Green synthesis of nitroenamines by γ-Fe2O3@SiO2-OSO3H nanoparticles as a highly efficient and magnetically separable catalyst

Article abstract of DOI:10.1007/s13738-014-0554-z

Some derivatives of nitroenamines were synthesized in moderate to good yields in the presence of sulfonic acid supported on silica-modified maghemite (γ-Fe₂O₃@SiO₂-OSO₃H) as a highly efficient and magnetically s

Full text of DOI:10.1007/s13738-014-0554-z

J IRAN CHEM SOC
DOI 10.1007/s13738-014-0554-z
ORIGINAL PAPER
Green synthesis of nitroenamines by γ‑Fe2O3@SiO2–OSO3H
nanoparticles as a highly efficient and magnetically separable
catalyst
Samaneh Mahdudi · Dariush Saberi · Akbar Heydari
Received: 19 August 2014 / Accepted: 31 October 2014
© Iranian Chemical Society 2014
Abstract Some derivatives of nitroenamines were syn-
thesized in moderate to good yields in the presence of
sulfonic acid supported on silica-modified maghemite
(γ-Fe2O3@SiO2–OSO3H) as a highly efficient and magnet-
ically separable catalyst. Both up to five times recyclabil-
ity and the magnetic separation are salient features of this
catalytic system.
in organic syntheses either as support or as catalyst [1–4].
Using them as support results in the straightforward separa-
tion of the catalyst from the reaction mixture (this is only
achieved using an external magnet). Therefore, we have
turned our attention to these systems [5–8]. This time, we
report the synthesis of the nitroenamines facilitated by
γ-Fe2O3@SiO2–OSO3H as an efficient catalyst. The fore-
going catalyst has recently been prepared in our laboratory
and used in the synthesis of miscellaneous organic com-
pounds [9].
Keywords Magnetic catalyst · Nitroenamine ·
Heterogeneous catalyst · Reusability
Nitroenamines, olefins with an electron-donating amino
group attached to one side and the strongly electron-with-
drawing nitro group to the other, are indispensable com-
pounds in organic synthesis. They show substantial phar-
macological properties [10, 11], and are also utilized as
synthetic intermediates [12–17]. Nevertheless, there are
only a few synthetic methods, reported in literatures, for the
synthesis of these compounds. Some of the recent methods
entail the reaction of β-halo- [18–20], β-alkylthio- [21], β-
alkylsulfinyl- [22] or β-alkoxynitroethenes [23, 24] with
an amine. Other methods are transamination [25], nitra-
tion of imines [26, 27] rearrangement of N-nitroenamines
[28] and the reaction of phenyl acetylene with aniline in the
presence of mercury (II) chloride [29]. Despite all merits,
the above-mentioned methods suffer from major draw-
backs such as the requirement for the pre-preparation of
the appropriate precursors, the toxicity of the catalyst and
low efficiency. Amongst all reports, the condensation reac-
tion of the nitromethane with orthoformates and amines in
the presence of p-toluene sulfonic acid as catalyst seems
to be superior, due to easily accessible starting materials
[30]. However, it still suffers from the shortcoming of low
reaction yields. Also this procedure does not benefit from
the feature of reusability and recovery of the catalyst. This
prompted us to innovate a heterogeneous, and reusable
Introduction
Due to both economical and environmental reasons, het-
erogeneous catalysts are gradually replacing homogeneous
catalysts in the most fields. Heterogeneous systems pos-
sess the inherent aptitude of recyclability, a matter that has
recently gained a great deal of attention. In this context,
assembling the catalyst on a suitable support is the most
facile way to render it as heterogeneous. In recent years,
magnetic nanoparticles have taken up a special position
Electronic supplementary material The online version of this
article (doi:10.1007/s13738-014-0554-z) contains supplementary
material, which is available to authorized users.
S. Mahdudi · A. Heydari (*)
Chemistry Department, Tarbiat Modares University,
P. O. Box 14155-4838, Tehran, Iran
e-mail: heydar_a@modares.ac.ir
D. Saberi (*)
Fisheries and Aquaculture Department,
College of Agriculture and Natural Resources,
Persian Gulf University, 75169 Bushehr, Iran
e-mail: saberi_d@pgu.ac.ir
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J IRAN CHEM SOC
1
catalyst which would easily be separated from the reaction
medium and also able to improve the yield, relative to the
methods using the homogeneous catalyst.
1253, 1319, 1629,2920, 3437 cm−1; H -NMR (250 MHz,
CDCl3) δ 3.32 (brs, 4H, (CH2)2N), 3.74 (brs, 4H, (CH2)2O),
3
3
6.69 (d, J = 7.4 Hz, 1H, CHNO2), 7.05 (d, J = 7.3 Hz,
1H, CHN); MS (EI, 70 eV): m/z (%) = 158 (M+, 66), 141
(54), 125 (16), 111 (71), 95 (64), 83 (100).
Experimental
1-Methyl-3-(2-nitrovinyl) -1H-indole j. Brown solid;
0.1 g, 50 % yield; mp 125–127 °C; IR (KBr) ν 1195,
1
General procedure for the direct synthesis of nitroenamines
in the presence of γ-Fe2O3@SiO2–OSO3H nanoparticles
as catalyst
1247, 1302, 1616, 2923, 3440 cm−1; H-NMR (250 MHz,
CDCl3) δ 3.87 (s, 3H, CH3), 7.26 (s, 1H, CHNMe), 7.4
(brs, 2H, Ar), 7.52 (s, 1H, CHCHNO2), 7.74–7.79 (m, 2H,
Ar), 8.26 (d, 3J = 13.2 Hz, 1H, CHNO2); MS (EI, 70 eV):
m/z (%) = 202 (M+, 100), 185 (23), 172 (6), 155 (87), 144
(26), 115 (50).
The amine (1 mmol) was added to a mixture of
γ-Fe2O3@SiO2–OSO3H (20 mg, 1.2 mol % of H+) and tri-
ethyl orthoformate (2 mmol). The mixture was stirred for
5 min and then nitromethane (5 mmol) was added. The mix-
ture was stirred at 80 °C under Ar atmosphere until com-
pletion of the reaction [monitored by TLC (n-hexane/ethyl
acetate, 80:20)]. Afterward, the catalyst was separated from
the reaction mixture using an external magnet, washed with
water and ethanol, respectively, and then oven dried at 80 °C
overnight for reuse in a subsequent run. The mixture was con-
centrated under vacuum, and the crude residue was purified
by column chromatography on silica gel eluting with n-hex-
ane/ethyl acetate (80:20, 500 ml) to afford the corresponding
nitroenamine. The products were identified by melting point
(in some cases), 1H-NMR, Mass, and IR spectra. The known
products were characterized by comparison of their spectro-
scopic data and their melting points with reported values.
4-Methyl-N-(2-nitrovinyl) aniline f. Brown solid; 0.12 g,
70 % yield; mp 102–104 °C; IR (KBr) ν 1180, 1263, 1353,
Results and discussion
Silica-coated uniform maghemite (γ-Fe2O3) core–shell
nanoparticles were synthesized by a chemical co-precipita-
tion technique of ferric and ferrous ions in alkaline solution
and then sulfonic acid functionalization of the iron oxide
was achieved by tetraethyl orthosilicate (TEOS) subse-
quently by chlorosulfonic acid [to γ-Fe2O3@SiO2 (1 g),
chlorosulfonic acid (1 g) was added dropwise at room tem-
perature over 15 min] and the loading of impregnated acid
was measured by back titration (0.60 mmol g−1). The syn-
thesized nanocatalyst (γ-Fe2O3@SiO2–OSO3H) was char-
acterized by FT-IR, SEM and XRD (see Supplementary
data).
1637, 2934, 3271 cm−1; H-NMR (250 MHz, CDCl3): δ
Table 1 Optimizing the reaction conditions in the presence of
γ-Fe2O3@SiO2–OSO3H nanoparticles as catalyst
1
3
2.36 (s, 3H, CH3), 6.65 (d, J = 5 Hz, 1H, CHNO2), 7.04
3
3
Entry Solvent Cat. (mg) Temp. (°C) Time (h) Yielda (%)
(d, J = 6.8 Hz, 2H, Ar), 7.2 (d, J = 6.8 Hz, 2H, Ar),
7.26–7.30 (m, 1H, CHNH), 10.72 (brs, 1H, CHNO2). MS
(EI, 70 eV): m/z (%) = 178 (M+, 100), 130 (55), 118 (40),
91 (52).
1
CH2Cl2 20
reflux
reflux
80
20
20
20
20
10
10
10
10
10
20
20
20
<5
30
40
25
80
60
80
60
80
80
-
2
EtOH
Toluen
CHCl3
Neat
20
20
20
20
20
20
10
30
20
20
20
3
N-Benzyl-N-methyl-2-nitroethenamine g. Yellowish
brown solid; 0.15 g, 80 % yield; mp 70–72 °C; IR (KBr)
4
reflux
80
ν 1180, 1260, 1312, 1622, 2925, 3121 cm−1; H-NMR
1
5
6b
7b
8
Neat
80
(250 MHz, CDCl3): δ 2.77 (s, 3H, CH3), 4.48 (s, 2H, CH2),
3
Neat
80
6.68 (d, J = 12.5 Hz, 1H, CHNO2), 7.08–7.41 (m, 5H,
3
Neat
80
Ar), 8.33 (d, J = 10 Hz, 2H, CHNMe). MS (EI, 70 eV):
m/z (%) = 192 (M+, 19), 176 (11), 146 (89), 131 (14), 91
9
Neat
80
10
11
12c
Neat
100
r.t
(100).
Neat
N-Benzyl-2-methyl-N-(2-nitrovinyl) propan-2-amine h.
Brown solid; 0.19 g, 80 % yield; mp 95–97 °C; IR (KBr)
Neat
80
20
ν 1185, 1265, 1313, 1612, 2924, 3448 cm−1; H-NMR
1
Reaction conditions diphenyl amine (1 mmol), triethyl orthoformate
(2 mmol), nitromethane (5 mmol), solvent (1 mL), under Ar atmos-
phere
(250 MHz, CDCl3) δ 1.45 (s, 9H, (CH3)3), 4.48 (s, 2H,
3
CH2), 6.46 (d, J = 8.9 Hz, 1H, CHN), 7.22–7.38 (m, 5H,
3
a
Ar), 8.65 (d, J = 8.9 Hz, 1H, CHNO2); MS (EI, 70 eV):
Isolated yield
m/z (%) = 235 (M+, 14), 188 (28), 132 (51), 91 (100).
4-(2-Nitrovinyl) morpholine i. Yellowish brown solid;
0.1 g, 60 % yield; mp 128–130 °C; IR (KBr) ν: 1195,
b
3 and 10 mmol of nitromethane was used in the entries of 6 and 7
respectively
c
γ-Fe2O3@SiO2 was used as catalyst
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J IRAN CHEM SOC
In the first step, to establish the optimum conditions,
the reaction of diphenylamine, triethyl orthoformate and
nitromethane was chosen as the model reaction. The vari-
able parameters investigated to reach the best result were:
the type of solvent, the reaction temperature, the amount
of the catalyst and nitromethane. The results have been
depicted in Table 1. As a start, the reaction was conducted
in some organic solvents such as CH2Cl2, EtOH, CHCl3
and toluene under the conditions shown in Table 1, but no
satisfactory results were obtained (Table 1, entries 1–4).
Surprisingly, performing the reaction under neat condi-
tions at 80 °C led to the best result (Table 1, entry 5). At
the next stage, the effect of the amount of nitromethane on
the reaction was considered. It was revealed that restricting
the amount of nitromethane to 3 mmol, led to a decline in
the yield. By contrast, no tangible change was observed by
increasing the amount of this reagent to 10 mmol (Table 1,
entries 6 and 7). The similar results were observed by
changing the amount of the catalyst in the same mode
(Table 1, entries 8 and 9). Finally, the impact of the tem-
perature on the reaction was evaluated. By elevating the
temperature up to 100 °C the yield remained unchanged.
However, no product was formed when the reaction was
carried out at room temperature (Table 1, entries 10 and
11). Note that the product was not formed in the absence of
R1
R1
Cat. (20 mg)
80 oC, Ar. atmospher
NO2
+
N
R2
+
CH(OEt)3
NH
CH3NO2
R2
1 mmol
2 mmol
5 mmol
a-k
Scheme 1 The synthesis of nitroenamine derivatives in the presence
of γ-Fe2O3@SiO2–OSO3H nanoparticles as catalyst
Yieldb
(%)
Table 2 Diversity of
Entry
Amine
Product
Time (h)
Ref.
γ-Fe2O3@SiO2–OSO3H-
catalyzed nitroenamines
synthesis
H
N
N
H
2
1
15
50
[31]
NO2
NO2
e
a
M
O
e
M
e
O
NH2
N
H
2
15
65
[31]
e
M
O
b
M
O
NO2
N
NH
3
10
80
[30]
c
H
N
4
10
80
[31]
NO2
N
d
N
NH
5
6
10
10
80
70
[30]
-
NO2
e
H
N
NH2
NO2
e
M
e
M
f
H
N
N
7
20
80
-
NO2
g
NO2
NH
N
8
9
15
15
80
60
-
-
h
NO2
NH
O
N
O
i
NO2
Reaction conditions amine
(1 mmol), triethyl orthoformate
(2 mmol), nitromethane
(5 mmol), catalyst (20 mg,
1.2 mol % of H+), 80 °C, under
Ar atmosphere; aIsolated yield
10
20
50
-
N
N
j
1 3

J IRAN CHEM SOC
conditions. Using this approach, our catalyst was reused for at
least five times without any further treatment while no appre-
ciable loss in the catalytic activity was observed (Fig. 1).
Conclusion
In conclusion, a simple and magnetically separable cata-
lytic system was introduced for the synthesis of nitro-
enamine derivatives. Different types of primary and
secondary amines were tested and the corresponding nitro-
enamines were prepared in acceptable yields. A series of
nitroenamines, that had previously been reported, were
synthesized in the presence of this heterogeneous catalytic
system with greater efficiency and shorter reaction time.
In addition, the new products in this family of compounds
were synthesized. Due to the magnetic properties, the cata-
lyst was easily separated from the reaction mixture using
an external magnet and up to five times reused without any
significant decrease in the activity.
Fig. 1 The recovery of γ-Fe2O3@SiO2–OSO3H nanoparticles in the
synthesis of compound c
catalyst even in trace amounts. Meanwhile, in the presence
of γ-Fe2O3@SiO2 nanoparticles as the catalyst, a 20 % effi-
ciency was obtained which it indicates that the presence of
sulfonic acid is essential as the catalyst for increasing the
reaction yield (Table 1, entry 12).
Thus, the following conditions were used for the syn-
thesis of the other nitroenamines: amine (1 mmol), triethyl
orthoformate (2 mmol), nitromethane (5 mmol), catalyst
(20 mg, 1.2 mol % of H+) at 80 °C under argon atmosphere
(Scheme 1).
Supporting information
Providing information about the preparation and char-
acterization of the catalyst and the FT-IR, Mass and H-
NMR spectra of new products are given as supporting
information.
1
Our findings from these experiments were as follow: the
primary and secondary aromatic amines were well coupled
with triethyl orthoformate and nitromethane to give the cor-
responding nitroenamine (Table 2). As shown, both aniline
and benzyl amine derivatives were converted to the corre-
sponding nitroenamine with moderate to good yields. Also,
N-methyl cyclohexyl amine led to the expected product in
a reasonable yield (Table 2, entry 5). To show the broad
scope of this reaction, morpholine as the cyclic amine
was also used as the substrate and gave rise to the corre-
sponding product (Table 2, entries 9). Another case exam-
ined was N-methyl indole which olefinated at C3 position
(Table 2, entry11). Note that the primary amines had a less
efficiency than the secondary amines probably because they
were more prone to oxidation and side reactions. Worthy of
note is that according to the previously reported reactions
[30, 31] and based on the coupling constants, it seems that
there is a possibility of formation of both isomers (E or Z),
depending on the substrate. Since the experiments have not
been done on our behalf to prove the type of isomer, it is
reasonable to draw the structure of the products as a whole.
Finally, the reusability of the catalyst was explored in the
model reaction. To this end, after the completion, the catalyst
was recovered using an external magnet, washed with H2O
and ethanol and oven dried at 80 °C overnight. A new reac-
tion was then performed with fresh reactants under identical
Acknowledgments The authors would acknowledge the financial
support provided by the Tarbiat Modares University for carrying out
this research.
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