Preparation of a Nafion-Teflon bimembrane-supported palladium catalyst and its use in the Heck reaction

Article abstract of DOI:10.1016/j.tetlet.2005.06.091

A novel palladium catalyst supported on the Nafion membrane, reinforced with poly(tetrafluoroethylene) fiber, has been prepared. The catalyst exhibits high activity and stability in the Heck arylation reactions of aryl iodides with olefins and Sonogashira couplings with phenylacetylene, and can be readily recovered and reused twenty times without significant loss of activity.

Full text of DOI:10.1016/j.tetlet.2005.06.091

Tetrahedron
Letters
Tetrahedron Letters 46 (2005) 6159–6162
Preparation of a Nafion–Teflon bimembrane-supported
palladium catalyst and its use in the Heck reaction
Yangzhou Li, Zhiming Li, Feng Li, Quanrui Wang* and Fenggang Tao
Department of Chemistry, Fudan University, 200433 Shanghai, PR China
Received 29 April 2005;revised 7 June 2005;accepted 13 June 2005
Available online 25 July 2005
Abstract—A novel palladium catalyst supported on the Nafion membrane, reinforced with poly(tetrafluoroethylene) fiber, has been
prepared. The catalyst exhibits high activity and stability in the Heck arylation reactions of aryl iodides with olefins and Sonogashira
couplings with phenylacetylene, and can be readily recovered and reused twenty times without significant loss of activity.
ꢀ 2005 Published by Elsevier Ltd.
Palladium catalyzed coupling of aryl and vinyl halides
with olefins, known as Heck reactions has become a
reliable way for Csp2–Csp2 bond formation.1 During
the past two decades, the Heck coupling reaction has
witnessed great success in the areas of natural products,
high-performance materials, and biologically active
compounds.2,3 Typical conditions include the use of var-
ious palladium species and phosphine ligands. However,
many employed phosphines are air- and moisture-sensi-
tive and can be converted to phosphine oxide species,
which poison the catalysts leading to loss of the catalytic
ability. Hence, the high cost of the catalyst and the metal
contamination of the product remain to be surmounted.
The search for recyclable catalysts with low leaching
level of metals has been constantly recognized as one
of the prime concerns from both academic and indus-
trial perspectives.4,5
col)-bound palladacycle7 and chitosan-based palladium
catalyst.8 While there have been many advances in this
area, new recyclable catalyst systems seem to be con-
stant desiderata.9
Nafion-Hꢁ, developed by DuPont in the early 1960s, is a
perfluorinated ion-exchange polymer containing tetra-
fluoroethylene and perfluorovinyl ether units terminated
with a –SO3H group. The high chemical and thermal
stability, in addition to its high mechanical strength,
make it ideal for use as a support.10 Indeed, acting as
an excellent support, a few cationic metal complexes
have been successfully immobilized onto the Nafion
motif,11 and in several instances exhibited catalytic
activities comparable to homogeneous conditions.12
However, metal complex attached to Nafion by ion
exchange can seldom be reused without loss of activities.
On the other hand, the precursor of Nafion-H, named as
Nafion-F that is terminated with a sulfuryl fluoride
group, provides the potential to combine an NH2-bear-
ing ligand via a sulfamide binding. Intrigued by the
relative inertness to corrosive environments, we expect
such kind of polymer-supported ligands and the resulted
palladium catalysts to be air stable. Herein, we wish to
describe the first use of a Teflon-reinforced Nafion mem-
brane (ꢀ0.003 in. thick, 0.3 mmol/g, Gore product) as
support for diphenylphosphine ligand, the preparation
of the palladium catalyst 4, and its use for the Heck
reactions.
Various supported catalysts are useful approaches
toward this goal. The linchpin of the strategies relies
on the immobilization of the ligand onto a functional-
ized polymer backbone through covalent binding. In
comparison to homogenous solution conditions, poly-
mer-bound catalysis can facilitate the catalyst separation
from the reaction mixture, simplify catalysts recovery
and recycling, and eventually be prospective to meet
the tendency of green chemistry. Many papers have
appeared describing polymer-supported Heck reac-
tions.6 Recent examples include an oligo(ethylenegly-
The preparation of (2-cyanoethyl)diphenylphosphine 1
was achieved with a slightly modified method13 by con-
jugate addition of the sodium salt of diphenylphosphine
Keywords: Palladium catalyst;Nafion film;Heck reaction;Sonogashira
couplings.
*
Corresponding author. E-mail: qrwang@fudan.edu.cn
0040-4039/$ - see front matter ꢀ 2005 Published by Elsevier Ltd.
doi:10.1016/j.tetlet.2005.06.091

6160
Y. Li et al. / Tetrahedron Letters 46 (2005) 6159–6162
O
S
O
b
a
Ph
Ph
Ph2P(CH2)3NH2
Pd(OAc)2/CH2Cl2
+
Ph2PCH2CH2CN
CH2=CHCN
Ph2PH
Nafion
TEFLON
NH(CH2)3P
Pd(OAc)2
2
1
3
rt, 2h
O
4
Nafion
S
NH(CH2)3PPh2
c
TEFLON
O
Scheme 2.
3
Scheme 1. Reagents and conditions: (a) 50% aq NaOH, 55 ꢂC, 1 h;(b)
LiAlH4, Et2O, reflux, 1 h;(c) Teflon–Nafion-F resin, DMF/H 2O, aq
Na2CO3, MW (250 W), 10 min.
be 0.665 wt % by Graphite Furnace Atomic Absorption
(GFAA) analysis.
It was found that catalyst 4 was effective for the cou-
pling of aryl iodides with alkenes (Heck reactions) and
also with alkynes (Sonogashira couplings) at 100 ꢂC
using Et3N as the base and acetonitrile as the medium.
An attractive feature was the low catalyst loading
(0.5 mol %) required. In the absence of a base, the reac-
tion did not proceed, indicating that it was indispensable
to the procedure. After screening a range of usual inor-
ganic and organic bases, triethylamine was the most
appropriate. By exploring the scope of the solvent, it
was found that the Teflon–Nafion bimembrane swelled
well in acetonitrile and satisfactory results were
obtained.
to acrylonitrile and subsequent reduction with LiAlH4
in 81% and 70% yields, respectively.14 The attachment
of the biphenylphosphine ligand to the polymer mem-
brane was accomplished through the sulfonamido link-
age under microwave heating condition by using
Na2CO3 as the base and DMF as the solvent to provide
the Nafion-bound diphenylphosphino ligand
(Scheme 1).15
3
The supported palladium catalyst was generated by
complexion of 3 with a palladium source. Two palla-
dium compounds, that is, palladium acetate and di(l-
chloro)bis(g3-allyl)dipalladium(II), were examined and
both were effective for Heck reactions. Since palladium
acetate is much cheaper, it has been applied throughout
the present research. The supported catalyst 4 was
obtained as a yellowish solid (Scheme 2) by stirring a
mixture of the bound ligand 3 and Pd(OAc)2 in dichlo-
romethane.16 The palladium content was determined to
Catalyst 4 was tested for its efficacy in the coupling reac-
tions of aryl iodides with alkenes or alkynes.17 The
results are summarized in Table 1. It is shown that all
of the aryl iodides employed could be coupled with alk-
enes or alkynes to give the products in good yields. If
the formation of E/Z isomers was possible, the exclusive
Table 1. Heck reactions and Sonogashira couplings catalyzed by 4a
CH2 CH X
Ar CH CH X
Ar
I
+
or
or
CH C Ph
Ar C C Ph
Entry
Aryl iodide
Alkene or alkyne
Time (h)
Product
Yield (%)b
I
CH CH CO2Et
1
2
CH2@CH–CO2Et
CH2@CH–CO2Et
16
24
96
84
Me
Me
CH=CH CO2Et
I
Me
Me
Me
3
CH2@CH–CO2Et
24
86c
CH=CH CO2Et
CH CH CO2Et
I
Me
4
5
CH2@CH–CO2Et
CH2@CH–CO2Et
20
24
97d
87
I
Me
Me
CH3O
CH=CH CO2Et
CH3O
I
HC CH CO2Et
6
CH2@CH–CO2Et
48
96e
I
I
HC CH CO2Et
CH CH CO2Et
I
7
8
CH2@CH–CO2Et
C6H5CH@CH2
16
18
98f
82
CH CH
I

Y. Li et al. / Tetrahedron Letters 46 (2005) 6159–6162
6161
Table 1 (continued)
Entry
Aryl iodide
Alkene or alkyne
Time (h)
18
Product
C C
Yield (%)b
I
9
C6H5C„CH
89
90
10
CH2@CH–CN
CH2@CH–CN
24
30
I
CH CH CN
CH CH CN
Me
Me
11
85
I
Me
Me
S
12
13
14
15
CH2@CH–CO2Et
CH2@CH–CN
C6H5CH@CH2
C6H5C„CH
15
15
20
15
CH CH CO2Et
CH CH CN
92
93g
80
90
I
S
I
S
S
CH
CH
I
S
S
C C
I
S
S
a Reagents and conditions: alkene or alkyne (1.5 equiv), Et3N (2.0 equiv), 0.8 mol % 4, CH3CN, 100 ꢂC.
b Isolated yields.
c [PdCl(g3-C3H5)]2 was used instead of Pd(OAc)2.
d When the catalyst was reused for second, third, fourth and fifth cycle, yields of 90%, 86% 85% and 80% were obtained, respectively.
e 3.0 equiv of alkene were employed.
f E:Z = 30:1.
g E:Z = 2:1.
or predominant product was the trans isomer by inspec-
tion of their olefinic H–H coupling constants. The scope
of the reaction was investigated using a variety of unsat-
urated substrates, including ethyl acrylate (entries 1–7
and 12), acrylonitrile (entries 10, 11, and 13), styrene (en-
tries 8 and 14), and phenylacetylene (entries 9 and 15). In
all of these reactions, the supported catalyst 4 provided
very high catalytic activities. However, our efforts on
employing aryl bromides as the substrates produced less
success, reflecting the substrate scope of the present pro-
tocol. The utility of supported [PdCl(g3-C3H5)]2 was also
verified by the coupling of 3-iodotoluene with ethyl acryl-
ate (entry 3). Notably, the reactions with 2-iodothio-
phene were also successfully achieved (entries 12–15).
This could provide a useful way for introduction of an
unsaturated group on the thiophene ring.
In summary, we have designed a kind of diphenyl-
phosphino ligand attached to the surface of a Teflon–
Nafion bimembrane and demonstrated its utility in the
Heck reactions and Sonogashira reactions. Under the
optimum conditions, the respective arylated products
have been produced in high to excellent yields. To the
best of our awareness, it represents an unprecedented
example of Nafion-supported catalyst, via covalent
bond, for palladium-catalyzed cross-coupling reaction.
Of particular note are the exceptional stability of the
ligand and the recyclability of the catalyst. The catalyst
has also been successfully applied in other coupling reac-
tions like Suzuki cross-couplings. These will be reported
separately in due course.
Acknowledgements
Experimentally, catalyst 4 was readily recovered from
the reaction mixture by filtration, and the product was
isolated following the usual aqueous workup. Since the
potentiality to recycle the catalyst containing expensive
metals is of considerable significance in rendering the
method green, the recovered catalyst 4 was reused with
the coupling between 5-iodoxylene and ethyl acrylate
as a model reaction (entry 4). Recycling of the catalyst
involved a simple filtration and washing. As shown by
entry 4 in Table 1, the catalyst provided sufficient activ-
ity until the fifth consecutive reaction. Less than 0.1%
Pd leaching into the products was observed. Addition
of traces of palladium source (here: Pd(OAc)2) could
retrieve the catalytic activities. Another merit of the
method lay in the fact that the ligand proved to be insen-
sitive to moisture and air, and amenable to survive un-
der the reaction conditions for up to 20 times.
We would like to thank W. L. Gore and Associates, Inc.
located at Elkton, USA, for their generous gift of the
Teflon–Nafion bimembrane, helpful comments and for
their interest. This work was partially supported by
Municipal Government of Shanghai (03DZ19209).
References and notes
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15. Procedure for attachment of 2 onto the bimembrane: 3-
(diphenylphosphino)propylamine 2 (0.24 g, 1 mmol) was
dissolved in DMF (10 mL), then the Teflon-Nafion-F
membrane (0.5 g, 0.3 mmol/g, 0.15 mmol) and 15 mL of
water were sequentially added. The mixture was adjusted
to about pH 11 by slow addition of aq Na2CO3. The vial
was heated under selected microwave conditions (ꢀ250 W
output) for two times, each for 10 min. After irradiation,
the polymer was taken out by filtration, washed succes-
sively with degassed water, acetone and ether, and dried in
vacuo at 50 ꢂC to give the supported ligand 3, which was
characterized by IR analysis. The loading value was
determined to be 0.314 mmol/g by nitrogen elemental
analysis.
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16. Preparation of the supported catalyst 4: a vial was charged
with the Nafion-bound diphenylphosphino ligand 3 and
20 mL of dichloromethane. Pd(OAc)2 (5 mg, 0.022 mmol)
was added in one portion to the resulted suspension at
ambient temperature, and the mixture was stirred at room
temperature for an additional 2 h. The polymer was taken
out from the mixture by filtration, washed successively with
dichloromethane, ethanol and acetone to ensure exclusive
removal of the unbound metal. Drying at 50 ꢂC in vacuum
afforded the supported catalyst 4 as a yellowish solid. The
palladium content was determined to be 0.665 wt % by
Graphite Furnace Atomic Absorption (GFAA) analysis.
17. General procedure for Heck arylation reactions: The
reaction was carried out at 100 ꢂC for 15–48 h under N2 in
CH3CN by using 1.0 equiv of aryl halide, 1.5 equiv of
alkene, 2.0 equiv of Et3N and the indicated amount of
catalyst 4 (0.665 wt % of palladium). After cooling to
room temperature, the polymer was filtrated and washed
successively with CH3CN for reuse. The mixture was then
diluted with water and extracted with ether. The combined
organic layers were washed with brine, dried with Na2SO4,
filtered, and then evaporated to dryness. The crude
product was purified by flash column chromatography to
afforded the pure product. The Nafion-bound catalyst can
be recycled several times. Addition of catalytic amount of
palladium sources refresh the catalytic activities and the
ligand itself proves to be extremely stable even after using
for 20 cycles.
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14. Freshly distilled acrylonitrile (1.80 g, 33.9 mmol) was
added slowly to a stirred mixture of diphenylphosphine
(3.80 g, 20.4 mmol), 50% aq NaOH (0.1 mL) and aceto-
nitrile (5 mL). The mixture was stirred for 1 h at 55 ꢂC.
The yellow solution was extracted with benzene
(3 · 10 mL) and the organic layer was separated and dried
over Na2SO4. Removal of the solvent left yellow oil which
was extracted into ether (50 mL) under reflux. Vacuum
evaporation afforded 3.91 g of (2-cyanoethyl)diphenyl-
phosphine (yield: 81%). IR (neat): 3057, 2250, 1591, 1481,
1438, 1184, 1118 cmÀ1; 1H NMR (500 MHz, CDCl3): 1.61
(m, 2H, PCH2), 2.63 (m, 2H, CH2CN), 7.51–7.75 (m, 10H,
2C6H5) ppm.
The selected spectroscopic data for the coupled products
are as follows:
Ethyl trans-cinnamate (entry 1). IR (film) 2931, 1737,
1495, 1451, 1361, 1230, 1029, 954 cmÀ1
.
1H NMR
(500 MHz, CDCl3): 1.34 (t, 7.1 Hz, 3H, CH3), 4.27 (q,
7.1 Hz, 2H, CH2), 6.44 (d, 16.0 Hz, 1H, @CH), 7.26–7.53
(m, 5H, C6H5), 7.69 (d, 16.0 Hz, 1H, @CH) ppm. 13C
NMR (100 MHz, CDCl3): 20.80, 64.90, 123.19, 126.52,
127.98, 128,51, 134.11, 136.27, 170.71 ppm.
trans-Stilbene (entry 8). IR (KBr): 3020, 1497, 1450, 966,
1
Reduction: To a suspension of LiAlH4 (1.0 g, 26.4 mmol)
in dry ether (50 mL) was added dropwise a solution of 2-
(cyanoethyl)diphenylphosphine in dry ether (50 mL) at
0 ꢂC with stirring under N2. The mixture was heated to
reflux for 24 h. After cooling to 0 ꢂC, 4 mL of degassed
water was added carefully to quench the reaction. The
precipitate was filtered and the organic layer was sepa-
rated and dried over Na2SO4. Evaporation of the solvent
in vacuo yielded the ligand 2 (2.80 g, 70%) as a colorless
viscous liquid. IR (film): 3069, 3051, 2929, 2855, 1585,
1481, 1435, 1026 cmÀ1. 1H NMR (500 MHz, CDCl3): 1.23
764 cmÀ1. H NMR (500 MHz, CDCl3): 7.10 (d, 16.5 Hz,
2H, 2 @CH), 7.25–7.59 (m, 10H, 2C6H5). 13C NMR
(100 MHz, CDCl3): 126.43, 127.31, 128.60, 129.01,
138.01 ppm.
Ethyl trans-3-(2-thienyl)acrylate (entry 12). IR (film):
2985, 1742, 1611, 1427, 1237, 841 cmÀ1
.
1H NMR
(500 MHz, CDCl3): 1.32 (t, 7.1 Hz, 3H, CH3), 4.24 (q,
7.1 Hz, 2H, CH2), 6.23 (d, 15.7 Hz, 1H, @CH), 6.78–7.36
(m, 3H), 7.77 (d, 15.7 Hz, 1H, @CH). 13C NMR
(100 MHz, CDCl3): 21.19, 61.38, 121.70, 125.34, 126.10,
127.01, 130.57, 139.55, 171.26 ppm.