Polystyrene-supported triphenylarsines: Useful ligands in palladium-catalyzed aryl halide homocoupling reactions and a catalyst for alkene epoxidation using hydrogen peroxide

Article abstract of DOI:10.1016/j.tet.2005.07.108

The utility of both soluble (non-cross-linked) and insoluble (cross-linked) polystyrene-supported triphenylarsine reagents were examined. These reagents were prepared by standard radical polymerization methodology and used in palladium-catalyzed homocoupling reactions of aryl halides. The insoluble reagent was also used as a catalyst precursor in heterogeneous alkene epoxidation reactions in which aqueous hydrogen peroxide was the stoichiometric oxidant. For the aryl halide homocoupling reactions, both reagents worked well and afforded similar results. Unhindered aryl iodides afforded the best yields in the shortest reaction times compared to aryl bromides. The epoxidation reactions of unfunctionalized alkenes were not very efficient. This was probably due to the hydrophobicity of the polystyrene matrix, which did not swell in the reaction medium. Thus, since a microporous, gel-type polystyrene matrix was used, the majority of the arsine groups were inaccessible to the reaction components and therefore incapable of participating in catalysis.

Full text of DOI:10.1016/j.tet.2005.07.108

Tetrahedron 61 (2005) 12053–12057
Polystyrene-supported triphenylarsines: useful ligands in
palladium-catalyzed aryl halide homocoupling reactions and
a catalyst for alkene epoxidation using hydrogen peroxide
*
Helen Song He, Chuanxin Zhang, Chris Ka-Wo Ng and Patrick H. Toy
Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong, People’s Republic of China
Received 14 June 2005; revised 7 July 2005; accepted 7 July 2005
Available online 6 October 2005
Abstract—The utility of both soluble (non-cross-linked) and insoluble (cross-linked) polystyrene-supported triphenylarsine reagents were
examined. These reagents were prepared by standard radical polymerization methodology and used in palladium-catalyzed homocoupling
reactions of aryl halides. The insoluble reagent was also used as a catalyst precursor in heterogeneous alkene epoxidation reactions in which
aqueous hydrogen peroxide was the stoichiometric oxidant. For the aryl halide homocoupling reactions, both reagents worked well and
afforded similar results. Unhindered aryl iodides afforded the best yields in the shortest reaction times compared to aryl bromides. The
epoxidation reactions of unfunctionalized alkenes were not very efficient. This was probably due to the hydrophobicity of the polystyrene
matrix, which did not swell in the reaction medium. Thus, since a microporous, gel-type polystyrene matrix was used, the majority of the
arsine groups were inaccessible to the reaction components and therefore incapable of participating in catalysis.
q 2005 Elsevier Ltd. All rights reserved.
1. Introduction
Triphenylarsine is an important reagent in organic synthesis
that is complimentary to much more widely used
triphenylphosphine.12 It has been found to be useful as a
ligand for palladium in a variety of palladium catalyzed
cross-coupling reactions,13 including the Suzuki–Miyaura
coupling14 of boronic acids with aryl and vinyl halides.15
Furthermore, triphenylarsine is useful for the generation of
arsonium ylides, which are more nucleophilic than the
corresponding phosphonium ylides.16 Due to the toxicity of
organoarsenic compounds in general, they are prime
candidates for immobilization onto a polymer support so
that they can be easily recovered and reused. Indeed, early
reports demonstrated that an arsenic acid functionalized
polystyrene resin was useful in Baeyer–Villiger oxidation17
and alkene epoxidation18 reactions using aqueous hydrogen
peroxide under conditions such that the polymers could be
recovered and reused. Subsequently, a polystyrene-sup-
ported triphenylarsine oxide19 was found to be an effective
reagent for the preparation of carbodiimides.20 Finally, prior
to our recent report, the only other description of a
supported arsine showed that a silica-supported alkyldiaryl-
arsine was a good ligand for palladium in Heck coupling
reactions.21 In this current report, we describe the
application of our soluble and insoluble polystyrene-
supported reagents in palladium-catalyzed homocoupling
reactions of aryl halides and in organocatalytic alkene
epoxidation reactions using aqueous H2O2.
The use of polymer-supported reagents and catalysts in
polymer-assisted solution-phase organic synthesis has
become commonplace since they can reduce product
purification to simple filtration and concentration operations
and are potentially easily recycled.1 A large variety of such
reagents and catalysts have been reported that utilize both
insoluble2 and soluble3 polymers as their carriers and new
ones are continually being developed in order to broaden
their utility and increase their efficiency. We have a long-
standing interest in the development of both soluble and
insoluble polymer-supported amine,4 fluorinated ketone,5
nitroxyl radical,6 phosphine,7 sulfide,8 sulfoxide,9 and
triflimide10 reagents for use in solution-phase organic
synthesis, especially in systems that simultaneously use
multiple polymers.4,6,7c Herein, we report an update on the
further utilization of our previously reported polystyrene-
supported triphenylarsine reagents that were previously
found to be useful as ligands for palladium in a variety of
Suzuki–Miyaura coupling reactions.11
Keywords: Triphenylarsine reagents; Homocoupling; Suzuki–Miyaura
coupling; Hydrogen peroxide; Epoxidation.
Corresponding author. Tel.: C852 2859 2167; fax: C852 2857 1586;
e-mail: phtoy@hku.hk
*
0040–4020/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2005.07.108

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H. Song He et al. / Tetrahedron 61 (2005) 12053–12057
Table 1. Homocoupling reactions of aryl halides
2. Results and discussion
The soluble, non-cross-linked polystyrene reagent,
NCPS–AsPh3 (1), and the insoluble, cross-linked poly-
styrene reagent, JandaJel-AsPh3 (JJ-AsPh3, 2),22,23 were
prepared as previously reported using functional monomer 3
for incorporation of the arsine groups (Scheme 1).11 The
JandaJel platform was chosen over other cross-linked
polymers because it has been shown by EPR,24 fluor-
escence,25 and NMR26 spectroscopy to provide better
performance in terms of reagent/substrate accessibility to
interior sites of the resin in solvents that it swells well in.
Furthermore, it has been used successfully as a synthesis
platform in a variety of synthetic applications27 and as a
support for chiral catalysts.28–31
Entry
Ar-X
Product
Time (h)a
Yield (%)
1
2
1b
4a
4a
16
40
90
92
88
2c
95
92
3b
4b
4c
16
16
80
60
4b
74
5c
4d
4e
60
60
65
21
40
11
6c
a Time for reactions using both 1 and 2.
b Two mole percentage of Pd(OAc)2, 2 mol% 1 or 2, 0.5 equiv
hydroquinone, 1.0 equiv Cs2CO3, 0.4 M, 75 8C.
c Four mole percentage of Pd(OAc)2, 4 mol% 1 or 2, 0.5 equiv
hydroquinone, 1.0 equiv Cs2CO3, 1.0 M, 100 8C.
Scheme 1. Reaction conditions: (a) PhMgBr, THF, 0 8C to rt; (b) HCl;
(c) p-BrMg–C6H4CH]CH2, THF, 0 8C to rt; (d) AIBN, PhMe, 85 8C;
(e) PhCl, H2O, acacia gum, NaCl, AIBN, 85 8C.
only moderate yields of 4d were obtained after prolonged
reaction times (Table 1, entry 5) and 2-bromotoluene
afforded poor yield 4e with both 1 and 2 (Table 1, entry 6). It
was found that the product yields obtained from the
reactions involving 2 were slightly higher than those with
1 (Table 1, entries 2–4). This was most likely due to the
easier removal of 2 from the reaction mixture (filtration)
compared to 1 (precipitation followed by filtration). On the
other hand, in the cases where low yield was observed, 1
afforded higher yields than 2 (Table 1, entries 5–6), most
likely due to its homogeneity.
Considering our previous success in using both 1 and 2 in
Suzuki–Miyuara coupling reactions, we wanted to examine
them in another palladium-catalyzed process. Since sym-
metrical biaryl compounds can be used to prepare useful
chiral phosphine ligands,32 and new methods for the
homocoupling of aromatic compounds are continuously
being developed,33 we chose to study the reaction system
reported by Rawal and co-workers in which they used a
sterically crowded triarylarsine, tri-o-tolylarsine, as a ligand
for palladium in the homocoupling of aryl bromides and
iodides.34 Thus, various aryl iodides and aryl bromides were
examined in the homocoupling reactions using reagents 1
and 2 as ligands for palladium (Table 1).
To investigate other application of reagent 2, a range of
alkenes were studied in epoxidation reactions using H2O2
catalyzed by the oxide of 2 (Table 2). It has been previously
shown that treatment of triphenylarsine with H2O2 affords
triphenylarsine oxide, which is a good epoxidation
reagent.35 Thus, by using an excess of H2O2, triphenylarsine
oxide can function as a catalyst. For the epoxidation
reactions with 2, 1,2-dichloroethane was chosen as solvent
since appeared to swell 2 to a large degree and a control
reaction in which 2 was omitted afforded only a small
amount of the desired product 5a (Table 2, entry 1). As seen
from the Table 2, entry 2, addition of 2 greatly improved the
conversion of cyclooctene to 5a. While good yield can be
The reactions were performed according to the published
procedure,34 using Pd(OAc)2, ligand, hydroquinone, and
Cs2CO3 in N,N-dimethylacetamide (DMA) at elevated
temperature. In general aryl iodides reacted more readily
than aryl bromides and aryl halides substituted at meta- or
para-positions afforded good to excellent yields of the
desired symmetrical biaryl compounds 4a–c (Table 1,
entries 1–4). When 1-bromonapthalene was the substrate,

H. Song He et al. / Tetrahedron 61 (2005) 12053–12057
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Table 2. Epoxide reactions of alkenes
4. Experimental
4.1. General
All reagents were obtained from the Aldrich, Lancaster or
Acros chemical companies and were used without further
purification. All moisture sensitive reactions were carried
out in dried glassware under a N2 atmosphere. Merck silica
gel 60 (230–400 mesh) was used for chromatography. Thin-
layer chromatography analysis was performed using glass
plates coated with silica gel 60 F254. Gas chromatographic
analyses were performed using a Thermo Finnigan Focus
chromatograph equipped with an RTX-5 column. NMR
spectra were recorded using either a Bruker DRX 300 or an
AV400 spectrometer. Chemical shift data is expressed in
ppm with reference to TMS. MS data was recorded on a
Finnigan MAT 96 mass spectrometer.
Entry
1b
Alkene
Time (h)
8
Product
Yield (%)a
12c
5a
2
8
5a
85 (98c)
3
4
5
12
24
28
5b
5c
5d
80 (92c)
38
47
4.2. General procedure for biaryl synthesis using 1
6
30
5e
28
To a mixture of aryl halide (1.5 mmol), hydroquinone
(0.083 g, 0.75 mmol), and Cs2CO3 (0.49 g, 1.5 mmol) was
added a pre-stirred DMA solution (3.8 or 1.5 mL) of
Pd(OAc)2 (0.007 g, 0.03 mmol or 0.013 g, 0.06 mmol) and
1 (0.038 g, 0.03 mmol or 0.076 g, 0.06 mmol, loadingZ0.8
AsPh3 mmol/g) in a 8 mL vial. The reaction mixture
darkened immediately upon addition of the catalyst solution
to the solid reagents. The vial was placed in a shaking
reaction block and heated at 75 or 100 8C for the indicated
time. The reaction mixture was cooled to room temperature,
quenched with 1 N HCl (20 mL), diluted with water
(20 mL), and extracted with EtOAc (2!40 mL). The
organic layers were combined and washed sequentially
with 10% NaOH (3!30 mL), and brine (2!30 mL), dried
over MgSO4, concentrated under reduced pressure, and
purified by flash column chromatography on silica gel
(100% hexane) to afford the desired product.
a Isolated yield.
b No catalyst 2.
c Determined by GC.
obtained using 2, it is a sluggish catalyst precursor
compared to other homogeneous, electron-rich arsines.35
Regardless, good isolated yield of 5b could also be obtained
from the epoxidation of cycloheptene (Table 2, entry 3).
Unfortunately the epoxidation of substituted styrenes and a
primary alkene afforded poor isolated yields of the desired
products (Table 2, entries 4–6). In these reactions, the
conversion rate of the alkene was slower than before and the
products seemed to be moderately unstable to the reaction
conditions since several unidentified, more highly polar
products were observed in addition to the desired product.
4.3. General procedure for biaryl synthesis using 2
3. Conclusions
To a mixture of aryl halide (1.5 mmol), hydroquinone
(0.083 g, 0.8 mmol), and Cs2CO3 (0.49 g, 1.5 mmol) was
added a pre-stirred DMA solution (3.8 or 1.5 mL) of
Pd(OAc)2 (0.007 g, 0.03 mmol or 0.013 g, 0.06 mmol) and
2 (0.021 g, 0.03 mmol or 0.042 g, 0.06 mmol, loadingZ
1.44 AsPh3 mmol/g) in a 8 mL vial. The reaction mixture
darkened immediately upon addition of the catalyst solution
to the solid reagents. The vial was placed in a shaking
reaction block and heated at 75 or 100 8C for the indicated
time. The reaction mixture was cooled to room temperature
and the polymer was filtrated off and washed with water and
EtOAc. The filtrate was extracted with EtOAc (2!40 mL).
The organic layers were combined and washed sequentially
with 1 N HCl (3!30 mL), 10% NaOH (3!30 mL), and
brine (2!30 mL), dried over MgSO4, concentrated under
reduced pressure, and purified by flash column chromatog-
raphy on silica gel (100% hexane) to afford the desired
product.
In summary, we extended the use of our previously reported
soluble and insoluble polystyrene-supported triphenylarsine
reagents, 1 and 2, respectively, to the homocoupling of
aryl halides to form symmetrical biaryl compounds and
in epoxidation reactions using H2O2 as the stoichiometric
oxidant. It was found that both reagents proved to be good
ligands for Pd and the resulting complex showed good
catalytic activity in the homocoupling reaction.
The performance of 2 as a catalyst precursor for epoxidation
reactions was not very efficient. This could stem from the
fact that polystyrene is hydrophobic and thus the H2O2 has a
difficult time accessing the arsine groups attached to our gel-
type, microporous polymer. Thus, only a small portion of
the potential catalytic groups are actually active. Due to this
limitation of 2, we are currently examining the immobiliz-
ation of electron-rich arsine groups to insoluble, hydrophilic
and macroporous polymers with the aim of identifying
better heterogeneous arsine catalysts for these expodiation
reactions.
1
4.3.1. Characterization data for biphenyl (4a). H NMR
(300 MHz, CDCl3) d 7.35 (m, 2H), 7.44 (d, JZ6.3 Hz, 4H),
7.58–7.61 (m, 4H). 13C NMR (75 MHz, CDCl3) d 127.2

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H. Song He et al. / Tetrahedron 61 (2005) 12053–12057
(2C), 127.3 (2C), 128.8 (2C), 141.3 (2C). EI-MS: calcd for
C12H10, 154.08; found, 154 (MC).
1H), 2.36–2.39 (m, 1H), 2.43–2.57 (m, 1H), 2.72–2.79 (m,
1H), 3.71 (t, JZ3.1 Hz, 1H), 3.83 (d, JZ4.2 Hz, 1H), 7.08
(d, JZ7.1 Hz, 1H), 7.19–7.25 (m, 2H), 7.37 (d, JZ1.5 Hz,
1H). 13C NMR (75 MHz, CDCl3) d 21.8, 24.4, 52.8, 55.1,
126.1, 128.4, 129.5, 132.6, 136.7. HR EI-MS: calcd for
C10H10O, 146.0732; found, 146.0736.
4.3.2. Characterization data for 4,40-dimethoxybiphenyl
1
(4b). H NMR (300 MHz, CDCl3) d 3.83 (s, 6H), 6.94 (d,
JZ8.8 Hz, 4H), 7.46 (d, JZ8.8 Hz, 4H). 13C NMR
(75 MHz, CDCl3) d 55.4 (2C), 114.2 (4C), 127.8 (4C),
133.5 (2C), 158.7 (2C). HR EI-MS: calcd for C14H14O2,
214.0994; found, 214.0988.
4.4.4. Characterization data for 1-phenylcyclohexene
epoxide (5d). 1H NMR (300 MHz, CDCl3) d 1.26–1.59 (m,
4H), 1.95–2.00 (m, 2H), 2.12–2.13 (m, 1H), 2.23–2.27 (m,
1H), 3.06 (s, 1H), 7.08–7.38 (m, 5H). 13C NMR (75 MHz,
CDCl3) d 19.8, 20.1, 28.9, 60.2, 61.9, 125.3, 127.2, 128.3,
142.5. EI-MS: calcd for C12H14O, 174.10; found, 174 (MC).
4.3.3. Characterization data for 3,30-dinitrobiphenyl
1
(4c). H NMR (300 MHz, CDCl3) d 7.69 (t, JZ8.0 Hz,
2H), 7.94–7.97 (m, 2H), 8.27 (d, JZ1.2 Hz, 2H), 8.48 (t,
JZ2.0 Hz, 2H). 13C NMR (75 MHz, DMSO) d 122.3 (2C),
123.7 (2C), 131.2 (2C), 134.2 (2C), 139.9 (2C), 148.9 (2C).
HR EI-MS: calcd for C12H8N2O4, 244.0484; found,
244.0489.
4.4.5. Characterization data for 2-(4-methoxybenzyl)-
oxirane epoxide (5e). H NMR (300 MHz, CDCl3) d 2.51
1
(dd, JZ5.0, 2.6 Hz, 1H), 2.73–2.83 (m, 3H), 3.09–3.11 (m,
1H), 3.77 (s, 3H), 6.82–6.87 (m, 2H), 7.15 (dd, JZ6.9,
2.0 Hz, 2H). HR EI-MS: calcd for C10H12O2, 164.0837;
found, 164.0839.
4.3.4. Characterization data for 1,10-binaphthyl (4d). 1H
NMR (300 MHz, CDCl3) d 7.28 (t, JZ8.0 Hz, 2H), 7.37 (d,
JZ8.5 Hz, 2H), 7.47–7.50 (m, 4H), 7.55–7.59 (m, 2H), 7.95
(dd, JZ8.1, 1.3 Hz, 4H). 13C NMR (75 MHz, CDCl3) d
125.4 (2C), 125.8 (2C), 126.0 (2C), 126.6 (2C), 127.9 (2C),
128.0 (2C), 128.2 (2C), 132.9 (2C), 133.6 (2C), 138.5 (2C).
HR EI-MS: calcd for C20H14, 254.1096; found, 254.1086.
Acknowledgements
This research was supported financially by the University of
Hong Kong, and the Research Grants Council of the Hong
Kong Special Administrative Region, P.R. of China (Project
No. HKU 7112/02P). We thank Prof. Kin Shing Chan
(Chinese University of Hong Kong) for helpful discussions.
4.3.5. Characterization data for 2,20-dimethylbiphenyl
1
(4e). H NMR (400 MHz, CDCl3) d 2.05 (s, 6H), 7.09 (d,
JZ6.7 Hz, 2H), 7.20–7.26 (m, 6H). 13C NMR (100 MHz,
CDCl3) d 19.8 (2C), 125.5 (2C), 127.1 (2C), 129.3 (2C),
129.8 (2C), 135.8 (2C), 141.6 (2C). HR EI-MS: calcd for
C14H14, 182.1096; found, 182.1094.
References and notes
4.4. General procedure for alkene epoxidation using 2
A solution of the alkene (1.0 mmol) was added to a mixture
of 2 (0.035 g, 0.05 mmol) and 50% H2O2 (0.5 mL,
8.7 mmol) in 1,2-dichloroethane (2 mL). The reaction
mixture was stirred at 75 8C and monitored by TLC and
GC analysis. After the complete disappearance of the
alkene, the suspension was filtered and the resin was washed
by diethyl ether (2!10 mL). The combined organic layer
was concentrated and the crude residue was filtered through
a plug of silica gel with diethyl ether to provide the
essentially pure epoxide product after solvent removal.
Reactions were also performed where the yields were
determined by GC analysis by comparison to an internal
standard.
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