Phosphoramidates: Features of the formation mechanism and the relationship structure-bioaction

Article abstract of DOI:10.1134/S1070363212050039

Mechanism of the synthesis of phosphoramidates by Todd-Atherton reaction is based on the primary interaction of a polyhaloalkane with the highly basic amine to form a 1:1 associate. The subsequent attack by the associate on the hydrophosphoryl compound of "symmetric" structure leads to the formation of the target compounds in high yields. The test of the effect of dialkyl hexamethylene- and dialkyl(pyridin-2-yl)-phosphoramidates in vitro against the strains of a number of the producents of pathogenic bacteria and mycotoxins showed that the high level of biological activity of the target compounds is correlated well with the physicochemical parameters characterizing their structure.

Full text of DOI:10.1134/S1070363212050039

ISSN 1070-3632, Russian Journal of General Chemistry, 2012, Vol. 82, No. 5, pp. 822–826. © Pleiades Publishing, Ltd., 2012.
Original Russian Text © V.I. Krutikov, A.V. Erkin, V.V. Krutikova, 2012, published in Zhurnal Obshchei Khimii, 2012, Vol. 82, No. 5, pp. 713–718.
Phosphoramidates: Features of the Formation Mechanism
and the Relationship Structure–Bioaction
V. I. Krutikov, A. V. Erkin, and V. V. Krutikova
St. Petersburg State Institute of Technology, Moskovskii pr. 26, St. Petersburg, 190013 Russia
e-mail: kruerk@yandex.ru
Received November 10, 2011
Abstract—Mechanism of the synthesis of phosphoramidates by Todd–Atherton reaction is based on the
primary interaction of a polyhaloalkane with the highly basic amine to form a 1:1 associate. The subsequent
attack by the associate on the hydrophosphoryl compound of “symmetric” structure leads to the formation of
the target compounds in high yields. The test of the effect of dialkyl hexamethylene- and dialkyl(pyridin-2-yl)-
phosphoramidates in vitro against the strains of a number of the producents of pathogenic bacteria and
mycotoxins showed that the high level of biological activity of the target compounds is correlated well with the
physicochemical parameters characterizing their structure.
DOI: 10.1134/S1070363212050039
Organic phosphoramidates exhibit a wide spectrum
of biological action, from pesticides to anticancer
drugs. The reasons for such diversity are different, and
include high alkylating ability of phosphoramidates,
their propensity to both spontaneous and enzymatic hydro-
lysis, and the ability of molecules with a P–N bond to
insert in the structure of natural nucleotides [1–3].
In this work we studied the mechanism of forma-
tion and biological action of some phosphoramidates:
(R1O)2P(O)H + (C2H5)3N + CCl4 + R2NH
(R1O)2P(O)R2 + CHCl3 + (C2H5)3N HCl
I
R1 = alkyl, R2 =
NH ,
N
.
Methods of synthesis of phosphoramidates are well
known. In particular, the classical four-component
one-step Todd–Atherton reaction provides the target
phosphoramidates of different structures under mild
conditions and in a high yield. This reaction is based
on the interaction of hydrogen phosphites or related
compounds with amines in a polyhalogenated solvent.
When the amine is not sufficiently basic, it is neces-
sary to add an equivalent quantity of triethylamine or
of other tertiary amine [4]. The popularity of Todd–
Atherton method has played a major role in the fact
that the chemistry of this process is known only in
general terms. Strictly speaking, nobody attempted to
prove it, and therefore into the textbooks a speculative
scheme is included [5]. It is assumed that initially
hydrophosphoryl compound interacts with triethyl-
amine to produce either a P(III)-form, or the corres-
ponding salt. Note that the cases of formation of salts
between dialkyl phosphites and amines of different
nature were described by Bakhtiyarova [6]. However,
the results obtained cannot be unambiguously
evaluated: the author wrongly claims that the forma-
tion of salts in the case of diethyl hydrogen phosphite
is impossible due to the dominance in solution of
“asymmetric” form (C2H5O)2P(O)H, in contrast to
other phosphites.
In the second stage of the proposed mechanism of
Todd–Atherton reaction [5] the symmetrical hydro-
phosphoryl compound or a salt thereof reacted as a
nucleophilic compound with carbon tetrachloride,
acting as a donor of the positively charged chlorine.
The latter statement is difficult to accept unreservedly.
Very often many authors consider a molecule of
822

PHOSPHORAMIDATES: FEATURES OF THE FORMATION MECHANISM
823
carbon tetrachloride as a source of ions or chlorine noted, has been reviewed in [4]. The view expressed
radicals and trichloromethyl groups. Unfortunately, the
reason for the formation of such species is not cleared.
Carbon tetrachloride is a sufficiently stable compound
under normal conditions, and to become a source of
charged particles it should interact with a nucleophilic
reagent or a quantum of light, or be irradiated with
other radiation source. Initially a transition state should
appear with the charge redistributed between the parts
of the molecule, in particular, the chlorine lone pair
must move to the non-bonding σ orbital. This occurs in
the case of the Kamai reaction [7].
there is commonly accepted. However, in this case, it
remains unclear why in the reaction mixture are not
detected trihlormethylphosphonates, the typical pro-
ducts of the Kamai reaction. This discrepancy is explain-
able assuming an alternative view on the mechanism of
the three-component interaction between hydrophos-
phoryl compound, carbon tetrachloride and highly
basic amine. In our opinion, it is completely incorrect
to restrict the role of triethylamine in the Todd–
Atherton reaction as converting hydrophosphoryl
compound into the “symmetric” form. The interaction
between carbon tetrachloride and highly basic amine
should not be ignored. Meanwhile, it is known [8] that
amines and ordinary halogenated hydrocarbons form
associates entering into the charge-transfer reactions:
Analysis of published data and results of our
experiments enable us to suggest two possible routes in
the Todd–Atherton reaction. The first way (preli-
minary phosphite interaction with amine), as already
H3C
H2C
_
hν
N+ HCl + CHCl3
N+
CCl4 + N(C2H5)3
Cl CCl3
H3C
H3C
H3C
H3C
Therewith, it was proved that the dark and photo- that of triethylamine (pKBH
+
are 11.1 and 10.87,
chemical reactions of some amines with carbon tetra- respectively [9]), it can be assumed that between the
chloride proceed with a quantum yield greater than molecules of carbon tetrachloride and hexahydro-
unity and, consequently, are the chain reactions. We azepine should occur a specific interaction.
believe that just the interaction of highly nucleophilic
The affinity of the trichloromethyl radical to the
amine with carbon tetrachloride initiates the Todd–
proton produced at the dissociation of a “symmetric”
Atherton reaction.
form of phosphite is undoubtedly higher than to any
Earlier [4] it has been proved experimentally that a electrophilic particle in solution, which results in the
stronger base provides a higher rate of the Todd– formation of a molecule of chloroform. Thus the lack
Atherton reaction action. Taking into account that the of the products of the Kamai reaction in the reaction
basicity of the hexahydroazepine is slightly higher than mixture is explained.
O
B:
OH
P
P
H
N+
Cl + _ CCl3
NH + CCl4
[(CH2)6NH CCl4]
H
II
NH
O
O
O_
N+ Cl
H
P
:
P
P
_HCl
N
Cl
B is a base (any highly basic amine).
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 82 No. 5 2012

824
ΔnD20
KRUTIKOV et al.
under mild conditions with a high yield, and hexa-
hydroazepine hydrochloride formed almost quanti-
tatively. In the synthesis of dialkyl (pyridine-2-yl)phos-
phoramidates Ih–Ij we used triethylamine as a com-
ponent of the associate II, as in the absence of
trietylamine we failed to isolate the target compounds
from the reaction mixture. Table 1 lists the parameters
of phosphoramidates Ia–Ij.
The phosphoramidates synthesized in this work
were tested in vitro for antibacterial and fungicidal
activity. Table 2 lists the data on the biological
activity. Attention is drawn to the fact that all the
target compounds showed significant antibacterial and
fungicidal action. The level of activity in most cases is
practically independent from the length of the alkoxy
radicals at the phosphorus atom. The level of activity
of various target compounds is approximately the
same, which makes it impossible to optimize their
structure for the synthesis of more effective drugs. An
exception concerns prediction of the level of activity
against the fungi Fusarium graminearum, the producer
of mycotoxins.
CCl4:HN(CH2)6 mole ratio
Refractometric titration curve in the system CCl4–HN(CH2)6
indicating the formation of associate II.
The search for new conditions of the Todd–
Atherton reaction continues to this day. In particular, a
new method was developed recently of the synthesis of
phosphoramidates using microwave irradiation [10,
11]. Studying the phosphorylation of the amino(aryl)-
methylphosphonates in the Todd–Atherton reaction
Minaeva et al. noted that the effect of microwave
irradiation of the reaction mixture increased the
reaction rate and the yield of the final compounds. We
think that the fact is an indirect confirmation of the
involvement of the charge transfer associates between
the amine and carbon tetrachloride in the synthesis of
phosphoramidates and their subsequent decomposition
at action of the microwave energy, with the formation
of desired products.
The found inhibition of the Fusarium graminearum
strain obeys the relation (1):
log 1/C50 = 6.20–1.27log p + 0.101(log P)2 + 0.112Еhydr, (1)
r 0.924, s 0.71.
In this work at the optimization of the level of phos-
phoramidates, we chose the following characteristics:
lipophilicity parameter log P, the value characterizing
the ratio of solubility in water and lipids, and Ehydr, the
energy of hydration of organic molecules. The choice
of these characteristics is due to the ease of sponta-
neous and enzymatic hydrolysis of phosphoramidates.
To prove the formation of associate II, we studied
the interaction of tetrachlorometane with hexahydro-
azepine by the method of refractometric titration. The
figure shows a curve characterizing the change in
refraction index of the CCl4–HN(CH2)6 mixture versus
the molar ratio of components. The nature of the curve
shows that carbon tetrachloride forms with hexa-
hydroazepine a 1:1 charge transfer associate. We note
that such a pattern was not observed in the case of 2-
aminopyridine. This is due, most likely, to the low
A significant improvement should be noted in the
correlation characteristics when the chemical shift of
compounds I in the 31P NMR spectra was used as an
additional correlation parameter [Eqs. (2) and (3)]. The
choice of δP is defined by the fact that this quantity is
an independent experimental value that largely
depends on the electronic effect of substituents at the
phosphorus atom. The latter circumstance makes it
more advantageous than the widely used Kabachnik
constants. Use of the latter is often limited due to the
impossibility of assessing the contribution of inductive
and resonance components.
basicity of this amine (pKBH = 6.86).
+
To prove that associate II lies on the reaction
coordinate, we carried out the synthesis of dialkyl-
phosphor(hexamethyleneamidates) Ib and Ie without
the use of triethylamine, but with a twofold molar
excess of hexahydroazepine. The reaction proceeded
log 1/C50 = 1.86 + 1.89log p – 0.189(log P)2 – 0.134Еhydr
– 0.203δР; r 0.967, s 0.12,
(2)
log 1/C50 = 4.26 + 0.172log p – 0.0327(log P)2
0.0961δР; r 0.955, s 0.11.
(3)
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 82 No. 5 2012

PHOSPHORAMIDATES: FEATURES OF THE FORMATION MECHANISM
Table 1. Yields and characteristics of phosphoramidates I
825
bp, °C
(mm Hg) or
mp, °C
Found, %
Calculated, %
Comp.
no.
Yield,
%
R1
R2
nD20
d420
Formula
С
Н
N
С
Н
N
CH3O
C2H5O
C3H7O
N(CH2)6
N(CH2)6
N(CH2)6
61
73а
56
41
1.4619 1.059
1.4525 1.063
1.4498 1.055
1.4469 1.064
1.4549 1.008
140(10)
46.1
51.2
54.7
54.7
8.39 6.69 C8H18NO3P
46.4
8.69 6.76
9.36 5.96
9.88 5.32
9.88 5.32
Iа
Ib
Ic
Id
144(19)
150(9)
100(2)
9.16 5.76 C10H22NO3P 51.1
9.81 5.19 C12H26NO3P 54.8
9.85 5.12 C12H26NO3P 54.8
i-C3H7O N(CH2)6
i-C4H9O N(CH2)6
C6H13O N(CH2)6
C8H17O N(CH2)6
85а
49
52
74
157(4)
153(4)
160(6)
89
57.5 10.1
62.2 10.7
66.6 11.4
4.69 C14H30NO3P 57.7 10.3
4.81
4.03
3.47
Ie
If
1.454
1.009
4.01 C18H38NO3P 62.2 10.9
3.19 C22H46NO3P 66.5 11.4
1.4301 1.018
Ig
Ih
CH3O
2-Amino-
–
–
–
–
–
–
41.3
5.89 13.8
C7H12N2O3P 41.4
5.91 13.8
pyridin-2-yl
i-C4H9O 2-Amino-
pyridin-2-yl
59
44
70
54.4
8.12 9.65 C13H24N2O3P 54.4
8.36 9.75
Ii
Ij
C6H13O 2-Amino-
pyridin-2-yl
190(6)
58.4 10.4
8.1
C17H37N2O3P 58.6 10.6
8.04
a Сompounds were obtained in two ways, the given yield corresponds to the method b.
Table 2. Antibacterial activity of phosphoramidates and correlation characteristics
log 1/C50
Comp.
Еhydr
log P
δР, ppm
Xant.
malv.
Sсl.
Fus.
Vent.
in.
Hel.
sat.
Rhiz.
no.
sol.
gram.
sol.
–13.2
–5.36
3.10
1.27
1.95
2.89
2.78
3.70
5.27
6.85
1.82
4.24
5.82
12.4
9.3
3.62
3.19
3.54
3.85
3.29
3.97
3.91
3.43
3.28
3.84
3.89
3.83
3.99
3.99
3.81
–
3.18
3.71
3.59
3.76
3.63
3.41
4.07
3.17
3.80
3.70
3.76
3.80
3.87
3.71
3.91
3.51
3.89
3.28
3.90
4.02
3.74
3.71
3.47
3.81
3.89
3.93
3.95
3.73
3.91
3.59
3.18
–
Iа
Ib
Ic
Id
Ie
If
8.1
3.29
3.59
3.81
3.71
3.95
3.47
3.80
3.40
2.76
7.2
6.28
9.3
9.83
8.6
16.3
–1.7
2.0
4.07
–
Ig
Ih
Ii
–34.6
–10.2
–6.69
1.0
4.03
3.88
–1.1
Ij
From these correlations a preliminary conclusion
can be made: for obtaining more effective fungicides it
is necessary to introduce into the structure of the
phosphoramidate electronegative groups and fragments
that ensure good solubility of the substance in the
lipids and water.
Russian Research Institute of the Plant Protection
Chemicals.
Individuality of the synthesized compounds was
proved by GLC (an LKhM-8MD chromatograph,
stationary phase XE-60 10% on Chezasorb or SE-30
5% on Inerton, detector katharometer, carrier gas
helium, 40 ml min–1) (for liquid products ) and TLC
(the plates Silufol UV 254, eluent acetone–hexane,
1:4) for crystalline substances. The 31P NMR spectra
were recorded on an instrument developed at
EXPERIMENTAL
The assessment of biological activity of the syn-
thesized compounds was carried out in the Krasnodar
Research Institute of Biological Plant Protection, and
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 82 No. 5 2012

826
KRUTIKOV et al.
St. Petersburg Technological Institute (operating fre-
quency 16.2 MHz, external reference 85% H3PO4).
Elemental analysis was performed on a Hewlett
Packard B-185 analyzer.
20 ml of carbon tetrachloride, and a solution of 1.9 g
of 2-aminopyridine in diethyl ether. To the resulting
mixture was added dropwise at room temperature 2.2 g
of dimethyl phosphite while stirring. The reaction
mixture was stirred for 3 h at 40°C. After cooling to
room temperature, triethylamine hydrochloride formed
was filtered off, and from the filtrate volatile com-
ponents were distilled off. The residual oil was washed
with diethyl ether. A colorless crystalline substance
formed. After filtration and drying 3 g (74%) of the
product was obtained.
The values of hydration energy and lipophilicity
parameter log P of the target compounds were
calculated using the software package HyperChem™
Release 8.08, after geometric optimization by the
Fletcher–Reeves method. Refractometric titration was
performed on an instrument of the URL type. The
processing of the titration results was performed using
the Origin Pro 8 SRO software package.
REFERENCES
Diethyl N,N-hexamethylenephosphoramidate (Ib).
a. In a reactor was placed 6.4 g of triethylamine, 9.8 g
of carbon tetrachloride, and 6.3 g of hexahydroazepine.
The reaction mixture was cooled to 0°C, and then 8.8 g
of diethyl phosphite was added dropwise while stirring
and maintaining the reaction mixture temperature
within 0–5°C. Then the reaction mixture was stirred at
room temperature for 2 h. The triethylamine hydro-
chloride formed was filtered off, from the filtrate
chloroform and carbon tetrachloride excess were
distilled off, the residual oil was distilled in a vacuum
at 144°C (19 mm Hg). Yield 58%. In a similar manner
were obtained other phosphoramidates.
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Cherkassy, 1983.
b. In a reactor was placed 10.3 g of hexa-
hydroazepine and 16 g of carbon tetrachloride. To the
mixture was added dropwise over 0.5 h 7.2 g of diethyl
phosphite while stirring and cooling to 0–5ºC. The
reaction mixture was stirred for 2 h at room
temperature. The resulting hexahydroazepine hydro-
chloride precipitate was filtered off (6.6 g, 94%). From
the filtrate were distilled off the volatile components
and then by fractional distillation at 140°C (14 mm
Hg) 8.8 g (73%) of target amidate was isolated. The
compound parameters coincided with those of the
compound Ib obtained by the above technique.
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Blinova, G.G., and Sochilin, E.G., Zh. Obshch. Khim.,
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Dimethyl (N-pyridine-2-yl)phosphoramidates (Ih).
The reactor was charged with 2 g of triethylamine,
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 82 No. 5 2012