Zn(OAc)2·2H2O-catalyzed synthesis of α-aminophosphonates under neat reaction

Article abstract of DOI:10.1002/hc.20765

A series of novel α-aminophosphonates have been prepared by one-pot three-component condensation in the presence of a zinc acetate (Zn(OAc) _2·2H₂O) catalyst at 50°C under solvent-free conditions with excellent yields. The major advan

Full text of DOI:10.1002/hc.20765

Heteroatom Chemistry
Volume 23, Number 2, 2012
•
Zn(OAc) 2H O-Catalyzed Synthesis of
2
2
α-Aminophosphonates under Neat Reaction
Uma Ravi Sankar Arigala, Chaitanya Matcha, and Kuk Ro Yoon
Department of Chemistry, College of Life Science and Nano Technology, Hannam University,
Daejeon 305-811, Korea
Received 20 July 2011; revised 21 August 2011, 2 September 2011
three-component synthesis starting from aldehydes,
ABSTRACT: A series of novel α-aminophosphonates
amines, and diethylphosphite or triethylphosphite
has been reported using Lewis and Brønsted
acid catalysts such as LiClO4 [6], InCl3 [7], lan-
thanide triflates/magnesium sulfate [8], TaCl5–SiO2
[9], Amberlyst-15 [10], Al2O3-MW [11], sulfonic acid
[12], scandium tris(-dodecyl sulfate) [13], M(OTf)n
have been prepared by one-pot three-component
condensation in the presence of a zinc acetate
◦
•
(Zn(OAc)2 2H2O) catalyst at 50 C under solvent-free
conditions with excellent yields. The major advantages
of the present method are high yields, a short reaction
time, and solvent-free reaction conditions. Their an-
timicrobial activity was evaluated, and some of them
•
[14], and M(ClO4)n [15]. More recently, CoCl2 6H2O
[16], ytterbium perfluorooctanate [Yb(PFO)3] [17],
ꢀ
C
showed significant activity.
2011 Wiley Periodicals,
•
nano Fe3O4 [18], Mg(ClO4)2[19], and BF3 SiO2[20]
Inc. Heteroatom Chem 23:160–165, 2012; View this article
online at wileyonlinelibrary.com. DOI 10.1002/hc.20765
were reported to be effective catalysts for the for-
mation of α-aminophosphonates using a three-
component system composed of aldehydes/ketones,
amines, and triethylphosphite under neat condi-
tions. However, the above-mentioned catalysts have
several disadvantages, such as a long reaction time,
low yield of the products, requiring a stoichiometric
amount of catalysts, costly, moisture-sensitive, cor-
rosive, toxic or volatile catalysts, and generating a
INTRODUCTION
α-Aminophosphonates have attracted much atten-
tion, owing to their biological activities. Their
utilities as enzyme inhibitors, antibiotics, pep-
tide mimics, herbicides, pharmacological agents,
and many other applications are well documented
[1–5]. A number of synthetic methods for the
preparation of α-aminophosphonates have been
carried out under various conditions. However,
the one-pot synthesis of α-aminophosphonates re-
mains a favorable method because of its versa-
tile route and high yielding reactions. Recently, the
•
large amount of waste products. Zn(OAc)2 2H2O is a
bench-top catalyst that is easy to handle, cheap, read-
ily available, ecofriendly, and versatile. Additionally,
it enables better accessibility of the reactants to the
active sites and is efficient for the promotion of many
acid-catalyzed organic reactions [21,22].
Hence, there is a need to develop a convenient,
environmentally benign, and practicably feasible
method for the synthesis of α-aminophosphonates.
In this article, we report for the first time an efficient
and environmentally benign protocol for the synthe-
sis of α-aminophosphonates (4a–l) by the condensa-
tion of various aldehydes, amines, and phosphites.
The method is a simple, one-pot, practical proto-
col for the synthesis of α-aminophosphonates in the
Correspondence to: Kuk Ro Yoon; e-mail: kryoon@hnu.kr.
Contract grant sponsor: National Research Foundation of
Korea. Grant funded by the Korean Government (MEST).
Contract grant number: NRF-2011-0005587.
Contract grant sponsor: Hannam University Research Fund.
2011 Wiley Periodicals, Inc.
c
ꢀ
160

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Zn(OAc)2 2H2O-Catalyzed Synthesis of α-Aminophosphonates under Neat Reaction 161
SCHEME 1 Synthesis of α-aminophosphonates 4a–1.
◦
•
presence of catalyst Zn(OAc)2 2H2O at 50 C under
solvent-free conditions.
tonitrile, dichloromethane, and ethanol were used
for the model reaction. The desired product was ob-
tained with 55, 65, 69, 70, 74, and 75% yields, re-
spectively, after a prolonged reaction time of 4 h,
whereas the neat condition afforded a product with
an excellent (94%) yield within a very short time
of 15 min. This shows that the use of solvent re-
tards the rate of reaction, which decreases the yield
of product. The results obtained are summarized in
Table 2.
RESULTS AND DISCUSSION
Initially, we attempted a three-component coupling
of p-hydroxybenzaldehyde (1), 2,4-dichloroaniline
(2), and dimethyl phosphite (3) in the presence of a
•
catalytic amount (12 mol%) of Zn(OAc)2 2H2O under
solvent-free conditions at 50◦C, and the desired prod-
uct, 4a, was obtained with a 94% yield (Scheme 1).
We have studied the catalyst concentration on the
model reaction. Various concentrations of the cata-
lyst 2, 4, 6, 8, 10, 12, and 14 mol% were used. The
results revealed that when the reaction was carried
out in the presence of 2, 4, 6, and 8 mol% of the
catalyst, it gave a lower yield of the product, even
after a prolonged reaction time. At the same time,
when the concentration of the catalyst was 12 or 14
mol%, an excellent yield of products was obtained
in a short span of time. Even after increasing the
catalyst concentration, the yield of the products was
found to be constant. Therefore, the use of 12 mol%
After optimizing the conditions, the generality
of this method was examined by the reaction of
several substituted benzaldehydes, anilines, and
•
diethyl phosphite using Zn(OAc)2 2H2O as the
catalyst under neat reaction conditions at 50◦C.
The results are presented in Table 3. Here, we
carried out a similar reaction with various aro-
matic aldehydes containing electron-donating or
electron-withdrawing functional groups at different
positions, but it did not show any remarkable differ-
ence in the yields of product and reaction time. This
result provided an incentive to extend this process to
various substrates. This methodology offers a signif-
icant improvement with regard to the scope of this
transformation, simplicity in operation, and green
aspects by avoiding expensive or corrosive catalysts.
The synthetic, analytical, and spectral data of α-
aminophosphonates (4a–l) are given in Tables 4–7.
•
of Zn(OAc)2 2H2O is sufficient to push the reaction
forward. The results obtained are summarized in
Table 1.
To evaluate the effect of the solvent, various sol-
vents such as tetrahydrofuran, dioxane, toluene, ace-
TABLE 1 Influence of the Catalyst on the Synthesis of
α-Aminophosphonates 4a–la
TABLE 2 Influence of the Solvent on the Synthesis of
α-Aminophosphonatesa
Entry
Catalyst (mol%)
Yield (%)b
Entry
Solvents
Yield (%)b
Time
1
2
3
4
5
6
7
2
4
6
55
66
67
70
80
94
94
1
2
3
4
5
6
7
Tetrahydrofuran
Dioxane
55
65
69
70
74
75
94
4 h
4 h
4 h
4 h
4 h
4 h
15 min
Toluene
8
Acetonitrile
Dichloromethane
Ethanol
10
12
14
Solvent-free
aReaction of p-hydroxybenzaldehyde, p-bromoaniline, and diethyl
aReaction of p-hydroxybenzaldehyde, p-bromoaniline, and diethyl
•
•
phosphite in the presence of Zn(OAc)2 2H2O under solvent-free con-
phosphite catalyzed by Zn(OAc)2 2H2O under solvent-free conditions
ditions at 50◦C for 15 min.
bIsolated yield.
at 50◦C for 15 min.
bIsolated yield.
Heteroatom Chemistry DOI 10.1002/hc

162 Arigala, Matcha, and Yoon
•
TABLE 3 Zn(OAc)2 2H2O Catalyzed Synthesis of α-Aminophosphonates 4a–l
Compound
R
Rꢁ
Rꢁꢁ
Time (min)
20
Yield (%)
94
4a
CH3
OH
4b
4c
C2H5
C6H5
OH
OH
18
22
90
85
4d
4e
4f
CH3
C2H5
C6H5
CH3
OH
OH
OH
OH
21
16
19
17
92
88
89
93
4g
4h
4i
C2H5
C6H5
OH
OH
15
18
94
93
4j
CH3
C2H5
C6H5
OMe
OMe
OMe
18
15
20
92
92
90
4k
4l
EXPERIMENTAL
ing KBr disks on a Nicolet-380 FT-IR spectrophoto-
1
meter. H, 13C, and 31P NMR spectra were recorded
Melting points were determined in open capillary
tubes on a Mel-temp apparatus and were uncor-
rected. IR spectra (νmax in cm−1) were recorded us-
on an AMX 400 MHz spectrometer operating at
1
400 MHz for H, 100 MHz for 13C, and 161.9 MHz
for 31P using deuterated chloroform as the solvent.
Heteroatom Chemistry DOI 10.1002/hc

•
Zn(OAc)2 2H2O-Catalyzed Synthesis of α-Aminophosphonates under Neat Reaction 163
TABLE 4 Synthetic, Analytical, Infrared, and 31P NMR Spectral Data of α-Aminophosphonates 4a–l [23,24]
Elemental Analysis (%)
IR (cm−1
)
31P NMR/
NH P O P Caliphatics δ (ppm)
Compound Molecular Formula MP (◦C)
C
H N
4a
4b
4c
4d
4e
4f
4g
4h
4i
C15H16 Cl2NO4P 142–144 47.81 (47.89) 4.23 (4.29) 3.69 (3.72) 3386 1215
C17H20 Cl2NO4P 138–140 50.45 (50.51) 4.96 (4.99) 3.42 3.47) 3394 1222
C25H20 Cl2NO4P 150–152 59.95 (60.02) 3.99 (4.03) 2.76 2.80) 3341 1223
759
749
763
756
756
760
745
786
757
757
753
759
25.10
24.83
24.55
25.44
25.82
24.95
24.63
24.85
24.52
26.94
26.75
26.88
C
19H20NO4P
135–137 63.82 (63.86) 5.60 (5.64) 3.89 3.92) 3397 1224
175–178 65.41 (65.45) 6.23 (6.28) 3.59 3.63) 3339 1209
162–164 72.30 (72.34) 5.00 (5.03) 2.89 2.91) 3386 1208
180–182 46.61 (46.65) 4.40 (4.44) 3.60 3.63) 3220 1224
167–169 49.25 (49.29) 4.68 (4.70) 3.32 3.38) 3346 1238
179–181 56.75 (56.83) 4.34 (4.38) 2.60 2.65) 3357 1206
C21H24NO4P
C29H24NO4P
C
15H17NO4PBr
C17H21NO4PBr
C25H21NO4PBr
4j
4k
4l
C16H18 Cl2NO4P 143–145 49.20 (49.250 4.61 (4.65) 3.53 3.59) 3389 1236
C18H22 Cl2NO4P 146–148 51.64 (51.69) 5.26 (5.30) 3.31 3.35) 3364 1233
C26H22 Cl2NO4P 184–186 60.68 (60.71) 4.29 (4.31) 2.70 2.72) 3378 1228
TABLE 5 1H NMR Spectral Data(δ/ppm) of α-Aminophosphonates 4a–l [25]
Compound
Ar H
P C H
N H
P OCH2CH3/P OCH3
P OCH2CH3
OH/OCH3
4a
4b
4c
4d
4e
4f
4g
4h
4i
6.71–7.46 (m, 7H)
6.69–7.43 (m, 7H)
6.76–7.54 (m, 17H)
6.68–7.34 (m, 11H)
6.63–7.22 (m, 11H)
6.72–7.41 (m, 21H)
6.64–7.38 (m, 8H)
6.58–7.36 (m, 8H)
6.54–7.48 (m, 18H)
6.74–7.42 (m, 7H)
6.79–7.44 (m, 7H)
6.52–7.26 (m, 17H)
4.86
5.08
4.94
5.02
5.13
4.96
4.83
4.87
4.99
5.06
4.92
4.72
5.76 (br, s)
5.89 (br, s)
5.72 (br, s)
5.56 (br, s)
5.66 (br, s)
5.42 (br, s)
5.88 (br, s)
5.33 (br, s)
5.28 (br, s)
5.49 (br, s)
5.27 (br, s)
5.62 (br, s)
3.42 (s, 6H)
3.62–3.87 (m, 4H)
–
10.2
9.8
1.02 (t)
–
3.44 (s, 6H)
3.74–3.88 (m, 4H)
–
3.71 (s, 6H)
3.66–3.82 (m, 4H)
–
3.45 (s, 6H)
3.71–3.86 (m, 4H)
–
–
–
9.01
10.10
10.20
9.99
10.12
9.23
10.02
4.7
1.16 (t)
–
–
1.22 (t)
–
4j
4k
4l
–
1.18 (t)
–
4.9
4.3
TABLE 6 13C NMR Spectral Data of α-Aminophosphonates 4a, 4b, 4g, and 4h [25b]
Compound
Chemical Shifts (δ, in ppm)
4a
136.2 (C-1), 115.5 (C2 and C6), 128.3 (C3 and C5), 156.2 (C-4), 141.2 (C-1ꢁ), 127.8 (C-2ꢁ) (10), 129.0 (C-3ꢁ),
125.0 (C-4ꢁ), 130.0 (C-5ꢁ), 128.0 (C-6ꢁ), 53.6 (d,1Jp-c = 152 Hz, C-7), 51.8 (d,2Jp-c = 7.2 Hz, OCH3), 54.3
(d,2Jp-c = 7.4 Hz, OCH3)
4b
137.0 (C-1), 115.3 (C2 and C6), 128.9 (C3 and C5), 156.9 (C-4), 141.1 (C-1ꢁ), 127.7 (C-2ꢁ), 128.2 (C-3ꢁ), 125.1
(C-4ꢁ), 129.0 (C-5ꢁ), 128.4 (C-6ꢁ), 58.9 (d,1Jp-c = 148.2 Hz, C-7), 64.2 (d,2Jp-c = 7.1 Hz, OCH2CH3), 62.8.
(d,2Jp-c = 6.9 Hz, OCH2CH3), 16.3 (d,J = 11.8 Hz, OCH2 CH3), 16.2 (d,J = 12.6 Hz, OCH2 CH3)
4g
4h
135.6 (C-1), 115.8 (C2 and C6), 132.1 (C3 and C5), 157.5 (C-4), 135.4 (C-1ꢁ), 126.9 (C-2ꢁ and C-6ꢁ), 129.1
(C-3ꢁ and C-5ꢁ), 129.4 (C-4ꢁ), 54.2 (d,1Jp-c = 152.4 Hz, C-7), 52.7 (d,2Jp-c = 7.4 Hz, OCH3), 50.9 (d, 2Jp-c
=
7.1 Hz, OCH3)
136.9 (C-1), 115.6 (C2 and C6), 132.6 (C3 and C5), 157.9 (C-4), 132.9 (C-1ꢁ), 126.3 (C-2ꢁ and C-6ꢁ), 128.7
(C-3ꢁ and C-5ꢁ), 129.7 (C-4ꢁ), 50.9 (d,1Jp-c = 147.2 Hz, C-7), 63.3 (d,2Jp-c = 6.7 Hz, OCH2CH3), 60.9.
(d,2Jp-c = 6.9 Hz, OCH2CH3), 17.6 (d,J = 11.4 Hz, OCH2 CH3), 17.4 (d,J = 11.3 Hz, OCH2 CH3)
TABLE 7 Mass Spectral Data of α-Aminophosphonates 4a, 4b, and 4k
Compound
m/z (% Relative Abundance)s
4a
4b
375 [69 (M+•)], 267 (82), 265 (100), 231 (29), 154 (20), 136 (15), 120 (10), 93 (8)
406 [40 (M+2)], 405 [55 (M+1)], 404 [75 (M+•)], 368 (20), 391 (25), 268 (97), 267 (15), 266 (100), 243 (50), 172
(14), 154 (37), 136 (23), 107 (20)
4k
390 [63 (M+•)], 284 (12), 280 (17), 229 (100), 215 (20), 196 (24), 139 (15), 74 (9)
Heteroatom Chemistry DOI 10.1002/hc

164 Arigala, Matcha, and Yoon
TABLE 8 Antibacterial Activity of α-Aminophosphonates
4a–l
TABLE 9 Antifungal Activity of α-Aminophosphonates 4a–l
Zone of Inhibition (%)
Zone of Inhibition (%)
A. niger
50
H. oryzae
50
E. coli
50
S. aureus
50
Compound
100
25
100
25
Compound
100
25
100
25
4a
10
11
13
12
9
10
14
13
13
9
7
8
9
10
5
6
10
9
10
7
10
8
7
5
4
6
8
3
4
9
8
8
6
8
7
–
11
11
13
15
13
9
13
10
11
12
14
12
12
6
9
10
9
11
8
12
9
9
7
10
10
9
5
5
7
4
9
–
8
7
5
8
7
7
–
4a
4b
4c
4d
4e
4f
14
13
15
15
10
10
12
8
8
8
12
12
6
5
8
5
7
6
9
4
4
8
7
5
3
6
5
6
4
6
6
–
11
10
12
15
10
14
11
–
9
9
10
9
10
8
8
9
10
8
11
8
–
7
7
8
6
6
5
8
5
8
5
–
5
6
6
6
–
4b
4c
4d
4e
4f
4g
4g
4h
4i
4h
4i
4j
8
4j
10
12
11
12
4k
4l
12
10
10
4k
4l
9
8
8
7
Griseofulvin
Penicillin
Antibacterial Activity
1
The H and 13C chemical shifts were referenced to
tetramethylsilane and 31P chemical shifts to 85%
H3PO4(ortho-phosphoric acid). Mass spectra were
recorded on a Jeol SX 102 DA/600 mass spectrom-
eter, using argon/xenon (6 KV, 10 mA) as the FAB
gas, and also on a Shimadzu QP-2000 GC-MS in-
strument.
The antibacterial activity of all the title compounds
4a–l was assayed [26] against the growth of S. aureus
(gram positive) and E. coli (gram negative) at con-
centrations of 100, 50, and 25 ppm (Table 8). The
majority of the compounds exhibited high activity
against both of the bacteria, and two compounds,
4c and 4d, were more effective than the standard
compound.
Penicillin was tested as a standard reference
compound to compare the activity of these com-
pounds.
General Procedure for the Preparation of
α-Aminophosphonates 4a–l
A mixture of p-hydroxybenzaldehyde (1 mmol),
2,4-dichloroaniline (1 mmol), dimethylphosphite
Antifungal Activity
•
(1 mmol), and Zn(OAc)2 2H2O (12%) was stirred vig-
orously at 50◦C for the appropriate time, as indicated
in Table 3. The progress of the reaction was moni-
tored by thin layer chromatography. After comple-
tion of the reaction, the mixture was washed with
rectified spirit and purified by column chromatog-
raphy using a short silica-gel column with hexane
and ethyl acetate (8:2) as the eluent. This procedure
was applied successfully for the preparation of other
compounds 4a–l (Table 3).
Compounds 4a–l (Table 9) were screened for their
antifungal activity against A. niger and H. oryzae
species, along with the standard fungicide, Griseo-
fulvin. The Disk diffusion method [27] was followed
for the screening of the compounds at three different
concentrations of 100, 50, and 25 ppm.
It is gratifying to observe that all the compounds
4a–l exhibited higher antifungal activity when com-
pared with the reference compound. All the com-
pounds exhibited very high activity against fungi,
and the compounds 4c and 4g were more effective
than the standard Griseofulvin.
ANTIMICROBIAL ACTIVITY
The Whatman no. 1 filter paper disk method [26] was
employed for the in vitro study of antibacterial and
antifungal activity against Escherichia coli, Staphylo-
coccus aureus, Aspergillus niger, and Helminthospo-
rium oryzae. The inhibitory effects of compounds
4a–l against these microorganisms are presented in
Tables 8 and 9.
CONCLUSION
•
In summary, Zn(OAc)2 2H2O was found to be an
efficient catalyst in the one-pot reaction of var-
ious aldehydes, amines, and phosphites to af-
ford α-aminophosphonates in excellent yields under
Heteroatom Chemistry DOI 10.1002/hc

•
Zn(OAc)2 2H2O-Catalyzed Synthesis of α-Aminophosphonates under Neat Reaction 165
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vantages, such as a short reaction time and a small
amount of catalyst used. The major advantages of
•
Zn(OAc)2 2H2O are that it is a reusable, eco-friendly,
inexpensive, and efficient catalyst. Short reaction
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Heteroatom Chemistry DOI 10.1002/hc