Synthesis of substituted nitroolefins: A copper catalyzed nitrodecarboxylation of unsaturated carboxylic acids

Article abstract of DOI:10.1039/c3ob41408f

A novel, mild and convenient method for the nitrodecarboxylation of substituted cinnamic acid derivatives to their nitroolefins is achieved using a catalytic amount of CuCl (10 mol%) and tert-butyl nitrite (2 equiv.) as a nitrating agent in the presence of air. This reaction provides a useful method for the synthesis of β,β-disubstituted nitroolefin derivatives, which are generally difficult to access from other conventional methods. Additionally, this reaction is selective as the E-isomer of the acid derivatives furnishes the corresponding E-nitroolefins. One more salient feature of the method is, unlike other methods, no metal nitrates or HNO₃ are employed for the transformation.

Full text of DOI:10.1039/c3ob41408f

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DOI: 10.1039/C3OB41408F
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ARTICLE TYPE
Synthesis of Substituted Nitroolefins: A Copper Catalyzed
Nitrodecarboxylation of Unsaturated Carboxylic Acids†
Balaji V. Rokade and Kandikere Ramaiah Prabhu*a
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
A
novel, mild and
a
convenient method for the
which is a one of the major limitations associated with these
nitrodecarboxylation of substituted cinnamic acid derivatives 50 methods. The decarboxylation of cinnamic acid is generally
to their nitroolefins is achieved using catalytic amount of
Table 1. Screening studies
CuCl (10 mol%) and tert-butyl nitrite (2 equiv) as a nitrating
10 agent in the presence of air. This reaction provides a useful
method for the synthesis of β,β-disubstituted nitroolefin
derivatives, which are generally difficult to access from other
conventional methods. Additionally, this reaction is selective
as E-isomer of acid derivatives furnish the corresponding E-
15 nitroolefins. One more salient feature of the method is, unlike
other methods, no metal nitrates or HNO3 are employed for
the transformation.
Catalyst
Oxidant
Ph
Ph
COOH
NO2
+
TBN
Ph
Ph
Solvent, Temp
2
1a
2a
TBN
(equiv)
1.5
1.5
1.5
1.5
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
catalyst
(10 mol%)
CuCl
solvent
(°C)
yielda
entry
oxidant
(%)
34
1
O2
O2
O2
O2
air
air
air
air
air
CH3CN (rt)
CH3CN (60)
CH3CN (80)
CH3CN (80)
CH3CN (80)
CH3CN (80)
CH3CN (80)
CH3CN (80)
CH3CN (80)
CH3CN (80)
CH3CN (80)
DCE (80)
2
CuCl
66
In recent times, decarboxylative coupling reactions have emerged
as a powerful tool for C-C1 bond and C-hetero2 bond forming
20 reactions. By adopting the decarboxylative coupling strategy, a
variety of aromatic acids and acrylic acids were coupled with
aromatic halides3a or triflates,3b hydrocarbons,3c alcohols,3d
ethers3d etc.1 to obtain a variety of compounds including olefin
derivatives. Nitroolefins represent a unique class of nitro
25 compounds, which have multifaceted utility in organic synthesis.4
Besides their applications in pharmaceuticals,5 they serve as
important precursors in organic synthesis.4 β-monosubstituted
nitroolefins and β,β-disubstituted nitroolefins also serve as vital
precursors for catalytic asymmetric conjugate addition6 and
30 asymmetric hydrogenation7 to furnish optically active,
synthetically valuable products.8 Henry reaction of condensing
aldehydes with nitroalkanes is one of the general methods to
access conjugated nitroolefins.9 Nitration of alkenes is another
useful method for synthesizing nitroolefins.10 Although
35 decarboxylative nitration is known in literature, it was achieved
using harsh reaction conditions such as high temperature,11a or
highly acidic reaction conditions (HNO3),11b-d or using
stoichiometric amounts of metal nitrates.11e-i Due to these
reasons, these strategies have restricted the scope of reactions
40 and lack of compatibility with oxidizable and/or acid sensitive
functional groups. To overcome these limitations, mild and
convenient methods to access nitroolefins are well sought. As
cinnamic acid and its derivatives are easily available or
synthesized the nitrodecarboxylation of cinnamic acid and its
45 derivatives serve as a convenient and attractive procedure for the
synthesis of corresponding nitroolefins. Although, nitroolefins are
synthesized from their corresponding olefins, the isolation of
such olefins is a cumbersome task, due to their volatile nature,
3
CuCl
77
4
CuCl
83
5
CuCl
96
6
CuCl
82b
7
none
44
8
CuI
90
9
CuCl2 ·2H2O
83
10
11
12
13
14
15
16
Cu(ClO4)·6H2O air
86
Cu(OTf)2
CuCl
air
air
air
air
air
air
77
50
CuCl
THF (80)
72
CuCl
Toluene (80)
DMF (80)
70
CuCl
trace
nd
CuCl
H2O (80)
a Isolated yields. b 5 mol % of CuCl.
55
performed using copper salt of cinnamic acid followed by
quenching with suitable radical partner.3c,3d tert-Butyl nitrite
(TBN) is a mild, and commercially available nitrating reagent
which is easy to handle, and is known to generate nitro radical in
60 the presence of air12a,12b, or oxygen.12c,12d In this context, we
envisioned a reaction of α, β-unsaturated acid with Cu salt and
TBN in the presence of air to obtain the corresponding nitro
compound. In this direction, and in continuation of our studies on
the utility of copper catalysts for C-N bond forming reaction,13
65 herein we present a novel, mild method to synthesize a variety of
β-monosubstituted and β,β-disubstituted nitrolefins by employing
catalytic amount of CuCl with TBN in the presence of air.14
β,β-Disubstituted nitrolefins are synthetically valuable
precursors, which cannot be easily synthesized by conventional
70 Henry reaction.9,7a,7c Due to this consideration, the preliminary
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[journal], [year], [vol], 00–00 | 1

Organic & Biomolecular Chemistry
Page 2 of 4
investigations were undertaken by reacting 3,3-diphenylacrylic
(60:40 ratio of 2ha and 2hb). The formation of 2-(3-nitroprop-1-
acid (1a) with Cu catalysts using TBN as a nitrating agent in the
presence of air or oxygen (Table 1). As revealed by these studies,
the combination of catalytic amount of CuCl with TBN, in the
en-2-yl)pyridine (2ha) as a major product can be explained on the
basis of isomerization of 2hb to 2ha, whicDhOcIa: n10b.1e0Va3it9et/wriCbA3urOttieBcld4e1Ot4on0li8nFe
the coordination of copper metal to the adjacent methyl group to
5
presence of air was good choice to affect this transformation 50 form
5
membered
complex.
Additionally,
the
(entries 1-5, Table 1). The optimal reaction conditions for this
transformation was achieved by performing the reaction of 1a
with TBN (2 equiv) and CuCl (10 mol%), in CH3CN at 80 °C
(entry 6, Table 1). Lowering the catalyst (CuCl) loading to 5 mol
nitrodecarboxylation reaction is selective as E-isomer of acids
resulted in the formation of the corresponding E-nitroolefins.
Table 3 Nitrodecarboxylation of α, β-unsaturated acid to nitroolefins
10 % resulted in decreasing the yield of the product (entry 7).
Interestingly, the reaction also proceeded in the absence of copper
catalyst to furnish the product 2a in low yield (44%, entry 8).
Further, it was also found that other copper catalysts such as CuI,
CuCl2 ·2H2O, Cu(ClO)4·6H2O, Cu(OTf)2 were useful for this
15 transformation, as these reactions resulted in the formation of 2a
in good yields (77-90%, entries 9-12). The solvent screening
studies indicated that the solvents such as THF, toluene and
dichloromethane were useful but resulted in formation of product
in low yields (entries 13-15) while the reaction did not proceed in
20 DMF or water (entries 16-17).
air
COOH
NO2
CuCl
TBN
+
R
R
CH3CN, 80 °C
1
3
NO2
NO2
NO2
O
3a, 92 %
3b, 82 %
3c, 52 %
3f, 60 %
O
NO2
NO2
NO2
O
BnO
Table 2 Nitrodecarboxylation of β-alkyl cinnamic acids to β-alkyl
nitroolefins
3d, 91 %
3e, 69 %
NO2
NO2
PhO
NO2
F3C
NC
3i, 36%c (32 %)
3g, 84%c (45 %)
3h, 46%c (38%)
NO2
NO2
NO2
Br
F
Cl
3j, 45 %
3k, 68 %
3l, 50 %
NO2
NO2
NO2
NO2
N
S
O
Bz
3n,41%
3p, 34%
3m, 72 %
3o, 75 %
55
a Reaction conditions: acid (0.50 mmol), TBN (1.0 mmol), CuCl (0.05
mmol), air, CH3CN (2 ml). b Isolated yield. c used 4 equiv of TBN
a Reaction conditions: acid (0.50 mmol), TBN (1.0 mmol), CuCl (0.05
25 mmol), air, CH3CN (2 ml). b Isolated yield.
The scope of this reaction was further expanded by
The scope of the copper catalyzed nitrodecarboxylation was
further examined with β,β-disubstituted acrylic acid derivatives
Table 2). (E)-3-Phenylbut-2-enoic acid on treatment with 2 equiv
of TBN in the presence of 10 mol% CuCl in CH3CN underwent a
30 smooth transformation to (E)-(1-nitroprop-1-en-2-yl)benzene
(2b) in 71% isolated yield. (E)-1-methyl-4-(1-nitroprop-1-en-2-
yl)benzene furnished the corresponding nitroolefin 2c in 74%.
While, (E)-3-(4-methoxyphenyl)but-2-enoic acid and (E)-3-(4-
bromophenyl)but-2-enoic acid under the similar reaction
35 conditions resulted in the formation 2d and 2e in moderate yields
(52%). β-Alkyl substituted naphthalene derivative such as (E)-3-
investigating the reaction of
a variety of cinnamic acid
60 derivatives and results are compiled in Table 3. As can be seen in
Table 3, a variety of substituted cinnamic acid derivatives such as
(E)-3-(4-methoxyphenyl)acrylic acid, and (E)-cinnamic acid
reacted well with TBN in the presence of CuCl to furnish
corresponding nitro derivatives 3a and 3b in excellent yields
65 (92% and 82% respectively). (E)-3-(p-tolyl)acrylic acid under the
similar reaction conditions afforded 3c in moderate yield (52%).
Similarly, (E)-3-(naphthalen-1-yl)acrylic acid reacted well to
furnish the product 3d in 91% yield, whereas (E)-3-(3,4-
dimethoxyphenyl)acrylic
acid
and
(E)-3-(4-
(naphthalen-2-yl)but-2-enoic
acid
underwent
the
70 (benzyloxy)phenyl)acrylic acid furnished 3e and 3f in 69% and
60% respectively. As can be seen, the reaction of (E)-3-(3-
phenoxyphenyl)acrylic acid needed excess of TBN (4 equiv) to
furnish (E)-1-(2-nitrovinyl)-3-phenoxybenzene 3g in 84%.
Whereas the reaction of (E)-3-(4-(trifluoromethyl)phenyl)acrylic
75 acid, and (E)-3-(4-cyanophenyl)acrylic acid with excess of TBN
(4 equiv) did not affect the yields of the products (E)-1-(2-
nitrovinyl)-4-(trifluoromethyl)benzene (3h, 46%) and (E)-4-(2-
nitrodecarboxylation reaction to form the corresponding nitro
derivative 2f in 70%. The heterocyclic α, β-unsaturated acid such
40 as (E)-3-(thiophen-2-yl)but-2-enoic acid was successfully
transformed in to its nitroolefin 2g in moderate yield (52 %). The
reaction of (E)-3-(pyridin-2-yl)but-3-enoic acid with TBN and
CuCl was interesting, as the reaction produced the mixture of 2-
(3- nitroprop-1-en-2-yl)pyridine (2ha) and (E)-2-(1-nitroprop-1-
45 en-2-yl)pyridine (2hb) in 80% yield with 2ha in major amount
2
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Journal Name, [year], [vol], 00–00
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Page 3 of 4
Organic & Biomolecular Chemistry
nitrovinyl)benzonitrile
(3i,
36%).
Further,
(E)-3-(2-
undergoes decarboxylation to form the corresponding nitroolefin.
bromophenyl)acrylic acid, (E)-3-(2-fluorophenyl)acrylic acid,
and (E)-3-(4-chlorophenyl)acrylic acid under the optimized
conditions furnished the corresponding nitroolefins 3j, 3k and 3l
in moderate to good yields (45%, 68%, and 50% respectively,
Table 3). It worth noting that acid sensitive substrates such as
furan and thiophene derivatives sustained very well under the
reaction conditions. As can be seen, the reaction was facile with
View Article Online
DOI: 10.1039/C3OB41408F
5
heterocyclic α, β-unsaturated acids, (E)-
3-(Thiophen-2-
10 yl)acrylic acid to furnish corresponding nitro derivative, (E)-2-(2-
nitrovinyl)thiophene (3m, 72% yield), whereas the similar
reaction with (E)-3-(furan-2-yl)acrylic acid resulted in the
formation of (E)-2-(2-nitrovinyl)furan (3n) in 41%. These
nitration reaction catalysed by CuCl appears to be versatile as
15 indole acrylic acid such as (E)-3-(1-benzoyl-1H-indol-2-
yl)acrylic acid under the optimal reaction conditions furnished the
corresponding nitroolefin 3o in good yield (75%). As the reaction
was proceeding readily with cinnamic acid, we thought it would
be interesting to subject dienoic acid for the nitrodecarboxylation
20 reaction. Hence, the reaction of (2E,4E)-5-phenylpenta-2,4-
dienoic acid under optimal conditions resulted in the formation of
50
Scheme 2. Tentative Mechanism
Conclusions
In conclusion, we report a novel, mild and a convenient method
for the nitrodecarboxylation of substituted cinnamic acid
55 derivatives to their correponding nitroolefins catalyzed by CuCl
in the presence of air. This nitrodecarboxylation reaction uses
TBN as a nitrating source. Besides these advantages, the reaction
provides a useful method for the synthesis of β,β-disubstituted
nitroolefin derivatives, which are generally difficult to access
60 from other conventional methods. Additionally, this reaction is
selective as E-isomer of acid derivatives resulted in the formation
of the corresponding E-nitroolefins. Apart from these advantages,
metal nitrates or HNO3 are not employed for the transformation.
One more salient feature of this reaction is that the acid sensitive
65 functionalities or compounds such as nitrile functionality and
thiophene and furan derivatives are well tolerated under the
reaction conditions.
corresponding
nitroolefin,
((1E,3E)-4-nitrobuta-1,3-dien-1-
yl)benzene 3p in 34%. Our attempts to convert (E)-4-(2-
carboxyvinyl)benzoic acid, 2-cyclohexylideneacetic acid, 4-
25 methoxybenzoic acid to their corresponding nitroolefins did not
meet with success and the starting materials were recovered
unchanged. The inertness of these acids for nitrodecarboxylation
can be attributed to the inability of carboxylic acid salts of
aromatic and aliphatic acids to undergo decarboxylation under the
30 present reaction conditions.
TBN (2 equiv)
Acknowledgements
Ph
Ph
CuCl (10 mol %)
+
TEMPO
Financial support by IISc and RL Fine Chem is acknowledged
70 with thanks. We thank Mr. G. Karthik for his assistance in
carrying out few experiments. Thanks are due to Dr. A. R.
Ramesha, for useful discussion. BVR thanks CSIR-UGC for a
senior research fellowship.
COOH
NO2
Ph
Ph
air, CH3CN, 80 °C
1a
(1.1 equiv)
40 %
2a
TBN (2 equiv)
Ph
CuCl (10 mol %)
Ph
BHT
+
COOH
NO2
Ph
Ph
air, CH3CN, 80 °C
Notes and references
no reaction
1a
(1.1 equiv)
2a
75 a Department of Organic Chemistry, Indian Institute of Science,
Bangalore 560 012, Karnataka, India.
b E-mail: prabhu@orgchem.iisc.ernet.in
†Electronic Supplementary Information (ESI) available: See
DOI: 10.1039/b000000x/
Scheme 1 Control experiments.
To follow the reaction pathway, few control experiments were
35 performed (Scheme 1). The reaction of 3,3-diphenylacrylic acid
(1a) with CuCl (10 mol %), and TBN (2 equiv) in CH3CN was
performed under the optimal conditions in the presence of radical
inhibitors such as TEMPO (2,2,6,6-tetramethylpiperidin-1-
yl)oxyl) and BHT (butylated hydroxytoluene). As can be seen,
40 the reaction in the presence of TEMPO has drastically reduced
the yield of 2a, whereas the similar reaction in the presence of
BHT did not proceed. These observations indicate that the
reaction may be proceeding through a radical intermediate. Based
on this information, and literature precedence,3d a tentative
45 mechanism is proposed (Scheme 2). α, β-Unsaturated acid reacts
with CuX to form the corresponding Cu (II) salt, which further
reacts with nitro radical, (which is generated by the reaction of
TBN in the presence of air) to form radical I. Further I
80
1
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14 During the preparation of this manuscript, a decarboxylative nitration
of cinnamic acid was reported using 80 mol% of TEMPO and TBN.
Interestingly, utility of TEMPO in the present copper catalyzed
nitrodecarboxylation reaction drastically reduces the formation of the
product (see Scheme 1): see S. Manna, S. Jana, T. Saboo, A. Maji
and D. Maiti, Chem Comm. 2013, 49, 5286.
4
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