Ritter reaction in subcritical water: An efficient and green method for amides synthesis

Article abstract of DOI:10.2174/157017812799304006

Ritter reaction was carried out efficiently in subcritical water with catalytic amount of trifluoromethanesulfonic acid. The amides were formed in good to excellent yields from secondary alcohols and tert-butanol with various nitriles.

Full text of DOI:10.2174/157017812799304006

24
Letters in Organic Chemistry, 2012, 9, 24-28
Ritter Reaction in Subcritical Water: An Efficient and Green Method for
Amides Synthesis
Shengqian Jiang, Zhouyu Wang*, Zhenju Jiang, Jianhui Li, Shulin Zhou and Long Pu
Department of Pharmaceutics Engineering, Xihua University, Chengdu, 610039, P.R. China
Received June 06, 2011: Revised September 02, 2011: Accepted September 16, 2011
Abstract: Ritter reaction was carried out efficiently in subcritical water with catalytic amount of trifluoromethanesulfonic
acid. The amides were formed in good to excellent yields from secondary alcohols and tert-butanol with various nitriles.
Keywords: Alcohol, amide, nitrile, ritter reaction, subcritical water, trifluoromethanesulfonic acid.
INTRODUCTION
physical properties. It has been used as a solvent in many
organic reactions and brought surprising and unforeseen
results [15]. In this paper, we wish to report a catalytic,
efficient and green Ritter reaction in subcritical water. This
is the first time that the subcritical water was used as a
solvent to get amides from alcohols.
The synthesis of amide through the reaction of nitrile
with alcohol or alkene in the presence of stochiometric
amount of sulfuric acid (H2SO4) is the typical Ritter reaction
[1]. Since the original report in 1948, it has gained much
attention due to its atomic economy and application in
biologically molecules synthesis [2]. However, the large
amount of toxic and corrosive acids is the major drawback
which results in environmental pollution and applicability
limitation. Thus, a number of methods have been explored to
decrease the amount of the acids in recent years. Catalytic
Ritter reaction has been realized by cobalt(II) chloride [3],
Fe3+-montmodlonite [4], 2,4-dinitrobenzenesulfonic acid
(DNBSA) [5], boron trifluoride [6], perfluorinated sulfonic
acid resin (Nafion-H) [7], silica sulfuric acid [8], o-
Benzenedisulfonimid [9], ferric trichloride [10], Iodine [11]
and so on [12]. These catalytic protocols are efficient and
cost-saving. But the longer reaction time, the limited
substrate spectrum, the using of one of the substrates (nitriles
or alcohols) or other toxic organic solvent as the reaction
medium are the common disadvantages. So it is still
desirable to develop more effective, economical and green
Ritter reaction.
RESULTS AND DISCUSSION
The reaction of 1-phenylethanol (1a) with acetonitrile
was taken as a model reaction to develop the optimum
reaction conditions. Firstly, we carried out the reaction in the
absence of any additional catalysts in water at 16ꢀꢁ for 5h.
But only trace product was obtained. We reasoned that the
acidity of water was not enough for promoting the Ritter
reaction which usually requires strong acid. So we added
catalytic amount of several strong acids to increase the
acidity of the reaction system. To our surprise, 36-55%
yields were obtained when 10 mol% H2SO4, 4-methylben-
zenesulfonicacid, methanesulfonic acid and trifluoromethan-
esulfonic acid (TfOH) were added (Table 1, entries 2-5).
Among these acids, the TfOH was the best catalyst. To
improve the yield, we increased the temperature to 200ꢁ.
Higher yield was obtained (Table 1, entry 6). However,
prolonging the reaction time to 10 hours, the yield of amide
was no obvious change (Table 1, entry 7). So we futher
increased the catalyst loading of TfOH to 20 mol% and the
yield of 2a was raised to 76% (Table 1, entry 8). Considering
the solubility of starting materials in water, we added 0.1
equiv sodium dodecyl sulfate (SDS) in the reaction system.
As expected, the yield was increased to 82% (Table 1, entry
9). To compare with the Firouzabadi`s catalyst, we carried
out a control catalytic Ritter reaction (Table 1, entry 10). The
results showed that the catalytic efficiency of dodecatun-
gstophosphoric acid was lower than TfOH at the same
reaction condition. We also took the reaction at reflux
condition with the dodecatungstophosphoric acid and TfOH
as the catalyst (Table 1, entries 11 and 12). But only trace
product was obtained in both reactions.
Nowadays, great attention has been drawn to the use of
water as solvent for organic synthesis [13]. Compared with
other organic solvents, water has many incomparable
advantages
such
as
inexpensive,
environmentally
compatible, nontoxic, widely available and so on. To the best
of our knowledge, there is only one report about water as the
solvent in the Ritter reaction. Firouzabadi et al. reported the
dodecatungstopsphoric
acid
(H3PW12O40)
catalyzed
amidation reaction of alcohols and protected alcohols in
water [14]. Although good to excellent yields were obtained,
the reaction time was long and the variety of alcohols was
limited. So the development of more effective, economical
and green Ritter reaction is still desirable.
As is known to all, subcritical water (150 < T < 370 ˚C,
0.4 < p < 22 MPa) possesses some special chemical and
With the optimal conditions in hand, the generality of
this protocol was explored. A series of secondary alcohols
(1a-i) and various nitriles were used in the reaction. As
shown in Table 2, the reaction of 1-(naphthalen-2-yl)ethanol
*Address correspondence to this author at the Department of Pharmaceutics
Engineering, Xihua University, Chengdu, 610039, P. R. China Tel: +86-28-
87720552; E-mails: zhouyuwang@mail.xhu.edu.cn,
zhouyuwang77@gmail.com
1875-6255/12 $58.00+.00
© 2012 Bentham Science Publishers

Ritter Reaction in Subcritical Water
Letters in Organic Chemistry, 2012, Vol. 9, No. 1
25
O
OH
HN
Water
+
MeCN
1a
2a
Scheme 1.
Table 1. Ritter Reaction of 1a and MeCN in Water with Different Catalysts
Entry
Catalyst
Temp (ꢀ)
T (h)
Yield (%) e
1
2
-
H2SO4
160
160
160
160
160
200
200
200
200
200
100
100
5
5
Trace
45
3
4-methylbenzenesulfonicacid
Methanesulfonic acid
TfOH
5
36
4
5
42
5
5
55
6
TfOH
5
62
7
TfOH
10
5
64
8 b
9 c
10 d
11 d
12 c
TfOH
76
TfOH, SDS
H3PW12O40, SDS
H3PW12O40, SDS
TfOH, SDS
5
82
5
55
5
trace
trace
5
aAlcohol (0.2 mmol), nitrile (0.8 mmol), water (0.2 mL), catalyst (10 mol%).
b20 mol% TfOH.
c20 mol% TfOH, 10 mol% SDS.
d20 mol% H3PW12O40, 10 mol% SDS.
eIsolated yield based on the alcohol.
O
OH
TfOH (20 mol%), SDS (10 mol%)
Water, 200 oC, 5 h
HN
R2
Ar
R1
R2CN
+
Ar
R1
2
1
Scheme 2.
Table 2. Ritter Reaction of Secondary Alcohols and Nitriles in Subcritical Water with TfOHa
Entry
Alcohol
Ar
R1
R2
Amideb
Yield (%) c
1
2
3
4
5
6
7
8
9
1a
1b
1c
1d
1e
1f
Ph
Me
Me
Me
Me
Me
Me
Me
Et
Me
Me
Me
Me
Me
Me
Me
Me
Me
2a1
2b1
2c1
2d1
2e1
2f1
82
86
70
73
75
72
-
2-naphthyl
2-ClC6H4
3-ClC6H4
4-ClC6H4
4-BrC6H4
4-MeOC6H4
Ph
1g
1h
1i
2g1
2h1
2i1
65
81
Ph
Ph

26 Letters in Organic Chemistry, 2012, Vol. 9, No. 1
Jiang et al.
(Table 2). Contd…..
Entry
Alcohol
Ar
R1
R2
Amideb
Yield (%) c
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
1a
1b
1c
1d
1e
1f
Ph
2-naphthyl
2-ClC6H4
3-ClC6H4
4-ClC6H4
4-BrC6H4
4-MeOC6H4
Ph
Me
Me
Me
Me
Me
Me
Me
Et
C6H5
C6H5
2a2
2b2
2c2
2d2
2e2
2f2
76
80
67
71
74
70
-
C6H5
C6H5
C6H5
C6H5
1g
1h
1i
C6H5
2g2
2h2
2i2
C6H5
62
78
71
81
68
72
74
79
trace
67
88
-
Ph
Ph
C6H5
1a
1b
1c
1d
1e
1f
Ph
Me
Me
Me
Me
Me
Me
Me
Et
C6H5CH2
C6H5CH2
C6H5CH2
C6H5CH2
C6H5CH2
C6H5CH2
C6H5CH2
C6H5CH2
C6H5CH2
CH2=CH
2a3
2b3
2c3
2d3
2e3
2f3
2-naphthyl
2-ClC6H4
3-ClC6H4
4-ClC6H4
4-BrC6H4
4-MeOC6H4
Ph
1g
1h
1i
2g3
2h3
2i3
Ph
Ph
1a
Ph
Me
2a4
aAlcohol (0.2 mmol), nitrile (0.8 mmol), water (0.2 mL), catalyst (20 mol%), SDS (10 mol%).
bAmides (entry 1-20, 23, 25, 27) are known and compared with authentic samples [refs. 5, 11, 12(e), 17].
cIsolated yield based on the alcohol.
O
TfOH (20 mol%), SDS (10 mol%)
OH
HN
R2
R2CN
+
Water, 200 oC, 5 h
1j
2j
Scheme 3.
Table 3. Ritter Reaction of Tertiary Butanol and Nitriles in Subcritical Water with TfOHa
Entry
Alcohol
R2
amidesb
Yield (%) c
1
2
3
4
Me
2j1
2j2
2j3
2j4
71
81
74
-
OH
C6H5
C6H5CH2
CH2=CH
aAlcohol (0.2 mmol), nitrile (0.8 mmol), water (0.2 mL), catalyst (20 mol%), SDS (10 mol%).
bAmides (entry 1-3) are known [11, 12(f)].
cIsolated yield based on the alcohol.
1b and acetonitrile gave the desired amides with good yield
(86 %, Table 2, entry 2). The electron-deficient benzylic
alcohols 1c-f also reacted well with acetonitrile and 70%,
73%, 75%, 72% yields were obtained respectively (Table 2,
entries 3-6). However, it should be noted that the electron-
rich benzylic alcohol 1g gave no product of amide (Table 2,
entry 7) [16]. The reason for this is unclear. Bulky alcohols
such as 1-phenylpropan-1-ol and diphenylmethanol were
also appropriate substrates for this protocol. Bigger steric
hindrance in 1-phenylpropan-1-ol give lower yield of 2h1
(65%, Table 2, entry 8). The existence of two phenyl groups
made the intermediate carbenium ions more stable which
lead to higher yield of 2i1 (81%, Table 2, entry 9). Different
nitriles such as benzonitrile and 2-phenylacetonitrile were
also proved to be efficient in the reaction. The desired
products were obtained with moderate to good yields. (Table

Ritter Reaction in Subcritical Water
Letters in Organic Chemistry, 2012, Vol. 9, No. 1
27
2, entries 10-15, 17-18, 19-24, 26-27). The acrylonitrile
however was an unsuitable substrate for this protocol and the
white polymer product was found (Table 2, entry 28).
mixtures were cooled to room temperature, the crude
solution was extracted with ethyl acetate (3 x 10 mL). The
combined organic layers were washed with brine and dried
over anhydrous Na2SO4. After removal of solvents under
reduced pressure, the residue was purified through column
chromatography on silica gel (eluent: hexane/EtOAc = 4/1)
to give the pure products.
In respect that the products N-tert-butyl amides are
important amine precursors in pharmaceuticals, we carried
out the reactions between tertbutyl alcohol 1j and nitriles.
The results are summarized in Table 3. Except for the
acrylonitrile, the other three nitriles were worked smoothly.
The desired N-tertbutyl-amides were obtained in 71%, 81%
and 74% respectively.
ACKNOWLEDGEMENTS
We are grateful for the financial supports from the
National Natural Science Foundation of China (21102115),
the youth founds of Sichuan Province (2010JQ0048),
Sichuan Education Department project (09ZA110) and the
Key Laboratory of Natural Medicine and biotechnology,
Xihua University (XZD0821-09-1).
Mechanistically, we think that the mechanism of this
protocol follows the traditional way. The alcohol firstly
generates the carbenium ion or ether (which can be translated
to the carbenium ion) under the effect of acid. Then the
carbenium ion reacts with the nitrile to produce a nitrilium
ion. The latter is trapped by water resulting in amides. Why
only trace products were obtained at reflux condition? We
thought that the subcritical water plays an important role.
Firstly, the subcritical water’s dielectric constant is much
lower than the ambient water which causes the solubility of
organics to increase. Secondly, the subcritical water has a
higher dissociation constant than the ambient water which
makes it easy to ionize and increase the acidity of the
reaction system greatly. These two reasons make the
differences.
SUPPLEMENTARY MATERIAL
Supplementary material is available on the publishers
Web site along with the published article.
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EXPERIMENTAL
All starting materials were of the commercially available
(analytical grade) and used without further purification.
Reactions were monitored by thin layer chromatography
using silica gel HSGF254 plates. Flash chromatography was
performed using silica gel HG/T2354-92. Melting poits were
1
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solvent resonance as the internal standard (CDCl3, ꢀ 77.0
ppm). ESIMS spectra were recorded on BioTOF Q.
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mmol), trifluoroacetic acid (0.2mmol) and water (2 mL) was
added to a sealed tube and heated to 200°C. Then the
temperature was maintained for five hours. After the reaction
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[17]
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