Synthesis of α-aminophosphonates using polystyrene supported Al(OTf)3 as a heterogeneous catalyst

Article abstract of DOI:10.1080/15533174.2010.538029

A mild and efficient method has been devised for the preparation of -aminophosphonates from three component condensation of an aldehyde, an amine, and diethyl phosphite in the presence of catalytic amounts of cross-linked polystyrene supported aluminium triflate (Ps-Al(OTf)₃) under solvent-free conditions in good to excellent yields. The catalyst is stable (as a bench top catalyst) and can be easily recovered and reused without appreciable change in its efficiency. Taylor & Francis Group, LLC.

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Synthesis and Reactivity in Inorganic, Metal-Organic,
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Synthesis of α-Aminophosphonates Using Polystyrene
Supported Al(OTf)3 as a Heterogeneous Catalyst
Kaveh Parvanak Boroujeni a
a Department of Chemistry , Shahrekord University , Shahrekord, I. R., Iran
Published online: 19 Feb 2011.
To cite this article: Kaveh Parvanak Boroujeni (2011) Synthesis of α-Aminophosphonates Using Polystyrene Supported
Al(OTf)3 as a Heterogeneous Catalyst, Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal Chemistry,
41:2, 173-176
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Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal Chemistry, 41:173–176, 2011
Copyright © Taylor & Francis Group, LLC
ISSN: 1553-3174 print / 1553-3182 online
DOI: 10.1080/15533174.2010.538029
Synthesis of ␣-Aminophosphonates Using Polystyrene
Supported Al(OTf)3 as a Heterogeneous Catalyst
Kaveh Parvanak Boroujeni
Department of Chemistry, Shahrekord University, Shahrekord, I. R. Iran
O
O
R2HN P(OEt)2
A mildandefficientmethodhas beendevisedforthepreparation
of α-aminophosphonates from three component condensation of an
aldehyde, an amine, and diethyl phosphite in the presence of cat-
alytic amounts of cross-linked polystyrene supported aluminium
triflate (Ps-Al(OTf)3) under solvent-free conditions in good to ex-
cellent yields. The catalyst is stable (as a bench top catalyst) and
can be easily recovered and reused without appreciable change in
its efficiency.
H
P(OEt)2
R1CHO
+ R2NH2
R1
H
Ps-Al(OTf)3 / rt
R1, R2: Alkyl or Aryl
SCH. 1.
cleophile, unsatisfactory yields, complicated operations, and
the use of moisture-sensitive, expensive, hazardous, difficult
to handle or unreusable catalysts. Sometimes, a part of men-
tioned catalysts do not work well with electron deficient amines
and/or secondary amines. On the other hand, the formation of
α-hydroxyphosphonate or a product of its rearrangement fre-
quently accompanies the formation of α-aminophosphonates.
However, the developments in this area demand further searches
for better catalysts that could be superior to the existing ones
with regard to toxicity, handling, selectivity, and recyclability.
In a continuation of our ongoing program to develop environ-
mentally benign methods using heterogeneous catalysts,[29,30]
we reported Ps-Al(OTf)3 as a new polymeric Lewis acid cat-
alyst for dithioacetalization of carbonyl compounds and trans-
dithioacetalization of acetals.[31] Along this line, we now wish
to report that Ps-Al(OTf)3 is also an effective and highly
chemoselective heterogeneous catalyst for the synthesis of α-
aminophosphonates under solvent-free conditions (Scheme 1).
Keywords α-aminophosphonates, aldehydes, aluminium triflate,
amines, diethyl phosphate
INTRODUCTION
α-Aminophosphonates as a kind of natural amino acid ana-
logues have been the focus of attention in recent years be-
cause of a wide range of applications in the areas of indus-
trial, biological, and medical chemistry.[1−3] Their potential
as plant growth regulators,[4] herbicidals,[5] antibiotics,[6] en-
zyme inhibitors,[7] HIV protease,[8] anti-thrombotic agents,[9]
and inhibitors of UDH-galactopyranose mutase[10] is well doc-
umented. Three component condensation of an aldehyde, an
amine, and a phosphite is the most convenient method for
the preparation of these compounds. For this purpose several
types of catalysts such as ZrCl4,[11] AlCl3,[12] LiClO4,[13] SiO2-
NH4HCO3,[14] Al(OTf)3,[15] ionic liquid [bmim]Cl-AlCl3,[16]
Mg(ClO4)2,[17] TiCl4,[18] Al2O3-SbCl3,[19] ZrOCl2.8H2O,[20]
TiO2,[21] amberlyst-15,[22] [Cu(3,4-tmtppa)](MeSO4)4,[23] sul-
famic acid,[24] FeCl3,[25] sodium dodecyl sulfate,[26] meso-
EXPERIMENTAL
Material and Techniques
porous aluminosilicate nanocage,[27] and H3BO[28] have been
3
Chemicals were either prepared in our laboratory or were
purchased from Merck and Fluka. Polystyrene (8% divinylben-
zene, prepared via suspension polymerization, polyvinylpyrroli-
done 90 K as suspension agent, grain size range: 0.25–0.6 mm)
was obtained from Iran Polymer and Petrochemical Institute. IR
spectra were run on a Shimadzu model 8300 FT-IR spectropho-
tometer. 1H NMR spectra were recorded on a Bruker DPX-300
(300 MHz) spectrometer using CDCl3 as solvent and TMS as
the internal standard. Capacity of the catalyst was determined
by gravimetric method and atomic absorption technique using
a Philips atomic absorption instrument. Melting points were
determined on a Fisher-Jones melting-point apparatus and are
used. A large number of these catalysts suffer from their
own drawbacks such as generation of environmentally per-
ilous waste material, tedious work-up, long reaction times, the
use of malodorous trialkyl phosphite as the phosphorus nu-
Received 4 June 2010; accepted 22 July 2010.
The author thanks the Research Council of Shahrekord University
for partial support of this work.
Address correspondence to Kaveh Parvanak Boroujeni, Depart-
ment of Chemistry, Shahrekord University, Shahrekord 115, I. R. Iran.
E-mail: parvanak-ka@sci.sku.ac.ir
173

174
K. P. BOROUJENI
TABLE 1
Ps-Al(OTf)3 catalyzed synthesis of α-aminophosphonate
derivatives
uncorrected. Reaction monitoring and purity determination of
the products were accomplished by GLC or TLC on silica-gel
polygram SILG/UV254 plates.
Entry
Aldehyde
Amine
Time (h) Yield (%)a,b
Preparation of Ps-Al(OTf)3[31]
1
2
3
4
5
6
0.9
1
92[26]
93[21]
93[26]
92[21]
95[21]
91[23]
In a two necked round bottomed flask (100 mL) equipped
with a reflux condenser, a solution of Ps-AlCl[330] (3 g, 8% DVB)
in Freon-113 (50 mL) was prepared. To the stirred solution was
added dropwise 5 mL of triflic acid over a period of 2 h, while
mentioning a gentle reflux. The mixture was then refluxed for
12 h. The polymer beads were filtered and washed with CH2Cl2
(50 mL) and then with Et2O (50 mL) and dried in a vacuum
oven overnight to give Ps-Al(OTf)3. The loading of Al(OTf)3
determined was 0.41 mmol/g.
CHO
NH2
NH2
NH2
NH2
NH2
Me
CHO
CHO
CHO
CHO
1.1
1.3
0.7
1.1
MeO
HO
O2N
Cl
Cl
NH2
CHO
Typical Procedure for ␣-Aminophosphonate Synthesis
To a stirred solution of benzaldehyde (1 mmol), aniline (1
mmol), and Ps-Al(OTf)3 (0.1 mmol) was added diethyl phos-
phite (1.2 mmol). The reaction mixture was stirred magnetically
at room temperature. After completion of the reaction (moni-
tored by TLC) the catalyst was filtered off and washed with
CH2Cl2 (2 × 10 mL). Then, the filtrate was washed with H2O (2
× 10 mL) and the organic layer was separated, dried (Na2SO4),
and concentrated on a rotary evaporator under reduced pressure
to give the crude product. Whenever required, the products were
purified by column chromatography (silica gel) using n-hexane-
EtOAc as eluent so as to afford the pure product. The spent cat-
alyst from different experiments was washed with Et2O, dried,
and used again.
7
8
0.9
1
91[26]
90[21]
Cl
CHO
CHO
NH2
NH2
Cl
91[15]
CHO
9
1.2
NH2
91[23]
83[26]
91[26]
83c,[16]
89c,[15]
93[11]
91[15]
82[23]
84[15]
CHO
10
11
12
13
14
15
16
17
18
1.5
1.5
1.3
2
NH2
NH2
CHO
CHO
Ph
NH2
NH2
NH2
O
COMe
O
2
0.7
0.8
1.8
2
CHO
CHO
CHO
MeO
NH2
NH2
NH2
RESULTS AND DISCUSSION
Me
Cl
Ps-Al(OTf)3 was prepared by the exchange reaction be-
tween cross-linked polystyrene supported aluminium chloride
(Ps-AlCl3) and triflic acid in Freon-113 under reflux condi-
tions. To exploit a simple and suitable condition for three com-
ponent condensation of aldehydes, amines, and diethyl phos-
phite using Ps-Al(OTf)3 as a catalyst, initially, we tried to con-
vert 4-methoxybenzaldehyde (1 mmol) to its corresponding α-
aminophosphonate with aniline (1 mmol) and diethyl phosphite
(1.2 mmol) using different catalytic amounts of Ps-Al(OTf)3 in
the presence of various solvents and also under solvent-free con-
ditions at different reaction temperatures. The best yield of the
corresponding α-aminophosphonate was obtained in the pres-
ence of 10 mol% of Ps-Al(OTf)3 under solvent-free conditions
at room temperature. Therefore, we employed the above con-
ditions for conversion of various aldehydes to the correspond-
ing α-aminophosphonates (Table 1). Both electron-deficient and
electron-releasing benzaldehydes react efficiently with aniline
to give the corresponding α-aminophosphonates (entries 1–8).
Naphthalene-2-carbaldehyde also gave the corresponding prod-
uct in high yield (entry 9). In addition, this method is even
effective with aliphatic aldehydes, which normally produce
low yields (entry 10). Acid sensitive substrates such as cin-
namaldehyde and furan-2-carbaldehyde gave the corresponding
Cl
CHO
NH2
19
20
2.2
2.3
86[21]
82[15]
MeO
CHO
O2N
NH2
O2N
CHO
CHO
CHO
CHO
CHO
CHO
NH2
21
22
23
24
25
1.1
1.3
1.5
1.7
1.5
95[15]
91[21]
90[17]
90[16]
90[25]
Ph
NH2
NH2
NH
(Et)2NH
PhMeNH
aRatio of catalyst:carbonyl compound:amine:diethyl phosphite is
0.1:1:1:1.2.
bIsolated yields. All products are known compounds and were identi-
fied by comparison of their physical and spectral data with those of the
authentic samples.
α-aminophosphonates without any decomposition, conjugate cThe reaction was carried out at 40◦C.

SYNTHESIS OF α-AMINOPHOSPHONATES
175
TABLE 2
Comparison of the efficiencies of a number of different
reported catalysts with that of Ps-Al(OTf)3 in condensation of
aniline and 4-methoxybenzaldehyde with diethyl phosphite
PhNH2 (1 mmol)
HPO(OEt)2 (1.2 mmol)
rt
O
PhHN P(OEt)2
PhCHO
H
Ph
(1 mmol)
Ps-Al(OTf)3 (0.1 mmol, 0.9 h)
Ps-AlCl3 (0.15 mmol, 3 h)
92%
18%
Entry Cat./Solv./Temp.
Time (h) Yield (%)
1
2
3
ZrCl4/MeCN/rt
5
3
2
90[11]
90[15]
95[16]
SCH. 2.
Al(OTf)3/neat/80◦C
ionic liquid
addition to the α, β-unsaturated carbonyl group or generation
of polymeric by-products under the present reaction conditions
(entries 11,12). Ketones also afforded the corresponding phos-
phates in relatively good yields (entries 13,14). In order to test
the generality of the protocol, we have reacted different substi-
tuted anilines and primary aliphatic amines with benzaldehyde
and diethyl phosphite (entries 15–22). As it is evident from the
results, the presence of electron-withdrawing substituents on the
aromatic ring in substituted anilines increases the reaction times
and decreases the yields (entries 17–20). Also, the coupling re-
action of secondary amines like piperidine, diethylamine, and
N-methylaniline with benzaldehyde and diethyl phosphite in
the presence of Ps-Al(OTf)3 at ambient temperatures resulted
in the formation of tertiary α-aminophosphonate in high yields
(entries 23–25). It is noteworthy that no competitive side re-
actions such as the formation of α-hydroxyphosphonates were
observed in these transformations.
[bmim]Cl–AlCl3/neat/rta
Mg(ClO4)2/neat/rt
Al2O3-SbCl3/MeCN/rt
ZrOCl2.8H2O/neat/rta
TiO2/neat/50◦C
4
5
6
7
8
5 min
3.5
5 min
2.5
95[17]
92[19]
95[20]
98[21]
9023
[Cu(3,4-tmtppa)](MeSO4)4/
3
H2O/80◦C
9
10
11
sulfamic acid/neat/rt
FeCl3/THF/60◦C
2
0.75
30 min
87[24]
92[25]
98[26]
sodium dodecyl
sulfate/H2O/50◦C
AlKIT-5b/MeCN/80◦C
Ps-Al(OTf)3/neat/rt
12
13
4
1.1
90[27]
93
aWith benzaldehyde
bMesoporous aluminosilicate nanocage catalyst
To find out whether the reaction takes place in the solid
matrix of Ps-Al(OTf)3 or whether Al(OTf)3 simply released in
the reaction medium is responsible for the coupling reaction,
Ps-Al(OTf)3 was added to benzaldehyde and the mixture was
stirred at roomtemperaturefor 2 h. Then, thecatalyst was filtered
off and the filtrate was analyzed for its aluminium content,
which showed a negligible release of Al(OTf)3. The filtrate was
found to be inactive for the coupling reaction of benzaldehyde,
aniline, and diethyl phosphite. These observations indicate that
Ps-Al(OTf)3 is stable under the reaction conditions and there is
no leaching of acid moieties during reactions.
phosphite, a sample of the solid catalyst was recovered by fil-
tration after the reaction and it was then reused in an identical
reaction of benzaldehyde with aniline and diethyl phosphite. As
is shown in Scheme 3, the efficiency of the recycled catalyst did
not change appreciably after five cycles.
A comparison of the present protocol, using Ps-Al(OTf)3,
with selected previously known protocols is collected in Table
2. As it is seen in addition to having the general advantages
attributed to the solid catalysts, Ps-Al(OTf)3 has a good effi-
ciency compared to many of those reported catalysts in the con-
densation of aniline and 4-methoxybenzaldehyde with diethyl
phosphite.
It is important to note that Ps-Al(OTf)3 shows a higher cat-
alytic activity than Ps-AlCl3 in the coupling reaction of aldehy-
des, amines, and diethyl phosphite (Scheme 2).
O
CONCLUSION
PhNH2 (1mmol)
PhCHO
(1 mmol)
PhHN P(OEt)2
In summary, three component coupling reaction of aldehy-
des, amines, and diethyl phosphite using Ps-Al(OTf)3 has been
achieved. This catalyst has comparable activity with Al(OTf)3,
but it has added advantages that involve its ability to be recycled,
non-toxicity, non-corrosiveness, ease of handling, high chemos-
electivity, and environmental compatibility. High yields, green
and mild reaction conditions, facile work-up, and short reaction
times are other notable features of this protocol.
HPO(OEt)2 (1.2 mmol)
H
Ph
Ps-Al(OTf)3 (0.1 mmol) / rt / 0.9 h
Usea
1
2
3
4
5
Yield (%)
92
92
91
90
90
aRecovered catalyst was used successively (Use 2,3,…).
SCH. 3.
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