tetramethylsilane as an internal standard. Infrared spectra
were recorded on a Perkin-Elmer FTIR spectrometer. The
samples were examined as KBr discs *5 % w/w. A JEOL-
6360 scanning electron microscope was used for scanning
electron microscopy (SEM) observations and Energy Dis-
persive X-ray Spectroscopy (EDS) analysis. TGA–DTA
analysis was recorded on SDS Q600 N20.9 in nitrogen.
Melting points were determined with a DBK melting point
apparatus and are uncorrected. All the chemicals were
obtained from Aldrich and were used without further
purification. The 3-n-propyl-1-azonia-4-azabicyclo[2.2.2]
octane chloride (DABCO-SIL) (1) was synthesized by
following literature procedure [36].
2.4 General Procedure for the Synthesis of Pyrido
[2,3-b]pyrazine Derivatives (10 a–c)
An oven-dried Schlenk flask, equipped with a magnetic stir bar,
septum and a condenser was charged with 7-bromo-2,3-diphe-
nylpyrido[2,3-b]pyrazine 9 (1.0 mmol), arylboronic acid
(1.2 mmol), K2CO3 (2 mmol), catalyst 3 (0.130 g, 1 mol%)
and 5 mL of THF. The reaction mixture was refluxed for 14 h.
Upon complete consumption of starting materials as determined
by TLC analysis, catalyst was separated out by filtration and the
water (20 mL) was added. The filtrate was extracted with die-
thyl ether (3 9 10 mL). The combined organic layers were
collected, dried over anhydrous Na2SO4, concentrated in vac-
uum to afford product which was purified by silica gel column
chromatography (eluent: n-hexane/ethyl acetate = 9:1).
2.2 Preparation of the Silica Tethered Pd–DABCO
Catalyst (3)
2.2.1 Synthesis of DABCO@SiO2 (2)
Mesoporous amorphous silica gel (average particle size
60–120 mesh) was activated in order to enhance the con-
tent of silanol groups on the silica surface. The silica gel
(10 g) was mixed with 6 M hydrochloric acid (100 mL)
and stirred at reflux temperature for 24 h. The solid product
was recovered by filtration, washed with doubly distilled
water to neutral pH and dried under vacuum at 80 °C for
12 h. The activated silica gel (10 g) was heated at 100 °C
with DABCO-SIL (3.52 g, 10 mmol) in dry toluene for
24 h. The product was filtered off and washed with hot
toluene for 12 h in continuous extraction apparatus
(Soxhlet) and then dried in oven at 100 °C for overnight to
give 2 (12.9 g).
2.5 Spectral Data for Selected Compounds
2.5.1 4-Methyl-1,10,40.100-terphenyl (Table 3,
product entry 8)
(White solid, mp 322–324 °C);1H NMR (CDCl3,
300 MHz): dH (ppm) 2.44 (s, 3H); 7.26–7.28 (m, 2H);
7.33–7.39 (m, 1H); 7.44–7.49 (m, 2H); 7.53–7.56 (m, 1H);
7.62–7.67 (m, 4H); 13C NMR (CDCl3, 75 MHz): dC (ppm)
21.18, 126.87, 127.01, 127.21, 127.27, 127.43, 128.75,
129.50, 136.92, 137.86, 139.84, 140.06, 140.82.
2.5.2 4-(40-Fluoro-phenyl) benzophenone (Table 3,
product entry 11)
2.2.2 Synthesis of Pd–DABCO@SiO2 (3)
(White solid, mp 145–148 °C); 1H NMR (CDCl3,
300 MHz): dH (ppm) 7.13–7.21 (m, 2H); 7.48–7.53 (m,
2H); 7.58–7.66 (m, 5H); 7.82 (d, 2H, J = 6.9 Hz); 7.90 (d,
2H, J = 8.1 Hz); 13C NMR (CDCl3, 75 MHz): dC (ppm)
116.07, 126.75, 128.26, 128.84, 128.94, 129.98, 130.75,
132.29, 136.13, 136.28, 137.71, 144.12.
A mixture of 2 (2.0 g) and Pd(OAc)2 (0.044 g, 0.19 mmol)
in dry acetone (20 mL) was stirred at room temperature for
24 h. The solid product was filtered by suction, washed
with acetone, distilled water and acetone successively and
dried under vacuum at 60 °C for 4 h to give yellow
palladium complex of Pd–DABCO@SiO2 (3) (2.03 g).
123

1390
A. Kumbhar et al.
2.5.3 2-Phenyl-9H-fluorene (Table 3, product entry 12)
135.75, 137.78, 138.10, 138.22, 138.62, 143.70, 149.06,
154.96, 156.70, 158.10; ESI–MS (M?= 409); Elemental
analyses; (Found: C = 85.1, H = 4.7, N = 10.2 %; Calc. for
C29H19N3: C = 85.0, H = 4.8, N = 10.2 %).
(White solid, mp 295–297 °C); 1H NMR (CDCl3, 300 MHz):
dH (ppm) 3.97 (s, 2H); 7.29–7.38 (m, 2H); 7.40–7.47(m, 2H);
7.53–7.65 (m, 4H);7.79(d, 2H, J = 7.0 Hz); 7.84(s, 1H);13
C
NMR (CDCl3, 75 MHz): dC (ppm) 36.94, 119.90, 120.04,
123.76, 124.94, 125.99, 126.65, 126.79, 127.00, 127.15,
128.66, 139.95, 140.93, 141.43, 141.56, 143.23, 143.67.
3 Results and Discussion
The process for preparation of the silica tethered
Pd–DABCO catalyst (acronymed as Pd–DABCO@SiO2) 3
is schematically described in Scheme 1.
2.5.4 10b
(Yellow solid, mp 182–184 °C); 1H NMR (CDCl3,
300 MHz): dH (ppm) 2.41 (s, 6H); 3.80 (s, 3H); 7.31–7.40
(m, 6H); 7.45 (s, 2H); 7.55 (d, 2H, J = 7.8 Hz); 7.64
(d, 2H, J = 6.6 Hz); 8.59 (d, 1H, J = 1.8 Hz); 9.41 (d, 2H,
J = 1.8 Hz); 13C NMR (CDCl3, 75 MHz): dC (ppm) 16.17,
59.46, 128.02, 128.27, 129.22, 129.43, 129.89, 130.40,
131.63, 132.10, 134.18, 134.39, 136.09, 137.91, 138.12,
138.59, 148.26, 152.89, 155.05, 156.73, 158.15; ESI–MS
(M?= 433); Elemental analyses; (Found: C = 80.4,
H = 6.2, N = 9.7, O = 3.7 %; Calc. for C28H23N3O:
C = 80.3, H = 6.3, N = 9.7, O = 3.7 %).
Initially, monoquaternization of DABCO with 3-chlor-
opropyltriethoxysilane in ethanol:acetone (1:1 v/v) mixture
under reflux gave silylated DABCO-SIL 1 in 99 % yield.
The hydroxyl groups on the surface of the silica gel were
enriched by refluxing silica in aqueous solution of acid
facilitating the surface modification. The resulting acti-
vated silica gel was then functionalized with DABCO by
heating with 1 in toluene at 100 °C to afford DAB-
CO@SiO2 2. The reaction of 2 with Pd(OAc)2 in dry
acetone lead to the formation of 3.
FT–IR spectroscopy was performed to confirm the
grafting of DABCO in the matrix of silica as well as for-
mation of 3 (Fig. 1).
2.5.5 10c
FT–IR spectra are rather inconclusive, mainly due to
relatively minor changes that reflect the low amounts of the
anchored organic groups and palladium (II) acetate and the
fact that the strongly dominant bands of the support (mainly
due to O–H and Si–O bonds) remain invariant. Neverthe-
less, in comparison with pure silica, 2 and 3 displayed new
peaks at 2,949 cm-1 [C-H methylene antisym. str.],
(Yellow solid, mp 162–165 °C); 1H NMR (CDCl3, 300 MHz):
dH (ppm)7.34–7.41(m,6H);7.49–7.63(m,6H);7.65–7.69(m,
2H); 7.92–7.98 (m, 3H); 8.63(d, 1H, J = 2.7 Hz); 9.33 (d, 1H,
J = 2.1 Hz); 13C NMR (CDCl3, 75 MHz): dC (ppm) 125.06,
125.41, 126.29, 127.01, 127.96, 128.14, 128.30, 128.64,
129.24, 129.38, 129.94, 130.37, 131.39, 133.96, 135.09,
Scheme 1 Synthesis of
Pd–DABCO@SiO2 3
123

Silica Tethered Pd–DABCO Complex
1391
100
95
90
85
80
0
200
400
600
800
1000
Fig. 1 FTIR spectra of (a) silica gel, (b) DABCO@SiO2 and
(c) Pd–DABCO@SiO2
Temperature (0C)
Fig. 3 TGA spectrum of Pd–DABCO@SiO2
2,876 cm-1 [C-H methylene sym. str.], 1,463 cm-1[tC–N
DABCO], 1,057 and 778 cm-1 [t(asym)NC3 and t(sym)NC3
of DABCO]. The peaks assigned to the silica network were
observed at 1,090, 809 and 472 cm-1 (t Si–O–Si). These
facts evidenced that the DABCO was successfully anchored
on the silica surface. The formation of 3 was confirmed by
detecting Pd species using energy dispersive X-ray spec-
troscopy (EDS) (Fig. 2).
silica gel. This loading was also supported by the elemental
analysis of the N content of 3.
The elemental analysis revealed the presence of C, H
and N content in 2 and 3. It was found that 0.84 mmol g-1
of DABCO was grafted on the silica surface in 2. The
content of anchored Pd complex in 3 as determined by
ICP–AES was suggested to be 0.079 mmol g-1 and for
0.130 g of catalyst, the loading of Pd was calculated to be
1 mol% with respect to 1 mmol of reactants.
TGA analysis was carried out to investigate the thermal
stability of 3. TGA profile of 3 is shown in Fig. 3. Weight
loss is mainly divided into three temperature regions:
below 160 °C, 160–420 °C and above 420 °C. Weight loss
during heating from 25 to 160 °C was assigned to the loss
of loosely bound water (3.4 %). The large weight loss of
around 12–13 % at temperatures between 160 °C and
620 °C was due to the decomposition of covalently bonded
organics. The TGA analysis of catalyst indicated that
0.85 mmol g-1 of the DABCO was immobilized on the
Scanning electron microscopy studies were undertaken
to observe the morphology of Pd supported on silica gel.
The micrograph obtained by SEM showed the particles
with irregular shape with diameters in the range of several
hundreds of nanometer up to a few micrometers (Fig. 4).
To evaluate the catalytic activity of 3 in mediating the
Suzuki–Miyaura reaction, coupling of bromobenzene 4 and
phenyl boronic acid 5 was examined to optimize various
Fig. 2 EDS spectrum of
Pd–DABCO@SiO2
123

1392
A. Kumbhar et al.
Fig. 4 Scanning electron microscopy image (a) normal view and (b) magnified view
Table 2 Effect of the amount of catalyst on the Suzuki–Miyaura
cross-coupling reaction
Table 1 Optimization studies of the Suzuki–Miyaura cross-coupling
reaction
B(OH)2
Yield (%)a
Br
Entry
Amount of
catalyst (mol%)
Time (h)
Pd-DABCO@SiO2
3
+
Base, solvent, reflux
1
2
3
4
0.1
0.5
1.0
2.0
4
2
1
1
80
86
95
96
6
5
4
Entry
Base
Solvent
Time (h)
Yield (%)a
1
2
K2CO3
NaOH
95 % EtOH
95 % EtOH
95 % EtOH
95 % EtOH
95 % EtOH
EtOH
1
4
2
4
5
1
6
3
6
3
96
75
90
79
72
92
70
84
69
75
Reaction conditions: bromobenzene (1.0 mmol), phenylboronic acid
(1.2 mmol), 3 (0.1–2.0 mol%), K2CO3 (2.0 mmol), 95 % ethanol
(5 mL) at 80 °C under aerobic conditions
3
K3PO4ꢀ3H2O
NEt3
a
Isolated yields based on the bromobenzene used
4
5
KOH
6
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
ethanol renders this protocol quite practical and amenable
to large scale synthesis.
7
CH3CN
8
DMF
In order to evaluate the potential of the catalyst, the
standard reaction between 4 and 5 was run with varying
catalyst amount and the results are shown in Table 2. The
results showed that among the different loading 1 mol%
proved to be the best and was chosen as the optimum
loading in the Suzuki–Miyaura cross-coupling reaction.
To explore the scope and generality of the catalyst, the
optimized protocol [aryl bromide (1 equiv.), aryl boronic
acid (1.2 equiv.), K2CO3 (2 equiv.), catalyst (1 mol%),
temperature 80 °C under air] was applied for reactions of
variety of differently substituted and non-substituted aryl
halides and aryl boronic acids (Table 3). Both activated
and deactivated aryl bromides were completely converted
to the corresponding products in 70–96 % yields within
1–4 h. The reaction showed good diversity in the presence
of both electron withdrawing and donating groups. It has
been found that 1–4 h are required for full conversion of
aryl bromides, but a much longer reaction time as 12–15 h
was required for less reactive aryl chlorides giving low
yield of the coupling product (Table 3, entries 18–21). For
9
CH2Cl2
10
THF
Reaction conditions: bromobenzene (1.0 mmol), phenylboronic acid
(1.2 mmol), 3 (1 mol%), base (2.0 mmol), solvent (5 mL) at 80 °C
under aerobic conditions
a
Isolated yields based on the bromobenzene used
conditions like selection of solvent, base and catalyst
loading. The choice of base was found to be crucial for the
reduction of catalyst degradation with enhancement of the
reaction rate [37]. Several organic as well inorganic bases
and various solvents were screened and the superior reac-
tion efficiency was observed with K2CO3 as base in 95 %
ethanol at 80 °C (Table 1). The other bases and solvent
were not as effective as K2CO3. There are various reports
[38, 39] emphasizing the important role played by the
solvent in the reaction system prompted us to screen var-
ious solvents. Screening of solvents indicated that 95 %
ethanol was the best solvent for this system. Use of 95 %
123

Silica Tethered Pd–DABCO Complex
1393
Table 3 The Suzuki–Miyaura
reaction of various aryl halides
and arylboronic acids over
Pd–DABCO@SiO2
Entry
1
Aryl halide
Arylboronic
Product
Time
(h)
1
Yielda
acid
(%)
96
Br
BOH)2
BOH)2
Br
2
3
4
5
6
1
1
94
90
92
90
89
F
F
Br
BOH)2
F
F
Br
BOH)2
1.5
2
BOH)2
Br
CHO
CHO
Br
BOH)2
2
O
O
BOH)2
7
1.5
1.5
2
93
92
70
94
91
93
Br
BOH)2
8
Br
Br
BOH)2
9
OH
OH
O
BOH)2
O
10
11
12
2
Br
O
BOH)2
O
2
Br
Br
F
F
BOH)2
2
123

1394
A. Kumbhar et al.
Table 3 continued
Entry
13
Aryl halide
Arylboronic
acid
Product
Time
Yielda
(%)
(h)
16
Br
BOH)2
70
Br
BOH)2
14
15
4
4
88
Br
Br
BOH)2
81
BOH)2
BOH)2
BOH)2
BOH)2
16
17
18
19
20
21
2
92
80
76
20
58
60
Br
Br
3
Br
Br
Cl
12
14
15
15
O
O
Cl
Cl
BOH)2
O
Reaction conditions: aryl halide
(1.0 mmol), arylboronic acid
(1.2 mmol), 3 (1 mol%), K2CO3
(2.0 mmol), 95 % EtOH (5 mL)
at 80 °C under aerobic
CH3
O
Cl
BOH)2
CHO
conditions
a
Isolated yields based on aryl
H
O
halide
the highly sterically hindered, deactivated 2,4,6-trimeth-
ylbromobenzene, a satisfactory yield could be obtained
within 16 h (70 %, Table 3, entry 13).
reactions of dibromoarenes with aryl boronic acids afforded
desired products in quantitative yields (Table 3, entries 16
and 17). Diphenylanthracene which is used as a fluorescer in
a peroxyoxalate chemiluminescence system [40] was also
obtained by coupling 9,10-dibromoanthracene with phen-
ylboronic acid (Table 3, entry 15) in 81 % yield.
To further extend the scope of this methodology, we
carried out reactions of dibromoarenes with phenyl boronic
acid. The results of these reactions were of particular interest
since their outcome involves formation of products which
are extensively used in the synthesis of molecular wires. The
The protocol was extended for the synthesis of annulated
pyrazines such as pyrido[2,3-b]pyrazine derivatives
123

Silica Tethered Pd–DABCO Complex
1395
R
Br
N
N
NH2
NH2
O
O
N
N
RB(OH)2
Br
5
BAHC
+
Pd-DABCO@SiO2
K2CO3,THF,reflux
3
N
N
N
Water, RT
, R = phenyl, 84%
10a
8
9
7
92 %
, R = (3,5-dimethyl,4-methoxy) phenyl, 82%
10b
10c
, R = naphthyl, 79%
Scheme 2 Suzuki–Miyaura reaction between 7-bromo-2,3-diphenylpyrido[2,3-b]pyrazine and arylboronic acids using Pd–DABCO@SiO2
10a–c (Scheme 2) as these are part of important anticancer
drugs like CQS (chloroquinoxalinesulphonamide) and
XK469 as well as present in a number of synthetic antibiotics
such as echinomycin. They are also recognized in a great
number of naturally occurring compounds and many fluo-
rescent materials [41–43]. The key precursor 7-bromo-2,3-
diphenylpyrido[2,3-b]pyrazine 9 required for the synthesis
of 10a–c was obtained in 92 % yield from commercially
available 5-bromo-2,3-diaminopyridine 7 and benzil 8 in
important features. The recyclability of 3 was investigated
with consecutive Suzuki–Miyaura reactions of bromoben-
zene and 4-bromobenzophenone with phenyl boronic acid.
After first cycle, the catalyst was recovered by filtration
and extensively washed with water, dichloromethane, and
acetone. The catalyst was then dried under vacuum over-
night before performing the reusability test. The recycling
results of 3 are summarized in Table 4. Investigations of
the catalyst recycling mirrored a slight decrease of the total
yield in every run. After reaction the amount of Pd in
catalyst was measured by ICP–AES and it was found to be
0.075 mol g-1. There was 5–6 % weight loss of catalyst
observed after the five repeated runs. Additionally, the
catalyst exhibited high stability in air. It was found that
there was no decrease in activity when the catalyst was
exposed to air for several days.
¨
water in the presence of Bronsted acid hydrotrope combined
catalyst [44]. In order to substitute functionalized aromatic
ring at position 7 of 9, Suzuki–Miyaura reactions (Scheme 2)
were carried out in presence of 3 with various aryl boronic
acids 5. Though we were able to effect these reactions, the
yields of products 10a–c were very scarce in ethanol. This
could be rationalized on the basis of less solubility of 9 in
ethanol. However, switching of solvent from ethanol to THF
resulted in excellent yields of 10a–c in 14 h.
In order to evaluate the heterogeneity of catalyst, hot
filtration test was performed by coupling bromobenzene
with phenylboronic acid in presence of the catalyst [45].
After the 30 min of reaction time, the catalyst was filtered
off and the filtrate was allowed to react further. It was
found that, after this hot filtration, no further reaction was
observed. This result suggests that the palladium catalyst
remains on the support during the reaction.
4 Conclusions
In conclusion, silica tethered DABCO–Pd complex was
successfully synthesized by immobilization of DABCO–Pd
on 3-chloropropyltriethoxysilane modified silica gel by
covalent bonding and efficiently used as heterogeneous
catalyst for Suzuki–Miyaura reactions involving various
mono and dibromoarenes. The catalyst showed interesting
features such as low Pd leaching, high efficiency and
economy gain by simple reaction processing and easy
recovery and re-use of the catalyst. Moreover, the catalyst
showed excellent catalytic activity for the synthesis of
pyrido[2,3-b]pyrazines.
For practical applications of heterogeneous systems, the
lifetime of the catalyst and its level of reusability are very
Table 4 Recycling of catalyst 3 in Suzuki–Miyaura coupling of aryl
halide with phenyl boronic acid
Entry
1
Aryl halide
Cycle
Time (h)
Yield (%)
Acknowledgments We gratefully acknowledge the financial sup-
port from the Department of Science Technology and University
Grants Commission for FIST and SAP respectively. One of the
authors Arjun Kumbhar thanks the University Grants Commission,
New Delhi, India for the award of Teacher Fellowship under the
F. I. P. of the XIth Plan.
1
2
3
4
5
1
2
3
4
5
1
96
94
95
93
90
94
92
91
89
87
Br
1
1.5
2.0
2.5
2
2
O
2.5
2.5
3
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