Cu(NO3)2-catalyzed nitrodecarboxylation of α,β-unsaturated acids: facile synthesis of (E)-nitroolefins under additive-free conditions

Article abstract of DOI:10.1007/s11164-016-2446-6

Abstract: An additive-free, facile, efficient, and ecofriendly protocol for synthesis of (E)-nitroolefins via nitrodecarboxylation of α,β-unsaturated acids has been developed. Cu(NO₃)₂ was used as both nitrating agent and catalyst. Furthermore, the presented methodology offers several advantages such as easily accessible and stable substrates, inexpensive catalyst, high to excellent yield, short reaction time, and simple posttreatment procedure. Graphical Abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s11164-016-2446-6

Res Chem Intermed
DOI 10.1007/s11164-016-2446-6
Cu(NO ) -catalyzed nitrodecarboxylation of a,b-
3
2
unsaturated acids: facile synthesis of (E)-nitroolefins
under additive-free conditions
1
1
1
•
Zai-Gang Luo
2
•
Feng Xu
1
•
Yu-Yu Fang
Cheng-Tao Feng
1
•
Peng Liu
Zhong Li
•
Xue-Mei Xu
1
Jie He
•
1
•
Received: 1 November 2015 / Accepted: 15 January 2016
Ó Springer Science+Business Media Dordrecht 2016
Abstract An additive-free, facile, efficient, and ecofriendly protocol for synthesis of
E)-nitroolefins via nitrodecarboxylation of a,b-unsaturated acids has been developed.
Cu(NO ) was used as both nitrating agent and catalyst. Furthermore, the presented
(
3
2
methodology offers several advantages such as easily accessible and stable substrates,
inexpensive catalyst, high to excellent yield, short reaction time, and simple posttreat-
ment procedure.
Graphical Abstract
CH CN
3
COOH
Cu(NO3)2
NO
2
Ar
Ar
1
10°C, 8h
Additive free
1
3 examples
Radical pathway
up to 94% yields
High regioselectivity
Keywords Nitrodecarboxylation Á a,b-Unsaturated acids Á Cu(NO ) Á
3
2
(
E)-Nitroolefins
Electronic supplementary material The online version of this article (doi:10.1007/s11164-016-2446-6)
contains supplementary material, which is available to authorized users.
&
&
1
Zai-Gang Luo
luozi139@163.com
Jie He
aust_jhe@163.com
College of Chemical Engineering, Anhui University of Science & Technology,
Huainan 232001, China
2
Guang Zhou Institutes of Biomedicine and Health, Chinese Academy of Sciences,
Guangzhou 510530, China
123

Z.-G. Luo et al.
Introduction
Nitroolefins play important roles in organic chemistry, serving not only as a key unit
in many pharmaceutical and bioactive compounds [1, 2], but also as versatile
intermediates in synthetic organic chemistry for construction of carbon–carbon
bonds, e.g., in the Michael reaction [3], Diels–Alder cycloaddition [4], and Morita–
Baylis–Hillman reaction [5]. In addition, the nitro group can be easily transformed
into other useful functional groups [6]. Hence, development of efficient methods for
their preparation has been an important research direction in organic chemistry. The
traditional method, namely the Henry reaction [7], relies upon base-mediated
condensation of carbonyl compounds with nitroalkanes followed by dehydration.
However, harsh reaction conditions and formation of undesired E/Z mixtures
demand an alternative protocol. Recently, numerous useful methods have been
developed for synthesis of nitroolefins, such as direct nitration of olefinic C–H
bonds [8–11], nitrodecarboxylation of aromatic a,b-unsaturated carboxylic acids
[
12], ipso-nitration of carboxylic acids [13], and so on.
Carboxylic acids are very useful compounds, being abundant in natural products;
they are commercially available with large structural variation. They are easy to
store, simple to handle, and convenient to prepare by means of a large number of
well-established methods. The ready availability of carboxylic acids makes them
extremely promising raw materials for chemical synthesis. Recently, the decar-
boxylative cross-coupling reaction has attracted much attention, since it opens a
new avenue for formation of carbon–carbon and carbon–heteroatom bonds [14].
Also, relevant studies have well explored research on decarboxylative nitration of
a,b-unsaturated carboxylic acids. In 2002, Roy et al. [15] reported a facile
nitrodecarboxylation of aromatic a,b-unsaturated carboxylic acids under the effect
of nitric acid and catalytic azodiisobutyronitrile (AIBN). In 2004, Fiorentino and
coworkers reported nitrodecarboxylation of cinnamic acids using cerium(IV)
ammonium nitrate supported on silica as nitrating reagent [16]. However, nitration
of the aromatic ring was also observed in all cases, dramatically limiting the
application of this reaction. In 2011, Saiprakash et al. [17] reported a polyethylene-
glycol-supported reaction for synthesis of nitroolefins from a,b-unsaturated
carboxylic acids and various metal nitrates [excluding Cu(NO ) ] under mineral-
3
2
acid-free and solvent-free microwave-irradiated and grinding conditions. Addition
of polyethylene glycol enormously accelerated the reaction rate, and the yield was
significantly increased. However, the ratio of a,b-unsaturated carboxylic acids and
metal nitrates was 1:12, and the metal nitrates could not be recycled and were
heavily wasted. In 2013, Maiti [18] and Prabhu [19] and their coworkers developed
a decarboxylative nitration protocol for synthesis of nitroolefins from a,b-
unsaturated carboxylic acids using tert-butyl nitrite as nitrating reagent, 2,2,6,6-
tetramethyl-1-piperidinyloxy (TEMPO), and CuCl as catalyst. Very recently, the
first cellulose-supported copper-nanoparticle-catalyzed decarboxylative nitration of
cinnamic and benzoic acids for synthesis of (E)-nitroolefins and substituted
nitrobenzenes was reported by Baruah et al. [20]. Inspired by these results, we
present herein a facile and effective method, without any additives, to prepare
1
23

3 2
Cu(NO ) -catalyzed nitrodecarboxylation of a,b-…
nitroolefins via Cu(NO ) -catalyzed nitrodecarboxylation of a,b-unsaturated car-
3
2
boxylic acids.
Results and discussion
Initially, cinnamic acid (1a) was used as model substrate to optimize the reaction
conditions, including the metal nitrate, solvent, and temperature under ambient air.
As shown in Table 1, five metal nitrates, viz. Fe(NO ) , Ni(NO ) , AgNO , NaNO ,
3
3
3 3
3
3
and Cu(NO ) , were investigated at 110 °C in 2 mL CH CN; Cu(NO ) was found
3
2
3
3 2
to be the most effective catalyst and nitrating reagent (86 %) (entries 1–5).
Furthermore, different amounts of Cu(NO3)2 were attempted, and 1.2 equiv
Cu(NO3)2 was found to be more suitable for the present reaction (compare
entries 5–8). Additionally, solvents including CH CN, 1,2-dichloroethane (DCE),
3
tetrahydrofuran (THF), dimethylformamide (DMF), and 1,4-dioxane were tested
using Cu(NO ) as catalyst and nitrating reagent at 110 °C; CH CN provided the
3
2
3
highest yield (entries 8–12). Interestingly, the yield decreased when the temperature
was decreased to 100 °C (entry 13) or elevated to 120 °C (entry 14).
The scope and generality of substrates for the decarboxylative nitration of a,b-
unsaturated carboxylic acids was investigated under the optimized conditions, and the
Table 1 Optimization of reaction conditions
COOH
NO
2
solvent
M(NO3)n
temp.8h
2
3a
1
a
a
Entry
Nitrating reagent (eq)
Solvent
T (°C)
Time (h)
Yield (%)
1
2
3
4
5
6
7
8
9
Fe(NO
3
3
)
3
2
(1.0)
(1.0)
(1.0)
(1.0)
(1.0)
(1.1)
(1.2)
(1.3)
(1.2)
(1.2)
(1.2)
(1.2)
(1.2)
(1.2)
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
1,4-Dioxide
DCE
110
110
110
110
110
110
110
110
110
110
110
110
100
120
8
8
8
8
8
8
8
8
8
8
8
8
8
8
Trace
Trace
Trace
Trace
86
Ni(NO
)
Ag(NO
3
)
2
2
2
2
2
2
2
2
2
2
2
2
Na(NO
Cu(NO
Cu(NO
Cu(NO
Cu(NO
Cu(NO
Cu(NO
Cu(NO
Cu(NO
Cu(NO
Cu(NO
3
3
3
3
3
3
3
3
3
3
3
)
)
)
)
)
)
)
)
)
)
)
90
94
90
12
1
1
1
1
1
0
1
2
3
4
28
DMF
Trace
Trace
65
THF
MeCN
MeCN
73
Reaction conditions: 1a (0.4 mmol), 2 (0.48 mmol), solvent (2 mL), 90–120 °C, 8 h
a
Isolated yield
123

Z.-G. Luo et al.
results are summarized in Table 2. To our delight, cinnamic acids with electron-rich
substituents (p-Me, p-OMe, 3,4,5-trimethoxy) or electron-poor substituents (p-CF3,
m-NO ), as well as the para- and meta-substituted groups both gave the desired
2
products in moderate to good yields (3b–d, 3h–j), showing no obvious electronic
effect in this reaction. The para- and meta-substituted halogen atoms such as fluorine
and chlorine were well tolerated, affording the corresponding products in 68, 65, and
6
2 % isolated yields, respectively (3e–g). Notably, 2-(2-nitrovinyl)naphthalene is also
a suitable substrate for this reaction, reacting smoothly to give the expected
nitrodecarboxylation products in good yield (3k, 93 %). Furthermore, application of
the present protocol for nitrodecarboxylation of other aromatic heterocyclic
compounds, including 3-(2-furyl)acrylic acid and 3-pyridyl acrylic acid, was explored
(
Scheme 1). The substrate 3-(2-furyl)acrylic acid also gave moderate yield of
decarboxylated and nitrated product 3l in 41 % yield, while no nitrated product of
-pyridyl acrylic acid was obtained under the optimized reaction conditions.
3
Table 2 Nitrodecarboxylation of a,b-unsaturated acids to (E)-nitroolefins
Cu(NO3)2 (1.2 equiv)
COOH
NO2
2
R
R
CH CN, 110°C, 8h
3
1
3
MeO
MeO
NO2
NO2
NO2
NO2
MeO
OMe
3a, 94%
3b, 88%
3c, 93%
NO2
NO2
OMe
F
Cl
3d, 77%
3e, 68%
3f, 65%
NO2
NO2
NO2
NO2
Cl
3
F C
3g, 62%
3h, 98%
3i, 89%
NO2
NO2
3j, 63%
3k, 93%
3 2 3
Reaction conditions: substituted a,b-unsaturated acids (0.4 mmol), Cu(NO ) (0.48 mmol), CH CN
(2 mL), reaction temperature (110 °C), reaction time (8 h). Isolated yield
1
23

3 2
Cu(NO ) -catalyzed nitrodecarboxylation of a,b-…
Gram-scale application of the present method was also explored. As shown in
Scheme 2, the proposed reaction of 1a was investigated under the standard conditions,
giving 1.33 g 3a in 89 % yield without significant loss of reactive efficiency. Thus, this
simple, efficient, and practical protocol has good prospects for industrialization.
To gain further insight into the reaction mechanism, we added the radical
scavenger TEMPO to the reaction system (Scheme 3); the nitrodecarboxylation
reaction was suppressed, and product 3a was obtained in only 15 % isolated yield,
indicating that a free-radical pathway might be involved in the present reaction. On
the basis of the preliminary results above and previous related literature [15, 18],
two plausible pathways for the present reaction are proposed (Scheme 4). In path I,
a,b-unsaturated carboxylic acids react with Cu(NO ) to give compound A and
3
2
HNO . Then, NO radical is generated by heating of HNO . Subsequently, the NO
radical reacts with intermediate A to give alkyl radical intermediate B, which can be
stabilized by the adjacent phenyl ring. Finally, CO is released from the radical
3
2
3
2
2
intermediate B to afford the desired product 3. In a complementary mechanism,
?
?
2
electrophile NO is generated by HNO . Subsequently, electrophile NO undergoes
2
3
electrophilic addition to intermediate A as shown in path II (Scheme 4). Finally, the
?
product 3 is obtained after releasing CO2 and elimination of H . Further
investigations on the more detailed mechanism are in progress in our laboratory.
Conclusions
We present an example of additive-free, highly regioselective nitrodecarboxylation
of a,b-unsaturated acids under mild conditions. The nitrodecarboxylation proceeded
in good yield and tolerated a wide range of functional groups. Tentative experiments
suggested that this reaction was likely to proceed via a radical pathway. The
presented protocol is an environmentally benign, simple, efficient, green procedure
for synthesis of (E)-nitroolefins with good prospects for industrialization.
Experimental
Unless otherwise noted, all materials were obtained from commercial suppliers and
dried and purified using standard procedures. The melting point was measured on an
SGW X-4 monocular microscope melting point apparatus with an unadjusted
NO2
COOH
O
Cu(NO3)2 (1.2 equiv)
O
3l, 41% isolated yield
2
1
l (0.4 mmol)
NO2
COOH
CH CN, 110°C, 8h
3
N
N
3
m, 0%
1
m (0.4 mmol)
Scheme 1 Nitrodecarboxylation of 3-(2-furyl)acrylic acid and 3-pyridyl acrylic acid
123

Z.-G. Luo et al.
Cu(NO3)2 (1.2 equiv)
COOH
NO
2
2
1
a
CH
3
CN, 110°C, 8h
3a
1
.33g, 89% isolated yield
1
0 mmol, 1.48 g
Scheme 2 Synthesis of 3a at gram scale
Cu(NO3)2 (1.2 equiv)
COOH
NO2
2
CH CN, 110°C, 8h
3a
1
a
3
1
5% isolated yield
TEMPO (2.0 equiv)
Scheme 3 Control reaction with addition of TEMPO
O
O
O
Cu
O
Ar
Ar
A
path II
path II
2
HNO3
NO2
NO3
-
CO2
2
H
NO2
COOH
Ar
Ar
1
3
Cu(II)
Cu(I)
NO3
NO
2
-CO2
path I
2HNO3
NO2
path I
3
COO
NO2
COO
Cu(II)
2
Cu(II)
2
Ar
Ar
path I
A
B
Scheme 4 Proposed mechanism of nitrodecarboxylation of a,b-unsaturated acids to (E)-nitroolefins
1 13
thermometer. H and C nuclear magnetic resonance (NMR) spectra were acquired
on a Bruker Avance III 500, at 500 and 125 MHz, respectively, in CDCl , with
3
tetramethylsilane (TMS) as internal standard. Column chromatography was
performed with silica gel (200–300 mesh).
1
23

3 2
Cu(NO ) -catalyzed nitrodecarboxylation of a,b-…
General procedure for synthesis of (E)-nitroolefins 3a–l
A 25-mL Schlenk tube was charged with a,b-unsaturated acid (0.4 mmol),
Cu(NO ) (0.48 mmol), and CH CN (2 mL). The tube was sealed, then the mixture
was stirred under air at 110 °C for 8 h. After completion of the reaction as
3
2
3
monitored by thin-layer chromatography (TLC), H O (5 mL) was added. The
2
mixture was extracted with dichloromethane (3 9 4 mL). The combined organic
layers were washed with saturated sodium bicarbonate solution (10 mL) and water
(10 mL), dried over anhydrous Na SO , then concentrated by rotary evaporator.
2 4
Finally, the residue was purified by column chromatography on silica gel (mobile
phase: petroleum ether/ethyl acetate 6:1) to provide the desired products (3).
1
(
E)-(2-Nitrovinyl)benzene (3a) [18, 19, 21] Yellow solid, m.p. 55–58 °C. H NMR
(
500 MHz, CDCl ) d = 8.05 (d, J = 13.5 Hz, 1H), 7.63 (d, J = 13.5 Hz, 1H),
3
1
.60–7.55 (m, 2H), 7.57–7.52 (m, 1H), 7.52–7.49 (m, 2H). C NMR (125 MHz,
3
7
CDCl ) d = 139.1, 137.2, 132.2, 130.1, 129.4, 129.2.
3
(
E)-1-Methoxy-4-(2-nitrovinyl)benzene (3b) [18, 19] Yellow solid, m.p. 82–85 °C.
1
H NMR (500 MHz, CDCl ) d = 8.02 (d, J = 13.5 Hz, 1H), 7.58–7.54 (m, 3H),
3
1
3
7
.00 (d, J = 8.0 Hz, 2H), 3.92 (s, 3H). C NMR (125 MHz, CDCl ) d = 163.0,
3
1
39.0, 135.1, 131.2, 122.6, 115.0, 55.5.
(
E)-1,2,3-Trimethoxy-4-(2-nitrovinyl)benzene (3c) [18] Yellow solid, m.p.
1
1
20–122 °C. H NMR (500 MHz, CDCl ) d = 7.97 (d, J = 13.5 Hz, 1H), 7.57
3
13
d, J = 13.5 Hz, 1H), 6.80 (s, 2H), 3.95 (s, 3H), 3.94 (s, 6H). C NMR (125 MHz,
(
CDCl ) d = 153.8, 141.9, 139.3, 136.4, 125.3, 106.6, 61.1, 56.3.
3
1
(
E)-1-Methoxy-3-(2-nitrovinyl)benzene (3d) [21] Yellow solid, m.p. 93–95 °C. H
NMR (500 MHz, CDCl ) d = 8.02 (d, J = 13.5 Hz, 1H), 7.62 (d, J = 13.5 Hz,
3
1
3
1
H), 7.40 (d, J = 5.0 Hz, 1H), 7.31 (s, 1H), 7.18 (d, J = 5.0 Hz, 1H), 7.08 (s, 2H),
1
3
.90 (s, 3H). C NMR (125 MHz, CDCl ) d = 160.2, 139.0, 137.4, 131.4, 130.4,
3
21.7, 118.0, 114.0, 55.5.
(
E)-1-Fluoro-4-(2-nitrovinyl)benzene (3e) [18, 21] Yellow solid, m.p.
1
1
7
1
03–105 °C. H NMR (500 MHz, CDCl ) d = 8.02 (d, J = 13.5 Hz, 1H),
3
13
.62–7.56 (m, 3H), 7.21–7.18 (m, 2H). C NMR (125 MHz, CDCl ) d = 166.0,
3
64.0, 137.8, 137.0, 131.3, 131.2, 126.4, 126.3, 116.9, 116.7.
(
E)-1-Chloro-4-(2-nitrovinyl)benzene (3f) [19, 21] Yellow solid, m.p.
1
1
09–110 °C. H NMR (500 MHz, CDCl ) d = 7.96 (d, J = 13.5 Hz, 1H), 7.56
3
13
d, J = 13.5 Hz, 1H),7.50 (d, J = 5.0 Hz, 2H), 7.44 (d, J = 8.0 Hz, 2H). C NMR
(
(
125 MHz, CDCl ) d = 138.4, 137.7, 137.4, 130.3, 129.8, 128.5.
3
(
E)-1-Chloro-3-(2-nitrovinyl)benzene (3g) [18, 21] Yellow solid, m.p. 50–52 °C.
1
H NMR (500 MHz, CDCl ) d = 7.98 (d, J = 13.5 Hz, 1H), 7.62–7.58 (m, 2H),
3
1
3
7
.53–7.49 (m, 1H), 7.47–7.43 (m, 2H). C NMR (125 MHz, CDCl ) d = 138.2,
3
1
37.4, 135.5, 132.0, 131.9, 130.7, 128.8, 127.2.
123

Z.-G. Luo et al.
(
E)-1-Methyl-4-(2-nitrovinyl)benzene (3h) [18, 19, 21] Yellow solid, m.p.
1
1
06–108 °C. H NMR (500 MHz, CDCl ) d = 8.03 (d, J = 13.5 Hz, 1H), 7.61
3
(
d, J = 13.5 Hz, 1H), 7.49 (d, J = 5.0 Hz, 2H), 7.30 (d, J = 5.0 Hz, 2H), 2.45 (s,
3
1
3
H). C NMR (125 MHz, CDCl ) d = 143.1, 139.1, 136.4, 130.2, 129.2, 127.4,
3
2
1.7.
(
E)-1-(2-Nitrovinyl)-4-(trifluoromethyl)benzene (3i) [19] Yellow solid, m.p.
1
9
7
1
3–95 °C. H NMR (500 MHz, CDCl ) d = 8.02 (d, J = 13.5 Hz, 1H),
3
13
.74–7.61 (m, 5H). C NMR (125 MHz, CDCl ) d = 138.9, 137.1, 133.6, 133.5,
3
29.3, 126.4, 126.3, 124.8, 122.1, 30.1.
1
(
E)-1-Nitro-3-(2-nitrovinyl)benzene (3j) [18] Yellow solid, m.p. 48–50 °C. H
NMR (500 MHz, CDCl ) d = 8.46 (s, 1H), 8.39 (d, J = 8.0 Hz, 1H), 8.10 (d,
3
1
J = 13.5 Hz, 1H), 7.92 (d, J = 5.0 Hz, 1H), 7.75–7.71 (m, 2H). C NMR
3
(
125 MHz, CDCl ) d = 148.9, 139.3, 136.2 134.4, 131.9, 130.6, 126.2, 123.5.
3
1
(
(
(
E)-2-(2-Nitrovinyl)naphthalene (3k) [18] Yellow solid, m.p. 89–91 °C. H NMR
500 MHz, CDCl ) d = 8.90 (d, J = 13.5 Hz, 1H), 8.19 (d, J = 8.0 Hz, 1H), 8.05
d, J = 5.0 Hz, 1H), 7.97 (d, J = 8.0 Hz, 1H), 7.81 (d, J = 8.0 Hz, 1H), 7.72 (d,
1
J = 5.0 Hz, 1H), 7.70–7.56 (m, 3H). C NMR (125 MHz, CDCl ) d = 138.6,
3
3
3
1
36.2, 133.9, 132.6, 131.6, 129.1, 127.8, 127.1, 126.8, 126.4, 125.4, 123.0.
1
(
E)-2-(2-Nitrovinyl)furan (3l) [19] Yellow solid, m.p. 68–70 °C. H NMR
(
500 MHz, CDCl ) d = 7.82 (d, J = 13.5 Hz, 3H), 7.63 (d, J = 5.0 Hz, 1H),
3
7
.56 (d, J = 13.5 Hz, 1H), 6.93 (d, J = 3.5 Hz, 3H), 6.62 (dd, J = 3.5, 1.5 Hz,
1
3
1
H). C NMR (125 MHz, CDCl ) d = 146.8, 146.7, 135.0, 125.4, 119.8, 113.3.
3
Supplementary material
1
13
H and C NMR spectra for all compounds are provided.
Acknowledgments The authors gratefully acknowledge the financial support from the National Natural
Science Foundation of China (21102003) and Scientific Research Foundation for the Introduction of
Talent of Anhui University of Science and Technology.
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