406
M. Bakherad et al. / Journal of Organometallic Chemistry 749 (2014) 405e409
Me
alkynes with aryl halides catalyzed by a quinoline-2-carboimine
palladium complex immobilized on MCM-41 under aerobic con-
ditions [23].
N
SH
N
DMF, 100 oC, 20 h
N
O
N
+
Cl
N
PS
PS
Me
NH2
N
O
H2N
S
Polystyrene is one of the most popular polymeric supports used
in synthetic organic chemistry because of its inexpense, ready
availability, mechanical robustness, chemical inertness, and facile
functionalization.
Polystyrene-supported palladium catalysts have successfully
been used for the Heck [24] and Suzuki [25e27] reactions, and have
shown lower levels of palladium leaching during cross-coupling. To
date, a few palladium complexes on functionalized polystyrene
support have been prepared and successfully used in the Sonoga-
shira reaction.
1
2
[PdCl2(PhCN)2]
EtOH, reflux, 20 h
Me
N
N
O
PS
N
H2N
S
Pd
Cl
Cl
3
S.M. Islam and co-workers have described synthesis and char-
acterization of the reusable polystyrene anchored Pd(II) azo com-
plex catalyst for the Suzuki and Sonogashira coupling reaction in
water medium [28]. Moreover, Ying He et al. have developed a
successful copper-free Sonogashira coupling reaction catalyzed by a
reusable polystyrene-supported macrocyclic Schiff base palladium
complex in water [29]. Although all the methods provide good
yields, some of the reactions are sluggish, requiring high temper-
ature for completion, and the reactions should be performed in the
presence of relatively large amounts of palladium.
Our strategy for developing a versatile heterogeneous catalyst is
based upon the concept of immobilizing an organometallic com-
plex onto polystyrene. This is because the usage of polystyrene
often offers enhanced catalytic activity due to its high surface area,
and can be expected to have negligible leaching of precious metal
species and high catalytic activity.
We have recently reported the synthesis and characterization of
the polystyrene-supported bidentate phosphine palladium(0)
complex and the polystyrene-supported palladium(II) 1-phenyl-
1,2-propanedione-2-oxime thiosemi-carbazone complex, and
found that these complexes are highly active and recyclable cata-
lysts for the Sonogashira reaction of aryl halides with terminal al-
kynes under aerobic conditions [30,31]. Moreover, we have
developed successful Suzuki, Heck, and copper-free Sonogashira
reactions catalyzed by 4-amino-5-methyl-3-thio-1,2,4-triazole-
functionalized polystyrene resin-supported Pd(II) under aerobic
conditions in water [32]. However, to the best of our knowledge, no
Sonogashira reactions catalyzed by PS-anchored Pd(II) triazine
complex have yet been reported.
Scheme 1. Preparation of the supported Pd-triazine complex.
conditions resulted in the covalent attachment of palladium onto
the functionalized polymer. The catalyst prepared, PS-triazine-
Pd(II), was characterized by FT-IR and SEM. The amount of palla-
dium incorporated into the polymer was also determined by
inductively coupled plasma (ICP), which showed a value of
0.12 mmol/g of the heterogenized catalyst.
In order to ascertain the functionalized polymer and its corre-
sponding Pd complex, IR spectra were recorded separately at
different stages of preparation. As it can be seen in Fig. 1, the
spectrum of chloromethylated polystyrene resin shows an ab-
sorption band at 1265 cmꢁ1, attributed to the CeCl bond, and
weakened after the introduction of 4-amino-5-methyl-1,2,4-
triazine. Also the stretching vibrations of the C]O band appeared
at 1697 cmꢁ1 for the PS-triazine-Pd(II). Moreover, the spectrum of
polystyrene-supported palladium complex shows an absorption
band at 3425 cmꢁ1, which is attributed to the NeH bond. Scanning
electron micrographs (SEM) were reported for a single bead of pure
chloromethylated polystyrene, and polymer-anchored complex to
observe the morphological changes. It can easily be seen in Fig. 2
that the resin beads have different size and roughness. The pres-
ence of Pd has caused changes, demonstrated by change in the
polymer particle size and roughness of the surface.
To check the potency of 1,2,4-triazine-functionalized poly-
styrene resin-supported Pd(II) complex, it was used in the Sono-
gashira coupling reaction. The coupling between phenyl acetylene
and iodobenzene was chosen as the model reaction. Initially, we
examined the effects of bases on the copper- and solvent-free
Sonogashira reaction at room temperature. The organic bases
Et3N, DIEA, pyridine, piperidine, and pyrrolidine were investigated
(Table 1, entries 1e5). As shown in Table 1, Et3N was the best base
for the reaction with high TON 990. Also the inorganic bases KOH,
K2CO3, and Na2CO3 were studied but unsatisfactory yields were
obtained (Table 1, entries 6e8). Then different catalyst loadings
were tested for the reaction. As illustrated in Table 1, 0.05 mol% of
the catalyst gave rise to extremely high TON 1800 (Table 1, entry 9)
but a lower yield was obtained. Thus we selected Et3N as the base
and 0.1 mol% of the catalyst as the optimal conditions for the re-
action. Since bromoarenes are cheaper and more readily available
than iodoarenes and hence are synthetically more useful as educts,
we examined the reaction of bromobenzene with phenyl acetylene
under the above conditions and found that it was not efficient since
it afforded only a 50% yield of diphenyl acetylene 6a (entry 10).
However, by changing the base to piperidine, bromobenzene could
be smoothly coupled with phenyl acetylene resulting in a high yield
(98%) of diphenyl acetylene (entry 13).
Thus we were encouraged to develop a further application to-
ward the sophisticated chemical transformations, especially the
carbonecarbon bond forming reactions.
Here, we report the heterogeneous copper-free Sonogashira
coupling reaction of terminal alkynes with aryl halides catalyzed by
the triazine-functionalized polystyrene resin-supported Pd(II)
complex.
2. Results and discussion
We report here anchoring of 4-amino-5-methyl-1,2,4-triazine
[33e35] on the cross linked polystyrene polymer (Scheme 1). The
chelating polymeric matrix was further used for the Sonogashira
reactions. A polystyrene resin (cross-linked with 2% divinylben-
zene, 4e5% Cl content, 1.14e1.40 mmol/g Cl) functionalized with
triazine groups was formed by heating a mixture of chloromethy-
lated polystyrene and triazine in DMF at 100 ꢀC for 24 h. PS-triazine
prepared was characterized by elemental analysis. The nitrogen
content of this resin was 3.67%. According to this value, the degree
of triazine introduced to the polymer was 0.65 mmol/g of the
support. This shows that only 46e57% of the total chlorine content
was substituted by triazine. The reaction of polymer-bound triazine
After the optimized conditions were found, we explored the
general applicability of the PS-triazine-Pd(II) complex 3 as a cata-
lyst for copper-free coupling of different alkynes with aryl halides
containing electron withdrawing or donating substituents.
with
a solution of PdCl2(C6H5CN)2 in ethanol under reflux