N,N′-dichlorobis(2,4,6-trichlorophenyl)urea (CC-2): An efficient reagent for the synthesis of chemical weapons convention-related dialkyl-N,N-dialkylphosphoramidates from dialkylphosphites

Article abstract of DOI:10.1071/CH07081

The present paper describes the one-pot synthesis of dialkyl N,N-dialkylphosphoramidates (DADAP) from dialkylphosphites and dialkylamines using N,N?-dichlorobis(2,4,6-trichlorophenyl)urea (CC-2) as chlorinating reagent. DADAP belong to the schedule 2.B.6 category of the Chemical Weapons Convention (CWC), as they are the important markers of the chemical warfare agent tabun and its analogues. The study was undertaken to develop the spectral database of DADAP for verification of CWC. The reported synthetic strategy can be adopted to rapidly synthesize several analogues of DADAP during official proficiency tests. CSIRO 2007.

Full text of DOI:10.1071/CH07081

Full Paper
CSIRO PUBLISHING
www.publish.csiro.au/journals/ajc
Aust. J. Chem. 2007, 60, 879–882
N,Nꢀ-Dichlorobis(2,4,6-trichlorophenyl)urea (CC-2): an
Efficient Reagent for the Synthesis of Chemical Weapons
Convention-Related Dialkyl-N,N-dialkylphosphoramidates
from Dialkylphosphites
Arvind K. Gupta,A Deepak Pardasani,A Hemendra K. Gupta,A
and Devendra K. DubeyA,B
AVertox Laboratory, Defence Research and Development Establishment, Gwalior, India.
BCorresponding author. Email: dkdubey@rediffmail.com
The present paper describes the one-pot synthesis of dialkyl N,N-dialkylphosphoramidates (DADAP) from dialkylphos-
phites and dialkylamines using N,Nꢀ-dichlorobis(2,4,6-trichlorophenyl)urea (CC-2) as chlorinating reagent. DADAP
belong to the schedule 2.B.6 category of the Chemical Weapons Convention (CWC), as they are the important mark-
ers of the chemical warfare agent tabun and its analogues. The study was undertaken to develop the spectral database of
DADAP for verification of CWC. The reported synthetic strategy can be adopted to rapidly synthesize several analogues
of DADAP during official proficiency tests.
Manuscript received: 16 March 2007.
Final version: 28 August 2007.
Introduction
their synthesis (in pure form by reported methods) and record-
ing of spectral data is a daunting task. This problem can be
obviated by the development of efficient synthetic procedures
that can provide pure compounds in the shortest possible period.
Rapid synthesis of these compounds in pure form is essential
to confirm the structures of analytes identified from their spec-
troscopic (e.g. gas chromatography–mass spectrometry coupled
technique, 31P NMR spectroscopy, and gas chromatography
infrared spectroscopy) analysis. The match of spectra with syn-
thesized compounds is one of the unique ways to report the
results of analysis. Furthermore, the spectra and chromato-
graphic data gathered by the synthesis of pure compounds are
also helpful in off-site analysis of real samples.
Alkyl N,N-dialkylphosphoramidocyanidates (ADAPC) are
analogues of the highly toxic nerve agent tabun (ethyl N,N-
dimethylphosphoramidocyanidate) that are placed in schedule
1.A.2 of CWC.[9] Based on commercially available C1–C10
alcohols, the estimated number of compounds belonging to
schedule 1.A.2 is 48 000. Dialkyl N,N-dialkylphosphoramidates
(DADAP) are often produced when tabun or its derivatives
are produced in any laboratory or plant.[5] Hence, DADAP are
considered as important chemical markers of tabun and its ana-
logues. Consequently, DADAPareincludedintheschedule2.B.6
category of CWC and hence are extremely important from a
verification view point of the CWC.[14]
Efficient syntheses of reference chemicals required for the
verification analysis of chemical warfare agents (CWA) is of
paramount importance.[1–6] This field of synthesis and analy-
sis has attracted the attention of several groups owing to the
strict verification regime of the Chemical Weapons Convention
(CWC).[7,8] The CWC entered into force on 29 April 1997 and
prohibits the development, production, stockpiling, and usage of
chemical weapons.[9] By April 2007, 184 countries had ratified
the convention. The Organisation for Prohibition of Chemical
Weapons (OPCW), an international organization, administers
the CWC through its stringent verification program. Verifica-
tion involves collection of samples from production, storage,
and suspected sites by inspectors appointed by the OPCW.[9]
Generally, inspectors carry out the analysis of collected samples
on site; in case of any ambiguity, the samples are sent to at least
two off-site laboratories, designated by the OPCW, for unam-
biguous identification of CWC-related chemicals.[10] Thus, a
laboratory willing to undertake the challenging task of off-site
analysis of CWC-related chemicals must first achieve the status
of ‘designated laboratory’.[9,10] A laboratory can become and
stay designated only by proving its analytical capabilities in Offi-
cial Proficiency Tests (PT) conducted by the OPCW.[11–13] The
performance of participating laboratories in PT mainly depends
on the availability of spectral data of CWC-related chemicals,
sample preparation, analytical skills of chemists, judgment, and
reporting. It is important to note that a 15-calendar-days time-
line is a crucial part in the analysis of PT samples and reporting
the results. As the schedules of compounds are based on com-
mercial and past use of chemicals as CWA, the total number
of scheduled compounds is several hundred thousand. Hence,
Our continued efforts in the field of synthetic and analyt-
ical advancement of scheduled chemicals[1–5] have prompted
us to further advance the synthesis of DADAP. A plethora
of effective chemical approaches have been devised for the
preparation of phosphoramidates.[1,2,15–24] Among these, two
approaches are worthy of mention here. The first approach
© CSIRO 2007
10.1071/CH07081
0004-9425/07/110879

880
A. K. Gupta et al.
Cl
Cl
Cl
O
P
Cl
O
P
Cl
Cl
RO
RO
RЈ
RЉ
Cl
Cl
O
RЈ
RЉ
RO
RO
Al2O3
O
H
ϩ
N
H
ϩ
ϩ
N
N
H
N
CH3CN, 20–50 min,
temp. 0–40°C
N
Cl
N
Cl
H
Cl
Cl
Cl
Cl
1
2
3
4
Scheme 1.
involves the condensation of dialkyl chlorophosphates with
dialkylamines in the presence of a base.[20–24] This method
requires the synthesis of dialkyl chlorophosphates that are
moisture-sensitive and unstable. The second approach involves
condensation of N,N-dialkylphosphoramidic dichloride with
alcohols in the presence of Al2O3 as a solid support.[2]
This method is straightforward, yet has a major drawback in
the preparation of N,N-dialkylphosphoramidic dichlorides. The
synthesis of these moisture-sensitive intermediates is time-
consuming, and requires a strong base and tedious workup. Yet
another method involves solution-phase condensation of N,N-
dialkylphosphoramidic dichloride with alcohols in the presence
of base,[1] yet it also has the same disadvantages as mentioned
above. Thus, state-of-the-art synthesis of DADAP requires a
strategy that would permit an expedious, mild, and efficient syn-
thesis of the title compounds. It would be an extra advantage if
the synthesis of these compounds could be carried out in one pot
without elaborate workup.
Because of this, reactions of dialkylamines with dimethyl phos-
phite were faster, and were slowest with diisopropyl phosphate
(entry 34,Table 1). Furthermore, while studying the effect of var-
ious amines with in situ-generated dialkyl chlorophosphates, we
found that the reactions of dimethylamine were faster than that
of ethylmethylamine. It was also noticed that the reactivity of the
amines decreased with an increase in the chain length. However,
diisopropylamine took a somewhat longer time (35–60 min) in
each case (Table 1). The results of the present study indicated
that the order of reactivity of amines is mainly governed by their
steric demand.
The important advantages of this reaction are: (i) its occur-
rence at room temperature; (ii) short reaction time; and (iii)
the by-product bis(2,4,6-trichlorophenyl)urea 4 does not react
with amines and is insoluble in any solvent, facilitating its easy
isolation and recycling. Recycling of 4 was made possible by
removing the alumina from the precipitate by dissolving in alkali
solution and obtaining 4 by filtration, which was further con-
verted to CC-2 as per our earlier reported procedure.[26] From the
filtrate, the desired products could be obtained in pure form by
distillation after distilling off the solvent. This method is advan-
tageous in comparison with the one developed by Atherton and
Todd,[20] as it takes place at room temperature and in a single
step,[4] whereas the classical method requires additional base
(two moles of amines) and a longer reaction time. The method is
an advancement over the one developed by Saunders[20b] using
N-chlorosuccinimide that bears one active chlorine atom per
molecule; CC-2 contains two active chlorines, and hence half
mole of this latter reagent is sufficient to complete the reaction.
The second advantage of using CC-2 is that its dechlorinated
product 4 is practically insoluble in the organic solvent, hence
facilitating its complete removal by filtration. In contrast, suc-
cinimide is soluble in organic solvents and requires extra efforts
to remove it from the reaction mixture. To examine the repro-
ducibility of this one-pot concept at higher mole ratio, one mole
of diethyl phosphite was treated with 0.5 mol of CC-2 in acetoni-
trile at room temperature. The reaction mixture was stirred for
15 min; 31P NMR spectroscopy revealed complete conversion of
the diethyl phosphite into the corresponding diethyl chlorophos-
phate within 15 min. The heterogeneous reaction mixture was
cooled to 0◦C, and alumina (1.5 mol) was added. Subsequently
ethylmethylamine (1 mol) was added slowly. After addition of
amine, the reaction mixture was warmed to room tempera-
ture and shaken on a vortex shaker. The 31P NMR spectrum
was recorded by drawing a few milligrams of sample from the
reaction mixture after 10 min and extracting it with ether. The
results of NMR indicated that the signal of diethyl chlorophos-
phate at δ 4.14 completely vanished and a new signal appeared
at δ 13.58, which clearly demonstrated the rapid conversion
of diethyl chlorophosphate into the corresponding O,O-diethyl
N-ethyl-N-methyl phosphoramidate. Filtration of the heteroge-
neous reaction mixture and concentration of the solvent provided
the pure product in 92% isolated yield.
We envisaged that if dialkyl chlorophosphate could be pro-
duced in situ from dialkylphosphites by an active chlorine-
releasing reagent, followed by condensation with dialkylamines,
it would generate the required DADAP. For this, we resorted
to N,Nꢀ-dichlorobis(2,4,6-trichlorophenyl)urea (CC-2) as an
active chlorine-generating reagent,[25–27] and herein report the
optimization of reaction parameters.
Results and Discussion
A typical reaction scheme adopted to generate the DADAP is
illustrated in Scheme 1. The dialkyl phosphates 1 were treated
with CC-2 2 to generate the dialkyl chlorophosphates, which
were used to phosphorylate different dialkylamines in the same
reaction mixture in the presence of activated alumina. In a typical
reaction, when dialkyl phosphite was mixed with a suspension
of CC-2 in acetonitrile, the corresponding dialkyl chlorophos-
phate was generated, which was further treated in the same vessel
with dialkylamine in the presence of alumina to produce the
desired phosphoramidate 3. Reactions with varying mole ratios
of reactants were carried out, and maximum yields were obtained
whendialkylphosphites, CC-2, alumina, anddialkylamineswere
mixed at molar ratios of 1:0.5:1.5:1.0, respectively. Details of the
reaction protocol are described in the Accessory Publication.
To investigate the effect of dialkyl groups on the phosphory-
lation of dialkylamines, various dialkyl phosphites were treated
with diethylamine in the presence of CC-2. Phosphorylation of
diisopropyl phosphite with diethylamine was found to be slower
(as it took 40 min to complete), in contrast to most of the reac-
tions, which were completed within 20 min. This observation is
in accordance with the higher steric demand of the transition
state formed in the reaction. Second, this slow reaction can also
be explained by the positive inductive effect of the diisopropyl
group, which reduces the electrophilic susceptibility of the P=O
bond responsible for the nucleophilic attack by dialkylamines.

Synthesis of DADAP
881
Table 1. One-pot synthesis of dialkyl N,Nꢀ-dialkyl phosphoramidates from dialkyl phosphites and
N,Nꢀ-dichlorobis(2,4,6-trichlorophenyl)urea (CC-2)A
Entry
R
Rꢀ
Rꢀꢀ
Time [min]
Temp. [◦C]
YieldB [%]
1
2
3
4
5
6
7
8
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
C2H5
C2H5
C2H5
C2H5
C2H5
C2H5
CH3
C2H5
CH3
15
20
30
35
25
30
25
25
30
30
20
30
30
35
25
25
30
25
30
30
30
30
30
45
30
30
35
30
35
30
35
40
45
60
30
40
35
40
35
35
25
35
30
35
30
30
35
30
35
30
25
30
30
40
25
30
35
25
35
30
30
30
30
45
30
35
30
35
35
35
30
40
50
50
35
35
35
35
40
40
88
86
86
85
81
80
85
82
85
80
86
81
85
84
92
83
82
83
82
84
88
86
84
80
82
88
81
84
82
88
82
83
80
85
81
83
83
84
85
86
C2H5
n-C3H7
i-C3H7
C2H5
C2H5
C2H5
CH3
n-C3H7
CH3
CH3
C2H5
n-C3H7
i-C3H7
C2H5
C2H5
C2H5
CH3
n-C3H7
CH3
CH3
C2H5
n-C3H7
i-C3H7
C2H5
C2H5
C2H5
CH3
n-C3H7
CH3
CH3
C2H5
n-C3H7
i-C3H7
C2H5
C2H5
C2H5
CH3
n-C3H7
i-C3H7
CH3
n-C3H7
i-C3H7
n-C3H7
i-C3H7
i-C3H7
CH3
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
C2H5
n-C3H7
i-C3H7
CH3
n-C3H7
i-C3H7
n-C3H7
i-C3H7
i-C3H7
CH3
C2H5
C2H5
C2H5
C2H5
n-C3H7
n-C3H7
n-C3H7
n-C3H7
n-C3H7
n-C3H7
n-C3H7
n-C3H7
n-C3H7
n-C3H7
i-C3H7
i-C3H7
i-C3H7
i-C3H7
i-C3H7
i-C3H7
i-C3H7
i-C3H7
i-C3H7
i-C3H7
C2H5
n-C3H7
i-C3H7
CH3
n-C3H7
i-C3H7
n-C3H7
i-C3H7
i-C3H7
CH3
C2H5
n-C3H7
i-C3H7
CH3
n-C3H7
i-C3H7
n-C3H7
i-C3H7
i-C3H7
n-C3H7
CH3
AReactions were monitored by 31P NMR in CDCl3 using 162 MHz NMR.
BReactions were carried out at ambient temperature and the products had satisfactory infrared, NMR, and gas
chromatography–mass spectrometry coupled technique (GCMS) data. GCMS data were compared with literature
values.[28]
It must be noted that a limitation of this new method
is the non-commercial availability of CC-2. Nevertheless, it
can be easily prepared and stored in the laboratory using the
straightforward method described in ref. [25].
consumption of dialkyl phosphite. A white amorphous precip-
itate of symmetrical bis(2,4,6-trichlorophenyl)urea 4 formed.
When precipitation of 4 ceased, the reaction mixture was cooled
to 0◦C and neutral alumina (30.60 g, 0.3 mol) was added. The
dialkylamine (0.2 mol) was added slowly while shaking the het-
erogeneous reaction mixture on a vortex shaker. The progress of
the reaction was further monitored by 31P NMR spectroscopy to
determine the consumption of dialkyl chlorophosphate by draw-
ing a few milligrams of reaction mixture and extracting with
ether. After completion of the reaction, the white amorphous
precipitate of symmetrical bis(2,4,6-trichlorophenyl)urea 4 and
alumina was removed by filtration and washed with acetonitrile
(3 × 10 mL). The solvent was removed from the filtrate and the
product was distilled under vacuum to get pure DADAP. General
Experimental
General Experimental Procedure
In a typical experimental procedure, dialkylphosphite (0.2 mol)
was added in a suspended solution of N,Nꢀ-dichlorobis(2,4,6-
trichlorophenyl)urea (48.8 g, 0.1 mol) in 150 mL acetonitrile,
and the reaction mixture was stirred for 10–30 min.The progress
of the reaction was monitored by 31P NMR spectroscopy by
drawing a few milligrams of reaction mixture to determine the

882
A. K. Gupta et al.
experimental details and data for each compound synthesized
can be found in the Accessory Publication.
(b) Work Instructions for the Evaluation of Results of OPCW Profi-
ciency Tests, QDOC/LAB/WI/PT3 17 September 2004.
(c) M. Rautio, Recommended Operating Procedures for Sampling and
Analysis in theVerification of Chemical Disarmament 1994 (Ministry
for Foreign Affairs of Finland: Helsinki).
Conclusions
In conclusion, we have developed a simple, rapid, economical,
and efficient one-pot procedure for the preparation of DADAP.
The method is very useful to build up the spectral database
of CWC-related phosphoramidates. The method could also be
extended to prepare other phosphoramidates having biological
activity. It will make a useful and an important addition to exist-
ing methodologies. In addition to this, the recycling and reusing
of the reagent and the convenient workup producing high yields
makes it an attractive methodology. Further application of CC-2
in the synthesis of various organic compounds is in progress and
will be reported in due course.
(d) Standard Operating Procedure for the Organization of OPCW
Proficiency Tests, QDOC/LAB/SOP/PT1 17 September 2004.
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General experimental details and data for each compound syn-
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Acknowledgements
We thank Dr R. Vijayaraghavan, Director, DRDE, Gwalior, for his keen
interest and encouragement, and also Ms Mamta Sharma for recording the
NMR spectra.
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