Metal-free diimide reduction of alkenylphosphonates: simple and efficient protocol for the synthesis of α-substituted ethylphosphonates

Article abstract of DOI:10.1016/j.tetlet.2016.02.057

A simple and straightforward method for the synthesis of α-substituted ethylphosphonates via diimide reduction strategy is described. With K₃PO₄ or Na₂CO₃ as the basic additive, a range of terminal alkenylphosph

Full text of DOI:10.1016/j.tetlet.2016.02.057

Tetrahedron Letters 57 (2016) 1368–1371
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Metal-free diimide reduction of alkenylphosphonates: simple and
efficient protocol for the synthesis of
ethylphosphonates
a
-substituted
b
c,
b
b,
a
a
Yewen Fang a, , Meijuan Yuan , Xiaoping Jin , Li Zhang , Ruifeng Li , Shaoshuai Yang , Mei Fang
⇑
⇑
⇑
a School of Chemical Engineering, Ningbo University of Technology, No. 89 Cuibai Rd, Ningbo 315016, China
b College of Chemistry and Chemical Engineering, Taiyuan University of Technology, No. 79 West Yingze Street, Taiyuan 030024, China
c Departmentc of Biology and Pharmaceutical Sciences, Zhejiang Pharmaceutical College, No. 888 Yinxian Avenue East, Ningbo 315100, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A simple and straightforward method for the synthesis of
a
-substituted ethylphosphonates via diimide
Received 30 December 2015
Revised 2 February 2016
Accepted 15 February 2016
Available online 15 February 2016
reduction strategy is described. With K3PO4 or Na2CO3 as the basic additive, a range of terminal
alkenylphosphonates underwent efficient reduction by diimide generated from 2-nitrobenzenesulfonyl-
hydrazide (NBSH) under room temperature. The high functional group tolerance of this methodology is
also demonstrated. Moreover, our method features high safety, mild reaction condition, low requirement
on equipment, and high chemoselectivity.
Keywords:
Diimide reduction
Ó 2016 Elsevier Ltd. All rights reserved.
2-Nitrobenzenesulfonylhydrazide
a-Substituted ethylphosphonate
Alkenylphosphonate
Metal-free
a
-Substituted ethylphosphonates continue to receive great
pressure reduction1,11 and catalytic hydrogen transfer (CHT) reac-
tion,12 there is no precedent for the diimide reduction13 of the ter-
minal alkenylphosphonates (Scheme 1).
attention because of their significant biological and pharmaceuti-
cal properties.1 They could be considered as phosphorus-contain-
ing analogues of
non-steroidal anti-inflammatory drugs such as naproxen and
ibuprofen.2 Therefore,
-substituted ethylphosphonates may find
applications in the search for new potent antipsychotic drugs.
a
-substituted propionic acids, well-known
Notably, diimide reduction offers the advantages that the han-
dling of gaseous hydrogen is unnecessary and the transition-
metal-free reaction condition is especially positive for the purifica-
tion processes.14 However, a large of excess amount of hydrazine,
long reaction time, high temperature, and an oxygen atmosphere
are required in noncatalytic hydrogenation of olefin with diimide
generated in situ from hydrazine.15 Recently, some catalytic meth-
ods and practical approaches in continuous flow or using a photo-
driven strategy were reported and the above issues could be
alleviated greatly.16 In addition to hydrazine, 2-nitrobenzenesul-
fonylhydrazide (NBSH),17 a free-flow solid, could be employed in
the diimide reduction of olefines in the absence of external catalyst
and oxidant at ambient temperature.18 Of note, the approach
toward selective C@C bond reduction by diimide generated by
NBSH found many applications in the synthesis of complex natural
products because of the mild reaction conditions and the opera-
tional simplicity.19 As a result, we hypothesized that the use of
a
Moreover,
a-arylalkylphosphonates have been demonstrated to
exhibit negative inotropic and calcium antagonistic activity.3 They
were also widely employed as haptens for reactive immunization.4
Owing to the great importance of a-substituted ethylphospho-
nates, several methodologies have been developed for their prepa-
ration, including the Arbuzov reaction of triethyl phosphate with
a
-arylether bromides,5 reaction of phosphorus-containing car-
benoids with organoboranes,6 photo-Arbuzov rearrangements of
benzyl phosphates,7 ZnBr2-mediated reactions of the alcohol with
triethyl phosphate,8 Cu-catalyzed reductive coupling of N-tosylhy-
drazones and H-phosphonate,9 and other processes.10 In addition
to the above-mentioned methods, hydrogenation of a-substituted
ethenylphosphonates represents the attractive and straightfor-
ward approach. Compared to various reports on the hydrogen
readily available NBSH would allow access to
a-substituted
ethylphosphonates. Herein, as a part of our program aiming at
developing practical methods for the synthesis of alkenylphospho-
nates and alkylphosphonates,12a,20 we describe a straightforward
⇑
Corresponding authors.
E-mail addresses: fang@nbut.edu.cn (Y. Fang), jinxp@mail.zjpc.net.cn (X. Jin),
rfli@tyut.edu.cn (R. Li).
http://dx.doi.org/10.1016/j.tetlet.2016.02.057
0040-4039/Ó 2016 Elsevier Ltd. All rights reserved.

Y. Fang et al. / Tetrahedron Letters 57 (2016) 1368–1371
1369
H2
particular note, halogens such as chloro, bromo, and fluoro are well
tolerated on the aromatic ring (2b–e), which highlights the good
functional group tolerance. This offers the opportunity for further
elaboration of these versatile functional groups by transition-
Pd, Ru, Ir, etc.
PO(OEt)2
Me
PO(OEt)2
R
HCO2NH4
Pd/C
HN=NH
metal-free
this work
metal-catalyzed cross-coupling reactions. Additionally,
a-aryl
R
ethylphosphonates containing electron-neutral (2f) and electron-
donating groups (Me, OMe) (2g–l) at the phenyl ring were nicely
accessible in 80–99% yields. It is worth noting that the choice of
the Na2CO3 as the additive is critical for achieving high yields with
alkenylphosphonates possessing strong electron-withdrawing
group as the substrates. The reactions of vinylphosphonates having
electron-withdrawing groups (CO2Me, CN) at the phenyl ring pro-
ceeded efficiently to afford the products 2m and 2n in 95% and
91% yields, respectively. The highly chemoselective reduction of
nitro-containing alkenylphosphonate could be realized, giving the
product 2o in a 92% isolated yield. Of note, alkenylphosphonates
possessing biphenyl and naphthyl groups also worked uneventfully
to afford 2p–r in excellent yields. Furthermore, the hydrogenation
Scheme 1. Synthesis of a-aryl ethylphosphonates via reduction strategy.
and efficient approach to the
based on diimide reduction under mild conditions.
a-substituted ethylphosphonates
With 1-(4-chlorophenyl)ethenylphosphonate 1a as the model
substrate,12a,20 we set out to optimize the reaction parameters
and the results are listed in Table 1. Interestingly, mixing
2.0 equiv of NBSH and 1a in acetonitrile at room temperature led
to the desired product 2a in 40% yield after 5 h (entry 1).
Encouraged by this preliminary result, we next turned our
attention to the inorganic additives.18b With 1.0 equiv of NaHCO3
as the additive, no improvement was observed (entry 2).
Interestingly, high yields could be achieved employing K2HPO4 or
KH2PO4 as the additive (entries 3 and 4). Furthermore, it was
found that the addition of 1.0 equiv of KHCO3, K2CO3, Na2CO3, or
K3PO4 to the reaction mixture improved the conversion
dramatically leading to the product 2a in nearly quantitative
yields (entries 5–8). Additionally, organic base DBU (1,8-
diazabicyclo[5.4.0]undec-7-ene) was also an effective additive,
affording the 2a in 91% yield (entry 9). Attempt to reduce the
amount of NBSH (1.5 equiv) significantly decreased the efficiency
of the reaction (entry 10). It is noteworthy that the chloride was
untouched under the present NBSH-based reductive conditions.
However, using a HCOONH4/Pd/C system, dehalogenation of the
aromatic ring was observed.11c Based on these results, the
combination of 2.0 equiv NBSH and 1.0 equiv K3PO4 or Na2CO3 in
CH3CN for 5 h emerged as the optimal conditions.
of alkene also took place when
a-thienyl vinylphosphonate was
used as substrate to give the product 2s in a 94% isolated yield. As
a limitation of the method, this NBSH-based reduction is sensitive
to steric hindrance. The hindered
a-2-tolyl, 2-chlorophenyl, and
1-naphthyl were reluctant to be reduced and low conversions were
observed under the standard conditions. Gratifyingly, moderate
yields of 2t–v could be achieved when the NBSH was added in por-
tions. Likewise, the dimethyl ethylphosphonate 2w was expectedly
obtained in 90% yield. Unfortunately, despite attempting to
increase the reaction temperature and the NBSH amount, the
hydrogenation of trisubstituted vinylphosphonate failed to give
the desired product 2x.
This method was further extended to the hydrogenation of
diethyl 1-(quinolin-6-yl)ethenylphosphonate 3. Under palladium-
assisted transfer hydrogenation conditions using ammonium for-
mate, chemoselective hydrogenation of 3 failed and a mixture
comprising of phosphonate 4 and 1-(1,2,3,4-tetrahydroquinolin-
6-yl)ethylphosphonate
5
was observed.12b To our delight,
To explore the scope of the reaction with respect to the a-substi-
tuted ethenylphosphonates, we carried out further reaction in CH3-
CN with 2.0 equiv NBSH and 1.0 equiv K3PO4 or Na2CO3 for 5 h and
the results are summarized in Table 2.
alkenylphosphonates having electron-neutral, electron-donating,
or electron-withdrawing groups at the phenyl ring were found to
undergo hydrogenation reaction in good to excellent yields. Mean-
chemoselective reduction of the exocyclic double C@C was
achieved to give 4 in 97% yield under our standard conditions
(Scheme 2).
A range of a-aryl
In addition to the chemoselective reduction of
ethenylphosphonates, this method is also effective for the prepara-
tion of -alkyl ethylphosphonates, producing the reductive prod-
ucts 7a–c in high yields (Table 3). To investigate our method
further, the array of vinylphosphonates was expanded to -halo
vinylphosphonates. Pleasingly, -bromo and chloro ethenylphos-
a-(hetero)aryl
a
while, our protocol not only worked with
a-aryl alkenylphospho-
nates but also with
a-heteroaryl vinylphosphonate as well. Of
a
a
phonates underwent smooth reduction to deliver 7d and 7e in
76% and 50% yields, respectively.
Table 1
Optimization of the reaction conditionsa
To ensure the versatility of the reduction, a-allyl ethenylphos-
phonate 8 bearing two exo C@C bonds was subjected to the reduc-
tion and it was found that both the terminal double bonds were
reduced in the presence of NBSH (4.0 equiv) to give the product
9 in 74% yield (Scheme 3). A similar result was obtained with
K3PO4 (70%) as the additive.
PO(OEt)2
PO(OEt)2
NBSH (2.0 equiv)
Me
Base (1.0 equiv)
CH3CN (2.0 mL), rt, 5 h
Cl
Cl
2a
1a
Scheme 4 shows the proposed mechanism. NBSH generates dii-
mide by in situ elimination of o-nitrobenzenesulfinic acid.21 The
presence of base could possibly facilitate the generation of
diimide.18b The cis isomer of diimides reduced vinylphosphonates
Entry
Basic additive
Yield (%)
1
2
3
4
5
6
7
8
—
40
42
82
89
94
97
96
94
91
61
NaHCO3
K2HPO4
KH2PO4
KHCO3
K2CO3
Na2CO3
K3PO4
DBU
to produce the a
-substituted ethylphosphonates.16b
The resulting halo-containing ethylphosphonates are syntheti-
cally useful substrates for further functionalization by Suzuki cou-
pling.22 Thus, 2d was treated with phenylboronic acid in the
presence of Pd(OAc)2/SPhos catalyst, which gave the product 2p
in 94% yield. Furthermore, ethenyl group could be efficiently intro-
duced via the Pd-catalyzed Suzuki vinylation of 2d with potassium
vinyltrifluoroborate (Scheme 5).23 It could be envisioned that the
compatibility of vinyl group would be a problem during the
9
10b
K3PO4
a
Reaction conditions: alkenylphosphonate 1a (0.3 mmol), NBSH (133 mg,
0.6 mmol), base (0.3 mmol), CH3CN (2.0 mL), room temperature, 5 h, isolated yields.
b
NBSH (0.45 mmol) was used.

1370
Y. Fang et al. / Tetrahedron Letters 57 (2016) 1368–1371
Table 2
Scope of
Table 3
Diimide reduction of
a
-aryl or
a
-heteroaryl alkenylphosphonatesa
a
-alkyl or halo ethenylphosphonatesa
Me
NBSH (2.0 equiv)
Me
NBSH (2.0 equiv)
R2
PO(OR1)2
R2
K3PO4 or Na2CO3 (1.0 equiv)
CH3CN, rt, 5 h
PO(OR1)2
R
PO(OEt)2
R
Base (1.0 equiv)
CH3CN, rt, 5 h
PO(OEt)2
1
6
2
7
R1 = Me, Et
2 = Aryl, Heteroaryl
R
Entry
1
Base
Product
Yield (%)
78
Me
PO(OEt)2
Me
PO(OEt)2
Me
PO(OEt)2
Me
PO(OEt)2
PO(OEt)2
K3PO4
Me
n-Bu
7a
Cl
Br
c
Br
F
2b 90%b
2c
, 93%
b
2e 92%b
,
2d
, 89%
,
PO(OEt)2
Me
PO(OEt)2
2
3
4
Na2CO3
K3PO4
93
72
76
Me
PO(OEt)2
Me
Me
Me
CH2Ph
Me
Me
Me
PO(OEt)2
PO(OEt)2
7b
PO(OEt)2
Me
Me
Me
b
CH2Bn
Me
2h
7c
b
b
2g
b
, 99%
2f
, 98%
2i
, 90%
, 99%
PO(OEt)2
Me
Me
PO(OEt)2
PO(OEt)2
Me
PO(OEt)2
PO(OEt)2
KHCO3
Br
7d
PO(OEt)2
OMe
O
b
5
KHCO3
50
Me
Cl
OMe
O
, 90%
OMe
2k
O
OCH3
c
b
b
7e
2l
2j
, 91%
, 80%
PO(OEt)2
2m
, 95%
Me
Me
a
PO(OEt)2
Me
Me
PO(OEt)2
Reaction conditions: alkenylphosphonate
6 (0.3 mmol), NBSH (133 mg,
PO(OEt)2
0.6 mmol), base (0.3 mmol), CH3CN (2.0 mL), room temperature, 5 h, isolated yields.
Ph
NO2
2o 92%c,d
CN
,
91%b
c
PO(OEt)2
2n 91%c
,
2q
, 97%
2p
,
NBSH (4.0 equiv)
Me
(EtO)2OP
(EtO)2OP
Na2CO3 (2.0 equiv)
CH3CN, rt, 5 h
Me
Me
, 74%
8
Me
Me
PO(OEt)2
Cl
PO(OEt)2
Me
9
Me
PO(OEt)2
Scheme 3. Diimide reduction of two exo double bonds.
S
MeO
2u 81%c,d,e
2t 71%c,d,e
,
2r 99%b
2s 94%b
,
,
,
SO2NHNH2
NO2
Me
PO(OMe)2
PO(OEt)2
Me
PO(OEt)2
Me
O
O-
S
b
2v 72%c,d,e
2w
, 90%
2x
,0%
,
O2N
CH3CN
base
a
Reaction conditions: alkenylphosphonate 1 (0.3 mmol), NBSH (133 mg, 0.6 mmol),
base (0.3 mmol), CH3CN (2.0 mL), room temperature, 5 h, isolated yields.
PO(OEt)2
Het
PO(OEt)2
Het
N
H
N
H
b K3PO4 (0.3 mmol) was used.
Me
+
N2
R
c Na2CO3 (0.3 mmol) was employed in place of K3PO4.
d The reaction temperature was 50 °C.
R
e NBSH (1.2 mmol) was added in four portions at intervals of 3 h and every portion
was 0.3 mmol, the reaction time was 12 h.
Scheme 4. Proposed reaction mechanism.
PO(OEt)2
Me
PO(OEt)2
PO(OEt)2
PO(OEt)2
Me
PhB(OH)2 (1.5 equiv)
Me
PO(OEt)2
Method A or B
Me
+
toluene (0.15 M)
Ph
Pd(OAc)2 (5 mol%)
SPhos (10 mol%)
N
2p
, 94%
N
N
H
3
4
5
Cs2CO3 (2.5 equiv)
Method A: Pd/C, HCOONH4:12b
Method B: NBSH, Na2CO3:
PO(OEt)2
Me
60%
27%
50 oC, 15 h
BF3K
Br
(3.0 equiv)
97%
2d
toluene/H2O
(4 : 1, 0.15 M)
Scheme 2. Diimide reduction versus hydrogen transfer reaction.
10
, 87%
Scheme 5. Further functionalization of bromo-containing ethylphosphonate.
hydrogenation process. This method based on sequential alkene
reduction/Suzuki vinylation reaction provides an alternative for
the diimide source and K3PO4 or Na2CO3 as the additive, a variety
the synthesis of a-aryl ethylphosphonates having unsaturated car-
of
nished chemoselective reduction of exo C@C double bonds, giving
-substituted ethylphosphonates in good to excellent yields.
a-aryl, heteroaryl, alkyl, and halo ethenylphosphonates fur-
bon-carbon bond at the phenyl ring.
In summary, we developed an efficient and simple method for
a
the synthesis of
a-substituted ethylphosphonates via diimide
Moreover, our method features great functional group tolerance,
high safety, mild reaction condition, and low requirement on
equipment.
reduction of -substituted vinylphosphonates. In this NBSH-based
reductive protocol, the proper choice of basic additive is crucial for
the efficiency and functional group compatibilities. With NBSH as
a

Y. Fang et al. / Tetrahedron Letters 57 (2016) 1368–1371
1371
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Acknowledgments
The National Natural Science Foundation of China (No.
21202090), the Zhejiang Provincial Natural Science Foundation of
China (No. LQ13B010004), the Xinmiao Talents Program (No.
2015R424004), and Ningbo University of Technology (NBUT) are
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