Chitosan as a support for heterogeneous Pd catalysts in liquid phase catalysis

Article abstract of DOI:10.1016/j.apcata.2010.02.029

Four different chitosan-supported palladium catalysts were prepared, whereby two of them were modified as Schiff base by reaction with salicylaldehyde and 2-pyridinecarboxaldehyde before complexation with palladium. The remaining differ in their preparation method: co-precipitation or adsorption. The properties of the catalysts were characterized by FTIR, XPS, ICP-MS, and TGA. Comparison of the catalysts activity was assessed in microwave-assisted Suzuki reactions in aqueous media, resulting in good yields and excellent selectivities concerning cross-coupling product. Additionally, the catalysts prove their activity under conductive heating conditions. The study was extended to microwave-assisted Heck and Sonogashira reactions in DMF, confirming the efficiency of chitosan-supported palladium derivatives as catalysts for C-C couplings. Experiments revealed that catalysts prepared by co-precipitation furnished inferior yields concerning the employed C-C coupling reactions. Modification of chitosan with 2-pyridinecarboxaldehyde and subsequent palladium deposition resulted in highly active catalysts affording high product selectivities and yields.

Full text of DOI:10.1016/j.apcata.2010.02.029

Applied Catalysis A: General 379 (2010) 30–37
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Chitosan as a support for heterogeneous Pd catalysts in liquid phase catalysis
Silke E.S. Leonhardta, Achim Stollea, Bernd Ondruschkaa,∗, Giancarlo Cravottob,
Cristina De Leob, Klaus D. Jandtc, Thomas F. Kellerc
a Institute for Technical Chemistry and Environmental Chemistry, Friedrich-Schiller University Jena, Lessingstr. 12, 07743 Jena, Germany
b Dipartimento di Scienza e Tecnologia del Farmaco, University of Turin, Via Pietro Giuria 9, 10125 Torino, Italy
c Institute of Materials Science and Technology, Friedrich-Schiller University Jena, Löbdergraben 32, 07743 Jena, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
Four different chitosan-supported palladium catalysts were prepared, whereby two of them were modi-
fied as Schiff base by reaction with salicylaldehyde and 2-pyridinecarboxaldehyde before complexation
with palladium. The remaining differ in their preparation method: co-precipitation or adsorption. The
properties of the catalysts were characterized by FTIR, XPS, ICP-MS, and TGA. Comparison of the cata-
lysts activity was assessed in microwave-assisted Suzuki reactions in aqueous media, resulting in good
yields and excellent selectivities concerning cross-coupling product. Additionally, the catalysts prove
their activity under conductive heating conditions. The study was extended to microwave-assisted Heck
and Sonogashira reactions in DMF, confirming the efficiency of chitosan-supported palladium deriva-
tives as catalysts for C–C couplings. Experiments revealed that catalysts prepared by co-precipitation
furnished inferior yields concerning the employed C–C coupling reactions. Modification of chitosan
with 2-pyridinecarboxaldehyde and subsequent palladium deposition resulted in highly active catalysts
affording high product selectivities and yields.
Received 15 December 2009
Received in revised form 10 February 2010
Accepted 22 February 2010
Available online 6 March 2010
Keywords:
Chitosan support
Palladium catalyst
C–C coupling reaction
Catalysis
Microwave
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
fishery waste is the easiest. Despite the insolubility of chitosan
in the majority of common solvents, it can generate films, col-
Palladium is one of the most common transition metals in
organic synthesis, especially for C–C and C–X coupling reactions
[1]. For a long time its use was limited to homogeneous catalysis
causing difficulties in recycling and purification. Environmental-
friendly heterogeneous catalysts strongly stimulate the growth of
this research area [2,3]. In the last two decades a variety of solid-
supported palladium catalysts has been developed by immobilizing
Pd on various supports. These novel catalysts are mainly based
on alumina, silica, and other inorganic oxides like zeolites, active
carbon, and polymers [4–9].
Chitosan is a linear copolymer in which 2-amino-2-deoxy-d-
glucopyranose (glucosamine) is linked to 2-acetamido-2-deoxy-d-
glucopyranose (N-acetyl glucosamine) by a ˇ-(1,4) glycosidic bond
(Scheme 1). This structure offers numerous modification possibil-
ities – the capabilities of this material seem unlimited. Chitosan
is produced by the N-deacetylation of chitin, which is the second
most abundant natural polymer after cellulose. It is estimated that
about 150,000 tons of chitin are available for commercial use annu-
ally. However, only a few thousand tons are actually used world
wide. There are also other possibilities to produce chitosan, but
loids, powders, flakes, gel-beads, and fibers which make chitosan
an excellent candidate as a support for liquid phase catalysis [10].
The increasing interest in chitosan is due to its chemo-physical and
biological properties (hydrophilic, positively charged, biodegrad-
able, non-toxic and biocompatible). It has a high sorption capacity
for metal ions and it is prone to chemical modification. As a result,
many applications for such materials are conceivable including
waste water treatment, pharmaceutical and cosmetic preparations,
heavy metal complexation, and heterogeneous catalysis [11].
Indeed, several applications have already proven the cat-
alytic activity of chitosan-supported metals in reduction reactions
(chromate, phenol, nitro-aromatics), asymmetric hydrogenations,
allylic substitution, cyclopropanation of olefins, oxidation reactions
(alkylbenzenes, aromatics) and carbonylation [12–17]. Macquarrie
and coworkers reported the use of chitosan as a support to anchor
a palladium–pyridyl imine complex to promote Suzuki and Heck
reactions [18], whereas Xu et al. used a chitosan-supported imine
palladacycle complex with salicylaldehyde as the anchoring group
[19].
In this paper the different preparation methods of chitosan-
supported Pd catalysts are compared with each other. Thus, four
different catalysts were prepared and analyzed as well as applied in
Suzuki, Heck, and Sonogashira cross-coupling reactions. Amongst
others the methods described by Hardy et al. [18] and Xu et al.
∗
Corresponding author. Tel.: +49 3641 948400; fax: +49 3641 948402.
E-mail address: Bernd.Ondruschka@uni-jena.de (B. Ondruschka).
0926-860X/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2010.02.029

S.E.S. Leonhardt et al. / Applied Catalysis A: General 379 (2010) 30–37
31
2.1.4. Cross-linking with 2-pyridinecarboxaldehyde (C1)
C1 was prepared following a modified procedure described in
[18]. 2-Pyridinecarboxaldehyde (12 mmol, 1.147 g) was added to
a chitosan suspension (2.0 g) in ethanol (100 ml). The mixture was
heated under reflux for 18 h. After cooling the mixture was filtered-
off and the resulting chitosan-Schiff base (C) was washed with
ethanol and dried at 60 ◦C overnight in a drying oven. C (1.0 g) was
added to a solution of palladium-(II)-acetate (0.5 mmol, 0.113 g,
5 wt.% Pd) in acetone (50 ml) and stirred overnight. The solid cata-
lyst was washed by heating under reflux in a sequence of solvents
(ethanol, toluene and acetonitrile, 3 h each) to remove any unbound
Pd species. Finally, the catalyst was dried at 90 ◦C overnight in a
drying oven to give catalyst C1.
Scheme 1. Structure of chitin (DD < 0.75) and chitosan (DD > 0.75). DD, degree of
deacetylation.
[19] were used for the catalyst preparation. Their application in
Suzuki reaction under thermal conditions and under microwave
irradiation was investigated. Additionally, the chitosan–palladium
catalysts were used in microwave-promoted Heck and Sonogashira
reactions. These experiments were carried out under conditions
similar to those recently reported for the microwave-promoted C–C
couplings using porous glass-supported Pd as catalyst [20].
2.2. Suzuki reaction
2.2.1. Thermal heating in o-xylene
The reaction was prepared following a modified procedure
described in [18]. C1 was suspended in o-xylene (20 ml) and
phenylboronic acid (7.5 mmol, 0.914 g), bromobenzene (5.1 mmol,
0.805 g), K2CO3 (10 mmol, 1.382 g), and n-undecane (4.4 mmol,
0.686 g) as internal standard were added. The mixture was heated
under reflux and monitored by gas chromatography (GC). For
reuse purposes the catalyst was filtered-off from the reacted mix-
ture, washed with water, ethanol and acetone and dried overnight
at 90 ◦C in a drying oven. The same protocol was used for p-
bromophenol and p-bromotoluene.
2. Experimental
All chemicals were purchased from Sigma–Aldrich and were
used without further purification. Chitosan of medium molecu-
lar weight (from crab shells, 448877, Sigma–Aldrich) was used as
support material.
2.1. Catalysts preparation
2.2.2. Thermal heating in water
Phenylboronic acid (4.2 mmol, 0.514 g), p-bromoacetophenone
(4 mmol, 0.796 g), Na2CO3 (16 mmol, 1.696 g), deionized water
(8 ml) and requested amount of catalyst, were added to a 100-ml
flask equipped with a magnetic stirrer. Reactions were performed
in the carousel 6-place reaction station. The heating-up (to boiling
temperature) tookapproximately20 min, andthen the mixture was
refluxed (outside temperature 130–140 ◦C) for 1 h. After the reac-
tion was complete the mixture was cooled down in a water bath
under stirring. A sample for GC-analysis was prepared by extrac-
tion with MTBE (30 ml) at room temperature. For reuse purposes
the catalyst was filtered-off from the reacted mixture, washed with
water, ethanol and acetone and dried overnight at 80 ◦C in a drying
oven.
2.1.1. Co-precipitation method (A1)
A1 was prepared following a modified procedure described in
[21]. PdCl2 (0.1 mmol, 0.0182 g, 1 wt.% Pd) which was dissolved in
0.1 M HCl was added to a 1.5 wt.% solution of 1 g chitosan in 0.1 M
HCl. The mixture was stirred at room temperature until a clear
yellow solution was obtained. This yellow solution of the homo-
geneous palladium–chitosan complex was added dropwise to a
beaker containing a 0.5 M NaOH solution (350 ml). This led to an
immediate coagulation of the drops into spherical globules (beads)
with a diameter of approx. 1–3 mm. The beads formed were fil-
tered and washed with deionized water until neutrality. Then, the
catalyst was air-dried at room temperature for 48 h.
2.1.2. Adsorption method (A2)
2.2.3. Microwave-assisted Suzuki reaction
A2 was prepared following a procedure described in [21]. A
1.5 wt.% chitosan solution (1 g chitosan solved in 0.1 M HCl) was
added dropwise to a beaker containing a 0.5 M NaOH solution
(350 ml). The resulting pure chitosan beads were filtered-off and
washed with deionized water until neutrality and were air-dried
at room temperature for 48 h. The PdCl2 (0.5 mmol, 0.083 g, 5 wt.%
Pd) was suspended in 20 ml deionized water and stirred with 1.0 g
of the dried pure chitosan beads for 20 min. The beads were then
filtered-off, washed with deionized water and dried for 24 h at room
temperature.
The reaction was prepared following a modified procedure
accomplished in former work of our group, cf. [20]. Phenylboronic
acid (2.1 mmol, 0.256 g), p-bromophenol (2 mmol, 0.346 g), Na2CO3
(8 mmol, 0.848 g), deionized water (4 ml) and the requested
amount of catalyst were added to a microwave-suited mono-prep
vessel (MLS GmbH Leutkirch) equipped with a magnetic stirring
bar. Reactions were performed in the microwave device praktika
(MLS GmbH Leutkirch; temperature program: up to 150 ◦C in 2 min
at 500 W, hold for 10 min at 150 ◦C at max. 400 W). After the reac-
tion was complete the mixture was cooled down under stirring.
A sample for GC-analysis was prepared by extraction with MTBE
(10 ml) at room temperature. The same protocol was followed for
p-bromoacetophenone.
2.1.3. Cross-linking with salicylaldehyde (B1)
B1 was prepared following a modified procedure described in
[19]. Chitosan (4 g), salicylaldehyde (63 mmol, 7.724 g) and glacial
acetic acid (6.5 ml) were refluxed in methanol (65 ml) for 10 h. After
completion of the reaction, the yellow chitosan-Schiff base (B) was
collected by filtration, washed five times with ethanol and dried at
70 ◦C. B (1.5 g) was then stirred for 10 h at room temperature with
palladium chloride (0.3 mmol, 0.053 g, 2 wt.% Pd) in MeOH (30 ml)
in the presence of NaOAc (0.3 mmol, 0.026 g). Then, the chitosan-
supported imine palladacycle catalyst B1 was filtered, washed with
ethanol, and dried at 70 ◦C over night.
2.3. Heck and Sonogashira reactions
Iodobenzene (2 mmol, 0.408 g), styrene (3.2 mmol, 0.333 g) or
phenyl acetylene (3.2 mmol, 0.328 g), NaOAc (0.264 g, 3.2 mmol),
n-undecane (1 mmol, 0.156 g) as internal standard, a quantity of
catalyst and 20 ml N,N-dimethylformamide (DMF) were trans-
ferred into a mono-prep vessel (MLS GmbH Leutkirch), placed in
the microwave device praktika (temperature program: up to 190 ◦C

32
S.E.S. Leonhardt et al. / Applied Catalysis A: General 379 (2010) 30–37
in 2 min at max. 500 W, hold at 190 ◦C for 10 min at max. 400 W).
The reaction was monitored by GC.
2.4. Characterization
2.4.1. Gas chromatography (GC)
The GC-analyses of the extracted C–C coupling reaction mix-
tures were carried out by GC-FID and GC-MSD. Conditions
GC-FID for Suzuki reaction: HP-5, 30 m × 0.32 mm × 0.25 ␮m, H2:
5 psi (for o-xylene) and 12 psi (for MTBE), program: 50 ◦C (hold
for 3 min), 5 K min−1 up to 150 ◦C, 20 K min−1 up to 200 ◦C
(hold for 5 min), injector temperature: 280 ◦C, detector tempera-
ture: 300 ◦C. Conditions GC-FID for Heck and Sonogashira: HP-5,
30 m × 0.32 mm × 0.25 ␮m, H2: 12 psi, program: 70 ◦C, 20 K min−1
up to 290 ◦C (hold for 14 min), injector temperature: 280 ◦C, detec-
tor temperature: 300 ◦C. The same settings were used for GC-MSD
but with He as carrier gas.
Fig. 1. Recorded FTIR-spectra (ATR-method) of chitosan (bottom), catalyst A2 (mid-
dle) and catalyst A1 (top).
2.4.2. Fourier-transform infrared spectroscopy (FTIR)
Transmission FTIR-spectra were recorded in the range of
4000–550 cm−1 at a resolution of 4 cm−1 using a “spectrum
100 series” spectrometer (PerkinElmer precisely, Wiesbaden, Ger-
many). The catalysts were placed in solid form on the ZnSe-crystal
(ATR-method, ATR-correction with contact: 0, software: spec-
trum).
and consequently catalysis is very small, because all weighed in Pd
(0.08 mmol g−1) is distributed in the whole particle (and so only a
very small portion is located on the particle surface), in agreement
with the surface-sensitive XPS analysis which showed weak peaks
from the fraction of Pd located on the particle surface (cf. Fig. 2).
Nevertheless, signals of a Pd(II)-species (341 eV and 336 eV) could
still be identified.
It should be noted that the PdCl2 employed has a low solubil-
ity in water, ethanol, and methanol and a coarse consistence which
leads to difficulties of preparing a suspension with a consistent par-
ticle distribution for the adsorption method (A2, cf. Section 2.1.2).
With the adsorption method (see Section 2) the Pd(II)-salt was
not adsorbed quantitatively. However, the amount of Pd adsorbed
is located on the support surface and is therefore available for
catalysis. The FTIR-spectrum of A2 in Fig. 1 shows in the range
of 3540–3080 cm−1 the OH-/NH-bands which are shifted to lower
wave numbers in comparison to the pure chitosan spectrum, indi-
cating that the complexation of PdCl2 influences either the OH- or
the NH-vibrations. Due to the Pd coordination the bond to the H-
atom becomes longer and weaker resulting in this shift. However,
since the band becomes wider (that means only the NH-band is
shifted) the Pd is mainly coordinated to the N-atom. With respect
to the lower Pd loading on the A1 surface the FTIR spectrum of A1
does not show this shift so clearly.
2.4.3. Inductively coupled plasma mass spectrometry (ICP-MS)
For ICP-MS analysis small amounts of the Pd catalysts were
digested with HNO3 and aqua regia (1 HNO3/3 HCl). The residue
was dissolved in 1 ml aqua regia and diluted with 10 ml deionized
water. The measurements were performed by a Quadrupole-ICP-
MS X Series II (ThermoFisher Scientific, Bremen, Germany).
2.4.4. X-ray photoelectron spectroscopy (XPS)
To define the oxidation state of the adsorbed palladium, the
energy of the binding electrons was determined by X-ray photo-
electron spectroscopy with a Quantum 2000 (PHI Co., Chanhassen,
MN, USA) with a focused monochromatic Al K␣ source (1486.7 eV)
for excitation. The pass energy was 23.5 eV and the reference was
the C1s peak of the C–C bond. Two signals at 341 eV and 336 eV
indicate Pd(II)-species and signals at 338 eV and 333 eV suggest
Pd(0)-species.
XPS analysis of A2 catalyst for the 3d3/2- (Eb = 341.2 eV) and
3d5/2-core level (Eb = 336.2 eV) of palladium indicated the pres-
ence of Pd(II)-species, cf. Fig. 2. ICP-MS analyses revealed that only
approximately 25% of the submitted Pd (5 wt.%) was adsorbed on
the chitosan beads (approx. 0.12 mmol g−1). That shows a quanti-
tative adsorption of the provided Pd cannot be assumed. It refers to
optical observations (confirmed by microscopical pictures, 5000-
fold magnification) that the Pd–chitosan layer of A2 is not closed.
During drying the catalyst the outer layer can burst and so there
are Pd-free points on the globules, what makes the loading inho-
mogeneous.
2.4.5. Elementary analysis
The C-, H-, N-, and Cl-contents of the catalysts were determined
by elementary analysis. The investigations were accomplished with
an Elementar Analysator Vario EL III (Analytik Jena GmbH, Ger-
many).
2.4.6. Thermo-gravimetric analysis (TGA)
TG curves (heating rate 10 ◦C min−1) of the support material and
the catalysts were recorded by the DTG-60 (Simultaneous DTA-TG
Apparatus, Shimadzu, Kyoto, Japan) under atmospheric conditions
(air 30 ml min−1).
The produced beads of A1 and A2 have a diameter of approx-
imately 2 mm and are not perfectly round. Elementary analysis
showed that the C/H-, C/N- and H/N-ratios of both catalysts are
similar to pure chitosan. No chemical modification of chitosan took
place as already seen in Fig. 1. The chloride content of A2 was
higher than that of A1 in elementary analysis. This is caused by the
higher Pd loading of A2 and the assumption that two of the four
coordinating centers of Pd are taken by chloride in the adsorption
complex, and according to Kramareva et al. residual halogen atoms
are removed by the alkaline treatment during the co-precipitation
method (A1) indicating chelate ligands instead of monodentate lig-
ands [21].
3. Results and discussion
3.1. Preparation and characterization of the catalyst systems
3.1.1. Unmodified chitosan–Pd catalysts
With the co-precipitation method (A1, cf. Section 2.1.1) PdCl2
was consistently distributed in the chitosan solution and pre-
cipitated together with the biopolymer, which implies that the
Pd is arranged homogeneously in the chitosan sphere. The FTIR-
spectrum for A1 in Fig. 1 does not differ much from that of
untreated chitosan. The available Pd amount for surface analysis

S.E.S. Leonhardt et al. / Applied Catalysis A: General 379 (2010) 30–37
33
Fig. 2. XPS-spectra of the catalyst systems A2, C1, A1 and B1. Peaks at 342 + 337 eV and 340 + 335 eV are assigned to Pd(II)- and Pd(0)-species, respectively.
3.1.2. Modified chitosan–Pd catalysts
750 (␦ C–H → substituted benzene), 1210 (␯–C–O → phenol), 1270
In Scheme 2 the modified chitosan supports B and C are shown.
It is known that chitosan already has good complexation properties
due to its structure (chelate ligand). In both cases the amino-group
of chitosan was functionalized by an aromatic aldehyde to offer the
Pd a more stable environment for complexation. On the one hand
chitosan was modified with salicylaldehyde (B) and on the other
hand with the pyridine derivative 2-pyridinecarboxaldehyde (C).
The FTIR-spectra in Fig. 3 indicate the differences between
the catalysts obtained from the two modes of modification.
Comparison of B to unmodified chitosan reveals differences at
(␯–C–H → amine), 1490 and 1625 (aromatics), and 3055 cm−1
(␯ C–H → aromatic compounds). These bands are due to a modifi-
cation with a substituted phenol derivative. In the region of OH-
and NH-vibrations it is evident that two bands (3350 cm−1 and
3280 cm−1 → prim. amine) became one band (at 3446 cm−1), which
is shifted to higher wave numbers.
The successful functionalization to an imine is indicated by sev-
eral observations. Firstly, the intensity of the NH-bands decreased
Scheme 2. Structure of the modified chitosan supports of catalyst systems B1 (sal-
icylaldehyde) and C1 (2-pyridinecarboxaldehyde).
Fig. 3. Recorded FTIR-spectra (ATR-method) of support B, catalyst B1, chitosan,
support C and catalyst C1.

34
S.E.S. Leonhardt et al. / Applied Catalysis A: General 379 (2010) 30–37
Fig. 4. Recorded TG-curves of chitosan (CTS) and the prepared catalyst systems A1, A2, B1, and C1 under atmospheric conditions.
Table 1
(mask by OH absorption), and no C O-bands (free aldehyde) are
visible in the FTIR-spectrum of Fig. 3. Secondly, elementary anal-
ysis confirmed the chemical modification, because the C/H-ratio
rose from 5.6 to 9.1 and the C/N-ratio also increased from 5.5 to
10.6. Furthermore, XPS data shown in Fig. 2 indicate the coexis-
tence of different oxidation states of palladium, Pd(II) and Pd(0).
ICP-MS analyses indicated 1.4 wt.% Pd (70% of the weighed in
Pd; 0.13 mmol g−1) for B1. The salicylaldehyde-modified chitosan
showed good adsorption properties – the imine offers the Pd a bet-
ter possibility for complexation than pure chitosan – but because
of the used flakes also a larger surface area than the chitosan beads.
The preparation of C1 as described in the experimental sec-
tion takes a long time because of the additional washing steps to
avoid leaching later. During the refluxing in ethanol the color of C1
turns from orange to dark brown with a whiff of anthracite gray,
indicating a change in valency of Pd. Comparison of the three FTIR-
spectra showed a new band at 770 cm−1 in the spectra of C and
C1, which is a characteristic band for disubstituted benzenes (cf.
Fig. 3). At 1645 cm−1 a noticeably stronger absorbance caused by
the additional C N vibration can be seen. It is also evident that the
wide OH/NH-band of chitosan shifted to higher wave numbers and
turned into one band. The band of C1 is less shifted than that of
C. This may be due to the removal of excess imine-functions dur-
ing the washing steps. The introduction of an aromatic nitrogen
compound in the catalyst system is also confirmed by elementary
analysis, causing a decrease of the H/N ratio from 1.0 to 0.8 and an
increase of the C/H-ratio from 5.6 to 6.9. As shown in Fig. 2, XPS anal-
ysis indicates well-defined Pd(0)-species on the catalyst surface.
ICP-MS indicated a Pd content of 3.3 wt.% (0.31 mmol g−1) which is
twice as high as that indicated in the publication of Macquarrie et
al. who performed the washing step three times [18]. Reproducibil-
ity of the Pd content depends mainly on the exact performing of the
washing step.
Overview of the thermal Suzuki coupling reaction in o-xylene with catalyst C1.
Ra
H
Catalyst use
t (h)
Y (%)
S (%)
1
2
1
1
83
90
97
>99
CH3
1
1
3
4
1
2
2
2
78
95
80
67
99
>99
>99
>99
OH
1
1
1
2
58
62
99
>99
COCH3
1
2
3
1
1
1
87
70
49
>99
>99
>99
Condition: 5.1 mmol aryl bromide, 7.5 mmol phenylboronic acid, 10 mmol K2CO3,
25 mg catalyst C1 (0.15 mol% Pd), 20 ml refluxing o-xylene.
a
Functional group of aryl bromide, cf. Scheme 3.
first mass decrease is caused by the evaporation of the assimilated
water. Then the complex begins to lose weight significantly due
to the oxidative decomposition of the polymer support chitosan.
That corresponds approximately to the thermal stability of pure
chitosan.
3.2. Catalyst activity in Suzuki coupling reactions
Phenylboronic acid was coupled to some aryl bromides in o-
xylene and water for thermal Suzuki reaction and also in water for
investigation of the Suzuki reaction under microwave irradiation
(cf. Scheme 3).
3.2.1. Thermal Suzuki reaction in o-xylene
The results of the thermal Suzuki reaction in o-xylene are listed
in Table 1. With respect to the results reported by Macquarrie
and coworkers the yields for bromobenzene and p-bromotoluene
could be confirmed [18]. Further, as expected good results (87%)
were found for the activated p-bromoacetophenone after 1 h in
Comparison of the catalysts A2, B1, and C1 the different oxida-
tion states of Pd after preparation attract attention (cf. Fig. 2). This is
probably due to the employment of alcohol during the preparation
step. After thermal treatment of C1 in boiling ethanol the change in
valency of Pd could be observed by change in colour (yellow to dark
brown). For A2 (Pd(II) species) no ethanol was used, but B1 (Pd(II)
and Pd(0) species) was stirred in methanol at room temperature for
a few hours. The stirring in alcohol at air lead to partial reduction
of Pd, which is furthermore supported by increased temperature.
After reaction only Pd(0) is detected by XPS analysis.
Thermal stability is necessary for the catalytic activity and recy-
clability of a catalyst, because C–C coupling reactions are carried
out under high temperature conditions. The TG-curves in Fig. 4
show that the catalyst systems are stable up to 230 ◦C. Therefore
the supports are suited for liquid phase catalysis up to 200 ◦C. The
Scheme 3. Schematic illustration of Suzuki model reactions.

S.E.S. Leonhardt et al. / Applied Catalysis A: General 379 (2010) 30–37
35
Table 2
was quantitative even after shorter reaction times (5 min) or lower
temperature (130 ◦C) (cf. Table 2). In a comparison between the
two different educts, surprisingly the deactivated p-bromophenol
seems to be the more active Suzuki reactant. However, the activ-
ity of C1 in the Suzuki reaction of p-bromophenol (cf. Table 2)
is much lower than in the reaction of p-bromoacetophenone
(cf. Table 3). Higher catalyst amounts of C1 decrease the yield
and selectivity of the Suzuki product considerably (with 100 mg
C1 only by-products of the educts were found). Longer reac-
tion times did not improve the yield significantly either although
selectivity decreased. A1 is also active in this reaction but needs
higher catalyst amounts for good yields because of the smaller
amount of available Pd on the catalyst surface, as mentioned
above.
Recently Yi et al. published results for microwave-promoted
Suzuki reactions in water catalyzed by chitosan-supported Pd(0)
[22]. They used cross-linked chitosan (cross-linking agents: glu-
taraldehyde or diglycidyl ether polyethylene glycol) for Pd-support
and tetrabutyl-ammonium bromide (TBAB) as phase-transfer cat-
alyst (PTC). Our results show, that for A2 and B1 it is not necessary
to use an additional PTC under these conditions.
Parameters and results of the microwave-promoted Suzuki coupling reactions of
p-bromophenol with phenylboronic acid in water (cf. Scheme 3).
Catalyst
mol% Pd
0.12
t (min)
Y (%)
S (%)
10
10
10
58
73
81
97
96
>99
A1
0.28
a
0.19
10
5
5
>99
>99
64
>99
>99
>99
A2
B1
0.19
a
0.20
10
10
5
>99
72
>99
95
>99
99
a
0.20
0.20
>99
10
99b
C1
0.25
0.48
10
10
55
45
98
95
Condition: 2 mmol p-bromophenol, 2.1 mmol phenylboronic acid, 8 mmol Na2CO3,
4 ml water; microwave settings: T = 150 ◦C, Pmax = 400 W.
a
2nd catalyst use.
b
Reaction temperature was 130 ◦C.
the first run of catalyst C1. Additionally, p-bromophenol was cou-
pled, but Table 1 shows that the activity of C1 for the deactivated
p-bromophenol is considerably weaker with 62% yield after 2 h.
However, the reaction was as selective as the other coupling reac-
tions (99% and more) with respect to the educt.
In comparison to the results of the Suzuki reaction in o-xylene
(cf. Section 3.2.1) and in water at nearly the same reaction temper-
ature the yields are similar for catalyst C1 (cf. Tables 1–3). However,
in direct comparison the approaches differ not only in the choice of
the solvent, but also in reaction time and in the ratio between Pd
and aryl compound. The reaction time in o-xylene is six times as
much as that in water but the ratio Pd/aryl in water is at least twice
as much as the ratio in o-xylene. The calculation of the TOF results
in 980 h−1 (0.48 mol% Pd) for the reaction of p-bromoacetophenone
in water and reaches 570 h−1 (0.15 mol% Pd) for the reaction in
o-xylene. For p-bromophenol TOF are 1330 (0.25 mol% Pd) and
560 h−1 (0.48 mol% Pd) in water and 380 h−1 (0.15 mol% Pd) for the
reaction of p-bromophenol in o-xylene.
Bromobenzene is very reactive but the Suzuki coupling product
biphenyl is also a by-product of the homocoupling of phenyl-
boronic acid, so we investigated the recycling properties of C1
with p-bromotoluene and p-bromoacetophenone. Despite wash-
ing the catalyst with aqueous ethanol, methyl-tert-butylether
(MTBE) and methanol before reuse, it was found that with every
recycling step the activity decreased (between the first and the
second run by nearly 50%) and with increasing reuse, the selec-
tivity decreased slowly as well. After using warm solvents for
washing the catalyst, better yields and selectivities were reached
(cf. Table 1), although GC-analyses showed no significant 2-
pyridinecarboxaldehyde leaching either in the reaction or in the
filtrate solution. However, elementary analyses indicate desubsti-
tution because of a decrease in C/H and increase of the C/N ratio.
3.2.3. Thermal Suzuki reaction in water
As mentioned, for Suzuki reaction in water these investigations
resulted in very good yields and selectivities, but the temperature
is very high, which of course in general increases the reaction rate.
Hence, also Suzuki reactions in water under thermal heating were
accomplished in a 6-place reaction station. The yields shown in
Fig. 5 are even higher after 1 h without the assistance by a PTC
and also the selectivities are very high (>99%). Except for catalyst
A1 the recycling behavior is much better than under microwave
irradiation, 74% and 84% for A2 and B1, respectively, in the 3rd cycle
(cf. Fig. 5). This could be caused by the lower reaction temperature.
It is known that chitosan is only stable up to 230 ◦C (cf. Fig. 4) and
maybe the inhomogeneous heating of the metal-loaded support
surface, caused by microwaves, leads to any modifications.
C1 even indicates an increase in activity with increasing reuse
independent from the catalyst amount. Thus, results underline
that the catalyst undergo activation during application and work-
up. This behavior is rather uncommon for Pd catalysts in Suzuki
reactions. Perhaps, a change of the coordination site of Pd during
reaction and work-up is responsible for this effect. In addition a
few approaches with one mol-equivalent TBAB were performed
for B1 and C1. As expected, for B1 the yield of p-acetylbiphenyl
became quantitative also in the 2nd cycle. For C1 the use of a PTC
was unfavorable, the reaction yields approx. 20% of the coupling
product with a selectivity of 93% for all three reaction cycles. At the
moment there are no verified explanations for these uncommon
results obtained for C1. For p-bromophenol the use of B1 (30 mg)
gave quantitative yield without any PTC and decreased to 86% with
PTC. Unlike C1 (30 mg) yielded for p-bromophenol 2% of the Suzuki
coupling product without and 11% with PTC.
3.2.2. Microwave-assisted Suzuki reaction in water
The microwave-assisted Suzuki reaction was investigated for all
four catalyst systems and the coupling of phenylboronic acid with
p-bromophenol and p-bromoacetophenone (cf. Tables 2 and 3). B1
and A2 showed the best activities. For p-bromophenol the yield
Table 3
Parameters and results of the microwave-promoted Suzuki coupling reactions of
p-bromoacetophenone with phenylboronic acid in water (cf. Scheme 3).
Catalyst
A1
mol% Pd
0.25
t (min)
Y (%)
S (%)
10
64
98
0.19
10
15
15
15
81
84
74
40
>99
>99
>99
>99
A2
0.19
a
b
B1
C1
0.20
10
10
15
89
63
94
>99
>99
99
a
0.20
0.48
10
10
10
79
71
57
98
>99
94
a
b
Condition: 2 mmol p-bromoacetophenone, 2.1 mmol phenylboronic acid, 8 mmol
Na2CO3, 4 ml water; microwave settings: T = 150 ◦C, Pmax = 400 W.
a
2nd catalyst use.
3rd catalyst use.
b

36
S.E.S. Leonhardt et al. / Applied Catalysis A: General 379 (2010) 30–37
Fig. 5. Results of the Suzuki coupling reaction of p-bromoacetophenone (4 mmol) with phenylboronic acid (4.2 mmol) in refluxing water (1 h) in the presence of Na2CO3
(16 mmol). All selectivities over 99% (1, 2, and 3 indicate the catalyst run).
Scheme 4. Schematic illustrations of Heck and Sonogashira model reaction: (a) Heck reaction of styrene and iodobenzene to cis- and trans-stilbene. (b) Sonogashira reaction
of iodobenzene and phenyl acetylene to tolane under microwave irradiation.
3.3. Catalyst activity in Heck reaction and Sonogashira reaction
agent: epoxy chloropropane) gave 97% yield at 80 ◦C in DMF after
3 h with an organic base [24]. In another paper Zhang et al. tested
a chitosan microsphere-resin in the Heck reaction (90 ◦C) with
sodium acetate as inorganic base in water, but even after 14 h no
Heck coupling product was found [25]. As expected, higher tem-
peratures increase the reaction rate, and as shown in Table 4 same
yields could be confirmed even after 20 min with good selectivities
and the use of an inorganic base in DMF.
Compared to Macquarrie and coworkers, who used for their cou-
pling of styrene and bromobenzene (100 ◦C, in anhydrous dioxane)
a C1-analogous catalyst yielding 88% of the coupling product within
42 h, the yields for C1 listed in Table 4 are just as good (87%) after
as little as 20 min [18]. Of course, this is mainly affected by the
increased reaction temperature and the higher Pd loading of the
catalyst. Additionally, NaOAc instead of triethylamine was used,
which is soluble in DMF at high temperatures without a phase-
transfer catalyst.
3.3.1. Microwave-assisted Heck reaction
N,N-Dimethylformamide (DMF) and N,N-dimethylacetamide
were used as solvents, and analysis of the product mixtures
revealed that DMF gave better styrene conversions in the Heck
reaction (cf. Scheme 4a). In contrast to the results of the Suzuki
reaction C1 resulted in the best yields (84%) and B1 resulted in the
worst yields (22%) after 10 min at 190 ◦C (cf. Table 4).
Liu et al. reported under thermal conditions a yield of 80% (DMF,
120 ◦C, 8 h) for the Heck coupling of iodobenzene and styrene
with their chitosan–salicylaldehyde–Pd catalyst (B1-analogous,
but other preparation) [23]. Their chitosan–palladium catalyst (A2-
analogous, but not in form of beads) formed after 5 h at 80 ◦C
in DMF 93% trans-cinnamic acid [19]. They used an organic base,
longer reactions times and milder reaction conditions. Cross-linked
chitosan–salicylaldehyde–Pd catalyst of Cui et al. (cross-linking
3.3.2. Microwave-assisted Sonogashira reaction
Table 4
Up to the present time, a chitosan-supported Pd catalyzed
Sonogashira reaction was not found in literature. However, Mac-
quarrie and coworkers published microwave-assisted Sonogashira
coupling reactions catalyzed by biomaterial-supported Pd cata-
lysts, for example with starch [9d,26]. In the Sonogashira reaction
of iodobenzene and phenyl acetylene without additional Cu(I) it
was observed that all chitosan-supported catalysts have a lower
selectivity than in the other C–C coupling reactions (cf. Table 5).
The conversion of iodobenzene is excellent even after 3 min of
microwave irradiation, but the reactions yielded in a lot of par-
tially unknown side-products (known side-products were benzene,
stilbene and triphenylethylene). Thus, it can be concluded that
the unintended products result either from dehalogenation of
Results of the Heck reaction assisted by microwave irradiation (cf. Scheme 4a).
Catalyst
A1
mol% Pd
0.24
t (min)
Ya (%)
Sa (%)
10
40
89
0.19
10
20
82
90
94
94
A2
B1
C1
0.20
0.48
10
20
20
82
74
93
10
20
84
87
96
94
Condition: 2 mmol iodobenzene, 3.2 mmol styrene, 3.2 mmol NaOAc, 20 ml DMF;
microwave settings: T = 150 ◦C, Pmax = 400 W.
a
cis- and trans-stilbene.

S.E.S. Leonhardt et al. / Applied Catalysis A: General 379 (2010) 30–37
37
Table 5
analysis, B. Fähndrich from the Institute for Technical Chemistry
and Environmental Chemistry in Jena for TGA, B. Lentvogt from the
Institute of Organic Chemistry and Macromolecular Chemistry in
Jena for elementary analysis and D. Merten from the Institute of
Geosciences in Jena for performing ICP-MS analysis. For the pro-
vided help special thanks goes to M. Hoffmann in Brockport, New
York.
Results of the microwave-promoted Sonogashira reaction of phenylacetylene with
iodobenzene to tolane (cf. Scheme 4b).
Catalyst
mol% Pd
t (min)
T (◦C)
Ya (%)
Sa (%)
0.40
0.80
10
10
170
190
70
64
60
64
A1
0.19
0.60
10
10
150
190
67
54
62
54
A2
B1
C1
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0.20
0.65
3
10
170
190
48
20
47
20
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Acknowledgements
The authors would like to thank R. Wagner from the Institute of
Materials Science and Technology in Jena for performing the XPS