CeCl37H2O-catalyzed one-pot Kabachnik-Fields reaction: A green protocol for three-component synthesis of α- aminophosphonates

Article abstract of DOI:10.1002/hc.20635

CeCl₃7H₂O has been utilized as an efficient Lewis acid catalyst for the three-component coupling of aldehydes, aromatic amines, and diethylphosphite to produce α-aminophosphonates under solvent-free conditions. The advantages of this protocol are high yield, mild reaction conditions, less environmental pollution, and simple work-up procedure.

Full text of DOI:10.1002/hc.20635

Heteroatom Chemistry
Volume 21, Number 6, 2010
CeCl ·7H O-Catalyzed One-Pot
3 2
Kabachnik–Fields Reaction: A Green Protocol
for Three-Component Synthesis of
α-Aminophosphonates
Abbas Ali Jafari, Mahshid Nazarpour, and
Mohammad Abdollahi-Alibeik
Department of Chemistry, Faculty of Science, Yazd University, Yazd 89195-741, Iran
Received 19 November 2009; revised 14 June 2010
Generally, α-aminophosphonates are prepared
ABSTRACT: CeCl3·7H2O has been utilized as an
by the addition of phosphorous nucleophiles to
efficient Lewis acid catalyst for the three-component
imines in the presence of acids [11]. Recently,
coupling of aldehydes, aromatic amines, and di-
three-component synthesis starting from aldehydes,
ethylphosphite to produce α-aminophosphonates
amines, and diethylphosphite or triethylphosphite
under solvent-free conditions. The advantages of
has been reported by using Lewis and Brønsted
this protocol are high yield, mild reaction con-
acid catalysts such as LiClO4 [12], InCl3 [13a], FeCl3
ditions, less environmental pollution, and simple
[13b], ZrCl4 [13c], ZrOCl2·8H2O [13d], lanthanide
ꢀ
C
work-up procedure. 2010 Wiley Periodicals, Inc. Het-
triflates/magnesium sulfate [14], SbCl3/Al2O3 [15a],
eroatom Chem 21:397–403, 2010; View this article online
at wileyonlinelibrary.com. DOI 10.1002/hc.20635
TaCl5-SiO2 [15b], BiNO3·5H2O [15c], amberlyst-15
[16a], montmorillonite clay-MW [16b], silica sul-
furic acid [16c], sulfamic acid [16d], Al2O3-MW
[17a], TiO2 [17b], CF3CO2H [18], Sc(DS)3 [19a],
β-cyclodextrine [19b], SDS [19c], BF3·Et2O [20],
INTRODUCTION
M(OTf)n [21], M(ClO4)n [22], in an ionic liquid [23]
Phosphonate-containing molecules are biologically
potent. Their diverse applications include as in-
hibitors of synthase [1], HIV protease [2], rennin
and under thermal conditions [24]. By consider-
ing the importance of α-aminophosphonates as bi-
ologically active compounds, there is a great de-
[3], PTPases [4–6], antibiotics [5], enzymes [6],
mand for the development of more convenient and
UDP-galactopyranose mutase [6a], and herbicides
[7] and as surrogates of α-amino acids [8]. α-
Aminophosphonates have attracted attention as
substrates in the synthesis of phosphonopeptides
(peptido mimetics) [9]. However, there are also a
few reports available in which antitumor activity of
α-aminophosphonates has been investigated [10].
practically efficient methods for the synthesis of α-
aminophosphonates.
Cerium(III)
chloride
heptahydrate
[25]
(CeCl3·7H2O) has emerged as a potential cata-
lyst in affecting various organic transformations
due to its high catalytic ability, water tolerance,
being a “friendly” reagent, economic viability, and
easy availability. To our knowledge, no Kabachnik–
Fields reaction catalyzed by CeCl3·7H2O has been
reported. In this paper, as a part of our ongoing
green organic chemistry program [26], we wish
to report an efficient and versatile procedure for
Correspondence to: Abbas Ali Jafari; e-mail: Jafari@
yazduni.ac.ir.
Contract grant sponsor: Research Council of Yazd University.
2010 Wiley Periodicals, Inc.
c
ꢀ
397

398 Jafari, Nazarpour, and Abdollahi-Alibeik
O
OEt
OEt
P
H
O
OEt
O
H
CeCl3.7H2O (5 mol-%)
P
N
OEt
Ar
+ ArNH2
+
H
OEt
Solvent-free
r.t.
X
H
X
HO
P
OEt
SCHEME 1
affecting the one-pot, three-component reaction of
an aldehyde, an amine, and diethylphosphite for
the preparation of α-aminophosphonates in the
presence of catalytic amounts of the CeCl3·7H2O
under solvent-free conditions (Scheme 1).
Then, to show the general application of
the method [25], several aldehydes, amines, and
diethylphosphate with similar molar ratios as
mentioned earlier were reacted smoothly in ap-
propriate reaction times (2.5–24 h) to yield the
corresponding α-aminophosphonates in high to
excellent isolated yields (87–95%). The examples
studied covered electron-rich and electron- defi-
cient aromatic aldehydes with aniline derivatives
containing strong electron-donating and electron-
withdrawing groups and also show that cyclohex-
anone as a ketone reacts smoothly to give the corre-
sponding α-aminophosphonate. The results of this
study are summarized in Table 2.
A reasonable pathway for the reaction of an
aldehyde with an amine and diethylphosphite con-
ducted in the presence of CeCl3·7H2O is presented
in Fig. 1. The reaction proceeds via formation of
imine that undergoes addition with diethylphosphite
to afford α-aminophosphonates. CeCl3·7H2O shows
dual activity: One action is to activate the in situ
generated imine, and its second function is to bring
diethylphosphite molecule close to the imine by the
template effect.
RESULTS AND DISCUSSION
At the outset, the three-component reaction of
benzaldehyde, aniline, and diethylphosphite was
investigated in various conditions at room tem-
perature (Table 1). To optimize the reaction con-
ditions, the model reaction was examined under
solvent-free conditions and in solvents such as
CH3CN, acetone, CH2Cl2, and EtOH in the pres-
ence of cerium salts. The best reaction condi-
tion resulted when the molar ratio of benzalde-
hyde (2 mmol)/aniline (2 mmol)/diethylphosphate
(2.2 mmol)/CeCl3·7H2O (5 mol%) was used at
room temperature under solvent-free conditions. In
these conditions, the reaction proceeded well and
was completed after 5 h to produce the desired
diethyl phenyl(phenylamino)methylphosphonate in
95% isolated yield. To show the merit of the catalyst,
a similar reaction in the absence of CeCl3·7H2O cat-
alyst was studied (Table 1, entry 1). The reaction did
not proceed even after 24 h.
Finally, the efficacy of CeCl3·7H2O was com-
pared with that of other catalysts reported earlier
H
R1 NH2
+
O
R
- H2O
TABLE 1 Optimization of Reaction Conditionsa
H
R
R1
N
Entry
Solvent
Catalyst (mol%) Time (h) Yield (%)
C
H
R
R1
C
N
1
2
3
4
5
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
–
24
24
14
12
12
<10
70
CeCl3
Ce(NO3)3
(NH4)Ce(NO3)6
CeCl3·7H2O (5)
CeCl3·7H2O +
NaI (5)
94
93
95
O
HP(OEt)2
CeCl3.7H2O
6
CH3CN
CeCl3·7H2O +
NaI/Al2O3 (5)
CeCl3·7H2O (5)
CeCl3·7H2O (5)
CeCl3·7H2O (5)
24
80
H
R1
R1
O
OEt
HN
C
N
O
HO
P
7
8
9
10
11
Acetone
CH2Cl2
Ethanol
10
24
24
5
94
70
70
95
92
R
CCl3
H
OEt
R
P
OEt
EtO
P
OEt
EtO
Solvent-free CeCl3·7H2O (5)
Solvent-free CeCl3·7H2O (2.5)
8
FIGURE 1 A plausible mechanism for the CeCl3·7H2O cat-
alyzed three-component reaction for the preparation of α-
aminophosphonates.
aReaction conditions: solvent (0.5 mL), room temperature, benzalde-
hyde (2 mmol), aniline (2 mmol), and diethylphosphate (2.2 mmol).
Heteroatom Chemistry DOI 10.1002/hc

CeCl3·7H2O-Catalyzed One-Pot Kabachnik–Fields Reaction 399
TABLE 2 One-Pot Synthesis of α-Aminophosphonates Catalyzed by CeCl3·7H2O at Room Temperature under Solvent-Free
Conditionsa
mp Found
(Reported)
Entry
1
Aldehyde
Amine
Productb
Time (h)
5
Yield (%)
95
Reference
[17b]
O
NH
2
87 (86)
H
HN
PO(OEt)2
O
2
3
2.5
94
92
77
–
–
NH2
H
HN
MeO
PO(OEt)2
OMe
H3CO
OCH3
OMe
O
NH2
5
120
H
OMe HN
PO(OEt)2
OMe
OMe
O
4
5
4.5
91
90
108
[12]
NH2
H
HN
MeO
PO(OEt)2
MeO
O
NH2
9
57 (57)
[17b]
H
HN
Cl
PO(OEt)2
Cl
6
7
8
14
88
94
89 (88)
[17b]
[15b]
[23]
O
NH
2
H
Cl HN
Cl
O
PO(OEt)2
8
91
NH2
H
OH HN
OH
PO(OEt)2
20 (9)c
94 (95)c
79
NH2
O
H
HN
F
PO(OEt)2
F
(Continued)
Heteroatom Chemistry DOI 10.1002/hc

400 Jafari, Nazarpour, and Abdollahi-Alibeik
TABLE 2 Continued
mp Found
(Reported)
Entry
10
Aldehyde
Amine
Productb
Time (h)
13
Yield (%)
90
Reference
[17b]
NH2
O
120 (120)
H
HN
O2N
PO(OEt)2
O2N
11
12
13
10.5
89
93
95
105
[15c]
[23]
NH2
O
H
HN
NC
PO(OEt)2
NC
NH
2
Cl
9
119
O
H
Cl
Cl
HN
PO(OEt)2
NH2
5
129
[17b]
O
Cl
H
HN
PO(OEt)2
14
15
17
18
Br
8
5
92
92
90
93
122
115
120
127
–
O
O
O
O
NH2
H
H
H
H
HN
Br
PO(OEt)2
NH2
OMe
[15b]
[17b]
[17b]
MeO
HN
PO(OEt)2
OH
HO
10
NH2
HN
PO(OEt)2
OH
8
NH2
HN
HO
PO(OEt)2
(Continued)
Heteroatom Chemistry DOI 10.1002/hc

CeCl3·7H2O-Catalyzed One-Pot Kabachnik–Fields Reaction 401
TABLE 2 Continued
mp Found
(Reported)
Entry
19
Aldehyde
Amine
Productb
Time (h)
8
Yield (%)
90
Reference
[13a]
NO2
118
O2N
NH2
O
H
HN
PO(OEt)2
NH2
CN
20
21
24
89
87
130
–
O
H
NC
HN
PO(OEt)2
NH
2
6
108 (104)
[17b]
O
HN
PO(OEt)2
aThe ratio of aldehyde (2 mmol), amine (2 mmol), and diethylphosphate (2.2 mmol).
bProducts were confirmed by physical and spectroscopic methods such as mp, IR, and NMR.
c10 mol% CeCl3·7H2O was used.
TABLE 3 Comparison of the Reaction of Benzaldehyde, Aniline, Diethylphosphate Catalyzed by CeCl3·7H2O with the Other
Catalysts Recently Used for This Reaction
Entry
Catalyst (mol%)
Solvent
Temperature (◦C)
Time (h)
Yield (%)
References
1
2
3
4
5
6
7
8
CF3CO2H (25)
ZrCl4 (10)
–
r.t.
r.t.
r.t.
50
r.t.
r.t.
r.t.
r.t.
80
r.t.
r.t.
24
4.5
11
2.5
8
10
3
20
0.5
24
5
96
92
93
98
84
93
91
90
92
[18]
[13c]
[13a]
[17a]
[23]
[15c]
[15a]
[15b]
[24a]
CH3CN
THF
InCl3 (10)
TiO2 (20)
[bmim]BF6 (1 mL)
Bi(NO3)3·5H2O (10)
SbCl3/ Al2O3 (5)
TaCl5-SiO2 (10)
–
–
–
–
CH3CN
CH2Cl2
9
10
11
–
–
–
–
N.R.
95
[23], [16d]
This work
CeCl3·7H2O (5)
r.t., room temperature; N.R., no reaction.
in the synthesis of α-aminophosphonates. The data
summarized in Table 3 clearly demonstrate the su-
periority of CeCl3·7H2O in terms of time, cost, as well
as operational simplicity of the reaction.
of mild reaction conditions, no environmental
pollution, avoiding use of any harmful organic
solvent in reaction media and/or in work-up pro-
cedure, use of an inexpensive, water-tolerant, and
environment-friendly catalyst, and simple work-up
procedure.
CONCLUSION
In conclusion, the results reported here show that
CeCl3·7H2O efficiently catalyzes the one-pot syn-
thesis of biologically active α-aminophosphonates
in excellent yields via a three-component coupling
reaction of aldehydes, amines, and diethylphosphite
at room temperature. This protocol has advantages
EXPERIMENTAL
General Remarks
Merck and Fluka chemicals were purchased from
Kimia Exir Chemical Co., Tehran, Iran. All the
products are known and were characterized by
Heteroatom Chemistry DOI 10.1002/hc

402 Jafari, Nazarpour, and Abdollahi-Alibeik
1
(Table 2, entry 14): white solid, mp 122◦C; H NMR
comparison of their physical data with those re-
ported in the literature. IR spectra were run on a Shi-
madzu model 8300 FT-IR spectrophotometer. NMR
spectra were recorded on a Bruker Avance DPX-250
or 500 MHz. The purity of the products and the
progress of the reactions were accomplished by thin-
layer chromatography (TLC) on silica-gel polygram
SILG/UV254 plates.
(500 MHz, CDCl3): δ = 7.47 (d, J = 7.2, 2H), 7.37 (d,
J = 7.2, 2H), 7.31 (t, J = 7.2, 1H), 7.20 (d, J = 8.5,
2H), 6.50 (d, J = 8.5, 2H), 4.92 (b, 1H, NH), 4.74
(d, J = 23.2, 1H), 4.31–4.28 (m, 2H), 3.99–3.97 (m,
1H), 3.69–3.70 (m, 1H), 1.32 (t, J = 7.0, 3H), 1.14
(t, J = 7.0, 3H) ppm; 13C NMR (125 MHz, CDCl3):
δ = 145.7, 135.8, 132.3, 129.1, 128.5, 128.2, 115.9,
110.6, 63.9, 63.7, 56.6 (d, J = 150.5), 16.8, 16.6 ppm;
IR (KBr): ν˜ = 3388, 2983,1615, 1516, 1235 cm−1;
C17H21BrNO3P (398): calcd C 51.27, H 5.32; found
C 51.24, H 5.30. [(4-Cyano-phenylamino)-phenyl-
methyl]-phosphonic acid diethyl ester (Table 2, en-
General Procedure for Preparation of
α-Aminophosphonates
To a mixture of aldehyde (2 mmol), amine (2 mmol),
and diethyl phosphite (2.2 mmol), CeCl3·7H2O
(0.02 g, 5 mol%) was added and stirred at room tem-
perature for the appropriate reaction time (Table 2).
After completion of the reaction (TLC), the mixture
was dissolved in hot EtOH (2 mL) and the prod-
uct was precipitated by adding ice water. The pure
α-aminophosphonates were collected with a sim-
ple filtration and consequently washed with water
(Table 2).
try 20): white solid, mp 130–131◦C; H NMR (500
1
MHz, CDCl3): δ = 7.49 (d, J = 7.5, 2H), 7.39–7.31(m,
5H), 6.64 (d, J = 8.6, 2H), 5.78 (b, 1H, NH), 4.80
(d, J = 24.1, 1H), 4.19–4.12 (m, 2H), 3.97–3.92 (m,
1H), 3.69–3.64 (m, 1H), 1.33 (t, J = 7.0, 3H), 1.13
(t, J = 7.0, 3H) ppm; 13C NMR (125 MHz, CDCl3):
δ = 150.3, 125.2, 134, 129.2, 128.8, 128.2, 120.5,
113.9, 100.5, 64.1 (d, J = 6.9), 63.7 (d, J = 6.9), 56.4
(d, J = 150.5), 16.8 (d, J = 5.6), 16.5 (d, J = 5.6)
ppm; IR (KBr): ν˜ = 3306, 2986, 2211, 1604, 1527,
1233, 1026 cm−1; C18H21N2O3P (344): calcd C 62.78,
H 6.15; found C 62.68, H 6.12.
Selected Spectral Data for New Compounds
[(3, 4 - Dimethoxy-phenyl) - phenylamino - methyl] -
phosphonic acid diethyl ester (Table 2, entry 2):
white solid, mp 77–78◦C; 1H NMR (500 MHz, CDCl3):
δ = 7.12 (dd, J1 = 8.4, J2 = 7.5, 2H), 7.05–7.02 (m,
2H), 6.84 (d, J = 7.9, 1H), 6.71 (t, J = 7.5, 1H),
6.63 (d, J = 8.1,2H), 4.85 (b, 1H, NH), 4.72 (d,
J = 23.8, 1H), 4.16–4.10 (m, 2H), 3.99–3.95 (m, 1H),
3.88 (s, 3H), 3.86 (s, 3H), 3.75–3.70 (m, 1H), 1.30
(t, J = 7.1, 3H), 1.16 (t, J = 7.1, 3H); 13C NMR
(125 MHz, CDCl3): δ = 149.5, 149.1, 146.9, 129.6,
128.6, 120.6, 118.8, 114.3, 111.5, 111.3, 63.6 (d,
J = 6.9), 63.6 (d, J = 6.9), 56.3, 56.2, 56.2 (d, J =
151.0), 16.8 (d, J = 5.6, CH3), 16.7 (d, J = 5.6, CH3);
IR = 3284, 2995, 1602, 1515, 1498, 1230, 1020
cm−1; C19H26NO5P (379): calcd C 60.15, H 6.91;
found C 60.03, H 6.90. [(2,5-Dimethoxy-phenyl)-
phenylamino-methyl]-phosphonic acid diethyl ester
ACKNOWLEDGMENTS
We are thankful to the Research Council of Yazd
University for financial support. We are also grateful
to Prof. H. Firouzabadi (University of Shiraz) for his
valuable advice.
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(Table 2, entry 3): white solid, mp 120–121◦C; H
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Heteroatom Chemistry DOI 10.1002/hc