One-pot solid phase synthesis of (E)-nitroalkenes

Article abstract of DOI:10.1016/j.tetlet.2013.07.146

An efficient one-pot protocol for the synthesis of (E)-nitroalkenes by reaction of aldehydes and nitroalkanes in the presence of polymer-bound triphenylphosphine, iodine and imidazole is described. Although the reaction works with similar efficiency with triphenylphosphine and its polymer-bound version, easy removal of the unwanted polymer-bound triphenylphosphine oxide and its recovery as triphenylphosphine provide the edge for practical application of the method.

Full text of DOI:10.1016/j.tetlet.2013.07.146

Tetrahedron Letters 54 (2013) 5500–5504
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Tetrahedron Letters
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One-pot solid phase synthesis of (E)-nitroalkenes
⇑
Lalthazuala Rokhum, Ghanashyam Bez
Department of Chemistry, North Eastern Hill University, Shillong 793022, India
a r t i c l e i n f o
a b s t r a c t
Article history:
An efficient one-pot protocol for the synthesis of (E)-nitroalkenes by reaction of aldehydes and nitroalk-
anes in the presence of polymer-bound triphenylphosphine, iodine and imidazole is described. Although
the reaction works with similar efficiency with triphenylphosphine and its polymer-bound version, easy
removal of the unwanted polymer-bound triphenylphosphine oxide and its recovery as triphenylphos-
phine provide the edge for practical application of the method.
Received 14 May 2013
Revised 24 July 2013
Accepted 28 July 2013
Available online 3 August 2013
Ó 2013 Elsevier Ltd. All rights reserved.
Keywords:
(E)-Nitroalkenes
Polymer-bound triphenylphosphine
Iodine
Imidazole
Solid phase synthesis
Nitroalkene is an important building block in organic synthesis.
Many b-nitrostyrene derivatives are valuable intermediates for the
preparation of numerous products including insecticides,1a fungi-
cides,1b,c and pharmacologically active substances.1d–i It can also
be used to synthesize a range of derivatives by 1,4-addition,2a–c
reduction,2d–g Diels–Alder reaction,2h–l etc. Recent studies revealed
that trans-b-nitrostyrene (TBNS) derivatives (Fig. 1) act as slow-
binding, reversible inhibitors of protein tyrosine phosphatases
(PTPs).3 Moreover various nitroalkenes derived from aromatic
aldehydes are found to be useful for natural product synthesis.4
Synthesis of nitroalkene from condensation (Henry condensa-
tion) of aldehyde and nitroalkane is a very important strategy for
carbon–carbon bond forming reaction. Generally, nitroalkenes are
synthesized by base-catalyzed reaction of the corresponding alde-
hydes with nitroalkanes5 in a two step-base catalyzed synthesis of
b-nitro alcohols followed by dehydration in the presence of dehy-
drating agents, such as phthalic anhydride,6 dicyclohexylcarbodi-
imide,7 PPh3-CCl4,8 Ac2O-AcONa,9CH3SO2Cl-NEt3,10 TFAA-NEt3,11
and Al2O3.12 In spite of being effective, the problems associated
with many of these methods include unwanted side products de-
rived from the reagent, highly hygroscopic nature of the reagents,
and requirement of high temperature. In search of one-step proto-
col, Ballini et al.13 reported an interesting method for one step
Henry condensation by heating a mixture of aldehyde and nitroal-
kane at 40–60 °C in the presence of heterogeneous catalysts based
on Al2O3 in super critical CO2 at 80–140 bar. But special apparatus
is required to carry out the reaction with super critical CO2 at high
pressure. Other methods from starting materials other than
conventional Henry adducts have also appeared in the literature.14
Concellón et al.15 reported a samarium-promoted synthesis of
(E)-nitroalkenes from 1-bromo-1-nitroalkan-2-ols in high yields,
but required two steps to achieve the desired nitroalkenes. There
are a few methods16 for the synthesis of nitroalkenes from nitric
oxide (NO) and alkenes, but they have regioselectivity issues unlike
Henry condensation. Recently, Pujol and co-workers17 have
reported a novel one-pot synthesis of nitroalkenes from aryl alde-
hydes using ammonium acetate as a catalyst without solvent un-
der microwave irradiation. This method often gives a mixture of
nitroalkenes and nitroalcohols and hence gives poor yield in many
cases. Moreover, the reaction is useful only for aromatic aldehydes,
not for aliphatic aldehydes. Given the existing literature, there is
every scope to develop new methodologies to simplify the reaction
conditions for the synthesis of nitroalkenes from aldehydes.
Solid-phase synthesis continues to evolve as a means to facili-
tate the manipulation of compound libraries via combinatorial
chemistry.18 The important features of solid-phase synthesis such
as purification of the product by filtration of the solid matrix, easy
handling, low moisture susceptibility, minimum side reaction, and
recyclability of the solid matrix for repeated use have drawn huge
attention from industry and academia.19 Although triphenylphos-
phine is considered one of the worst atom-economic reagents
due to its high carbon content, polymer-bound triphenylphosphine
is getting a lot of applications in recent years.20 The commonly
encountered problems in solution-phase chemistry involving tri-
phenylphosphine, such as removal of excess triphenylphosphine,
triphenylphosphine complexes, and the by-product triphenylphos-
phine oxide can be overcome easily with polymer-bound triphen-
ylphosphine. Moreover, for the reactions where polymer-bound
triphenylphosphine acts as an oxygen-acceptor, the byproduct
⇑
Corresponding author.
E-mail addresses: ghanashyambez@yahoo.com, bez@nehu.ac.in (G. Bez).
0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2013.07.146

L. Rokhum, G. Bez / Tetrahedron Letters 54 (2013) 5500–5504
NO2
5501
triphenylphosphine oxide can be reduced to triphenylphosphine
by treatment with trichlorosilane.21 Here we wish to report a
two-step one-pot methodology involving polymer-bound triphen-
ylphosphine/iodine/imidazole reagent system for the synthesis of
nitroalkenes in very good yields (Scheme 1). To our knowledge, so-
lid phase synthesis of nitroalkene from the reaction of aldehyde
and nitroalkane is not known in the literature.
NO2
NO2
MeO
OMe
R
R = H, CH3, OCH3, COOH
Figure 1. Structures of PTP inhibitors.
In continuation of our recent interest in the catalytic application
of triphenylphosphine (TPP) and iodine22 in useful organic trans-
formations, we assumed that if a mild base is used to abstract
the b-hydrogen of b-nitroalcohol, nitroalkenes can be synthesized
without use of commonly used dehydrating agents. Here, the hy-
droxy group was expected to react with the in situ generated iod-
otriphenylphosphonium iodide, from the reaction of TPP with
iodine, to make itself a good leaving group. We initially wanted
to test our assumption in solution phase to make sure that the re-
agent system works. For that purpose, we took a mixture of benz-
aldehyde (1 mmol) and nitromethane (1.5 equiv) in dry
dichloromethane (5 mL) and stirred with 0.2 equiv of imidazole
(ImH). After stirring for 4 h, the formation of b-nitroalcohol was
found to be complete. Without isolating the product, triphenyl-
phosphine (1.5 equiv) and iodine (1.5 equiv) were added to the
solution and stirred for additional 30 min. Slight formation of the
desired nitroalkene was observed, which did not increase even
after stirring for 3 h. Then we added additional imidazole (1 mmol)
and stirred for 30 min to find that the starting nitroalcohol got
completely converted into its nitroalkene derivative. We reasoned
that if the imidazole is used in excess, both C–C coupling and dehy-
dration can occur simultaneously to generate the desired nitroalk-
ene. With that idea in mind, we added 2.0 equiv of imidazole to a
solution of benzaldehyde (1 mmol) and nitromethane (1.5 equiv)
in anhydrous dichloromethane (5 mL) and stirred for 10 min. To
the resulting solution, triphenylphosphine (0.5 g, 1.5 mmol) and
iodine (1.5 equiv) were added under argon atmosphere at 0 °C.
The reaction took only 30 min to generate nitroalkene in 82% iso-
lated yield. We extended the method23 for the synthesis of a few
R2
NO2
R1
O
PPh2/I2/ImH
+
R2
NO2
R1
H
CH2Cl2, RT
Scheme 1. Synthesis of nitroalkene.
Table 1
Synthesis of nitroalkenes in solution phasea,b
R2
R1
1a-h
O
PPh3/I2/ImH
CH2Cl2, RT
+ R2
NO2
R1
H
NO
)
(
2
Entry
Aldehyde
PhCHO
R2
Product
Yieldc (%)
1
2
H
1a
1b
82
85
CH3
3
4
CHO
H
1c
1d
78
82
CH3
MeO
O2N
5
6
H
1e
1f
85
88
CHO
CHO
CH3
7
8
H
1g
1h
89
84
Br
CH3
a
b
c
Aldehyde:nitroalkane:triphenylphosphine:iodine:imidazole = 1.0:1.5:1.5:1.5:2.0.
Reaction time is 30 min.
Yields are calculated from pure isolated product.
Table 2
Solid phase synthesis of nitroalkenesa
R1
1a-t
O
PPh2/I2/ImH
CH2Cl2, RT
R2
+
R2
NO2
R1
H
NO2
)
(
Entry
1
Aldehyde
PhCHO
R2
H
t/h
Product
Yieldb (%)
90
NO2
NO2
Ph
Ph
1
1a
2
3
CH3
H
1
1
89
84
1b
CHO
CHO
NO2
NO2
1c
MeO
MeO
O2N
4
5
6
7
CH3
H
1
1
1
1
84
93
91
1d
1e
1f
MeO
O2N
NO2
NO2
O2N
Br
CH3
CH3
Br
CHO
NO2
89
1g
(continued on next page)

5502
L. Rokhum, G. Bez / Tetrahedron Letters 54 (2013) 5500–5504
Table 2 (continued)
Entry
Aldehyde
R2
H
t/h
Product
Yieldb (%)
55
MeO
CHO
MeO
NO2
NO2
8
3
1h
MeO
MeO
MeO
9
CH3
CH3
H
3
3
1
54
58
86
1i
MeO
O
CHO
NO2
O
O
10
11
1j
O
CHO
CHO
NO2
NO2
1k
Me
Me
12
13
14
H
H
H
1
1
1
93
93
90
1l
O2N
O2N
CHO
CHO
NO2
NO2
1m
Cl
Cl
1n
Br
Br
CHO
NO2
Ph
Ph
15
16
H
1
1
84
89
1o
1p
CHO
NO2
Ph
Ph
CH3
CHO
NO2
3
3
17
H
H
3
3
75
81
1q
CHO
NO2
5
5
18
19
1r
CHO
CHO
NO2
Ph
Ph
Ph
Ph
H
3
3
87
89
1s
1t
NO2
20
CH3
a
Aldehyde:nitroalkane:triphenylphosphine:iodine:imidazole = 1.0:1.5:1.5:1.5:2.0.
Isolated yield.
b
more nitroalkenes starting from aromatic aldehydes (Table 1, en-
tries 2–8) with nitromethane/nitroethane. In all the cases, we
found good yield of the corresponding nitroalkenes without forma-
tion of any side product.
comparing the chemical shift values of the vinylic protons with lit-
erature data. It may be noted that, the vinylic proton of (E)-isomer
appears downfield than the corresponding proton of the (Z)-isomer
because of the strong anisotropic effect of the nitro group.14c, 25 We
observed similar efficiency in the synthesis of nitroalkene from
Encouraged by the result in solution phase synthesis and the
advantage of using polymer-bound triphenylphosphine (PBTPP)
over simple triphenylphosphine (vide infra), we carried out the
reaction of benzaldehyde with nitromethane under similar reac-
tion conditions except for changing triphenylphosphine with
PBTPP (1.5 equiv). For this purpose, polymer-bound triphenylphos-
phine, a diphenylphosphinated copolymer of styrene and 2% divi-
nylbenzene (DVB) with loading of 3.2 mmol/g was used. Here it
took comparatively longer time (1 h) to drive the reaction into
completion and gave very good yield (85%). The reaction was
generalized24 for diverse aromatic aldehydes (entries 2–19) and
found to work well under similar conditions to give very good
yield. Only a few less electrophilic aldehydes, such as m, p-dime-
thoxybenzaldehyde (entries 8 and 9) and m, p-methylenedioxy-
benzaldehyde (entry 10) took longer reaction time (3 h) for
complete conversion. All the products revealed exclusive
formation of only the (E)-nitroalkenes. The (E)-configuration for
1,2-disubstututed nitroalkenes was confirmed from the coupling
constant values of trans-coupled vicinal protons, while (E)-config-
uration of the trisubstituted nitroalkenes was determined by
a
,b-unsaturated aldehyde (entries 15 and 16) and aliphatic
aldehydes (Table 2, entries 17–20) as well under similar reaction
conditions.
The probable mechanism for the exclusive formation of (E)-al-
kenes might be due to the formation of cyclic intermediate ‘B’,
where the nitro group is syn to the leaving group, that is, the phos-
phonium salt activated hydroxy group (Fig. 2). The absence of (Z)-
nitroalkene, if formed, may additionally be explained by Stanetty
and Kremslehner’s report26 that the (Z)-nitroalkenes undergo
isomerization to give (E)-nitroalkenes in the presence of poly-
mer-bound triphenylphosphine.
In summary, we have developed the first solid phase method for
the synthesis of (E)-nitroalkenes from both aliphatic and aromatic
aldehydes. Unlike most of the methods used for the synthesis of
nitroalkene, the isolation of b-nitroalcohol is not required before
converting it into nitroalkene in the successive step. Use of
resin-bound triphenylphosphine simplified the purification pro-
cess, because the triphenylphosphine oxide byproduct could be re-
moved by simple filtration as against tedious purification process

L. Rokhum, G. Bez / Tetrahedron Letters 54 (2013) 5500–5504
5503
I
PPh2
I
I
+
I
I
PPh2
I
PPh2
+
PPh2
O
N
O
O
H
OH
H
O
PIPh2
O
B
NO2
R1
B
NO2
R2
O
+
R1
R1
NO2
NO2
R1
R1
R2
H
R2
HI
BH + I
H
R2
R2
A
B
Figure 2. Plausible mechanism for the formation of exclusively (E)-nitroalkene.
10. (a) Melton, J.; McMurry, J. E. J. Org. Chem. 1975, 40, 2138; (b) Palomo, C.;
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Minnaard, A. J.; Feringa, B. L. Tetrahedron 2002, 58, 5773.
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conditions, no inert atmosphere, and two-step one-pot strategy are
some major advantages over conventional dehydrating reagents
used in solution chemistry.
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Thanks are due to the Department of Science and Technology
(DST), New Delhi [Grant No. SR/S1/OC-25/2007] for providing
financial supports to carry out this work. The analytical services
provided by the Sophisticated Analytical Instrumentation Facility
(SAIF), North Eastern Hill University, Shillong and Central Instru-
mentation Facility (CIF), Indian Institute of Technology, Guwahati,
India are acknowledged. The authors gratefully acknowledge the
suggestions made by the reviewers to improve the content.
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Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2013.
07.146.
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23. Typical procedure for solution phase synthesis of nitroalkene: To a solution of
benzaldehyde (0.106 g, 1.0 mmol) and nitromethane (0.091 g, 1.5 mmol) in
anhydrous dichloromethane (5.0 mL) was added imidazole (0.136 g, 2.0 mmol)
and stirred for 10 min. The resulting mixture was cooled to 0 °C and
triphenylphosphine (0.5 g, 1.5 mmol) and iodine (0.380 g, 1.5 mmol) were
added under argon atmosphere. After stirring for additional 30 min, the
reaction was complete. The solution was diluted with dichloromethane
References and notes
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(20.0 mL) and washed with
a saturated solution of sodium thiosulphate
(20.0 mL) to remove the unreacted iodine. The organic layer was separated,
washed with brine (20 mL), dried over anhydrous sodium sulfate, and
concentrated under vacuo. The resultant crude product was purified by
column chromatography using 10% ethyl acetate in hexane to get the pure
product in 82% (0.122 g, 0.82 mmol) yield.
24. Typical procedure for solid phase synthesis of nitroalkene: To
a solution of
benzaldehyde (0.106 g, 1 mmol) and nitromethane (0.091 g, 1.5 mmol) in
anhydrous dichloromethane (10.0 mL) was added imidazole (0.136 g,
2.0 mmol) and stirred for 10 min. To the resulting mixture, polymer-bound
triphenylphosphine (0.5 g, 1.5 mmol) and iodine (0.380 g, 1.5 mmol) were
added and allowed to stir for additional 50 min. After completion of the
reaction, the solution was filtered through a sintered funnel and the resin was
washed successively with dichloromethane (30.0 mL) and saturated aqueous
sodium thiosulfate solution (10.0 mL) thrice. The combined dichloromethane
and sodium thiosulfate solution was vigorously shaken in a separating funnel
to remove any unreacted iodine. The organic layer was washed with brine
(20.0 mL) and separated, dried over anhydrous sodium sulfate, and
concentrated under vacuo. The resultant crude product was purified by
column chromatography using 10% ethyl acetate in hexane to get the pure
product in 85% (0.127 g, 0.85 mmol) yield.
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Kostelitz, O.; Le Gall, T.; Mioskowski, C. Eur. J. Org. Chem. 1999, 2583.
8. Saikia, A. K.; Barua, N. C.; Sharma, R. P.; Ghosh, A. C. Synthesis 1994, 685.
9. Hass, H. B.; Susie, A. G.; Heider, R. L. J. Org. Chem. 1950, 15, 8.
Spectroscopic data of the new compounds: 1-Bromo-3-((E)-2-nitroprop-1-
enyl)benzene, 1g: Pale yellow liquid; IR (KBr):
m
3025, 2952, 1643, 1600,
1527, 1434, 1381, 1222, 1122, 1043, 930, 758, 678, 545, 446 cmÀ1
.
1H NMR

5504
L. Rokhum, G. Bez / Tetrahedron Letters 54 (2013) 5500–5504
(400 MHz, CDCl3): d 7.92 (s, 1H), 7.64–7.23 (m, 4H), 2.36 (s, 3H). 13C NMR
(100 Hz, CDCl3): d 134.44, 132.79, 132.50, 131.86, 130.40, 128.37, 122.95,
14.02. ESI-MS: m/z 242.2 (M+). Anal. Calcd for C9H8BrNO2: C, 44.66; H, 3.33; Br,
33.01; N 5.79. Found: C, 44.62; H, 3.35; Br, 33.03; N, 5.77.
(d, J = 8.4 Hz, 2H), 7.36 (d, J = 8.8 Hz, 2H). 13C NMR (100 MHz, CDCl3): d 138.36,
137.71, 137.41, 130.27, 129.78, 128.52. MS-EI:m/z (%) 183.03 (M+). Anal. Calcd
for C8H6ClNO2: C, 52.34; H, 3.29; Cl, 19.31; N, 7.63. Found: C, 52.36; H, 3.31; Cl,
19.29; N, 7.65.
5-((E)-2-Nitroprop-1-enyl)benzo[d][1,3]dioxole, 1j: Pale yellow solid. Mp 101–
((1E,3E)-4-Nitropenta-1, 3-dien-1-yl)benzene, 1p: Yellow solid. Mp 65–66; IR
102; IR (KBr):
m
3605, 3025, 2402, 1643, 1525, 1434, 1374, 1210, 1116, 1043,
(KBr): m .
3015, 2974, 2331, 1645, 1526, 1401, 1121, 1075, 865, 781, 656 cmÀ1
930, 758, 545, 543 cmÀ1
.
1H NMR (400 MHz, CDCl3): d 8.01 (s, 1H), 6.99–6.88
1H NMR (400 MHz, CDCl3): d 7.75 (d, J = 12.4 Hz, 1H), 7.52–7.36 (m, 5H), 7.04
(d, J = 12.8, 1H), 6.90 (dd, J = 12.0, 2, 1H), 2.35 (s, 3H). 13C NMR (100 MHz,
CDCl3): d 146.29, 143.97, 135.65, 133.76, 129.88, 128.98, 127.54, 121.26, 13.06.
MS-EI: m/z (%) 198.03 (M+). Anal. Calcd for C11H11NO2: C, 69.83; H, 5.86; N,
7.40. Found: C, 69.62; H, 5.75; N, 7.68.
(m, 3H), 6.04 (s, 2H), 2.45 (s, 3H). 13C NMR (100 Hz, CDCl3): d 149.29, 148.25,
146.15, 133.67, 125.95, 125.51, 109.53, 108.86, 101.77, 14.17. ESI-MS: m/z
207.2 (M+). Anal. Calcd for C10H9N04: C, 57.97; H, 4.38; N, 6.76. Found: C, 57.94;
H, 4.37; N, 6.79.
1-Chloro-4(2-nitrovinyl)benzene, 1m: Yellowish solid. Mp 113–114 °C; IR (KBr):
m
25. Hayama, T.; Tomoda, S.; Takeuchi, Y.; Nomura, Y. Chem. Lett. 1982, 1109.
26. Stanetty, P.; Kremslehner, M. Tetrahedron Lett. 1998, 39, 811.
3041, 2932, 2849, 1633, 1573, 1452, 1192, 1093, 824, 769, 751, 694 cmÀ1 1H
.
NMR (400 MHz, CDCl3): d 7.89 (d, J = 13.6 Hz, 1H), 7.49 (d, J = 13.6 Hz, 1H), 7.42