Highly efficient N-formylation of amines with ammonium formate catalyzed by nano-Fe3O4 in PEG-400

Article abstract of DOI:10.1039/c3ra43273d

A simple and efficient protocol for the formylation of amines with ammonium formate with excellent yields catalyzed by Fe₃O₄ nanoparticles in PEG-400 is described. Notably, this method shows excellent activity and chemoselectivity for the formylation of primary and secondary arylamines. Moreover, the nano-Fe₃O₄ catalyst and PEG-400 could be easily recovered and reused.

Full text of DOI:10.1039/c3ra43273d

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Highly efficient N-formylation of amines with
ammonium formate catalyzed by nano-Fe O in
3
4
Cite this: RSC Adv., 2014, 4, 1234
PEG-400
Zhao-gang Wang and Ming Lu*b
a
Received 28th June 2013
Accepted 11th October 2013
A simple and efficient protocol for the formylation of amines with ammonium formate with excellent yields
catalyzed by Fe nanoparticles in PEG-400 is described. Notably, this method shows excellent activity
and chemoselectivity for the formylation of primary and secondary arylamines. Moreover, the nano-
Fe catalyst and PEG-400 could be easily recovered and reused.
3 4
O
DOI: 10.1039/c3ra43273d
www.rsc.org/advances
3 4
O
reaction temperatures, long reaction times, and the use of toxic
solvents. Thus, there is still a need to explore an environmen-
Introduction
Formylation of amines is an important transformation in tally friendly method for this classic reaction.
organic synthesis, and has been widely used for amine protec-
In recent years, with the popularity of green chemistry, envi-
tion in the synthesis of the complex organic molecules of ronmentally benign chemical processes or methods have been of
natural products. Also, formamides are valuable intermediates great concern and extensive research has been carried out in this
for the synthesis of pharmaceutically active compounds such as area. Polyethylene glycol, as an inexpensive, thermally stable,
1
2
substituted aryl imidazoles, 1,2-dihydroquinolines, oxazolidi- non-toxic, and recyclable polymer, has been widely used espe-
3
4,5
nones etc. The formylation of amines is usually achieved by cially in the eld of drug discovery. Moreover, polyethylene
reacting amines with formic acid catalyzed by ZnO, [TMG] ,
ZnCl , sulfated titania, Amberlite IR-120, indium metal,
2
iodine, and sulfonic acid supported hydroxyapatite encapsu- they could become a substitute for volatile organic solvents. In
6
+
7
glycols with low molecular weights (<800) are employed as envi-
ronmentally benign solvents in organic synthesis, it is hoped that
8
9
10
11
12
13
lated g-Fe
the presence metal triates, Bi(III) salts, heteropoly acids,
2 3
O .
Derivatives of formic acid like formate esters in addition, PEGs act as powerful phase transfer catalysts, like
1
4
15
16
crown ethers, due to their special properties such as their cation
17
18
19,20
Al(HSO ) , silphos[PCl -(SiO ) ], or orthoformates
have complexation ability, which can lead to coordination with metal
4
3
3
2 n
also been used for the conversion of amines into formamides. cations of different sizes and has been utilized in many useful
Notably, of these formylating agents, formic acid is corrosive organic transformations under mild reaction conditions, such as
2
3
24–26
and toxic, so it may affect acid-sensitive functional groups such an N-arylation of amines, cross-coupling reaction,
asym-
27
28
as ether and ester bonds, and the number of side reactions may metric dihydroxylation, aldol reaction, Baylis–Hillman reac-
be increased, which is especially detrimental in the amine- tion, multi-component condensation.
protecting steps in the synthesis of complex natural products.
Magnetic nanoparticles (MNPs), as a new class of catalyst,
29
30
There are several disadvantages of using formate esters and have been suggested to be a bridge between homogeneous and
orthoformates, such as longer reaction times, lower yields and heterogeneous catalysis and are expected to exhibit special
the relatively high cost of the reagents. In addition to the above catalytic performance due to their ultrane size and high
3 4
mentioned methods, there have been reports in the litera- surface area. Among the various magnetic nanoparticles, Fe O
21,22
ture
of formylation with formate salts as less expensive and magnetic nanoparticles have been used as a high performance
efficient N-formylating agents for anilines and secondary catalyst and support, owing to them being inexpensive, envi-
amines in the absence of catalysts. Despite these methods being ronmentally benign, chemically stable and conveniently recov-
suitable for specic synthetic conditions, unfortunately, they ered aer the reaction by simple magnetic separation. They
still suffer from one or more disadvantages such as high have also been demonstrated to be an efficient catalyst for many
useful chemical transformations, including thiolysis of epox-
3
1
32
ides, synthesis of diverse N-heterocycles, synthesis of
a
33
34
School of Chemical Engineering, Nanjing University of Science and Technology, a-aminonitriles, synthesis of decahydroacridine derivatives,
Nanjing, 210094, China. E-mail: wangzhaogang1985@126.com; Fax: (+086)-025-
35
36
synthesis of quinoxalines, Sonogashira–Hagihara reactions,
dehydrogenation of ethylbenzene, Friedel–Cras acylation
reaction, trimethylsilyl protection of hydroxyl groups and
three-component coupling of an aldehyde, alkyne and amine.
8
4315514; Tel: (+086)-025-84315030
37
b
School of Chemical Engineering, Nanjing University of Science and Technology,
Nanjing, 210094, China. E-mail: luming302@126.com; Fax: (+086)-025-84315514;
Tel: (+086)-025-84315030
38
39
40
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Considering the above points and our efforts towards the
RSC Advances
development of a new formylating synthetic method, here we
report a green, mild and efficient method for the synthesis of
formamides from amines with ammonium formate catalyzed by
3 4
nano-Fe O in PEG-400. The protocol presented here provides
an alternative and efficient strategy to achieve both chemo-
selectivity and catalyst recycling and would be a particularly
good approach from the view point of green chemistry.
Results and discussion
The X-ray diffraction patterns of Fe
3 4
O MNPs, are shown in
Fig. 1. The position and relative intensities of all peaks (2q ¼
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
3
0.1 , 35.4 , 43.1 , 53.4 , 57 and 62.6 ), which are assigned to
the (2 2 0), (3 1 1), (4 0 0), (4 2 2), (5 1 1) and (4 4 0), were
41
consistent with the database in the JCPDS le (JCPDS19-0629),
Fig. 2 FT-IR adsorption of Fe
after six uses.
3
O
4
magnetic nanoparticles. (a) Fresh, (b)
indicating the crystalline cubic spinel of Fe O MNPs.
3
4
Fig. 2 shows IR spectra for Fe O MNPs, the strong absorp-
3
4
ꢁ1
tion at 578 cm was characteristic of the Fe–O stretching
ꢁ1
vibration. The broad band at around 3500–3000 cm
attributed to adsorbed water. Also, the FT-IR adsorption showed
no obvious differences in the characteristic adsorption peaks
was
with different molecular weights have recently been used as
green stabilizers to protect metal nanoparticles preventing
42,43
aggregation,
3 4
so we speculated that the Fe O magnetic
between fresh Fe
3 4 3 4
O MNPs and Fe O MNPs aer six uses.
nanoparticles could be efficiently stabilized by PEG-400 during
the reaction.
Initially, the reaction of aniline with ammonium formate
was chosen as the model reaction (eqn (1)) to optimize the
reaction conditions for the synthesis of benzamide. The results
are summarised in Table 1.
Thermogravimetric analysis (TGA) and DTA curves of Fe
3 4
O
MNPs showed the mass loss of the organic substances as they
decomposed with heating (Fig. 3). The initial weight loss from
ꢀ
the Fe O MNPs up to 170 C is due to the thermo desorption of
3
4
solvents from the sample, while the weight loss between 170
ꢀ
and 420 C may be associated to the thermal crystal phase
transformation from Fe
that the catalyst was very stable under the reaction conditions.
TEM images of nano-Fe are shown in Fig 4. It can be seen
3 4 2 3
O to g-Fe O . These results revealed
3 4
O
that the synthesized nanoparticles were uniform spheres with
an average size of about 30 nm. Also, the catalysts were well
dispersed in spite of the existence of some bigger structures that
were likely due to the slight aggregation/coalescence of indi-
vidual nanoparticles (Fig 4a). To our delight, the aggregation
phenomenon was not clearly present aer the catalyst had been
used six times (Fig 4b). It is known that polyethylene glycols
4
4
Recently, Das and co-workers reported that the formylation
of aniline was carried out at room temperature in excellent
yields by treatment with formic acid in PEG-400. Inspired by
this good result, we surmised that PEG-400 might act as a phase
transfer catalyst to promote the reaction and reasoned that use
3 4
Fig. 3 Thermo gravimetric analysis of Fe O magnetic nanoparticles.
Fig. 1 X-Ray diffraction pattern of Fe
3
O
4
magnetic nanoparticles.
(a) TGA curve. (b) DTA curve.
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Table 1 (Table 1, entries 6–9) show that the catalyst amount of
10 mol% gave the highest product yield. When the catalyst
amount was reduced to 1 mol%, aniline was converted to
benzamide aer a relatively long reaction time of 1.5 h. On the
other hand, when the amount was above 10 mol%, there was no
signicant change in the conversion. Considering the economy
of the reaction, an optimum catalyst amount of 10 mol% was
chosen. In addition, the effect of reaction temperature was also
investigated (Table 1, entries 10–14), and the highest yield of the
ꢀ
ꢀ
product was obtained at 70 C. A lower temperature (<50 C) led
to a sharp decrease in conversion because of the drop in mass
transfer efficiency and insufficient mixing during the reaction.
In contrast, a higher reaction temperature might lead to more
side reactions due to the ease of oxidation of the amine group.
As control experiments, we studied the impact of solvent using
commercial solvents such as acetonitrile, DMF, toluene,
PEG-200 and PEG-600 under the same conditions, the highest
yield was obtained in PEG-400 (Table 1, entries 15–19). It was
speculated that the polarity of solvents played an important role
for the dissociation of ammonium formate, and polar solvents
3 4
Fig. 4 TEM of Fe O magnetic nanoparticles. (a) Fresh, (b) after six
uses.
Table 1 The effect of catalytic conditions on the N-formylation of
aniline
Temperature
( C)
ꢀ
c
Run
Catalyst (mol%)
Solvent
Yields (%)
a
1
—
PEG-400
PEG-400
PEG-400
PEG-400
PEG-400
PEG-400
PEG-400
PEG-400
PEG-400
PEG-400
PEG-400
PEG-400
PEG-400
PEG-400
rt
—
97
55
70
78
69
86
97
96
35
62
83
95
80
89
85
46
67
71
b
2
Nano-Fe
CuO(10)
ZnO(10)
3
O
4
(10)
70
70
70
70
70
70
70
70
40
50
60
80
90
70
70
70
70
70
3
4
5
6
7
8
9
3
like CH CN and DMF could increase the polarization of the
formic group to facilitate the nucleophilic attack of the amine.
However, PEG might facilitate a different catalytic pathway. It is
known that liquid-phase polyethylene glycol can be considered
to be an open chain of a crown ether, which has a helical
structure, and it could fold into holes of different sizes. Not only
could it complex with ions with different radii by changing the
size of these holes, but some salts and most of the organic
molecules are more soluble in it due to polar hydroxyl groups at
both ends of the polyethylene glycol. Besides, polyethylene
glycols with different molecular weights have an obvious
distinct complexation ability with cations of different sizes.
Therefore, we thought that PEG-400 might act as a more
powerful phase transfer catalyst, due to its improved complex-
ation ability with ammonium cations compared with other
reaction media, which could promote the maximum activity of a
formate anion during the reaction (Scheme 1).
2 3
Al O (10)
Nano-Fe
Nano-Fe
Nano-Fe
Nano-Fe
Nano-Fe
Nano-Fe
Nano-Fe
Nano-Fe
Nano-Fe
Nano-Fe
Nano-Fe
Nano-Fe
Nano-Fe
Nano-Fe
3
3
3
3
3
3
3
3
3
3
3
3
3
3
O
4
O
4
O
4
O
4
O
4
O
4
O
4
O
4
O
4
O
4
O
4
O
4
O
4
O
4
(1)
(5)
(15)
(20)
(10)
(10)
(10)
(10)
(10)
(10)
(10)
(10)
(10)
(10)
10
11
12
13
14
15
16
17
18
19
45
3
CH CN
DMF
Toluene
PEG-200
PEG-600
a
Reaction conditions: aniline (10 mmol), ammonium formate (15
b
mmol), PEG-400 (15 mL), rt, 24 h. Reaction conditions: aniline (10
mmol), ammonium formate (15 mmol), PEG-400 (15 mL), nano-Fe
3 4
O
ꢀ
c
(1.0 mmol), 70 C, the reaction progress was monitored by TLC. The
Based on the above good results, we continued to study the
scope of this reaction protocol extending it to a series of various
aromatic substituted primary and secondary arylamines. As
isolated yield is stated.
of ammonium formate might lead to similar results. Unfortu-
nately, when we mixed aniline (10 mmol) and ammonium
formate (15 mmol) for 24 h at room temperature, no products
were observed (Table 1, entry 1). Then, N-formylation was
carried out in the presence of various metal oxides like CuO,
ZnO, Al O and nano-Fe O under the same conditions. It was
2
3
3 4
3 4
surprising to nd that the nano-Fe O could rapidly induce N-
formylation, which was complete in 30 min to give a quantita-
tive yield of 99.2% (crude yield, the isolated yield was 97%)
(Table 1, entries 2–5). The high efficiency here could be attrib-
uted to the large surface area and ultrane size offered by the
Fe O nanoparticles, which dispersed well in the PEG-400
3
4
during the reaction process to form a pseudo-homogeneous
system and alleviate the disadvantages of the corresponding
heterogeneous catalysts. Then, the inuence of catalyst amount
Scheme 1 Phase transfer effect of PEG-400 by complexing with an
was investigated in the range of 1–20 mol% and the results in ammonium cation.
1236 | RSC Adv., 2014, 4, 1234–1240
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Table 2 The scope of the N-formylation reaction of arylamines
a
b
c
ꢀ
Entry
Substrate
Product
Time (min)
30
Yields
mp ( C)
1
97
47–48
58–59
2
25
92
3
4
5
6
7
25
30
35
20
50
90
94
91
93
80
55–57
129–131
135–137
81–82
81–82
8
9
30
86
56–57
60
50
75
83
90
84
100–102
110–112
174–175
1
1
1
1
0
1
2
3
90
60
85
87
194–196
167–168
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Table 2 (Contd.)
a
b
c
ꢀ
Entry
Substrate
Product
Time (min)
120
Yields
mp ( C)
1
4
5
77
70–71
1
60
90
84
123–124
1
6
81
71–73
a
ꢀ
Reaction conditions: substrate (10 mmol), ammonium formate (15 mmol), nano-Fe
3
O
4
(1.0 mmol), PEG-400 (15 mL), 70 C, the reaction progress
b
was monitored by TLC. All products were puried by column chromatography on a silica gel using petrol ether–dichloromethane (5 : 2) as the
eluent to afford the pure product and determined by melting point and HPLC analysis. The isolated yield is stated.
c
3 4
Scheme 2 The mechanism of nano-Fe O catalyzed formylation.
heteroatom ortho to the amino group, which have a lone pair
electrons, showed low reactivity and were converted to the
corresponding formamides aer a longer reaction time
(
Table 2, entry 14).
The recycling of the catalyst was examined for the N-for-
mylation reaction of aniline. When the reaction was complete,
ether was added to the mixture, the nano-Fe catalyst could
be recycled using a magnetic eld and used six times without
any appreciable loss in its catalytic activity (Fig. 5).
3 4
O
3 4
Fig. 5 Recycling of Fe O MNPs for the N-formylation.
Hence, the reaction mechanism of nano-Fe O catalyzed
3
4
shown in Table 2, the reactions of all the substituted amines N-formylation as a Lewis acid was shown in Scheme 2. We
proceeded very efficiently and no side-reactions occurred. speculated that the reaction involved the complexation of
However, the reactions were largely dependent on the the catalyst with the formic group, nucleophilic attack of the
substituent on the aromatic ring in terms of reaction time lone pair of electrons present in the amine to the carbonyl
and product yield. The reactions of substrates bearing an carbon of the formic group, then the removal of OH– occurs
8
,46
electron-donating group like methyl, methoxy, or hydroxyl to afford the corresponding product.
In order to conrm
(
Table 2, entries 2–6) proceeded more effectively than the this, we tested the pH of the reaction mixture and were
reactions of arylamines with an electron-withdrawing delighted to nd that the pH of the reaction mixture varied
group such as chlorine, bromine, carboxyl or nitro (Table 1, from 8.0 to 10.0 before and aer the reaction, supporting our
entries 7–13). Moreover, the heterocyclic arylamines with proposed reaction mechanism (Scheme 2).
1238 | RSC Adv., 2014, 4, 1234–1240
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Conclusions
Acknowledgements
In conclusion, we have explored an efficient method for for- This work was supported by the National Natural Science
mylation of arylamines using nano-Fe as the catalyst in PEG- Foundation of China (no. 11076017).
00. The mild reaction conditions and eco-friendly practicability
3 4
O
4
made this protocol an attractive approach to prepare formamide
derivatives. Furthermore, it is worth noting that this protocol is
especially attractive because of the cheap and recyclable catalyst
from the view point of green and sustainable chemistry.
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