Synthesis of nitroalkenes involving a cooperative catalytic action of iron(III) and piperidine: A one-pot synthetic strategy to 3-alkylindoles, 2H-chromenes and N-arylpyrrole

Article abstract of DOI:10.1002/ejoc.201300172

An efficient and simple strategy has been developed to synthesize various substituted nitroalkenes involving a cooperative catalytic system of FeCl ₃ and piperidine. This dual catalytic protocol simultaneously activates both electrophile and nucleophile and works under mild reaction conditions so that many sensitive functional groups were tolerated. Moreover, this cooperative catalytic reaction is also suitable for various one-pot reactions involving nitroalkenes such as, 2H-chromenes, N-arylpyrrole and Michael reaction with indole. Notably, this method is low-cost, efficient and environmentally friendly. Copyright

Full text of DOI:10.1002/ejoc.201300172

Job/Unit: O30172
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Date: 15-06-13 10:30:18
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FULL PAPER
Efficient Nitroalkene Synthesis
An efficient and simple strategy has been
developed to synthesize substituted nitro-
alkenes involving a cooperative catalytic sy-
stem of FeCl3 and piperidine. The dual
catalytic protocol simultaneously activates
both electrophile and nucleophile and pro-
vides an efficient route to various nitro-
alkenes in high yields. Moreover, this
method facilitates tandem reactions in-
volving nitroalkenes.
S. Jalal, S. Sarkar, K. Bera, S. Maiti,
U. Jana* ........................................... 1–7
Synthesis of Nitroalkenes Involving a Co-
operative Catalytic Action of Iron(III) and
Piperidine: A One-Pot Synthetic Strategy
to 3-Alkylindoles, 2H-Chromenes and N-
Arylpyrrole
Keywords: Synthetic methods / Domino re-
actions / Nitrogen heterocycles / Cooperat-
ive catalysis / Alkenes
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S. Jalal, S. Sarkar, K. Bera, S. Maiti, U. Jana
FULL PAPER
DOI: 10.1002/ejoc.201300172
Synthesis of Nitroalkenes Involving a Cooperative Catalytic Action of Iron(III)
and Piperidine: A One-Pot Synthetic Strategy to 3-Alkylindoles, 2H-
Chromenes and N-Arylpyrrole
Swapnadeep Jalal,[a] Soumen Sarkar,[a] Krishnendu Bera,[a] Sukhendu Maiti,[a] and
Umasish Jana*[a]
Keywords: Synthetic methods / Domino reactions / Nitrogen heterocycles / Cooperative catalysis / Alkenes
An efficient and simple strategy has been developed to syn-
thesize various substituted nitroalkenes involving a coopera-
tive catalytic system of FeCl3 and piperidine. This dual cata-
lytic protocol simultaneously activates both electrophile and
nucleophile and works under mild reaction conditions so that
many sensitive functional groups were tolerated. Moreover,
this cooperative catalytic reaction is also suitable for various
one-pot reactions involving nitroalkenes such as, 2H-
chromenes, N-arylpyrrole and Michael reaction with indole.
Notably, this method is low-cost, efficient and environmen-
tally friendly.
preparations of nitroalkenes have been described, which in-
clude the use of microwave and ultrasound techniques in
ammonium acetate,[16] amphiphilic ionic liquids,[17] ethyl-
enediamine[18] and direct nitration of alkenes by using
NaNO2 in the presence of various oxidizing agents.[19] In
addition, a few heterogeneous catalysis have also been em-
ployed for the direct synthesis of nitroalkenes from aro-
matic aldehydes and nitroalkanes.[20] Very recently, a one-
pot synthesis of aliphatic nitroalkenes by using a secondary
amine in combination with molecular sieves has been re-
ported.[21] Although some of these methods are efficient,
many suffer from one or more drawbacks: for example, the
need a large excess of nitroalkane and expensive and toxic
reagents, low product yields, complicated reaction assembly,
harsh reaction conditions, laborious isolation procedure,
and large amounts of acid to neutralize the bases. There-
fore, the development of an efficient, general and environ-
mentally friendly process, which enables rapid and easy ac-
cess to nitroalkene derivatives, is of great importance, par-
ticularly for the synthesis of complex molecules.
Our aim was to develop a protocol that overcomes the
problems associated with the traditional methods of nitro-
alkene preparation and to extend it to develop new tandem
reactions involving nitroalkenes in one pot. In doing so, we
believe that the concept of dual catalysis, i.e. the combina-
tion of metal catalysis and organocatalysis[22] in one pro-
cess, will be effective for nitroalkene formation. Suitable
Lewis base/Lewis acid will help to form the carbon–carbon
bond, and the Lewis acid would help the elimination step
and subsequent reactions with other suitable substrates. As
part of our continuous interest in the area of iron catalysis
towards the development of environmentally friendly and
sustainable reactions,[23] herein we wish to report a new
Introduction
Nitroalkenes are important building blocks in organic
synthesis[1] and are commonly used as substrates in a wide
variety of transformations including Michael,[2] Friedel–
Crafts alkylation,[3] Diels–Alder[4] and 1,3-dipolar cycload-
dition[5] reactions. They are also very important substrates
for the synthesis of natural products,[6] heterocyclic com-
pounds[7] and useful in the area of asymmetric synthesis.[8]
Furthermore, the nitro group can easily be converted into
other valuable functional groups such as amines and carb-
onyl groups, and hence nitroalkenes are important sub-
strates in organic synthesis.[9] In addition, nitroalkenes are
well known for their biological activity such as insecti-
cides,[10] fungicides[11] and in various pharmacologically
active[12] substrates. Very recently β-nitrostyrene motifs have
also been reported as pro-apoptotic anticancer[13] and anti-
bacterial agents.[14] Accordingly, a number of synthetic
methods have been developed for the preparation of conju-
gated nitroolefins.
The most common method for the preparation of nitro-
alkenes involves a Henry condensation reaction of a carb-
onyl compound with a nitroalkane that provides a β-nitro
alcohol, which upon dehydration affords a nitroalkene.[15]
The Henry reaction is generally performed under mildly ba-
sic conditions with a variety of bases, but the dehydration
step requires harsh reaction conditions. Only a few one-pot
[a] Department of Chemistry, Jadavpur University,
Kolkata 700 032, West Bengal, India
Fax: +91-33-2414-6584
E-mail: jumasish2004@yahoo.co.in
umasish@gmail.com
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201300172.
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Nitroalkene Synthesis: A One-Pot Synthetic Strategy
method for the synthesis of nitroalkenes by the reaction be- superiority of FeCl3 in combination with piperidine under
tween aldehydes and nitroalkanes by using a combination the same reaction conditions. All the products were charac-
of iron and secondary amines in one process under mild terized spectroscopically. With regard to stereochemistry,
conditions (Scheme 1). This reaction offers an efficient, en- the reaction gave exclusively the E-isomers.
vironmentally friendly and straightforward strategy to
Table 1. Catalysts screening studies.[a]
achieve nitroalkenes. Moreover, we have demonstrated that
the cooperative catalytic conditions are also suitable for car-
rying out the one-pot synthesis of 2H-chromenes, multisub-
stituted N-arylpyrroles and 3-substituted indoles involving
nitroalkenes.
Scheme 1. One-pot preparation of nitroalkenes.
Results and Discussion
The present idea stems from our recently developed
iron(III)-salt catalyzed four-component synthesis of pyr-
role. We demonstrated that this reaction proceeded through
in situ generation of nitroalkenes,[23a] however, we could not
isolate it during the course of the reaction because it was
very reactive and immediately underwent subsequent steps.
In connection with those studies, we decided to apply a co-
operative catalytic reaction by using Lewis acids and organ-
ocatalysts to prepare nitroalkenes.
To optimize the reaction conditions, first we studied the
reaction between benzaldehyde and nitromethane in the
presence of different Lewis acids and bases. After screening
a large number of catalysts under similar conditions
(Table 1), we observed that the reaction in the presence of
only FeCl3 (10 mol-%) did not afford any nitroalkene 3a.
Desired nitroalkene 3a is obtained in 15% yield upon heat-
[a] Reaction conditions: Benzaldehyde (1 mmol), nitromethane
(6 mmol), piperidine (0.10 mmol), FeCl3 (0.10 mmol), toluene
(1 mL), gentle reflux, 4 h. [b] Pure, isolated yield. [c] Without tolu-
ene.
ing to reflux for 24 h in the presence of piperidine (10 mol-
%) in toluene solvent. But, when the reaction was carried
out in the presence of a combination of anhydrous FeCl3
(10 mol-%) and piperidine (10 mol-%), target nitrostyrene simple and straightforward.
The experimental procedure for this reaction was very
mixture of aldehyde
A
3a formed in an excellent 95% yield (Table 1, Entry 2). Fur- (1 mmol), nitroalkane (6 mmol), piperidine (0.1 mmol) and
ther studies indicated that reducing the amount of either of FeCl3 (0.1 mmol) in toluene (1 mL) was gently heated to
the catalysts to 5 mol-% reduced the yield of the product reflux for a set period of time. After completion of the reac-
(Table 1, Entry 3). Then a series of other iron catalysts such tion (TLC) and removal of the solvent, the crude product
as FeCl3·6H2O, FeBr3, Fe(OTf)3 and Fe(acac)3 were also was directly purified by silica gel column chromatography
tested, however, they did not improve the results. Among to afford the desired product with high purity.
the various organic bases tested in combination with FeCl3,
Following the optimized reaction conditions, we then ex-
piperidine was found to be the best organocatalyst for this plored the substrate scope of this reaction. The results are
transformation. Bases, such as primary amine, tertiary summarized in Table 2. Different aldehydes were success-
amine and K2CO3, were less effective under similar reaction fully reacted with nitromethane to afford the respective ni-
conditions. This reaction was also studied without toluene trostyrenes in very good yields. Aromatic, heteroaromatic
as solvent but lower yields resulted of desired product and aliphatic aldehydes condensed efficiently with nitro-
(Table 1, Entry 7). Without toluene, formation of a dinitro methane leading exclusively to the desired nitroalkenes in
derivative through Michael reaction of nitrostyrene and ni- high yield. Aromatic aldehydes containing electron-donat-
tromethane was observed. Finally, relative to other com- ing groups such as methyl and varieties of ethers substitu-
monly available metal catalysts such as Yb(OTf)3, AlCl3, ents at ortho-, meta- or para- gave the desired products in
ZnCl2 and InCl3(Table 1, Entries 14–17), anhydrous FeCl3 high yields. Free –OH bearing substrates such as 1f and 1g
proved to be the best Lewis acid. These results show the also reacted smoothly with nitromethane under optimized
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reaction conditions in good yields (Table 2, Entries 6 and desired product. The strong electron-withdrawing effect of
7). Notably, Lewis-acid-sensitive functional groups such as, the p-NO2 group may make the substrate and the product
1,3-dioxolane, benzyl ether, allyl ether and propargyl ethers more reactive so that many unwanted side reactions take
were also tolerated and gave excellent yields, 75, 80, 85 and place. Naphthyl-2-carbaldehyde 1r, and ortho-biphenylcarb-
96%, respectively (Table 2, Entries 8–11). Moreover, the aldehydes 1s and 1t were also smoothly converted into the
combination of FeCl3 (10 mol-%) and piperidine (10 mol- desired products in good yields (Table 2, Entries 18–20).
%) also worked efficiently in the case of aromatic aldehydes
Next, we investigated various heterocyclic carbaldehydes
containing weakly electron-withdrawing substituents such such as furan-2-carbaldehyde 1u and thiophene-2-carbal-
as, p-Cl, o-Br, o-F and p-F (Table 2, Entries 12–15), and dehyde 1v (Table 2, Entries 21 and 22). They were smoothly
strong electron-withdrawing groups such as p-CN and m- converted into corresponding nitroolefins 3u and 3v in 85
NO2 (Table 2, Entries 16 and 17), and gave exclusively de- and 95% yield, respectively. Although, indole-3-carbal-
sired products 3l, 3m and 3n–3q in high yields. However, dehyde 1w completely converted into the desired nitroal-
in contrast to m-nitrobenzaldehyde, para-nitrobenzaldehyde kene, the concomitant Michael addition product, 2,2-bi-
produced mixtures of products including trace amounts of (indolyl)nitroethane, was also observed with the reaction of
Table 2. Reaction of various aldehydes and nitromethane in the presence of a combination of FeCl3 and piperidine.[a]
[a] Reaction conditions: Aldehyde (1 mmol), nitromethane. (6 mmol), piperidine (0.10 mmol), FeCl3 (0.10 mmol), toluene (1 mL), gentle
reflux, 4 h. [b] Pure, isolated yield. [c] A mixture of 3w and 2,2-bis(indolyl)nitroethane (82:18) were isolated.
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Nitroalkene Synthesis: A One-Pot Synthetic Strategy
nitromethane (Table 2, Entry 23). Interestingly, no Michael groups, such as p-F, p-CN and p-NO2 containing aldehydes
addition product was observed in the case of N-methylin- (Table 3, Entries 8–10), worked more efficiently with nitro-
dole-3-carbaldehyde, which gave the desired nitroalkene as ethane relative to nitromethane and gave higher yields of
a single product in 84% yield (Table 2, Entry 24).
Encouraged by the above results, we further studied a obtained in all the examples given in Table 3.
few aliphatic aldehydes to show the versatility of this Thus we have developed a simple and efficient method
the desired nitroalkenes. Pleasingly, only (E)-isomers were
method (Table 2, Entries 25–28). As shown, these reactions for the preparation of structurally varied nitroalkenes in the
proceeded successfully to afford the corresponding aliphatic presence of a combination of metal catalysis and organoca-
nitroalkenes in moderate to good yields. It is noteworthy talysis through a cooperative action. The present reaction
that 2-bromocyclohexene-1-carbaldehyde (1dd) also reacted did not work with ketones. Although, all the above studies
with nitromethane and gave desired product 3dd in 65% were performed by using 1 mmol of aldehyde, we also
yield (Table 2, Entry 28). The tolerance of a halide substitu- checked the applicability of the method for large-scale syn-
ent (Table 2, Entries 12, 13 and 28) might provide an ad- thesis with aldehydes 1a (18.87 mmol), 1f (10 mmol) and 1v
ditional benefit for further synthetic transformations (10 mmol). Pleasingly, the present protocol worked for
through cross coupling reactions.
large-scale syntheses and gave corresponding nitroalkenes
Finally, the reaction of nitroethane instead of nitrometh- 3a, 3f and 3v in 75, 58 and 61% yields, respectively (see
ane with varieties of aromatic aldehydes was also explored Supporting Information).
to show the generality of this reaction for the preparation
Finally, to make this cooperative catalysis more attract-
of α-substituted nitroalkenes. We were delighted to find that ive, we investigated a few one-pot syntheses of complex mo-
the optimized reaction conditions were also suitable for the lecules such as 3-substituted indoles, 3-nitrochromenes and
preparation of α-substituted nitroalkenes (Table 3, En- substituted pyrroles in the same catalytic conditions.
tries 1–10). Interestingly, strong electron-withdrawing
One-Pot Synthesis of 3-Alkylindoles
Table 3. Reaction of various aldehydes and nitroethane in the pres-
Indole and its derivatives are widely distributed in nature
and possess a diverse range of biological and pharmacologi-
cal activity. Therefore, the synthesis of 3-substituted indoles
has received much attention in modern organic synthesis.[24]
A Michael addition reaction between indole and nitroalk-
enes in the presence of various Lewis acids is an important
tool for the synthesis of 3-substituted indole derivatives. We
thought our reaction conditions would allow a one-pot syn-
thesis of nitroalkenes and a subsequent Michael addition
reaction because the Lewis acid was in the reaction me-
dium. To our delight, this one-pot reaction worked ef-
ficiently and gave desired products 4a and 4b in good yields
(Scheme 2).
ence of a combination of FeCl3 and piperidine.[a]
Scheme 2. Tandem nitroalkenes formation/Michael addition reac-
tion with indole.
One-Pot Synthesis of 3-Nitrochromenes
3-Nitrochromenes are another important class of hetero-
cyclic compounds owing to their biological activity and
their importance as precursors of flavonols, amines, and
other important targets. Normally it is prepared from sal-
icylaldehydes and pre-formed nitroalkenes in the presence
[a] Reaction conditions: Aldehyde (1 mmol), nitroethane (6 mmol),
piperidine (0.10 mmol), FeCl3 (0.10 mmol), toluene (1 mL), gentle
reflux. [b] Pure, isolated yield.
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of various organocatalysts and alumina.[25] We demon- to account for the formation of nitro olefins. Aldehyde 1
strated that this cooperative catalysis reaction condition reacts with piperidine, catalyzed by FeCl3 to give iminium
was well suited for the preparation of 3-nitrochromenes in ion intermediate 1Ј.[27] Lewis acid may also activate nitroal-
one pot through the sequential formation of nitroalkene/ kane 2 in combination with the organobase to generate re-
Michael addition/aldol condensation reactions, and af- active metalo aci-nitronate intermediate 2Ј.[28] The activated
forded good yields of 3-nitrochromenes 5a and 5b (Scheme electrophilic species 1Ј, and nucleophilic species 2Ј are then
3).
efficiently coupled to form a β-amino nitro derivative A.
Intermediate B is then converted into desired nitroolefin 3
by the elimination of piperidine through an EIcB-type
mechanism regenerating the catalysts (Scheme 5).
Scheme 3. Tandem nitroalkenes formation/Michael-aldol reaction
for 3-nitrochromenes.
One-Pot Synthesis of N-Arylpyrrole
Scheme 5. A plausible mechanism.
N-Arylpyrrole containing compounds exhibit a wide ar-
ray of biological activities and it is also an important build-
ing block for the synthesis of many complex molecules.[26]
With this in mind, we next turned our attention to develop
an efficient method for the synthesis of N-arylpyrroles by
using the present catalytic conditions. The one-pot genera-
tion of nitroalkenes, followed by addition of aromatic
amine and 1,3-diketone gave the corresponding substituted
N-arylpyrroles in good yield (Scheme 4). This present pro-
tocol is an improved method for the preparation of N-aryl-
pyrrole relative to our previous report.[23a]
Conclusions
In conclusion, we have developed a simple and practical
method for the one-pot synthesis of nitroalkenes catalyzed
by a cooperative catalysis system involving piperidine and
iron(III) chloride. To the best of our knowledge this is the
first report of nitroalkenes preparation involving coopera-
tive catalysis. This method has several advantages: (a) use
of inexpensive and environmentally friendly catalysts;
(b) simple work up and isolation of procedure; (c) tolerance
of many sensitive functional groups and molecules; (d) ap-
plicable to aromatic, heteroaromatic and aliphatic alde-
hydes; (e) applicable for large-scale synthesis; (f) useful to
design many tandem reactions. The above advantages and
experimental simplicity make this cooperative catalytic sys-
tem very attractive for the efficient preparation of nitroalk-
enes and their application towards the synthesis of complex
molecules in one pot.
Experimental Section
Representative Experimental Procedure for the Synthesis of (E)-(2-
Nitrovinyl)benzene (3a): Aldehyde 1a (106 mg, 1 mmol), nitroalkane
2a (366 mg, 6 mmol) and piperidine (8.5 mg, 0.1 mmol) were added
sequentially to a round-bottomed flask containing toluene (1 mL).
To this mixture anhydrous FeCl3 (16.2 mg, 0.1 mmol) was added,
and the mixture was heated to reflux slowly for 4 h. Progress of the
reaction was monitored by TLC. After completion of the reaction,
Scheme 4. One pot nitroalkenes/N-arylpyrroles synthesis.
Based on the experimental observations and other stud-
ies by using cooperative catalysis of secondary amines and
Lewis acids, we proposed a tentative mechanistic pathway the mixture was cooled to room temperature. The excess solvent
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Received: January 31, 2013
Published Online: ᭿
Eur. J. Org. Chem. 0000, 0–0
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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