Fe3O4@MgO nanoparticles as an efficient recyclable catalyst for the synthesis of phosphoroamidates via the Atherton-Todd reaction Dedicated to Professor Tsutomu Yokomatsu from Tokyo University of Pharmacy and Life Sciences on the occasion of his 65th birthday

Article abstract of DOI:10.1016/j.tetlet.2015.09.129

A simple and efficient method is presented for the synthesis of phosphoroamidates in moderate to good yield via the Atherton-Todd coupling of primary amines with H-dialkyl phosphites using Fe₃O₄@MgO nanoparticles as a recyclable catalyst.

Full text of DOI:10.1016/j.tetlet.2015.09.129

Accepted Manuscript
Fe3O4@MgO nanoparticles as an efficient recyclable catalyst for the synthesis
of phosphoroamidates via the Atherton-Todd reaction
Babak Kaboudin, Foad Kazemi, Fereshteh Habibi
PII:
S0040-4039(15)30171-4
DOI:
http://dx.doi.org/10.1016/j.tetlet.2015.09.129
TETL 46801
Reference:
Tetrahedron Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
8 April 2015
24 August 2015
26 September 2015
Please cite this article as: Kaboudin, B., Kazemi, F., Habibi, F., Fe3O4@MgO nanoparticles as an efficient recyclable
catalyst for the synthesis of phosphoroamidates via the Atherton-Todd reaction, Tetrahedron Letters (2015), doi:
http://dx.doi.org/10.1016/j.tetlet.2015.09.129
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1
Tetrahedron Letters
Fe3O4@MgO nanoparticles as an efficient recyclable catalyst for the synthesis of
phosphoroamidates via the Atherton-Todd reaction
Babak Kaboudin,* Foad Kazemi, Fereshteh Habibi
Department of Chemistry, Institute for Advanced Studies in Basic Sciences (IASBS), Gava Zang, Zanjan 45137-66731, Iran
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
A simple and efficient method is presented for the synthesis of phosphoroamidates in moderate to
good yield via the Atherton-Todd coupling of primary amines with H-dialkyl phosphites using
Fe3O4@MgO nanoparticles as a recyclable catalyst.
Dedicated to Professor Tsutomu Yokomatsu from Tokyo University of Pharmacy and Life Sciences on
the occasion of his 65th birthday
Available online
Keywords:
Phosphoroamidate
Fe3O4@MgO nanoparticles
H-Dialkyl phosphite
Atherton-Todd reaction
2015 Elsevier Ltd. All rights reserved.
Phosphorus-heteroatom bond formation is an active and
important research area for the preparation of organophosphorus
compounds.1 Many investigations have been conducted in order
to develop new procedures and methods for the synthesis of
biologically and materially important organophosphorus
formation of a dialkyl chlorophosphate via the reaction of
dialkylphosphites with CCl4. This is then followed by
nucleophilic addition of an amine to the resulting dialkyl
chlorophosphate 2 (Scheme 2). 11
However, the use of triethylamine as the base has associated
problems that include harsh reaction conditions, long reaction
times and the formation of side-products. Additionally, it is well-
known that the formation of salt 3 accompanies the formation of
dialkyl chlorophosphate in the presence of a base (Scheme 2).12
compounds.2
Among
organophosphorus
compounds,
phosphoroamidates (phosphorus analogues of amides containing
a tetrahedral pentavalent phosphorus atom) are particularly
important (Scheme 1). They are stable analogues of the high-
energy tetrahedral transition states for many enzyme-catalyzed
reactions.3 Phosphoroamidate containing peptides and lipid
chains are used as inhibitors of HIV protease4 and nucleoside
reverse transcriptase.5 These compounds have also been applied
as a flame retardant for rigid polyureathane foams.6 Recently
chiral phosphoroamidates have been used as chiral catalysts in
asymmetric transformations.7
Scheme 1: Structures of amides and phosphoroamidates
Scheme 2: The Atherton-Todd reaction via the formation of
intermediate 2 and as well as potential side product 3
One-pot, multi-step and multi-component reactions are attractive
since they can significantly lower the cost of synthetic routes by
reducing the number of separation and purification steps.8 The
Atherton-Todd reaction, which involves the one-pot reaction of a
dialkyl phosphite with a primary amine and carbon tetrachloride
(CCl4) in the presence of base, is a typical method for the
In the past decade, numerous publications have been reported for
organic transformations regarding the development of catalysts
bound to inorganic solids, due to environmental and economical
considerations.13 Magnetic nanoparticles have great potential in
view of their easy recovery and accessibility, and can be prepared
synthesis of phosphoroamidates.9,10 The key step is the in-situ
———
* Corresponding authors. Tel.: +98 241 4153220; fax: +98 241 4214949; e-mail: kaboudin@iasbs.ac.ir

2
from inexpensive materials, and easily supported on organic and
inorganic materials.14 For example, Fe3O4 nanoparticles (NPs)
supported on metal oxides have been extensively studied in the
fields of chemical catalysis,15a-e environmental protection,15f
sensors15g and magnetic storage medias.15h Recently, Marquina
and co-workers reported the preparation and characterization of
Fe3O4 NPs supported on magnesium oxide (MgO) using the sol-
gel method.16 As part of our efforts to explore the utility of solid
phase reactions for the synthesis of organophosphorus
compounds,17 we recently reported Fe3O4 NPs supported on MgO
(Fe3O4@MgO) as a novel, recyclable solid base catalyst for the
synthesis of 1-hydroxyphosphonates.18 To the best of our
knowledge, there are no reports on conducting the Atherton-Todd
reaction in the presence of a recyclable solid base catalyst.
Therefore, we decided to study the feasibility of the Atherton-
Todd reaction in the presence of Fe3O4@MgO NPs as a base
catalyst.
The superparamagnetic Fe3O4@MgO NPs were prepared
according to Marquina’s method and our previous report (see the
literature16,18 and ESI for preparation). The Fe3O4/MgO mol ratio
of the catalyst was calculated from chemical analysis by atomic
absorption spectroscopy giving a mole ratio of 2.78 Fe3O4 to 1.0
MgO. The NPs size were determined from the XRD pattern using
Scherrer equation (the NPs sizes evaluted by Scherrer’s equation
were 9.2 nm). Initially, the one-pot reaction of aniline 1a with
diethylphosphite in the presence of carbon tetrachloride was
chosen as a model reaction to be studied under various conditions
(Scheme 3 and Table 1).
in 36% yield (Table 1, Entry 7). The reaction yield also
decreased when the reaction was carried out in the presence of
only 1 equiv. of carbon tetrachoride (Entry 8). Treatment of 1a
with a mixture of diethylphosphite and CCl4 at room temperature
in the presence of Fe3O4 NPs or MgO powder gave the
corresponding phosphoroamidate 4a in 54% and 20% yield
respectively (Entries 9, 10). Finally, we found that the dropwise
addition of a mixture of diethylphosphite and CCl4 to aniline 1a
at ambient temperature in the presence of 15 mol% catalyst led to
an acceleration of the reaction rate and an increase in the yield of
4a (Entry 11).
This process was successfully applied to other amines as
summarized in Table 2. Substituted anilines reacted with a
mixture of diethylphosphite and CCl4 in the presence of 15 mol%
of Fe3O4@MgO NPs as the basic catalyst to afford the desired
products 4b-4k in moderate to good yields. Benzylamine and
cyclohexyl amine also reacted to give compounds 4l and 4m in
70% and 85% yield respectively.
Table 2. Reaction of amines (1 equiv.) with a mixture of dialkyl phosphite (1
equiv.) and CCl4 (2 equiv.) in the presence of Fe3O4@MgO NPs (15 mol%).
Entry
R
R'
Time (h)
Product
Yield
(%)a
1
C6H5
C2H5
3
4a
4b
4c
4d
4e
4f
80
78
85
52
70
71
65
65
75
70
68
70
85
50
67
67
2
p-MeC6H4
p-MeOC6H4
p-NO2C6H4
p-ClC6H4
p-BrC6H4
m-MeOC6H4
m-NO2C6H4
o-MeC6H4
o-EtC6H4
o-NO2C6H4
PhCH2-
C2H5
2
3
C2H5
1
4
C2H5
12
12
12
8
Scheme 3 Reaction of aniline (1 equiv.) with diethylphosphite (1
equiv.) and CCl4
5
C2H5
6
C2H5
7
C2H5
4g
4h
4i
Table 1. Reaction of aniline (1 equiv.) with diethylphosphite (1 equiv.) and
CCl4
8
C2H5
10
12
12
12
1
Catalyst
loading
(mol%)
9
C2H5
Yield of 4a
Entry
Temp.
Time (h)a
(%)b
10
11
12
13
14
15
16
C2H5
4j
C2H5
4k
4l
1
2
-
rt
rt
8
8
15
55
63
72
73
72
36
70
54
20
80
C2H5
5
cyclohexyl
p-MeC6H4
p-MeOC6H4
PhCH2-
C2H5
1
4m
4n
4o
4p
3
10
15
20
15
15
15
15d
15e
15
rt
8
(CH3)2CH-
(CH3)2CH-
(CH3)2CH-
3
4
rt
8
3
5
rt
8
3
6
rt
24
24
6c
24
24
3f
aYield (based on amine equiv. with regard to phosphite) refers to yield after
column chromatography
7
reflux
rt
8
9
rt
The Atherton-Todd reaction of amines with a mixture
diisopropyl phosphite and CCl4 was also studied (entries 14-16).
The reaction of aromatic and aliphatic amines with a mixture of
diisopropyl phosphite and CCl4 at ambient temperature, gave the
desired products 4n-4p in moderate yields.
10
rt
11
rt
a 2 equiv. of CCl4; b isolated yield; c 1 equiv. of CCl4; d Reaction carried out in
the presence of Fe3O4 nanoparticles; e Reaction carried out in the presence of
MgO powder; f Dropwise addition of a mixture of diethylphosphite and CCl4
to aniline 1a
We found that it was also possible to carry out this reaction
using
N-methylaniline
to
give
the
corresponding
phosphoroamidate 5 in 63% yield (Scheme 4).
Treatment of 1a with a mixture of diethylphosphite and CCl4 (2
equiv.) at room temperature in the absence of base gave the
corresponding phosphoroamidate 4a in 15% yield after 8 h
(Entry 1). The yield of the reaction increased to 55% yield in the
presence of 5 mol% Fe3O4@MgO (Entry 2). When the catalyst
loading was raised from 5% to 15%, the yield of 4a increased to
72% (Entries 3 and 4) however this did not increase futher upon
increasing the catalyst loading or reaction time (Entries 5 and 6).
Heating diethyl phosphite, aniline (1a), and CCl4 (as solvent) at
reflux for 24 hours led to the formation of the desired product 4a
Scheme 4: Reaction of N-methylaniline (1 equiv.) with a mixture of
diethylphosphite (1 equiv.) and CCl4 (2 equiv.) in the presence of
Fe3O4@MgO NPs (15 mol%).

3
2.
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reaction of 1a with diethylphosphite in the presence of CCl4. The
NPs were collected using a magnet and washed four times with
deionized water and methanol and reused after drying at 60 °C
for 3 h for further reactions. The catalytic activity did not
considerably decrease after four catalytic cycles (5% decreases
after four catalytic cycles).
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A
proposed
mechanism
for
the
synthesis
of
phosphoroamidates via the Atherton-Todd coupling using
Fe3O4@MgO NPs is outlined in Scheme 5. The process is
thought to proceed via the reaction of dialkyl phosphite with
CCl4 to give dialkyl chlorophosphate 2, a known intermediate,
followed by nucleophilic substitution by the amine to give
phosphoroamidate 4. The reaction was carried out in an open
flask and the generation of HCl gas was detected with wet pH
paper test.
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Scheme 5: Proposed mechanism for the synthesis of
phosphoroamidate 4
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In conclusion, we have reported a simple, and convenient
method for the synthesis of phosphoroamidates via the Atherton-
Todd coupling reaction of amines with dialkyl phosphites. A
simple work-up, mild reaction conditions, moderate to good
yields, and clean reactions should make this method an attractive
and a useful contribution to present methodologies.
Acknowledgements
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The authors thank the Institute for Advanced Studies in Basic
Sciences for support of this work. Thanks are also given to
Professor Behzad Haghighi for the atomic absorption analysis of
the catalyst and Ms Shahla Heydari for assistance in the
preparation of the paper.
15. a) Zhang, Q.; Su, H.; Luo, J.; Wei, Y. Green Chem. 2012, 14, 201-208.
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Zhang, Q. Chem. Commun. 2011, 47, 10317-10319. e) Kaboudin, B.;
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Supplementary Material
1
Spectroscopic characterization data and copies of H NMR,
13C NMR, and 31P NMR for compounds 4a-4p. Supplementary
data associated with this article can be found, in the online
version
References and notes
1.
a) Alonso, C.; de losSantos, J. M.; Vicario, J.; Palacios, F. Arkivoc
2011, 3, 221-253. b) Allen, D. W. Organophosphorus Chemistry 2014,
43, 1-51. c) Sues, P. E.; Lough A. J.; Morris, R. H. Chem. Commun.
2014, 50, 4707-4710. d) Toy, A.; Walsh, E. N. Phosphorus Chemistry
in Everyday Living, American Chemical Society, Washington, DC, 2nd
edn., 1987. b) Quin, L. D. A Guide to Organophosphorus Chemistry,
Wiley, New York, 2000. c) Gallo, M. A.; Lawryk, N. J. Organic
Phosphorus Pesticides. The Handbook of Pesticide Toxicology,
Academic Press, San Diego, 1991. c) Engel, R. Chem Rev. 1977, 77,
18. Kaboudin, B.; Kazemi, F.; Habibi, F. J. Iran. Chem. Soc. 2015, 12, 469-
475.
General procedure for the synthesis of phosphoroamidates:
A
349-367. d) Frank, A. W. Chem. Rev. 1961, 61, 389-424. e)
mixture of dialkyl phosphite (5 mmol) and CCl4 (1 mL, 10 mmol) were
added dropwise to a stirred mixture of the amine (5 mmol) in the
presence of Fe3O4@MgO NPs (15 mol%, 0.17 g, calculated according
to molecular weight of Fe3O4-MgO) in an open flask. The resultant
mixture was stirred for 1-12 h at room temperature (see Table 2).
EtOAc (50 mL) was added to reaction mixture and stirred for 10 min.
Hilderbrand, R. L., in “The Role of Phosphonates in Living Systems”,
CRC Press: Boca Raton, 1982. f) Redmore, D. in “Topics in
Phosphorus Chemistry”; Griffith, E. J.; Grayson, M. Eds.; Vol. 8;
Wiley: New York, 1976. g) Moonen, K.; Laureyn, I.; Stevens, C. V.
Chem. Rev. 2004, 104, 6177-6215.

4
The solution was decanted and the catalyst washed three times with
EtOAc (10 mL). The combined organic solutions were washed with
water, dried with CaCl2, and the solvent evaporated to give the crude
product. Chromatography on silica gel eluting with EtOAc/n-hexane
(4:6 to 10:1) gave the products in 52-85% yield. The recovered catalyst
was washed with H2O (50 mL) and MeOH (30 mL) and reused after
drying for the next reaction. All the products are known and gave
satisfactory spectral data in accordance with the assigned structures and
literature reports (see NMR spectra of products in the ESI).17e,20,21 For
example: O,O′-diethyl phenylamidophosphate (4a): White crystal;
1
mp: 94-96 °C [Lit. mp17 95-96 °C]; H-NMR (400 MHz, CDCl3): 1.35
(6H, t, JHH = 7.2 Hz), 4.07-4.23 (4H, m), 6.95 (1H, t, JHH = 7.8 Hz), 7.02
(1H, -NH, br), 7.06 (2H, d, JHH = 7.8 Hz ), 7.26 (2H, m); 31P-NMR
(162.0 MHz, CDCl3, H3PO4): 2.77 ppm; 13C-NMR (100.6 MHz,
CDCl3): 16.1 (d, JCP = 7.0 Hz), 62.6 (d, JCP = 4.0 Hz), 117.2 (d, JCP = 8.0
Hz),
121.3,
129.2,
140.0.
O,O′-Diethyl
methyl(phenyl)amidophosphate (5), colorless viscous oil.20 1H-NMR
(250 MHz, CDCl3):  1.38 (t, 6H, J = 6.5 Hz), 3.15 (d, 3H, JPH = 11.2
Hz), 3.85-4.15 (4H, m), 6.95-7.30 (5H, m); 31P-NMR (101.2 MHz,
CDCl3, 85% H3PO4): 5.99; 13C-NMR (62.9 MHz, CDCl3): 16.0 (d, JPC
= 6.9 Hz), 36.7 (d, JPC = 4.4 Hz), 62.5 (d, JPC = 5.0 Hz), 121.8 (d, JPC
=
3.8 Hz), 123.4, 128.8, 143.9.
19. Jardine, A. M.; Vather, S. M.; Modro, T. A. J. Org. Chem. 1988, 53,
3983.
20. Mielniczak, G.; Bopusinski, A. Synth. Commun. 2003, 33, 3851.

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Fe3O4@MgO nanoparticles as an efficient
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Babak Kaboudin,* Foad Kazemi, Fereshteh Habibi