Solid supported platinum(0) nanoparticles catalyzed chemo-selective reduction of nitroarenes to N-arylhydroxylamines

Article abstract of DOI:10.1039/c3gc41179f

Solid supported platinum(0) (SS-Pt) nanoparticles were developed as a heterogeneous catalyst following a reduction/deposition method and characterized by SEM, TEM, EDX and XRD analysis. The SS-Pt catalyst was applied in the chemo-selective reduction of nitroarenes to N-arylhydroxylamines using hydrazine hydrate as a hydrogen source. A wide variety of reducible functional groups such as halides, carboxylic acids, esters, amides, nitriles, keto, alkenes, alkynes and N-benzyl were well tolerated under the reaction conditions. This process was further successfully employed in 10 g scale reactions. N-Arylhydroxylamines were further applied for catalyst free synthesis of azoxybenzenes. Moreover, use of PEG-400 as cheap reaction medium, additive free methodology and the recyclability of SS-Pt catalyst up to ten times without significant loss of catalytic activity evidently follow the principles of green chemistry.

Full text of DOI:10.1039/c3gc41179f

Green Chemistry
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Solid supported platinum(0) nanoparticles catalyzed
chemo-selective reduction of nitroarenes to
N-arylhydroxylamines†
Cite this: Green Chem., 2013, 15, 3421
Arun K. Shila,b and Pralay Das*a,b
Solid supported platinum(0) (SS-Pt) nanoparticles were developed as a heterogeneous catalyst following
a reduction/deposition method and characterized by SEM, TEM, EDX and XRD analysis. The SS-Pt catalyst
was applied in the chemo-selective reduction of nitroarenes to N-arylhydroxylamines using hydrazine
hydrate as a hydrogen source. A wide variety of reducible functional groups such as halides, carboxylic
acids, esters, amides, nitriles, keto, alkenes, alkynes and N-benzyl were well tolerated under the reaction
conditions. This process was further successfully employed in 10 g scale reactions. N-Arylhydroxylamines
were further applied for catalyst free synthesis of azoxybenzenes. Moreover, use of PEG-400 as cheap
reaction medium, additive free methodology and the recyclability of SS-Pt catalyst up to ten times
without significant loss of catalytic activity evidently follow the principles of green chemistry.
Received 18th June 2013,
Accepted 12th September 2013
DOI: 10.1039/c3gc41179f
www.rsc.org/greenchem
Introduction
The controlled reduction of nitroarenes is one of the impor-
tant strategies for the synthesis of industrially valuable N-aryl-
hydroxylamines which are potential precursors as well as
intermediates for polymerization inhibitors,1 reagents2 and
bioactive pharmaceuticals.3 The conventional procedures for
the synthesis of N-arylhydroxylamines include a coupling reac-
tion of aryl halides with hydroxylamine,4 reduction of nitro
compounds using stoichiometric reducing agents such as Zn
or Sn,5 Rh-complex catalyst/hydrazine6 and reduction of alde-
hyde oxime with boronhydrides.7 Usually, reduction of nitroar-
enes is a consecutive process where a number of intermediates
are involved for the formation of anilines as the final products
(Scheme 1). In our previous report, we described solid sup-
ported palladium (SS-Pd) catalyzed reduction of nitroarenes to
anilines as the end product.8 The formation of each intermedi-
ate and their stability in the reaction media generally depend
upon the kinetics of the step involved, hydrogen source con-
centration and also the activity of catalyst. In most of the
cases, reduction of nitro functionality occurs via a nitroso and
hydroxylamine pathway in which it is very difficult to stop the
reaction at hydroxylamine. Moreover, the condensation of
Scheme 1 Possible pathway for the reduction of nitroarenes.
nitrosoarenes and N-arylhydroxylamines or the disproportiona-
tion reaction of N-arylhydroxylamines during the nitro
reduction process greatly hinders the possibility to obtain
N-arylhydroxylamines in high yields.9 So there is a limited
scope for the selective formation of N-hydroxylamines through
hydrogenation of nitroarenes.
From the seminal work of Rylander, in later publications
authors have tried the selective hydrogenation of nitroarenes
to N-arylhydroxylamines by employing different additives. The
deactivation of the catalyst is necessary to stop the reduction
after absorption of two equivalents of hydrogen, and addition
of DMSO was found to greatly reduce the catalytic activity of
Pt/C.10 A high pressure hydrogenation reaction of nitrobenzene
in the presence of Pt and Pd on charcoal in DMSO has also
been reported, but simultaneous formation of aniline was the
main disadvantage.11 Yasumasa et al. have reported commer-
cially available Pt–SiO2 (5 wt%) for selective reduction of
nitroarenes to N-arylhydroxylamines under H2 gas (1 bar)
using DMSO-triethylamine as additives.12 In most of the pre-
vious reports the authors described a complicated hydrogen-
ation process, use of costly metal–ligand combination and
aNatural Plant Products Division, CSIR-Institute of Himalayan Bioresource
Technology, Palampur-176061, H. P., India. E-mail: pdas@ihbt.res.in,
pdas_nbu@yahoo.com; Fax: +91-1894-230433
bAcademy of Scientific and Innovative Research, CSIR-IHBT, Palampur-176061,
H. P., India
†Electronic supplementary information (ESI) available: Further experimental
details and spectral data. See DOI: 10.1039/c3gc41179f
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additives to furnish the controlled reduction of nitroarenes to
N-arylhydroxylamines. The application of additives often
creates problems in product isolation and regeneration of the
catalyst. Therefore, the development of an additive free envir-
onmentally clean practical method for the chemo-selective
reduction of nitroarenes to N-arylhydroxylamines is of high
demand in academia as well as industry.
In recent years, our group has been dedicated to developing
solid supported transition metal nano/microparticles as het-
erogeneous catalysts and their applications in several areas of
organic transformations.8,13 Herein, for the first time we
report the development of highly efficient SS-Pt nanoparticles
as a heterogeneous catalyst and their application in additive
free chemo-selective reduction of nitroarenes to N-arylhydroxy-
lamines under milder reaction conditions using hydrazine
hydrate as a reducing source.
Fig. 2 Scanning electron micrograph (SEM) image of fresh SS-Pt.
XRD pattern showed the presence of Pt(0) nanoparticles into
the polymer matrix as compared with literature (Fig. 4).15 The
well characterized SS-Pt catalyst was further applied for chemo-
selective reduction of nitroarenes to N-arylhydroxylamines
synthesis.
Initially, in order to optimize the reaction conditions we
took nitrobenzene as a model substrate and screened for cata-
lyst loading, different solvents and reducing sources (Table 1).
Among these, SS-Pt (1 mol% Pt) and N2H4·H2O (3 eq.) in
PEG-400 at 60 °C for 70 min was found to be the optimum
conditions to give N-phenylhydroxylamine 1 in 92% yield
(Table 1, entry 4).
Results and discussion
Transition metal nanoparticles supported on a heterogeneous
surface got enormous interest in recent topics of organic trans-
formations due to their multifold advantages. Generally tran-
sition metal nanoparticles are prepared by reduction of metal
salts in the presence of a stabilizer which inhibits the metal
aggregation during reduction but sometimes with moderate
catalytic activity.14 The SS-Pt was prepared using Amberlite IRA
900 Cl− form resin as a heterogeneous phase. Initially Amber-
lite IRA 900 Cl− form resin (1 g) was partially exchanged with
−
The optimized reaction conditions were further explored for
different nitroarenes to see the versatile application and
chemo-selectivity of the SS-Pt catalyst for N-hydroxylamines
synthesis. Reduction of 4-nitrotoluene under classic conditions
gave N-hydroxy-4-methylbenzenamine 2 in 96% yield (Table 2,
entry 1). 4-Bromo-, 4-chloro-, 3-chloro-, 2-chloro- and 4-iodo-
nitrobenzene were selectively reduced to corresponding hydro-
xylamines 3–7 in good to excellent yields and no dehaloge-
nated product was observed under these conditions (Table 2,
entries 2–6). Nitroarenes containing different reducible func-
tional groups such as nitrile, carboxylic acid ester, carboxylic
acid, amides (both the primary and secondary) were chemo-
selectively reduced to their corresponding hydroxylamines
8–14 in good yields (Table 2, entries 7–13). The chemo-selec-
tive reduction of 4-nitroacetophenone was also achieved in a
good yield leaving the –COCH3 functional group intact which
otherwise is very prone to hydrazone formation or reduction
reactions (Table 2, entry 14). The electron withdrawing group
(–CF3) attached 3-nitrobenzotrifluoride was also reduced
smoothly to give 3-trifluoromethyl phenylhydroxylamine 16 in
93% yield (Table 2, entry 15). o-Hydroxymethyl and o-amino-
methyl nitroarenes were also found to be tolerated under the
same reaction conditions and gave 17 and 18 in excellent
yields (Table 2, entries 16 and 17). The amino substituted
nitroarenes were efficiently reduced to afford 19 and 20 in
excellent yields regardless of +R effect of amino group
(Table 2, entries 18 and 19). Nitro biphenyls and heterocycles
BH4 ion by treatment with NaBH4 (25 mg in 7.5 ml H2O i.e.
0.09 (M)) in water at room temperature (Fig. 1). Further, dried
borohydride exchanged resin was treated with PtCl2 (10 mg) in
DMF at 100 °C for 1 h or until the brown colour of the solution
changes to colorless with a simultaneously white solid surface
of the bead changing to brownish (which is an indication of
the reduction of Pt(II) to Pt(0) and deposition). Scanning elec-
tron micrograph (SEM) analysis showed the deposition and
distribution of Pt(0) nanoparticles over the solid matrix
(Fig. 2). Transmission electron micrograph (TEM) and EDX of
SS-Pt catalyst was also investigated to analyze the Pt nanoparti-
cles’ distribution (highest average size in between 1.5–2 nm,
Fig. 3C) and their formation into the polymer matrix (Fig. 3A
and B). Infrared spectral analysis of SS-Pt showed slightly
greater absorption at near 1000 cm−1 than that of the corres-
ponding resin matrix (ESI†) which might be due to the mere
impregnation of Pt into the solid support. Furthermore, the
Fig. 1 Preparation of solid supported platinum (SS-Pt) catalyst; (A) partially
BH4− ion exchanged Amberlite resin, (B) SS-Pt catalyst.
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Fig. 3 (A) Transmission electron micrograph image of SS-Pt, (B) EDX of SS-Pt, (C) particle size distribution.
also participated in the same reaction to give 21–23 in high ESI†) and produced 1, 2, 4, 8 and 9 in considerably high yields
yields (Table 2, entries 20–22). Interesting and most challen- (Scheme 2).
ging alkene and alkyne unsaturated nitroarenes participated in
In all the above mentioned reactions, interestingly no effer-
the same reaction to obtain the desired alkenyl and alkynyl vescence was observed prior to the addition of the required
N-arylhydroxylamines 24–26 in 78–95% yields (Table 2, entries amount of nitroarenes in the reaction mixture containing
23–25). The N-benzyl functionality is susceptible to the cataly- SS-Pt, N2H4·H2O and PEG-400, which indicated that the
tic hydrogenation reaction, but pleasingly, in the case of H-bonding between the –NO2 group of the substrate and N2H4
N-benzyl-4-nitroaniline we got 4-(benzylamino)-N-hydroxybenzen- might be playing a crucial role to weaken the H–N bond for
amine 27 in 87% yield under the same conditions (Table 2, easy decomposition to 2H2 and N2 gases on the SS-Pt catalyst
entry 26). The complete reduction of 4-nitrotoluene to 4-tolu- surface (Scheme 3). Yet now, in all the reported methods gen-
idine 28 was also observed using SS-Pt (2 mol%) and N2H4·H2O erally the reduction of nitrobenzene to N-phenylhydroxylamine
(7 eq.) in PEG-400 at 100 °C with satisfactory yield (Table 2, followed the usual pathway of nitrosobenzene intermediate
entry 27).
formation (Scheme 1) but with some exceptions where the
The up-scaling of a synthetic methodology has huge indus- reaction goes without the intermediacy of nitrosobenzene.16
trial demand for process development. In this regard a few To prove the mechanism of the present reaction when nitroso-
reported reactions (Table 1, entry 4 and Table 2, entries 1, 3, 7 benzene was treated under the standard reaction conditions
and 8) were reinvestigated for 10 g scale reactions (procedure: for N-arylhydroxylamine synthesis, several minor by-products
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Table 2 SS-Pt catalyzed chemo-selective reduction of nitroarenes to N-aryl-
hydroxylamines
Entry
1
Product
Time [min]
50
% Yielda
96
2
3
4
60
55
40
98
95
87
5
6
7
8
45
65
40
80
91
90
99
92
Fig. 4 XRD diffractogram of SS-Pt.
Table 1 Optimization of the reaction conditions for reduction of nitrobenzene
to N-phenylhydroxylaminea
Catalyst
Reducing
Time
Entry [mol% Pt] agent [eq.]
Solvent
[min] % Yielda
9
60
70
50
86
87
92
1
2
PtCl2 [1]
N2H4·H2O [2.5] PEG-400 90
50
75
SS-Pt [0.5] N2H4·H2O [2.5] PEG-400 80
10
11
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
SS-Pt [1]
SS-Pt [1]
SS-Pt [2]
SS-Pt [1]
—
SS-Pt [1]
SS-Pt [1]
SS-Pt [1]
SS-Pt [1]
SS-Pt [1]
SS-Pt [1]
SS-Pt [1]
SS-Pt [1]
SS-Pt [1]
SS-Pt [1]
N2H4·H2O [2.5] PEG-400 70
81
92
92
90
N2H4·H2O [3]
N2H4·H2O [3]
PEG-400 70
PEG-400 70
N2H4·H2O [3.5] PEG-400 70
N2H4·H2O [3]
N2H4·H2O [3]
N2H4·H2O [3]
N2H4·H2O [3]
N2H4·H2O [3]
N2H4·H2O [3]
N2H4·H2O [3]
NaBH4 [3]
PEG-400 70
No reaction
76
78
82
85
87
40
Trace
Trace
Trace
Trace
EtOH
CH3CN
PhCH3
70
70
70
Dioxane 70
12
13
14
15
60
50
35
60
90
98
78
93
DMF
H2O
70
90
PEG-400 70
PEG-400 70
PEG-400 70
PEG-400 60
HCOONH4 [3]
HCOOH [3]
Et3SiH [4]
a All are isolated yields.
with two major compounds hydrazobenzene (25% yield), and
aniline (15% yield) were detected. As per our previous report,8
this is quite obvious where nitrosobenzene and in situ con-
verted N-phenylhydroxylamine under these conditions readily
condenses to azoxybenzene and is further reduced to hydrazo-
benzene and aniline. Therefore starting from nitrosobenzene
16
70
99
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Table 2 (Contd.)
Entry
17
Product
Time [min]
60
% Yielda
83
Scheme 2 SS-Pt catalyzed gram scale synthesis of N-arylhydroxylamines.
18
19
75
60
92
89
20
21
22
40
55
50
97
94
91
Scheme 3 Hydrogen bonding driven decomposition of hydrazine on SS-Pt
surface. (A) Hydrazine hydrate, SS-Pt and PEG-400 mixture; (B) effervescence
started after addition of nitroarenes in hydrazine hydrate, SS-Pt and
PEG-400 mixture at room temperature.
23
40
87
24
25
90
30
78
95
Scheme 4 Possible mechanistic pathway for the reduction of nitrosobenzene
to N-phenylhydroxylamine.
26
27
60
88
120
75b
intermediate which further decomposes to N-aryl-
hydroxylamines.
The recyclability experiment of the SS-Pt catalyst was per-
formed using 4-nitrotoluene as a test substrate and the catalyst
was recycled up to ten consecutive reactions without signifi-
cant loss of catalytic activity (96–91% yields of 2) (Fig. 5). The
SEM analysis after the 10th run showed the presence of Pt(0)
a All are isolated yields. b Isolated yield; reaction conditions: N2H4·H2O
(7 eq.), SS-Pt (2 mol% Pt), temp. 100 °C.
it is very difficult to stop the reaction at N-phenyl- nanoparticles on the solid support (Fig. 6) and ICP-MS analy-
hydroxylamine.
sis (ESI†) indicated the negligible leaching of platinum metal
This indicated that, probably the reduction of nitrobenzene during the reaction.
occurs through a transfer hydrogenation process and coordi-
Interestingly, under the same PEG-400 mediated catalyst
nation of –NO2 group with Pt+2 (in situ converted from Pt(0)) free condition at comparably higher temperature (100 °C) the
and it plays a crucial role in partial hydrogenation to –NHOH N-arylhydroxylamines derivatives further participated in an oxi-
synthesis (Scheme 4). The chemisorbed hydride transfer to dative condensation reaction to produce azoxyaryls 29 and 30
nitroarenes may produce a coordinated hydroxyammonium in good yields (Scheme 5).
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N-arylhydroxylamines derivatives with comparatively lower
catalyst loading which showed the robust-efficiency of the
process and can fulfill the industrial demands. The syn-
thesized arylhydroxylamines were further transformed into
azoxyarenes under catalyst free reaction conditions. Moreover,
the air/moisture stable SS-Pt catalyst was recycled upto ten
runs for successive reactions without significant loss of cata-
lytic activity.
Experimental
General information
Fig. 5 Recyclability experiment of SS-Pt using 4-nitrotoluene as
a test
Reagents of high quality were purchased from Sigma Aldrich.
Silica gel (230–400 mesh size) for column chromatography was
procured from Sd Fine-chem Ltd. Reagents and solvents were
of analytical grade and were purified by standard procedures
prior to use. The sample for SEM analysis was mounted on an
aluminium stub using double sided tape. The completely
dried sample was coated with gold by a sputter coating unit at
10 Pascal vacuum for 10 second (E1010 ion sputter Hitachi,
Japan). A TEM image was taken using a carbon coated copper
grid (Microscopy sciences) in a transmission electron micro-
scope JEOL 2100F. XRD analysis of the prepared powdered
sample was done by a Goniometer (Ultima3 theta–theta gonio,
under 45 kV/40 mA – X-ray, 2 theta/theta Scanning mode,
Fixed Monochromator). ICP-MS analysis was performed using
an iCAP6300DUO, Thermo Fisher instrument. Data were taken
for the 2Θ range of 5 to 80 degrees. Thin layer chromatography
was performed using precoated silica gel plates 60 F254 (Merck)
in a UV light detector. ESIMS spectra were determined using a
Waters Micromass Q-TOF Ultima Spectrometer. All the melting
points are uncorrected and were determined on a Barnstriethyl-
amined Electrothermal 9100 capillary melting point appar-
atus. The samples were prepared on an anhydrous KBr disc for
FT-IR analysis in a Thermo Nicolate-6700 infra red spectropho-
tometer and the λmax are expressed in cm−1. The electronic
spectra were recorded on a Perkin Elmer Lambda-35 UV/Vis
spectrophotometer and the λmax are expressed in nanometers.
1H NMR and 13C NMR spectra were recorded using a Bruker
Avance 300 spectrometer/Bruker Avance 600 spectrometer
operating at 300 MHz (1H) and 75 MHz (13C)/600 MHz (1H)
and 150 MHz (13C) respectively. Spectra were recorded at 25 °C
in CDCl3 and MeOD [residual CHCl3 (δH 7.26 ppm) or CDCl3
(δC 77.0 ppm), MeOD (δH 3.30, 4.78 ppm) or (δC 49.0 ppm) and
DMSO-d6 (δH 2.50) or (δC 39.5) as international standards] with
TMS as the internal standard. Chemical shifts were recorded
in δ (ppm) relative to the TMS, coupling constants (J) are given
in hertz (Hz) and multiplicities of signals are reported as
follows: s, singlet; d, doublet; t, triplet; m, multiplet; brs,
broad singlet.
substrate.
Fig. 6 Scanning electron micrograph image of SS-Pt after the 10th cycle.
Scheme 5 Transformation of N-arylhydroxylamines into azoxybenzenes.
Conclusions
In summary, we developed solid supported platinum(0) nano-
particles (SS-Pt) as an efficient heterogeneous catalyst, charac-
terized it (IR, SEM, TEM and XRD analysis) and applied it to
the chemo-selective reduction of nitroarenes to N-arylhydroxyl-
amines under milder reaction conditions. Hydrazine hydrate
was used as a milder and suitable reducing agent of choice
which on decomposition gives N2 and H2 and after reaction
produced H2O as a harmless by-product in the presence of
SS-Pt and PEG-400 mediated conditions. Reductively vulner-
able functional groups such as alkene, alkyne, keto etc. were
Experimental procedures
Preparation of solid supported platinum(0) catalyst. The
well tolerated under the reaction conditions. The same reac- solution of 100 mg of NaBH4 in 30 ml of water was added to
tion strategy was also employed for gram scale synthesis of 4 g of Amberlite IRA 900 resin (chloride form) (Across, BE) in a
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100 ml flask. The mixture was stirred for 4 h at room tempera- and EDX analysis. The authors thank CSIR, New Delhi for
ture. Then the resin was washed with water till the pH became financial support as part of XII Five Year plan programme
neutral and then with acetone to remove water from the solid under title ORIGIN (CSC-0108) and the Department of Science
surface. The resin beads (partially borohydride exchanged) & Technology (Nano Mission), New Delhi (grant no. SR/NM/
were dried under reduced pressure. The dried borohydride NS-95/2009). AKS (CSIR-SRF) thanks CSIR New Delhi for award-
exchanged resin beads (solid surface) (1 g) were added into the ing a fellowship. Note: IHBT communication no. 3414.
warm (100 °C) solution of platinum chloride (10 mg) in DMF
(3 ml) then the mixture was stirred for 1 h or till the dark
brown color of the solution changed to colorless and simul-
taneously white solid beads turned brownish. After cooling,
the beads were filtered through a cotton bed, washed with
Notes and references
water and acetone, and dried under reduced pressure.
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General experimental procedure for N-aryl hydroxylamine
synthesis. A mixture of nitroarene (1 mmol), SS-Pt (1 mol%
Pt) and PEG-400 (1.5 ml) were taken in a 25 ml round bot-
tomed flask. Hydrazine hydrate (2.5 mmol) was added to the
reaction mixture at room temperature under stirring con-
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addition of 2 ml of water and dried over anhydrous Na2SO4.
Evaporation of the combined organic layer followed by column
chromatography (hexane: ethyl acetate gradient) over silica gel
(mesh 60–120) afforded N-arylhydroxylamines.
Typical experimental procedure for non catalytic synthesis
of azoxybenzene from N-phenyl hydroxylamine. A mixture of
N-phenylhydroxylamine 1 (200 mg, 1.834 mmol) and 1.5 ml of
PEG-400 was taken in a 10 ml round bottom flask. The reac-
tion mixture was stirred magnetically at 100 °C for 2 hours.
Progress of the reaction was monitored by TLC. On com-
pletion, 3 ml of distilled water was added to the reaction
mixture and extracted with ethylacetate (3 × 3 ml) and dried
over anhydrous Na2SO4. Evaporation of the combined organic
layer and followed by column chromatography (hexane) over
silica gel (60–120 mesh) afforded azoxybenzene 29 as light
yellow crystalline solid (297.90 mg, 82%); mp 35–37 °C; 1H
NMR (300 MHz, MeOD) δ 7.34–7.51 (m, 6H), 8.09–8.13 (m,
2H), 8.21–8.24 (m, 2H); 13C NMR (75 MHz, MeOD) δ 123.18
(2C), 126.50 (2C), 129.68 (2C), 129.94 (2C), 130.70, 132.80,
145.31, 149.52. ESIMS data; m/z calc. for [M + H]+ C12H11N2O
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Acknowledgements
Authors are grateful to Director CSIR-IHBT for providing
necessary facilities during the course of the work. We also
thank Dr G. Saini and AIRF, JNU-New Delhi, India for the TEM
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