Solvent-free Heck and copper-free Sonogashira cross-coupling reactions catalyzed by a polystyrene-anchored Pd(II) phenyldithiocarbazate complex

Article abstract of DOI:10.1016/j.tetlet.2012.08.065

A new polystyrene-anchored Pd(II) phenyldithiocarbazate complex is synthesized and characterized. This Pd-complex behaves as an efficient heterogeneous catalyst in the Heck coupling and copper-free Sonogashira coupling reactions under aerobic conditions. Furthermore, the catalyst shows good thermal stability and recyclability.

Full text of DOI:10.1016/j.tetlet.2012.08.065

Tetrahedron Letters 53 (2012) 5773–5776
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Solvent-free Heck and copper-free Sonogashira cross-coupling reactions
catalyzed by a polystyrene-anchored Pd(II) phenyldithiocarbazate complex
⇑
Mohammad Bakherad , Ali Keivanloo, Shahrzad Samangooei
School of Chemistry, Shahrood University of Technology, Shahrood, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 15 May 2012
Revised 27 July 2012
Accepted 15 August 2012
Available online 24 August 2012
A new polystyrene-anchored Pd(II) phenyldithiocarbazate complex is synthesized and characterized. This
Pd-complex behaves as an efficient heterogeneous catalyst in the Heck coupling and copper-free Sono-
gashira coupling reactions under aerobic conditions. Furthermore, the catalyst shows good thermal sta-
bility and recyclability.
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Heck reaction
Sonogashira reaction
Aryl halides
Polystyrene-anchored Pd(II)
phenyldithiocarbazate complex
Palladium-catalyzed cross-coupling reactions for the formation
of carbon–carbon bonds have emerged as a powerful method in or-
Immobilization methods used to deposit palladium onto heter-
ogeneous solid beds have been studied extensively, and diverse
1
2
8
9
ganic synthesis. Among them, the Mizoroki–Heck and Sonogash-
supports such as clay, carbon nanofiber, Montmorillonite K-
3
10
11
12
13
ira reactions play important roles in modern synthetic chemistry.
10, magnetic mesoporous silica, zeolite, and metal oxides
have been investigated.
The reactions generally proceed in the presence of a homogeneous
palladium catalyst, which makes separation and recovery of the
catalyst tedious, if not impossible, and might result in unaccept-
able palladium contamination of the products.
From the standpoint of environmentally benign organic synthe-
sis, the development of highly active and easily reusable immobi-
lized catalysts, and the use of solvent-free reactions instead of
organic solvents are of significant interest to chemists. Solid-phase
organopalladium metal complexes, having high activity and selec-
tivity, offer several practical advantages in synthetic and industrial
chemistry, among which, the ease of separation of the catalyst
from the desired reaction products and the ease of recovery and re-
use of the catalyst are most important.
Polystyrene-supported palladium catalysts have successfully
1
4
15
been used for Heck and Suzuki reactions, and have shown
lower levels of palladium leaching during cross-coupling. To date,
several palladium complexes on functionalized polystyrene sup-
ports have been prepared and successfully used in Sonogashira
1
6
reactions.
Our approach was guided by three imperatives: (1) the support
should be easily accessible; (2) the reaction should be carried out
using readily available and cheap reagents; and (3) the ligand an-
chored on the support should be air-stable at room temperature,
which should allow its storage with a very long shelf-life in stan-
dard bottles.
Palladacycles have emerged as a promising class of catalysts or
catalyst precursors for Pd-catalyzed C–C bond forming reactions
such as the Heck–Mizoroki and Sonogashira reactions.
In continuation of our interest in the development of new
ligands for Pd-catalyzed C–C bond forming reactions,17 we report
the synthesis and characterization of a new polystyrene-anchored
Pd(II) dithiocarbazate complex catalyst 2, and illustrate its applica-
tion in Heck and Sonogashira coupling reactions under aerobic
conditions. The catalyst shows high activity in the coupling reac-
tions of various aryl halides. Furthermore, its ease of recovery
and excellent recycling efficiency make it an ideal system for cou-
pling reactions. The procedure followed to obtain the catalyst is
shown in Scheme 1.
4
5
To date, many efforts have been made to search for more effi-
cient ligands, the most common ligands used for these coupling
reactions being phosphine-based examples.6 Since most of the
phosphine-based ligands are air and/or moisture-sensitive, phos-
phine-free ligands such as N-heterocyclic carbenes (NHCs) have
7
been employed.
Chloromethylated polystyrene (cross-linked with 2% divinyl-
benzene, 4–5% Cl content, 1.14–1.40 mmol/g Cl) was treated with
⇑
Corresponding author.
E-mail address: m.bakherad@yahoo.com (M. Bakherad).
0
040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.08.065

5
774
M. Bakherad et al. / Tetrahedron Letters 53 (2012) 5773–5776
S
Table 2
a
Heck reactions of aryl halides with alkenes using Pd-catalyst 2
KOH
PhNHNH2
+
CS2
+
Cl
PS
PhNHNH
S
PS
2
o
2
DMF, 80
C
R
R
Pd-catalyst 2
1
1
R
R
X
+
H
2
C
o
Et N, 70
3
C
PdCl (PhCN)
2
2
o
DMF, 80
C
3
4
5
S
S
Entry
R1
R2
X
Product
Yieldb (%)
NH
Ph NH
1
2
3
4
5
6
7
8
9
H
Cl
NO
MeO
H
NO
H
NO
MeO
H
NO
CO
CO
CO
CO
Ph
Ph
CO
CO
CO
Ph
Ph
2
2
2
2
Me
Me
Me
Me
I
I
I
I
I
I
Br
Br
Br
Br
Br
5a
5b
5c
5d
5e
5f
5a
5b
5d
5e
5f
98 (95)
99 (95)
99 (96)
97
96
97 (93)
96
98
88 (83)
78
PS
Pd
2
Cl
Cl
Scheme 1.
2
2
Me
2
Me
2
Me
2
1
1
0
1
Table 1
2
92
Optimization of the conditions for the Heck reaction of iodobenzene with methyl
acrylatea
a
Conditions: aryl halide (1.0 mmol), alkene (1.2 mmol), Pd-catalyst
0.01 mmol), Et N (1.0 mmol), 5 h, 70 °C.
GC yield. Isolated yields are given in parentheses.
2
(
3
CO Me
CO Me
2
b
2
Pd-catalyst 2
I
+
H
2
C
o
base, 70
C
3
a
4a
5a
which was accompanied by extension of the reaction time to
10 h with production of the desired product in 60% yield (entry
9). A lower palladium concentration gave a decreased yield of the
product (entry 10).
Entry
Base
DIPEAc
Et
Pyridine
Piperidine
Pyrrolidine
KOH
Cat (mol %)
Yieldb (%)
1
2
3
4
5
6
7
8
1
1
1
1
1
1
1
1
87
98
90
80
83
81
75
88
60
90
3
N
The optimized conditions were as follows aryl halide
(
3
1.0 mmol), alkene (1.2 mmol), Et N (1.0 mmol), and polystyrene-
anchored Pd(II) phenyldithiocarbazate complex 2 (1.0 mol %) un-
der solvent-free conditions at 70 °C for 5 h.
As shown in Table 2, a range of aryl iodides was reacted with
methyl acrylate or styrene to give the desired products 5 in excel-
lent yields. The nature of the substituent, either an electron-donat-
ing group such as methoxy (entry 4), or an electron-withdrawing
Na
2
CO
3
K
Et
Et
2
CO
3
d
9
3
N
N
1
0.5
1
0
3
a
Conditions: iodobenzene (1.0 mmol), methyl acrylate (1.2 mmol), base
1.0 mmol), 5 h, 70 °C.
GC yield.
Diisopropylethylamine.
(
2
group such as Cl or NO (entries 2, 3, and 6) on the phenyl ring
b
c
of 3 had no significant effect on the reaction outcome. The reac-
tions of iodobenzene and p-nitroiodobenzene with styrene gave
trans-stilbenes 5e and 5f in 96% and 98% GC yields, respectively
d
Reaction at 25 °C.
(Table 2, entries 5 and 6).
phenylhydrazine and carbon disulfide in the presence of KOH in
DMF at 80 °C to produce the corresponding polystyrene-anchored
phenyldithiocarbazate ligand 1. This was then reacted with
PdCl (PhCN) in DMF at 80 °C to yield the polystyrene-anchored
2 2
Pd(II) phenyldithiocarbazate complex 2. Catalyst 2 was character-
As expected, aryl iodides were more reactive than aryl bro-
mides, and the substituent effects on the aryl iodides appeared to
be less significant than those of the aryl bromides. As shown in Ta-
ble 2, activated aryl bromides such as p-nitrobromobenzene under-
went the Heck reaction with methyl acrylate and styrene under
similar conditions to afford the corresponding products in 98%
and 92% GC yields, respectively (entries 8 and 11), whereas, reac-
tion of p-bromoanisole (an unactivated aryl bromide) with methyl
acrylate gave product 5d in 88% GC yield (entry 9).
ized by FT-IR, SEM, and ICP.
The IR spectrum of polystyrene-anchored phenyldithiocarbaz-
À1
ate ligand 1 showed a band around 3460 cm due to the N–H
vibrations. The sharp C–Cl peak (due to –CH
2
Cl groups) at
À1
1
264 cm in the starting polymer was not present or was ob-
Another important Pd-catalyzed coupling reaction is the alky-
nylation of aryl halides, that is, the Sonogashira reaction. The appli-
cation of this reaction is to be found in the synthesis of numerous
natural products including enediyne antibiotics. Thus we investi-
gated the polystyrene-anchored Pd(II) phenyldithiocarbazate com-
plex catalytic system 2 for the copper-free Sonogashira coupling
reaction. Since no copper salt was used, the undesired formation
of oxidative homocoupling diyne products was avoided. We em-
ployed the solvent- and copper-free coupling reaction of iodoben-
zene (1.0 mmol) with phenylacetylene (1.0 mmol) as a model
reaction using complex 2 (1 mol %) as the catalyst at room temper-
ature to study the effect of the base on the reaction (Table 3).
Among various bases, pyridine was found to be the best (entry
3). The reaction worked well when organic bases were used. Inor-
ganic bases such as KOH, K CO , and Na CO were less effective. A
served as a weak band after introduction of the phenyldithiocar-
bazate ligand. The N-content of the resin was calculated to be
2
.01% (0.72 mmol/g), which indicated that only 51–63% of the total
chlorines had been substituted by phenyldithiocarbazate. The
amount of palladium incorporated into the catalyst 2 was deter-
mined by inductively coupled plasma (ICP), which gave a value
of 0.24 mmol/g for the heterogeneous catalyst.
We applied the polystyrene-anchored Pd(II) phenyldithiocar-
bazate complex 2 as the catalyst in Heck and Sonogashira reac-
tions. Initial studies were performed on the Heck coupling
reaction of iodobenzene (3a) with methyl acrylate (4a) as a model
reaction using complex 2 (1 mol %) as the catalyst at 70 °C over five
3
hours (Table 1). Of the bases screened, Et N gave the best result,
and the corresponding coupled product 5a was obtained in 98%
GC yield (Table 1, entry 2). The reaction was also studied at 25 °C
2
3
2
3
lower palladium concentration gave a decreased yield (entry 9).

M. Bakherad et al. / Tetrahedron Letters 53 (2012) 5773–5776
5775
Table 3
Table 5
a
Optimization of the conditions for the Sonogashira reaction of phenylacetylene with
Heck and Sonogashira reactions catalyzed by the recycled catalyst
iodobenzenea
Entry
Cycle
Heck yieldb (%)
Sonogashira yieldb (%)
Entry
Base
Et
Cat (mol %)
Yieldb (%)
1
2
3
4
5
1
2
3
4
5
98
98
97
95
93
99
99
98
96
95
1
2
3
4
5
6
7
8
9
3
N
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
0.5
87
85
99
78
93
65
70
50
82
DIPEA
Pyridine
Piperidine
Pyrrolidine
KOH
a
Reaction conditions: iodobenzene (1.0 mmol), methyl acrylate (Heck reaction)
(
(
1.2 mmol), iodobenzene (1.0 mmol), phenylacetylene (Sonogashira reaction)
1.0 mmol), Pd-catalyst 2 (0.01 mmol), base (1.0 mmol), 70 °C (for Heck reactions),
K
2
CO
3
Na
CO
2 3
room temperature (for Sonogashira reactions).
Pyridine
b
GC yield.
a
Conditions: phenylacetylene (1.0 mmol), iodobenzene (1.0 mmol), base
(
1.0 mmol), room temperature, 3 h.
GC yield.
b
reactive acetylenes, 1-hexyne, and propargyl alcoho1 were used,
the expected coupling products were produced efficiently.
The coupling of para-substituted iodobenzenes having nitro and
methoxy groups took place with 1-hexyne to give the correspond-
ing products 8i and 8j in 98% and 95% GC yields, respectively
Table 4
a
Copper-free Sonogashira reactions of terminal alkynes with aryl halides
(
entries 9 and 10). The coupling of propargyl alcohol with the more
Pd-catalyst 2
pyridine, r.t
X
+
HC
R
R
reactive electron-withdrawing p-nitroiodobenzene gave an excel-
lent yield of product (entry 12). Under the same conditions, less ac-
tive, electron-rich p-iodoanisole produced a very high 95% GC yield
of alkyne 8m (entry 13).
<
<
6
7
8
We next investigated the coupling of various aryl bromides
with terminal alkynes. As shown in Table 4, high catalytic activity
was observed in the coupling of aryl bromides possessing electron-
donating groups such as p-bromoanisole (entries 19, 22, and 25).
Moreover, p-nitrobromobenzene, m-nitrobromobenzene, and
p-bromobenzonitrile having electron-deficient aromatic rings also
underwent the Sonogashira coupling reaction with terminal
alkynes under similar conditions to afford the corresponding
products in excellent yields.
The reusability of the catalyst was tested in reactions of
iodobenzene with methyl acrylate (Heck reaction), and iodoben-
zene with phenyl acetylene (Sonogashira reaction) as representa-
tives in the presence of 1.0 mol % of polystyrene-anchored Pd(II)
phenyldithiocarbazate complex 2. After the first run, the catalyst
was separated by filtration, washed thoroughly with ethanol and
acetonitrile, and dried at room temperature. The dried catalyst
was then reused with a fresh reaction mixture without any further
activation. The catalyst could be almost completely recovered and
was recycled an additional four times without any significant loss
of activity (Table 5).
In conclusion, we have developed a clean and safe protocol for
Heck and copper-free Sonogashira reactions catalyzed by polysty-
rene-anchored Pd(II) phenyldithiocarbazate complex 2 under aero-
bic conditions. The catalyst shows not only high catalytic activity,
but also offers many practical advantages such as air and thermal
stability and it can be recycled. The catalyst was used for five
consecutive cycles with consistent activity. The excellent catalytic
performance and easy preparation and separation of the catalyst
make it a valuable heterogeneous system, and potentially a useful
alternative to other heterogeneous palladium catalysts.
Entry
R
X
Y
Product
Yieldb (%)
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
Ph
Ph
Ph
Ph
Ph
Ph
Ph
n-C
n-C
n-C
CH
CH
CH
Ph
Ph
Ph
Ph
Ph
Ph
n-C
n-C
n-C
CH
CH
CH
I
I
I
I
I
I
I
I
I
I
I
I
H
8a
8b
8c
8d
8e
8f
8g
8h
8i
99 (97)
100 (96)
100 (98)
97
96
99
97
90
98 (95)
95
94
98 (96)
95
96
98
97 (94)
98
94
94
93
95 (91)
91
4-NO
3-NO
4-Cl
4-Br
4-COCH
4-OCH
H
4-NO
4-OCH
H
4-NO
4-OCH
H
4-NO
3-NO
4-CN
4-F
4-OCH
H
4-NO
4-OCH
H
2
2
3
3
3
3
4
4
4
H
H
H
OH
OH
OH
9
9
9
2
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
8j
8k
8l
2
2
2
2
I
8m
8a
8b
8c
8n
8o
8g
8h
8i
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
2
2
3
3
3
4
H
4
H
4
H
9
9
9
2
8j
8k
8l
2
OH
OH
OH
93
97 (93)
95
2
2
4-NO
4-OCH
2
8m
a
Reaction conditions: aryl halide (1.0 mmol), terminal alkyne (1.0 mmol), Pd-
catalyst (0.01 mmol), pyridine (1.0 mmol), 3 h, room temperature, aerobic
conditions.
2
b
GC yield. Isolated yields are given in parentheses.
The heterogeneous catalytic copper-free Sonogashira coupling
was also examined with a variety of iodo and bromoarenes under
the reaction conditions identified above, which exhibited wide
substrate tolerance. Representative results are summarized in Ta-
ble 4. Reaction of aryl iodides with electron-withdrawing substitu-
ents such as nitro and acetoxy (Table 4, entries 2, 3, and 6) with
phenylacetylene gave excellent yields of expected products, while
aryl iodides with electron-donating groups such as methoxy (entry
Preparation of polystyrene-anchored Pd(II) phenyldithiocarbaz-
ate complex 2
To a 250 ml round bottom flask equipped with a magnetic stir
bar and containing DMF (30 ml) were added chloromethylated
polystyrene (2 g, 1.25 mmol/g of Cl), phenylhydrazine (5.0 mmol),
KOH (5.0 mmol), and carbon disulfide (5.0 mmol). The mixture
was stirred for 10 h at 80 °C and was subsequently filtered and
washed thoroughly with DMF, and dried in vacuo for 12 h. Phen-
yldithiocarbazate-functionalized polymer 1 (2.0 g) was treated
7
) gave the corresponding coupling product in a slightly lower
yield.p-Chloroiodobenzene and p-bromoiodobenzene also under-
went the Sonogashira coupling reaction with phenylacetylene
under similar conditions to afford the corresponding biarylacetyl-
enes 8d and 8e in good yields (entries 4 and 5). When the less
2 2
with PdCl (PhCN) (2.0 g) in DMF, and the mixture heated at

5
776
M. Bakherad et al. / Tetrahedron Letters 53 (2012) 5773–5776
8
0 °C for 15 h. The resulting bright yellow colored polymer,
References and notes
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4
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The aryl halide (1.0 mmol) and terminal alkyne (1.0 mmol)
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(
stirred at room temperature for 3 h under aerobic conditions. The
mixture was filtered to recover the catalyst and the polymer was
washed with EtOH and MeCN, vacuum dried, and reused. After
GC analysis, the solvent was removed under vacuum, and the crude
product was subjected to silica gel column chromatography using
14. Phan, N. T. S.; Brown, D. H.; Adams, H.; Spey, S. E.; Styring, P. Dalton Trans. 2004,
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1
1
5. (a) Phan, N. T. S.; Brown, D. H.; Styring, P. Tetrahedron Lett. 2004, 45, 7915; (b)
Byun, J. W.; Lee, Y. S. Tetrahedron Lett. 1837, 2004, 45; (c) Vassylyev, O.; Chen, J.;
Panarello, A. P.; Khinast, J. G. Tetrahedron Lett. 2005, 46, 863; (d) Shimizu, K.;
Koizumi, S.; Hatamachi, T.; Yoshida, H.; Komai, S.; Kodama, T.; Kitayama, Y. J.
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3 3
CHCl –CH OH (98:2) as eluent to afford the pure product.
16. (a) Bakherad, M.; Bahramian, B.; Keivanloo, A.; Kamali, A. T. J. Braz. Chem. Soc.
2009, 20, 907; (b) Bakherad, M.; Bahramian, B.; Isfahani, H. N.; Keivanloo, A.;
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Bahramian, B.; Isfahani, H. N.; Keivanloo, A.; Sang, G. Chin. J. Chem. 2009, 27,
Compound 8a
3
53; (d) Datta, A.; Ebert, K.; Plenio, H. Organometallics 2003, 22, 4685; (e)
1H NMR (CDCl
H).
3
, 300 MHz) d 7.55–7.51 (m, 4H), 7.36–7.29 (m,
Uozumi, Y.; Kobayashi, Y. Heterocycles 2003, 29, 1255; (f) Lin, C.-A.; Luo, F.-T.
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1
6
049.
1
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Compound 8b
1H NMR (CDCl
3
, 300 MHz) d 7.34–7.44 (m, 3H), 7.52–7.58 (m,
2
H), 7.60 (d, J = 8.3 Hz, 2H), 7.92 (d, J = 8.3 Hz, 2H).
Acknowledgement
We are grateful to the Research Council of Shahrood University
of Technology for the financial support of this work.