Biodynamic organothiophosphates of pyridine: Synthesis and bioactivity

Article abstract of DOI:10.1080/10426500701641098

The biological importance and practical significance of thiophosphates and pyridine nucleus prompted us to synthesize organothiophosphate of pyridine. In this regard, we have synthesized new series of such derivatives adopting the synthetic methodology, which we have evolved in our laboratory using cycloiminium derivatives of pyridine as starting material and PCl3 as phosphorylating agent. These representatives were also tested for their insecticidal activity against a notorious polyphagus insect Helicoverpa armigera. Copyright Taylor & Francis Group, LLC.

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Biodynamic
Organothiophosphates of
Pyridine: Synthesis and
Bioactivity
Vijaya Kabra a , Arpana Meel a , Swaroop Singh b ,
Sunita Mitharwal a & Suneeta Ojha a
a Department of Chemistry , University of
Rajasthan , Jaipur, India
b Department of Entomology , Agricultural research
Station, Durgapura , Jaipur, India
Published online: 05 Jun 2008.
To cite this article: Vijaya Kabra , Arpana Meel , Swaroop Singh , Sunita Mitharwal
& Suneeta Ojha (2008) Biodynamic Organothiophosphates of Pyridine: Synthesis
and Bioactivity, Phosphorus, Sulfur, and Silicon and the Related Elements, 183:6,
1301-1312, DOI: 10.1080/10426500701641098
To link to this article: http://dx.doi.org/10.1080/10426500701641098
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Phosphorus, Sulfur, and Silicon, 183:1301–1312, 2008
Copyright © Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426500701641098
Biodynamic Organothiophosphates of Pyridine:
Synthesis and Bioactivity
Vijaya Kabra,1 Arpana Meel,1 Swaroop Singh,2
Sunita Mitharwal,1 and Suneeta Ojha1
1Department of Chemistry, University of Rajasthan, Jaipur, India
2Department of Entomology, Agricultural research Station, Durgapura,
Jaipur, India
The biological importance and practical significance of thiophosphates and pyri-
dine nucleus prompted us to synthesize organothiophosphate of pyridine. In this
regard, we have synthesized new series of such derivatives adopting the synthetic
methodology, which we have evolved in our laboratory using cycloiminium deriva-
tives of pyridine as starting material and PCl as phosphorylating agent. These
3
representatives were also tested for their insecticidal activity against a notorious
polyphagus insect Helicoverpa armigera.
Keywords Insecticide; phosphorylation; thiophosphates
INTRODUCTION
A number of pyridine derivatives occur in nature and some of them play
very vital biochemical roles. Pyridine derivatives have been found to be
useful as antitumor,1 antileukemic,2 antimicrobial,3 and anthelmintic4
agents. Due to this versatile behavior of pyridine, we have selected
2-aminopyridine as the starting material for the synthesis of the previ-
ously mentioned derivatives. N-alkyl-2-aminopyridinium halides have
been prepared with a view to generate new series of aminophosphines,
in situ, which led to a variety of amidothiophosphates and trisami-
dothiophosphates with potential biocidal properties. In an extension to
our previous work,5−8 the new series of thiophosphates have been syn-
thesized using few activating groups at nitrogen atom of the pyridine
ring.
Received 18 June 2007; accepted 30 July 2007.
Address correspondence to Vijaya Kabra, Department of Chemistry, University of
Rajasthan, Jaipur, India 302 004. E-mail: sgkabra@sancharnet.in
1301

1302
V. Kabra et al.
RESULTS AND DISCUSSION
The N-alkyl-2-aminopyridinium halides (1) reacted with an equimolar
amount of phosphorus trichloride in the presence of two equivalents
of triethylamine in methylene chloride, at 0–5◦C. After 3–4 h, stirring
at ambient temperature in situ, the oxidation of aminodichlorophos-
phines (2) was accomplished with equimolar amount of elemental sul-
fur, which led to N-alkyl-2-pyridinylidenamidothiophosphoryl dichlo-
rides (3) (Scheme 1).
SCHEME 1
3a–c were subsequently subjected to nucleophilic substitution with
different secondary amines viz. diethylamine, diphenylamine, and N-
methylcyclohexylamine (Scheme 2).
Through this synthetic strategy, two series of alkylidenamidoth-
iophosphoric esters, namely, cycloiminylidenami-dothiophosphoric
dichlorides (3a–c), and cycloiminylidenamidobis(dialkyl/diaryl/
alkylalicyclicamido)thiophosphates (4–6) have been obtained in good
yield.
All these newly synthesized products 3–6 were white to yellow, sharp
melting crystalline solids, and were soluble in slightly polar solvents,
e.g., chloroform, methylene chloride, diethylether, etc. The physical and
analytical data and bioactivity data of these products have been pre-
sented in Tables I and II.
CHARACTERIZATION
31P NMR Spectra
The 31P NMR spectroscopy was found to be quite promising for deter-
mining the reaction progress and identification of the products.

Biodynamic Organothiophosphates of Pyridine
1303
SCHEME 2
When phosphorus trichloride and triethylamine were added to a sus-
pension of the quaternary cycloiminium salts in methylene chloride and
the mixture was examined for its phosphorus signal, the 31P NMR sig-
nal appeared at δ 179.0–191.8 ppm, which was characteristic signal for
tri-coordinated phosphines (2), which on course of further reaction, was
shifted to upfield region, i.e. δ 44.1–50.0 ppm (3), and δ 44.2–63.8 ppm
(4–6). Pure solids also gave signals in the region δ 40.6–63.7 ppm.
1H NMR Spectra
The structures of the products were further confirmed by 1H NMR
spectroscopic studies. The methyl protons of diethylamido moiety i.e.
PNCH2CH3, resonated as a triplet at δ 1.01–1.09 ppm due to three-bond
coupling with adjacent methylene protons (3 J = 6.9–7.2 Hz). The
HH
methylene protons of the same moiety (PNCH2CH3), however, showed
interesting splitting pattern, a doublet of doublet of quartet was ob-
served due to coupling with geminal and vicinal protons as well as
three-bond coupling with phosphorus atom (3 J = 13.2–6.9 Hz). The
PH
two set of such signals were observed in the range δ 3.07–3.12 and δ
3.25–3.31 ppm. This observation suggested the diastereotopic nature
of the methylene protons.
The NCH2 protons (attached to the pyridine ring) of 3–6b,c were
observed as a singlet in the range δ 4.25–4.32 ppm while the same

1304

1305

1306

Biodynamic Organothiophosphates of Pyridine
1307
protons in 3–6a resonated as a quartet at δ 4.09 ppm (3 J = 7.1 Hz)
HH
due to coupling with three-bond apart methyl protons (δ 1.39 ppm).
The OCH3 protons of 3–6b resonated as a singlet at δ 3.87 ppm while
the OCH2CH3 protons resonated as a characteristic set of triplet at δ
1.34 ppm (3 J = 6.1 Hz) and quartet at 4.22 ppm (3 J = 6.1 Hz).
HH
HH
Other aromatic protons have resonated in the expected regions, δ
7.01–7.62 ppm. The H-5 proton resonated as a doublet of triplet in the
range δ 5.28–6.25 ppm (3 J = 6.0–7.2, 4 J = 1.2 Hz) due to three and
HH
HH
four-bond coupling with H-3, H-4 and H-6. The H-3 proton resonated as
a doublet of doublet or merged triplet in the range of δ 7.25-7.61 ppm
due to vicinal coupling with H-4 and four-bond coupling with H-5 with
coupling constants 3 J = 6.9–7.2 Hz, 4 J = 1.2 Hz, respectively. The
HH
HH
H-4 proton also showed a doublet of doublet in the range δ 7.25–7.61
ppm due to three-bond coupling with H-3 proton (3 J = 6.1–7.0 Hz).
HH
Since there is very less difference in the chemical shift values of the
H-4 and H-6 protons, a merged multiplet was observed for these pro-
tons in few cases. The ring protons of PN(CH3)c-C6H11 were shielded
and appeared as a multiplet in the region δ 0.82–1.65 ppm, suggesting
the nonequivalent nature of these protons. The equatorial protons have
been found more deshielded in comparison to axial protons with a differ-
ence of δ 0.1–0.7 ppm.9 The methyl protons of PN(CH3)c-C6H11moiety
resonated as a doublet in the range δ 2.43–2.51 ppm due to three-bond
coupling with phosphorus (3 J = 12.0–12.5 Hz).
PH
13C NMR Spectra
The NCH2CH3 carbon is highly shielded and appeared at δ 14.0 ppm,
while the NCH2CH3 carbon resonated at δ 39.6 ppm. The NCH2 carbon
absorbed at δ 60.2 ppm. The aromatic carbons, C-3, C-4, C-5, and C-6,
resonated at δ 138.0, 120.4, 107.5, and 142.0 ppm, respectively. The C-2
carbon being quaternary gave a very low intensity peak at δ 153.9 ppm.
The highly deshielded CO appeared at δ 167.9 ppm. The 13C NMR data
ꢀ
were also helpful in establishing the proposed structure. The Cα Cβ,β
,
ꢀ
Cγ,γ , and Cδ of PN(CH3)c-C6H11, appeared at δ 25.8–30.9 ppm. The
PN(CH3)c-C6H11 showed signal at δ 54.0 ppm. The NCH2C6H5 carbon
resonated at δ 54.8 ppm. The meta carbon of NCH2C6H5moiety gave a
doublet at δ 127.4 and δ 127.7 ppm. Similarly, ortho carbons also gave a
doublet at δ 136.3 and δ 137.4 ppm. The para carbon showed a singlet
at δ 128.8 ppm.
BIOLOGICAL ACTIVITY
The protection of agricultural crops and boosting up of their products
through the wise and considerate use of appropriate agrochemicals,

1308
V. Kabra et al.
e.g. pesticides, fertilizers, growth promoters etc., have played a very
crucial and needful role in ensuring sufficient food supply to an ever
exploding world population. Historically, highly chlorinated insecti-
cides have been predominantly used and the most important and clas-
sical examples were undoubtedly, DDT, 2,2-bis(p-chlorophenyl)-1,1,1-
trichloroethane, which were now regarded as hard insecticides,10−13 i.e.,
stable in environment and exist as such for many years, not only in soil
and water, but in plants and animal tissues as well.14−15 Since, pests
develop resistance against the commonly used pesticides in due course
of time, we need to continually design and synthesize new organophos-
phorus pesticides with potential broad spectrum of bioactivity and less
toxicity to higher animals.
In this light, four synthesized novel organothiophophates (4–5bc)
have been screened for their insecticidal activity against Helicoverpa
armigera, a polyphagus insect that severely damages almost all the
crops throughout the year in India. The study has been conducted on the
second and third instar larval stages of the said insect. Two concentra-
tions, viz., 0.025% and 0.05% of the test compounds were selected along
with the standard check, the endosulfan 35, together with an untreated
control. The mortality counts of the insect pests were recorded daily up
to fourteen days. All the test compounds initially were found to exhibit
low bioactivity up to 3 days but after 7 and 14 days time interval their
activity were increased to be at par with the check compound (Figure 1,
Table II). This observation has indicated that the metabolites of the test
compounds may be even more potent than the original compound them-
selves. It has been already reported in literature11 that under natural
conditions, these compounds suffers a P S → P O bond change in due
course of time, that is, the thiophosphates is oxidized to phosphates by
the air in soil. Furthermore, nitrogen of the diethylamido substitutent
(4b,c) is converted into nitrogen oxide in the similar conditions, i.e., in
air and soil. Hence, it is just possible that the enhanced insecticidal be-
havior of all the test compounds after 7 days onwards arises due to the
above mentioned chemical changes in the molecules and due to electron
withdrawing chlorine and ester moieties.11 This property also seems to
be a plus point for test compounds which on longer stay on the crops,
ensures killing of all the stages (I, II, III, and also the pupal stages) of
this insect. This study thus confirmed the potential insecticidal nature
of the compounds.
EXPERIMENTAL
All the glassware used was washed first with alkali, followed by dilute
acid solution then plenty of water, and further rinsed with acetone or

1309

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V. Kabra et al.
alcohol, dried in an electric oven. Good quality dry needles and sy-
ringes were used for adding reactants during the course of reactions.
All the solvents used in this work were carefully purified prior to use
while fine chemicals were used as obtained from Aldrich, Lancaster,
and Merck. Elemental analyses were carried out on Heraeus Carlo
Erba 1108 analyzer. 31P NMR spectra were recorded on Jeol AL300
at 121.50 MHz (obset 156 KHz) using 85% H3PO4 as an internal stan-
1
dard. H NMR spectra were recorded in CDCl3 solution also on Jeol
AL300 at 300.4 MHz (obset 130 KHz) using TMS as the internal refer-
ence. 13C NMR spectra were similarly recorded in deuterated solvent on
Jeol AL300 at 75.45 MHz (Obset 124 KHz) using TMS as the internal
standard.
General Procedure for the Synthesis of
N-alkyl-2-pyridinylidenamido-dichlorothiophosphates (3a–c)
To a well-stirred suspension of N-alkyl-2-aminopyridinium halide (0.01
mol) in methylene chloride, was added phosphorus trichloride (0.01 mol)
followed by dropwise addition of triethylamine (0.02 mol) in methylene
chloride at 0–5◦C. After 3–4 h stirring, powdered sulfur (0.01 mol) was
added to reaction mixture and stirring was continued. After 15–16 h,
the reaction mixture was then filtered off. The solvent of filtrate was
removed under vacuo and the crude was extracted with diethyl ether
and was left in refrigerator whereupon a white to yellow colored solid
was obtained.
General procedure for the Synthesis of (N-alkyl-2-
aminopyridinyli-denamido)bis(diethylamido)thiophosphates
(4a–c)
2-aminopyridinium halide (0.01 mol), was stirred in methylene chloride.
Phosphorus trichloride (0.01 mol) was added to this suspension in the
presence of triethylamine (0.02 mol) in methylene chloride at 0–5◦C.
After 2–4 hrs. sulfur powder (0.01 mol) was added at room tempera-
ture after 24 hrs., four equivalents of diethylamine (0.04 mol) mixed in
methylene chloride at 0–5◦C was further added and stirring was con-
tinued for another 20–24 hrs. The reaction mixture was then filtered
off and dried under vacuo, extracted with ether and left in refrigerator
whereupon a white to yellow solid was deposited.

Biodynamic Organothiophosphates of Pyridine
1311
General Procedure for the Synthesis of
N-alkyl-2-pyridinylidenamido)
bis(N-methylcyclohexylamido)thiophosphates (5a–c)
To a well-stirred suspension of salt in methylene chloride was added
phosphorus trichloride (0.01 mol) directly and triethylamine (0.02 mol)
at 0–5◦C. After 3–4 h stirring, sulfur (0.01 mol) was added at ambi-
ent temperature. A solution of N-methylcyclohexylamine (0.02 mol) in
methylene chloride was added dropwise at 0–5◦C, after 24–30 h. stir-
ring. The stirring was continued for 24–30 h at room temperature. Then
it was filtered off and solvent of the filtrate was removed in vacuo. The
solid was extracted with diethyl ether and kept in refrigerator, where-
upon white to cream crystals were obtained.
General Procedure for the Synthesis of
(N-alkyl-2-pyridinylidenamido)bis
(diphenylamido)thiophosphates (6a–c)
To a suspension of N-alkyl-2-aminopyridinium halide (0.01 mol) in
methylene chloride, was added phosphorus trichloride (0.01 mol) in
presence of dropwise addition of triethylamine (0.02 mol) in methy-
lene chloride at 0–5◦C. After 3–4 h, sulfur (0.01 mol) was added to
the reaction mixture. Triethylamine (0.02 mol) was added directly and
diphenylamine (0.02 mol) in methylene chloride then added dropwise
after 20–24 hrs. stirring at 0–5◦C. After 20–24 h stirring, reaction mix-
ture was filtered off and the solvent was removed in vacuo. Residue was
extracted with diethyl ether and left in refrigerator whereupon a cream
to yellow solid was obtained.
Insecticidal Activity on H. armigera
Two concentrations 50 ppm and 25 ppm of four test compounds were
made. For this purpose 250 mg of compound was dissolved in 1–2 ml
of acetone and the solution was made up to 500 ml by adding distilled
water. Thus, a solution of 0.05% was obtained 250 ml of this solution
was taken in a flat bottom flask and further diluted to 500 ml to obtain
the solution of 0.025% concentration.
The third instar larvae were selected for the study. For this 2–3 g of
gram shoots were soaked in the test solution that was made earlier and
were dried, then introduced in plastic container with larvae. Twenty
such containers were prepared for each replication and there were four
replicates for each treatment. A parallel set of four replicates of twenty
insects each, representing the standard check (endosulfan 35 ec, 0.05)

1312
V. Kabra et al.
and untreated control was also run. All the insects were placed at room
temperature and percent mortality was checked every day.
CONCLUSION
A series of novel thiophosphates incorporating pyridine ring have been
synthesized through one-pot method. The insecticidal study of these
compounds confirmed their potential insecticidal nature.
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