Iodine catalyzed solvent-free cross-dehydrogenative coupling of arylamines and H-phosphonates for the synthesis of N-arylphosphoramidates under atmospheric conditions

Article abstract of DOI:10.1016/j.tetlet.2014.01.064

Aerobic oxidative coupling of various arylamines and H-phosphonates to the corresponding N-arylphosphoramidates has been achieved under solvent-free conditions using molecular iodine. This protocol works at room temperature furnishing the corresponding P-

Full text of DOI:10.1016/j.tetlet.2014.01.064

Tetrahedron Letters 55 (2014) 1544–1548
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Iodine catalyzed solvent-free cross-dehydrogenative coupling
of arylamines and H-phosphonates for the synthesis
of N-arylphosphoramidates under atmospheric conditions
Bashir Ahmad Dar a, Nisar A. Dangroo a,b, Amit Gupta a, Aarti Wali a, Mohammad Akbar Khuroo b,
Ram A. Vishwakarma a, , Baldev Singh a,
⇑
⇑
a Indian Institute of Integrative Medicine (CSIR), Canal Road, Jammu 180001, India
b Department of Chemistry, University of Kashmir, Hazratbal, Srinigar 190006, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Aerobic oxidative coupling of various arylamines and H-phosphonates to the corresponding N-aryl-
phosphoramidates has been achieved under solvent-free conditions using molecular iodine. This protocol
works at room temperature furnishing the corresponding P–N coupling products in moderate to high
yields.
Received 2 December 2013
Revised 13 January 2014
Accepted 15 January 2014
Available online 23 January 2014
Ó 2014 Elsevier Ltd. All rights reserved.
Keywords:
Phosphoramidates
Molecular iodine
Anilines
Neat conditions
Room temperature
Phosphoramidates are present as structural motifs in a number
of biologically active natural products like agrocin 84,1 phosmido-
sine,2 and microcin C7.3 Bioactive compounds like N-phosphoryl
peptides, N-phosphoryl amino acids, and phosphoramidate substi-
tuted nucleosides also contain phosphoramidate functional groups
as a key structural unit.4 Phosphoramidates are used in industry as
pharmaceutical prodrugs.5 Several aryloxy triester phosphorami-
dates (GS-9140, GS-9131, thymectacin (NB1011), and stampidine)
are in clinical trials.6 In the domain of medicinal chemistry, phos-
phonamidates have been used as surrogates for amide bonds in the
synthesis of pharmaceutically active peptide-based protease inhib-
itors since the P(@O)ANH moiety mimics the tetrahedral transition
state during amide bond hydrolysis.7 Phosphoramidates are also
used as flame retardants8 and are being used in analytical chemis-
try to improve ionization efficiency and suppress matrix-related
ion effects in MALDI-TOF mass spectrometry.9 N-arylphosphoram-
idates have also been employed for the synthesis of azetidines,10
aziridines,11 imines12 and various heterocycles such as
(i) reactions of amines with dibenzyl or dialkyl phosphites using
CCl4 or a better activating agent14 and (ii) reaction of amines with
phosphorodichloridates or phosphorochloridates using a base.4b,15
Other methods include the oxidation of phosphite triesters with I2
in the presence of alkylamines and the reduction of nitroarenes
with triethyl phophite followed by phosphorylation with triethyl
phosphite.16 These procedures are all based on the prefunctional-
ization–defunctionalization strategy. Consequently these proce-
dures use additional active agents or produce undesired
by-products. Preparation and handling of hazardous phosphoryl
halides, stoichiometric use of CCl4, use of excess base and long
reaction time present an obvious limitation. The Staudinger-phos-
phite reaction offers one alternative for phosphoramidate synthe-
sis,17 but this method is not ideal as it relies on the preparation
and use of organic azides. More recently Cu(I) salts like CuBr18
and CuI19 have successfully been employed for oxidative coupling
of amines and H-phosphonates to generate phosphoramidates.
quinazolinediones.13 As
a consequence of their broad range
applications, a plethora of methods have been developed for phos-
phoramidate synthesis. The two most common standard proce-
dures used extensively for the synthesis of phosphoramidates are
H
N
O
P
O
O
P
O
NH2
O
Air,
R.T.
H
I2 ,
O
+
Neat
⇑
Corresponding authors. Tel.: +91 (0191) 2572002; fax: +91 (0191) 2548607.
E-mail address: drbaldev1@gmail.com (B. Singh).
Scheme 1.
0040-4039/$ - see front matter Ó 2014 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2014.01.064

B. A. Dar et al. / Tetrahedron Letters 55 (2014) 1544–1548
1545
Table 1
Most of the methods reported so far involve the use of organic sol-
vents, metallic catalysts, high temperature and prolonged reaction
time. However organic reactions operated with green solvents or
without use of organic solvents are a field of active attention for or-
ganic and medicinal chemists. For economic and environmental
reasons, solvent-free protocols20 are being developed to modernize
old procedures by making them cleaner, safer, and more efficient.
The demand for both neat and efficient chemical syntheses is
becoming more popular21 and it is often claimed that the best sol-
vent is no solvent.22 In continuation of our research interest for
green chemistry protocols23 and inspired by the recent examples
of iodine-catalyzed aerobic oxidative coupling24 and oxidative
aromatization25 methods, we herein report a simple, efficient,
and solvent-free iodine catalyzed synthesis of N-arylphosphoram-
idates through aerobic oxidative cross-coupling reactions between
arylamines and H-phosphonates at room temperature.26
Optimization of catalyst amounta
Entry
Iodine (mol %)
Atmosphere
Yieldb (%)
1
2
3
4
5
6
7
8
0
Air
Air
Air
Air
Air
Air
O2
N.D
27
63
86
86
85
88
Traces
0.52
1.04
1.55
2.06
2.56
1.55
1.55
N2
a
Reaction conditions: aniline (1 mmol), diethyl phosphite (2 mmol), room tem-
perature, solvent free conditions time 60 min.
b
Isolated yield.
Table 2
Effect of solvents on the condensation of aniline and diethyl phosphitea
In order to establish the optimum conditions for the synthesis
of N-arylphosphoramidates different amounts of iodine and vari-
ous solvents were examined. Reaction of aniline (1 equiv) with
diethylphosphite (2 equiv), was chosen as a model reaction to
study the effect of iodine at atmospheric conditions (Scheme 1).27
All catalyst-screening reactions were performed in solvent-free
conditions. We observed that molecular iodine catalyzes the model
reaction at room temperature with constant stirring (Table 1). The
reaction did not occur under catalyst-free conditions even after
24 h (Table 1, entry 1). The conversion rate and yields were found
to increase with larger amounts of molecular iodine. With
0.52 mol % of molecular iodine we observed 27% yield of the de-
sired product (Table 1, entry 2), which increased to 63% when
1.04 mol % of iodine was used (Table 1, entry 3). The optimum
amount of iodine for the model reaction was found to be
1.55 mol %, which yielded 86% of the desired product (Table 1, en-
try 4). Increasing the catalyst above this amount could not bring
any significant increase in the product yield (Table 1, entries 5
Entry
Solvent
Yieldb (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
Acetonitrile
DCM
65
62
52
46
67
72
42
60
61
62
78
47
53
86
45
Methanol
n-Butanol
Toluene
Chloroform
Water
DMF
THF
DMSO
Benzene
Ethanol
Hexane
No solvent
Ethyl acetate
a
Reaction conditions: aniline (1 mmol), diethyl phosphite (2 mmol) catalyst
(1.55 mol %), room temperature, time 60 min.
b
Isolated yield.
Table 3
Iodine catalyzed synthesis of N-arylphosphoramidatesa
Entry
Arylamine
Dialkyl phosphite
Product
Time (min)
30
Yieldb (%)
79
HO
NH2
NH2
O
H
N
O
P
O
HO
O
H
H
H
P
O
O
O
O
1
Cl
O
P
O
H
O
Cl
N
O
P
O
2
3
4
30
30
30
83
75
68
O
P
O
H
N
O
P
O
NH2
O
NH2
O
P
O
H
N
O
P
O
O
H
H
O
O
F
F
NO2
O
P
O
NO2
H
N
O
P
O
NH2
O
5
6
30
30
80
O
O
O
H
N
O
H
NH2
O
P
O
O
P
O
74
(continued on next page)

1546
B. A. Dar et al. / Tetrahedron Letters 55 (2014) 1544–1548
Table 3 (continued)
Entry
Arylamine
Dialkyl phosphite
Product
Time (min)
30
Yieldb (%)
NH2
O
H
N
O
P
O
O
H
P
O
O
O
O
7
8
9
61
O
O
NH2
O
P
O
H
N
O
P
O
O
H
H
O
O
30
30
64
62
OH
O
P
O
OH
H
O
P
O
NH2
N
O
Cl
Cl
Cl
O
P
O
Cl
H
N
O
P
O
NH2
H
O
O
10
30
43
NH2
O
P
H
N
O
P
O
H
H
H
H
H
H
H
O
O
O
O
O
O
O
11
12
13
14
15
16
30
30
30
30
30
30
86
O
O
NH2
NH2
O
P
O
H
N
O
P
O
O
O
O
Traces
88
HO
NC
HO
NC
O
P
O
H
N
O
P
O
NH2
O
P
O
H
N
O
P
O
82
O
O
O2N
NH2
O
P
O
H
O
O2N
N
O
P
O
80
NC
NH2
O
P
O
H
O
P
O
NC
N
O
80
CN
O
P
O
CN
H
N
O
P
O
NH2
O
17
30
75
NH2
O
P
O
H
N
O
P
O
O
O
O
O
O
H
H
O
O
19
20
30
60
52
44
O
P
O
O
O
H
N
O
P
O
O
NH2
NH2
O
P
O
H
O
P
O
N
O
H
H
21
22
60
60
30
28
O
O
F
F
Cl
O
P
O
Cl
H
N
O
P
NH2
O
O

B. A. Dar et al. / Tetrahedron Letters 55 (2014) 1544–1548
1547
Table 3 (continued)
Entry
Arylamine
NH2
Dialkyl phosphite
Product
Time (min)
60
Yieldb (%)
O
H
N
O
P
O
O
O
O
H
H
P
O
23
24
32
O
O
Br
O
P
O
Br
H
N
O
P
O
NH2
NH2
60
60
60
Traces
OH
O
P
O
OH
H
N
O
P
O
H
H
O
25
Traces
50
NO2
NO2
NH2
O
P
O
H
O
P
N
O
26
O
Cl
O
Cl
a
Reaction conditions: arylamine (1 mmol), dialkyl phosphite (2 mmol) catalyst (1.55 mol %), room temperature.
Isolated yield.
b
I2
O
PH
O
O
PH
O
O
P
O
NH2
OH
P
O
I2
O2
H
N
H
N
O
O
O
O
-I2
-H2O
H
Scheme 2. Plausible mechanism of iodine catalyzed cross-dehydrogenative coupling of arylamines and H-phosphonates.
and 6). Under nitrogen atmosphere, the desired product was ob-
tained in traces which indicate that oxygen is essential to this reac-
tion (Table 1, entry 6). Supplying pure oxygen to the reaction
system leads to a slight increase in the product yield (Table 1, entry
6), but it was more convenient to conduct the reaction under aer-
obic conditions.
amines were found to react sluggishly and the desired products
were obtained with comparatively lower yields (Table 3, entries
4, 7–9, 12), Comparatively, ortho and meta substituted aniline re-
acted fast and better yields were obtained (Table 3, entries 1–3,
5 and 6, 10, 15–17). Dimethyl phosphite was also a good substrate
for this cross-coupling reaction. Interestingly, diethyl phosphite
(Table 3, entries 1–19) performed more effectively and afforded
the desired product in higher yield than dimethyl phosphite
(Scheme 2 Table 3, entries 20–26).
In conclusion we have developed a facile oxidative cross cou-
pling reaction for the synthesis of phosphoramidate products using
solvent-free conditions. Operational simplicity, mild conditions,
and the environmental friendliness are the features of this proto-
col. Only water is produced as a by-product.
With 1.55 mol % iodine we proceeded to screen a range of sol-
vents like THF, toluene, benzene, DCM, ethylacetate, methanol,
ethanol, acetonitrile, DMF, DMSO, and water to determine the sol-
vent effect on the present protocol. As a general observation, the
reaction takes place in every solvent at varying degrees. Polar,
non-protic solvents like chloroform, acetonitrile, DMF, DMSO,
DCM, and the THF (Table 2, entries 1, 2, 6, 8–10) and non polar
solvents like toluene, benzene, and hexane (Table 2, entries 5,
11, 13) resulted in better yields of the desired product. However,
in protic solvents like methanol, ethanol, n-butanol, and water,
lesser yields were obtained and reaction time was longer to reach
completion (Table 2, entries 3, 4, 5, 12). Conducting the model
reaction in ethyl acetate could not bring any improvement in
the product yield (Table 2, entry 15). When the reaction was car-
ried under solvent-free condition, to avoid the use of organic sol-
vents, even better yields were obtained with shorter reaction
time (Table 2, entry 14).
When the reaction time for the model reaction was extended to
3 h, the yield dropped to 64%. With optimized conditions in hand,
this protocol was explored using an assortment of arylamines (Ta-
ble 3). To study the scope and the limitations of this novel method,
we investigated the reaction using several electron-donating and
electron-withdrawing substituted arylamines with diethylphosph-
ite under solvent-free conditions. The reaction was found to be
compatible with various functional groups such as Cl, Br, F, OMe,
OEt, NO2, CN, and OH. It was observed that aniline substituted with
electron withdrawing group required less time than the amine
substituted with electron donating group. para Substituted aryl-
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2014.01.
064.
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reaction mixture was diluted with ethyl acetate and washed with water. The
organic phase was dried over anhydrous Na2SO4 and removed under reduced
vacuum. The crude product thus obtained was purified by silica gel column
chromatography eluting with ethyl acetate and petroleum ether (2:1) to afford
the desired products.
8919.
20. (a) Herna´ndez, J. G.; Juaristi, E. J. Org. Chem. 2010, 75, 7107; (b) Li, S.; Wang, J.
X.; Wen, X.; Ma, X. Tetrahedron 2011, 67, 849.