N-cycloalkyl- and N-arylimidoylphosphonates

Article abstract of DOI:10.1134/S107036321509008X

Reaction of acetyl- and fluorobenzoylphosphonates with 2- and 3-trifluoromethyl-substituted cyclohexylamines or 2- and 3-trifluoromethylanilines has resulted in the formation of the corresponding iminophosphonates and/or in the cleavage of the C-P bond yielding the dialkyl phosphites. The σ constants of N-[(trifluoromethyl)cyclohexyl]- and N-[(trifluoromethyl)phenyl]-substituted imidoylphosphoryl group have been determined by means of ¹⁹F NMR spectroscopy.

Full text of DOI:10.1134/S107036321509008X

ISSN 1070-3632, Russian Journal of General Chemistry, 2015, Vol. 85, No. 9, pp. 2065–2070. © Pleiades Publishing, Ltd., 2015.
Original Russian Text © P.P. Onys’ko, D.V. Klyukovskii, A.V. Bezludnyi, 2015, published in Zhurnal Obshchei Khimii, 2015, Vol. 85, No. 9, pp. 1484–1489.
To the 85th Anniversary of birthday of late Yu.G. Gololobov
N-Cycloalkyl- and N-Arylimidoylphosphonates
P. P. Onys’ko, D. V. Klyukovskii, and A. V. Bezludnyi
Institute of Organic Chemistry, National Academy of Sciences of Ukraine, ul. Murmanskaya 5, Kiev, 02094 Ukraine
e-mail: onysko_@ukr.net
Received July 17, 2015
Abstract—Reaction of acetyl- and fluorobenzoylphosphonates with 2- and 3-trifluoromethyl-substituted cyclo-
hexylamines or 2- and 3-trifluoromethylanilines has resulted in the formation of the corresponding imino-
phosphonates and/or in the cleavage of the C–P bond yielding the dialkyl phosphites. The σ constants of N-
[(trifluoromethyl)cyclohexyl]- and N-[(trifluoromethyl)phenyl]-substituted imidoylphosphoryl group have been
determined by means of 19F NMR spectroscopy.
Keywords: imidoylphosphonate, oxophosphonate, cyclohexyl, trifluoromethyl, E/Z isomerism, σ constants
DOI: 10.1134/S107036321509008X
Imidoylphosphonates are recognized as new
promising precursors of aminophosphonic acids. The
presence of the “oxidized” fragment of α-
aminophosphonic acid >P(O)C=N (capable of smooth
reductive functionalization) in imidoylphosphonates
opens versatile synthetic routes to new biologically
important acyclic and heterocyclic derivatives
containing aminophosphonate pharmacophore [1–5].
The emerging interest to the mentioned derivatives is
due to the wide range of their practically important
properties. In particular, they potentially can act as
enzymes activity regulators or inhibitors of HIV
protease; they have been applied as antibacterial,
antiviral, and anticancer agents [6, 7]. We have earlier
developed synthetic procedures for the preparation of
NН-, N-alkyl-, N-aryl-, N-acyl-, N-sulfonyl-, and N-
phosphoryliminophosphonates [1–5, 8, 10–12].
However, cycloalkyliminophosphonates have re-
mained almost unknown so far [8, 9]. The presence of
a cycloalkyl group is expected to improve physico-
chemical and biological properties of the discussed
compounds. The overwhelming majority of the
synthetic approaches to the imidoylphosphonates have
been based on phosphorylation of imidoyl chlorides
[8]. We have recently applied this route to prepare the
first representatives of trifluoroacetyl- [9] and benz-
imidoylphosphonates [10] containing a cyclopropyl
group at the nitrogen atom. The application of this
method to obtain N-cycloalkyliminophosphonates has
been limited by the poor availability of the starting
imidoyl chlorides and their relatively low reactivity.
On the other hand, the condensation of available α-
oxophosphonates with amines appears as the most
straightforward and the easiest route to the imidoyl-
phosphonates; however, only a few examples of suc-
cessful application of this method have been reported
[11, 13, 14]. We have earlier demonstrated that the
outcome of this reaction dramatically depends on the
nature of the amine component: in the case of benz-
ylamine, the C–P bond cleavage yielding the corres-
ponding N-benzylamide is the primary pathway [11].
The less nucleophilic aminophosphonates readily react
with oxophosphonates giving the target imidoylphos-
phonates in a high yield [11]. This work aimed to
demonstrate the application of this approach by the
study of the acetyl- and fluorobenzoylphosphonates
reactions with isomeric trifluoromethylcyclohexyl-
amines and trifluoromethylanilines of quite different
nucleophilicity.
The outcome of the reaction of О,О-diethyl acetyl-
phosphonate 1 with isomeric trifluoromethylcyclo-
hexylamines appeared to depend on the amine
structure. In particular, cis-2-trifluoromethylcyclo-
hexylamine 2a [15] reacted with phosphonate 1 under
conditions described in [14] (Et2O, АсОН, 20°С) to
form exclusively iminophosphonate 3a. In contrast, the
reaction of the isomeric cis-3-trifluoromethyl-
2065

2066
ONYS’KO et al.
Scheme 1.
O P(OEt)2
O
N
а
R
Me
Me
P(OEt)2
−H2O
R
O
1
OH
3а, 3b
N
Me
+
NH2
H
O P(OEt)2
O
O
b
А
(EtO)2PH +
R
Me
N
R
4
H
2а, 2b
O
P(OEt)2
OH
Me
+
1
4
P(OEt)2
O
5
R = 2-CF3 (а), 3-CF3 (b).
cyclohexylamine 2b [15] under the same conditions
was not selective: according to the 31Р NMR data,
iminophosphonate 3b was a minor product (<20%),
the major oxygen-containing products being diethyl
phosphite 4 and 1-hydroxyethylidene-1,1-bisphos-
phonate 5 (3b : 4 : 5 ≈ 1 : 1.5 : 3). The obtained results
may be understood in the frame of Scheme 1.
compound 1 to give the hydroxybisphosphonate 5. The
interaction between compounds 1 and 6a under the
more severe conditions (refluxing in benzene in the
presence of TsOH with simultaneous azeotropic distilla-
tion off of water) afforded the complete conversion
within 48 h, the reaction mixture contained iminophos-
phonate 7a, hydroxybisphosphonate 5, and diethyl
phosphite 4 (7a : 5 : 4 ≈ 10 : 6 : 1; 31P NMR data).
Intermediate А formed in the first stage could
decompose via two competing pathways. With R = 3-
CF3 the elimination of dialkyl phosphite anion
occurred (route b), owing to the influence of strongly
electron-donor amino and hydroxy groups (cf. [11]). In
the case of amine 2a, the donor properties of amine
substituent were significantly reduced due to the
inductive effect of the CF3 group, the С–Р bond in the
intermediate А was stronger, and the major pathway of
its decay was dehydration (route a).
3- and 4-fluorobenzoylphosphonates 8a and 8b
reacted with 2-trifluoromethylcyclohexylamine sig-
nifycantly faster. Other conditions being the same
(benzene, TsOH, refluxing), the reaction was complete
within 2 h but was accompanied by the formation of
side products: the content of iminophosphonates 9a
and 9b in the reaction mixture was of 30 and 70%,
respectively. The interaction of fluorobenzoylphos-
phonates 8a and 8b with 2-trifluoromethylaniline 6a
was more selective. The reaction was complete within
48 h under the same conditions, and the reaction
It could be expected that the far less nucleophilic
trifluoromethylanilines 6a and 6b would mainly react
via the path a. Indeed, the reaction of ketophosphonate
1 with anilines 6a and 6b initially occurred selectively
to give the iminophosphonates 7a and 7b, but the
reaction was however very slow: the conversion of
34% was attained after 2 days, other conditions being
the same (Et2O, АсОН, and 20°С) (Scheme 2).
Scheme 2.
O
P(OEt)2
NH2
N
R
Me
R
Further keeping of the reaction mixture resulted in
hydrolysis of the iminophosphonates 8a and 8b, and
the formed diethyl phosphite 4 reacted with the starting
6а, 6b
7а, 7b
R = 2-CF3 (а), 3-CF3 (b).
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 85 No. 9 2015

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N-CYCLOALKYL- AND N-ARYLIMIDOYLPHOSPHONATES
Scheme 3.
O P(OMe)2
2а
N
R
CF3
O
P(OMe)2
O
9а, 9b
R
8а, 8b
O P(OMe)2
N
6а
R
CF3
10а, 10b
R = 3-F (а), 4-F (b).
Scheme 4.
O
P(OEt)2
CF3
CF3
H
O
N
NH2
P(OEt)2
O
+ (EtO)2PH + HC(COOEt)3
2а
4
11
mixture contained >90% of iminophosphonates 10a
and 10b, the content of dimethyl phosphite not ex-
ceeding 5–7% (31P and 19F NMR data) (Scheme 3).
The spectral data confirmed the suggested struc-
tures of iminophosphonates 3, 7, 9, and 10. The most
indicative feature was the position of phosphorus
nuclei signal (3.2–8.8 ppm), typical of alkyl- [19, 20]
and benzimidoylphosphonates [16, 20] and allowing
for easy identification of those compounds, including
the reaction mixtures case. 13С NMR spectrum of
compound 3a contained a downfield doublet signal of
the imine carbon atom (165 ppm) exhibiting the strong
spin-spin coupling (1JСР = 218 Hz) typical of the
phosphorus atom adjacent to the carbon one; that
unambiguously confirmed the presence of the P–C=N
fragment. Positions of the phosphorus nucleus signals
in the acetylimidoylphosphonates 3 and 7 are in
accordance with the Е configuration of the C=N bond
in those compounds [14, 19, 20]. At the same time,
benzimidoylphosphonates 9 and 10 (similarly to other
representatives of that class) existed as mixtures of E
and Z isomers, the Е form being predominant [E : Z ≈
(3–5) : 1]. Signals of phosphorus atoms of the E
isomers of compounds 9 and 10 (7.8–8.8 ppm) were
However, pure iminophosphonates 10a and 10b
could not be isolated (even by means of chromato-
graphy), for in the course of workup they are subject to
ambiguous transformations resulting in the formation
of complex mixtures of products. We believe that the
low stability of compounds 10 was not due to the
presence of the o-CF3 group, but rather owing to the
capability of the P–C=N fragment to the cleavage of
the Р–С bond, especially in the presence of basic or
acidic admixtures [4, 8, 16, 17]. For example, the
aminobisphosphonate 11 prepared via Scheme 4 was
sufficiently stable under ordinary conditions. Note-
worthily, the N-(cycloalkyl)aminobisphosphonic acids
have been recognized as efficient phosphatase
inhibitors and are therefore regarded as promising
candidates for development of drugs for malaria and
dysentery treatment [18].
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2068
ONYS’KO et al.
Chemical shifts δF (ppm) of the fluorosubstituted benzenes XC6H4F and σ constants of N-substituted imidoylphos-phonate
groups
δF(XC6H4F)
Run no.
1
X
σI
σR
σp
3-F
4-F
CF3
N
2.15
4.55
0.39
0.08
0.47
P(O)(OMe)2
P(O)(OMe)2
2
3
1.10
0.76
4.82
1.6
0.24
0.19
0.13
0.03
0.37
0.22
N
F3C
CF3
N
P(O)(OMe)2
detected down field from those of the Z isomers (3.2–
5.5 ppm), in accordance with the literature data
[16, 17, 20].
(the major form of the imidoylphosphonates 10a and
10b) was significantly stronger than that of the
corresponding Z form. The similar effect of the C=N
bond geometry has been earlier revealed for the
imidoylphosphonates containing substituted cyclo-
propyl substituents at the nitrogen atom [10]. In
general, the electron-acceptor properties of the imino-
phosphonates followed the following series of the
substituents at the nitrogen atom: ArSO2, (RO)2P(O)
[23, 24] > Ar > PhCH2 [25] > cyclohexyl > cyclo-
propyl [10].
Electronic nature of N-[(trifluoromethyl)cyclo-
hexyl]- and N-[(trifluoromethyl)phenyl]-substituted
imidoylphosphoryl groups. We took advantage of
sensitivity of fluorine nucleus to the electronic effects
of the substituents to quantify the effects of N-[(tri-
fluoromethyl)cyclohexyl]- and N-[(trifluoromethyl)-
phenyl]-substituted imidoylphosphoryl groups. In par-
ticular, we measured chemical shifts of fluorine atom
of the fluorophenyl group in imidoylphosphonates 9
and 10 (in CDCl3, relative to fluorobenzene as internal
reference) and applied the Taft equations (1) and (2)
[21, 22] to elucidate the inductive and resonance σ
constants of imidoylphosphoryl groups containing aryl
and cycloalkyl substituents at the nitrogen atom.
In summary, we showed that preparative
application of the reaction between α-ketophos-
phonates and amines to synthesize N-aryl- and N-
cyclohexyl-substituted iminophosphonates was sig-
nificantly limited. The outcome was affected by the
amine nature: The lower amine nucleophilicity favored
the formation of the iminophosphonates, whereas the
higher nucleophilicity led to the cleavage of the C–P
bond. The introduction of 3- and 4-fluorobenzoyl-
phosphonates in the reaction gave the first-time
reported estimation of electronic effects of the
iminophosphonate groups containing cyclohexyl or
aryl substituents at the nitrogen atom.
σI = (δmF + 0.6)/7.1,
σR = (δpF – δmF)/29.5.
(1)
(2)
Analysis of the results (see table) showed that the
imidoylphosphonate groups containing trifluoromethyl-
phenyl substituent at the nitrogen atom (nos. 1 and 2)
were strong electron acceptors (σp 0.37–0.47). The
substitution of phenyl fragment with cyclohexyl one
significantly deteriorated the electron-acceptor properties
(σp = 0.22). Noteworthily, the σ constants were
affected by the molecule geometry: The electron-
acceptor ability of the Е-imidoylphosphonate group
EXPERIMENTAL
1Н, 19F, 31Р, and 13C NMR spectra were recorded
using a Varian VXR–300 spectrometer at 299.95,
282.20, 121.42, and 75.429 MHz, respectively. The
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 85 No. 9 2015

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N-CYCLOALKYL- AND N-ARYLIMIDOYLPHOSPHONATES
chemical shifts were reported relative to the internal
TMS (1Н and 13C), CFCl3 (in Experimental) or PhF (in
table) for 19F or external 85% Н3РО4 (31Р) references.
All reactions were performed in anhydrous conditions
under argon atmosphere.
and 19F spectroscopy. Iminophosphonates 7, 9, and 10
were not isolated and were identified by the spectral
methods.
O,O-Diethyl-N-[(2-trifluoromethyl)phenyl]acet-
imidoylphosphonate (7a). 31P NMR spectrum (Et2O):
δР 6.83 ppm.
O,O-Diethyl-N-[(cis-2-trifluoromethyl)cyclohexyl]-
acetimidoylphosphonate (3a). A drop of glacial
acetic acid was added to a solution of О,О-diethyl-
acetylphosphonate 1 (1 mmol) and 2-(trifluoromethyl)-
cyclohexylamine 2a (1 mmol) in 5 mL of anhydrous
diethyl ether. The mixture was stirred at room
temperature under argon atmosphere during 24 h; the
reaction course was monitored by means of 31P and 19F
NMR spectroscopy. The solvent was evaporated; the
residue was washed with a small amount of petroleum
ether and kept in a high vacuum to give the
iminophosphonate 3a containing >90% of the main
component (yield 95%, NMR data). The crude product
was distilled in a vacuum to afford the pure product.
mp 101–102°C (0.12 mmHg). 1H NMR spectrum
O,O-Diethyl-N-[(3-trifluoromethyl)phenyl]acet-
imidoylphosphonate (7b). 31P NMR spectrum (Et2O):
δР 7.54 ppm.
O,O-Dimethyl-N-[(2-trifluoromethyl)cyclohexyl]-
3-fluorobenzimidoylphosphonate (9a). 19F NMR
3
spectrum (C6H6), δF, ppm: –69.50 d (CF3, JFH 8.0 Hz,
3
E isomer), –69.38 (CF3, JFH 9.0 Hz, Z isomer),
–112.23 (ArF, E isomer), –111.34 (ArF, Z isomer). 31P
NMR spectrum (C6H6), δP, ppm: 4.59 (Z), 8.07 (E),
E : Z ≈ 5 : 1.
O,O-Dimethyl-N-[(2-trifluoromethyl)cyclohexyl]-
4-fluorobenzimidoylphosphonate (9b). 19F NMR
spectrum (C6H6), δF, ppm: –69.50 d (CF3, 3JFH 10.0 Hz,
3
(CDCl3), δ, ppm (J, Hz): 1.34 t (6H, OCH2CH3, JHH
3
E isomer), –69.37 (CF3, JFH 8.0 Hz, Z isomer),
6.0), 1.39–2.05 m (8H, CH2, cyclohexyl), 2.09 d (3H,
–111.40 (ArF, E isomer), –109.35 (ArF, Z isomer). 31P
NMR spectrum (C6H6), δP, ppm: 3.19 (Z), 8.75 (E),
E : Z ≈ 5 : 1.
3
CH3C=N, JHP 10.6), 2.33 m (1H, CHCF3), 4.16 m
(4H, OCH2). 13C NMR spectrum (CDCl3), δC, ppm (J,
2
Hz): 15.79 d (CH3C=N, JCP 43.0), 16.22 d (CH3CH2,
3
3JCP 6.0), 16.26 d (CH3CH2, JCP 7.0); 20.00, 20.15,
O,O-Dimethyl-N-[(2-trifluoromethyl)phenyl]-3-
fluorobenzimidoylphosphonate (10a). 19F NMR
spectrum (C6H6), δF, ppm: –61.09 (CF3, E isomer),
–60.89 (CF3, Z isomer), –111.04 (ArF, E isomer),
–111.95 (ArF, Z isomer). 31P NMR spectrum (C6H6),
δP, ppm: 5.13 (Z), 7.81 (E), E : Z ≈ 3 : 1.
25.26, 32.54 (C4, C5, C3, C6, cyclohexyl), 46.38 q
(CHCF3, 2JCF 24.0), 54.02 d (CHN, 3JCP 32.0), 62.76 d
2
2
(OCH2, JCP 7.0), 63.28 d (OCH2, JCP 6.0), 126.94 q
(CF3, JCF 280.0), 165.29 q (C=N, JCP 218.0). 19F
NMR spectrum (CDCl3), δF, ppm: –70.52 d (3JCF
8.0 Hz). 31P NMR spectrum (CDCl3): δP 7.64 ppm.
Calculated, %: N 4.25; P 9.41. C13H23F3NO3P. Found,
%: N 4.08; P 9.63.
1
1
O,O-Dimethyl-N-[(2-trifluoromethyl)phenyl]-4-
fluorobenzimidoylphosphonate (10b). 19F NMR
spectrum (C6H6), δF, ppm: –61.07 (CF3, E isomer),
–60.89 (CF3, Z isomer), –108.80 (ArF, E isomer),
–108.43 (ArF, Z isomer). 31P NMR spectrum (C6H6),
δP, ppm: 5.5 (Z), 8.1 (E), E : Z ≈ 3 : 1.
The reaction of cis-3-trifluoromethylcyclohexyl-
amine 2a with compound 1 under the same conditions
yielded a mixture of iminophosphonate 3b (δР
8.15 ppm), diethyl phosphite 4 (δР = 7.2 ppm, JРН
=
=
1
O,O,O,O-Tetraethyl-({[2-(trifluoromethyl)cyclo-
695 Hz), and О,О-diethyl-1-hydroxyethylidenebis-
phosphonate 5 (δР = 21.7 ppm; δР = 21.02 ppm [26]) in
the 3b : 4 : 5 ≈ 1 : 1.5 : 3 ratio.
hexyl]amino}methylene)bisphosphonate (11).
A
mixture of compound 2a (0.334 g, 2 mmol), triethyl
orthoformate (0.556 g, 8 mmol), and diethyl phosphite
(1.104 g, 8 mmol) was heated at 150°C during 15 h.
The volatile products were evaporated in a high
vacuum, and the residue was purified by preparative
TLC (CH3OH : CHCl3 = 1 : 49, Rf 0.1). Yield 28%. 1H
NMR spectrum (CDCl3), δ, ppm: 1.22 m (12H, CH3),
1.35–1.95 m? (8H, CH2, cyclohexyl), 2.08 m (1H,
Interaction of fluorobenzoylphosphonates 8a
and 8b with trifluoromethylcyclohexylamines 2 or
trifluoromethylanilines 6. 1 mmol of the correspond-
ing amine and 15 mg of TsOH were added to a
solution of 1 mmol of the compound 8 in 15 mL of
anhydrous benzene. The mixture was refluxed during
48 h in an apparatus equipped with a Dean–Stark trap.
The reaction course was monitored by means of 31P
2
CHCF3), 3.25 t (CHP, JHP 21.7 Hz), 3.51 br.s (1H,
NH), 4.10 m (8H, OCH2). 13C NMR spectrum
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ONYS’KO et al.
Curr. Org. Chem., 2008, vol. 12, no. 1, p. 2. DOI:
10.2174/ 138527208783330091.
(CDCl3), δС, ppm (J, Hz): 16.18 (CH3), 16.23 (CH3),
16.30 (CH3), 18.81 (CH24), 19.53 (CH25), 24.81 (CH23),
28.16 (CH263), 45.68 q (CHCF3, JCF 25.0), 49.72 d
2
13. Ryglowski, A. and Kafarski, P., Tetrahedron, 1996,
1
vol. 52, no. 32, p. 10685.
(CH2CHN, JCP 11.0), 50.67 d. d (CHP, JCPА 138.0,
3
1JCPB 152.0), 62.46 d (OCH2, JCP 7.0), 62.84 (OCH2,
14. Jelaiel, N., Said, N., Touil, S., and Efrit, M.L.,
Phosphorus, Sulfur, Silicon, Relat. Elem., 2010, vol. 185,
no. 11, p. 2382. DOI: 10.1080/10426501003671429.
3
1
3JCP 6.0), 63.00 (OCH2, JCP 7.0), 127.34 q (CF3, JCF
280.0). 19F NMR spectrum (CDCl3), δF, ppm: –68.71 d
(3JFH 10.0 Hz). 31P NMR spectrum (CDCl3): δP
20.06 ppm. Calculated, %: N 3.09; P 13.66.
C16H32F3NO6P2. Found, %: N 3.21; P 13.37.
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