A practical and highly efficient reductive dehalogenation of aryl halides using heterogeneous Pd/AlO(OH) nanoparticles and sodium borohydride

Article abstract of DOI:10.1016/j.tet.2016.08.027

The reductive dehalogenation of aryl halides was performed by using commercially available aluminium oxy-hydroxide-supported palladium (Pd/AlO(OH)) nanoparticles of about 3?nm size (0.5?wt.?% Pd) with sodium borohydride. The dehalogenated products were obtained with absolute conversion in a mixture of H₂O/MeOH (v/v=1/1) under ultrasonic conditions at room temperature. All aryl halides were successfully converted to halogen-free compounds within 1.5–4?h with yields of over 95%. The one-pot catalytic method is presented as a new process for the reductive dehalogenation of halogenated compounds. This method is quite simple, highly efficient and eco-friendly, and has an exceptional recovery rate.

Full text of DOI:10.1016/j.tet.2016.08.027

Tetrahedron 72 (2016) 5898e5902
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Tetrahedron
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A practical and highly efficient reductive dehalogenation of aryl
halides using heterogeneous Pd/AlO(OH) nanoparticles and sodium
borohydride
a
a
a
b,
*
Belguzar Yasemin Kara , Melike Yazici , Benan Kilbas , Haydar Goksu
a
Department of Chemistry, Faculty of Sciences, Duzce University, 81620 Duzce, Turkey
Kaynasli Vocational College, Duzce University, D u€ zce 81900, Turkey
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 12 May 2016
Received in revised form 28 July 2016
Accepted 9 August 2016
Available online 10 August 2016
The reductive dehalogenation of aryl halides was performed by using commercially available aluminium
oxy-hydroxide-supported palladium (Pd/AlO(OH)) nanoparticles of about 3 nm size (0.5 wt. % Pd) with
sodium borohydride. The dehalogenated products were obtained with absolute conversion in a mixture
of H
2
O/MeOH (v/v¼1/1) under ultrasonic conditions at room temperature. All aryl halides were suc-
cessfully converted to halogen-free compounds within 1.5e4 h with yields of over 95%. The one-pot
catalytic method is presented as a new process for the reductive dehalogenation of halogenated com-
pounds. This method is quite simple, highly efficient and eco-friendly, and has an exceptional recovery
rate.
Keywords:
PdAlO(OH)
Heterogeneous catalyst
Dehalogenation
Reduction
Ó 2016 Elsevier Ltd. All rights reserved.
NaBH
4
1. Introduction
The toxicity effects of halogenated compounds have led scien-
tists to carry out the removal of the halogens by cleaving the car-
8
Halogenated organiccompoundshave beenrecognizedasbiocidal
and reagent/intermediate in organic synthesis. However, they are
bonehalogen bond.
Thus, novel catalytic systems for
1
dehalogenation of aryl halides have been developed using catalysts
9
10
11
12
known to be serious pollutants due to their environmental impact in
2 3
such as Pd/HAP-ɣ-Fe O , In, Pd/SBA-15, Pd(OAc)2, Zn/THF-
2
13
14
15
the chemical industry. For instance, (R)-5-(azetidin-2-ylmethoxy)-2-
NH Cl, PdPt/TiO2, PdCl2 and various hydrogen sources. Re-
4
3
chloropyridine, (E)-1-(3-(chloromethylene)-6-hydroxy-2,3-dihydro
cently, instead of free hydrogen gas, preference has been shown to
4
16
benzo[b]oxepin-7-yl)ethanone, 2,3,7,8-tetrachlorodibenzo[b,e][1,4]
hydrogen sources such as ammonia borane (AB, 19.6 wt. %), di-
5
6
17
dioxine and 2-chloro-6-(1H-pyrrol-2-yl)aniline halogen derivative
methyl ammonia borane (DMAB, 3.5 wt. %) and sodium borohy-
1
8
chemicals are toxic materials in which chlorine atoms are located in
dride (NaBH ,10.8 wt. %) due to their ability to be used under mild
4
7
19
the aromatic and olefin systems.
conditions and their high hydrogen density. Among these, NaBH4
is quite stable, cheap, eco-friendly and water-soluble. It also pro-
2
0
duces hydrogen gas in the presence of catalyst/water (Eq. 1).
Catalyst
NaBH þ 2H O
NaBO þ 4H
(1)
4
2
2
2
Although Pd/AlO(OH) nanoparticles (NPs) are commercially
available, relatively little interest in these nanoparticles as hetero-
geneous catalysts has been shown in the literature. However, more
recently, exponential growth has been observed in the number of
2
1
publications dealing with the alkylation of ketones with alcohols,
2
2
reduction of olefins and dynamic kinetic resolution of primary
amines. Furthermore, Pd/AlO(OH) NPs were utilized for the ef-
fective reduction of nitro compounds into primary amines by our
2
3
18a
*
Corresponding author. Fax: þ90 380 544 2812; e-mail address: haydargoksu@
group for the first time. This nanocatalyst is preferable because it
duzce.edu.tr (H. Goksu).
http://dx.doi.org/10.1016/j.tet.2016.08.027
0
040-4020/Ó 2016 Elsevier Ltd. All rights reserved.

B.Y. Kara et al. / Tetrahedron 72 (2016) 5898e5902
5899
is more accessible, recoverable, and reusable in addition to being
Table 1
Determination of reaction conditions for dehalogenation of 2-bromopyridinea
more stable in different environmental conditions. Thus, it can be
employed for the dehalogenation of aryl halides.
Herein, we report that various aryl halides underwent reaction
with NaBH
4
in the presence of aluminium oxy-hydroxide-
incorporated palladium nanoparticles. The dehalogenation re-
actions were performed with full conversion in short reaction times
in a low volume of water/methanol mixture under ultrasonic
conditions at room temperature.
Entry Solvent
Catalyst (mg) NaBH
4
(mmol) Time (h) Yieldb (%)
1
2
IPA
30
30
30
30
1.25
1.25
0.75
0.75
0.75
1.25
0.75
1.25
0.75
0.75
1
2
2
4
4
2
2
2
2
3
2
3
3
1.5
5
4
50
70
95
90
60
80
50
70
60
70
95
90
>95
2
IPA/H O (1:1)
3
2
H O
2
. Results and discussion
4
MeOH
5
6
7
8
9
10
H
2
H
2
H
2
H
2
H
2
H
2
H
2
H
2
H
2
H
2
O/MeOH (2:3) 20
O/MeOH (2:3) 20
O/MeOH (1:4) 20
O/MeOH (1:4) 20
O/MeOH (1:2) 20
O/MeOH (1:1) 20
O/MeOH (1:1) 20
O/MeOH (1:1) 30
O/MeOH (1:1) 40
The morphologies of Pd/AlO(OH) NPs both before the reaction
and after reusing five times were examined by SEM, XRD and TEM.
The SEM images show that the catalyst has a nanocrystalline
Boehmite structure. Under high magnification, the surface mor-
phology of a nanocluster can be clearly observed in Fig. 1a and b.
Both images have an irregular spread. The agglomeration of the
catalyst surface can be detected after the catalyst was reused five
times, as seen in Fig. 1b. The elemental composition of the catalyst
consists of carbon, aluminium and palladium, as seen in EDX
spectrums in the ESI (Figs. S1 and S2). It is likely that the abundance
of carbon is from the air. An XRD image of the Pd/AlO(OH) NPs is
also presented in the ESI (y Fig. S3). Diffraction peaks indicated that
the face-centred cubic (fcc) structure of the palladium is well
established on the surface of the AlO(OH), as was mentioned in
11
12
13
14
0.75
0.75
0.75
0.75
O/MeOH (1:1)
d
No reaction
>95
c
15
2
H O/MeOH (1:1) 40
a
Reaction conditions: 2-bromopyridine (0.25 mmol), Pd/AlO(OH) NPs and room
temperature.
b
c
Calculated by GC yield.
Without ultrasonic conditions.
formate salts as a hydrogen source.2a Within 4 h, 4-bromotoluene
(1 mmol) was converted to toluene with a 94% yield in the pres-
ence of an excess amount of Zn/NH
2
4
a previously published work. Transmission electron microscopic
images revealed that PdAlO(OH) NPs had an average size of about
13
4
Cl in THF. In the literature,
21
other commercially available catalysts such as Pd/C, Pd(OAc) ,
2
3
nm (y Fig. S5).
Fig. 1. SEM images: (a) Pd/AlO(OH) NPs before the reaction; (b) Pd/AlO(OH) NPs after reusing five times.
In the dehalogenation of aryl halides, the effects of PdAlO(OH)
NPs as catalysts, NaBH as a hydrogen source and the use of dif-
ferent solvents were investigated. For optimization of the reaction
conditions, 2-bromopyridine was used as a substrate. The reaction
conditions were optimized with 0.25 mmol of substrate, 40 mg of
PdCl
2 2
, Pd(acac) have also been tested in dehalogenation reactions.
4
However, their catalytic efficiency in water was observed to be
poor.
2
a,12
In addition, they are homogeneous, except for Pd/C, and
they cannot be efficiently reused. On the contrary, Pd/AlO(OH) NPs
work quite well in water and can be reused at least five times, as
seen at Tables 2 and 3.
Table 2 shows that all the aryl and heteroaryl halides were
successfully reduced by the PdAlO(OH)/NaBH Most of the
2
2
PdAlO(OH) NPs, 0.75 mmol of NaBH
der mild conditions (Table 1, entry 13). The dehalogenation reaction
did not occur without a catalyst (Table 1, entry 14).
4
in H
2
O/MeOH (v/v¼1/1) un-
4
.
Ultrasonic agitation (100 W, 50 Hz) was then applied to accel-
erate the developing process. The dehalogenation of 2-
bromopyridine was accomplished within nearly 4 h at room tem-
perature (Table 1, entry 15), while the reaction was completed in
halogen-free compounds were quantitatively obtained within
1.5e4 h using ultrasonic agitation. Chloro-, bromo- and iodo-
benzenes (1, 3, 4) were all reduced to benzene with high yields
(Table 2, entries 1e3). It is recognized that the ease of dehaloge-
nation conveniently follows a fallen of I>Br>Cl. As shown in Table
2, the CeCl bond is difficult to activate with naked Pd(0) at room
1.5 h when the ultrasonic bath was utilized.
The catalytic efficiency of PdAlO(OH) NPs was relatively high
compared to previously published works dealing with dehaloge-
nation of aryl halides (see ESI,y Table S1). The use of a correspond-
ing catalyst has priority in terms of the product yield, product
diversity, reaction time and temperature. For instance, the syn-
thesis of benzene from bromobenzene was achieved with only 30%
2
5,26
temperature, but it is not impossible.
This effect is due to the
support material (AlO(OH)). Consequently, the iodobenzene was
reduced in less time (Table 1, entry 3). Aryl halides containing al-
dehyde groups were reduced to halogen-free alcohols by using
excess amounts of the reducing agent (Table 2, entries 4e6). In 3 h,
4-iodo toluene (9), 4-iodo anisole (11) and (6-bromopyridin-3-yl)
ꢀ
conversion within 2 h at 90 C in the presence of PdeC, using

5900
B.Y. Kara et al. / Tetrahedron 72 (2016) 5898e5902
Table 2
PdAlO(OH) NP effects in the dehalogenation of various aryl halides
a
Entry
1
Substrate
Product
Time, h
4
Yieldc
%
Entry
10
Substrate
Product
Time, h
1.5
Yieldc
%
>95
>95
2
15
14
1
2
3
>95
11b
3
>95
2
2
3
1
6
17
19
3
2.5
>95
>95
12
13
3
2
>95
>95
4
5
18
20
4b
4
6
6
2
1
5b
3.5
>95
14
1.5
>95
7
23
25
2
2
6b
3
3
>95
>95
15
16
1.5
1.5
>95
>95
2
4
6
8
9
7
10
12
26
27
27
8
9
3
>95
>95
17
18
1.5
2.5
>95
>95
2
8
1
1
1.5
1
3
14
3
0
2
9
a
Unless otherwise stated, 0.25 mmol of substrate, 0.75 mmol of NaBH
4
and 0.16 mmol% of Pd/AlO(OH) NPs (0.5 wt % Pd) were used with 1 mL of H
2
O/MeOH (v/v¼1/1) under
ultrasonic conditions.
b
1.25 mmol of NaBH
4
was used.
c
Calculated by GC yield.
methanol (16) were easily converted to the corresponding halogen-
free compounds (10, 11, 17) with high yields (Table 2, entries 7, 8,
easily reduced with high yields within 1.5 h at room temperature
(Table 2, entries 9, 10, 14e17). Within h, 4-amino-2-
3
1
1). The electronic effect of heteroatoms such as nitrogen and sulfur
bromopyridine (18) was converted to 4-aminopyridine (19) with
the yields being above 95% due to the electron donating amine
group (Table 2, entry 12). The dehalogenation process of 1-(5-
enabled the dehalogenation of heteroaromatic halides to be com-
pleted more quickly. Thus, the corresponding aryl halides were

B.Y. Kara et al. / Tetrahedron 72 (2016) 5898e5902
5901
Table 3
Hydrogenehalogen substitution is then observed, followed by re-
a
Reusability test of Pd/AlO(OH) NPs
ductive elimination. Thus, palladium (II) is reduced to palladium
2
5,26
Entry
Substrate
Product
1st run
5th run
(0).
Yieldb (%)
Time, h
Yieldb (%)
Time, h
3. Conclusions
1
2
>95
>95
1.5
4
90
87
1.5
4
In summary, the current catalytic process showed that highly
effective palladium nanoparticles on the surface of aluminium oxy-
hydroxide provided dehalogenation of various aryl halides via so-
dium borohydride hydrolysis within 1.5e4 h under mild conditions
with a high conversion rate. Moreover, the Pd/AlO(OH) NPs
remained stable, even when reused more than five times. This
novel process has many advantages: (i) a safe hydrogen supply; (ii)
an eco-friendly solvent system and (iii) potential use in large-scale
application in industry.
a
Unless otherwise stated, substrate (0.25 mmol), NaBH
4
(0.75 mmol) and Pd/
O/MeOH (v/v¼1/
AlO(OH) NPs (0.16 mmol%, 0.5 wt. % Pd) were used with 1 mL of H
2
1
) at room temperature.
b
Calculated by GC yield.
4. Experimental
bromothiophen-2-yl)ethanone (29) is slower (2.5 h) than for the
other halogenated heteroaromatics due to the competitive re-
duction process of both the carbonyl and the halogen groups (Table
4.1. Materials
The halogenated aromatic compounds and Pd/AlO(OH) NPs
2
, entry 18).
used in the study were obtained from SigmaeAldrich. The NaBH
4
The Pd/AlO(OH) NPs exhibited high efficiency in the dehaloge-
nation of aryl halides. Furthermore, when the catalyst was reused
five times, there was not a great loss in the yield of the reaction
was also purchased from SigmaeAldrich and used as received. Ul-
trasonic agitation was carried out using a Laborgerate GmbH 100 W
(50 Hz) unit.
(Table 3). The ICP-MS analyses determined the amount of palla-
dium (w2.0 ppm) leached into the reaction solution after reuse
with very low. ICP analysis was made after purification of products
in Table 3. It was found that the amount of palladium in products is
in an accepted range (w0.03 ppm).
The reaction mechanism of the dehalogenation process has not
been entirely explained in previous works. Because there is not
a radical initializer such as heat and light in reaction medium, the
mechanism is thought to proceed via concerted pathway, not
radical-based. A proposed mechanism is depicted in Scheme 1.
Scheme 1 shows the activation, probably by adsorption, of AreX as
the first step. We think that palladium (0) is inserted between the
halogen atoms with the aryl group and it oxidizes to palladium (II).
4.2. Characterization methods
Scanning electron microscopy (SEM) analysis was performed via
a JEOL SEM5800 device equipped with an EDX probe having an
accelerator voltage of 20 keV. The X-ray diffraction (XRD) was
conducted using a Panalytical Empyrean diffractometer with
Ultimaþtheta-theta high resolution goniometer and an X-ray
¼1.54056 ꢀA ) operating at 45 kV and
generator (Cu K
a
radiation,
l
40 mA. TEM images of Pd/AlO(OH) NPs have been obtained by
a JEOL 200 kV TEM instrument. The amounts of the metals in the
monodisperse Pd/AlO(OH) NPs were determined by a Leeman Lab
1
inductively coupled plasma (ICP) spectrometer; the H NMR spec-
tra were recorded on a Jeol ECS 400 MHz spectrometer.
4.3. General procedure for the dehalogenation reactions
First, Pd/AlO(OH) NP catalyst (40.0 mg, 0.16 mmol%) and 1 mL of
H
2
O/MeOH (v/v¼1/1) were added to a Schlenk tube. Next, the ha-
logenated compound (0.25 mmol) was added. Finally, NaBH
0.75 mmol) was added to the reaction mixture and the vessel was
4
(
closed. The reaction then continued during vigorous stirring under
ultrasonic conditions at room temperature and was monitored by
GC. Most of the reactions were completed over a time period of
1.5e4 h. After completion of the reaction, the catalyst was removed
via simple centrifugation at 6000 rpm and then washed three times
with methanol and water and allowed to dry for further use. The
solvent was evaporated under vacuum. The products were purified
by flash column chromatography and the dehalogenated products
1
were then determined by H NMR.
Acknowledgements
This research was supported by Duzce University Research Fund
(grant nos. 2014.05.03.243 and 2015.26.04.371).
Supplementary data
Scheme 1. Proposed mechanism for dehalogenation reaction.
Supplementary data associated with this article can be found in
the online version, at http://dx.doi.org/10.1016/j.tet.2016.08.027.

5902
B.Y. Kara et al. / Tetrahedron 72 (2016) 5898e5902
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