TfOH/C-catalyzed one-pot three-component synthesis of α-amino phosphonates under solvent-free conditions

Article abstract of DOI:10.1007/s13738-012-0200-6

The TfOH/C was readily prepared via simple absorption of triflic acid onto activated charcoal. This solid acid was used as an efficient catalyst for the synthesis of α-aminophosphonates through the Kabachnik-Fields reaction of carbonyl compound, amine and

Full text of DOI:10.1007/s13738-012-0200-6

J IRAN CHEM SOC (2013) 10:677–684
DOI 10.1007/s13738-012-0200-6
ORIGINAL PAPER
TfOH/C-catalyzed one-pot three-component synthesis
of a-amino phosphonates under solvent-free conditions
•
Abbas Ali Jafari Sakineh Amini Fatemeh Tamaddon
•
Received: 28 July 2012 / Accepted: 30 November 2012 / Published online: 25 January 2013
Ó Iranian Chemical Society 2013
Abstract The TfOH/C was readily prepared via simple
absorption of triflic acid onto activated charcoal. This solid
acid was used as an efficient catalyst for the synthesis of
a-aminophosphonates through the Kabachnik–Fields reac-
tion of carbonyl compound, amine and diethyl phosphite
under solvent-free conditions.
and free-flowing powder with superior thermal and
mechanical stability under catalytic conditions [3–10]. To
the best of our knowledge, there has been no report on the use
of unsupported and supported TfOH as catalyst to promote
the synthesis of a-aminophosphonates via the Kabachnik–
Fields reaction.
From organic chemistry to biology, molecules contain-
ing phosphonate moiety have attracted the interest of
organic chemists for their structural modifications and
synthetic developments [11–13]. a-Aminophosphonates are
important in medicinal chemistry, since they are consid-
ered as structural analogs of the corresponding a-amino
acids and their utilities as enzyme inhibitors, antibiot-
ics, pharmacological agents and peptidomimetics which are
well documented [14–16]. Among the various synthetic
approaches for their preparation, the Kabachnik–Fields
reaction is established as the most useful method. This one-
pot procedure has been carried out using InCl3 [17],
In(OTf)3/MgSO4 [18], GaI3 [19], BiCl3 [20], I2 [21],
SbCl3/Al2O3 [22], metal perchlorates [23–27], metal tri-
flates [28], CeCl3 [29], Na2CaP2O7 [30], Sulfamic acid
[31], TFA [32], (bromodimethyl)sulfonium bromide [33],
TsCl [34], Amberlyst-15 [35], Cd(ClO4)2.xH2O [36], ZnO
nanoparticle [37], NaHSO4–SiO2 [38] and acidic ionic
liquids [39]. A few catalyst-free as well as solvent-free
protocols have been developed which necessitates thermal
[40–42], ultrasonic [43] or MW [44, 45] activation. How-
ever, some of the reported procedures are associated with
disadvantages like costly and moisture-sensitive catalysts,
use of trialkylphosphite as phosphorus nucleophile with a
nasty smell, use of additional reagents and long reaction
times. Furthermore, in all of the reported methods, a non-
environmental-friendly organic solvent was uses as reaction
media and/or for workup. Keeping in view the increasing
importance of a-aminophosphonates in pharmaceutical/
Keywords a-Aminophosphonates ꢀ Kabachnik–Fields
reaction ꢀ Three-component synthesis ꢀ Solvent free ꢀ Triflic
acid supported on carbon
Introduction
Solid-supported acids are unique catalysts that have
become popular over the last two decades. The activity and
selectivity of a catalyst dispersed on the surface of a sup-
port are improved, as its effective surface area is increased
significantly and hence is more effective than the individ-
ual acidic catalysts [1, 2]. Low toxicity, moisture resis-
tance, air tolerance and low prices are other common
features that make the use of solid-supported reagents
attractive alternatives to conventional Lewis acids or metal
triflates.
Triflic acid (TfOH) absorbed on solid inorganic or organic
support has received considerable attention as an inexpen-
sive, recyclable, easily handled, low toxic, non-corrosive
Electronic supplementary material The online version of this
article (doi:10.1007/s13738-012-0200-6) contains supplementary
material, which is available to authorized users.
A. A. Jafari (&) ꢀ S. Amini ꢀ F. Tamaddon
Department of Chemistry, College of Sciences,
Yazd University, Yazd 89195-741, Iran
e-mail: Jafari@yazduni.ac.ir
123

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J IRAN CHEM SOC (2013) 10:677–684
Scheme 1 TfOH/C catalyzed
synthesis of
a-aminophosphonates
O OEt
P
R2
H
OEt
CHO
HN
NH2
TfOH/C (10 mol %)
O
P
OEt
Solvent-free
R1
R2
OEt
EtO
HO
P
r.t.
R1
OEt
industrial chemistry, there is a need to develop their
synthesis. Therefore, in continuation of our researches on
developing environmentally benign methodologies [29, 46–
48], herein we describe a convenient and simple procedure
via a one-pot reaction of aldehydes, amines and diet-
hylphosphonate for the preparation of a-aminophospho-
nates using trifluoromethanesulfonic acid supported on
carbon (TfOH/C) under solvent-free conditions as a reus-
able and efficient solid catalyst (Scheme 1).
phosphonates were achieved in high to excellent yields
(85–98 %).
Whenaldehydes withelectron-donatinggroupsand aniline
withelectron-withdrawinggroupswereused, the reactionwas
performed with shorter reaction times and higher yields
(Table 2, entries 5, 6, 11, 12, 13, 14, 16, 18 and 19). In addi-
tion, new diethyl(3-methylphenyl)(phenylamino) methyl
phosphonate (3c), diethyl (3,4-dihydroxyphenyl)(phenyla-
mino)methyl phosphonate (3h) and diethyl(4-bromophe-
nylamino) (3,4 dimethoxyphenyl) methyl phosphonate (3m)
were produced under the same conditions to show the selec-
tivity and the scope of this green procedure. In all cases, after
the reaction completion, the reaction mixture was dissolved in
ethanol. The catalyst was filtered off and the pure product was
isolated by adding ice-water to stirring residue and simple
filtration. Thus the workup procedure is very simple and the
use of choromatography and harmful organic solvents for
isolation and purification of products is not necessary.
Cyclohexanone as a ketone was also coupled with aniline and
diethylphosphite to produce the corresponding a-amino-
phosphonates in 70 % isolated yield after 10 h.
Results and discussion
At first, a one-pot three-component reaction of benzalde-
hyde (2.0 mmol), aniline (2.0 mmol) and diethylphosphite
(2.5 mmol) was studied as a model reaction in the presence
of supported TfOH catalyst at room temperature. As the
results show, 10 mol% of TfOH/C was the most effective
catalyst for this reaction, which led to the production of the
desired diethyl phenyl (phenyl amino)methyl phosphonate
in 97 % isolated yield after 3 h. In the presence of solvents,
the product was achieved in low yield overnight (Table 1,
entries 5–7). To show the merit of the supported catalyst, a
similar reaction was studied in the presence of unsupported
TfOH and charcoal and in the absence of any catalyst
(Table 1, entries 8–10). These results showed the strong
synergistic effect of charcoal on the catalytic activity of
TfOH in this reaction (Table 1).
Furthermore, the possibility of this method for prepa-
ration of bis(a-aminophosphonates) was investigated.
Thus, the reaction of 4,40-methylenedianiline (1.0 mmol)
with benzaldehyde (2.0 mmol) and diethyl phosphite
(2.5 mmol) was completed in 5 h to produce the desired
bis(a-aminophosphonate)(3u) in 90 % yield (Scheme 2).
To show the efficiency of this protocol for Kabachnik–
Fields reaction, the catalytic activity of TfOH/C with some of
the other recent catalysts was compared. As demonstrated in
Table 3, TfOH/C is an effective catalyst for performing this
reaction at ambient temperature and under solvent-free
conditions.
Under the optimized conditions, the scope of this
method was examined for various aromatic aldehydes and
amines. By this method, the corresponding a-amino
Table 1 Optimization of the reaction conditions
In conclusion, we have succeeded in developing TfOH/
C as an effective heterogeneous TfOH catalyst for one-pot
three-component reaction of aldehyde, amine and diet-
hylphosphite. This novel catalyst provides a clean and
convenient alternative for the Kabachnik–Fields reaction in
view of the following advantages. The reaction proceeds
smoothly in the presence of moisture under solvent-free
condition and produces a-aminophosphonate in high to
excellent yield at room temperature. The catalyst is stable
and a non-polluting solid that offers easy handling and
ready workup. In addition, no harmful organic solvent was
used as reaction media or in the workup procedure.
Entry Solvent Catalyst (mol%)
Time (h) Conversion (%)
1
2
–
–
–
–
PEG-SO3H (5)
TfOH/SiO2 (5)
TfOH/C (5)
6
6
40
80
3
6
98
4
TfOH/C (10)
3
100
5
CH3CN TfOH/C (10)
CH2Cl2 TfOH/C (10)
Ethanol TfOH/C (10)
24
24
24
6
70
6
30
7
60
8
–
–
–
TfOH (10)
80
9
Activated charcoal 24
24
Trace
10
–
N.R. [31, 54]
123

J IRAN CHEM SOC (2013) 10:677–684
679
Table 2 TfOH/C-catalyzed
synthesis of a-amino
phosphonates under solvent-free
Time Yield
Entry
Aldehyde
Amine
Product
M. P.
Ref.
[49]
(h)
(%)
conditions at room temperature
O
H
NH2
HN
88-89
PO(OEt)2
1
3
97
(87-89)
2a
2a
1a
3a
O
NH2
HN
H
99-100
(64-65)
H3C
PO(OEt)2
2
3
5
5
95
[50]
H3C
3b
1b
O
H
HN
NH2
PO(OEt)2
90
80-82
CH3
CH3
2a
1c
3c
O
HN
H
NH2
PO(OEt)2
4
5
3
90
94
121
[51]
[52]
2a
1d
1e
3d
O
104-105
NH2
H
HN
2.25
H3CO
(102-103)
PO(OEt)2
H3CO
2a
3e
HN
NH2
O
H3CO
PO(OEt)2
OCH3
H3CO
H
6
7
1.5
90
96
126-127
[29]
[53]
OCH3
1f
2a
2a
3f
O
H
NH2
HN
PO(OEt)2
HO
4
88-89
HO
1g
3g
123

680
J IRAN CHEM SOC (2013) 10:677–684
Table 2 continued
Time Yield
Entry
Aldehyde
Amine
Product
M. P.
Ref.
(h)
6
(%)
85
O
HO
HO
NH2
HN
H
HO
HO
PO(OEt)2
8
137-138
2a
2a
2a
1h
3h
O
NH2
HN
H
84-85
PO(OEt)2
F
9
9
85
[54]
[53]
F
(78-79)
1i
3i
O
NH2
HN
H
68-69
PO(OEt)2
Cl
10
4.5
87
Cl
(58-59)
1j
3j
Br
O
NH2
HN
H
123-124
Br
PO(OEt)2
11
2
93
[52]
[41]
(121-123)
2b
1a
3k
Br
NH2
O
HN
109-110
H
Br
12
13
2
1
92
95
PO(OEt)2
H3CO
(107-108)
H3CO
1e
2b
2b
3l
Br
O
NH2
HN
H
PO(OEt)2
Br
92-94
H3CO
OCH3
H3CO
OCH3
1k
3m
Cl
Cl
O
H
HN
130-131
(129)
14
H2N
1.25
98
[29]
PO(OEt)2
1a
2c
3n
123

J IRAN CHEM SOC (2013) 10:677–684
681
Table 2 continued
Time Yield
Entry
Aldehyde
Amine
Product
M. P.
Ref.
[41]
(h)
(%)
Cl
O
O
H
HN
H
123-124
Cl
PO(OEt)2
15
3
90
Cl
(117-118)
Cl
2d
2e
1j
1i
3o
NH2
NO2
O
114-116
(115)
HN
O2N
H
16
17
2.5
93
85
[55]
[54]
PO(OEt)2
H3CO
H3CO
3p
NO2
O
NH2
NH2
NH2
HN
H
138-140
O2N
PO(OEt)2
Cl
10
Cl
(130-132)
2f
1e
1a
3q
NO2
O2N
O
H
127-128
HN
18
1.5
89
[53]
PO(OEt)2
(129-130)
2g
3r
NO2
O2N
O
HN
151-152
H
PO(OEt)2
19
20
1
86
70
[54]
H3CO
H3CO
(150-152)
1e
O
2g
2a
3s
NH2
HN
PO(OEt)2
10
108 (104) [53]
1l
3t
123

682
J IRAN CHEM SOC (2013) 10:677–684
Table 3 Comparison of the effect of catalysts in the reaction of benzaldehyde, aniline and diethylphosphite
Entry
Catalyst (mol%)
Solvent
Temperature (°C)
Time (h)
Yield (%)
References
1
2
CF3CO2H (25)
b-CD (10)
–
r.t.
100
r.t.
50
r.t.
r.t.
60
80
80
r.t.
60
60
r.t.
24
24
11
3.5
10
20
0.75
4
96
61
92
90
93
90
92
86
90
91
100
95
97
[31]
H2O
[56]
3
InCl3 (10)
THF
[17]
4
TiO2 (20)
–
[53]
5
Bi(NO3)3ꢀ5H2O (10)
TaCl5–SiO2 (10)
FeCl3
–
[57]
6
CH2Cl2
THF
CH3CN
H2O
CH3CN
–
[58]
7
[59]
8
Mesoporous alumino silicate nano cage
[Cu(3,4-tmtppa)](MeSO4)4
SbCl3/Al2O3 (5)
Al(ClO4)3 (5)
[60]
9
1
[61]
10
11
12
13
3
[22]
12
0.5
3
[54]
NbCl5 (5)
–
[55]
TfOH/C (10)
–
This method
O
O
TfOH/C (10 mol %)
H
P OEt
OEt
+
NH2
+
H
H2N
(EtO)2OP
N
N
H
PO(OEt)2
H
2h
2a
3u
Scheme 2 The reaction of 4,40-methylenedianiline with benzaldehyde and diethyl phosphite
Experimental
consequently washed with water to achieve good to
excellent yields.
Preparation of carbon-supported triflic acid (TfOH/C)
Selected spectral data for new compounds
The activated carbon (4.0 g) was activated by refluxing in
HNO3 (100 mL, 5.0 M) for 6 h. Activated carbon was
filtered off and washed with deionized water until pH 6–7.
The residue was dried at 120 °C for 12 h [62]. Then, triflic
acid (3.0 mmol) was added dropwise to activated carbon
(3.0 g) and the mixture was stirred magnetically for 6 h at
room temperature to yield TfOH/C (1.0 mmol/g).
Diethyl(3-methylphenyl)(phenylamino)methyl phosphonate
(3c): white solid; m.p.: 80–82 °C; FT-IR: vmax = 3,291 (NH
stretching), 2,984 (CH stretching), 1,603 (C=C stretching),
1,521 (NH bending), 1,499 (C=C stretching), 1,232 (P=O
stretching), 1,021 (C–O stretching), 963 (C–O stretching),
1
744, 691 cm-1; H-NMR(400 MHz, CDCl3): d = 1.12(t,
J = 6.6 Hz, 3H, OCH2CH3), 1.3(t, J = 6.6 Hz, 3H,
OCH2CH3), 2.34(s, 3H, Ar-CH3), 3.63–3.69 (m, 1H,
OCH2CH3), 3.91–3.97 (m, 1H, OCH2CH3), 4.09–4.15 (m,
2H, OCH2CH3), 4.73 (d, J = 25.2 Hz, 1H, HC-P), 4.8 (br,
1H, NH), 6.61 (d, J = 7.2 Hz, 2H, Ar–H), 6.7 (t, J = 7.2 Hz,
1H, Ar–H), 7.07–7.14 (m, 3H, Ar–H), 7.22–7.29 (m, 3H, Ar–
H) ppm; 13C-NMR (100 MHz, CDCl3): d = 16.21(d,
General procedure for the preparation of a-
aminophosphonates
TfOH/C (0.20 mmol, 0.20 g) was added to a stirring
mixture of benzaldehyde (2.0 mmol), aniline (2.0 mmol)
and diethylphosphite (2.5 mmol) at room temperature for
an appropriate time (Table 2). The progress of the reaction
was monitored by TLC using hexane and ethyl acetate as
eluent. After completion, the reaction mixture was dis-
solved in hot EtOH. The catalyst was filtered off and the
ethanol volume was decreased. The pure products were
solidified by adding ice water and, after a simple filtration,
3
3JPC = 5.5 Hz), 16.46 (d, JPC = 5.5 Hz), 21.5, 56 (d,
2
JCP = 149 Hz), 63.2 (d, JPC = 7.5 Hz), 63.3 (d, JPC
2
=
7.5 Hz), 113.5, 113.8, 118.3,125, 128.5, 128.8, 129.2, 135.8,
138.2, 146.5(d, 3JPC = 14 Hz) ppm.
Diethyl(3,4 dihydroxyphenyl)(phenylamino) methyl
phosphonate(3 h): white solid; m.p.: 137–138 °C; FT-IR:
123

J IRAN CHEM SOC (2013) 10:677–684
683
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methyl phosphonate (3m): white solid; m.p.: 92–94 °C;
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(C=C stretching), 1,229 (P=O stretching), 1,143, 1,020 (C–
O stretching), 973 (C–O stretching) cm-1
;
1H-NMR
(400 MHz, CDCl3): d = 1.13 (t, J = 7 Hz, 3H
OCH2CH3,), 1.29 (t, J = 7 Hz, 3H, OCH2CH3), 3.66–3.7
(m, 1H, OCH2CH3), 3.85 (s, 3H, OCH3), 3.86 (s, 3H,
OCH3), 3.86–3.97 (m, 1H, OCH2CH3), 4.07–4.13 (m, 2H,
OCH2CH3), 4.62 (dd, J1 = 25.6 Hz, J2 = 7.2 Hz, 1H,
NH-CH-P), 4.81–4.85(dd, J1 = J2 = 7.2 Hz, 1H, NH),
6.48 (d, J = 8.4 Hz, 2H, Ar–H), 6.82 (d, J = 8.4 Hz, 1H,
Ar–H), 6.96 (br s, 2H, Ar–H), 7.18 (d, J = 8.4 Hz, 2H,
Ar–H) ppm; 13C-NMR (100 MHz, CDCl3): d = 16.39 (d,
3
6 Hz), 16.4 (d, JPC = 6 Hz), 55.7 (d, JCP
3JPC
=
=
2
152 Hz), 55.8, 55.9, 63.2 (d, JPC = 7 Hz), 63.3 (d,
2JPC = 7 Hz), 110, 110,7 111, 115.5, 120.2, 127.6, 131.8,
3
145.5 (d, JPC = 15 Hz), 148.7, 149.1 ppm.
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