Protic ionic liquids as catalysts for a three-component coupling/hydroarylation/dehydrogenation tandem reaction

Article abstract of DOI:10.1515/znb-2018-0084

Protic ionic liquids with nitrate anions were used as solvents and catalysts for a three-component oxidative dehydrogenation tandem reaction via the coupling and hydroarylation of benzaldehyde, aniline, and phenylacetylene to a quinoline derivative. The reaction was supported by air and microwave irradiation. The presence of nitrate as counter anion in the protic ionic liquids was essential for the reaction.

Full text of DOI:10.1515/znb-2018-0084

Z. Naturforsch. 2018; 73(7)b: 515–519
Maren Muntzeck and René Wilhelm*
Protic ionic liquids as catalysts for a three-
component coupling/hydroarylation/
dehydrogenation tandem reaction
https://doi.org/10.1515/znb-2018-0084
ILs can be divided into different subclasses. In addition
to the more common aprotic ILs, also protic ionic liquids
(PILs) have been reported. In fact, the first recognized IL
was a PIL. Ethylammonium nitrate (EAN) was described
in 1914 [11]. PILs are prepared via the stoichiometric neu-
tralization reaction of Brønsted acids and Brønsted bases.
In general, they possess a relatively weakly acidic proton
at the cation. They should not be mistaken with Brønsted
acidic ILs. The latter possess typically an acidic anion like
HSO4 with an aprotic cation or incorporate in the aprotic
cation, for example, a sulfonic acid moiety. The field of
Received May 10, 2018; accepted May 14, 2018
Abstract: Protic ionic liquids with nitrate anions were
used as solvents and catalysts for a three-component oxi-
dative dehydrogenation tandem reaction via the coupling
and hydroarylation of benzaldehyde, aniline, and phenyl-
acetylene to a quinoline derivative. The reaction was sup-
ported by air and microwave irradiation. The presence of
nitrate as counter anion in the protic ionic liquids was
essential for the reaction.
Keywords: ammonium salts; C–H activation; multicompo- Brønsted and PILs has been reviewed [12–14]. PILs can be
nent reaction; oxidation; protic ionic liquids.
directly prepared and their pH tuned by using the appro-
priate Brønsted acid–base combinations. This would be
beneficial for many organic reactions that are catalyzed by
a proton transfer or via hydrogen-bond formation. Hence,
it is remarkable that there are fewer examples of PIL sol-
vents applied for organic reactions compared to aprotic
ILs in the literature.
1 Introduction
Ionic liquids (ILs) have received increased attention in
recent years as novel solvents for reactions and electro-
chemical processes [1]. Several of these ILs, which are salts
that melt below 100°C, can be considered to be “green sol-
vents” since they can be efficiently recovered [2]. The com-
monly used ILs in the 1960s were composed of organic
cations and chloroaluminate anions [3]. Since AlCl3 was
present in these liquids, the latter were also used as cata-
lysts in Lewis acid-catalyzed reactions [3]. However, they
are also highly moisture sensitive. Many of the ILs devel-
oped in the 1990s incorporated relatively inert lipophilic
anions and have been used as solvents for catalytic reac-
tions [4–9]. However, it is also known that these ILs are
capable of catalyzing reactions either in substoichiomet-
ric amounts or as reaction media because of their stronger
interactions with the reaction partners compared to some
standard solvents [10].
PILs have been mainly applied for reactions catalyzed
by protons. Hence, PILs have been used for esterification
reactions [15–17] and aldol condensation [18]. A mixture
of ammonium acetate and Bu-pyridinium nitrate was
employed to catalyze a Knoevenagel condensation [19]. In
addition, a Knoevenagel reaction could be carried out in
EAN as solvent, which resulted in a significant improve-
ment compared to the reaction with standard aprotic ILs
[20]. EAN has also been used as a Brønsted acid catalyst in
common solvents for the synthesis of imines [21]. EAN and
+
N-methyl-2-pyrrolidone hydrogensulfate [NMPH] [HSO4]−
were applied as a solvent and catalyst system for a three-
component condensation reaction [22, 23].
The specific utilization of NO3 counter anions in ILs
has been limited so far. Aprotic ILs have been used with
NO2BF4 for the nitration of arenes [24]. In aprotic ILs, a
mixture of HNO3 and Ac2O gave nitroarenes in high yields
[25–27]. A Brønsted acidic IL was reported for the nitra-
tion of arenes with HNO3 [28]. Aprotic ILs with a nitrate
counterion supported the oxidative halogenation or oxi-
dation of toluene to benzoic acid [29]. Brønsted acidic ILs
have been reported to oxidize benzylic alcohols to alde-
hydes in the presence of sodium nitrate because of the
oxidative behavior of the in situ formed HNO3 [30]. This
*Corresponding author: René Wilhelm, Department of Chemistry,
University of Paderborn, Warburger Straße 100, 33098 Paderborn,
Germany, Fax: +49 5251 603 245,
E-mail: rene.wilhelm@uni-paderborn.de
Maren Muntzeck: Department of Chemistry, University of Paderborn,
Warburger Straße 100, 33098 Paderborn, Germany
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516ꢀ ꢀM. Muntzeck and R. Wilhelm: PILs as catalysts for oxidative dehydrogenation tandem reaction
mixture could also be used to oxidize Baylis–Hillman the rare earth salt Yb(OTf)3 has been published [62]. Yet,
adducts [31]. Pyridinium nitrate catalyzed this oxidation investigations with pure PILs have not been reported for
+
under an oxygen atmosphere [32]. [HMIM] [NO3]− served this reaction so far.
as a catalyst in an oxidative aromatic chlorination of
mesitylene with hydrochloric acid [33] and the oxidative
formation of oximes from benzyl alcohols under micro-
wave conditions [34].
2 Results and discussion
Based on our work with ILs [35–45] and in three-
component
coupling/hydroarylation/dehydrogenation First, PILs (Fig. 1) were prepared by the neutralization
tandem reactions [45], we present here an investigation of an amine with nitric acid or acetic acid in an aqueous
of the catalytic behavior of nitrate-based acidic and oxi- solution at 0°C according to the procedures in the litera-
dative PILs in the synthesis of a quinoline derivative ture [13]. Thereafter, the water was removed. Contrary to
(Scheme 1). Quinoline derivatives are important structural aprotic ILs, the prepared PIL systems are in equilibrium,
moieties in biologically active natural products and often and in addition to the protonated amine, small amounts
also starting materials for the chemical and pharmaceu- of neutral amine and acid are present depending on the
tical industry [46–48]. The standard method for creating strength of the applied Brønsted base and acid. Hence, in
quinolines is the Friedländer reaction, which is based the nitrate systems, a small amount of free HNO3 can be
on an aldol condensation of 2-aminobenzaldehydes with present. The latter is known to be an oxidative agent. The
ketones. Because of their instability, these precursors are prepared PILs were EAN 1 [63], which is a clear colorless
generated in situ by the reduction of 2-nitrobenzaldehyde liquid, and diethylammonium nitrate (DEAN) 2[64], which
derivatives, which are not readily available [49–52]. Hence, forms a white gel-like solid. Pyrrolidinium nitrate (PyrN) 3
the development of different methods of quinoline syn- [65], piperidinium nitrate (PipN) 4 [66], und N-methylimi-
thesis has been attracting increasing attention recently. In dazolium nitrate (MethN) 5 [67] are clear yellow liquids. In
addition to methods based on the use of transition-metal addition to the nitrate-based PILs, also diethylammonium
catalysts [53–57], also some examples using different iron acetate (DEAA) 6 [68] and pyrrolidinium acetate (PyrA) 7
salt-based catalysts have been reported [45, 58–61]. The [69] were prepared in order to evaluate the influence of
first example of this reaction with copper(I) complexes as another anion.
Lewis acid was published by Takai et al. in 2007 [54]. One
The model reaction was the transformation of benzal-
+
report with the aprotic [bmim] [BF4]− in the presence of dehyde 8, aniline 9, and phenylacetylene 10 to 2,4-diphe-
nylquinoline 11 (Scheme 1). The results with different
conditions and PILs are summarized in Table 1.
First the reaction was investigated with 1 equiv.
O
EAN 1. It was decided from the beginning to explore the
NH2
PIL
reaction under microwave irradiation since ILs are very
+
+
suited to be heated under such condition. However, after
20 min, no desired product could be found (Table 1, Entry
1). However, when EAN 1 was prepared in the molar ratio
of 1:1.1 from ethylamine and nitric acid and applied in the
reaction, a yield of 33% was achieved (Table 1, Entry 2).
N
11
8
9
10
Scheme 1:ꢀReaction of benzaldehyde 8, aniline 9, and phenylacety-
lene 10 with 2,4-phenylquinoline 11.
NO3
NO3
NO3
NO3
N
NH3
N
N
H2
PipN 4
H2
H2
PyrN 3
EAN 1
DEAN 2
N
NH NO3
AcO
AcO
N
N
H2
H2
PyrA 7
MethN 5
DEAA 6
Fig. 1:ꢀApplied PILs.
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M. Muntzeck and R. Wilhelm: PILs as catalysts for oxidative dehydrogenation tandem reactionꢂ
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Table 1:ꢀReaction of benzaldehyde 8, aniline 9, and phenylacety-
lene 10 to 2,4-phenylquinoline 11 under microwave conditions
(150 W).
Entry 16). The latter results show that microwave heating
is favorable for the presented system. A control reac-
+
tion with a mixture of [BMIM] Cl− and HNO3, as well as
in water with 1 equiv. HNO3, afforded no product (Table
1, Entries 17 and 18). The PIL was needed to obtain 11.
The important influence of oxygen was found when the
reaction was performed under an oxygen atmosphere.
The yield increased from 20% under normal heating in
air to 29% (Table 1, Entry 19). Finally, when the amount
of added HNO3 to DEAN was increased to 0.5 equiv.,
the yield decreased to 19% (Table 1, Entry 20). Only a
slight excess of HNO3 appears to be the optimum for the
reaction.
A proposed mechanism is depicted in Scheme 2. ILs
are known to contain a certain amount of water, and the
course of the reaction may therefore vary depending on
the amount of water present in the system [1].
However, taking into consideration that the first step
of the reaction is the formation of an imine species T
(Scheme 2), water is formed during the reaction. Since the
imine was present after the reaction and could also be iso-
lated, it can be concluded that the imine formation is not
the limiting step in the reaction. The ammonium cation
coordinates to this imine T and activates it via a hydro-
gen bond. Phenylacetylene 10 is then added to the adduct
to form the intermediate X. Evidence for the ammonium
cation–π interactions with alkynes has been reported
recently [70]. The propargylamine Y is formed through the
addition of acetylene 10 to the imine T. It forms the dihy-
droquinoline intermediate Z by performing an intermo-
lecular hydroarylation. Z is oxidized by the nitrate, which
is reduced to nitrite in this process. The nitrite can be
regenerated by oxygen in the air or in an enriched oxygen
atmosphere.
Entryaꢁ PIL (ratio of
amine to acidb)
ꢁ
Equiv. PILꢁ T (°C)ꢁ t (min)ꢁ Yield (%)c
1
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
EAN (1:1)
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
1ꢃ
1ꢃ
110ꢃ 20
110ꢃ 20
110ꢃ 20
110ꢃ 20
110ꢃ 20
130ꢃ 20
110ꢃ 40
110ꢃ 20
110ꢃ 20
110ꢃ 20
110ꢃ 20
110ꢃ 20
110ꢃ 20
110ꢃ 20
110ꢃ 24 h
110ꢃ 24 h
110ꢃ 20
110ꢃ 20
110ꢃ 24 h
110ꢃ 20
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
–
33
10
–
2
EAN (1:1.1)
EAN (1:1.1)
DEAN (1:1)
3
0.1ꢃ
1ꢃ
4
5
DEAN (1:1.1)
DEAN (1:1.1)
DEAN (1:1.1)
PyrN (1:1.1)
PipN (1:1.1)
MethN (1:1.1)
DEAA (1:1.1)
DEAA (1:1.5)
PyrA (1:1.1)
PyrA (1:1.1)
EAN (1:1.1)
1ꢃ
40
42
26
16
17
–
6
1ꢃ
7
1ꢃ
8
1ꢃ
9
1ꢃ
10
11
12
13
14
15c
16c
17
18
19d,e
20
1ꢃ
1ꢃ
–
1ꢃ
–
1ꢃ
–
1ꢃ
–
1ꢃ
11
20
–
DEAN (1:1.1)
1ꢃ
+
[Bmim] Cl−/HNO3ꢃ
1ꢃ
HNO3(aq)
DEAN (1:1.1)
DEAN (1:1.5)
ꢃ
ꢃ
ꢃ
1ꢃ
–
29
19
1ꢃ
1ꢃ
aReaction conditions: 1 equiv. benzaldehyde 8, 1.05 equiv. aniline
9, 1.5 equiv. phenylacetylene 10, 150 W microwave; bHNO3 or AcOH;
cisolated yield; dunder normal heating; eunder O2 atmosphere.
Obviously, the reaction benefits from a small excess of
free nitric acid. When a smaller amount of 0.1 equiv. was
used in this mixture, the yield decreased to 10% (Table
1, Entry 3). Thereafter, DEAN 2 was evaluated (Table 1,
Entry 4). As with EAN 1, only with a slight excess of nitric
acid could the desired product be isolated. However, the
yield improved to 40% (Table 1, Entry 5) compared to EAN
1. A small increase of the temperature to 130°C resulted
in a similar yield of 42% (Table 1, Entry 6). An extension
of the reaction time to 40 min decreased the yield to 26% 3 Conclusion
(Table 1, Entry 7) as a result of the decomposition of the
product. Next, the other PILs were applied in the reaction In this work, the influence of PILs based on nitrate
under the optimized conditions. PyrN 3 gave the product counter anions in the oxidative tandem and hydroaryla-
in 16% yield (Table 1, Entry 8), and PipN 4 resulted in an tion reaction of benzaldehyde 8, aniline 9, and phenyla-
isolated yield of 17% (Table 1, Entry 9). The aromatic PIL cetylene 10 was investigated. It was possible to use PILs
MethN 5 did not give the desired product (Table 1, Entry as catalysts and solvents. Compared to other ILs, higher
10). In order to investigate the importance of the counter yields were obtained. The reaction could be performed
anion, DEAA 6 and PyrA 7 were also applied in reac- under normal atmosphere. The PIL is a hydrogen-bond-
tions with different excesses of acetic acid. In all cases, activating catalyst and oxidant while atmospheric oxygen
no product was obtained (Table 1, Entries 11–14). There- is a co-oxidant. Although the maximum yield was a mod-
after, the influence of standard heating was evaluated. erate 42%, this study will allow the development of more
With EAN 1, a yield of 11% was observed after 24 h (Table reactions involving an oxidation step with PILs as cata-
1, Entry 15), while DEAN 2 gave a yield of 20% (Table 1, lysts and solvents.
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518ꢀ ꢀM. Muntzeck and R. Wilhelm: PILs as catalysts for oxidative dehydrogenation tandem reaction
O
H2N
+
10
8
9
+ H2O
N
R2NH2
T
N
X
R2NH2
N
R2NH2
Y
O2
HNO3
HNO2
+ H2O
N
[O]
N
Z
11
Scheme 2:ꢀProposed mechanism.
were added to a 10-mL flask under an atmosphere of air.
The reaction mixture was stirred at 110°C for 20 min in a
microwave oven (150 W). Afterward, the reaction mixture
was cooled to room temperature. Purification was per-
formed by flash chromatography (petroleum ether/
4 Experimental section
All chemicals were commercially available and used
without further purification. The solvents were purified
prior to use by standard procedures. Thin-layer chroma-
tography was performed on plates from Merck (Silica gel
60, F254), and the substances were detected under UV
light at 254 nm. NMR spectra were recorded at 30°C on a
Bruker Avance 500 instrument (1H: 500 MHz; 13C: 125 MHz)
referenced to the residual proton or carbon signals of the
deuterated solvent (1H and 13C NMR). The NMR signals are
reported as ppm relative to TMS. Compounds 1 [63], 2 [64],
3 [65], 4 [66], 5 [67], 6 [68], and 7 [69] were synthesized
according to literature procedures.
1
AcOEt 20:1) to afford 11. H NMR (CDCl3, 500 MHz, ppm):
δꢀ=ꢀ8.31–8.29 (d, Jꢀ=ꢀ10 Hz, 1H), 8.25–8.24 (m, 2H), 7.95–7.93
(d, Jꢀ=ꢀ10 Hz, 1H), 7.86 (s, 1H), 7.78–7.75 (m, 1H), 7.60–7.48
13
(m, 9H). C NMR (CDCl3, 125 MHz, ppm): δꢀ=ꢀ156.8, 149.1,
148.8, 139.6, 138.4, 130.1, 129.5, 129.4, 129.3, 128.8, 128.5,
128.4, 127.6, 126.3, 125.8, 125.6, 119.3. The spectral data of the
product were consistent with their literature values [58].
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