Reactivity of aryl halides for reductive dehalogenation in (Sea)water using polymer-supported terpyridine palladium catalyst

Article abstract of DOI:10.3390/molecules20069906

A polymer-supported terpyridine palladium complex was prepared. The complex was found to promote hydrodechlorination of aryl chlorides with potassium formate in seawater. Generally, reductive cleavage of aryl chlorides using transition metal catalysts is more difficult than that of aryl bromides and iodides (reactivity: I > Br > Cl); however, the results obtained did not follow the general trend. Therefore, we investigated the reaction inhibition agents and found a method to remove these inhibitors. The polymeric catalysts showed high catalytic activity and high reusability for transfer reduction in seawater.

Full text of DOI:10.3390/molecules20069906

Molecules 2015, 20, 9906-9914; doi:10.3390/molecules20069906
OPEN ACCESS
molecules
ISSN 1420-3049
www.mdpi.com/journal/molecules
Communication
Reactivity of Aryl Halides for Reductive Dehalogenation in
Sea)water Using Polymer-Supported Terpyridine
(
Palladium Catalyst
Toshimasa Suzuka *, Hiromu Sueyoshi, Shohei Maehara and Hiroaki Ogasawara
Department of Chemistry, University of the Ryukyus, Okinawa 903-0213, Japan;
E-Mails: e123346@eve.u-ryukyu.ac.jp (H.S.); e113331@eve.u-ryukyu.ac.jp (S.M.);
rei_sz@yahoo.co.jp (H.O.)
*
Author to whom correspondence should be addressed; E-Mail: suzuka@sci.u-ryukyu.ac.jp;
Tel./Fax: +81-98-895-8531.
Academic Editor: Fernando Albericio
Received: 18 March 2015 / Accepted: 26 May 2015 / Published: 28 May 2015
Abstract: A polymer-supported terpyridine palladium complex was prepared. The complex
was found to promote hydrodechlorination of aryl chlorides with potassium formate in
seawater. Generally, reductive cleavage of aryl chlorides using transition metal catalysts is
more difficult than that of aryl bromides and iodides (reactivity: I > Br > Cl); however, the
results obtained did not follow the general trend. Therefore, we investigated the reaction
inhibition agents and found a method to remove these inhibitors. The polymeric catalysts
showed high catalytic activity and high reusability for transfer reduction in seawater.
Keywords: palladium; polymer-support; hydrodechlorination; seawater
1
. Introduction
Following the adoption of the “Stockholm Convention on Persistent Organic Pollutants” in 2001
many countries prohibited the production and use of chloroarenes, including poly(chlorobiphenyls)
PCB), 3-(3,4-dichlorophenyl)-1,1-dimethylurea and dioxins [1,2]. However, environmental pollution
(
caused by past use of chloroarenes persists. For example, in 2000, Haynes et al. reported that 3-(3,4-
dichlorophenyl)-1,1-dimethylurea [3], which was widely used in sugarcane (Saccharum) plantations,
has caused atrophy of coral and seaweed beds in the Australian Great Barrier Reef [4–6]. The

Molecules 2015, 20
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chloroarenes released into the environment are present at levels of only parts per million in soils and
seawater, and the collection and decomposition of chloroarenes is difficult. Therefore, we have
attempted to develop a decomposition reaction for chloroarenes in seawater and establish a method to
collect the low concentrations of chloroarenes released in seawater [7]. Hydrodechlorination of aryl
chlorides has received considerable attention in chemistry, particularly from an environmental perspective.
A large amount of research has been devoted to the synthetic application of the reaction as well as to
improving its efficiency in homogeneous and heterogeneous conditions [1,2]. In addition, considerable
research into the Pd-catalysed dehalogenation reaction in organic solvent and aqueous media [8–20]
has been undertaken since Helquist first reported the reduction of aryl bromides by sodium formate
catalysed by Pd(PPh
3
4
) in 1978 [21].
Our first study into hydrodechlorination of various aryl chlorides in seawater were conducted
in the presence of an amphiphilic polystyrene-poly(ethylene glycol) (PS-PEG) resin-bound
terpyridine-palladium catalyst and unexpectedly gave dechlorinated products in lower yield than yields
in pure water (Table 1) [7]. In these studies, we were surprised to find that the reactive order of halides
for the hydrodehalogenation of aryl halides in pure water was Ar–Cl > Ar–Br > Ar–I. For example, the
hydrodehalogenation of p-bromoacetophenone and p-iodoacetophenone for 24 h gave dehalogenated
products in 74% and 0.5% yield (Table 3, runs 2–3), although a dechlorinated product was obtained in
quantitative yield in the case of p-chloroacetophenone [22]. In general, the reductive cleavage of aryl
chlorides is more difficult than that of aryl bromides and iodides (reactivity: I > Br > Cl); however, the
results obtained did not follow the general trend [8,10,13,23–26]. Here, we report the anionic influence
on the reactivity of hydrodehalogenation in pure water by using a polymer-supported Pd catalyst
(
Scheme 1).
PEG 3000
N
O
+
PS
O
n
N C
N Pd Cl
H
Cl-
n = 60~74
N
1
cat. 1 (5 mol% Pd)
Cl + HCO K
H
2
R1
anion, water, 25 °C, 3 h
R1
-
-
-
-
-, HCO
-, H
-
2
anion: F , Cl , Br , I , HSO
4
3
2
PO
4
3
Scheme 1. Polystyrene-poly(ethylene glycol)-supported terpyridine-Pd complex and
application to the hydrodehalogenation of aryl halides in the presence of anion inhibitors
in water.
2
. Results and Discussion
We developed the hydrodehalogenation of aryl chlorides, bromides and iodides in water using
the polymeric catalyst 1, which was readily prepared from methyl 4-formylbenzoate, 2-acetylpyridine,
NH OH, PS–PEG–NH resin and (C CN) PdCl according to previously reported procedures [27].
The hydrodechlorination of p-chloroacetophenone (2a) was performed in pure water with HCO K and
polymeric catalyst 1 (5 mol % Pd) at 25 °C for 3 h to provide acetophenone (3a) in 94% yield. The
4
2
6
H
5
2
2
2

Molecules 2015, 20
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hydrodechlorination of p-chloroacetophenone (2a) with HCO
2
K was examined in seawater rather than
pure water under similar reaction conditions. The seawater was collected on Kirakira beach in Okinawa.
Representative results that allow a comparison between the results for seawater and those for water
are summarised in Table 1. The reduction of 2a–c with HCO K and polymeric catalyst 1 (5 mol % Pd)
2
in seawater provided acetophenone (3a) in 44%–65% yield, whereas, when performed in water, a
similar reaction gave 94%–96% yields (Table 1, runs 1–3). The reduction of chlorobenzophenones 2d–f
in seawater provided benzophenone (3b) in 52%–84% yield (Table 1, runs 4–6, respectively).
Reduction of chlorobenzene derivatives 2g–l, which have electron-donating substituents in the ortho-,
meta- and para-positions, provided aniline 3c or phenol 3d in yields of 9%–90% (Table 1, runs 7–12).
Dechlorination of highly chlorinated compounds 2m and 2n in seawater gave the corresponding
product 3a in 58% and 90% yield, respectively (Table 1, runs 13–14). It was surprising to find that the
reaction efficiency was noticeably affected by whether water or seawater was used. In general, various
inorganic salts dissolve in seawater in contrast to pure water. Therefore, we examined halides that
dissolve in seawater as catalytic inactivating agents to determine the reactivity of hydrodechlorination
in water using a polymer-supported palladium catalyst. The results are summarised in Table 2. The
hydrodechlorination of p-chloroacetophenone (2a) was performed in NaBr solution and NaI solution
with HCO
2
K and polymeric catalyst 1 (5 mol % Pd) at 25 °C for 3 h to provide acetophenone (3a) in
6
3% and 66% yields (Table 2, runs 4–5), whereas a similar reaction provided 94%–95% yield in water,
NaF solution and NaCl solution (Table 2, runs 1–3).
Table 1. Hydrodechlorination of aryl chlorides using polymeric catalyst 1 in (seawater) a.
cat. 1 (5 mol% Pd)
Cl + HCO K
H
2
R1
(sea)water, 25 °C, 3 h
R1
2
3
1
1
Run
1
R :2
R :3
Ac:3a
3a
Yield (%) (Water) Yield (%) (Seawater)
Yield (%) (Seawater)
p-Ac:2a
m-Ac:2b
o-Ac:2c
p-B:2d
94
96
95
94
94
93
93
95
96
94
94
94
90
96 e
56
65
44
52
84
68
9
94 c
95 c
91 c
87 c
84 c
68 c
94 c
95 c
95 c
94 c
94 c
94 c
58 c
90 e
2
3
3a
4 b
5 b
6 b
7 b
8
Bz:3b
3b
m-Bz:2e
o-Bz:2f
3b
p-NH
2
:2g
:2h
:2i
2
NH :3c
m-NH
2
3c
3c
44
33
90
77
59
21
43
9
o-NH
2
1
1
1
1
0
1
2
3
p-OH:2j
m-OH:2k
OH:3d
3d
o-OH:2l
3d
Ac, 2,5-Cl:2m
4 d Ac, 2,3,5-Cl:2n
3a
1
3a
a
All reactions were performed with 2 (0.2 mmol) and HCO
2
K (0.4 mmol) using polymeric catalyst 1 in H
2
O
(
1.5 mL) at 25 °C under N
2
. Yields were determined by gas chromatography based on n-dodecane as
b
c
d
an internal standard; 50 µL of toluene was used; Reaction time was 12 h; 12 mole equivalents (2.4 mmol)
e
of HCO
2
K vs. aryl chloride were used; Reaction time was 24 h.

Molecules 2015, 20
Table 2. Ionic influence on hydrodechlorination in water using polymeric catalyst 1.
9909
cat. 1 (5 mol% Pd)
Ac
Cl + HCO K
Ac
H
2
salt solution (100 ppm)
2
5 °C, 3 h
2a
3a
-
-
-
-
-
-
-
anion: F , Cl , Br , I , HSO , HCO , H PO
4
3
2
4
Run
1
Salts
none
NaF
Amount of Salts (mol) 100 ppm Yield (%) a
---
94
94
95
63
60
96
95
95
92
96
0
−
−
−
−
−
−
−
−
−
6
6
6
6
6
6
6
6
6
2
3.57 × 10
2.57 × 10
1.46 × 10
1.00 × 10
0.47 × 10
1.42 × 10
0.96 × 10
1.59 × 10
1.50 × 10
1.59 × 10
---
3
NaCl
NaBr
NaI
4
5
6
Na
2
SO
Na
NaH PO
AgNO
AgNO , NaBr
4
·10H
CO
·2H
2
O
7
2
3
8
2
4
2
O
9
3
1
0
3
b
−6
1
1
AgNO
None
3
b,c
1
2
94
a
2
All reactions were performed with 2 (0.2 mmol) and HCO K (0.4 mmol) using polymeric catalyst 1 in salt
solution of 100 ppm (1.5 mL) at 25 °C under N
2
. The yields were determined by gas chromatography based
b
c
on n-dodecane as an internal standard; Seawater was used; Reaction time was 12 h.
The reaction proceeded smoothly when a silver bromide ion was precipitated in a silver nitrate
−
−
solution (Table 2, run 10). These results suggested that Br and I were reaction inhibiting
−
−
agents for the hydrodechlorination of aryl halides in water. Due to the generated Br or I , the
hydrodehalogenation of p-bromoacetophenone (2o) and p-iodoacetophenone (2p) did not proceed
smoothly, and the desired product was obtained in 74% and 0.5% yield (Table 3, runs 2–3). However,
unexpectedly, a quantitative yield of the dechlorinated product was obtained in the case of p-chloro-
acetophenone (Table 3, run 1). We proceeded to examine the reduction of aryl chlorides by increasing
the reaction time under similar conditions. Thus, the reduction of 2a with HCO
polymeric precatalyst 1 (5 mol % Pd) in seawater for 12 h to provide acetophenone (3a) in 94% yield
Table 2, run 12). All reactions were also carried out in seawater for 12 h to give dechlorinated
2
K was performed with
(
products in 68%–95% yield (Table 1).
Table 3. Hydrodehalogenation of acetophenone-4-halide with ammonium formate in water.
cat. 1 (5 mol% Pd)
Ac
X + HCO K
Ac
H
2
water
X = Cl, Br, I
25 °C, 24 h
2
3a
Entry
Aryl Halide
Yield (%) a
quant.
74
1
4-Cl-acetophenone 2a
4-Br-acetophenone 2o
4-I-acetophenone 2p
b
2
b
3
0.5
a
All reactions were performed with 2 (0.2 mmol) and HCO
2
K (0.4 mmol) using polymeric precatalyst 1 in
H
2
O (1.5 mL) at 25 °C under N . The yields were determined by gas chromatography based on n-dodecane as
2
b
an internal standard; 50 μL of toluene was used.

Molecules 2015, 20
9910
The recyclability of PS–PEG terpyridine Pd (II) 1 was examined for the hydrodechlorination of
p-chloroacetophenone (2a) in seawater. After the first reaction, which provided acetophenone (3a) in
9
4% yield, the precatalyst was recovered by simple filtration, washed with H
and reused five times under similar reaction conditions to provide 3a in 95%, 96%, 95%, 95% and
6% yields, respectively. Inductively coupled plasma atomic emission spectroscopy analysis of the
2
O, dried under vacuum,
9
aqueous phase revealed barely detectable levels of palladium residue (<1 ppm).
To investigate the reduction of meta-chlorobiphenyl (2q) as a representative PCB using polymeric
precatalyst 1, we also tested the reaction with HCO K with polymeric precatalyst 1 in water at 100 °C
2
for 24 h. This reaction provided biphenyl (3e) in 91% yield. This result demonstrates the efficient
detoxification reaction of PCB in seawater, which is an environmental endocrine disrupter (Scheme 2).
cat. 1 (5 mol% Pd)
Cl + HCO K
H
2
seawater, 100 °C, 24 h
Ph
Ph
2q
3e
Scheme 2. Reduction of PCB.
3. Experimental Section
3.1. General Methods
All manipulations were carried out under aerobic conditions. Water was deionised using a MilliQ
1
13
gradient A10 apparatus (Millipore, Bedford, MA, USA) to Milli-Q grade. H- and C-NMR spectra
were recorded in dimethyl sulfoxide-d
6
(DMSO-d
6
) at 25 °C on an Avance spectrometer (400 MHz for
1
13
13
H and 100 MHz for C, Bruker, Karsruhe, Germany). C-NMR chemical shifts were given relative
to the residual DMSO-d peaks used as an internal standard (77.0 ppm and 39.7 ppm, respectively).
6
Mass spectral data were measured on a JMS-T100GCV mass spectrometer (MS) detector and a
JMS-T100LP MS detector (JEOL, Tokyo, Japan); the abbreviation “bp” is used to denote the base peak.
Gas chromatography (GC) analysis was performed on a GC-2014 GC (Shimadzu, Kyoto, Japan). Infrared
spectroscopy (IR) analysis was performed on a FTIR-410 spectrometer (JASCO, Tokyo, Japan).
3.2. Materials
The PS-PEG-supported terpyridine ligand was prepared from PS-PEG amino resin (TentaGel S-NH ,
2
average diameter 90 µm, 1% divinylbenzene cross-linked, loading value of amino residue 0.31 mmol/g,
purchased from Rapp Polymere, Tuebingen, Germany) and terpyridine-COOH ligand according to
previously reported procedures [28,29].
3
.3. Synthesis of Polymer-Supported Ligand
A solution of aqueous NH
was added to a solution of 4-methoxycarbonylbenzaldehyde (164 mg, 1.0 mmol) and 2-acetylpyridine
242 mg, 2.0 mmol) in EtOH (4.1 mL). After adding NaOH, the solution turned red after 1 h. The
solution was stirred vigorously at room temperature in a flask exposed to air for 17 h, after which a
4
OH (28%, 0.2 mL) and NaOH (80 mg, 2.0 mmol) in minimum water
(

Molecules 2015, 20
9911
yellow suspension was obtained. Water (50 mL) was added, and the solution was neutralised with
concentrated HCl to yield a pale yellow precipitate and a red solution. The precipitate was filtered and
washed with water. The precipitate was further purified by heating under reflux for 1 h in EtOH
1
(
(
10 mL). The terpyridine-COOH ligand (77.6 mg, 22% yield) was collected by filtration. H-NMR
DMSO-d
.05 (td, J = 7.6, 1.8 Hz, 4H), 7.56–7.53 (m, 2H); C-NMR (DMSO-d
2C), 149.0 (2C), 148.4, 141.0, 136.9 (2C), 132.4, 129.8 (2C), 126.6 (2C), 124.0 (2C), 120.7 (2C),
6
): δ13.2 (br s, 1H), 8.79–8.76 (m, 4H), 8.69 (d, J = 7.9 Hz, 2H), 8.14 (d, J = 8.4 Hz, 2H),
1
3
8
6
): δ166.6, 155.8 (2C), 154.9
(
−1
1
17.9 (2C); IR (ATR) (cm ): v 3414 (br), 3122, 1684, 1565; HR-ESI-MS: calculated for
Na (M + Na) 376.1062, found 376.1061. CAS registry number: 158014-74-5.
C
22
H
15
N
3
O
2
3
.4. Preparation of PS–PEG Resin-Supported Terpyridine Palladium Complex 1
A Merrifield vessel was charged with PS-PEG-NH
ligand (127 mg, 0.36 mmol), 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (138 mg, 0.72 mmol),
-hydroxybenzotriazole (146 mg, 0.96 mmol) and dimethyl sulfoxide (DMSO) (10 mL). The reaction
2
(0.77 g, 0.24 mmol), terpyridine-COOH
1
mixture was shaken at 25 °C for 16 h. The consumption of the primary amino residue of the resin was
monitored by the Kaiser negative test. The reaction mixture was filtered, and the resin was washed
with DMSO and CH
2
Cl . The resin was dried under reduced pressure to provide the polymer-supported
2
terpyridine (loading value of terpyridine: 0.27 mmol/g, as determined by elemental analysis).
Another Merrifield vessel was charged with resin-supported terpyridine ligand (444 mg, 0.12 mmol)
and toluene (10 mL). To this suspension, (C
the mixture was shaken (CM-1000) at 25 °C for 2 h. The mixture was filtered, and the resin was
washed with toluene and CH Cl . The resulting resin was dried under reduced pressure to provide the
polymer-supported palladium complex 1 (loading value of Pd: 0.26 mmol/g). IR (ATR) (cm ); 1;
6
H
5
CN)
2
PdCl
2
(72.7 mg, 0.19 mmol) was added, and
2
2
−1
1
3
v 2868, 1653, 1453, 1094. Gel-phase C-NMR (DMSO-d
6
) δ 164.8, 156.1 (2C), 154.8 (2C), 150.6
(
2C), 146.6, 142.1, 137.5 (2C), 136.7, 132.5 (2C), 129.8 (2C), 128.6 (2C), 125.4 (2C), 111.0 (2C),
7
0.7–69.0 (br, poly(ethylene glycol)).
3
.5. Hydrodechlorination of Aryl Chlorides
HCO
.010 mmol) and p-chloroacetophenone (2a) (31 mg, 0.20 mmol) in seawater (1.5 mL). The reaction
O and
2
K (67 mg, 0.40 mmol) was added to a mixture of the polymeric precatalyst 1 (40 mg,
0
mixture was shaken at 25 °C for 3 h and filtered. The recovered resin beads were rinsed with H
2
extracted five times with EtOAc (5 mL). The EtOAc layer was separated, and the aqueous layer was
extracted with EtOAc (3 mL). The combined EtOAc extracts were washed with brine (2 mL) and dried
over MgSO ; then, n-dodecane (20 mg) was added. The GC sample was transferred to a GC vial
4
from the organic layer. The yield of acetophenone (3a) determined by GC analysis was 94%, with
n-dodecane as an internal standard. The compounds and CAS registry numbers are as follows:
acetophenone (3a), benzophenone (3b), aniline (3c) and phenol (3d). CAS registry numbers: 98-86-2,
1
19-61-9, 62-53-3, and 108-95-2.

Molecules 2015, 20
. Conclusions
9912
4
A new polymer-supported terpyridine palladium complex 1 has been developed through the
formation of a covalent bound between the resin and the amino group. The resulting resin efficiently
catalysed the hydrodechlorination of aryl chlorides with HCO K in seawater under mild conditions to
2
provide the corresponding products in up to 95% yield. We also describe the catalysis inactivation
agents and a method to remove these inactivating agents, which were dissolved in seawater for the
presented reaction. This is the first example of the hydrodechlorination of aryl chlorides in seawater in
which a polymeric precatalyst has been employed. We are continuing to investigate the scope of the
hydrodechlorination and possible applications of the precatalyst in other organic transformations.
Acknowledgments
We acknowledge the financial support provided by the International Research Hub project of the
Faculty of Science, University of the Ryukyus and the Okinawa Science and Technology Promotion
Center. We thank Ichino Fukuzato, an undergraduate student, for assistance with some of the
analytical experiments.
Author Contributions
Hiromu Sueyoshi, Shohei Maehara, and Hiroaki Ogasawara performed research, analysed the data
and discussed. Toshimasa Suzuka designed and performed the research, did the data analysis,
manuscript writing and revision. All authors read and approved the final manuscript.
Conflicts of Interest
The authors declare no conflict of interest.
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