M. Bakherad et al. / Journal of Organometallic Chemistry 740 (2013) 78e82
79
aerobic conditions [29]. However, to the best of our knowledge, no
Sonogashira reaction catalyzed by the PVC-anchored Pd(II) phe-
nyldithiocarbazate complex has yet been reported.
Herein, we report the synthesis and characterization of a new
polyvinyl chloride-supported palladium(II) phenyldithiocarbazate
complex, and the application of this complex in the copper-free
Sonogashira reactions under aerobic conditions.
we examined the reaction of bromobenzene with phenylacetylene
under the above conditions and found that it was not efficient since
it afforded only an 80% yield of diphenylacetylene 5a (entry 15).
However, by changing the base to ethanolamine, bromobenzene
could be smoothly coupled with phenylacetylene resulting in a high
yield (95%) of diphenylacetylene (entry 20).
After the optimized conditions were found, we explored the
general applicability of the PVC-anchored Pd(II) phenyl-
dithiocarbazate complex 2 as a catalyst for the copper- and solvent-
free coupling of different alkynes 3 with aryl halides 4 containing
electron-withdrawing or donating substituents. The results ob-
tained are shown in Table 2. The electron-neutral, electron-rich or
electron-poor aryl iodides reacted with phenylacetylene to generate
the corresponding cross-coupling products in high yields under the
standard reaction conditions (Table 2, entries 1e7). The Sonogashira
coupling of the less reactive acetylene, 1-hexyne, and propargyl
alcohol with aryl iodides bearing electron-donating or electron-
withdrawing groups all gave the corresponding products in high
yields (entries 8e14). To extend the scope of our work, we next
investigated the coupling of various aryl bromides with terminal
alkynes. As expected, aryl iodides were more reactive than aryl
bromides, and the substituent effects in aryl iodides appeared to be
less significant than in aryl bromides. However, as shown in Table 2,
high catalytic activity was observed in the coupling of activated aryl
bromides such as p-nitrobromobenzene (entries 16, 21, and 25) and
unactivated aryl bromides such as p-bromoanisole (entries 19 and
23) as well as p-nitroiodobenzene and p-iodoanisol. It should be
noted that the coupling reactions of the aryl chlorides (entries 27e
29), also took place under similar copper-free conditions, though the
reactivity was much lower than their iodo and bromo counterparts.
The recovery and reusability of the catalyst were investigated
using the reaction of iodobenzene with phenylacetylene as a model
system. After completion of the reaction, CHCl3 was added. The
mixture from the first-run reaction was filtered, and the solid
substance obtained was washed alternately with water, methanol,
and acetonitrile. After drying under vacuum for 15 h, the recovered
catalyst was reused in the same reaction under identical conditions.
The recycling process was repeated for five cycles with some
decrease in the catalytic activity of the catalyst. We found that the
product yield decreased slightly over five recycling runs (Table 3).
The ICP analysis shows that the palladium content of the catalyst
does not change after catalysis, indicating that no significant
2. Results and discussion
A PVC resin functionalized with phenyldithiocarbazate group was
formed by heating a mixture of PVC with phenylhydrazine and car-
bon disulfide in the presence of KOH in DMF at 80 ꢀC to produce the
corresponding PVC-anchored phenyldithiocarbazate ligand 1, which
was then reacted with PdCl2(PhCN)2 in DMFat 80 ꢀC to yield the PVC-
anchored Pd(II) phenyldithiocarbazate complex 2 (Scheme 1). Cata-
lyst 2 was characterized by the FT-IR, scanning electron micrograph
(SEM), and inductively coupled plasma (ICP) techniques.
Presence of the phenyldithiocarbazate ligand on polyvinyl
chloride was confirmed by FT-IR spectra. The stretching vibrations
of the C]C band appeared at 1598 cmꢁ1 for the PVC-phenyldithi-
ocarbazate ligand. The NH group in PVC-phenyldithiocarbazate
appeared, in the IR spectrum, at 3432 cmꢁ1 (Fig. 1). The N con-
tent of the resin was obtained to be 2.3%. The amount of palladium
incorporated into the catalyst 2 was also determined by ICP, which
showed a value of about 2.5%.
SEM was also recorded to understand the morphology of the
surface of the support and catalyst. It can easily be seen in Fig. 2 that
the resin beads have different size and roughness. The presence of
Pd has caused changes, demonstrated by change in the polymer
particle size and roughness of the surface.
To check the potency of the polyvinyl chloride resin supported
phenyldithiocarbazate palladium catalyst, it was used in a solvent-
and copper-free Sonogashira coupling reaction. The coupling be-
tween iodobenzene and phenylacetylene was chosen as the model
reaction. Our optimization data is shown in Table 1. Out of the bases
screened, pyridine gave the best results, and the corresponding
coupled product 5a was obtained in 98% GC yield (Table 1, entry 3).
The inorganic bases KOH, K2CO3, and Na2CO3 were less effective.
A low palladium concentration gave a decreased yield (entry 12).
Since bromoarenes are cheaper and more readily available than
iodoarenes and hence are synthetically more useful as educts,
S
KOH
DMF, 80oC
PVC
S
CS2
Cl
PVC
PhNHNH2
+
+
PhNHNH
1
PdCl2(PhCN)2
o
DMF, 80 C
S
NH
PVC
S
Ph NH
Pd
Cl
Cl
2
Scheme 1. Preparation of the supported Pdephenyldithiocarbazate complex.