Highly enantioselective hydrogenation of α,β-unsaturated phosphonates with iridium-phosphinooxazoline complex: Synthesis of a phosphorus analogue of naproxen

Article abstract of DOI:10.1016/S0957-4166(03)00224-6

A number of pharmaceutically interesting optically active 1-arylethylphosphonates, including a phosphorus analogue of naproxen, has been synthesized with ee 92-95% via asymmetric hydrogenation under mild conditions using [Ir(cod)(phosphine oxazoline)]

Full text of DOI:10.1016/S0957-4166(03)00224-6

TETRAHEDRON:
ASYMMETRY
Pergamon
Tetrahedron: Asymmetry 14 (2003) 1397–1401
Highly enantioselective hydrogenation of a,b-unsaturated
phosphonates with iridium–phosphinooxazoline complex: synthesis
of a phosphorus analogue of naproxen
Natalia S. Goulioukina,a Tat’yana M. Dolgina,a Grigorii N. Bondarenko,a Irina P. Beletskaya,a,*
Mikhail M. Ilyin,b Vadim A. Davankovb and Andreas Pfaltzc,*
aDepartment of Chemistry, Moscow State University, Leninskie Gory, GSP-2, 119992 Moscow, Russia
bN. Nesmeyanov Institute of Organoelement Compounds of the Russian Academy of Sciences, Vavilov St. 28,
117813 Moscow, Russia
cDepartment of Chemistry, University of Basel, St. Johanns-Ring 19, CH-4056 Basel, Switzerland
Received 21 January 2003; accepted 7 March 2003
Abstract—A number of pharmaceutically interesting optically active 1-arylethylphosphonates, including a phosphorus analogue of
naproxen, has been synthesized with ee 92–95% via asymmetric hydrogenation under mild conditions using [Ir(cod)(phosphine
oxazoline)]+ [BArF]− catalysts. © 2003 Elsevier Science Ltd. All rights reserved.
tion of enantiopure 1-arylethylphosphonates is neces-
sary for correct evaluation of their structure–activity
relationships. Optically active 1-phenylethylphospho-
nates have been obtained via enantioselective methyla-
tion of benzylphosphonic acid derivatives (phosphona-
mides or 1,3,2-oxazaphosphorinanes) bearing chiral
auxiliaries7–9 and by photo-Arbuzov rearrangement of
optically active 2-(1-phenylethoxy)-1,3,2-dioxaphospho-
rinanes.10–12 Recently a series of optically active 1-aryl
substituted ethylphosphonic acids was prepared with
high enantioselectivity by homogeneous catalytic
hydrogenation of corresponding a,b-unsaturated pre-
cursors, using chiral ruthenium complexes.13 However,
Ru-catalyzed hydrogenation of diethyl 1-arylethenyl-
phosphonates using chiral Ru(II)-catalysts required vig-
orous conditions and gave only moderate enantioselec-
tivities. Herein we report our results on iridium(I)-
catalyzed hydrogenation of prochiral diethyl 1-aryl-
ethenylphosphonates 1 to corresponding diethyl 1-aryl-
ethylphosphonates 2 under mild conditions with good
catalytic activity and enantioselectivity (Scheme 1).
1. Introduction
Phosphonic acid analogues of a-arylpropionic acids (a
well-known class of non-steroidal anti-inflammatory
and analgesic drugs such as naproxen and ibuprofen)
deserve attention since many of these derivatives exhibit
interesting biological activity profiles. 1-Arylethylphos-
phonates were reported to possess negative inotropic
and Ca2+-antagonistic activity,1 as well as cyclooxyge-
nase inhibitor properties.2 They were successfully used
as haptens for reactive immunisation as a strategy for
the generation of catalytic antibodies for enantioselec-
tive hydrolysis.3,4 Recently it was shown5 that some
1-arylethylphosphonic acids reveal antidopaminergic
and neuroprotective functions. Therefore, 1-aryl-
ethylphosphonates may be considered as a promising
class of compounds in the search of new potent antipsy-
chotic drugs.
Synthetic approaches to racemic 1-arylethylphosphonic
acids and esters are well elaborated.6 Since, in general,
the biological activity of chiral compounds is highly
dependent on the absolute configuration, the prepara-
2. Results and discussion
The starting prochiral diethyl 1-arylethenylphospho-
nates 1 were easily prepared by palladium-catalysed
hydrophosphorylation of terminal arylacetylenes14,15 or
* Corresponding authors. Fax: +7-095 939-1854 (I.P.B.); fax: +41-61
267-1103 (A.P.); e-mail: beletska@org.chem.msu.ru; andreas.
pfaltz@unibas.ch
0957-4166/03/$ - see front matter © 2003 Elsevier Science Ltd. All rights reserved.
doi:10.1016/S0957-4166(03)00224-6

1398
N. S. Goulioukina et al. / Tetrahedron: Asymmetry 14 (2003) 1397–1401
Scheme 1.
by esterification of 1-arylethenylphosphonic acids,
which are readily available by a Conant reaction.6 As
catalysts, air-stable cationic iridium(I) complexes bear-
ing either the phosphinooxazoline ligand La or oxazo-
line–phosphinite ligands Lb or Lc were tested.16,17
Tetrakis[3,5-bis(trifluoromethyl)phenyl]borate ([BArF]−)
was used as the counterion because this anion is known
to reduce the sensitivity of the Ir complexes against
moisture and to prevent deactivation of the catalyst
during the reaction.16,17
The catalyst [Ir(cod)(La)]+[BArF]− was highly effective
and quantitative conversions were observed in all reac-
tions (Table 1, entries 1–7).
The rate and enantioselectivity of this reaction were
found to increase significantly with the decrease of
hydrogen pressure from 60 to 5 bar (Table 1, entries
1–5). A similar pressure dependence of the ee has been
previously observed in the hydrogenation of terminal
olefins with Ir catalysts.17 Variation of the temperature
between 20 and 60°C had a small effect on enantioselec-
tivity (entries 5–7). The highest ee (94%, entry 6) was
observed at 40°C and a hydrogen pressure of 5 bar.
Under these conditions complexes [Ir(cod)(Lb)]+[BArF]−
and [Ir(cod)(Lc)]+[BArF]− bearing oxazoline–phos-
phinite ligands showed very low activity (entries 8 and
9).
Initial optimisation of reaction conditions was per-
formed using the iridium–phosphinooxazoline complex
[Ir(cod)(La)]+[BArF]− (1 mol%) and diethyl 1-
phenylethenylphosphonate 1a as a model substrate. The
reactions were carried out in CH2Cl2 and monitored
using 31P NMR spectroscopy. Disappearance of the
peak of starting 1a at l (CDCl3) 16.8 ppm with increase
of the peak of final product 2a at 29.5 ppm13 indicated
a clean reaction with no detectable side products.
The absolute configuration (R) of 2a was assigned
based on the specific rotation of the corresponding
Table 1. Optimisation of enantioselective hydrogenation of 1a in CH2Cl2 with use of [Ir(cod)(L*)]+ [BArF]− catalysts
Entry
1
L*
Pressure (bar)
60
Temperature (°C)
Time (h)
Conversion (%) (31P NMR)
Ee (%)
La
20
6
24
24
3
82
100
100
88
70
72
2
3
La
La
50
30
20
20
6
3
6
3
6
3
6
3
100
89
100
95
100
94
100
93
90
91
91
94
91
4
5
6
7
8
9
La
La
La
La
Lb
Lc
10
5
20
20
40
60
40
40
5
5
6
3
100
7
5
28
3
7
8
5
216
8

N. S. Goulioukina et al. / Tetrahedron: Asymmetry 14 (2003) 1397–1401
1399
1-phenylethylphosphonic acid ([h]D +4.6 (c 0.9,
3.2. Synthesis of starting materials
MeOH)), obtained by McKenna procedure,18 in com-
parison with literature data [h]D +4.8 (c 0.9, MeOH) for
the (R)-enantiomer.7
3.2.1. Diethyl 1-phenylethenylphosphonate 1a. A mixture
of 1-phenylethenylphosphonic acid (3.26 g, 17.7 mmol)
and triethyl orthoformate (13.0 mL, 78.2 mmol) was
slowly heated in a gentle stream of argon, ethyl formate
and ethanol being allowed to distil out. As volatile
materials were removed the reaction mixture was
heated to 140–145°C and maintained at reflux for 1–2 h
until the completion of the reaction (TLC control). The
excess HC(OEt)3 was removed under reduced pressure
and the residue was distilled in vacuo to afford 1a (4.09
Under optimum conditions a range of substrates 1 was
investigated and high enantioselectivities and good con-
versions were achieved in most cases (Table 2). Only a
small steric effect of the aromatic substituent was
observed and reaction rates and enantioselectivities
were practically the same when aryl was phenyl, 4-
diphenyl, 1- or 2-naphthyl (entries 1–4). Substrate 1e
showed very low reactivity and deactivation of the
catalyst was observed in the course of the reaction.
Such a dramatic effect of the MeO-group is difficult to
explain since several 4-methoxyphenyl substituted
olefins have been previously used as test substrates for
hydrogenation with iridium-phosphinooxazoline cata-
lysts without any problems with conversion. However,
as a possible cause of the low reactivity, traces of
impurities in the substrate cannot be ruled out. We
could improve the yield of 2e (phosphorus analogue of
naproxen) up to 78% after 115 h when the catalyst
(total amount 2 mol%) was added in two portions; the
second portion after 27 h of the reaction. Diethyl
1-(3-pyridyl)ethenylphosphonate proved to be totally
unreactive under the reaction conditions, which is not
surprising, because coordinating species are known to
deactivate Ir catalysts of this type.17
g, 96%) as a colourless viscous liquid, Bp2
mmHg
132°C,15 Rf 0.6 (Silufol UV 254, CHCl3/EtOH, 10:1).
3.2.2. Diethyl 1-(4-diphenyl)ethenylphosphonate 1b.
Diethyl 1-(4-diphenyl)ethenylphosphonate 1b was pre-
pared in 80% yield analogously to 1a from 1-(4-
diphenyl)ethenylphosphonic acid6 (0.94 g, 3.6 mmol)
and HC(OEt)3 (4.4 mL, 26.5 mmol). Yellowish thick
oil, Bp0.1 mmHg 185°C, Rf 0.75 (Silufol UV 254, CHCl3/
EtOH, 10:1). 31P NMR (CDCl3): l (ppm) 16.8. 1H
NMR (CDCl3): l (ppm) 1.29 (t, 3JHH 7.1 Hz, 6H, CH3),
2
3
4.12 (m, 4H, OCH2), 6.19 (dd, JHH 1.5 Hz, JHP 45.7
2
3
Hz, 1H, trans-PCꢀCH), 6.34 (dd, JHH 1.5 Hz, JHP
3
21.9 Hz, 1H, cis-PCꢀCH), 7.33 (m, JHH 7.3 Hz, 1H,
(C-4¦)H), 7.42 (m, 2H, Har), 7.55–7.62 (m, 6H, Har). 13
C
3
NMR (CDCl3): l (ppm) 16.0 (d, JCP 4.8 Hz, CH3),
2
62.0 (d, JCP 5.3 Hz, OCH2), 126.7 (CHar), 126.8
3
(CHar), 127.2 (C-4¦), 127.6 (d, JCP 4.7 Hz, C-3% and
2
C-5%), 128.6 (CHar), 131.2 (d, JCP 6.7 Hz, C-2), 135.3
2
1
Table 2. Enantioselective hydrogenation of diethyl 1-
arylethenylphosphonates 1 catalyzed by [Ir(cod)(La)]+
[BArF]− (1 mol%) (CH2Cl2, 5 bar, 40°C)
(d, JCP 11.2 Hz, C-4%), 139.0 (d, JCP 174.9 Hz, C-1),
140.1 (Car), 140.7 (Car).
3.2.3. Diethyl 1-(1-naphthyl)ethenylphosphonate 1c.
Diethyl 1-(1-naphthyl)ethenylphosphonate 1c was pre-
pared in 91% yield analogously to 1a from 1-(1-naph-
thyl)ethenylphosphonic acid6 (1.70 g, 7.3 mmol) and
HC(OEt)3 (6.0 mL, 36.1 mmol). Yellowish viscous liq-
uid, Bp0.1 mmHg 135°C, Rf 0.6 (Silufol UV 254, CHCl3/
EtOH 10/1). 31P NMR (CDCl3): l (ppm) 15.4. 1H
NMR (CDCl3): l (ppm) 1.17 (t, 3JHH 7.0 Hz, 6H, CH3),
Entry
Substrate
Time (h)
Conversion (%) ee (%)
(
31P NMR)
1
2
3
1a
1b
1c
6
6
5
24
6
6
100
95
90
(+)94
94
93
96
92
93
4
5
1d
1e
2
3
B10
4.03 (m, 4H, OCH2), 6.00 (dd, JHH 2.0 Hz, JHP 46.8
115
78a
95
2
3
Hz, 1H, trans-PCꢀCH), 6.66 (dd, JHH 2.0 Hz, JHP
22.4 Hz, 1H, cis-PCꢀCH), 7.42–7.48 (m, 4H, Har),
7.77–7.84 (m, 2H, Har), 7.99–8.03 (m, 1H, Har). 13C
a An additional portion of [Ir(cod)(La)]+ [BArF]− (1 mol%) was added
after 27 h of the reaction.
3
NMR (CDCl3): l (ppm) 16.0 (d, JCP 7.0 Hz, CH3),
2
62.2 (d, JCP 6.2 Hz, OCH2), 124.7 (CHar), 125.5
3
3. Experimental
(CHar), 125.7 (CHar), 125.8 (CHar), 126.2 (d, JCP 4.7
3
Hz, C-2%), 128.0 (CHar), 128.1 (CHar), 131.4 (d, JCP 4.2
2
3.1. Physical measurements
Hz, C-9%), 133.5 (C-10%), 134.3 (d, JCP 6.1 Hz, C-2),
2
1
134.4 (d, JCP 9.7 Hz, C-1%); 138.7 (d, JCP 177.1 Hz,
C-1); IR (film): 2983, 1251, 1234, 1051, 1024, 968, 808,
779.
1H, 13C and 31P NMR spectra were recorded in CDCl3
with a Varian VXR-400 (1H at 400, 13C at 100 and 31P
at 162 MHz) spectrometer. Chemical shifts of H spec-
1
tra are referenced to tetramethylsilane internal standard
(l=0), 13C spectra are referenced to CDCl3 (l=77.00),
31P spectra are referenced to H3PO4 (85%) external
standard (l=0). IR spectra were obtained on a IKAR
FT spectrophotometer (Microtek, Russia). HPLC
analyses were performed with a Bischoff (Germany)
liquid chromatograph. Optical rotation was recorded
using a Perkin–Elmer 241 polarimeter.
3.2.4. Diethyl 1-(2-naphthyl)ethenylphosphonate 1d.
Diethyl 1-(2-naphthyl)ethenylphosphonate 1d was pre-
pared in 85% yield analogously to 1a from 1-(2-naph-
thyl)ethenylphosphonic acid6 (2.34 g, 10.0 mmol) and
HC(OEt)3 (6.7 mL, 40.3 mmol). Yellowish viscous liq-
uid, Bp0.1 mmHg 167°C, Rf 0.8 (Silufol UV 254, CHCl3/
EtOH, 10:1). 31P NMR (CDCl3): l (ppm) 16.7. 1H
NMR (CDCl3): l (ppm) 1.27 (t, 3JHH 7.1 Hz, 6H, CH3),

1400
N. S. Goulioukina et al. / Tetrahedron: Asymmetry 14 (2003) 1397–1401
2
3
3
4.12 (m, 4H, OCH2), 6.25 (dd, JHH 1.5 Hz, JHP 45.4
Hz, trans-PCꢀCH), 6.40 (dd, 2JHH 1.5 Hz, 3JHP 22.0 Hz,
cis-PCꢀCH), 7.46 (m, 2H, (C-6%)H and (C-7%)H), 7.62
4.05 (m, 2H, OCH2), 7.32 (m, JHH 7.4 Hz, 1H, (C-
4¦)H), 7.42 (m, 4H, Har), 7.57 (m, 4H, Har). 13C NMR
2
(CDCl3): l (ppm) 15.5 (d, JCP 5.4 Hz, CHCH3), 16.2
3
3
3
(d, JCP 6.8 Hz, CH2CH3), 16.4 (d, JCP 6.4 Hz,
(m, JHH 8.5 Hz, 1H, (C-3%)H), 7.78–7.85 (m, 3H, Har),
1
8.02 (brs, 1H, (C-1%)H). 13C NMR (CDCl3): l (ppm)
CH2CH3), 38.0 (d, JCP 137.2 Hz, CHCH3), 61.9 (d,
2JCP 5.9 Hz, OCH24), 62.4 (d, JCP 8.6 Hz, OCH2), 126.9
2
16.0 (d, 3JCP 6.7 Hz, CH3), 62.0 (d, 2JCP 6.5 Hz, OCH2),
3
(CHar), 127.0 (d, JCP 2.2 Hz, C-2% and C-6%), 127.2
124.9 (d, JCP 5.1 Hz, C-3%), 126.0 and 126.1 (C-6% and
3
3
(C-4¦), 128.7 (CHar), 129.0 (d, JCP 6.8 Hz, C-3% and
C-7%), 126.5 (d, JCP 6.7 Hz, C-1%), 127.3 (CHar), 127.8
2
2
C-5%), 136.9 (d, JCP 6.1 Hz, C-4%), 139.8 (Car), 140.6
(CHar), 128.1 (CHar), 131.7 (d, JCP 8.7 Hz, C-2), 132.7
(Car), 132.9 (Car), 133.7 (d, 2JCP 12.3 Hz, C-2%), 139.4 (d,
1JCP 173.3 Hz, C-1).
(Car).
3.4.2. Diethyl ( )-1-(1-naphthyl)ethylphosphonate ( )-2c.
Yellowish liquid, Rf 0.3 (Silufol UV 254, Et2O). 31P
NMR (CDCl33): l (ppm) 29.8. 1H NMR (CDCl3): l
3.2.5. Diethyl 1-(6-methoxy-2-naphthyl)ethenylphos-
phonate 1e. Diethyl 1-(6-methoxy-2-naphthyl)ethenyl-
phosphonate 1e was prepared according to the litera-
ture procedure.15
3
(ppm) 0.92 (t, JHH 7.2 Hz, 3H, CH2CH3), 1.26 (t, JHH
3
3
7.2 Hz, 3H, CH2CH3), 1.72 (dd, JHH 7.4 Hz, JHP 18.2
Hz, 3H, CHCH3), 3.57 (m, 1H, OCH2), 3.82 (m, 1H,
OCH2), 4.00–4.16 (m, 3H, OCH2 and CH), 7.48 (m,
2H, Har), 7.54 (m, 1H, Har), 7.72 (m, 1H, Har), 7.76 (m,
3.3. General procedure for the enantioselective
hydrogenation
3
3JHH 8.0 Hz, 1H, Har), 7.85 (m, JHH 8.0 Hz, 1H, Har),
8.12 (m, JHH 8.4 Hz, 1H, Har). 13C NMR (CDCl3): l
3
The autoclave was deaerated with argon and filled with
hydrogen. A Schlenk flask was charged with 1 (0.4
mmol), [Ir(cod)(L*)]+ [BArF]− (4 mmol) and deaerated
CH2Cl2 (20 mL, freshly distilled over CaH2) under
argon and the mixture was stirred at room temperature
for 15 min. The resulting orange solution was trans-
ferred by syringe to autoclave under hydrogen stream.
The autoclave was sealed and pressurised with H2, and
the mixture was stirred under the conditions (tempera-
ture and time). The samples of reaction mixture (ca. 0.8
ml) were taken at times and analysed by 31P NMR to
assess the conversion. After completing of the reaction
CH2Cl2 was removed and replaced with Et2O (2–2.5
mL). The solution was passed through a short column
(Silpearl 10×1 cm, Et2O) to afford 2 in essentially
quantitative yield. The identity of products 2 was
confirm by 1H NMR. The enantiomeric excesses of
samples obtained were determined by HPLC (Chiral-
pak AD-H 25×0.46 cm column, hexane/i-PrOH 9/1, 1.0
mL min−1, 254 nm). Retention times of two enan-
tiomers: 2a, 7.1 and 5.2 min (ratio 33/1); 2b, 8.9 and 8.2
min (ratio 30/1); 2c, 10.4 and 6.6 min (ratio 23/1); 2d,
25.5 and 11.6 min (ratio 26.5/1); 2e, 19.8 and 13.3 min
(ratio 38/1).
2
3
(ppm) 16.0 (d, JCP 4.3 Hz, CHCH3), 16.2 (d, JCP 4.5
3
Hz, CH2CH3), 16.4 (d, JCP 6.3 Hz, CH2CH3), 32.2 (d,
1JCP 139.8 Hz, CHCH3), 61.8 (d, JCP 7.5 Hz, OCH2),
2
2
62.4 (d, JCP 7.6 Hz, OCH2), 123.1 (CHa3r), 125.3 (d,
4JCP 5.4 Hz, C-3%), 125.4 (CHar), 125.9 (d, JCP 6.5 Hz,
2
C-2%), 126.0 (CHar), 127.4 (d, JCP 3.1 Hz, C-8%), 128.8
(CHar), 131.6 (d, JCP 7.1 Hz, Car), 133.7 (Car), 134.2 (d,
JCP 5.7 Hz, Car).
3.4.3. Diethyl ( )-1-(2-naphthyl)ethylphosphonate ( )-2d.
Colourless viscous liquid, Bp0.1
171°C, Rf 0.4
mmHg
(Silufol UV 254, Et2O). 31P NMR (CDCl3): l (ppm)
1
3
29.3. H NMR (CDCl3): l (ppm) 1.09 (t, JHH 7.0 Hz,
3
3H, CH2CH3), 1.25 (t, JHH 7.0 Hz, 3H, CH2CH3), 1.66
3
3
(dd, JHH 7.4 Hz, JHP 18.4 Hz, 3H, CHCH3), 3.33 (dq,
3JHH 7.4 Hz, JHP 22.4 Hz, 1H, CH), 3.77 (m, 1H,
2
OCH2), 3.91 (m, 1H, OCH2), 4.03 (m, 2H, OCH2), 7.42
3
(m, 2H, Har), 7.50 (d, JHH 8.4 Hz, 1H, Har), 7.75–7.81
(m, 4H, Har). 13C NMR (CDCl3): l (ppm) 15.4 (d, JCP
2
3
6.2 Hz, CHCH3), 16.0 (d, JCP 5.7 Hz, CH2CH3), 16.2
3
1
(d, JCP 5.4 Hz, CH2CH3), 38.3 (d, JCP 138.0 Hz,
2
2
CHCH3), 61.6 (d, JCP 6.3 Hz, OCH2), 62.1 (d, JCP 7.2
3
Hz, OCH2), 125.4 (CHar), 125.8 (CHar), 126.6 (d, JCP
3
4.5 Hz, CHar), 127.0 (d, JCP 7.9 Hz, CHar), 127.3
2
(CHar), 127.5 (CHar), 127.7 (CHar), 135.3 (d, JCP 4.8
3.4. Synthesis of racemic diethyl 1-arylethylphosphon-
ates ( )-2
Hz, C-2%), 132.2 (Car), 133.1 (Car).
Synthesis of racemic ( )-2a and ( )-2e has been
described previously.6,13 In the same manner racemic
( )-2b–d were prepared in 77–86% yield by hydrogen
transfer reduction of 1b–d with ammonium formate (6
equiv.) in the presence of 10% Pd/C (2.9 mol%) in
absolute MeOH; reaction time 4–9 h. Spectroscopic
data for these compounds are given below.
Acknowledgements
This study was financially supported by the Program
‘Leading Scientific Schools’ (Project N 00-15-97406).
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