Oxidation of secondary amines to nitrones using magnetically separable tungstophosphoric acid supported on silica-encapsulated γ-Fe 2O3 nanoparticles

Article abstract of DOI:10.1016/j.tetlet.2013.09.090

Superparamagnetic tungstophosphoric acid supported on silica-encapsulated γ-Fe₂O₃ was used as an efficient catalyst for the direct oxidation of secondary amines to nitrones with hydrogen peroxide as the oxidant. The catalyst could be recycled up to four times without significant loss of activity.

Full text of DOI:10.1016/j.tetlet.2013.09.090

Accepted Manuscript
Oxidation of secondary amines to nitrones using magnetically separable tung-
stophosphoric acid supported on silica-encapsulated γ-Fe2O3 nanoparticles
Fatemeh Nikbakht, Akbar Heydari, Dariush Saberi, Kobra Azizi
PII:
S0040-4039(13)01655-9
DOI:
http://dx.doi.org/10.1016/j.tetlet.2013.09.090
TETL 43590
Reference:
Tetrahedron Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
22 May 2013
2 September 2013
20 September 2013
Please cite this article as: Nikbakht, F., Heydari, A., Saberi, D., Azizi, K., Oxidation of secondary amines to nitrones
using magnetically separable tungstophosphoric acid supported on silica-encapsulated γ-Fe2O3 nanoparticles,
Tetrahedron Letters (2013), doi: http://dx.doi.org/10.1016/j.tetlet.2013.09.090
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Oxidation of secondary amines to nitrones
using magnetically separable
Leave this area blank for abstract info.
tungstophosphoric acid supported on silica-
encapsulated γ-Fe2O3 nanoparticles
Fatemeh Nikbakht, Akbar Heydari* , Dariush Saberi, Kobra Azizi
γ-Fe2O3@SiO2-H3PW12O4

1
Tetrahedron Letters
journal homepage: www.elsevier.com
Oxidation of secondary amines to nitrones using magnetically separable
tungstophosphoric acid supported on silica-encapsulated γ-Fe2O3 nanoparticles
Fatemeh Nikbakht, Akbar Heydari *, Dariush Saberi, Kobra Azizi
Chemistry Department, Tarbiat Modares University, Tehran, P.O. Box 14155-4838, Iran
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
Superparamagnetic tungstophosphoric acid supported on silica-encapsulated γ-Fe2O3 was used
as an efficient catalyst for the direct oxidation of secondary amines to nitrones with hydrogen
peroxide as the oxidant. The catalyst could be recycled up to four times without significant loss
of activity.
Available online
Keywords:
Oxidation
2009 Elsevier Ltd. All rights reserved.
Secondary amines
Nitrones
Tungstophosphoric acid
Magnetic separation
Previously, we reported the preparation of a novel version of
solid tungstophosphoric acid supported on silica-encapsulated γ-
Fe2O3 (γ-Fe2O3@SiO2-H3PW12O40)14 as a magnetic, recoverable
catalyst. Metal complexes of tungsten and molybdenum, are
active catalysts for the oxidation of secondary amines to nitrones.
Oxidative transformations of amines offer potentially
interesting new routes to important chemical building blocks.1
Selective catalytic oxidation of secondary amines to nitrones,2
imines3 and hydroxylamines4 have been explored. Nitrones have
long been regarded as useful intermediates for the synthesis of
heterocyclic compounds.5 Moreover, they have relevant
application as spin traps6 and therapeutics in age-related
diseases.7 Conventional strategies for the synthesis of these
compounds include condensation of carbonyl compounds with
monosubstituted hydroxylamines,8 N-alkylation of oximes9 and
oxidation of hydroxylamines,10 imines11 and secondary amines.2
Among these methods, selective oxidation of imines and
secondary amines to nitrones is the most attractive because
amines are readily accessible compared to hydroxylamines.
Oxidation of secondary amines to nitrones has been performed
with various types of peroxides as the oxidant: hydrogen
peroxide, urea-hydrogen peroxide and alkyl hydroperoxide in the
presence of various catalysts such as Ti(IV),2k W(VI),2p,2s
Mo(VI),2r Pt(II),2m and CH3ReO3.13,2u Excellent yields have been
obtained for the catalytic oxidation of certain amines. However,
in some cases, there are disadvantages such as low selectivity,2h-i
problems with catalyst recovery and procedures that demand
harsh conditions. In addition, with highly reactive nitrones, over-
oxidation and hydrolysis can be significant problems.2d,12
Consequently, it is desirable to use a cheap and environmentally
benign oxidant in combination with suitable catalysts in order to
overcome these problems.
2d,r
Hence it was decided to investigate the catalytic activity of
this magnetic compound for the oxidation of amines. Moreover,
among the various peroxides, aqueous hydrogen peroxide was
chosen as the most suitable oxidant as it is cheap,
environmentally safe and easy to handle. Facile separation of the
catalyst by using an external magnet, the recyclability of the
catalyst (up to four times) and selectivity are other important
benefits of this system.
In an initial study, dibenzylamine was used as a model
substrate to optimize the conditions for the oxidation of
secondary amines to nitrones. Oxidation of this substrate in
methanol as the solvent at room temperature, in the absence of a
catalyst, was tested and formation of a small amount of the
desired product showed that the presence of a catalyst was
necessary for this reaction (Table 1, entry 1). Adding silica-
encapsulated γ-Fe2O3 nanoparticles as the catalyst had little
impact on the progress of the reaction (Table 1, entry 2). In
contrast, addition of γ-Fe2O3@SiO2-H3PW12O40 [10 mg, 1.1
mol% (determined by back titration)] as the catalyst enabled the
oxidation to give the corresponding nitrone, (Z)-N-
benzylidenebenzylamine N-oxide in 85% yield (Table 1, entry 4).
Next, we examined the effects of parameters such as the amount
of catalyst, temperature and solvent. Screening organic solvents
(MeOH, CH3CN, CH2Cl2, DMSO) as well as water, showed that
the best yield was obtained in MeOH (Table 1, entries 3-8).
———
* Corresponding author Tel.: (+98)-21-82883444; Fax: (+98)-21- 82883455; email: heydar_a@modares.ac.ir

2
Tetrahedron Letters
Raising the reaction temperature had a negative effect on the
Generally speaking, the presence of an electron-releasing
group such as methyl or methoxy on the benzyl moiety increased
the product yield (Table 2, entries 2 and 3). N-Benzyl-tert-
butylamine was converted into the corresponding nitrone in good
yield (Table 2, entry 4). Oxidation of less activated dibutylamine
was investigated and the related nitrone was obtained in a good
yield, but required more time compared to benzyl derivatives
(Table 2, entry 5). In the case of unsymmetrical amines, there is
the possibility of forming two products, but using these reaction
conditions, only the thermodynamically more stable product was
formed (Table 2, entries 6-8).
yield (Table 1, entry 5). Increasing the amount of catalyst gave
no change in the reaction rate, but decreasing the amount of
catalyst prolonged the reaction time. All the reactions were
carried out under an argon atmosphere (reaction under
atmospheric pressure gave a mixture of products). Therefore, the
optimum conditions for this reaction are as follows: 1.1 mol% of
catalyst, MeOH as the solvent, room temperature, argon
atmosphere.15
Table 1
Optimization of the reaction conditions using the model
system.a
The reusability of the catalyst was examined in the synthesis
of compound 2a. When the reaction was complete, the catalyst
was adsorbed on to the side-wall of the reaction vessel with the
aid of an external magnet. The catalyst was washed with
methanol three times and each time the catalyst separated easily
from the solution by attaching an external magnet onto the
reaction vessel, followed by decanting the solution. The
remaining catalyst was dried in a vacuum and reused in a
subsequent reaction. More than 95% of the catalyst could usually
be recovered from each reaction. The catalyst could be reused in
four runs without significant loss of its catalytic activity (see
entry 1, Table 2).
Entry
Catalyst (mol%)
Solvent
Temp.
(°C)
23
Time (h)
/Yield (%)b
24/trace
24/10
1
No catalyst
MeOH
MeOH
H2O
2
γ-Fe2O3@SiO2 (1.1)
23
3
γ-Fe2O3@SiO2- PWc (1.1)
γ-Fe2O3@SiO2- PW (1.1)
γ-Fe2O3@SiO2- PW (1.1)
γ-Fe2O3@SiO2- PW (1.1)
γ-Fe2O3@SiO2- PW (1.1)
γ-Fe2O3@SiO2- PW (1.1)
γ-Fe2O3@SiO2- PW (2.2)
γ-Fe2O3@SiO2- PW (0.55)
23
2/70d
4
MeOH
MeOH
DMSO
CH3CN
CH2Cl2
MeOH
MeOH
23
2/85
5
60
24/40
6
23
24/10
7
23
24/50d
Mechanistic investigations on tungsten-based oxidation
systems with hydrogen peroxide have shown that
[PO4{WO(O2)2}4]3− is a catalytically important species generated
by the reaction of H3PW12O40 with excess hydrogen peroxide,
and is the active complex for the oxidation of variety of organic
compounds.18 Tomaseli and co-workers reported that the
oxidation of secondary amines by anionic mononuclear Mo(VI)
and W(VI) peroxo complexes, involves a rate determining
nucleophilic attack of the amine onto a peroxide oxygen with
formation of the corresponding hydroxylamine, followed by a
faster step in which the hydroxylamine is oxidized to a nitrone.19
In order to confirm the above-mentioned mechanism in the
presence of γ-Fe2O3@SiO2-H3PW12O40 nanoparticles, oxidation
of N,N-dibenzylhydroxylamine was performed under the
optimized reaction conditions and (Z)-N-benzylidenebenzylamine
N-oxide was formed almost immediately. This observation seems
to support the possibility that the hydroxylamine may represent
the first intermediate generated by nucleophilic attack of the
amine on the peroxide oxygen. This intermediate is then
converted into the nitrone in a much faster subsequent step
(Scheme 2). It has to be pointed out that when an imine was
exposed to the oxidation conditions, only degradation products
were obtained. This result eliminates the hypothesis of an imine
as intermediate in the reaction mechanism.
8
23
48/0
9
23
2/85
10
23
24/85
a Reaction conditions: dibenzylamine (1 mmol), H2O2 (3 equiv.), solvent (2
mL), Ar atm.
b Isolated yield
c γ-Fe2O3@SiO2-H3PW12O40
d Benzaldehyde was formed as a by-product
Under these conditions, a variety of secondary amines were
used as substrates for the synthesis of the corresponding nitrones
(Scheme 1). As can be seen in Table 2, various nitrones were
synthesized in moderate to good yields. All the products are
known compounds (except 2b and 2c)16 and were characterized
by comparison of their IR, H NMR and 13C NMR spectroscopic
1
data and respective melting points with reported values.2d, 8f, 17
γ-Fe2O3@SiO2-H3PW12O4
Scheme 1. Synthesis of nitrone derivatives in the presence of γ-
Fe2O3@SiO2-H3PW12O40 as the catalyst.
Scheme 2. Proposed mechanism for the γ-Fe2O3@SiO2-H3PW12O40-catalyzed oxidation of secondary amines to nitrones.

3
Table 2
Catalytic oxidation of secondary amines by hydrogen peroxide in the presence of γ-Fe2O3@SiO2-H3PW12O40 nanoparticles as the
catalyst.a
Entry
Substrate
Product
Time (h)
Yield (%)b
TONc
TOF (h-1)d
Ref.
8f
1
2
85 (85, 83, 80)e
77.2
38.6
1a
2a
2
3
2
1
90
87
81.8
79.1
40.9
79.1
-
-
1b
2b
2c
1c
4
85
70
77.2
63.6
19.3
5.3
4
5
6
17
2d
2d
2d
1d
12
1e
2e
12
65
58.8
4.9
1f
2f
7
8
8f
3
53
55
48.3
49.7
16.1
14.2
1g
2g
3.5
17
2h
1h
a Reaction conditions: amine (1 mmol), H2O2 (3 equiv.), CH3OH (2 mL), r.t., Ar atm.
b Isolated yield.
c Turnover number (average number of product molecules produced per mole of the catalyst).
d Turnover frequency (turnover number per hour).
e The reaction was accomplished with recycled catalyst.
In summary, this report describes an efficient method for the
References and notes
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Fe2O3@SiO2-H3PW12O40 nanoparticles can be easily separated
from the products and can be reused. The reactions were
complete after 1-12 hours producing good yields of the nitrones,
which were comparable with yields obtained by known methods.
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We are thankful to Tarbiat Modares University for financial
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