Synthesis of polystyrene-supported Pd(II)-NHC complex derived from theophylline as an efficient and reusable heterogeneous catalyst for the Heck-Matsuda cross-coupling reaction

Article abstract of DOI:10.1016/j.molcata.2016.03.045

The polystyrene-supported palladium(II)-N-heterocyclic carbene complex PS-NHC-Pd(II) was successfully synthesized from theophylline as an environmentally benign NHC precursor, using chloromethylated polystyrene resin. The PS-NHC-Pd(II) complex was characterized by various techniques including Fourier transform infrared spectroscopy (FTIR), energy dispersive X-ray spectroscopy (EDX), thermal analysis (TGA-DTG), inductively coupled plasma (ICP), and CHN elemental analysis. The morphology of the polymer beads was also studied using scanning electron microscope. The polymer supported Pd(II)-NHC complex exhibits excellent catalytic activity and stability for the Heck-Matsuda cross-coupling reaction under mild conditions. A variety of arenediazonium tetrafluoroborate salts was coupled with the olefinic substrates in ethanol, in the presence of the 0.9 mol% of the catalyst to afford the corresponding cross-coupling adducts in high yields under aerial conditions. Moreover, the heterogeneous catalyst can be easily recovered by simple filtration and reused for seven cycles without significant loss in its activity.

Full text of DOI:10.1016/j.molcata.2016.03.045

Accepted Manuscript
Title: Synthesis of polystyrene-supported Pd(II)-NHC
complex derived from theophylline as an efficient and
reusable heterogeneous catalyst for the Heck-Matsuda
cross-coupling reaction
Author: Elmira Mohammadi Barahman Movassagh
PII:
S1381-1169(16)30122-4
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.molcata.2016.03.045
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10-3-2016
31-3-2016
Please cite this article as: Elmira Mohammadi, Barahman Movassagh,
Synthesis of polystyrene-supported Pd(II)-NHC complex derived from
theophylline as an efficient and reusable heterogeneous catalyst for the Heck-
Matsuda cross-coupling reaction, Journal of Molecular Catalysis A: Chemical
http://dx.doi.org/10.1016/j.molcata.2016.03.045
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Synthesis of polystyrene-supported Pd(II)-NHC complex derived
from theophylline as an efficient and reusable heterogeneous
catalyst for the Heck-Matsuda cross-coupling reaction
Elmira Mohammadi, Barahman Movassagh*
Department of Chemistry, K.N. Toosi University of Technology, P.O. Box 16315-1618, Tehran, Iran; Tel:
+98-21-23064323; Fax: +98-21-22853650
E-mail: bmovass1178@yahoo.com
Graphical abstract
Highlights
 A new polystyrene-supported Pd(II)-NHC catalyst based on theophylline was synthesized.
 Heck-Matsuda arylation of olefins with arenediazonium salts performed by the catalyst.
 Desired products obtained in good to excellent yields.
 The heterogeneous catalyst was reused for seven runs.

Abstract: The polystyrene-supported palladium(II)-N-heterocyclic carbene complex PS-NHC-
Pd(II) was successfully synthesized from theophylline as an environmentally benign NHC
precursor, using chloromethylated polystyrene resin. The PS-NHC-Pd(II) complex was
characterized by various techniques including Fourier transform infrared spectroscopy (FTIR),
energy dispersive X-ray spectroscopy (EDX), thermal analysis (TGA-DTG), inductively coupled
plasma (ICP), and CHN elemental analysis. The morphology of the polymer beads was also
studied using scanning electron microscope. The polymer supported Pd(II)-NHC complex
exhibits excellent catalytic activity and stability for the Heck-Matsuda cross-coupling reaction
under mild conditions. A variety of arenediazonium tetrafluoroborate salts was coupled with the
olefinic substrates in ethanol, in the presence of the 0.9 mol% of the catalyst to afford the
corresponding cross-coupling adducts in high yields under aerial conditions. Moreover, the
heterogeneous catalyst can be easily recovered by simple filtration and reused for seven cycles
without significant loss in its activity.
Keywords: N-Heterocyclic carbene, Heterogeneous catalyst, Theophylline, Polystyrene, Heck-
Matsuda cross-coupling.
1. Introduction
Over the past decades, palladium-catalyzed transformations have proved to be powerful
tools in both microscale laboratory and large scale syntheses [1]. Among various Pd-mediated
bond-forming reactions developed to date, the cross-coupling of an aryl moiety with an olefin
skeleton, referred to as the Heck reaction, is an important synthetic route since the resultant
functionalized olefins are ubiquitous in numerous natural products [2], biologically active
pharmaceuticals [3], and as precursor for materials science applications [4]. The traditional
Mizoroki-Heck reaction involves a cross-coupling reaction between aryl and alkenyl halides and
olefins. However, these reactions are strongly limited by harsh reaction conditions, low
functional group tolerance, and high cost of the reagents. A breakthrough has been made by
Matsuda and co-workers who utilized arenediazonium salts instead of aryl halides as the
electrophilic coupling fragments [5]. The practical benefits of using arenediazonium salts over
common electrophiles such as aromatic halides and triflates contribute the economic,
environmental, and synthetic accessibility due to the fact that these species can be easily
prepared from cheap and available aromatic amines in high yields [6]. Furthermore, the Heck-

Matsuda reaction is usually performed at lower temperatures (between 20-50 ºC) compared to
that of the typical Mizoroki-Heck reaction. This is due to the high reactivity of the diazonium
salts in comparison to aryl halides and evolution of the neutral nitrogen gas as the leaving group
[6c, 7]. Moreover, in the presence of arenediazonium salts, the coupling reaction can be
conducted under additive- and base-free conditions which expand the scope of the methodology
to the substrates with sensitive functional groups [8]. Various homogeneous palladium catalysts
have been designed for the cross-coupling reactions of the arenediazonium tetrafluoroborate
salts with olefins [9]. However, limitations in terms of the recovery of the catalyst and
contamination of the final product with toxic residues have lessened the utility of this class of
catalysts in organic reactions. These shortcomings can be efficiently overcome by anchoring the
palladium ions on suitable solid supports. Thus, diverse inorganic and organic solid supports for
immobilizing Pd- species have been developed, including carbon nanotubes (CNTs) [10], silica-
based supports [11], magnetic nanoparticles [12], polymers [13], bio-polymers [14], zeolites
[15], molecular sieves [16], and metal oxides [17]. Among several kinds of polymers,
chloromethylated polystyrene has attracted particular attention as potential supports for transition
metal complexes due to the thermal stability, chemical inertness, low cost, and availability. In
addition, metal species can be attached to the polymer matrix via covalent, or non-covalent
bonding through ionic, hydrogen bond, or hydrophobic interactions [18]. Accordingly, many
palladium complexes with different types of ligands, such as Schiff-bases [19], pincer-type
ligands [20], N- and O-containing macrocyles [19a, 21], etc., that are anchored onto the
polystyrene beads have found considerable synthetic applications. Therefore, designing or
employing organic ligands with accessible binding sites has received a great deal of interest. In
this context, N-heterocyclic carbenes (NHCs) are considered as an important class of ligands in
inorganic and organometallic chemistry [22] ever since they were discovered by Arduengo’s
group [23]. Indeed, NHCs were first recognized as air- and moisture-stable phosphine mimics,
but further studies revealed that NHC-metal complexes are superior to their phosphine-based
counterparts in both activity and stability [24]. Moreover, NHCs have shown higher dissociation
energies which enable them to bind to the metal ions regardless of their oxidation states [25].
Recently, polymer supported NHC-metal complexes, especially palladium complexes of these
species, have been utilized in many organic reactions, including cross-coupling reactions [26],
reductive amination of carbonyl compounds [27], and aminocarbonylation of aryl halides [28].

Despite the significant progress that has been made in the development of the NHC ligands
based on which they can be immobilized on polymer matrix, there still exists a need for
designing new, cheap, and environmentally benign N-heterocyclic carbenes. In this regard, using
natural products as sustainable ligands has been growing increasingly. As shown in Fig. 1,
caffeine and theophylline, which are the derivatives of xanthine and contain an imidazole ring in
their structures, can be considered as highly-valued plant-derived precursors for the
imidazolium-based N-heterocyclic carbenes. Major source of theophylline is cocoa beans, and
caffeine is commonly found in the tea bush, coffee beans, and cola nuts. Previously,
homogeneous bis-NHC-Pd catalyst based on caffeine has been reported by Luo and Lo in 2011,
which suffers from the above-mentioned drawbacks of homogeneous catalysts [29].
Continuing our longstanding interest in developing efficient heterogeneous catalysts [30],
herein we have designed a new Pd(II)-NHC catalyst which is derived from theophylline
immobilized onto chloromethylated polystyrene as an efficient and recyclable catalyst for the
Heck-Matsuda reaction. The catalyst is not only readily prepared from cheap reagents but also
compatible with a wide variety of arenediazonium salts and olefins. Furthermore, this air-stable
catalyst can be easily recovered from the reaction mixture by simple filtration.
2. Experimental
2.1. General remarks
All materials were commercial grade and purchased from Merck, Aldrich, and Fluka.
Chloromethylated polystyrene resin cross-linked with 2% DVB (200-400 mesh, 2.8-3.2 mmol/g
of Cl) was a product of Acros chemicals. Benzenediazonium tetrafluoroborate salt and its
derivatives were all known and prepared by a previously described method [6]. The NBP test
procedure was applied according to the literature [30]. FTIR spectra were recorded using an
1
ABB Bomem model FTLA 2000 spectrophotometer. H (300 MHz) and 13C NMR (75 MHz)
spectra of the final Heck-Matsuda products were recorded on Bruker AQS-300 Avance
spectrometer. Thermogravimetry analysis and differential thermogravimetry analysis were
carried out, using a thermal gravimetric analysis instrument (NETZSCH STA 409 PC/PG) under
nitrogen atmosphere. The metal content of the catalyst was determined with Varian Vista-MPX

inductively coupled plasma (ICP) instrument. The surface morphology of the polystyrene
supported species was analyzed by scanning electron microscope (Vega, TESCAN-Model)
equipped with energy dispersive X-ray (EDX) facility. The CHN analysis was carried out, using
a Perkin-Elmer, 2004 (II) elemental analyzer. X-ray photoelectron spectroscopy (XPS) was
performed with 8025-BesTec twin anode XR3E2 X-ray source system. The complete
characterizations of the catalyst (3) and the NMR spectra of the selected compounds which are
shown in Table 3 are actually given in electronic supporting information (ESI).
2.2. Preparation of the catalyst
2.2.1. Preparation of polymer supported theophylline (PS-theo, 1)
In a 100 mL round-bottom flask, chloromethylated polystyrene resin (1.0 g, almost 3 mmol Cl)
was allowed to swell in DMSO (20 mL) for 24h prior to the reaction. Then, theophylline (1.08 g,
6 mmol) and K2CO3 (0.90 g, 6.5 mmol) were added to the flask. The mixture was stirred at 70 ºC
for 72h. Then, polymer beads were filtered and washed with hot DMSO (10 mL), hot
DMSO:H2O (1:1, 20 mL), hot H2O (10 mL), MeOH (20 mL), and CH2Cl2 (10 mL) and dried in
an oven for 5h.
2.2.2. Synthesis of IL type species (PS-IL, 2)
The obtained PS-theo beads were swollen in CH3CN (2 mL) for 4h; then, a solution of
iodoethane (3 mL) in CH3CN (1 mL) was added. The mixture was stirred at room temperature
for 20h. The resulting beads were filtered and washed with CH3CN (20 mL), and CH2Cl2 (20
mL), respectively, and dried in an oven at 100 ºC for 12h.
2.2.3. Preparation of polymer anchored Pd(II)-NHC complex (PS-NHC-Pd(II), 3)
The functionalized beads (2, 0.6 g) were allowed to swell in dry THF (15 mL) for 1h; then,
Pd(OAc)2 (0.075 g, 0.33 mmol) was added to the above suspension. The solution was heated to
40 ºC for 16h. Afterwards, the resin was filtered and rinsed with THF (10 mL) and methanol (15
mL). The final polymer beads were dried in an oven at 80 ºC for 5h.
2.3. General procedure for the Heck-Matsuda cross-coupling reaction
A test tube was charged with arenediazonium tetrafluoroborate salt (0.5 mmol), PS-NHC-Pd(II)
complex (3) (0.9 mol%, 15 mg), and EtOH (1 mL). To this mixture an olefin substrate (0.75
mmol) was added; the mixture was stirred at room temperature for an appropriate time. The

progress of the reaction was monitored by TLC. After the completion of the reaction, the mixture
was filtered to recover the catalyst and the filtrate was concentrated in a vacuum. Further, the
reaction mixture was extracted with CH2Cl2 (2 × 5 mL) and the combined organic layer was
washed with water (2 × 5 mL). The organic layer was dried over Na2SO4 and the solvent
evaporated under reduced pressure. The residue was purified by preparative thin layer
chromatography plate, using 10% EtOAc in n-hexane as eluent.
3. Results and discussion
3.1. Synthesis and characterization of catalyst
The steps involved in the synthesis of polystyrene resin (PS) supported Pd(II)-NHC catalysts
have been outlined in Scheme 1. Firstly, a mixture of commercially available chloromethylated
polystyrene (CMPS) resin and theophylline was heated in the presence of DMSO and K2CO3 at
70 ºC for 72h to produce the corresponding PS-theo (1). Respective treatment of polymer-
bounded theophylline with iodoethane in acetonitrile in 3:1 ratio for 20h at room temperature
afforded the immobilized ionic liquid (IL) type species PS-IL (2) as the NHC ligand precursor.
Reaction of PS-IL (2) with Pd(OAc)2 in dry THF for 16h at 40 ºC resulted in the covalent
attachment of palladium (II) and the formation of PS supported NHC-Pd(II) complex 3. In
order to directly form imidazolium type IL and shorten the reaction pathway, caffeine was also
selected to react with CMPS; the reaction was performed in DMSO at 80 ºC for 5 days to form
PS-caff (4). The subsequent reaction of 4 with Pd(OAc)2 gave PS-NHC-Pd(II) (5). The
immobilization of theophylline and caffeine onto the PS matrix was confirmed by FTIR analysis;
IR spectra were recorded separately at different stages, and a comparative study was made (Fig.
2). The IR spectrum of CMPS exhibits two characterstic bands at 1261.93 and 670.54 cm-1,
which contribute to the stretching and bending vibrations of C ̶ Cl group (Fig. 2a). They were
practically unobservable after the functionalization of the polymer with theophylline (Fig 2b).
New absorption bands at around 1711.6 and 1653.1 cm-1 due to the C═O bond and the C=N
stretching vibration of imidazole ring, respectively, confirms the binding of the ligand species
(Fig. 2b). The broad band at 3452.4 cm-1 contributes to the tertiary nitrogen group [31],
supporting the formation of the supported imidazolium salt PS-IL (Fig. 2c). Furthermore, on
complexation with palladium, a new peak at 1476.6 cm-1 may be assigned to the stretching

vibration of metal-Caromatic-Nimidazole bond [32] in PS-NHC-Pd(II) complex (Fig. 2d).
Additionally, in the IR spectrum of the PS-caff (4), the intensity of the C Cl peaks were slightly
̶
decreased, confirming that most of the –CH2Cl groups on the CPMS have remained intact (Fig.
2e).
In order to ascertain the functionalization of the polymer and confirm that no adsorption has
occurred during the immobilization of the ligand molecules, 4-(4'-nitrobenzyl)pyridine (NBP)
tests [33] were also applied to the PS-theo, PS-IL, and PS-caff species (Fig. 3). This colorimetric
test has been developed by Luis et al., and enabled us to monitor the presence of chloride groups
on the polymer structure in each step. As depicted in Fig. 3a, the color of the CMPS beads turned
to deep purple under the NBP test conditions, which gave a positive result answer to the test.
However, under the same conditions with PS-theo and PS-IL species, no change in color was
observed, which indicates that most of the chloride groups have been substituted by theophylline
(Fig. 3b and 3c). On the other hand, the result of the NBP test was positive for the PS-caff,
showing the presence of numerous unreacted chloride groups on polymer matrix (Fig. 3d).
Also, elemental analysis was applied at the end of each step to confirm the immobilization
of the corresponding NHC preligands. The results are summarized in Table 1. Based on the
elemental analysis, the nitrogen content introduced into the polymer was 15.41 wt% (11.0
mmol/g), for the PS-theo (1) suggesting that 90-96% of the total chloride content has been
substituted by theophylline (Table 1, entry 1), whereas for the PS-caff (4) only 4.21 wt% (3.0
mmol/g) of nitrogen was found, affirming that only trace amounts of chloride groups been
replaced by caffeine molecules (Table 1, entry 4).
As shown in Table 1, the content of palladium incorporated into the polymer matrix was
determined by inductively coupled plasma (ICP) for PS-NHC-Pd(II) (3) as well as PS-NHC-
Pd(II) (5) species and was found to be 3.31 wt% (0.31 mmol/g) and 1.1 wt% (1.0 mmol/g) Pd,
respectively (Table 1, entries 3 and 5). According to the above-mentioned results, theophylline in
comparison to caffeine was more effectively immobilized into the polystyrene beads; therefore,
it was selected for further studies.
The scanning electron micrograph (SEM) images of the polymer supported NHC ligand
precursor (2) and Pd(II)-NHC (3) catalyst were recorded to examine the morphological changes

that have occurred on the polystyrene beads (Fig. 4). It can be clearly observed that the cubic
morphology of the PS-IL species (Fig. 4a) was changed after the complexation with palladium
diacetate (Fig. 4c). Furthermore, energy dispersive X-ray spectroscopy (EDX) analysis data for
the polymer anchored IL (2) and palladium catalyst, PS-NHC-Pd(II) (3), are shown in Fig. 4b
and 4d, respectively. The EDX data also infers the attachment of theophylline and its
corresponding imidazolium salt as well as palladium metal on the polymer matrix.
To investigate the thermal behavior of the PS-NHC-Pd(II) catalyst, thermogravimetric and
differential thermal analysis (TG-DTA) were also carried out under nitrogen atmosphere (Fig. 5).
The negligible weight loss of the catalyst on heating below 200 ºC might be attributed to the
release of physisorbed water. Meanwhile, the DTG curve shows that the decomposition of the
polymeric materials and the Pd(II)-NHC complex occurred in one step from 320 ºC to
460 ºC, which corresponds to the main weight loss. Furthermore, the peak at 405 ºC in the DTG
curve exhibits that the fastest mass loss has happened at this temperature which indicates that the
supported catalyst is thermally stable and can be easily used in a broad range of temperatures.
To study the structure of the PS-NHC-Pd(II) catalyst, X-ray photoelectron spectroscopy
(XPS) analysis was employed (Fig. 6). C, N, O, I, and Pd were detected in the as-synthesized
polymer supported Pd(II)-NHC complex (Fig. 6a). As shown in Fig. 6b, two prominent peaks at
337.12 eV and 342.48 eV can be readily assigned to Pd(II) 3d5/2 and 3d3/2, which correspond to
Pd(II) ions in the PS-NHC-Pd(II) catalyst [34]. The shift of Pd(II) 3d5/2 of Pd(OAc)2 binding
energy peak from 338.7 eV to 337.1 eV confirms the formation of Pd(II)-NHC complex [35].
The spectrum of carbon 1s clearly shows the peaks of C═C, C ̶ C, C═N, C ̶ N ̶ C, C═O, and
carbene to Pd bonds at 284.5, 285.1, 286.0, 286.8, 287.5, and 288.0 eV, respectively (Fig. 6c)
[36]. Fig. 6d, exhibits N 1s XPS spectra of the as-synthesized catalyst; the spectrum was divided
into two components at binding energies of 399.2 and 401.2 eV, which can be attributed to the
sp2 and quaternary nitrogens, respectively [34a, 37].
3.2. Catalytic activity of the PS-NHC-Pd(II) (3) in Heck-Matsuda cross-coupling reaction
To investigate the potency of the polystyrene supported Pd(II)-NHC precatalyst, it was used in
the Heck-Matsuda cross-coupling reaction. It is known that the catalytically active Pd(0) species
+
̶
can be generated in the presence of olefin molecules; thus, an excess of olefin (olefin : ArN2 BF4
= 1.5:1) was used in all studies. Optimization of the reaction conditions, including the solvent,

the catalyst loading, and the temperature were carried out for the reaction of benzenediazonium
tetrafluoroborate (7a) and ethyl acrylate (8a) as model substrates. The role of the solvent was
screened by testing different protic and aprotic solvents such as MeOH, EtOH, H2O, THF, and
CH3CN. As can be seen in Table 2, the reaction was significantly affected by the nature of the
solvent. It was observed that among all of the solvents employed, EtOH was found to be the best
reaction medium and the corresponding product was obtained in 98% yield (Table 2, entry 4).
However, performing the reaction in pure water or in the organic/aqueous cosolvent system
delivered poor results (Table 2, entries 5 and 6). Next, we examined the effect of different
catalyst concentrations on the Heck-Matsuda reaction. As illustrated in Table 2, while 0.9 mol%
of the catalyst worked remarkably well (98%, Table 2, entry 4), the yield of the coupling adduct
dropped to 84% when 0.7 mol% of the catalyst was used (Table 2, entry 8). Furthermore,
utilizing 1.1 mol% of the heterogeneous PS- NHC-Pd(II) (3) had no significant effect on either
the yield or the reaction rate (Table 2, entry 7). To determine the effect of the temperature, the
reaction was performed at 40 ºC but no marked change in the rate of the reaction was observed
(Table 2, entry 9). Additionally, the cross-coupling reaction with the in situ generated
benzenediazonium tetrafluoroborate salt 6a was examined. Due to the poor solubility of the salt
in THF, low yield of the product was obtained (Table 2, entry 10). In order to compare the
catalytic activity of PS supported NHC-Pd(II) complexes derived from caffeine 5 with the
catalyst 3, the coupling of three different arenediazonium tetrafluoroborate salts with ethyl
acrylate (7) were studied. As seen from Table 3, the reaction of benzenediazonium
tetrafluoroborate with 7 gave 80% of the olefinated product in 2.5h in the presence of the catalyst
5. On the other hand, the yield of the reaction was dramatically decreased in the second run
(59%), while the catalyst 3 successfully performed the second cycle (98%). Also, PS-NHC-
Pd(II) (5) conducted the cross-coupling reaction of 4-methoxy- and 4-nitrobenzenediazonium
tetrafluoroborates with ethyl acrylate (Table 4, entries 2 and 3), but exhibited lower catalytic
activity than that of PS-NHC-Pd(II) (3) catalyst.
The efficiency of the reaction protocol described above encouraged us to extend our studies
to different olefins and arenediazonium tetrafluoroborate salts; the results are outlined in Table 4.
In all cases, the coupling between arenediazonium tetrafluoroborate salts bearing electron

donating as well as electron withdrawing groups with acrylate substrates, proceeded readily to
afford the corresponding products in 78-98% yields (Table 4, entries 1-14). Sterically demanding
2-methylbenzenediazonium tetrafluoroborate salt reacted smoothly with ethyl acrylate and acryl
amide to give the corresponding product in 93% and 78% yields, respectively (Table 4, entries 4
and 14). The coupling reactions of arenediazonium salts containing a halogen substituent gave
exclusively the Heck-Matsuda adducts (Table 4, entries 7, 10, and 17), and no typical Mizoroki-
Heck coupling was observed. We also studied the coupling of diverse styrenes with
arenediazonium salts under the optimized reaction conditions. Due to the poor reactivity of the
styrene substrates, the yields of the coupling reactions were decreased (Table 4, entries 15-19).
However, high yields were obtained for the arenediazonium salts, bearing strong electron
withdrawing groups (Table 4, entries 16 and 19). Moreover, reaction of arenediazonium salts
with internal olefins was investigated; for instance, reaction of ethyl crotonate with
arenediazonium salt bearing electron withdrawing group, such as 4-nitrobenzenediazonium
tetrafluoroborate under the same reaction conditions occurred slowly and afforded only 24%
yield of the desired product.
3.3. Recyclability test
The reusability of a catalyst is an attractive attribute, mainly for commercial applications.
Therefore, the recovery and recyclability of our catalyst was studied by conducting the reaction
of phenyldiazonium tetrafluoroborate salt and ethyl acrylate under standard reaction conditions.
After the completion of the reaction, which was monitored by thin layer chromatography (TLC),
the catalyst was removed from the reaction media by filtration and washed thoroughly with water
and ethanol. Then, it was allowed to dry in an oven at 80 °C for 1h. The catalyst activity was not
considerably diminished even after 7 cycles (Fig. 7). The degree of the palladium leaching from
the heterogeneous catalyst was also investigated after the first and eight cycles. After the
separation of the catalyst by filtration, the palladium content of the filtrate was determined by
ICP. At the end of the first run, only 0.2% of palladium was lost into the solution during the
course of the reaction. The total leaching of palladium was also measured and it was found that
1.9% of palladium was lost after eight reaction cycles. The result of the leaching test confirmed
the heterogeneous nature of the catalyst.

3.4. The plausible mechanism of the Heck-Matsuda reaction
The proposed mechanism for the Heck-Matsuda cross-coupling reaction has been depicted in
Scheme 2 [38]. The catalytic cycle was started by the reduction of Pd(II) to the active Pd(0)
species in the presence of the olefin substrate. Then, oxidative addition of the arenediazonium
salt to Pd(0) generated complex 8, and subsequent elimination of N2 create a cationic ArPd+L2
species (9). As shown in Scheme 2, unlike a typical Mizoroki-Heck reaction, the Heck-Matsuda
reaction occurs in a polar-cationic mechanism. The next step involves migratory insertion of the
alkenyl substrate, followed by syn β-elimination to produce cationic palladium(II)-hydrido
complex (12), which rapidly undergoes reductive elimination to reproduce the active catalyst.
4. Conclusion
In summary, we have successfully prepared a recoverable and heterogeneous PS-NHC-
Pd(II) catalyst based on theophylline as an environmentally benign NHC precursor. In fact,
theophylline in combination with polystyrene can be considered as a cheap solid support for
recycling of the costly palladium metal. This catalyst shows high efficiency in the Heck-Matsuda
arylation of olefins in EtOH at room temperature. This reaction is a synthetically useful method
for syntheses of arylated olefins because of its tolerance of many functional groups, short
reaction times, high yields, mild, and nature-friendly conditions. Furthermore, the catalyst could
be reused for seven successive cycles without noticeable loss of its catalytic activity. These
advantages make the PS-NHC-Pd(II) complex valuable for synthetic and environmental aspects.
Acknowledgements
This research was supported by the Research Council of the K. N. Toosi University of
Technology. We also gratefully acknowledge Mr. Joseph Clarkson, University of British
Columbia, and Dr. Sogand Noroozizadeh for editing the English content of this article.
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O
N
O
N
H
N
N
N
N
N
N
O
O
caffeine
theophylline
Figure 1. Xanthine derivatives.

Figure 2. IR spectra of chloromethylated polystyrene resin (a), polystyrene-supported theophylline (b),
polystyrene-supported IL (c), polystyrene-supported Pd(II)-NHC complex (d), and polystyrene-supported
caffeine (e).
(a)
(d)
(c)
(b)
Cl
N
I
N
N
N
N
N
O
O
HN
O
N
N
N
O
N
N
O
H
O
Figure 3. The NBP test result for the chloromethylated polystyrene resin (a), polystyrene-supported
theophylline (b), polystyrene-supported IL (c), and polystyrene supported caffeine (d).

(a)
(b)
(c)
(d)
Figure 4. SEM images of polystyrene-supported IL 2 (a), and polystyrene-supported Pd(II)-NHC (3)
complex (c) and EDX data of PS-IL 2 (b), and PS-NHC-Pd(II) (3) (d).

Figure 5. Thermogravimetric weight loss plot for the PS-NHC-Pd(II) (3) catalyst.

(a)
(b)
(d)
(c)
Figure 6. Full range XPS spectrum (a), the Pd 3d (b), C 1s (c), and N 1s (d) of PS-NHC-Pd(II) (3),
respectively.

O
O
PS-NHC-Pd(II) (3) (0.9 mol%)
O
N2BF4
+
EtOH, r.t., 1h
O
6a
7a
8a
Figure 7. Recycling activity of the PS-NHC-Pd(II) (3) catalyst; reaction conditions: benzenediazonium
tetrafluoroborate salt (0.5 mmol), ethyl acrylate (0.75 mmol), PS-NHC-Pd(II) (3) (0.9 mol%),
in EtOH (1 mL), at room temperature.

Scheme 1. Synthesis of functionalized supported NHC ligands and palladium(II)-NHC complexes.

Pd(II)-L2 (L = NHC ligand)
ArN2+ BF4
HBF4
reductive elimination
BF4
Pd(0)-L2
oxidative addition
HPd+L2
ArPdN2L2
12
8
Ar
R
syn -elimination
Pd+L2
N2
ArPd+L2
H
H
R
9
Pd+L2
Ar
11
Ar
R
R
H
H
migratory insertion
10
Scheme 2. Plausible mechanism for the Heck-Matsuda cross-coupling reaction.

Table 1. Chemical composition of various PS supported compounds.
Entry
Compound
PS-theo (1)
PS-IL (2)
C %
H %
6.0
5.37
5.97
6.91
6.65
N %
15.41
13.94
12.34
4.21
Pd %
-
-
3.31
-
1.1
1
2
3
4
5
65.70
64.50
68.04
71.35
72.96
PS-NHC-Pd(II) (3)
PS-caff (4)
PS-NHC-Pd(II) (5)
5.32
Table 2. Optimization of the reaction conditions for Heck-Matsuda cross-coupling reaction.a
O
PS-NHC-Pd(II) (3) Cat.
O
O
N2BF4
+
Solvent, Temp.
O
8a
6a
7a
Entry
Solvent (ratio)
THF
Catalyst (mol% of Pd)
Temp. (ºC)
Time (h)
Yieldb (%)
1
2
0.9
0.9
0.9
0.9
0.9
0.9
1.1
0.7
0.9
0.9
25
25
25
25
25
25
25
25
40
25
1
1
15
26
CH3CN
MeOH
EtOH
3
1
72
4
1
98
5
H2O
24
24
1
trace
57
6
EtOH:H2O (1:1)
EtOH
7
98
8
EtOH
1
84
9
EtOH
1
98
10
THF
24
33c
a
Reaction conditions: arenediazonium tetrafluoroborate salt (0.5 mmol), olefin (0.75 mmol), and catalyst (3) in
solvent (1 mL) at room temperature. b Isolated yields. c Benezendiazonium tetrafluoroborate (6a) was generated
in situ using aniline, BF3.OEt2 and tBuONO.

Table 3. Comparison of catalytic activity of PS-NHC-Pd(II) (3) and (5)a
Catalyst 3
Time (h)
Catalyst 5
Time (h)
Entry
R1
R2
Yieldb (%)
98, 98c
92
Yieldb (%)
80, 59c
72
1
2
3
H
4-MeO
4-NO2
CO2Et
CO2Et
CO2Et
1
2.5
0.67
2.5
4
1.5
97
84
a
Reaction conditions: arenediazonium tetrafluoroborate salt (0.5 mmol), olefin (0.75 mmol), and PS-NHC-Pd(II)
catalyst (0.9 mol%) in EtOH (1 mL) at room temperature. b Isolated yields. c Yield of the 2nd cycle.
Table 4. Heck-Matsuda arylation of the olefins with arenediazonium saltsa
Entry
R1
R2
product
8a
Time (h)
Yieldb (%)
1
2
CO2Et
CO2Et
CO2Et
CO2Et
CO2Et
CO2Et
CO2Et
CO2Me
CO2Me
CO2Me
H
1
1
98
96
95
93
92
97
91
95
93
93
8b
8c
4-Me
3-Me
2-Me
4-MeO
4-NO2
4-I
3
1
4
8d
8e
1
5
2.5
0.66
2
6
8f
7
8g
8
8h
8i
4-Me
4-MeO
4-Br
1
9
2.25
2
10
8j

n
11
12
13
14
15
16
17
18
19
8k
8l
CO2 Bu
4-Me
H
1
1
94
86
93
78
51
82
71
66
88
CONH2
CONH2
CONH2
8m
8n
8o
8p
8q
8r
8s
4-NO2
2-Me
H
0.75
3
Ph
Ph
8
4-NO2
4-Cl
H
4
Ph
4
4-ClC6H4
6
4-MeOC6H4
4-NO2
4
a Reaction conditions: arenediazonium tetrafluoroborate salt (0.5 mmol), olefin (0.75 mmol), PS-NHC-Pd(II) (3)
catalyst (0.9 mol%), in EtOH (1 mL), at room temperature. b Isolated yields; all products were analyzed by NMR
technique.