Economical and efficient aqueous reductions of high melting-point imines and nitroarenes to amines: Promotion effects of granular PTFE

Article abstract of DOI:10.1039/c4ra04528a

The reductions of high melting-point imines and nitroarenes to amines in aqueous media using Zn powder, granular PTFE (polytetrafluoroethylene), catalytic Aliquat 336 and NH₄Cl or 5% NaOH at room temperature have been achieved. A major advantag

Full text of DOI:10.1039/c4ra04528a

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Economical and efficient aqueous reductions of
high melting-point imines and nitroarenes to
amines: promotion effects of granular PTFE†
Cite this: RSC Adv., 2014, 4, 33599
*
Tao Li,‡ Xiaoxue Cui,‡ Lili Sun‡ and Chunbao Li
The reductions of high melting-point imines and nitroarenes to amines in aqueous media using Zn powder,
granular PTFE (polytetrafluoroethylene), catalytic Aliquat 336 and NH4Cl or 5% NaOH at room temperature
have been achieved. A major advantage of this procedure is that the cost of the catalyst is only 1/7200 of
that of a previously reported nitroaromatic aqueous reduction catalyst. Altogether 13 imines and 11
nitroarenes were reduced to the corresponding amines with excellent yields. The effects of the amount
of granular PTFE, and solubilities and melting points of the substrates, and melting points of the products
on the reaction rates are discussed. For the first time, the relationship between the aqueous reaction
rates and melting points of the products was investigated, which leads to a conclusion that lower
melting-point products form faster than higher melting-point ones in the aqueous reductions. The
Aliquat 336, granular PTFE and water are all recyclable.
Received 14th May 2014
Accepted 14th July 2014
DOI: 10.1039/c4ra04528a
www.rsc.org/advances
have been reported. They include catalytic hydrogenations
using Raney Ni,25 Pd/C,26 PdCl2,27 Pd(OH)2,28 Pd/NaBH4,29
Rh/C,30 Ru/Al2O3,31 Au/TiO2,32 Au/SiO2,32 PtO2,33 Ag/NaBH4,34
Introduction
Amino functional groups are common in bioactive natural
products and many pharmaceutically important substances.1
Amines are usually obtained by the reduction of the corre-
sponding imines.2 Imines have been reduced by a wide variety
of reagents including metal hydrides or metallic reagents, such
as NaBH4,3 NaBH3(CN),4 NaBH(OAc)3,5 NaBH4/Al2O3,6 LiAlH4,3a
Zn(BH4)2/SiO2,7 CaH2/ZnX2,8 Bu2SnClH,9 (h-C5H5)MoH2,10
RuHCl(PPh3)3,11 silane/MoO2Cl2,2 Ni(acac)2/Et2Zn,1a Pd(AcO)2/
Et3SiH,12 In/NH4Cl,13 ZnB2H8,14 and SmI2.15 In addition, metal-
free hydrogen donors like NH3–BH3,16 hydrogenation catalysts
like Pd–C,17 b-CD/PdCl2,18 Pt,17 Co,19 Ru,20 and CF3SO3Zn,21 and
lanthanides such as Yb22 have been used. All these reductions
were performed in organic solvents. An aqueous enzymatic
reduction has been reported for the synthesis of asymmetric
amines23 and an aqueous reduction using Zn powder has been
used to produce amines in excellent yields.24 However, in the
aqueous systems only low melting-point substrates were
investigated.
Pt/g-Fe2O3,35 or Co–Mo2C/AC;36 dissolving metal reductions
using Fe/HCl,37 or Sn/HCl;38 and metallic reducing reagents
using In,39 Sm,40 or B10H14.41 In addition, FeCl3/N2H4,42
Se/NaOAc,43 and TEOA/eosin Y44 have been used to reduce nitro
compounds to amines. Two elegant aqueous reductions have
been reported by Tsukinoki45 and Lipshutz,46 but these require
heating or expensive catalysts. In the other reduction systems,
highly toxic, expensive, or moisture-sensitive reagents as well as
organic solvents are necessary.
For the reduction of high melting-point imines and nitro-
arenes, it would be extremely benecial to be able to replace the
expensive catalysts and organic solvents with cheap reductants
and green reaction media like water. As a solvent, water has a
multitude of advantages including safety, nontoxicity, inam-
mability, cheapness, and environmental friendliness.
Results and discussion
Another method leading to amines is the reduction of
nitroarenes. A variety of procedures for these transformations
When high melting-point imine (E)-N-(4-methoxybenzylidene)-
1-naphthalenamine (1h) (2 g, mp: 100–102 ꢀC, Table 1, entry 8)
was subjected to the aqueous reduction conditions previously
Department of Chemistry, College of Science, Tianjin University, Tianjin, 300072, P. R.
China. E-mail: lichunbao@tju.edu.cn; Fax: +86-22-27403475; Tel: +86-022-27892351 reported in the literature (Zn powder, 5% aq. NaOH solution at
room temperature),24 no reaction occurred aer several hours
and only a 50% conversion was achieved aer 3 days. This
† Electronic supplementary information (ESI) available: General experimental
information, general procedure for the reduction of imines, control
experiments for the function of Aliquat 336 and granular PTFE in the reduction
indicates that the aqueous reductions of high melting-point
of imines, control experiments for the function of PTC in the reduction of
imines, general procedure for the reduction of nitroarenes, references, 1H NMR
and 13C NMR spectra for new products. See DOI: 10.1039/c4ra04528a
‡ T. L., X. C., and L. S. contributed equally.
imines are different from those of low melting-point imines.
In an attempt to solve this problem, benzyltriethylammonium
chloride was added to 1h and the mixture was stirred under the
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Table 1 Reduction of high melting-point imines to amines in water at room temperaturea
Solubilityb
(mol Lꢁ1
Mp of imine Reaction
Mp of amine Yieldc
Entry Imine
)
(ꢀC)
time (h)
Amine
(ꢀC)
(%)
1
7.0 ꢂ 10ꢁ4 51–52
0.67
Syrup at rt
85
2
3
3.4 ꢂ 10ꢁ5 74–76
0.8
1.5
66–68
88
89
1.5 ꢂ 10ꢁ5 85–86
Syrup at rt
4
9.4 ꢂ 10ꢁ6 84–85
0.75
60–61
90
5
6
6.2 ꢂ 10ꢁ6 85–86
1.5
88–90
86
90
1.3 ꢂ 10ꢁ5 90–92
0.67
Syrup at rt
7
1.8 ꢂ 10ꢁ5 99–101
0.8
Syrup at rt
89
8
9
3.7 ꢂ 10ꢁ5 100–102
2.8
1.3
67–69
88
93
1.4 ꢂ 10ꢁ6 111–113
Syrup at rt
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Table 1 (Contd.)
Solubilityb
(mol Lꢁ1
Mp of imine Reaction
Mp of amine Yieldc
Entry Imine
)
(ꢀC)
time (h)
Amine
(ꢀC)
(%)
10
4.3 ꢂ 10ꢁ7 117–118
1.8 ꢂ 10ꢁ5 128–129
4.0 ꢂ 10ꢁ6 130–132
1.2
125–128
95
11
12
1.8
1.5
78–79
69–72
89
90
13
5.2 ꢂ 10ꢁ6 134–137
0.75
Syrup at rt
92
a
All conversions were 100%. b 1a–1i, 1k–1m: data from SciFinder; 1j: data calculated using Soware Wskow. c Isolated yields. Reaction conditions:
imine (2 g), 5% aq. NaOH (10 mL), Aliquat 336 (0.06 equiv.), Zn powder (5 equiv.), and granular PTFE (5 g) at room temperature.
same conditions as above. Aer several hours, the reduction of the imines was not an important factor in determining the
still did not take place. Other solid PTCs (phase transfer cata- reaction rates. This is different from our previous observation in
lysts) such as tetraethylammonium chloride, and tetrabutyl- the hydrolysis of steroidal epoxides.48 One possible reason for
ammonium bromide did not work either. When liquid PTC this is that the differences in solubilities were not large enough
Aliquat 336 and 5 g of granular PTFE (PTFE sand, 70 pieces per and other factors dominated. However, the reaction rates of
g) were added to the reaction mixture with mechanical some of the reactions that produced liquid products (2a, 2f, 2g, 2m)
stirring,47 the reduction was complete in 2.8 h and gave an yield (Table 1, entries 1, 6, 7, 13) were faster than those that produced
of 88%.
solid products (2e, 2h, 2j, 2k, 2l) (Table 1, entries 5, 8, 10–12).
ꢀ
Other imines (1a–1m, 2 g, mp: 70–140 C) with low to high The reason is that the syrup products can serve as a solvent to
melting-points were suspended on a 5% aq. NaOH solution dissolve the solid high melting-point imines, which accelerates
(10 mL) and treated with Aliquat 336 (0.06 equiv.), Zn powder the reduction. Halogen groups such as F, Cl, and Br remained
(5 equiv.) and granular PTFE (5 g) at room temperature with intact aer the reduction (Table 1, entries 3–5, 7, 9, 11–13).
mechanical stirring.47 The corresponding amines were obtained
The reduction of nitroarenes 3a–3k using 7 equiv. of Zn
in 0.6–3 h (Table 1, entries 1–13). The products of the reaction powder, 2 equiv. of NH4Cl, 0.06 equiv. of Aliquat 336, 5 g of
adhered to the granular PTFE. So when the reduction was granular PTFE and water with mechanical stirring47 at room
complete, the aqueous solution was decanted and the residue temperature are presented in Table 2. The conditions are
was extracted with a small amount of ethyl acetate which was applicable to both high and low melting-point nitroarenes. The
acidied with conc. HCl to give amine hydrochlorides 2a (85%), reaction times ranged from 0.25 to 13 h and the yields were
2b (88%), 2c (89%), 2d (90%), 2e (86%), 2f (90%), 2g (89%), 2i excellent. Nitroarenes with lower melting points tended to react
(93%), and 2m (92%). Alternatively the product was crystallized faster (3j vs. 3k vs. 3i) (Table 2, entries 10 vs. 11 vs. 9). However,
with ethanol aer the ethyl acetate was evaporated to give 2h there were large differences in the reaction times for substrates
(88%), 2j (95%), 2k (89%), and 2l (90%). The reduction of 1d was with similar melting points (3a vs. 3b) (Table 2, entries 1 vs. 2).
scaled up to 11 gram with similar yield (91%) in 8 h. The imines The difference in the reaction rates between 3a and 3b is
with lower melting points tended to react faster (1a, 1b, 1d, 1f, probably due to the lower solubility of 3b. Although the solu-
1g vs. 1h–1l) (Table 1, entries 1, 2, 4, 6, 7 vs. 8–12). The solubility bilities of 3c and 3d (Table 2, entries 3 and 4) are higher than
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Table 2 Reduction of nitroarenes to amines in water at room temperaturea
Solubilityb
(mol Lꢁ1
Mp of
Reaction
time (h)
Mp of amine
Yieldc
(%)
Entry
1
Nitroarene
)
nitroarene (ꢀC)
Amine
(ꢀC)
2.0 ꢂ 10ꢁ4
60–61
60–61
0.5
1.2
48–50
87
92
2
2.7 ꢂ 10ꢁ5
Syrup at rt
3
4
5.0 ꢂ 10ꢁ3
4.1 ꢂ 10ꢁ3
80–81
90–91
13
100–102
98–100
86
88
4.0
5
6.1 ꢂ 10ꢁ4
96–97
1.4
112–114
90
6
7
1.5 ꢂ 10ꢁ2
5–6
2.0
1.7
Syrup at rt
Syrup at rt
82
88
3.2 ꢂ 10ꢁ3
ꢁ9–10
8
9
3.2 ꢂ 10ꢁ3
15–16
50–52
2.7
2.0
Syrup at rt
86
92
3.2 ꢂ 10ꢁ3
41–42
10
2.3 ꢂ 10ꢁ3
30–32
0.25
Syrup at rt
91
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Table 2 (Contd.)
Solubilityb
(mol Lꢁ1
Mp of
Reaction
time (h)
Mp of amine
Yieldc
(%)
Entry
Nitroarene
)
nitroarene (ꢀC)
Amine
(ꢀC)
11
1.9 ꢂ 10ꢁ3
40–42
0.4
Syrup at rt
88
a
All conversions were 100%. b 3a–3k: data from SciFinder. c Isolated yields. Reaction conditions: nitroarene (2 g), Aliquat 336 (0.06 equiv.), NH4Cl
(2 equiv.), Zn powder (7 equiv.), granular PTFE (5 g), and water (20 mL) at room temperature.
those of 3a and 3b (Table 2, entries 1 and 2), longer reaction comparison of reduction conditions between the two methods
times for 3c and 3d than those for 3a, 3b, and 3f–3k (Table 2, mentioned above and this work is listed in Table 3. Our
entries 1–2, 6–11) were required since the melting points of reduction rates are generally higher than that reported by the
products 4c and 4d (Table 2, entries 3 and 4) are higher than Lipshutz's group but their yields are slightly higher than ours.
those of products 4a, 4b, and 4f–4k (Table 2, entries 1–2, 6–11). Catalyst TPGS-750-M requires a two-step synthesis or is
Obviously, lower melting-point products form faster than commercially available from Sigma-Aldrich.46,50 A comparison
higher ones in the reductions. This is similar to what was of the cost between Aliquat 336 and TPGS-750-M is shown in
observed in the imine reductions discussed above (Table 1). To Table 4. The per millimolar reaction cost of TPGS-750-M is 7200
our knowledge, this represents the rst example that the effect times of that of Aliquat 336.
of the melting points of products on the aqueous reaction rates
The effect of the amount of granular PTFE on the reaction
has been investigated.49 The procedure is chemoselective for rate was investigated and the results are presented in Table 5.
nitro groups and carbonyl (4c), acetal (4d), benzyl ether (4e) and When 100, 500 or 5000 mg of imine 1d was stirred in the
halogen (4j, 4k) groups remained intact (Table 2, entries 3–5, presence of 5 g of granular PTFE and 0.5, 2.5 or 25 mL of 5% aq.
10–11).
NaOH respectively at room temperature, the reactants were
The procedure for the reduction of nitroarenes previously 100% converted in 0.83, 1.0, and 4.33 h respectively (Table 5,
reported in the literature requires heating to 80 ꢀC with Zn entries 1–3). With 1 g of granular PTFE, the corresponding times
powder, NH4Cl and H2O.45 Under our reaction conditions, the were 1.5, 1.66, and 11 h respectively (Table 5, entries 1–3). With
reductions took place at room temperature. Recently, the no granular PTFE, the 100- and 500 mg scale reactions were
Lipshutz's group reported a method for the reductions of complete in 11 and 12 h respectively (Table 5, entries 1–2) and
nitroaromatics which uses Zn, NH4Cl and catalytic TPGS-750-M the 5000 mg scale reaction was not complete even aer 24 h
in water at room temperature with excellent yields.46 The (Table 5, entry 3). For the 5000 mg scale reaction, reducing the
Table 3 Comparison of two aqueous nitroarene reduction conditions with this work
Method
Zn (eq.)
NH4Cl (eq.)
Catalyst
Granular PTFE
Stirring
Temp.
Time (h)
Yield (%)
Tsukinoki'sa
Lipshutz'sb
This work
7
5
7
2
1.2
2
None
TPGS-750-M
Aliquat 336
None
None
5 g/2 g of nitroarene
Magnetic
Magnetic
Mechanical
80 ꢀC
rt
rt
0.5–1
0.5–8
0.25–13
81–97
88–99
82–92
a
Ref. 45. b Ref. 46.
Table 4 Price comparison between Aliquat 336 and TPGS-750-M
Price
Catalyst (CAS no.)
Aldricha
Xiamen Pioneer Technology Co., Ltd.b
Cost of 1 mmol reaction
Aliquat 336 (5137-55-3)
2 wt % TPGS-750-M in H2O (1309573-60-1)
—
￥0.12 per g
—
￥0.003 per 0.024 g
￥21.6 per 2c mL
￥10.8 per mL
a
http://www.sigmaaldrich.com/catalog/search?interface¼All_ZH&term¼733857&N¼0&mode¼match%20partialmax&focus¼product&lang¼
zh&region¼CN. b http://detail.1688.com/offer/1204581242.html. c Ref. 46.
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Table 5 The effect of granular PTFE on the reaction rate of imine 1d reduction
Time, conversiona (granular
PTFE, 5 g)
Time, conversiona (granular
PTFE, 1 g)
Time, conversiona (granular
PTFE, 0 g)
Entry
Amount of 1d
1
2
3
100 mg
500 mg
5000 mg
0.83 h, 100%
1.0 h, 100%
4.33 h, 100%
1.5 h, 100%
1.66 h, 100%
4.33 h, 75%
11 h, 100%
11 h, 100%
12 h, 100%
4.33 h, 58b%
a
Determined by 1H NMR. b The reaction was not complete aer 24 h. Reaction conditions: 1d (100 mg, 500 mg or 5000 mg), Zn powder (5 equiv.),
5% aq. NaOH (0.5 mL for the 100 mg scale reaction; 2.5 mL for the 500 mg scale reaction; 25 mL for the 5000 mg scale reaction), Aliquat 336 (0.06
equiv.), and granular PTFE at room temperature.
amount of granular PTFE from 1 to 0 g, resulted in a decrease in
the yield at 4.33 h from 75% to 58% (Table 5, entry 3). These
data indicate that the granular PTFE has a large promoting
Experimental
General experimental information
effect on the reaction rate.
All of the chemicals were obtained from commercial sources or
prepared according to standard methods. The 1H NMR (400 or
600 MHz) and 13C NMR (100 or 150 MHz) were recorded on a
Bruker AM-400 spectrometer or a Bruker Avance III spectrom-
eter respectively. Chemical shis (d) are reported relative to
TMS (1H) or CDCl3 (13C). IR spectra were recorded on BIO-RAD
FTS 3000 spectrometer. Melting points were recorded on an X-4
Micro-melting Point Apparatus. High resolution mass spectra
(ESI) were obtained on a Bruker micrOTOF-QII. All the aqueous
reductions were performed in 100 mL asks and agitated by a
modied stirring rod. The stirring rod was modied by insert-
ing a 25 cm PTFE wire into the blades and has been described
previously.47 The granular PTFE is 70 pieces per g.
Aliquat 336 probably serves as the reaction medium since it
can dissolve the substrates; although it was only used in cata-
lytic amount. Other PTCs such as benzyltriethylammonium
chloride, tetraethylammonium chloride, tetrabutylammonium
bromide, sodium dodecanesulfonate or liquid N-butyl-N-methyl
imidazolium chloride are too hydrophilic and are not capable of
dissolving imines and nitroarenes at room temperature (see
ESI†). The granular PTFE functions as a co-stirrer to promote
the efficiency of the stirring.51 In the control experiments, the
reduction did not occur when only Aliquat 336 was used as the
solvent or in the absence of both Aliquat 336 and granular PTFE
(see ESI†). Aliquat 336 may facilitate the electron transfer from
the solid zinc to the imines (or nitroarenes) in this multiphasic
system (solid Zn, solid substrate, solid granular PTFE, water,
and liquid Aliquat 336).
General procedure for the reduction of imines
Zn powder (5 equiv.) was added to a mixture of imine (1a–1m, 2 g),
Aliquat 336 (0.06 equiv.), granular PTFE (5 g) and 5% aq. NaOH
solution (10 mL). The reaction mixture was mechanically
stirred47 for 0.6–2.8 h at room temperature. TLC (thin layer
chromatography) was used to determine when the reaction was
complete. The aqueous solution was decanted and the solid
residue was extracted with a small amount of ethyl acetate
followed by acidied with conc. HCl to give amine hydrochlo-
rides: 2a (85%), 2b (88%), 2c (89%), 2d (90%), 2e (86%), 2f
(90%), 2g (89%), 2i (93%), and 2m (92%). This procedure was
applied to the synthesis of 2d (91%, 8 h) from 11 gram of 1d.
Alternatively the product was crystallized with ethanol aer
ethyl acetate was evaporated to give 2h (88%), 2j (95%), 2k
(89%), and 2l (90%).
In order to evaluate the reusability of the Aiquat 336, gran-
ular PTFE and water, the reduction of 1k was investigated as a
representative reaction (Table 6). Aer the rst reduction cycle
was complete, the reaction mixture was extracted three times in
a ask with a minimal amount of toluene. The product was then
crystallized with ethanol aer toluene was evaporated. The
concentrated mother liquor, fresh Zn powder and 1k were then
reintroduced to the ask. With each subsequent cycle, the
amounts of zinc were gradually reduced from 5 to 3 equiv. and
the reaction times were shortened from 2.83 to 1.08 h (Table 6).
For all six cycles, the conversions were 100% and the reaction
yields were excellent, indicating that the Aliquat 336, granular
PTFE and water are highly recyclable in this reduction system.
General procedure for the reduction of nitroarenes
Table 6 Recycling of Aliquat 336, granular PTFE and water in the
reduction of imine 1ka
Zn powder (7 equiv.) was added to a mixture of nitroarene
(3a–3k, 2 g), Aliquat 336 (0.06 equiv.), NH4Cl (2 equiv.), granular
PTFE (5 g) and H2O (20 mL). The reaction mixture was
mechanically stirred47 for 0.25–13 h at room temperature. TLC
(thin layer chromatography) was used to determine when the
reaction was complete. The aqueous solution was decanted and
the solid residue was extracted with a small amount of ethyl
acetate followed by acidied with conc. HCl to give amine
hydrochlorides: 4a (87%), 4b (92%), and 4e (90%). Alternatively
Cycle No.
1
2
3
4
5
6
Zn (equiv.)
Time (h)
5
2.83
88
5
2
86
5
1.17
85
5
1.33
82
4
1.08
83
3
1.08
81
Yieldb (%)
a
b
All conversions were 100%. Isolated yields. Reaction conditions: 1k
(200 mg), 5% aq. NaOH (10 mL), Aliquat 336 (0.06 equiv.), Zn powder,
and granular PTFE (5 g) at room temperature.
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aer ethyl acetate was evaporated the product was crystallized 15 C. N. Rao and S. Hoz, J. Am. Chem. Soc., 2011, 133, 14795–
with ethanol to give 4c (86%), and 4d (88%) or distillation under 14803.
vacuum to give 4f (82%), 4g (88%), 4h (86%), 4i (92%), 4j (91%), 16 X. Yang, L. Zhao, T. Fox, Z.-X. Wang and H. Berke, Angew.
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Conclusions
In conclusion, we have developed a new method using Aliquat 18 W. A. Silva, M. T. Rodrigues Jr, N. Shankaraiah,
336, granular PTFE and Zn powder for the reduction of high
melting-point imines and nitroarenes in aqueous media at
R. B. Ferreira, C. K. Andrade, R. A. Pilli and L. S. Santos,
Org. Lett., 2009, 11, 3238–3241.
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inexpensive, and safe, this procedure should be easily adaptable
for industrial production.
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