Tungsten- and molybdenum-based coordination polymer-catalyzed N-oxidation of primary aromatic amines with aqueous hydrogen peroxide

Article abstract of DOI:10.1002/adsc.200700224

Recyclable tungsten- and molybdenum-based coordination polymers efficiently catalyzed the oxidation of primary aromatic amines to the corresponding nitroso derivatives with 30% aqueous hydrogen peroxide in high yields at room temperature.

Full text of DOI:10.1002/adsc.200700224

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DOI: 10.1002/adsc.200700224
Tungsten- and Molybdenum-Based Coordination Polymer-
Catalyzed N-Oxidation of Primary Aromatic Amines with
Aqueous Hydrogen Peroxide
Ankur Bordoloia and S. B. Halligudia,*
a
Inorganic Chemistry & Catalysis Division, National Chemical Laboratory, Pune – 411 008, India
Phone: (+91)-20-2590-2107; fax: (+91)-20-2590-2633; e-mail: sb.halligudi@ncl.res.in
Received: May 3, 2007; Published online: September 11, 2007
Supporting information for this article is available on the WWW under http://asc.wiley-vch.de/home/.
Abstract: Recyclable tungsten- and molybdenum-
based coordination polymers efficiently catalyzed
the oxidation of primary aromatic amines to the
corresponding nitroso derivatives with 30% aque-
ous hydrogen peroxide in high yields at room tem-
perature.
used in the vulcanization of rubber, stabilization of
halogenated materials and as antioxidants in lubricat-
ing oil.[9]
Few homogeneous metal-catalyzed methods report-
ed in the literature are able to yield aromatic nitroso
compounds.[10] These include the use of Caroꢀs acid, 3-
chloroperoxybenzoic acid (MCPBA), potassium per-
manganate, peracetic acid, peroxyformic acid, peroxy-
benzoic acid, oxaziridinium salts, oxygen difluoride,
nitrous acid, trifluoroperacetic acid and, more recent-
ly, molybdic peroxo complexes. Catalytic methods
using H2O2 or tert-butyl hydroperoxide as oxidant and
sodium tungstate, oxomolybdate complexes, phospho-
tungstate, phosphomolybdate and zirconate salts were
Keywords: coordination polymers; hydrogen perox-
ide; oxidation; primary aromatic amines; recyclable
catalyst
Oxidative biotransformation is amongst the most published later. Dimeric products due to condensa-
useful of all identified biologically mediated conver- tion reactions such as azo or azoxy derivatives as well
sions. Environmentally friendly technologies accepta- as over-oxidation to the nitro compounds are often
ble with efficient formation of the desired products observed as by-products or sometimes as main prod-
and with negligible waste and recyclability of the cata- ucts. Some methods for the direct oxidation of aro-
lyst systems are the major aspects which must be con- matic amines to nitro derivatives with peracids were
sidered when designing a new catalyst for selective described and, more recently dioxirane, tert-butyl hy-
oxidation reactions. One of the most attractive ap- droperoxide and H2O2 have been used as oxidants
proaches is the catalytic oxidation reaction by hetero- whereas metal peroxo complexes, FeCl3, Pb(AcO)4
A
geneous means.[1–3] Oxidation of amines is widespread and chromium silicalite have been employed as cata-
in the area of biological science, reflecting the diverse lysts.[11] Nitroso compounds can also be produced by
roles for amine compounds. Amine compounds play the direct reduction of nitro compounds with
important roles in neurotransmission,[4] cell growth Mn3O4.[12] But organic synthetic methods have not yet
and differentiation,[5] and neoplastic cell prolifera- provided an easy and mild path to obtain the aromat-
tion.[6,7] On the other hand, the oxidation of amines is ic nitroso compounds. Our earlier report shows that
a fundamentally important reaction for industrial ap- [(n-Bu3Sn)2MO4], (M=Mo and W) is able to transfer
plications, particularly for the synthesis of oxygenated one peroxidic oxygen atom to an olefin to yield epox-
derivatives such as hydroxylamine, nitroso, nitro, ides.[13] Hence, to the best of our knowledge we pres-
oxime, azo and azoxy compounds. Among these, the ent here for the first time a heterogeneous catalytic
preparations of nitro, oxime and azoxy compounds system based on tungsten and molybdenum coordina-
have assumed special importance as synthetically tion polymers [(n-Bu3Sn)2MO4] for the selective oxi-
useful intermediates. Nitroso derivatives hold a key dation of primary aromatic amines to the correspond-
position in the chemistry of heterocycles offering ing nitroso derivatives with 30% aqueous H2O2 as ox-
functional group manipulation and structural modifi- idant at ambient conditions such as room temperature
cation possibilities that are not accessible by any and under an air atmosphere (Scheme 1). This is a
other methods.[8] Aromatic nitroso compounds are simple and clean process where the desired com-
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2085

Ankur Bordoloi and S. B. Halligudi
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alytic activity of [(n-Bu3Sn)2WO4] and [(n-
Bu3Sn)2MoO4] catalysts in the oxidation of aniline
shows that both the catalysts have almost similar ac-
tivity (Table 1) However, Na2WO4 and Na2MoO4 cat-
alysts show approximately half the activity compared
to their corresponding polymers while Bu3SnCl does
not show any activity in the oxidation of aniline.
Scheme 1. Oxidation of aniline.
pounds are formed free from any contamination with Hence, we can conclude that the reaction was pro-
by-products. moted mainly by tungsten and molybdenum species,
The organometallic coordination polymers [(n- and that tin enhances the activity.
Bu3Sn)2MO4], with M=Mo and W were obtained in
To study the scope of the above catalysts for gener-
analytically pure form as colorless precipitates by the al applications, the oxidation of other amines were
addition of
a
saturated aqueous solution of studied with [(n-Bu3Sn)2WO4] catalysts (Table 2).
Na2MO4·2H2O to a saturated solution of n-Bu3SnCl Aniline was oxidized to the corresponding nitrosoben-
at room temperature. The products were recovered zene in high yield. Similarly, 4-methylaniline, 2,4-di-
by filtration, washed with water, and air-dried at methylaniline, 2,4,6-trimethylaniline, 4-ethylaniline, 2-
1008C for several hours.[14] The good crystallinity of ethyl-6-methylaniline, isopropylaniline, 4-tert-butylani-
[(n-Bu3Sn)2MoO4] and [(n-Bu3Sn)2WO4] could be line, 4-methoxyaniline, 1-napthylamine could be oxi-
seen by their powder XRD patterns. The powder dized to the respective nitroso compounds. Amines
XRD patterns of [(n-Bu3Sn)2MoO4], and [(n- containing electron-withdrawing ring substituents,
Bu3Sn)2WO4] showed characteristic peaks of MoO4 such as 2-bromoaniline, 4-bromoaniline, 2-chloroani-
and WO4 of the materials in the range of 2q, 22 and line, 4-chloroaniline, 2-nitro aniline, 4-nitroaniline are
28, which are similar to the data reported in the liter- less reactive. These results clearly reveal that the
ature. By fitting various peaks to Scherrer equation present method could be applied for the oxidation of
and taking into account the instrumental broadening, a variety of aromatic primary amines with 30% aque-
the average crystallite sizes were found to be 200 nm. ous H2O2 by heterogeneous means. The products
N2 sorption studies show that the surface area and were detected by gas chromatography-mass spectrom-
1
pore volume of the materials are quite low. FT-IR etry and quantified with FT-IR and H NMR after
and FT-Raman spectra of [(n-Bu3Sn)2MoO4], and [(n- work-up of the solutions and separation by flash chro-
Bu3Sn)2WO4] show all characteristic bands of organic matography. In order to obtain the highest yields of
and metal organic functionalities present in the coor- the nitroso derivatives, it is necessary to avoid the
dination polymers. 119Sn MAS NMR spectra of possible subsequent oxidation to the corresponding
[(Bu3Sn)2MoO4] show a tetrameric structure in which nitro derivatives, by frequent checking of the reaction
each Sn atom has a trigonal bipyramidal coordination profile, and by stopping the reaction immediately
geometry. Four main signals in 13C CP MAS NMR after the complete consumption of amine. The results
were observed for the four methyl groups, two peaks given in the Table 2 represent the conditions for ob-
each for the beta and gamma carbons, however, for taining the best yields. The nitroso derivatives ob-
the alpha carbon, the signal is clearly the sum of two tained in this way are monomeric and the FT-IR spec-
components. The two small broad signals at 24 and
17 ppm are probably related to the coupling between
Table 1. Oxidation of aniline with different catalysts.[a]
13C and 117Sn. The endothermic peaks around 2408C
and 2698C, respectively for [(n-Bu3Sn)2WO4] and [(n-
Bu3Sn)2MoO4] in the DTA curve indicate the decom-
position of the coordination polymers.The typical
average particle size estimated by SEM for both the
catalysts is around 500 nm.[17,18]
Entry Catalyst
Substrate Product
Yield
[%][b]
1
2
3
Na2WO4
Na2MoO4
Aniline
Aniline
Aniline
Nitrosobenzene 38
Nitrosobenzene 31
Nitrosobenzene 82
A
The catalysts [(n-Bu3Sn)2MO4], (M=Mo and W)
were then investigated for the oxidation of aniline in
the presence of 30% aqueous H2O2. We were pleased
to find that the oxidation occurred to afford nitroso-
benzene in quantitative yield when the reaction mix-
ture (4 mmol of aniline and 8 mmol of 30% aqueous
H2O2) was allowed to stir in the presence of 0.4 mmol
of catalyst, at room temperature and under atmos-
pheric air for 1 h in acetone. A blank experiment
showed that no oxidation occurred in the absence of
catalyst and the amine remained unchanged. The cat-
4
5
6
U
Aniline
Aniline
Nitrosobenzene 77
-
-
No reac-
tion
No reac-
tion
[a]
[b]
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Adv. Synth. Catal. 2007, 349, 2085 – 2088

N-Oxidation of Primary Aromatic Amines with Aqueous Hydrogen Peroxide
Table 2. Oxidation of primary amines using 30% H2O2 cata-
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In conclusion, the N-oxidation of primary aromatic
amines is described using tungsten- and molybdenum-
based coordination polymers (heterogeneous catalyst)
in the presence of 30% aqueous H2O2 at room tem-
perature and atmospheric air to give the correspond-
ing nitroso derivatives in high yield. It is a clean pro-
cess and the catalyst is recyclable without any loss of
activity.
lyzed by [(n-Bu3Sn)2WO4].[a]
Entry Substrates Product
Yield
[%][b]
1
2
3
Aniline
Nitrosobenzene
82
4-Methylaniline 4-Methylnitrosobenzene 86
2,4-Dimethylani- 2,4-Dimethylnitrosoben- 90
line
zene
4
2,4,6-Trimethyl- 2,4,6-Trimethylnitroso-
92
89
aniline
benzene
Experimental Section
5
6
4-Ethylaniline
4-Ethylnitrosobenzene
2-Ethyl-6-meth- 2-Ethyl-6-methylnitroso- 83
ylaniline benzene
Isopropylaniline Isopropylnitrosobenzene 81
Preparation of Catalysts
7
8
A
4-tert-Butylani-
line
4-tert-Butylnitrosoben-
zene
85
solved in a mixture of water (6 mL) and acetone (27 mL). A
saturated aqueous solution of Na2MoO4·2H2O (1.21 g,
5 mmol in 8 mL of H2O) was added dropwise with magnetic
stirring to n-Bu3SnCl solution. A white precipitate was
formed immediately and stirring was continued for 5 min.
The precipitate was filtered, washed thoroughly with water,
and dried in air at 1008C for 6 h to give (nBu3Sn)2MoO4;
yield: 90%.
9
4-Methoxyani-
line
2-Bromoaniline 2-Bromonitrosobenzene 70
2-Chloroaniline 2-Chloronitrosobenzene 74
4-Bromoaniline 4-Bromonitrosobenzene 63
4-Chloroaniline 4-Chloronitrosobenzene 68
2-Nitroaniline
4-Nitroaniline
1-Napthylamine 1-Napthylnitrosoben-
zene
4-Methoxynitrosoben-
zene
87
10
11
12
13
14
15
16
2-Nitronitrosoaniline
4-Nitronitrosoaniline
35
20
72
AHCTREUNG
Na2MoO4·2H2O. The general reaction for the preparation of
the organotin-oxometalate coordination polymer is as
shown below:
[a]
Reaction conditions: amine (4 mmol), catalyst: [(n-
Bu3Sn)2WO4] (0.4 mmol) and 30% aqueous H2O2
(8 mmol) were stirred in acetone (7 mL) at room temper-
ature; reaction time=1 h.
[b]
Yield [%]=actual yield/expected yield”100%.
Elemental analysis confirmed that the above materials are
as per the stoichiometric composition of the coordination
polymers [(n-Bu3Sn)2MoO4] and [(n-Bu3Sn)2WO4].
tra in KBr showed a weak band close to 1500 cmÀ1,
attributable to the stretching mode of the N=O bond
of the monomer. These assignments are in good
agreement with reported values.[15] The oxidation path
of this reaction could follow the steps from amine to
hydroxylamine and then to nitrosoamine. The oxygen-
ation of the nitrogen atom of the substrate is accom-
plished by the formation of an oxo-tungsten complex,
originating from nucleophilic attack of the amine
onto peroxidic oxygen or SET processes.[16] Early lit-
erature reports and a recent communication on this
subject support this hypothesis.[17]
Typical Procedure for Oxidation
Amine (4 mmol), catalyst (0.4 mmol) and 30% aqueous
H2O2 (8 mmol) were stirred in acetone (7 mL) at room tem-
perature for the appropriate time (Table 1). After the com-
pletion of the reaction, the catalyst was filtered and the fil-
trate was subjected to reduced pressure to remove the sol-
vent. Then 10 mL of CH2Cl2 were added and the mixture
was washed with 10 mL of distilled water and brine. The or-
ganic layer was separated and dried over anhydrous Na2SO4.
The catalyst could be filtered and recycled without The mixture was again filtered and concentrated under re-
duced pressure to afford a residue, which was purified by
flash chromatography over silica (n-hexane:CH2Cl2, 4:6).
The catalyst could be recycled without a loss of activity. All
a loss of activity. After completion of the reaction,
the catalyst was filtered, washed with acetone, dried
at 1008C and reused in the oxidation of aniline with
fresh 30% aquoeus H2O2 for four runs. No loss of ac-
tivity was observed and the corresponding nitrosoben-
zene was obtained in high yield. In an another experi-
ment, the filtrate was used for the oxidation of aniline
in the presence of 8 mmol of 30% aqueous H2O2 and
no reaction was observed. This indicates that the
active ingredient has not leached out of the solid cata-
lyst during the reaction.
1
compounds were analyzed by GC, GCMS, IR and H NMR.
Acknowledgements
Ankur Bordoloi gratefully acknowledges CSIR, New Delhi
(India) for the research fellowship.
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Ankur Bordoloi and S. B. Halligudi
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