Superb efficient and recycle polymer-anchored systems for palladium catalyzed Suzuki cross-coupling reactions in water

Article abstract of DOI:10.1016/j.apcata.2012.09.048

A set of three new polymer-anchored palladium(II) Schiff base catalysts have been synthesized, characterized and their catalytic activity was investigated in the Suzuki cross-coupling reaction between aryl halides and arylboronic acid in the presence of Cs₂CO₃ as a base. They show excellent catalytic activity in coupling of aryl bromides or aryl iodide with phenylboronic acid under the optimized reaction conditions in water. Polymer-anchored Pd(II) complexes provided turnover frequency of 29,700 or 58,200 h^-1 in Suzuki coupling reactions of phenylboronic acid with p-bromoacetophenone or p-iodobenzene, respectively, which are the highest values ever reported for the Suzuki coupling reactions in water as sole solvent. The catalyst 1 could be used for 15 reaction cycles in the Suzuki coupling of p-acetobromobenzene at 100 °C with no loss of catalytic activity.

Full text of DOI:10.1016/j.apcata.2012.09.048

Applied Catalysis A: General 449 (2012) 172–182
Contents lists available at SciVerse ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Superb efficient and recycle polymer-anchored systems for palladium
catalyzed Suzuki cross-coupling reactions in water
a
a,∗
b
a
a
Fatma Siga , Hamdi Temel , Murat Aydemir , Yusuf Selim Ocak , Salih Pasa ,
Akın Baysalb
a
Department of Chemistry, Faculty of Education, University of Dicle, 21280 Diyarbakir, Turkey
Department of Chemistry, Faculty of Science, University of Dicle, 21280 Diyarbakir, Turkey
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 22 May 2012
Received in revised form
A set of three new polymer-anchored palladium(II) Schiff base catalysts have been synthesized, charac-
terized and their catalytic activity was investigated in the Suzuki cross-coupling reaction between aryl
halides and arylboronic acid in the presence of Cs2CO3 as a base. They show excellent catalytic activity in
coupling of aryl bromides or aryl iodide with phenylboronic acid under the optimized reaction conditions
1
8 September 2012
Accepted 28 September 2012
Available online 24 October 2012
−
1
in water. Polymer–anchored Pd(II) complexes provided turnover frequency of 29,700 or 58,200 h in
Suzuki coupling reactions of phenylboronic acid with p-bromoacetophenone or p-iodobenzene, respec-
tively, which are the highest values ever reported for the Suzuki coupling reactions in water as sole
solvent. The catalyst 1 could be used for 15 reaction cycles in the Suzuki coupling of p-acetobromobenzene
Keywords:
Polymer-anchored Pd(II) complex
Catalysis
Suzuki cross-coupling reaction
Water
◦
at 100 C with no loss of catalytic activity.
© 2012 Elsevier B.V. All rights reserved.
Aryl halides
1
. Introduction
The palladium-catalyzed Suzuki cross-coupling reaction of aryl-
in water, even though this may be less efficient than that in a
suitable organic solvent [18–21]. The palladium catalysts with
phosphine ligands [22], carbene ligands [23], palladacycle [24] and
other coordinates [25] have shown high activity in the Suzuki cou-
pling reactions in water. However, problems such as expensive
poisonous phosphine ligands and non-reusability of the homoge-
neous catalyst extremely limited the industrial applications. Thus,
the development of an easily available or readily prepared, inex-
pensive palladium catalyst with a high activity toward Suzuki
cross-coupling reactions in the sole solvent of water is a highly
desirable goal.
There have been many successful demonstrations of homo-
geneous catalysis for coupling reactions [26–30]. However, these
homogeneous catalyst systems have some basic problems in terms
of the separation and recycling of the catalysts [31]. Additionally,
they induce contamination of the ligand residue in the products.
Polymer metal complexes continuously attract the interest of a
growing part of the scientific community for the advantages they
offer with respect to their soluble counterparts [32–37]. Func-
tionalized polymers have also been used as carriers in various
organic and inorganic synthetic transformations [38]. Such het-
erogeneous catalytic systems have several advantages, such as
robustness, increased air and moisture stability, fast recovery and
simple recycling of the catalysts by filtration, which in turn prevent
the contamination of the ligand and decrease the environmen-
tal pollution caused by residual metals in the waste. Although
boronic acids with aryl halides is one of the most powerful and
versatile methods for the selective construction of carbon–carbon
bonds and has been widely used in the synthesis of pharma-
ceuticals, herbicides, natural products, and advanced materials
[
1–5]. Although nickel [6] and copper [7] catalysts have also been
reported, the palladium appears to be the best catalyst for the C
C
coupling reactions due to the higher reaction yield and shorter
reaction time [8,9]. In recent years, there has been a considerable
interest in the preparation of highly active palladium catalysts to
facilitate such transformations [10–13]. Most of the palladium cat-
alyzed Suzuki coupling reactions have been performed in organic
solvents. However, the use of an environmentally benign reaction
medium, minimization of steps, better yields, and faster reaction
remained challenges in the context of green chemistry for the
Suzuki coupling reaction [14]. In this regard, the use of water,
the most abundant and non-toxic solvent for organic reactions,
is reclaiming its importance due to the pressing environmental,
economical, and safety concerns [15–17]. Thus far, various cat-
alytic systems have been reported for the Suzuki coupling reaction
∗ Corresponding author. Tel.: +90 412 2488320; fax: +90 4122488320.
E-mail address: htemel@dicle.edu.tr (H. Temel).
0
926-860X/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.apcata.2012.09.048

F. Siga et al. / Applied Catalysis A: General 449 (2012) 172–182
173
various supports including silica [39], zeolites [40–44], sepio-
lites [45] and polymers [46,47] have been used to immobilize
metal complexes [48], polymer supports showed better control on
catalytic activities in comparison to inorganic supports [49–51].
Polymer support in the presence of a solvent provides prefer-
ential stereochemistry around the catalytic sites to enhance the
catalytic activity of the immobilized catalyst [52]. Many catalytic
reactions involving polymer bound metal complexes have shown
dependence on the structure of the polymer support because the
geometry of the polymer bound metal complex depends upon the
structure, rigidity and flexibility of the polymer backbone [53]. The
catalytic activity of the complex on polymer support also depends
upon the distance of metal complex from the polymer backbone so
that accessibility of the substrate to the metal ion will be maximum
beads were filtered off, washed with hot dioxane, followed by hot
ethanol and dried in an air oven. L (deep yellow): IR (KBr pellet,
1
−
1
cm ) ꢀ(CN); 1634; Anal. found: C 84.06; H 7.52; N 1.87; L (yel-
low): IR (KBr pellet, cm ) ꢀ(CN): 1634; Anal. found: C 85.03; H
7.36; N 1.30; L3 (deep yellow): IR (KBr pellet, cm ) ꢀ(CN): 1628;
2
−
1
−
1
Anal. found: C 85.69; H 7.29; N 1.39.
2.2.2. General method for the synthesis of the polystyrene
anchored Schiff base Pd(II) complexes 1, 2 and 3
The polymeric Schiff base ligand (2 g), preswollen in dioxane
(20 mL) for 1 h, was refluxed with the metal ion solution (1 g in
20 mL) for 24 h under stirring. The polymer-anchored Pd(II) Schiff
base complex formed was then suction filtered, washed thoroughly
with dioxane, water, alcohol and acetone, respectively. The com-
plex was then dried in a vacuum desiccator at room temperature.
[
54]. These investigations have clearly suggested that the activity of
−
1
a catalyst is highly influenced by the polymer matrix and geometry
of the metal complex on polymer support.
Pd-L1(1) (yellowish green): IR (KBr pellet, cm ) ꢀ(CN): 1600,
ꢀ(PdN): 277, ꢀ(PdO): 324, ꢀ(PdCl): 310, EDX quantitative element
analysis (atomic%): C 83.26; O 5.15; Cl 4.70; Pd 0.30; ICP (induc-
tively coupled plasma): Pd 0.29%; Anal. found: C 79.77; H 6.98; N
Recently, polymer anchored various metal complexes have been
used as catalyst in various Suzuki cross-coupling reactions in dif-
ferent solvents [55,56], but the reported activities of these catalysts
using water [57–65] as sole solvent are too low for them to be
reused for practical organic synthesis [66]. This prompted us to
extend our catalytic investigations to Pd(II) complexes anchored
with P(S-DVB) polystyrene divinyl benzene beads and to study the
influence of anchoring on catalytic reactions. We extended this
study to cover use of them as catalyst in Suzuki cross-coupling
reaction in water as sole solvent without any purification process.
Herein, we report the further characterizations and the superb cat-
alytic performance of inexpensive polymer-anchored palladium(II)
Schiff base catalysts, especially Pd complex (1) in Suzuki cross-
coupling reactions of arylboronic acid with various aryl bromides
or aryl iodide in aqueous solution under the relatively mild reaction
conditions.
−1
2.03; Pd-L2(2) (green): IR (KBr pellet, cm ) ꢀ(CN): 1600, ꢀ(PdN):
453, ꢀ(PdO): 532, ꢀ(PdCl): 254, EDX quantitative element analy-
sis (atomic%): C 82.24; O 7.15; Cl 4.40; Pd 0.40; ICP (inductively
coupled plasma): Pd 0.38%; Anal. found: C 80.55; H 7.03; N 1.68;
−
1
Pd-L (3) (black): IR (KBr pellet, cm ) ꢀ(CN): 1605, ꢀ(PdN): 537,
3
ꢀ(PdO): 351, ꢀ(PdCl(bridge): 318, EDX quantitative element anal-
ysis (atomic%): C 71.75; O 6.53; Cl 3.19; Pd 6.53; ICP (inductively
coupled plasma): Pd 6.49%; Anal. found: C 67.16; H 6.03; N 1.27.
2.3. Catalytic test
2.3.1. General procedure for the Suzuki coupling reaction
Polymer-anchored Schiff base palladium complexes 1, 2 or 3
(0.001 mmol, 0.1% mmol), aryl bromide (1.0 mmol), phenylboronic
acid (1.5 mmol), Cs CO3 (2 mmol), aqua (3 mL) were added to a
2
Schlenk tube under air atmosphere and the mixture was heated at
2
. Experimental
◦
1
00 C for the indicated time. To study the progress of the reaction,
the reaction mixtures were collected at different time intervals and
quantified by GC analysis. At the end of the reaction, the catalyst
was separated by simple filtration. The filtrate was extracted with
diethyl ether and passed through a pad of silica gel. The organic
2
.1. Materials
Analytical grade solvents and chemicals were used throughout.
Aminomethylated polystyrene (2% cross linked with divinylben-
zene) was obtained from (Aldrich), 2-hydroxy-1-naphthaldehyde
phase thus collected was dried over Na SO , filtered, concentrated
2
4
and the residue was purified by flash column chromatography on
silica gel. All the prepared compounds are known and were com-
pared with the authentic samples. The purity of the compounds was
also checked immediately by GC and NMR and yields are based on
aryl halides.
(
Fluka), dimethylformamide (DMF) (Fluka), ethanol (Riedel-de
Haën), methanol (Riedel-de Haën), dioxane and PdCl (Sigma). FTIR
2
spectra were recorded from the KBr pellet on a PerkinElmer 1600
Fourier transform spectrophotometer. UV–vis electronic spectra
were performed on a Shimadzu UV 60 spectrometer (Lambda
2
5). Scanning electron micrographs were recorded on a FEI Nova
2
.3.2. GC analysis
GC analyses were performed on a Shimadzu GC 2010 Plus Gas
NanoSEM 430 and thermogravimetric analysis of the catalysts was
carried out on a Shimadzu DSC 60 A thermal analyzer up to 600 C
at a heating rate of 20 C min under nitrogen atmosphere. Palla-
dium content of the catalyst was measured by ICP on an Optima
◦
Chromatograph equipped with a capillary column (5% biphenyl,
9
ness). The GC parameters for Suzuki coupling reactions were as
follows: initial temperature, 50 C; initial time, 1 min; solvent
delay, 3.70 min; temperature ramp 1, 10 C/min; final temperature,
1
final time, 20.67 min; injector port temperature, 250 C; detector
temperature, 250 C, injection volume, 2.0 ␮L.
◦
−1
5% dimethylsiloxane; 30 m × 0.32 mm i.d. × 0.25 ␮m film thick-
2
100 DV analyzer. Elemental analysis was carried out on a Fisons
◦
EA 1108 CHNS-O instrument.
◦
◦
◦
◦
50 C; temperature ramp 2, 15 C/min; final temperature, 250 C;
◦
2.2. Preparation and characterization of the ligands and their
◦
complexes
2.2.1. General method for the synthesis of the polystyrene
3. Results and discussion
anchored Schiff base (L , L and L3)
1
2
Aminomethylated polystyrene (10 g), preswollen in diox-
ane (20 mL) for 1 h, was refluxed with a dioxane solution of
aldehyde [(a) 1,6-bis(2-carboxy aldehyde phenoxy)hexane, (b)
3.1. Synthesis and characterization of polymer-anchored
palladium Schiff bases and their Pd(II) complexes
1
1
,5-bis(2-carboxy aldehyde phenoxy)pentane and (c) 2-hydroxy-
-naphthaldehyde] [67] (10 g in 30 mL) for 24 h. The flesh colored
Schiff bases of amino methylated polystyrene (Scheme 1) and
their metal complexes (Scheme 2) were synthesized and character-
ized by thermal analysis, IR, UV–vis spectral analysis, SEM and EDX
resin was then cooled to room temperature. The dark yellow resin

1
74
F. Siga et al. / Applied Catalysis A: General 449 (2012) 172–182
Scheme 1. Preparation of polymer-anchored ligands: (a) 1,6-bis(2-carboxy aldehyde phenoxy)hexane Schiff base of amino methylated polystyrene, L1; (b) 1,5-bis(2-carboxy
aldehyde phenoxy)pentane Schiff base of amino methylated polystyrene, L2; (c) 2-hydroxy-1-naphthaldehyde Schiff base of amino methylated polystyrene, L3.
studies. Analytical data of the complexes show 1:1 geometry with
respect to metal and ligand. The IR spectra of the polymer anchored
spectrum of the polymer-anchored Pd(II) Schiff base complexes
were recorded over the 200–700 nm range in diffuse reflectance
mode. Because of low metal loading, the intensities of the bands
were affected. Low-intensity bands at 270 and 310 nm were
observed for all of our catalysts, which were assigned to Pd2+ → N
coordination [78]. Ligand to- metal charge-transfer bands were
not observed in this region. In the electronic spectrum of the Pd-
P(S-DVB)L 500–600 nm the absorption band at 1A1g → A ve
−1
Schiff bases showed peaks at 1634–1621 cm characteristic of the
N group [68]. No bands characteristic of free carbonyl or free
C
amine were observed. The above information strongly suggests that
the proposed ligand framework is formed. In all the complexes, the
−
1
ꢀ
(C N) group absorption shifts to lower frequency by 30–15 cm
,
1
indicating coordination of the imine nitrogen atoms of the Schiff
3
2g
1
1
bases [69]. In the case of L , disappearance of OH band upon poly-
A1g → B may be due to a spin allowed transition. From the
3
1g
merization of the ligand indicates the breakdown of H bonding
followed by deprotonation of phenolic OH group [70]. Further, in
the far-IR spectra of 1 and 2, observed new low intensity bands at
different microanalytical and spectral data, a structure for the
polymer-anchored Pd(II) complex is proposed.
−
1
−1
3
10 cm and 254 cm were assigned to ꢁ(Pd–Cl) [71] vibrations.
3.2. Thermal analysis
−
1
The spectrum of 3 also shows a new band at 318 cm which can be
attributed to ꢁ(Pd–Clbridge) [72]. Furthermore, the IR spectra of the
complexes 1-3 show new bands at 277 cm [73], 453 cm [74]
and 537 cm [75], respectively which are probably due the for-
mation of M–N bands. The supported Schiff base Pd(II) complexes
also exhibit M–O absorption bands at 324 cm , 532 cm [76] and
51 [77], respectively. In addition, the infrared spectra of fresh and
Thermal stability is an important criterion to be met for high
temperature applications. So, the thermal studies have been car-
ried out to determine the applicability of polymer supported Pd(II)
complexes in catalysis reactions operating at high temperature.
The thermal stability of the catalysts was investigated by TGA
and DTA. The thermal data are in well agreement with the pro-
posed structures. Thermogravimetric analyses were carried out in
−1
−1
−1
−1
−1
3
reused catalysts were taken and it was found that they are almost
the same (supporting information).
The electronic spectra of the polymeric complexes were
recorded in diffuse reflectance spectrum mode as a BaSO4 disk due
to the solubility limitations and the structures of the complexes
were confirmed by reflectance electronic spectra. An electronic
◦
−1
air with a heating rate of 10 C min
over a temperature range
◦
30–600 C. Thermo- gravimetric analysis shows stability of the
polymer-anchored complexes up to 300 C, above which there is
a continuous weight loss, which extends to 600 C. DTA analysis
◦
◦
reveals that all the decomposition stages are exothermic in nature.

F. Siga et al. / Applied Catalysis A: General 449 (2012) 172–182
175
Scheme 2. Preparation of polymer-anchored Pd(II) complexes: 1, 2 and 3.
The TGA profiles suggest that the polymer-anchored Pd(II) complex
degrades at high temperature like the precursor polymer anchored
the metal was observed by SEM. According to the EDX spectra in
(Fig. 3a: L , 3b: 1, 3c: L , 3d: 2, 3e: L , 3f: 3) it is seen that only C and
1
2
3
ligands (Fig. 1a: L , 1c: L , 1e. L ). As it is clear from the data, the
N were present in the ligands, while, the polymer-anchored metal
complex contained Pd along with C, N and Cl (a, b and c). Thus, we
can conclude that the palladium was immobilized specifically to
the ligands on the surface of the beads.
1
2
3
complexation of the polymer supported Pd(II) compounds (Fig. 1b:
, 1d: 2, 1f: 3) has shown a dramatic increase in thermal stability
1
which has clearly indicate that these complexes could be used as
catalysts for high temperature reactions.
3.4. The Suzuki coupling reactions
3.3. Scanning electron micrographs studies
The palladium-catalyzed cross-coupling of aryl halides with aryl
Scanning electron microscopy can be used to detect the presence
of the complex on the surface of the polymer. Scanning elec-
tron micrographs (SEM) of pure amino methylated polystyrene
boronic acids (Suzuki reaction) is one of the most powerful and
popular methods for the construction of unsymmetrical biaryls,
which are widely used for the synthesis of valuable organic com-
pounds such as pharmaceuticals and agrochemicals [2]. Although
several other cross-coupling reactions are available to produce
biaryls, the Suzuki reaction has widely been used over the course
of the last few years, since it has several advantages compared
to other available methods. One particular advantage of this reac-
tion is that it can be performed under mild conditions in aqueous
solutions that tolerate a broad range of functional groups [79].
Furthermore, boronic acids are generally nontoxic, stable, and envi-
ronmentally benign, and the boron containing by-products can
easily be removed from the reaction medium after the reaction
[80,81].
anchored ligands L –L (Fig. 2a: L , 2c: L , 2e: L ) and polymer
1
3
1
2
3
anchored Pd(II) Schiff base complexes, 1–3 (Fig. 2b: 1, 2d: 2, 2f:
) were recorded taking single bead of each to understand the
3
morphological changes occurring at various levels of synthesis. A
light roughening of the top layer of bead having polymer-anchored
ligands compared to relatively smooth and flat surface of neat
polystyrene bead suggests the change of nature of bead on covalent
bonding. Images of metal complexes beads show further roughen-
ing of the top layer which is possibly due to interaction of metal
ions with ligand to arrange in the fixed geometry of the complex.
A clear change in morphology of the polymer after introduction of

1
76
F. Siga et al. / Applied Catalysis A: General 449 (2012) 172–182
Fig. 1. Termogravimetric weight loss and differential thermal analysis of the polymer anchored ligands (1a: L1; 1c: L2; 1e: L3) and polymer anchored Pd(II) complexes (1b:
1
; 1d: 2; 1f: 3).
To investigate the applicability of the polymer-anchored Pd(II)
probe reaction. From the Table 1, it can be seen that the efficiency
of complexes is different from each other. For example, the Suzuki
reaction with catalyst 1 always afforded higher catalytic activity
than that with catalysts 2 or 3. These results have clearly demon-
strated that the activity of the catalyst is highly influenced by the
polymer matrix and also by geometry of the metal complex on poly-
mer support. The complex 1 is structurally different from 2 and 3
complexes, we first tested the Suzuki cross-coupling reaction
between aryl bromides with boronic acid [82]. The reaction
parameters for the Suzuki cross-coupling reaction were optimized
through a series of experiments. The influences of temperature,
base, solvent and atmosphere were systematically studied by using
the coupling of p-acetobromobenzene and phenylboronic acid as

F. Siga et al. / Applied Catalysis A: General 449 (2012) 172–182
177
Fig. 2. SEM spectra of polymer anchored ligands (2a: L1; 2c: L2; 2e: L3) and polymer anchored Pd(II) complexes (2b: 1; 2d: 2; 2f: 3).
and is more flexible than complex 2 since it has longer alkyl moi-
ety. This geometry may introduce ideal flexibility for catalyst 1 to
afford higher catalytic activity than 2 and 3.
In addition, the replacement of the reaction atmosphere from an
inert gas to an ambient atmosphere had no negative influence on
the activity of the catalysts (Table 1, Entries 4–6 and 10–12). There-
fore, the coupling reactions were performed in air. Optimization
studies were performed to determine how solvent and tempera-
ture affect the coupling reaction. As demonstrated in Table 1, the
best catalytic activities were not obtained until the Suzuki reaction
◦
was carried out at 100 C in aqua medium (Table 1, Entries 13–15).
In conclusion, we found that under identical conditions, the cou-
pled product obtained in higher yield at elevated temperature and
no reaction occurred at room temperature (Table 1, Entry 16).

1
78
F. Siga et al. / Applied Catalysis A: General 449 (2012) 172–182
Fig. 3. EDX spectra of polymer anchored ligands (3a: L1; 3c: L2; 3e: L3) and polymer anchored Pd(II) complexes (3b: 1; 3d: 2; 3f: 3).
Optimization studies were also performed to determine how
least three times. As seen in Table 2, the best yield was obtained in
◦
bases affect the coupling reaction. As illustrated in Table 2, ini-
tial studies were performed by using different bases (Cs CO ,
the Suzuki reaction when it was carried out at 100 C using Cs CO
2
3
as base, which demonstrated that Cs CO3 as the base was the best
2
3
2
t
KOH, NaOH, K CO3 and KO Bu) in order to optimize the reaction
choice for the cross-coupling reactions and other bases such as
2
t
conditions for the coupling of p-acetobromoacetophenone with
phenylboronic acid in the presence of complexes, 1–3 (0.001 mmol
Pd) under aerobic conditions. The reproducibility of each catalytic
reaction was confirmed by carrying out the catalytic reaction at
KOH, Na CO3 and K OBu were slightly less effective. In addition,
2
the effect of base-to-substrate mole ratio on the coupling reaction
was investigated. Using Cs CO3 at a base-to-substrate mole ratio
2
of 2:1 afforded the highest conversion, while the conversion was

F. Siga et al. / Applied Catalysis A: General 449 (2012) 172–182
179
Table 1
The Suzuki coupling reactions of p-acetobromobenzene with phenylboronic acid catalyzed by the polymer-anchored palladium(II) Schiff base catalysts 1, 2 and 3.
TOF (h 1
−
)
Entry
Cat.
Time
Solvent
Base
Temp.
Atm.
Conv. (%)
Yield (%)
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
1
2
3
1
2
3
1
2
3
1
2
3
1
2
3
1
1
3 min
5 min
10 min
3 min
5 min
10 min
3 min
5 min
10 min
3 min
5 min
10 min
2 min
4 min
8 min
24 h
dioxane
dioxane
dioxane
dioxane
dioxane
dioxane
methanol
methanol
methanol
methanol
methanol
methanol
aqua
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
110
110
110
110
110
110
65
65
65
65
65
Ar
Ar
Ar
Air
Air
Air
Ar
Ar
Ar
Air
Air
Air
Air
Air
Air
Air
Air
99
100
99
100
98
100
99
99
97
98
97
98
97
97
99
96
96
97
99
98
99
98
99
<10
78
19,400
11,760
5820
19,600
11,640
5820
19,800
11,520
5760
19,400
11,880
5880
98
99
1
1
1
1
1
1
1
1
100
100
100
99
100
<10
80
65
100
100
100
rt
29,700
14,700
7425
aqua
aqua
aqua
aqua/DMF(1/1)
<3
<5
24 h
rt
Reaction conditions: 1.0 mmol of p–CH3C(O)–C6H4Br aryl bromide, 1.5 mmol of phenylboronic acid, 2.0 mmol Cs2CO3, 0.001 mmol cat., solvent 3.0 (mL). Purity of compounds
−
1
was checked by NMR and yields are based on arylbromide. All reactions were monitored by GC for 1, 2 and 3. TOF = (mol product/mol Cat.) × h
.
lower at a mole ratio of 1:1. However, increasing the mole ratio
of the base-to-substrate did not increase the conversion (Table 2,
Entries 22–24).
With the best conditions at hand, we decided to explore the
scope of Suzuki cross-coupling of catalyst 1 with various sub-
strates, including aryl bromides, chlorides and iodides having
electron-withdrawing or electron-donating substituents (Table 3).
Aryl iodides could react with phenylboronic acid with outstanding
shorter time approximately 1 min and the yields of the corre-
sponding products were very satisfactory (Table 3, Entries 1 and
Compared to aryl iodides, the reaction took longer (Table 3),
thus we can easily say that the electronic nature of the aryl
bromides has a clear effect on the coupling reactions (Table 3,
Entries 3–6). For example, the coupling reaction of electron-rich
4-methyl-bromobenzene with phenylboronic acid provided with
a TOF value of 11,760, but for the electron-deficient 4-aldehyde-
bromobenzene, the desired coupled product was obtained with
a
TOF value of 58,200. After obtaining such a high activ-
ity in Suzuki coupling reactions of 4-aldehyde-bromobenzene,
p-iodoacetophenone or iodobenzene with phenylboronic acid cat-
alyzed by complexes, 1–3 in water, we studied the catalytic
activity of 1 in the Suzuki coupling reactions of arylchlorides with
2
). Encouraged by these results, we tried to study the reactiv-
ity between substituted aryl bromides and phenylboronic acid.
Table 2
The Suzuki coupling reactions of p-acetobromobenzene with phenylboronic acid catalyzed by polymer-anchored palladium(II) Schiff base catalysts, 1–3.
Cat / (0.001 mmol)
B(OH)2
Br
C(O)CH3
C(O)CH3
Base
◦
−1
)
Entry
Cat.
Time
Solvent
Base
Temp. ( C)
Conv. (%)
Yield (%)
TOF (h
1
2
3
4
5
6
7
8
9
1
2
3
1
2
3
1
2
3
1
2
3
1
2
3
1
2
3
1
2
3
1
1
1
8 h
DMF
DMF
DMF
toluene
toluene
toluene
CH3CN
CH3CN
CH3CN
aqua
aqua
aqua
aqua
aqua
aqua
aqua
aqua
aqua
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
K2CO3
K2CO3
K2CO3
NaOH
NaOH
NaOH
KOH
110
110
110
110
110
110
110
110
110
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
99
100
100
100
99
98
97
98
97
98
97
96
98
98
97
96
96
96
96
94
95
96
96
94
96
95
97
99
88
123
97
70
320
200
97
320
200
98
11,640
3840
1920
5760
2880
1250
11,400
5760
1920
11,280
5760
3800
29,100
29,700
26,400
10 h
14 h
3 h
5 h
10 h
3 h
5 h
10 h
5 h
15 h
30 h
10 h
20 h
45 h
5 h
10 h
30 h
5 h
10 h
25 h
2 h
2 h
2 h
99
100
100
99
98
100
99
100
97
98
98
100
98
100
99
98
100
92
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
KOH
KOH
t
aqua
aqua
aqua
aqua
aqua
aqua
K OBu
t
K OBu
t
K OBu
a
b
c
Cs2CO3
Cs2CO3
Cs2CO3
Reaction conditions: 1.0 mmol of p–CH3C(O)–C6H4Br aryl bromide, 1.5 mmol of phenylboronic acid, 2.0 mmol base, 0.001 mmol cat., solvent 3.0 (mL), in air. Purity of compounds
−
1
was checked by NMR and yields are based on arylbromide. All reactions were monitored by GC for 1, 2 and 3. TOF = (mol product/mol Cat.) × h
.
a
Base-to-substrate mole ratio: 1:1.
b
Base-to-substrate mole ratio: 2:1.
c
Base-to-substrate mole ratio: 5:1.

1
80
F. Siga et al. / Applied Catalysis A: General 449 (2012) 172–182
Table 3
The Suzuki coupling reactions of aryl halides with phenylboronic acid catalyzed by polymer-anchored palladium(II) Schiff base catalyst, 1.
1
, (0.001 mmol)
B(OH)2
X
R
R
Cs CO (2 equiv.)
2
3
TOF (h 1
−
)
Entry
Substrate
Time (min)
Conv. (%)
Yield (%)
I
H
1
2
3
1
1
1
99
98
96
97
97
96
57,600
58,200
58,200
I
C(O)CH3
C(O)H
H
Br
Br
100
4
5
2
3
99
98
28,800
19,200
96
Br
Br
OCH
CH3
3
14a
82
(
(
18)
86)
b
6
5
100
(11)
(
(
98
11,760
c
8
47
d
51)
88)
e
85
Cl
H
7
30
45
42
840
Reaction conditions: 1.0 mmol of p–R–C6H4X aryl halogens, 1.5 mmol of phenylboronic acid, 2.0 mmol Cs2CO3, 0.001 mmol cat. (1), water 3.0 (mL). Purity of compounds was
◦
−1
.
checked by NMR and yields are based on arylbromide. All reactions were monitored by GC; 100 C for 1. TOF = (mol product/mol Cat.) × h
a
1
2
1
3
4
min.
min.
min.
min.
min.
b
c
d
e
phenylboronic acid in water. However, the highest conversion was
reached up to 45% in the presence of Cs CO within 30 min in water
catalyst does not change after catalysis, indicating that no sig-
nificant leaching of palladium to the solution occurs during the
catalytic process. SEM spectra of the reused polymer anchored
Pd(II) complexes were also given in SI, Figs. 5–7). Additionally, the
filtrates obtained from the filtration of the catalysts after catalysis
reactions do not exhibit any catalytic activity.
2
3
◦
at 100 C (Table 3, Entry 7) and the longer reaction time did not
give any further conversion. It is well-known that chlorides were
generally less reactive toward Suzuki coupling reaction under the
same conditions used for the coupling of bromides, iodides, and
triflates. The low reactivity of chlorides is usually attributed to the
strength of the C Cl bond (bond dissociation energies for Ph-X: Cl:
3.5. Hot filtration experiments
9
6 kcal/mol; Br: 81 kcal/mol; I: 65 kcal/mol) which leads arylchlo-
The hot filtration experiments were performed to elucidate
the true catalytic species in the heterogeneous Pd-catalyzed
rides to be reluctant in adding oxidatively to palladium(0) centers,
a critical initial step in Pd-catalyzed coupling reactions [83,44,84].
An important point concerning the use of heterogeneous cat-
alysts is their lifetime, particularly for the industrial applications
of cross-coupling reactions. So, one of the main objectives of
our study was to investigate the recycling and reuse of the
catalysts. We explored the reusability of the polymer-anchored
palladium Schiff base catalyst, 1 using the reaction of between
p-acetobromobenzene and phenylboronic acid as model reaction.
Fig. 4 shows the fifteen runs reusability test performed in the pres-
ence of 0.001 mmol polymer-anchored palladium(II) Schiff base
catalyst, 1 in the Suzuki cross-coupling of p-acetobromobenzene
with phenylboronic acid. The catalyst, 1 maintains 97% of their
initial catalytic activity at the end of the fifteen cycle of Suzuki
coupling reaction, which indicates that the polymer-anchored pal-
ladium(II) Schiff base catalyst could be reused in several successive
runs without significant loss of initial activity for Suzuki cross-
coupling reactions in water. In addition, the excellent embodiment
is the outstanding recyclablility of this catalyst system which is
promising for the applications of industrial chemical synthesis. Fur-
thermore, the ICP analysis shows that palladium content of the
Fig. 4. A reusability test for polymer-anchored palladium(II) Schiff base catalyst,
1
in Suzuki cross-coupling reaction of p-acetobromobenzene with phenylboronic
acid in aqueous solution under optimized reaction conditions. GC yield of Suzuki
reaction between p-acetobromobenzene with benzeneboronic acid in every cycle.
0.001 mmol cat. (1), water 3.0 (mL); 100 ◦C; 2 min.

F. Siga et al. / Applied Catalysis A: General 449 (2012) 172–182
181
Suzuki–Miyaura reaction, whether the reaction takes place on the
surfaces of the solid palladium catalyst or whether the active cata-
lysts are palladium species leached out from the support. So, Suzuki
coupling reaction was carried out for a certain time (app. 50%). The
catalyst was then removed by filtration at the reaction temperature.
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2
[
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