Palladium-iminodiacetic acid immobilized on pH-responsive polymeric microspheres: Efficient quasi-homogeneous catalyst for Suzuki and Heck reactions in aqueous solution

Article abstract of DOI:10.1002/adsc.200800304

The pH-responsive core-shell microspheres of poly(styrene-co-methylacrylic acid) (PS-co-PMAA) containing a polystyrene (PS) core and a poly(methylacrylic acid) (PMAA) shell are synthesized by one-stage soap-free copolymerization and the catalyst system palladium-iminodiacetic acid (IDA-Pd) is immobilized on the outer shell-layer of the core-shell microspheres to form the quasi-homogeneous and easily accessible catalyst PS-co-PMAA-IDA-Pd. This quasi-homogeneous PS-co-PMAA-IDA-Pd catalyst is highly dispersed in the reaction medium just like a homogeneous one and can be separated like a heterogeneous catalyst by adjusting the pH of the reaction medium. Suzuki reactions employing the quasi-homogeneous PS-co-PMAA-IDA-Pd catalyst are efficiently performed in water as the sole solvent under mild conditions such as room temperature. The PS-co-PMAA-IDA-Pd catalyst is also used in Heck reactions of a wide range of aryl halides with styrene and proves to be efficient in aqueous solution. The PS-co-PMAA-IDA-Pd catalyst has a low leaching loss and can be reused at least 4 times without loss of activity.

Full text of DOI:10.1002/adsc.200800304

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DOI: 10.1002/adsc.200800304
Palladium-Iminodiacetic Acid Immobilized on pH-Responsive
Polymeric Microspheres: Efficient Quasi-Homogeneous Catalyst
for Suzuki and Heck Reactions in Aqueous Solution
Jianzheng Zhang,a Wangqing Zhang,a,* Yao Wang,a and Minchao Zhanga
a
Key Laboratory of Functional Polymer Materials of Ministry of Education, Institute of Polymer Chemistry, Nankai
University, Tianjin, 300071, Peopleꢀs Republic of China
Fax : (+86)-22-2350-3510, phone; (+86)-22-2350-9794; e-mail: wqzhang@nankai.edu.cn
Received: May 15, 2008; Published online: August 19, 2008
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.200800304.
Abstract: The pH-responsive core-shell microspheres the pH of the reaction medium. Suzuki reactions em-
of poly(styrene-co-methylacrylic acid) (PS-co- ploying the quasi-homogeneous PS-co-PMAA-IDA-
PMAA) containing a polystyrene (PS) core and a Pd catalyst are efficiently performed in water as the
poly(methylacrylic acid) (PMAA) shell are synthe- sole solvent under mild conditions such as room tem-
sized by one-stage soap-free copolymerization and perature. The PS-co-PMAA-IDA-Pd catalyst is also
the catalyst system palladium-iminodiacetic acid used in Heck reactions of a wide range of aryl hal-
(IDA-Pd) is immobilized on the outer shell-layer of ides with styrene and proves to be efficient in aque-
the core-shell microspheres to form the quasi-homo- ous solution. The PS-co-PMAA-IDA-Pd catalyst has
geneous and easily accessible catalyst PS-co-PMAA- a low leaching loss and can be reused at least 4 times
IDA-Pd. This quasi-homogeneous PS-co-PMAA- without loss of activity.
IDA-Pd catalyst is highly dispersed in the reaction
medium just like a homogeneous one and can be Keywords: core-shell microspheres; Heck reaction;
separated like a heterogeneous catalyst by adjusting leaching; palladium; polymerization; Suzuki reaction
Introduction
abundant, non-toxic, non-flammable, and readily sep-
arable from organic compounds.
The Suzuki reaction and the Heck reaction are the
Although great developments have been achieved,
À
two most important palladium catalyzed C C cou- there are two key problems to limit their industrial
pling reactions,[1,2] which have been applied in a applications. First, these coupling reactions are usually
number of areas ranging from natural product, mate- performed with a Pd catalyst along with phosphine li-
rials to fine chemical synthesis.[3] The general condi- gands, which are expensive, toxic and air/moisture
tions for performing the Suzuki and Heck reactions sensitive. Many extensive efforts have been made to
include a palladium catalyst and a base such as develop Pd catalysts that utilize inexpensive phos-
K2CO3, K3PO4, triethylamine (Et3N), etc. Recently, a phine-free ligands such as nucleophilic carbenes,[11]
major study on Suzuki reactions has focused on devel- carbapalladacycles,[12] imidazolium ions,[13] ethylenedi-
oping active Pd catalysts.[4] The major advance ach- amine-N,N,N’,N’-tetraacetic acid (EDTA),[14] quino-
ieved by increasing the catalytic activity is the exten- line-8-carboxylate,[15] and hydrazones.[16] Second, the
sion of Suzuki reaction to unactivated aryl chlorides, homogeneous Pd catalyst is difficult to be recovered
as noted by the research groups of Buchwald,[5] Fu,[6] and reused. The general method to construct a recov-
and Herrmann,[7] as well as several other groups. An- erable catalyst is to immobilize the active palladium
other major focus has been the use of an environmen- on a polymer or inorganic solid, such as silica,[17] NaY
tally benign medium. Suzuki reactions in supercritical zeolite,[18] diatomite,[19] polymeric microspheres and
CO2,[8] in ionic liquids,[9] and in water[10] have all been microcapsules and resin,[20] magnetic nanoparticles,[21]
developed. The use of water as solvent for chemical dendrimer,[22] and soluble single polymer chains.[23]
reactions clearly has both economical and environ-
Recently, we have focused on using polymeric col-
mental advantages because water is inexpensive, loids as the catalyst scaffold and found that the col-
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loid-loaded palladium catalyst combines the features
of an easily recoverable heterogeneous catalyst and a
highly efficient homogeneous one.[24] Herein, we im-
mobilize a non-phosphine Pd catalyst of palladium-
iminodiacetic acid (IDA-Pd) on the outer shell of the
pH-responsive core-shell microspheres of poly(styr-
ene-co-methylacrylic acid) (PS-co-PMAA). It is found
that the immobilized catalyst PS-co-PMAA-IDA-Pd
can be highly dispersed in basic reaction medium just
like a homogeneous one and can be separated and
reused like a heterogeneous catalyst by adjusting the
pH of the reaction medium. Suzuki and Heck reac-
tions performed in water or aqueous solution demon-
strate that the quasi-homogeneous PS-co-PMAA-
IDA-Pd catalyst is efficient and easily reusable.
Results and Discussion
Synthesis and Characterization of the Quasi-
Homogeneous PS-co-PMAA-IDA-Pd Catalyst
The PS-co-PMAA core-shell microspheres containing
a polystyrene (PS) core and a poly(methylacrylic
acid) (PMAA) shell were synthesized by the one-
stage, soap-free polymerization of styrene (St) and
methylacrylic acid (MAA) in water as described else-
where.[25] To enhance the stability of the polymeric
scaffold, 5 wt% of the cross-linker divinylbenzene
were added. Compared with the non-cross-linked
core-shell microspheres,[25] the present microspheres
(Figure 1A) have a larger size ranging from 410 to
480 nm. As shown in Figure 1A, the core-shell struc-
ture can be clearly distinguished. The shell thickness
ranges from 30 to 65 nm and the average diameter of
the core is about 350 nm. The average ratio of the
shell thickness to the core diameter, 0.1, is well con-
sistent with the theoretical value, which is calculated
based on the ratio of the feed monomers of St/MAA
and the diameter of the PS-co-PMAA core-shell mi-
crospheres observed by transmission electron micro-
scope (TEM). To load the ligand iminodiacetic acid
(IDA) onto the shell-layer of PMAA to form IDA-
functionalized core-shell microspheres (PS-co-
PMAA-IDA), part of the carboxyl groups in the
PMAA segment were firstly activated with 1-ethyl-3-
(3-dimethylaminopropyl)carbodiimide hydrochloride
Figure 1. TEM images of the PS-co-PMAA core-shell micro-
spheres (A) and Pd nanoparticles synthesized on the shell-
layer of the IDA-functioned core-shell microspheres of PS-
co-PMAA-IDA (B).
(EDC·HCl) followed by treatment with N-hydroxy- 860 cmÀ1 ascribed to the C=O stretching vibration of
sulfosuccinimide sodium salt (sulfo-NHS) as shown in acylamide group in the pendant IDA can be clearly
Figure 2.[26]
observed in the spectrum B, which confirms formation
Figure 3 shows the IR spectra of the core-shell PS- of the IDA-functionalized microspheres of PS-co-
co-PMAA microspheres before (spectrum A) and PMAA-IDA. Figure 4 shows the solid state 13C
after being functionalized with IDA (spectrum B), re- CPMAS NMR spectrum of the IDA-functionalized
spectively. Compared with the spectrum
A in core-shell microspheres of PS-co-PMAA-IDA. As in-
Figure 3, the characteristic absorption at 1650 cmÀ1 dicated by the insets in Figure 4, the characteristic
due to the C=O stretching vibration of the acylamide chemical shifts due to the carbon in the PS and
group amide and the characteristic absorption at PMAA-IDA segments are well recorded, which fur-
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Figure 2. Schematic synthesis of the PS-co-PMAA-IDA-Pd catalyst.
Figure 3. The IR spectra of the PS-co-PMAA (A) and PS-
co-PMAA-IDA (B) core-shell microspheres.
Figure 4. The solid state 13C CPMAS NMR spectrum of the
PS-co-PMAA-IDA core-shell microspheres.
ther confirms the constitution of the PS-co-PMAA-
IDA core-shell microspheres.
in the shell-forming PMAA segment are helpful to
Because IDA can strongly coordinate with various disperse the PS-co-PMAA-IDA-Pd catalyst in basic
transition metal ions such as Cu(II), Ni(II), Pd(II), aqueous solution to form a stable colloidal dispersion.
etc.,[27] the PS-co-PMAA-IDA-Pd catalyst forms when It is found that the PS-co-PMAA-IDA-Pd catalyst is
the PS-co-PMAA-IDA core-shell microspheres are uniformly dispersed in aqueous solution at pH>10
dispersed in PdCl2 aqueous solution. The concentra- and deposition takes place when the pH of the aque-
tion of Pd loaded onto the PS-co-PMAA-IDA core- ous solution is adjusted below 5. The high dispersion
shell microspheres, 0.85 mmolgÀ1, is calculated based of the PS-co-PMAA-IDA-Pd catalyst in basic aque-
on the Pd concentration in the centrifugal aqueous so- ous solution is mainly ascribed to the unmodified car-
lution analyzed by atomic absorption spectrum boxyl groups in the shell-forming PMAA segment of
(AAS). This suggests that only about 60% of the car- the core-shell microspheres. As is known, the Suzuki
boxyl groups in the shell-forming PMAA segment of reaction and the Heck reaction are usually performed
the PS-co-PMAA core-shell microspheres have been in the presence of a base such as K2CO3, K3PO4, or
modified with IDA. The unmodified carboxyl groups Et3N.[1–10] Thus, when the PS-co-PMAA-IDA-Pd cata-
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lyst is used for Suzuki and Heck reactions, the pH re- neous PS-co-PMAA-IDA-Pd catalyst is highly dis-
sponse of the present catalyst provides a great poten- persed in basic aqueous medium just like a homoge-
tial to bridge the gap between the easy recoverable neous one, Suzuki reactions of aryl halides with phe-
heterogeneous catalyst and the highly efficient homo- nylboronic acid are carried out in water in the pres-
geneous one. That is to say, during the reaction, the ence of 0.10 mol% Pd catalyst.
PS-co-PMAA-IDA-Pd catalyst is highly dispersed in
As is known, the base has a strong influence on
the reaction medium just like a homogeneous one; Suzuki reactions.[1–10] In a preliminary catalysis study,
while after the reaction is completed, the PS-co- the base was optimized. As shown in Table 1, the
PMAA-IDA-Pd catalyst can be separated and reused base has great effect on the Suzuki reaction of 4-bro-
just like a typical heterogeneous one.
moacetophenone. Of all the bases tested, K2CO3,
To explore the exact site of the immobilized IDA- Na2CO3, NaHCO3, KOH and Et3N afford similarly
Pd catalyst on the core-shell microspheres, excess high yields of 4-acetylbiphenyl at 908C. In the follow-
NaBH4 aqueous solution was added into the disper- ing study, all Suzuki reactions are performed in water
sion of the PS-co-PMAA-IDA-Pd catalyst. Under employing K2CO3 as the base unless especially point-
these conditions, metal nanoparticles of Pd are ed out.
formed. As shown in Figure 1B, the 3 nm nanoparti-
With the optimized conditions in hand, Suzuki reac-
cles are mainly embedded in the outer shell-layer of tions of a wide range of substrates such as iodinated,
the PS-co-PMAA-IDA core-shell microspheres. This brominated, and chlorinated aromatic compounds
suggests that the IDA-Pd catalyst is immobilized on were tested. The catalysis results are summarized in
the outer shell-layer of the core-shell microspheres. Table 2. The catalysis results suggest that the quasi-
The immobilization of the IDA-Pd catalyst on the homogeneous PS-co-PMAA-IDA-Pd catalyst is highly
outer shell-layer of the PS-co-PMAA core-shell mi- active for both aryl bromides and aryl iodides and all
crospheres provides a potential for easy access to the the Suzuki reactions in water as the sole solvent
active site of the Pd catalyst and therefore high effi- afford satisfactory yields higher than 92% in 1 h
ciency. In the following, the application of the quasi- (Table 2, entries 1–18) except for the Suzuki reaction
homogeneous PS-co-PMAA-IDA-Pd catalyst in of the electron-rich 4-bromoanisole (Table 2,
Suzuki reactions is studied.
entry 19). However, when the reaction time is in-
creased or TBAB is used, the Suzuki reaction of 4-
bromoanisole also affords 88% or 98% yield (Table 2,
entry 19). Besides, the quasi-homogeneous PS-co-
PMAA-IDA-Pd catalyst is especially active for hydro-
philic substrates such as 4-iodobenzoic acid and 4-io-
Suzuki Reaction Employing the Quasi-Homogeneous
PS-co-PMAA-IDA-Pd Catalyst in Water
The Suzuki reaction is usually performed in an organ- dophenol, and 99% yields are achieved in 15 min
ic solvent. However, when a hydrophilic ligand is (Table 2, entries 5 and 7).
used or the Pd catalyst is immobilized on a hydrophil-
Up to now, several elegant systems have been pro-
ic sacffold, Suzuki reactions can be performed in posed to efficiently catalyze Suzuki reactions at room
water as the sole solvent, in solvent mixtures of water temperature in an organic solvent or in water.[29] To
and a polar organic solvent such as N, N-dimethylfor- further evaluate the quasi-homogeneous PS-co-
mamide (DMF), or in water with a suitable surfactant PMAA-IDA-Pd catalyst, Suzuki reactions employing
such as tetrabutylammonium bromide (TBAB) being 0.10 mol% Pd catalyst in water as the sole solvent at
added.[28] Considering that the present quasi-homoge- room temperature were also tested. As shown in
Table 1. Base effect on the Suzuki reaction in water employing the quasi-homogeneous PS-co-PMAA-IDA-Pd catalyst.[a]
Entry
Base
Yield (%)a
Entry
Base
Yield [%][b]
1
2
3
4
5
K2CO3
99
98
99
96
18
6
7
8
9
NaOH
KOH
CH3COONa
Et3N
90
98
90
99
95
Na2CO3
NaHCO3
K3PO4
KH2PO4
10
Bu3N
[a]
Reaction conditions: 4-bromoacetophenone (1.0 mmol) and benzenboronic acid (1.5 mmol, 1.5 equiv.), base (3.0 mmol,
3 equiv.), 4.4 mL of dispersion containing 0.10 mol% PS-co-PMAA-IDA-Pd catalyst, 908C, 1 h.
Isolated yield and purity of the isolated product being confirmed by H NMR.
[b]
1
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