Palladium-modified functionalized cyclodextrin as an efficient and recyclable catalyst for reduction of nitroarenes

Article abstract of DOI:10.1039/c5ra23271f

A palladium-modified functionalized cyclodextrin (DACH-Pd-β-CD) catalytic system was synthesized and characterised. It showed high catalytic performance in the reduction of different nitroarenes to the corresponding anilines with the presence of sodium borohydride in water at room temperature. The yields of the desired products are up to 99%. Furthermore, the catalyst can be easily separated and still could maintain high catalytic activity after five cycles and no leaching of Pd into solution occurred.

Full text of DOI:10.1039/c5ra23271f

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Palladium-modified functionalized cyclodextrin as
an efficient and recyclable catalyst for reduction of
nitroarenes†
Cite this: RSC Adv., 2016, 6, 7950
Yafei Guo,ab Jiuling Li,b Fen Zhao,a Guineng Lan,a Liang Li,a Yuqi Liu,a Yunsen Si,a
Received 5th November 2015
Accepted 11th January 2016
a
b
a
*
Yubo Jiang, Bo Yang and Rui Yang
DOI: 10.1039/c5ra23271f
www.rsc.org/advances
A palladium-modified functionalized cyclodextrin (DACH-Pd-b-CD) Pd/C, usually have excellent activity for reduction of nitroarenes
catalytic system was synthesized and characterised. It showed high under the atmosphere of H2.7 In addition, the reducing agent of
catalytic performance in the reduction of different nitroarenes to the hydrazine, silanes and sodium borohydride play a signicant
corresponding anilines with the presence of sodium borohydride in role in the reduction reactions as hydrogen donors.8
water at room temperature. The yields of the desired products are up
Palladium is a great transition metal for reduction of nitro-
to 99%. Furthermore, the catalyst can be easily separated and still arenes, which has been successfully used in different catalytic
could maintain high catalytic activity after five cycles and no leaching forms, such as Pd–polymer, Pd–silicon hydride, Pd/C nanober
of Pd into solution occurred.
and so on.9 These Pd catalysts show a great catalytic perfor-
mance for chemoselective reduction reaction of nitroarenes. On
the other hand, they can be used repeatedly for many times and
still maintain excellent catalytic performance. Moreover, the Pd
catalysts are oen used together with NaBH4 in the reduction of
nitroarenes.10 NaBH4 has less ability and chemical selectivity
while reducing nitroarenes to corresponding amines in the
absence of metals and the reactions usually stay at intermediate
stage of azo, azoxy, and hydrazo.11 Hence, lots of work had been
done by our team to explore some water-soluble and recyclable
Pd complexes so as to achieve their catalytic activity with NaBH4
for reduction of nitroarenes in water.
Due to the special structure of lipophilic central cavity and
hydrophilic outer surface of cyclodextrin (CD), the water solu-
bility of CD is excellent which can provide a lipophilic micro-
reactor for organic molecules in water. In addition, it was
reported that cyclohexanediamine (DACH) as a perfect ligand
could successfully complex with several metals, such as Pt
and Pd.12
Aromatic amines are a signicant class of compounds which are
widely used as organic intermediates in the preparation of dye
polymers, pharmaceuticals, agrochemicals and natural prod-
ucts.1 The most common and conventional route for preparing
the aromatic amines is through the reduction of corresponding
nitroarenes using the method of catalytic hydrogenation.2 In the
progress of catalytic hydrogenation, mental catalysts are very
important. Common metal catalysts include noble metals (Pt,
Ru, Rh and Pd) and non-noble metals (Fe, Co, Ni, Cu and Zn).3 It
shows that there are many disadvantages to using them from
previous studies, such as high H2 pressure, long reaction time,
low yield, high temperature and serious environmental pollu-
tion.4 It is well known that the traditional catalyst for reduction
nitroarenes is Fe–HCl system. However, the catalytic system
exerts many disadvantages, such as poor yield, larger iron
dosage and Fe–FeO sludge.5
In recent years, the concept of “Green and Sustainable
Chemistry” has been accepted by more and more researchers of
modern organic synthesis.6 More efficient and recyclable cata-
lysts are needed for the reduction of nitroarenes. Many reports
revealed that noble metals nanoparticles, such as Pt/C, Rh and
In this paper, a palladium-modied functionalized cyclo-
dextrin (DACH-Pd-b-CD) was synthesized and characterized by
NMR, XRD, TEM and EDS. The catalytic activity and the recy-
clability for the reduction of nitroarenes in the presence of
NaBH4 in water were researched. Moreover, the leaching
experiment was carried out.
The TEM images of DACH-b-CD (Fig. 1A, 100 nm), DACH-Pd-
b-CD and its histogram for particle size distribution (Fig. 1B,
200 nm) as well as HR-TEM image of DACH-Pd-b-CD (Fig. 1C)
are shown in Fig. 1. The sample was onto a piece of copper grid
and dried at room temperature under vacuum. Comparing with
Fig. 1A and B, many black spots appeared to be observed in
aFaculty of Science, Kunming University of Science and Technology, Kunming, 650500,
P. R. China. E-mail: yangrui9111@163.com
bFaculty of Life Science and Technology, Kunming University of Science and
Technology, Kunming, 650500, P. R. China
† Electronic supplementary information (ESI) available: Experimental details,
characterizations, NMR, XRD, EDS, FT-IR, HR-MS and TG spectra, and
measurements. See DOI: 10.1039/c5ra23271f
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Fig. 1 TEM image: (A) DACH-b-CD at 100 nm (B) DACH-Pd-b-CD at 200 nm (C) HR-TEM image of DACH-Pd-b-CD at 5 nm.
Fig. 1B, but not in Fig. 1A. The result may indicate that the 1 h and 3 h (entries 14 and 15, 8). As a result, the reaction
formation of the catalyst. In order to count the size of particles, could complete during 3 h.
we enlarged the image (Fig. S11†) and the histogram inserted in
In order to further explored the effect of other reduction
Fig. 1B. The average diameter of the particles was 6 Æ 0.5 nm. In agents, H2 (1 atm, entry 16) and hydrazine hydrate (entry 17)
order to further demonstrate the presence of Pd, the HR-TEM was used. The results showed that the condition of H2 was
image of DACH-Pd-b-CD was presented (Fig. 1C). The average signicant which could lead to an excellent yield of 95%.
interplanar distance of DACH-Pd-b-CD was measured to be However, under the same conditions, the yield was only 8%
ꢀ0.22 nm, which corresponds well to the plane of Pd. Moreover, by using hydrazine hydrate. Then using the open vessel
the present of Pd was also demonstrated by means of EDS instead of sealed tube and the other conditions did not
(Fig. S4†) and XRD (Fig. S3†). The detailed synthetic steps and change, there was only 38% aniline obtained (entry 18). So it
other characterization results are in ESI.†
Owing to study the catalytic activity of DACH-Pd-b-CD,
different nitroarenes, DACH-Pd-b-CD and NaBH4 were Table 1 Optimization of the reaction conditionsa
employed in water in a sealed tube at room temperature
(Scheme 1). Preliminary investigation of the reduction of
nitrobenzene was carried out to optimise the reaction condi-
tions (Table 1). At rst, DACH-Pd-b-CD was absent and the
usage of NaBH4 ranged from 3 to 10 equiv. (entries 1–3).
However, the yields were very low from 5–12%. Without
DACH-Pd-b-CD
(mol%)
Entry
NaBH4 (eq.)
Time (h)
Yieldb (%)
NaBH4, no products were detected (entries 4–6). The result
indicated that NaBH4 had less ability and chemical selectivity
to reduce nitroarenes to the corresponding amines in the
absence of DACH-Pd-b-CD. Then, the consumption of NaBH4
was xed (3 mmol) and the amount of DACH-Pd-b-CD was
gradually increased from 0.1 to 5 mol% (entries 7–10). The
results showed that the yields of aniline are up to 99% when
DACH-Pd-b-CD is from 0.5 to 5 mol% and a dosage of more
than 0.5 mol% did not cause a distinct increase yields. In
addition, the usage of NaBH4 is also very crucial. From entries
10–12 and 8, the optimization is 3 equiv. Considering the
inuence of reaction time, the yields were studied aer 0.5 h,
1
2
3
4
5
6
7
8
—
—
—
1
3
3
3
12
3
3
3
5
3
3
3
3
3
3
3
0.5
1
8
6
6
5
8
10
—
—
—
3
3
3
3
1
12
—
—
—
52
99
99
99
63
99
98
82
93
95
8
5
10
0.1
0.5
1
9
10
11
12
13
14
15
16c
17d
18e
19f
5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
5
10
3
3
—
—
3
38
96
3
3
a
Reaction conditions: 1.0 mmol of nitrobenzene (0.123 g, 0.102 mL),
given amounts of DACH-Pd-b-CD and NaBH4 were stirred in a sealed
tube (2 mL H2O) at room temperature for the appropriate amount of
b
c
time. Isolated yields. Reaction carried out under H2 (1 atm) in the
d
e
absence of NaBH4. Hydrazine hydrate was used. The open vessel
conditions have been carried out. 10 mmol of nitrobenzene (1.23 g,
f
1.02 mL), 30 mmol NaBH4 (1.135 g) and 20 mL H2O.
Scheme 1 Reduction of nitroaromatic catalyzed by DACH-Pd-b-CD.
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Table 2 Reduction of various nitroarenes catalyzed by DACH-Pd-b-CDa
Entry
1
Reactant
Product
Yieldb (%)
99
Selectc (%)
99
2
3
99
99
99
99
4
5
6
7
8
99
99
96
98
95
99
99
98
96
97
9
99
96
96
98
10
11
12
96
93
93
95
13
14
15
16
95
94
96
98
96
98
96
96
17
93
97
a
Reaction conditions: 1.0 mmol nitroarene, 3 mmol of NaBH4 (0.1135 g), 0.5 mol% DACH-Pd-b-CD (11.9 mg, containing 4.5%w Pd), 2 mL water, 3 h
at room temperature. Isolated yields. c Selectivity were determined by GC.
b
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was necessary to complete the reduction reaction with the
sealed condition. To test the practical application of the
catalyst, a higher scale was very necessary. Then we enlarged
each quality of the reactants to ten times and the yield was up
to 96% (entry 19).
As a consequence, the optimum reaction conditions are
nitrobenzene (1 mmol), DACH-Pd-b-CD (0.5 mol%), NaBH4 (3
mmol), 3 h reaction time and H2O (2 mL) in the sealed tube at
room temperature.
A series of nitroarenes were used to further explore the
generality of catalytic performance of DACH-Pd-b-CD in Table 2.
To ensure the high yields and chemical selectivity of
substances, the reactions were performed in the optimum
conditions (Table 1, entry 8).
The general experimental procedure is that 1.0 mmol of
nitroarenes, 3 mmol of NaBH4 (0.1135 g) and 0.5 mol% DACH-
Pd-b-CD (11.9 mg, containing 4.5%w Pd) were added in a sealed
tube with 2.0 mL H2O. The mixture was stirred at room
temperature for 3 h and monitored by TLC. Aer the reaction
completion, the aqueous phase was extracted with ethyl acetate
for 3 times (3 Â 5 mL). Then the combined organic layers were
dried by anhydrous MgSO4 and the crude product was further
puried by column chromatography.
At rst, the inuence of the position of substituents on the
benzene ring had been explored. From entries 2–4, regardless of
the methyl at ortho-, meta- or para-positions of nitroarenes, the
catalytic system was found to be very efficient. The yields and
chemical selectivity of corresponding products are about to
99%. Then the inuence of electron-donating groups and
electron-withdrawing groups was studied (entries 4–10). The
consequence indicated that no matter the substrates were
bearing electron-donating groups or electron-withdrawing
groups, the yields were very high. While the other nitroarenes
with functional groups (entries 11–17), DACH-Pd-b-CD still
showed strong activity. All the results reveal that the catalytic
system DACH-Pd-b-CD/NaBH4 possesses strong catalytic activity
to reduce nitroarenes.
Besides reusability is also an important factor in heteroge-
neous catalytic systems. The recycling experiment was con-
ducted with the optimization reaction conditions in Table 1
(entry 8). Aer every cycle, 3 Â 5 mL ethyl acetate was added to
separate the organic phase and the aqueous phase was used in
the next run with adding 3 mmol NaBH4. The recycling results
are shown at Fig. 2. In the rst cycle, the yield was 99% and the
catalytic system DACH-Pd-b-CD/NaBH4 still behaved high
catalytic activity with a yield of 93% aer ve cycles. The
detailed progress of separation and reusing of catalyst was
mentioned in the ESI.† Furthermore, the content of Pd in the
product was determined by ICP-OES and no leaching of Pd was
detected.
Fig. 2 Recyclability of DACH-Pd-b-CD in the reduction of nitroben-
zene in H2O.
III (hydroxylamine) and then the aniline (route A) as well as the
formation of intermediates IV, V and VI (route B) (Fig. 3).10 The
major route (route A) through intermediates II and III is the
preferred pathway for the reduction of nitroarenes promoted by
Pd-based catalysts.13 Another route (route B) is the unwished
pathway which usually generates some azobenzene by-products
which directly reduce the chemical selectivity of catalysts.
During whole progress of the reduction of nitrobenzene by
DACH-Pd-b-CD/NaBH4 system, arylhydroxylamine (III) was the
major intermediate (>99%) detected by TLC and the diazo-
intermediate (V) was only in trace amounts. The concentra-
tion of intermediate III gradually decreased with time to yield
the pure aniline and there were no by-products in the nal
product. Most importantly, the reaction probably happened in
the cavity of DACH-Pd-b-CD in neat water. In order to prove the
present of arylhydroxylamine (III), we tried best to isolate the
pure intermediate III. However, the intermediate was close to
aniline (product) on the TLC and it was difficult to be separated.
On the other hand, the arylhydroxylamine (III) was easily to turn
to aniline during the progress of isolation. Hence, the mixture
of the intermediate and aniline were researched by 1H NMR
(Fig. S12†). In spite of the peaks of aniline, the other peaks of
the intermediate were studied and it was true that the inter-
mediate was compound III. The result suggested that the
possible mechanistic pathway of nitroarene reduction is mainly
through route A in the cavity of DACH-Pd-b-CD.
During the reaction, NaBH4 was excess and there was
quantitative H2. Therefore, the state of palladium metal in
the reaction was easily reduced from Pd(II) to Pd(0). It is
possible that Pd(0) played an important role in the reduction
reaction.
It is reported that the possible mechanism for the reduction
of nitrobenzene may go through the formation of intermediate Fig. 3 Possible mechanistic pathways of nitroarene reduction.
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In conclusion, the palladium-modied functionalized cyclodex-
trin catalytic system DACH-Pd-b-CD can efficiently promoted the
reduction of nitroarenes in water at room temperature in the
presence of NaBH4. The yields of the desired products are
excellent. What's more, the catalyst can be easily separated from
the reaction media and still maintain high catalytic activity aer
ve cycles. Furthermore, the Pd did not leach out in the solution
during reaction. TEM analyses showed that the average diameter
of the particles was 6 Æ 0.5 nm. All the results indicate that
DACH-Pd-b-CD/NaBH4 is found to be an environmentally-
friendly, efficient and recyclable catalytic system for the reduc-
tion of nitroarenes.
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