Convenient synthesis of fluoroalkyl α- And β-aminophosphonates

Article abstract of DOI:10.1016/j.jfluchem.2011.05.005

Addition of both alkyl phosphites and phosphonate α-carbanions to N-substituted aldimines derived from fluoroalkyl aldehydes presents a convenient method for synthesis of fluoroalkyl α- and β-aminophosphonates in good yield (55-86%) under mild conditions.

Full text of DOI:10.1016/j.jfluchem.2011.05.005

Journal of Fluorine Chemistry 132 (2011) 834–837
Contents lists available at ScienceDirect
Journal of Fluorine Chemistry
journal homepage: www.elsevier.com/locate/fluor
Convenient synthesis of fluoroalkyl
a- and b-aminophosphonates
Gerd-Volker Ro¨schenthaler a, Valery P. Kukhar b, Irene B. Kulik b, Alexander E. Sorochinsky b,
,
*
Vadim A. Soloshonok c,d
a School of Engineering and Science, Jacobs University Bremen, P.O. Box 750 561, D-28725 Bremen, Germany
b Institute of Bioorganic Chemistry & Petrochemistry, National Academy of Sciences of Ukraine, Murmanska 1, Kyiv 02660, Ukraine
c University of the Basque Country UPV/EHU, San Sebastian, Spain
d IKERBASQUE, Basque Foundation for Science, 48011, Bilbao, Spain
A R T I C L E I N F O
A B S T R A C T
Article history:
Addition of both alkyl phosphites and phosphonate
a-carbanions to N-substituted aldimines derived
Received 6 April 2011
from fluoroalkyl aldehydes presents a convenient method for synthesis of fluoroalkyl
aminophosphonates in good yield (55–86%) under mild conditions.
a- and b-
Received in revised form 4 May 2011
Accepted 9 May 2011
ß 2011 Elsevier B.V. All rights reserved.
Available online 14 May 2011
Keywords:
a
- and b-Aminophosphonic acids
Fluorinated compounds
Addition
Imines
Fluorine-containing biologically active
compounds
1. Introduction
fluoroalkyl
a- and
b
-iminophosphonates and their isomeric
enamines, base-catalyzed [1,3]-proton shift reaction of the N-
benzyl substituted fluoroalkyl - and -iminophosphonates [7],
Recently fluoroalkyl
a
- and
b
-aminophosphonic acids and their
a
b
phosphonate esters have been the subject of considerable
attention in organic and bioorganic chemistry because of their
potential activity as metabolically stable transition state analogue
inhibitors of different peptidases, antimicrobial, antibacterial,
antihypertensive and anticancer agents as well as peptidomimetic
units [1]. The efficient examples of aminophosphonates containing
fluoroalkyl groups as inhibitors of serine esterases, alanine
racemase and pyrimidine phosphorylases have been demonstrated
[2]. Furthermore, considering the unique mode of hydrogen
bonding observed in free aminophosphonic acids and their
derivatives [3], these compounds might be very interesting models
for studying self-disproportionation of enantiomers via achiral
chromatography [4] and sublimation [5]. Despite the fact that
several research groups have carried out extensive studies for
exchange of hydroxyl group in fluoroalkyl N-acyl hemiaminals for
phosphonate group via phosphite-phosphonate rearrangement,
addition of ammonia or benzylamine to fluoroalkyl
a,b-unsatu-
rated phosphonates or ring opening of fluoroalkyl aziridine-2-
phosphonates [8]. Although nucleophilic addition of alkyl phos-
phites and phosphonate
approach for preparation of fluorine-free
a
-carbanions to imines is a common
- and -aminopho-
a
b
sphonates [9] analysis of the literature has revealed that potential
of imines derived from polyfluoroalkyl aldehydes [10] in these
reactions remains largely unexplored. Only multi-step procedure
for assumed generation in situ of the corresponding N-phenyl
imine of fluoral followed by addition of diethyl phosphite giving
rise to corresponding phosphonotrifluoroalanine derivative have
been developed [11]. Synthesis of phosphonotrifluoroalanine was
also accomplished by treatment of O-trifluoroacetyl derivatives
derived from hemiaminals of fluoral with dialkyl trimethylsilyl
phosphites in pyridine [12]. It was suggested that intermediate N-
acyl imines were trapped by dialkyl trimethylsilyl phosphites
synthesis of fluoroalkyl
a- and b-aminophosphonic acid deriva-
tives, their preparation is not trivial and usually includes multi-
step procedures, which were summarized in recent comprehensive
review [6]. General methods for preparation of fluoroalkyl
a-and
b
-aminiophosphonates involve hydrogenation or reduction of
leading to corresponding a-trifluoromethyl a-aminophospho-
nates in quantitative yields. In contrast addition of dialkyl (or
diaryl) phosphites or carbanions generated from phosphonates to
the Schiff bases derived from hexafluoroacetone [13], trifluor-
omethyl ketones and trifluoropyruvates [14] as well as cyclic
* Corresponding author. Tel.: +38 044 573 25 31; fax: +38 044 558 25 52.
E-mail address: sorochinsky@bpci.kiev.ua (A.E. Sorochinsky).
0022-1139/$ – see front matter ß 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jfluchem.2011.05.005

G.-V. Ro¨schenthaler et al. / Journal of Fluorine Chemistry 132 (2011) 834–837
835
NH2
P(OEt)2
fluoroalkyl imines [15] in the presence of bases or Lewis acids was
widely investigated. In this context it seemed quite reasonable to
consider the addition of alkyl phosphites and -carbanions derived
from alkyl phosphonates to fluoroalkyl aldimines as a potentially
convenient, straightforward and general approach for preparation
for 5
F3C
a
H2, 10%Pd/C
EtOH, 20oC
O
RHN
F3C
O
8
P(OEt)2
81.0%
of fluoroalkyl
a- and b-aminophosphonates.
n
O
NH2
4 n = 0, R = PMP
for 4 and 7
2. Results and discussion
P(OEt)2
5
n = 0, R = Bn
7 n = 1, R = PMP
F3C
n
CAN
MeCN-H2O, 20oC
Initially the addition of diethyl phosphite to N-substituted
fluoroalkyl aldimines 1–3 readily obtained by condensation of
hydrate or hemiacetal of corresponding aldehydes with amines
was studied. We found that reaction did not proceed in CH2Cl2 or
ether at room temperature without any catalyst. However, the
addition of diethyl phosphite on fluoroalkyl aldimines 1–3 in
CH2Cl2 proceeded smoothly in the presence of boron trifluoride
8
n = 0, 54.7%
9 n = 1, 68.9%
Scheme 1.
reaction product in excellent yield (Scheme 1). In spite of the
presence of strong electron-withdrawing trifluoromethyl substit-
uent, deprotection of 4 and 7 was effectively realized by treatment
with excess of cerium ammonium nitrate (CAN) in a mixture of
acetonitrile and water at room temperature giving rise to the N-
dearylated product 8 and 9 in good chemical yield according to the
literature procedure [16]. The structure and purity of the known
products were confirmed by spectroscopic analysis and the new
products were fully characterized.
etherate (1 equiv.) at room temperature to give the desired
fluoroalkyl -aminophosphonates 4–6 (Table 1). After simple
work-up procedure and purification by column chromatography
-fluoroalkyl -aminophosphonates 4–6 were isolated in good
a-
a
a
a
yield. However, the purity of product 4 thus obtained was 96%
(according the 19F NMR data) and additional purification by
recrystallization from hexane was required. The scaling up of the
reactions did not affect the yield of products. The study of reaction
of aldimine 2 with triethyl phosphite in CH2Cl2 showed that
addition also proceeded in the presence of boron trifluoride
3. Conclusions
etherate at room temperature to result in the formation of
a-
trifluoromethyl -aminophosphonate 5. The addition reaction
a
In summary, we have reported that nucleophilic addition of
between aldimine 1 and lithium salt of diethyl methylphosphonate
proceeded smoothly in THF at À78 8C within 1 h to afford, after
alkyl phosphites or phosphonate
a-carbanions to aldimines
derived from polyfluoroalkyl aldehydes under mild reaction
conditions followed by subsequent deprotection constitutes a
mild acidic work up, the corresponding
b-trifluoromethyl b-
aminophosphonate 7, which was purified by crystallization. In
general, the best results were obtained when the solution of the
carbanion was added via cannula to a pre-cooled solution of the
aldimine 1. Variation of the solvents did not improve the efficiency
of addition and resulted in incomplete conversion of starting
aldimine 1 as indicated by 19F NMR and TLC analysis of crude
products.
convenient synthetic route leading to fluoroalkyl
aminophosphonates. Scope and limitation of this method are
currently under investigation and will be reported in due course.
a- and b-
4. Experimental
For the removal of the substituent on the nitrogen, various
methods were used depending of the chemical structure of
substrates. Catalytic hydrogenolysis of 5 in EtOH in the presence of
20% palladium hydroxide on carbon under hydrogen atmosphere
NMR spectra were recorded on a Bruker DRX 500 or Varian
Union Plus 400 spectrometers using TMS (1H) and CFCl3 (19F) as an
internal standards and H3PO4
Chemical shifts ( ) are reported in ppm. IR spectra were recorded
(
31P) as an external standard.
d
(1 atm) occurred smoothly within 1 h and afforded the desired
aminophosphonate 8 with primary amino group as a single
a
-
on Bruker Fourier-transform spectrometer VERTEX 70. Melting
points are uncorrected. Analytical TLCs were performed with
Merck silica gel 60 F254 plates. Visualization was accomplished by
ultraviolet light (254 nm). Flash chromatography was carried out
using Merck silica gel 60.
Table 1
Addition of phosphorus reagents to N-substituted fluoroalkyl aldimines 1–3
The starting N-substituted fluoroalkyl aldimines 1–3 were
prepared according to known procedures in 60–80% yields by
heating a toluene solution of hemiacetal or hydrate of corre-
sponding fluoroalkyl aldehydes with amines in the presence of
catalytic amount of p-toluenesulfonic acid in a Dean-Stark
apparatus [17].
4.1. N-Substituted diethyl
general procedure
a-fluoroalkyl a-aminophosphonates 4–6;
.
To a solution of corresponding N-substituted imine of
fluoroalkyl aldehyde (4.92 mmol) in dichloromethane (10 mL)
were added alkyl phosphite (9.84 mmol) and BF3ÁEt2O (0.62 mL,
4.92 mmol) at 0 8C. After being stirred at the room temperature for
24 h, the reaction mixture was poured into a saturated aqueous
solution of NaHCO3 (5 mL). The aqueous layer was extracted with
dichloromethane (2 Â 5 mL), and the combined organic extracts
were washed with brine, dried over Na2SO4 and concentrated
under reduced pressure.
Entry N-substituted RF
R
Phosphorus
reagent
Product Yielda
(%)
Aldimine
1
2
3
4
5
1
2
3
2
1
CF3
CF3
PMP HP(O)(OEt)2
4
5
6
5
7
55.4
86.3
63.3
58.5
83.5
Bn
HP(O)(OEt)2
HP(O)(OEt)2
P(OEt)3
H(CF2)6 Bn
CF3
CF3
Bn
PMP CH3P(O)(OEt)2
a
Isolated yield.

836
G.-V. Ro¨schenthaler et al. / Journal of Fluorine Chemistry 132 (2011) 834–837
4.1.1. Diethyl 2,2,2-trifluoro-1-(4-methoxyphenylamino)-
ethylphosphonate (4)
4.3. Diethyl 1-amino-2,2,2-trifluoroethylphosphonate (8)
Purification by flash chromatography, eluent hexane–ether
1:1, followed by recrystallization from hexane gave 4 in 55.4% as
white solid, mp 72–73 8C. Spectral properties are in agreement
To solution of diethyl 1-benzylamino-2,2,2-trifluoroethylpho-
sphonate 5 (700 mg, 2.15 mmol) in absolute ethanol (19 mL) was
added 20% palladium hydroxide on carbon (450 mg). The reaction
mixture was stirred under hydrogen (1 atm) at room temperature
for 1 h and then was filtered through Celite, washed with ethanol
(3 Â 10 mL) and concentrated under reduced pressure. Purification
by flash chromatography, eluent ether-ethyl acetate 2:1, gave
410 mg (81.0%) of 8 as colorless oil. Spectral properties are in
agreement with those reported in the literature [16,18]. IR
with those reported in the literature [16]. IR (CH2Cl2):
3058, 2986, 2837, 1515, 1243, 1173, 1114, 1024, 979 cmÀ1
NMR (500 MHz, CDCl3): 1.27 (t, J 7.0 Hz, 3H), 1.33 (t, J 7.0 Hz, 3H),
3.75 (s, 3H), 4.00–4.03 (m, 1H), 4.11–4.23 (m, 5H), 6.69 (d, J 8.2 Hz,
2H), 6.80 (d, J 8.2 Hz, 2H); 19F NMR (376 MHz, CDCl3):
À69.78
(m); 31P {1H} NMR (202 MHz, CDCl3):
19.57 (q, J 8,6 Hz).
n
3412,
;
1H
d
d
d
(CH2Cl2): 3416, 3004, 1263, 1183, 1118, 1053, 1026, 978 cmÀ1
1H NMR (500 MHz, CDCl3):
1.37 (t, J 7.0 Hz, 6H), 1.73 (s, 2H), 3.56
;
4.1.2. Diethyl 1-benzylamino-2,2,2-trifluoroethylphosphonate (5)
Purification by flash chromatography, eluent hexane–ether
1:1, gave 5 as colorless oil in 86.3% yield using diethyl phosphite or
d
(dq, J 19.4 and 8.4 Hz, 1H), 4.21–4.26 (m, 4H); 19F NMR (470 MHz,
CDCl3):
À72.03 (dd, J 8.4 and 7.7 Hz); 31P {1H} NMR (202 MHz,
CDCl3): d 21. 13 (q, J 7.7 Hz).
d
58.5% using triethyl phosphite. IR (CH2Cl2):
1257, 1168, 1114, 1051, 1026, 977 cmÀ1
CDCl3): 1.34 (t, J 7.1 Hz, 3H), 1.35 (t, J 7.1 Hz, 3H), 2.07 (br. s, 1H),
n
3365, 3001, 1455,
;
1H NMR (400 MHz,
d
4.4. Diethyl 2-amino-3,3,3-trifluoropropylphosphonate (9)
3.43 (dd, J 21.0 and 8.4 Hz, 1H), 3.99 (d, J 13.1 Hz, 1H), 4.09 (d, J
13.1 Hz, 1H), 4.12–4.22 (m, 4H), 7.28–7.35 (m, 5H); 19F NMR
To a stirred solution of diethyl 3,3,3-trifluoro-2-(4-methox-
yphenylamino)propylphosphonate 7 (1.51 g, 4.25 mmol) in CH3CN
(110 mL) and H2O (50 mL) at 0 8C, CAN (14.0 g, 25.50 mmol) was
added in one portion. Then the resulting solution stirred at room
temperature for 18 h, and quenched with saturated aqueous
NaHCO3 solution (75 mL). The aqueous layer was extracted with
CH2CI2 (2 Â 50 mL) and the combined organic layer was washed
with 5% aqueous Na2SO3 solution (50 mL), dried over anhydrous
Na2SO4, and concentrated under vacuum. The resultant crude
residue was purified by recrystallization from hexane to obtain
0.73 g (68.9%) of 9 as a white solid, mp 51–52 8C. Spectral
properties are in agreement with those reported in the literature
(470 MHz, CDCl3):
(202 MHz, CDCl3):
d
d
À68.32 (dd, J 9.4 and 8.4 Hz); 31P {1H} NMR
21.12 (q, J 9.4 Hz). Calcd. for C13H19F3NO3P:
C, 48.00; H, 5.89; N, 4.31; P 9.52. Found: C, 48.26; H, 5.93; N, 4.54;
P, 9.47.
4.1.3. Diethyl 1-benzylamino-2,2,3,3,4,4,5,5,6,6,7,7-dodeca-
fluoroheptylphosphonate (6)
Purification by flash chromatography, eluent hexane–ethyl-
acetate 2:1, gave 63.3% of 6 as colorless oil. IR (CH2Cl2):
2985, 2910, 1454, 1203, 1142, 1050, 1024, 975 cmÀ1 1H NMR
(500 MHz, CDCl3): 1.39 (m, 6H), 1.84 (br. s, 1H), 3.67–3.75 (m,
n 3357,
;
d
1H), 3.93 (d, J 11.0 Hz, 1H), 4.12 (d, J 11.0 Hz, 1H), 4.24 (m, 4H),
6.07 (t, J 52.4 Hz, 1H), 7.29–7.36 (m, 5H); 19F NMR (470 MHz,
[8f]. IR (CH2Cl2):
n
3683, 3411, 2985, 1252, 1169, 1124, 1055, 1027,
1.35 (t, J 7.1 Hz, 3H), 1.36
967 cmÀ1; 1H NMR (500 MHz, CDCl3):
d
CDCl3):
d
À110.50 (d, J 285.0 Hz, 1F), À116.70 (d, J 285.0 Hz, 1F),
(t, J 7.1 Hz, 3H), 1.71 (s, 2H), 1.89 (ddd, J 17,1, 15.4 and 11.4 Hz, 1H),
2.15 (ddd, J 21.1, 15.4 and 2.0 Hz, 1H), 3.60–3.73 (m, 1H), 4.09–4.22
À119.61 (d, J 296. 9 Hz, 1F), À121.20 (d, J 296.9 Hz, 1F), À121.56
(d, J 302.1 Hz, 1F), À123.08 (d, J 302.4 Hz, 1F), À123.24 (d, J
302.1 Hz, 1F), À124.68 (d, J 302.4 Hz, 1F), À130.14 (s, 2F), À136.95
(dd, J 309.7 and 52.4 Hz, 2F), À138.19 (dd, J 309.7 and 52.4 Hz, 1F);
(m, 4H); 19F NMR (470 MHz, CDCl3):
d
À80.38 (d, J 6.6 Hz); 31P {1H}
NMR (202 MHz, CDCl3):
d 27.6 (s).
31P {1H} NMR (202 MHz, CDCl3):
d 18.62 (s). Calcd. for
Acknowledgment
C
18H20F12NO3P: C, 38.79; H, 3.62; N, 2.51; P 5.56. Found: C,
38.81; H, 3.65; N, 2.63; P, 5.51.
This work was supported by Deutsche Forschungsgemeinschaft
(Grant 436 UKR 113/92/0-1).
4.2. Diethyl 3,3,3-trifluoro-2-(4-methoxyphenylamino)-
propylphosphonate (7)
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n
2985, 1516, 1239, 1170,
1.19 (t, J
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