Efficient synthesis of α-substituted ethylphosphonates via CuH-catalyzed conjugate reduction of terminal alkenylphosphonate

Article abstract of DOI:10.1016/j.tetlet.2017.10.045

An unprecedented approach toward synthesis of α-substituted ethylphosphonates based on CuH-catalyzed conjugate reduction of vinylphosphonates has been successfully developed. This protocol features mild conditions, broad substrate scope, good functional group compatibility, high overall efficiencies, and easy gram-scale synthesis. The Cu-catalyzed reduction takes place in a highly selective manner on the phosphono substituted C[dbnd]C bond in the case of the reaction of alkenylphosphonates bearing both phosphono and alkyl or aryl substituted alkene moieties. Furthermore, the result of competitive reaction indicates that the Cu-catalyzed conjugate reduction of vinylphosphonate is more challenging and reproducible than the corresponding acrylate's reaction.

Full text of DOI:10.1016/j.tetlet.2017.10.045

Tetrahedron Letters xxx (2017) xxx–xxx
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Efficient synthesis of
a
-substituted ethylphosphonates via CuH-
catalyzed conjugate reduction of terminal alkenylphosphonate
c,
d
a,
d
b
b
Li Zhang a, Yewen Fang b,e, , Xiaoping Jin , Ting Guo , Ruifeng Li , Yan Li , Xie Li , Yi Yang ,
⇑
⇑
⇑
Meijuan Yuan a, Zongming Tian c
a College of Chemistry and Chemical Engineering, Taiyuan University of Technology, No. 79 West Yingze Street, Taiyuan 030024, China
b School of Materials and Chemical Engineering, Ningbo University of Technology, No. 201 Fenghua Road, Ningbo 315211, China
c Department of Biology and Pharmaceutical Sciences, Zhejiang Pharmaceutical College, No. 888 Yinxian Avenue East, Ningbo 315100, China
d Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials and Ministry-of-Education Key Laboratory for Synthesis and Application of Organic
Functional Molecules, Hubei University, No. 368 Youyi Dadao, Wuhan 430062, China
e Key Laboratory of Organofluorine Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345 Lingling Road, Shanghai 200032, China
a r t i c l e i n f o
a b s t r a c t
Article history:
An unprecedented approach toward synthesis of
a-substituted ethylphosphonates based on CuH-cat-
Received 29 August 2017
Revised 16 October 2017
Accepted 17 October 2017
Available online xxxx
alyzed conjugate reduction of vinylphosphonates has been successfully developed. This protocol features
mild conditions, broad substrate scope, good functional group compatibility, high overall efficiencies, and
easy gram-scale synthesis. The Cu-catalyzed reduction takes place in a highly selective manner on the
phosphono substituted C@C bond in the case of the reaction of alkenylphosphonates bearing both phos-
phono and alkyl or aryl substituted alkene moieties. Furthermore, the result of competitive reaction indi-
cates that the Cu-catalyzed conjugate reduction of vinylphosphonate is more challenging and
reproducible than the corresponding acrylate’s reaction.
Keywords:
a-Substituted ethylphosphonate
Conjugate reduction
CuH catalysis
Ó 2017 Elsevier Ltd. All rights reserved.
Chemoselective reduction
Introduction
several years, many efforts have been devoted to developing effi-
cient metal-catalyzed methods. As for the Ni-catalyzed Hiyama
Amongst the various phosphorus compounds,
a-substituted
coupling,6 only one example was reported on the coupling of
a-
ethylphosphonates continue to receive great attention because
they could be considered as phosphorus-containing analogues of
brominated ethylphosphonate with p-toly silane. Furthermore, in
the case of Cu-catalyzed reductive coupling of N-tosylhydrazones
and H-phosphonates,7 moderate to high yields were observed
under somewhat harsh conditions. Additionally, the method based
on the Pd-catalyzed hydrophosphorylation of olefins is attractive
because of its high atom economy and efficiency. However, the
expected Markovnikov products were always contaminated by
the anti-Markovnikov adducts.8 In comparison with the above-
a
-substituted propionic acids, well-known non-steroidal anti-
inflammatory drugs such as naproxen and ibuprofen.1 Therefore,
-substituted ethylphosphonates may find applications in the
search for new potent antipsychotic drugs. Moreover, -ary-
a
a
lalkylphosphonates have been demonstrated to exhibit negative
inotropic and calcium antagonistic activity.2 They were also widely
employed as haptens for reactive immunization.3 As a conse-
quence, significant progress has been achieved for the preparation
mentioned methods, hydrogenation of
a-substituted vinylphos-
phonates represents the versatile and straightforward approach.
These methods fall into the following three categories based on
the resource of hydrogen: (i) hydrogen pressure reduction9; (ii)
hydrogen transfer reduction9c,10; (iii) diimide reduction.11
Given the dangerous handling of gaseous hydrogen and the
employment of a large excess amount of hydrazine in noncatalytic
diimide reduction, reduction of alkenes based on catalytic hydro-
gen transfer offers advantages such as operational simplicity, low
requirement of equipment, and economical fashion. However, in
the case of the hydrogen transfer reduction using a HCOONH4/
Pd/C system, hydrodehalogenation as a side reaction often occurs
over most metal catalysts employed.9c We hypothesized that the
of
a
-substituted ethylphosphonates.
Compared to the Arbuzov reaction of triethyl phosphite with
arylethyl bromides4 and other noncatalytic protocols,5 the catalytic
approaches toward the synthesis of -substituted ethylphospho-
a-
a
nates are undoubtedly more powerful and appealing. In the last
⇑
Corresponding authors at: School of Materials and Chemical Engineering,
Ningbo University of Technology, No. 201 Fenghua Road, Ningbo 315211, China (Y.
Fang).
E-mail addresses: fang@nbut.edu.cn (Y. Fang), jinxp@mail.zjpc.net.cn (X. Jin),
rfli@tyut.edu.cn (R. Li).
https://doi.org/10.1016/j.tetlet.2017.10.045
0040-4039/Ó 2017 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Zhang L., et al. Tetrahedron Lett. (2017), https://doi.org/10.1016/j.tetlet.2017.10.045

2
L. Zhang et al. / Tetrahedron Letters xxx (2017) xxx–xxx
above mentioned drawbacks could be addressed via CuH-catalyzed
conjugate reduction with silanes as the hydrogen source. Since the
pioneering works of Stryker and co-workers,12 Stryker’s reagent,
[(Ph3P)CuH]6 effect highly regioselective conjugate reductions of
various carbonyl derivatives, including unsaturated ketones, esters,
aldehydes, and related substrates.13 Surprisingly, there is no prece-
dent for the copper-catalyzed reduction of the terminal
alkenylphosphonates. Herein, as a part of our program aiming at
catalyst (entry 8). Significant drop of the yield was observed with
CuCl or Cu(acac)2 as the copper source (entries 9–10). Other
common copper complexes like CuI, CuCN, CuCl2Á2H2O, CuBr2,
and CuSO4 did not catalyze the reaction at all (entries 11–15).
Expectedly, control experiments indicated that both Cu
(OAc)2ÁH2O and PPh3 were required for this reductive protocol
(entries 16–17). On the basis of the screening, the combination of
10 mol% Cu(OAc)2ÁH2O, 10 mol% PPh3, 3 equiv of t-BuOH and 3
equiv of PMHS in toluene at 40 °C for 3 h emerged as the best
reaction conditions.
developing practical methods for the synthesis of
phosphonates,10a,11,14 we describe a straightforward and efficient
approach toward the synthesis of -substituted
a-substituted
a
With the optimized reaction conditions in hand, we then set out
ethylphosphonates based on CuH-catalyzed reduction under mild
to examine the scope of the reduction with respect to the a-aryl
conditions.
ethenylphosphonates. As shown in Table 2, the copper-catalyzed
conjugate reduction of various terminal vinylphosphonates pro-
ceeded smoothly and furnished
a-aryl ethylphosphonates in good
Results and discussion
to excellent yields. -Aryl ethylphosphonates possessing elec-
a
tron-donating groups (Me, OMe) (2b–f) at the phenyl ring were
efficiently accessible in 82-95% yields. The reactions of vinylphos-
phonates having electron- withdrawing groups (CO2Me, NO2, CN)
at the phenyl ring proceeded efficiently to provide the products
2g–i in 87–91% yields. Notably, the highly chemoselective reduc-
tion of acetyl-containing alkenylphosphonate could be realized,16a
giving the product 2j in a 87% isolated yield. The reaction of
alkenylphosphonate containing biphenyl group afforded 2k in
At the outset, we selected a-phenyl ethenylphosphonate 1a as
the model substrate to optimize the reaction conditions and the
results are listed in Table 1. Subjecting 1a to a solution of CuH cat-
alyst (generated from Cu(OAc)2ÁH2O, PPh3, and a stoichiometric
amount of poly(methylhydrosiloxane) (PMHS) as the hydride
source)15 resulted in the formation of the desired product 2a in
22% yield. Gratifyingly, the presence of 1 equiv t-BuONa as the
additive accelerated the conversion greatly,16 affording the 2a in
81% yield (entry 2). The yield could be further improved to 95%
with 3 equiv of t-BuOH as the additive13c (entry 3). Reducing the
amount of the catalyst to 5 mol% resulted in a lower yield of 87%
(entry 4). Changing the solvent to THF, dioxane, or CH3CN
dramatically decreased the efficiency (entries 5–7). Furthermore,
the significance of the copper source in this reaction was
demonstrated by control experiments. Similar yield was obtained
using anhydrous Cu(OAc)2 in place of Cu(OAc)2ÁH2O as the
94%
yield.
However,
the
2-naphthyl
substituted
vinylphosphonate was reluctant to be reduced under the
standard conditions. Fortunately, the conjugate reduction could
undergo smoothly when PPh3 was replaced with 1,2-bis
(diphenylphosphino)benzene (BDP),13c delivering 2l in 96% yield.
Furthermore, the hydrogenation of alkene also took place when
a-thienyl vinylphosphonate was used as substrate to give the
corresponding product 2m in 96% yield. Likewise, the dimethyl
Table 1
Optimization of reaction conditions.a
entry
catalyst
ligand
additive
solvent
yield of 2ab (%)
1c
Cu(OAc)2ÁH2O
Cu(OAc)2ÁH2O
Cu(OAc)2ÁH2O
Cu(OAc)2ÁH2O
Cu(OAc)2ÁH2O
Cu(OAc)2ÁH2O
Cu(OAc)2ÁH2O
Cu(OAc)2
Cu(acac)2
CuCl
PPh3
PPh3
PPh3
PPh3
PPh3
PPh3
PPh3
PPh3
PPh3
PPh3
PPh3
PPh3
PPh3
PPh3
PPh3
PPh3
–
–
toluene
toluene
toluene
toluene
THF
dioxane
CH3CN
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
22
81
95
87
2d
t-BuONa
t-BuOH
t-BuOH
t-BuOH
t-BuOH
t-BuOH
t-BuOH
t-BuOH
t-BuOH
t-BuOH
t-BuOH
t-BuOH
t-BuOH
t-BuOH
t-BuOH
t-BuOH
3
4e
5c
23
6c
trace
trace
92
33
20
trace
trace
trace
trace
trace
0
7c
8
9c
10c
11c
12c
13c
14c
15c
16c
17c
CuI
CuCN
CuCl2Á2H2O
CuBr2
CuSO4
–
Cu(OAc)2ÁH2O
0
a
Reaction conditions: a reaction mixture of 1a (0.4 mmol), copper catalyst (0.04 mmol), PPh3 (10.5 mg, 0.04 mmol), PMHS (266 lL, 1.2 mmol), t-BuOH (127 lL, 1.2 mmol),
and solvent (2 mL) was stirred at 40 °C for 3 h.
b
Yield of the isolated product.
c
The yield was determined using 1H NMR spectroscopy with 4-nitroacetophenone as an internal standard.
d
With 1 equiv of t-BuONa as the additive in place of t-BuOH.
A combination of Cu(OAc)2ÁH2O (5 mol%)/PPh3(5 mol%) was used.
e
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L. Zhang et al. / Tetrahedron Letters xxx (2017) xxx–xxx
3
Table 2
of these versatile functional groups by transition-metal-catalyzed
cross-coupling reactions. However, in the case of reduction of alke-
nes under hydrogen transfer conditions in the presence of carbon-
supported metal catalysts, hydrodehalogenation is a competing
reaction. Considering the mild reaction conditions, we speculate
that our protocol could be extended to the reduction of halo-con-
taining vinylphosphonates (Table 3). To our delight, aryl chlorides,
aryl bromide, and aryl fluorides turned out to be compatible. In
contrast to the moderate yield of 2s, high yields of 2-chloro- and
2-fluorophenyl substituted ethylphosphonates (4e and 4f) were
observed with BDP as the ligand.
Scope of a
-substituted vinylphosphonates.a,b
One limitation of NBSH mediated reduction of alkenes arises
from the difficulties associated with differentiation between phos-
phono and aliphatic olefins.11 To determine whether this method
displays acceptable levels of olefin selectivity, conjugate reduction
of 5, which contain both phosphono and alkyl substituted alkene
moieties, was investigated (Scheme 1). The exclusive formation
of the respective ethylphosphonate 6 indicates that CuH-catalyzed
conjugate reduction takes place in a highly selective manner on the
phosphono substituted C@C bond. A similar result occurs when 7
bearing two exo C@C bonds was subjected to the standard reaction
conditions. In this case, the phosphono substituted olefin was
reduced exclusively while the other aryl substituted C@C bond
remained intact.
Interestingly, in addition to the 1-aryl ethenylphosphonate, the
above CuH-catalyzed conjugate reduction could be extend to 2-
(aryl)vinylphosphonate. As demonstrated in Scheme 2, with BDP
(5 mol%) as the ligand at 60 °C, styrene phosphonate 9 was nicely
reduced to give the diethyl 2-phenylethylphosphonate 10 in 89%
yield. This result further expanded the substrate scope of the
reaction.
Having learned the scope of the conjugate reduction, we sought
to explore the effect of increasing the scale of this reductive reac-
tion (Scheme 3). Subjecting 1.44 g of 1a yielded 1.40 g (96% yield)
of the desired reduced product 2a without decreasing the effi-
ciency compared with the scale used in the optimization studies.
This gram-scale reaction highlights the practicality of this new
Cu-catalyzed conjugate reduction of vinylphosphonates.
a
Reaction conditions: a reaction mixture of 1 (0.4 mmol), Cu(OAc)2ÁH2O (0.04
L, 1.2 mmol), t-BuOH (127 L, 1.2
mmol), PPh3 (10.5 mg, 0.04 mmol), PMHS (266
l
l
mmol), and toluene (2 mL) was stirred at 40 °C for 3 h.
b
Yield of the isolated product.
The reaction was conducted in the presence of Cu(OAc)2ÁH2O (10 mol%) and BDP
c
(5 mol%) at 60 °C for 12 h.
d
Reaction run at 80 °C for 14 h in the presence of Cu(OAc)2ÁH2O (10 mol%) and
BDP (8 mol%).
ethylphosphonates 2n and diisopropyl ethylphosphonate 2o could
be accessed efficiently under the standard reaction conditions. As a
limitation of this method, this Cu-catalyzed reduction is sensitive
to steric hindrance. In the case of the reduction of
a-2-tolyl
ethenylphosphonate, only moderate yield of 2p could be
achieved. Of note, with BDP as the ligand, this protocol could be
extended
to
the
reduction
of
a-alkyl
substituted
vinylphosphonates, albeit under somewhat harsh reaction
conditions, producing 2q and 2r in 90% and 93% yield, respectively.
The halo group is a kind of fundamental and important func-
tional groups, which offers the opportunity for further elaboration
Table 3
Scheme 1. Chemoselective reduction of C@C double bonds.
Cu-catalyzed conjugate reduction of halo-containing vinylphosphonates.a,b
Scheme 2. Reduction of 2-(phenyl)vinylphosphonate.
a
Reaction conditions: a reaction mixture of 3 (0.4 mmol), Cu(OAc)2ÁH2O (0.04
mmol), PPh3 (10.5 mg, 0.04 mmol), PMHS (266 lL, 1.2 mmol), t-BuOH (127 lL, 1.2
mmol), and toluene (2 mL) was stirred at 40 °C for 3 h.
b
Yield of the isolated product.
BDP (3 mol%) was used.
c
Scheme 3. Gram-scale synthesis.
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Research reported in this publication was supported by the
National Natural Science Foundation of China (No. 21202090),
the Zhejiang Provincial Natural Science Foundation of China (Nos.
LY12B02001, LQ13B010004, and LY17B020004), the Zhejiang
Provincial Project of Applied Public Welfare Technology (No.
2016C33254), the Ningbo Natural Science Foundation (Nos.
2016A610234, 2016A610237, and 2016A610097), the National
Training Programs for Innovation and Entrepreneurship for Under-
graduates (No. 201611058006), and Ningbo University of Technol-
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