Synthesis and characterization of a polymer-anchored palladium(II) Schiff base complex and its catalytic efficiency in phosphine-free Sonogashira coupling reactions

Article abstract of DOI:10.1007/s11243-010-9328-3

A polymer-anchored Pd(II) Schiff base complex has been synthesized by reacting a polymeric amine with 2-pyridinecarboxaldehyde to get the polymer-anchored Schiff base, which was then reacted with palladium acetate. The catalyst was characterized by physicochemical and spectroscopic methods. It shows excellent catalytic activity in the Sonogashira coupling of phenylacetylene with aryl halides using triethylamine as a base and copper iodide as a co-catalyst in water under open air at 70 °C. We have also studied the effects of base and solvent on the coupling reaction. Sonogashira reactions of phenylacetylene with a variety of functionalized aryl halides were performed under the optimized reaction conditions. This catalyst gives excellent yields without the use of phosphine ligands. Further experiments showed that the catalyst can be used five times without much loss in the catalytic activity. Springer Science+Business Media B.V. 2010.

Full text of DOI:10.1007/s11243-010-9328-3

Transition Met Chem (2010) 35:305–313
DOI 10.1007/s11243-010-9328-3
Synthesis and characterization of a polymer-anchored
palladium(II) Schiff base complex and its catalytic efficiency
in phosphine-free Sonogashira coupling reactions
•
S. M. Islam Sanchita Mondal Anupam Singha Roy
•
•
•
•
•
Paramita Mondal Manir Mobarak Dildar Hossain
Palash Pandit
Received: 26 October 2009 / Accepted: 10 January 2010 / Published online: 27 January 2010
Ó Springer Science+Business Media B.V. 2010
Abstract A polymer-anchored Pd(II) Schiff base complex
has been synthesized by reacting a polymeric amine with
2-pyridinecarboxaldehyde to get the polymer-anchored
Schiff base, which was then reacted with palladium acetate.
The catalyst was characterized by physicochemical and
spectroscopic methods. It shows excellent catalytic activity
in the Sonogashira coupling of phenylacetylene with aryl
halides using triethylamine as a base and copper iodide as a
co-catalyst in water under open air at 70 °C. We have also
studied the effects of base and solvent on the coupling
reaction. Sonogashira reactions of phenylacetylene with a
variety of functionalized aryl halides were performed under
the optimized reaction conditions. This catalyst gives
excellent yields without the use of phosphine ligands.
Further experiments showed that the catalyst can be used five
times without much loss in the catalytic activity.
[1–5]. This reaction is frequently utilized as a key step in
natural product chemistry [6–11] and for the synthesis of
acetylene compounds which are potentially useful in opto-
electronic applications [12–14]. This coupling reaction also
offers a powerful tool for the preparation of oligomers and
polymers [15–17]. These conjugated systems have received
much attention recently as a result of their optical and elec-
tronic applications [18–21].
Palladium metal has been employed as a catalyst for
various organic reactions such as oxidation [22], hydro-
genation [23–25], carbonylation [26], C–C couplings [27–
29], etc. The Sonogashira reaction generally proceeds in
the presence of homogeneous palladium catalysts which
possess high efficiency and are suitable for the study of the
reaction mechanism but their susceptibility to drastic
reaction conditions and the difficulties associated with their
isolation from the product mixture restrict their reusability.
This is a great disadvantage in Sonogashira couplings using
homogeneous catalysts. Recycling of catalysts is a task of
great economic and environmental importance in the
chemical and pharmaceutical industry, especially when
expensive or toxic heavy metal complexes are employed
[30]. A way to overcome these difficulties would be the use
of heterogeneous palladium catalysts such as a solid-sup-
ported metals. Nowadays, efficient and recyclable hetero-
geneous catalysts have gained much attention [31–36]. The
most readily available form of supported catalyst is palla-
dium on charcoal, which is widely used for heterogeneous
hydrogenation and also has a growing importance in car-
bon–carbon bond-forming reactions [37–41]. There are
many examples of heterogeneous catalysts for C–C cou-
pling reactions, and they are prepared by different
approaches like encapsulation or immobilization of a cat-
alytically active metal complex on solid supports such
as zeolites [42–46], covalent grafting of such active
Introduction
The Sonogashira coupling, a palladium-copper-catalyzed
reaction of aryl halides with terminal acetylenes, is a pow-
erful method for the formation of substituted acetylenes
S. M. Islam (&) ꢀ S. Mondal ꢀ A. S. Roy ꢀ P. Mondal ꢀ
M. Mobarak ꢀ D. Hossain
Department of Chemistry, University of Kalyani,
Kalyani, Nadia, West Bengal 741235, India
e-mail: manir65@rediffmail.com
P. Pandit
Department of Chemistry, University of Calcutta,
92 A.P.C. Road, Kolkata, West Bengal 700009, India
123

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Transition Met Chem (2010) 35:305–313
complexes onto reactive polymer surfaces [47–49] or
inorganic porous matrices [50, 51], ordered mesoporous
silica such as MCM-41[52–54].
Results and discussion
Synthesis of catalyst
In recent years, much attention has been paid toward
using water as solvent under aerobic conditions for
organic and organometallic reactions [55–59]. Typical
literature procedures for Sonogashira coupling utilize
catalytic palladium with a metal co-catalyst, phosphine
ligands and a base in polar toxic medium under nitrogen
atmosphere [60–66]. The most widely employed solvents
are DMF, DMSO, MeCN, THF and toluene, although
there are a few reports of Sonogashira couplings in
aqueous medium using Pd-catalysts under aerobic condi-
tions [56, 67].
The outline for the preparation of polymer-anchored Pd(II)
Schiff base complex is given in Scheme 1.
Characterization of the polymer-anchored Pd(II) Schiff
base complex
Due to insolubilities of the polymer-anchored palladium
complex in all common organic solvents, characterization
was limited to physicochemical properties, chemical anal-
ysis, SEM, TGA, IR and UV–Vis spectroscopic data.
Elemental analysis of the free ligand and complex (Table 1)
supports the formulation of the complex. Chemical analysis
suggests 12.10 wt% Pd in the immobilized Pd(II) complex.
Fig. 1 shows SEM images of the polymer-anchored Schiff
base (Fig. 1a) and the immobilized palladium complex on
modified polystyrene (Fig. 1b). The SEM pictures show a
morphological difference between the free Schiff base and
the catalyst. The presence of the Pd was also indicated by
energy dispersive of X-ray spectroscopy analysis (EDX)
(Fig. 2).
In this present work, Sonogashira reactions of aryl
iodides and bromides with phenylacetylene were carried
out in water with copper iodide as co-catalyst and
triethylamine as base, without the need of any organic
co-solvent or additives such as phase transfer catalysts.
We have avoided the use of phosphine ligands because
the phosphines in palladium complexes are often air-
sensitive. Interestingly, the prepared catalyst is capable of
activating aryl chloride in Sonogashira coupling reactions
under truly heterogeneous conditions and gives better
results than previously reported palladium catalysts [68,
69]. The experimental results reveal that the anchoring of
the complex on a solid support not only exhibits
improved catalyst activity, stability and selectivity of the
product but also enables easy recovery and reuse of the
catalyst.
Thermal stability of the complex was investigated using
TGA-DTA at a heating rate of 10 °C/min in air over a
temperature range of 30–600 °C. TGA-DTA curves of the
polymer-anchored Schiff base and supported palladium
catalyst are shown in Fig. 3. The complex is stable up to
200 °C, and above this temperature it decomposes.
Scheme 1 Synthesis of the
polymer-anchored Pd(II)
complex
SnCl2 + HCl
AcOH
AcOH
+
HNO3
P
NH3Cl
P
NO2
P
°C
50
, 6 h
polystyrene
polystyrene
p-nitropolystyrene
(1)
amino hydrochloride
(2)
OHC
N
NaOH
MeOH
P
N
P
NH2
Toluene, reflux
N
(L)
p-aminopolystyrene
(3)
Pd(OAc)2
AcOH
P
N
P=Polystyrene framework
L=
(P-N=CHC5H4N)
N
Pd
AcO
OAc
PdL(OAc)2
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Transition Met Chem (2010) 35:305–313
307
Table 1 Analytical data for the
polymer-anchored ligand and
complex
Compound
Color
Cl (%)
C (%)
H (%)
N (%)
Pd (%)
P
Colorless
–
–
92.5
74.1
72.0
84.1
83.2
67.8
7.6
5.7
6.7
7.3
6.3
5.2
–
–
–
P–NO2
Light yellow
Yellow
6.0
5.8
6.7
9.1
6.8
P–NH3Cl
13.3
–
–
P–NH2
Pale yellow
Brownish yellow
Deep brown
–
P–N=CHC5H4N
P–N=CHC5H4NPd(OAc)2
–
12.1
12.1a
P Polystyrene
a
Used catalyst
in the Schiff base, which also confirms the attachment of
carbonyl compound to the polymer matrix. On complexa-
tion with Pd, the frequency of C=N (azomethine) is
reduced in intensity and present at 1,615 cm-1 (vC=N,
coordinated) indicating the formation of a metal–ligand
bond. Two new peaks at 1,629 cm-1 (vC=O, free),
1,313 cm-1 (vC–O, coordinated) [72], a weak peak at
450 cm-1 (vPd–Nazomethine) [73] and a Pd–Npyridine
ring
stretching vibration at 567 cm-1 [74] are present in the
spectra of the complex.
The electronic spectrum (Fig. 4) of the polymer-
anchored Pd(II) complex was recorded in diffuse reflec-
tance mode as a MgCO3/BaSO4 disk due to the solubility
limitations noted above. The main absorptions are at ca.
310, 350–410 and 480 nm. The absorption around 310 nm
may be attributed to p ? p* transitions of the pyridine
and phenyl moieties. The absorption at 350–410 nm is
due to p ? p* and n ? p* transition of imines. In the
complex, the band around 480 nm is assigned to spin—
allowed d–d transition [75]. From the microanalytical and
spectroscopic data, a probable structure of the polymer-
anchored palladium(II) complex may be proposed
according to Scheme 1.
Catalytic activity
The polymer-anchored complex was investigated as a
catalyst in the Sonogashira coupling of phenylacetylene
with aryl halides. To optimize the reaction conditions, we
have chosen the reaction between phenylacetylene and
iodobenzene as a model, and various parameters, such as
solvent and base, were varied to get maximum product.
As shown in Table 3, our initial goal was to evaluate the
effect of solvent on a model reaction using our catalyst.
The reaction was conducted using 0.05 mmol of CuI as a
co-catalyst, 1.2 mmol of Et3N as a base and various sol-
vents like water, DMF, DMSO, THF, toluene, etc. The
reaction in water was found to be better than other polar
and non-polar solvents. Polar solvents, such as DMSO and
DMF were found to be good for this reaction, while the less
Fig. 1 FE SEM images of the polymer-anchored Schiff base (a) and
the Pd(II) catalyst (b)
Thermogravimetric study suggests that the polymer-
anchored Pd(II) complex degrades at considerably higher
temperature.
The IR spectra of the various functionalized polysty-
renes and the polymer-anchored Pd(II) complex are given
in Table 2. The polymer-anchored Schiff base exhibits an
absorption at 1,640 cm-1 assigned to C=N stretching. A
band at 1,600 cm-1 indicates the presence of pyridyl C=N
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Transition Met Chem (2010) 35:305–313
Fig. 2 EDAX data for the
polymer-anchored Schiff base
(a) and Pd(II) catalyst (b)
polar solvents, such as toluene, afforded the product only in
very poor yields.
bromoanisole and 4-bromotoluene with phenylacetylene
proceeded very slowly (entries 8, 9). In contrast, bromides
with electron-withdrawing substituents on the aromatic
ring are very good substrates. 4-Methyl chlorobenzene is
the least reactive substrate; longer reaction time is required
for 4-methylchlorobenzene than for chlorobenzene or 4-
nitrochlorobenzene. Iodobenzene shows higher activity
than bromobenzene or chlorobenzene. From the above we
conclude that electron-withdrawing groups in the aryl
halides increase the yield, while electron-donating groups
decrease the yield.
The effect of base also plays an important role in the
Sonogashira reaction. Table 4 illustrates the effect of base
on the model reaction using the catalyst. The reaction was
carried out using 0.05 mmol of CuI as a co-catalyst in
water with bases such as triethylamine, cesium carbonate,
potassium carbonate, potassium hydroxide, etc. The best
yield was observed with Et3N. CuI plays a crucial role in
our optimized reaction; without CuI, low yield of the
diphenylacetylene was obtained after 12 h (entry 5).
From the above results it can be seen that the best yield
was obtained by using Et3N and CuI in water with the
catalyst at 70 °C under open air. The Sonogashira reaction
is shown in Scheme 2.
We compared the activity of our catalyst in the Sono-
gashira reaction with other reported palladium catalysts
(Table 6). It can be seen that the activity of our catalyst is
more than the other reported systems. Also, the present
system requires lower temperature (70 °C) and water for
solvent, which are other advantages.
Under the optimized reaction conditions, the couplings
of various other aryl halides with phenylacetylene were
carried out to afford the corresponding diphenylacetylene
derivatives in moderate to high yields (Table 5). The cat-
alyzed coupling reaction tolerated a variety of functional
groups, with variable yields and reaction rates. Bromides
with electron-donating substituents on the aromatic ring are
less reactive in these reactions. Thus, reaction of 4-
Heterogeneity tests
To determine whether the catalyst was actually functioning
in a heterogeneous manner, a hot-filtration test [76] was
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Transition Met Chem (2010) 35:305–313
309
Table 2 IR spectral data of the compounds (cm-1
)
Compound
P–NO2
dNH2 mNO2
1,520(s)
mC=N (azomethine)
1,350(s)
1,520(w)
P–NH3Cla
1,350(w)
P–NH2
1,625 1,520(w)
1,350(w)
P–N=CHC5H4N
P–N=CHC5H4NPd(OAc)2b
1,625 1,520(w) 1,640
1,350(w)
1,520(w) 1,615
1,340(w)
P Polystyrene
a
Additional bond: m(NH3Cl): 2,550 cm-1
b
Additional bonds: m(C=O) (free): 1,629 cm-1; v(C–O) (coordinated):
1,313 cm-1; v(Pd–N) (azomethine nitrogen): 450 cm-1; v(Pd–N)
(pyridine nitrogen): 567 cm-1
Fig. 3 TGA-DTA curves for the polymer-anchored Schiff base (a)
and supported palladium catalyst (b)
Fig. 4 DRS-UV–visible absorption spectrum of the Pd(II) catalyst
Table 3 Effect of solvent on the coupling of phenylacetylene with
iodobenzene
performed in the coupling of phenylacetylene with iodo-
benzene. The solid catalyst was filtered out after the
reaction had proceeded for 6 h and the conversion of
iodobenzene was determined by GC as 49%, with 96%
selectivity for diphenylacetylene (4% of the diyne was also
obtained). Atomic absorption analysis of the liquid phase
of the reaction mixtures collected by filtration confirmed
that Pd was absent from the reaction mixture. The obtained
filtrate was stirred under the reaction conditions for a fur-
ther 3 h, after which the conversion of iodobenzene was
determined to be still 49%, but the selectivity for diphen-
ylacetylene was decreased to 86%. The proportion of diyne
was increased to 14%. This may be due to the presence of
CuI in the filtrate which accelerated the Hey type homo-
coupling reaction. From this result it is clear that the cross-
coupling reaction occurs only in the presence of the Pd(II)
Entry
Solvent
Time (h)
Temperature (°C)
Yielda (%)
1
2
3
4
Water
DMF
12
16
16
20
70
120
115
100
96
93
92
45
DMSO
Toluene
Reaction conditions: Pd(II) catalyst (50 mg, 3.72 mol% Pd), iodo-
benzene (1.5 mmol), phenylacetylene (1 mmol), CuI (0.05 mmol),
Et3N (1.2 mmol), solvent (10 mL), open air
a
Yield refers to GC and GC–MS analysis, based on the iodobenzene
using decane as internal standard
catalyst. These results also suggest that the Pd is not being
leached out from the catalyst during the Sonogashira
reactions and that the catalyst is truly heterogeneous.
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Transition Met Chem (2010) 35:305–313
Table 4 Effect of base on the coupling of phenylacetylene with
iodobenzene
Table 5 Sonogashira reactions of phenylacetylene with aryl halides
Entry Aryl halide
Product
Time Yielda
Entry
Base
Time (h)
Yielda (%)
(h)
(%)
1
Et3N
12
14
12
13
12
96
87
82
58
54
1
C6H5I
C6H5C:CPh (5a)
12
8
96 (90)b
99
2
Cs2CO3
K2CO3
KOH
2
p-HO2CC6H4I
p-OHCC6H4I
p-NO2C6H4I
p-MeOC6H4I
p-MeC6H4I
C6H5Br
p-HO2CC6H4C:CPh
p-OHCC6H4C:CPh
p-NO2C6H4C:CPh (5b)
3
3
8
98
4
5b
4
8
99(91)b
92(80)b
94(82)b
86(77)b
48(33)b
45(36)b
88(72)b
90
55(47)b
32(18)
51(45)b
96
Et3N
5
p-MeOC6H4C:CPh (5c) 10
6
p-MeC6H4C:CPh (5d)
C6H5C:CPh (5a)
10
12
Reaction conditions: Pd(II) catalyst (50 mg, 3.72 mol% Pd), iodo-
benzene (1.5 mmol), phenylacetylene (1 mmol), CuI (0.05 mmol),
base (1.2 mmol), water (10 mL), 70 °C, open air
7
8
p-MeOC6H4Br p-MeOC6H4C:CPh (5c) 14
a
Yield refers to GC and GC–MS analysis, based on the iodobenzene
using decane as internal standard
9
p-MeC6H4Br
p-NO2C6H4Br
p-MeC6H4C:CPh (5d)
14
10
11
12c
13c
14c
15
16
p-NO2C6H4C:CPh (5b) 10
b
Without CuI
p-HO2CC6H4Br p-HO2CC6H4C:CPh
9
16
20
C6H5Cl
C6H5C:CPh (5a)
p-MeC6H4Cl
p-NO2C6H4Cl
p-MeC6H4C:CPh (5d)
p-NO2C6H4C:CPh (5b) 14
Catalyst recycling
m-HO2CC6H4I m-HO2CC6H4C:CPh
o-C6H4NBr o-C6H4NC:CPh (5e)
8
10
92(83)b
One of the main advantages of a heterogeneous catalyst is
enhanced lifetime. To investigate the reusability of our
polymer-anchored Pd(II) Schiff base complex, the catalyst
was separated by filtration after the first run, washed, dried
under vacuum and then subjected to a second run under the
same reaction conditions, with further addition of sub-
strates in appropriate amounts. The nature and yields of the
final products were comparable to the original run. As
shown in Table 7, the catalyst could be reused up to five
times, without any appreciable loss of its catalytic activity.
Reaction conditions: Pd(II) catalyst (50 mg, 3.72 mol% Pd), aryl
halide (1.5 mmol), phenylacetylene (1 mmol), CuI (0.05 mmol),
Et3N (1.2 mmol), water (10 mL), 70 °C, open air
Yield refers to GC and GC–MS analysis, based on the aryl halide
using decane as internal standard
a
b
Isolated yield after column chromatography
c
DMF is used as a solvent
Preparation of p-nitro polystyrene (1)
A suspension of macroporous polystyrene beads (5.0 g) in
a mixture of acetic anhydride (20 mL), nitric acid (*70%,
2 mL) and glacial acetic acid (4 mL) was stirred for
30 min at 5 °C followed by 5 h at 50 °C [71]. The corre-
sponding p-nitro polystyrene was washed successively with
acetic acid, water and methanol and finally dried under
vacuum.
Experimental
Analytical-grade reagents and freshly distilled solvents
were used throughout. All reagents and substrates were
purchased from Merck. Liquid substrates were predis-
tilled and dried by molecular sieve, and solid substrates
were recrystallized before use. Distillation, purification of
the solvents and substrates were done by standard pro-
cedures [70]. Macroporous polystyrene beads cross-linked
with 2% divinylbenzene and 2-pyridinecarboxaldehyde
were purchased from Aldrich. Palladium acetate was
procured from Arora Matthey and used without further
purification.
Preparation of p-amino polystyrene (3)
A mixture of acetic acid (20 mL), stannous chloride
(5.0 g), concentrated hydrochloric acid (6 mL) and p-nitro
polystyrene (5.0 g) was stirred for 72 h at room tempera-
ture to reduce the nitro-compound to the corresponding
amine hydrochloride (2) [71]. The residue was washed
C
X
H
R
C
Ph
C
R
C
Ph
Scheme 2 Sonogashira reactions of phenylacetylene with aryl halides. Reaction conditions: Pd(II) Schiff base catalyst (50 mg, 3.72 mol% Pd),
CuI (0.05 mmol), Et3N (1.2 mmol), water (10 mL), 70 °C, open air. X = I, Br, Cl, R = CH3, COOH, NO2, CHO, OMe
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Transition Met Chem (2010) 35:305–313
311
Table 6 Catalytic activities of different palladium catalysts in the Sonogashira coupling of iodobenzene with phenylacetylene
Entry
Catalyst
Reaction conditions
Yield%
Reference
1
2
3
P–N=CHC5H4NPd(OAc)2
Water, CuI, Et3N, 70 °C, 12 h
Pyrrolidine, CuI, 90 °C, 5 h
DMF, CuI, K3PO4, 80 °C, 5 h
96
75
94
This study
[84]
Oxime Palladacycle (0.5 mol% Pd)
PdCl2(MeCN)2 with phosphine-free hydrazone ligand
[85]
Perkin–Elmer FTIR 783 spectrophotometer using KBr
pellets. A Mettler Toledo TGA/SDTA 851 instrument
was used for the Thermogravimertric (TGA) analysis.
Morphologies of the functionalized polystyrene and com-
plex were analyzed using a scanning electron microscope
(SEM) (ZEISS EVO40, England) equipped with EDX
facility. Palladium content in the catalyst was determined
using a Varian AA240 atomic absorption spectrophotom-
eter (AAS).
Table 7 Recycling activity of the recovered Pd(II) catalyst toward
the Sonogashira reaction of phenylacetylene with iodobenzene
No. of cycle
Yielda (%)
1
2
3
4
5
96
95
95
93
92
Reaction conditions: recovered Pd(II) catalyst (50 mg, 3.72 mol%
Pd), iodobenzene (1.5 mmol), phenylacetylene (1 mmol), CuI
(0.05 mmol), Et3N (1.2 mmol), water (10 mL), 12 h, 70 °C, open air
Yield refers to GC and GC–MS analysis, based on the iodobenzene
using decane as internal standard
Reaction procedure for Sonogashira reaction
a
Aryl halide (1.5 mmol), phenyacetylene (1.0 mmol), tri-
ethylamine (1.2 mmol), decane (1.5 mmol), copper iodide
(0.05 mmol) and Pd(II) catalyst (50 mg, 3.72 mol% Pd)
were added to water (10 mL) under aerobic conditions. The
resulting mixture was stirred at 70 °C in an oil bath for
12 h. Aliquots were withdrawn at different time intervals
and analyzed by GC and GC–MS using decane as an
internal standard. After completion of the reaction, the
reaction mixture was extracted three times with ethyl
acetate, and the combined organic layer was dried with
anhydrous sodium sulfate. Solid residues were obtained on
evaporation of the organic solvent under vacuum at 25 °C.
The residue was purified by flash chromatography to give
the desired product. Product identity was confirmed by
comparison of mp and NMR spectra with authentic sam-
ples or with those reported in the literature.
several times with hydrochloric acid (12 M) and glacial
acetic acid (1:4) mixture and then with methanol. The
product on repeated treatment with dilute alcoholic NaOH
(5%) gave the corresponding free amine. This was washed
with alcohol and dried under vacuum.
Preparation of the polymer-anchored ligand (L)
2-Pyridinecarbaldehyde (5 mL) in dry toluene (20 mL)
containing macroporous p-amino polystyrene beads (3 g)
was refluxed for 48 h under nitrogen atmosphere. After
cooling to room temperature, the polymer-anchored ligand
was filtered off, washed thoroughly with toluene followed
by methanol and finally dried under vacuum.
Characterization of the used catalyst and reaction
products
Preparation of polymer-anchored catalyst {PdL(OAc)2}
A brownish yellow suspension of the polymer-anchored 2-
pyridinecarboxaldehyde ligand (1 g) in glacial acetic acid
(10 mL) and a 1% (w/v) glacial acetic acid solution of
palladium(II) acetate (0.1 g) were refluxed for 24 h when
the color changed slowly to deep brown. The brown pal-
ladium(II) Schiff base complex catalyst was filtered off,
washed with acetic acid and then with dry ethanol and
finally dried under high vacuum.
Characterization of the used catalyst was limited to
chemical analysis, IR and UV–Vis spectroscopic data.
Chemical analysis results suggest 12.1 wt% Pd in the used
catalyst. The FTIR spectrum for the used catalyst was
identical to that of the original catalyst. Thus, peaks at
1,629 cm-1 (vC=O, free), 1,313 cm-1 (vC–O, coordi-
nated), a weak peak at 450 cm-1 (vPd–Nazomethine) and a
Pd–Npyridine ring stretching vibration at 567 cm-1 were all
present in the spectrum of the used catalyst. The UV–Vis
spectrum of the used catalyst was also very similar to that
of the original catalyst. From the spectroscopic data, we
conclude that the structure of the used catalyst is identical
with that of the original.
Instrumentation
A Perkin–Elmer 2400 C elemental analyzer was used to
collect micro analytical data (C, H and N). FTIR spectra of
the samples were recorded from 400 to 4,000 cm-1 on a
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Transition Met Chem (2010) 35:305–313
Melting points were recorded on a melting point appa-
1
ratus. H NMR spectra were recorded at 300 or 250 MHz,
Sonogashira coupling reactions of aryl halides with phen-
ylacetylene has been developed. Compared with previous
reports, several interesting features are apparent on the
basis of the present results. Thus, the catalyst is recyclable
and also able to activate chlorobenzene with a high yield.
Furthermore, water is employed as a solvent, and the
reaction is free of phosphine. Further efforts to extend the
applications of the system to other coupling transforma-
tions are under way in our laboratory. There are only a few
catalysts that can catalyze C–C coupling reactions in truly
heterogeneous catalytic conditions [82, 83]. Aryl chlorides
normally remain inactive toward this type of catalyst,
especially in heterogeneous conditions. Because the reac-
tion medium is heterogeneous, our catalyst might be useful
for industrial applications.
using a Bruker DPX-300 and Bruker AC 250 MHz spec-
trometer in CDCl3 with TMS as internal standard, respec-
tively. Chemical shifts were given as d values with
reference to tetramethylsilane (TMS) internal standard. The
reaction products were quantified (GC data) using a Varian
3400 gas chromatograph equipped with a 30 m CP-
SIL8CB capillary column and a flame ionization detector
and identified by Trace DSQ II GC–MS equipped with a
60 m TR-50MS capillary column. Standardization of the
products was done by calibration using decane as an
internal standard. Melting point and 1H and 13C NMR data
of the five products are given below.
Diphenylacetylene (5a). mp 59 °C (lit. 60–62 °C [77]).
1H NMR (CDCl3, 250 MHz) d: 7.54–7.50 (m, 4H),
7.32–7.28 (m, 6H); 13C NMR (CDCl3, 62.5 MHz) d: 131.6,
128.3, 128.2, 123.3, 89.4; MS m/z (relative intensity, %):
178 (MC, 100), 152 (14), 126 (7), 89 (13).
Acknowledgments The authors acknowledge the Indian Associa-
tion for the Cultivation of Science for providing the instrumental
support. We also wish to thank Department of Science and Tech-
nology (DST), Council of Scientific and Industrial Research (CSIR)
and University Grant Commission (UGC), New Delhi, India for
funding.
(4-Nitrophenyl)phenylacetylene (5b). mp 118 °C (lit.
1
120–121 °C [78]). H NMR (CDCl3, 250 MHz) d: 8.19
(d,JZ8.83 Hz, 2H), 7.64 (d, JZ8.73 Hz, 2H), 7.56–7.52 (m,
2H), 7.39–7.37 (m, 3H); 13C NMR (CDCl3, 62.5 MHz)
d:146.9, 132.2, 131.8, 130.2, 129.2, 128.5, 123.6, 122.0,
94.7,87.5; MS m/z (relative intensity, %): 223 (MC, 100),
193(40), 176 (80), 165 (33), 151 (40), 139 (8), 126 (14).
(4-Methoxyphenyl)phenylacetylene (5c). mp 55 °C (lit.
57–61 °C [79]).1H NMR (CDCl3, 300 MHz) d: 7.53–
7.46(m, 4H), 7.37–7.30 (m, 3H), 6.87 (dd, JZ8.70, 2.10 Hz,
2H), 3.82 (s, 3H); 13C NMR (CDCl3, 75 MHz) d:
159.5,133.0, 131.4, 128.3, 127.9, 123.5, 115.3, 113.9, 89.3,
88.0,55.3; MS m/z (relative intensity, %): 208 (MC, 100),
193(48), 165 (39), 139 (8).
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