Protic ionic liquids: A lucid, rational tool for synthesis of phthalazinediones, quinoxalines and benzopyrans

Article abstract of DOI:10.1007/s11164-015-2014-5

Protic ionic liquids (PILs), which are easily produced through the combination of a Bronsted acid and Bronsted base, such as [Mim]Ac and 1,4-diazabicyclo[ 2.2.2]octane (DABCO):AcOH:H₂O (1:1:3), were found to be lucid, tunable tool for synthesis

Full text of DOI:10.1007/s11164-015-2014-5

Res Chem Intermed
DOI 10.1007/s11164-015-2014-5
Protic ionic liquids: a lucid, rational tool for synthesis
of phthalazinediones, quinoxalines and benzopyrans
A. G. Mulik1 D. R. Chandam2 D. R. Patil2
•
•
•
P. P. Patil2 G. N. Mulik1 S. T. Salunkhe1
M. B. Deshmukh2
•
•
•
Received: 13 January 2015 / Accepted: 11 March 2015
Ó Springer Science+Business Media Dordrecht 2015
Abstract Protic ionic liquids (PILs), which are easily produced through the combi-
nation of a Brønsted acid and Brønsted base, such as [Mim]Ac and 1,4-diazabicy-
clo[2.2.2]octane (DABCO):AcOH:H2O (1:1:3), were found to be lucid, tunable tool for
synthesis of various heterocyclic motifs such as phthalazinediones, quinoxalines and
benzopyrans. These PILs were found to be efficient for synthesis of diverse heterocyclic
derivatives, along with demonstrating noteworthy aspects such as high yields, isolation
of pure products without column chromatography and recyclable reaction media.
Electronic supplementary material The online version of this article (doi:10.1007/s11164-015-2014-5)
contains supplementary material, which is available to authorized users.
&
A. G. Mulik
abhi_mulik1@rediffmail.com
1
2
PG Department of Chemistry, Balwant College, Vita 415311, Maharashtra, India
Heterocyclic Laboratory, Department of Chemistry, Shivaji University,
Kolhapur 416004, Maharashtra, India
123

A. G. Mulik et al.
Graphical abstract
Keywords Ionic liquids Á Phthalazinediones Á Benzopyran Á Quinoxaline
Introduction
To synthesize diverse range of organic molecules under a single catalytic umbrella
is a cornerstone of contemporary synthetic chemistry. With respect to this, protic
ionic liquids (PILs) were found to be a tunable solvent–catalyst system for various
organic transformations, such as synthesis of substituted 1,2,4-triazoles [1], the
Diels–Alder reaction [2], formation of diphenylmethyl ethers [3] and synthesis of
oxoisoindolines [4]. PILs, liquid acid–base salts that can readily be prepared and
recycled, perfectly answer this constraint. Proton transfer from the acid to the base
creates proton-donor as well as proton-acceptor sites, establishing hydrogen-bonded
networks that distinguish PILs from other ILs [5]. Having unanimous properties
such as negligible vapour pressure, thermal stability and reusability makes them an
impressive alternative for conventionally used toxic organic solvents [6].
Heterocyclic moieties such as phthalazinediones, quinoxalines and benzopyrans
have a wide range of applications (Fig. 1). Several derivatives of quinoxaline (e.g.-
HBY-097 and s-2720) display interesting activity against HIV, as non-nucleosidic
inhibitors of reverse transcriptase (RT) (Fig. 1) [7, 8]. In addition, they are well
known for their applications as dyes [9], electroluminescent material [10], organic
semiconductors [11, 12], building blocks for the synthesis of anion receptors [13],
cavitands [14, 15], dehydroannulenes [16], DNA cleaving agents [17, 18], and also
as pesticides [19]. Phthalazine derivatives are an important structural motif, as they
have been reported to possess anticonvulsant [20], cardiotonic [21] and vasorelaxant
[22] activities. Besides these, they are endowed with anti-inflammatory, analgesic,
antihypoxic and antipyretic properties [23]. Pyran framework are usually found in a
variety of important natural compounds including carbohydrates, alkaloids,
polyether antibiotics, pheromones and iridoids [24].
123

Protic ionic liquids: a lucid, rational tool for synthesis…
Cl
NH2
N
O
S
O
O
N
NH
O
N
N
Chloroquinoxaline sulfonamide (CQS)
NH
CO2H
CH3
O
N
N
Ar
Antihypoxic and Antipyretic agen
O
Cl
XK469
Fig. 1 Bioactive quinoxalines and phthalazinediones
CH
3
N
N+
+
N
H
O
O
H
N
O-
H3C
-
O
H C
3
1-Methyl imidazolium acetate [Mim]Ac
1,4-diaza bicyclo[2.2.2]octanium acetate
Fig. 2 Protic ionic liquids (PILs) used
Thus, synthesis of all these structural units under one catalytic umbrella of protic
ionic liquids has been demonstrated in this report.
1,4-diaza bicyclo[2.2.2]octanium acetate and 1-methylimidazolium acetate
([Mim]Ac) (Fig. 2) PILs were found to be a proficient solvent–catalyst system for
various one-pot organic transformations, depicted by the successful synthesis of
phthalazinediones, quinoxalines and benzopyrans.
Result and discussion
The first reaction we examined was one-pot, three-component syntheses of
phthalazinediones by condensation reactions of phthalhydrazide, malononitrile
and various aryl aldehydes. Very few catalytic systems, such as p-TSA in [bmim]Br
[25], triethylamine using ultrasound [26] and recently using mild basic ionic liquids
[27], have been reported to be effective for the synthesis of these vital moieties.
However, most of these systems suffer from drawbacks such as use of hazardous
organic solvents, longer reaction time, resolute toxicity of the catalyst, etc. Thus,
there is an immense need for the development of a newer synthetic route for their
synthesis.
To show the efficacy of PILs and in continuation of our work in ILs [28], we
initially investigated the synthesis of phthalazinediones by the reaction of
123

A. G. Mulik et al.
Table 1 Synthesis of 1H-pyrazolo[1,2-b]phthalazinediones promoted by different DABCO and Imida-
zolium salts of weak acids
Entry
Catalysta
Time (min)
Yield (%)b
1
DABCOc
55
80
2
DABCO–PhCOOH–3H2O
DABCO–Cinnamic acid–3H2O
DABCO–Hippuric acid–3H2O
DABCO–AcOH–3H2Od
DABCO–AcOH–5H2O
DABCO–AcOH–7H2O
First run
20
85
3
3 h
3 h
\5
15
15
4
00
5
95
6
70
7
40
70
8
\5e/10f
5/10
5/10
2 h
10
95e/90f
92/88
91/85
00
9
Second run
10
11
12
Third run
Acetic acid
1-methylimidazole-AcOH (1:1)
90
Reaction run at the 1.0 mmol scale with respect to each component of phthalhydrazide, malononitrile and
aldehyde
a
1.0 equiv used based on DABCO
b
Isolated yield
c
Reaction carried out at 80 °C using water as co-solvent and 20 mol % of DABCO
2.0 equivalent used based on DABCO
d
e
Yield and time when DABCO PIL is used
f
Yield and time when imidazolium PIL is used
phthalhydrazide, benzaldehyde and malononitrile using 1,4-diazabicyclo[2.2.2]oc-
tane (DABCO) in aqueous media at 80 °C, which afforded an 80 % yield within 1 h
(Table 1, entry 1). When the same reaction was carried out using salt of DABCO
and acetic acid with water in proportion (1:1:3) [41], respectively, it afforded a
95 % yield within 5 min (Scheme 1; Method A). Furthermore, the product was
analyzed without purifying with column chromatography and was found to be
correct. Water was deliberately added to the mixture to make it viscous. The
reaction was so rapid and efficient that as soon as the third component was added to
the reaction mixture, completion of reaction was confirmed within 5 min, affording
an excellent yield (95 %). Enthused with these astonishing results, we tried various
proportions of DABCO, acetic acid and water (Table 1, entries 5–7) and found that
1:1:3 was the most efficient and rapid proportion, possibly due to its viscosity.
Besides this, we also tried the salts of DABCO with various weak acids such as
cinnamic acid, hippuric acid, benzoic acid (Table 1, entries 2–4). Salt of DABCO
with benzoic acid showed efficacy over cinnamic acid and hippuric acid salts and
afforded an 80 % yield after 20 min.
With these optimized reaction conditions, to check the substrate scope of the
protocol, we reacted various aryl aldehydes with electron-donating and withdrawing
substituents. Interestingly, all the aldehydes participated well in the reaction and
123

Protic ionic liquids: a lucid, rational tool for synthesis…
O
N
+
-
H C
O
3
N
H
A- DABCO:AcOH:H
O (1:1:3)
2
O
O
O
O
Ar
NH
NH
CN
CN
N
N
+
Ar-CHO
+
CN
NH2
Solvent free, Room temperature
2
1
3
B- 1-methylimidazole:AcOH (1:1)
O
4a-j
+
-
N
N
H C
3
O
H
H C
3
Scheme 1 Synthesis of 1H-pyrazolo[1,2-b]phthalazinediones using ionic liquids a DABCO:AcOH:H2O
(1:1:3) and b 1-methylimidazole:AcOH as catalyst at ambient temperature under solvent-free conditions
there was no subsequent effect of the substituents on the reaction time and yield
(Table 2).
Encouraged by these results with DABCO-based PILs, we decided to pursue the
concerns for synthesis of phthalazinediones using imidazolium based PILs. Thus,
we carried out a model reaction of phthalhydrazide, malononitrile and benzaldehyde
(Scheme 1) using imidazolium salt with acetic acid [Mim]Ac obtained using
literature method [40]. To our surprise, like DABCO-based PILs, imidazolium-
based PIL [Mim]Ac also proved to be rapid and efficient catalyst, since the reaction
was completed within 10 min. In order to prove the catalytic activity of [Mim]Ac,
we also carried out the reaction in only acetic acid, which afforded no yield
(Table 1, entry 11).
With the optimistic reaction conditions in hand, we reacted a variety of
structurally diverse aromatic aldehydes to understand scope and versatility of
imidazolium-based, PILs-promoted synthesis of 1H-pyrazolo[1,2-b]phthalazine-
diones (Scheme 1). It was observed that all the aldehydes reacted with the same
efficiency attributing to yields and time (Table 2).
Recyclability and recovery of a catalyst is an important aspect from a commercial
point of view. Thus, we studied the recyclability of the DABCO–AcOH–H2O and
[Mim]Ac solvent–catalyst system. After completion of the reaction, 5 ml water was
added to the reaction mixture to separate the product, which was then filtered and
recrystallized. Removal of extra water from the filtrate under reduced pressure
recovered the catalyst system (95 %), which was washed with toluene and reused
for three times, showing no significant loss of activity (Table 1, entries 8–10;
Fig. 3).
Enthused with the above-mentioned gratifying results, we decided to circumvent
the concerns of synthesis of quinoxaline by using the same protic ionic liquid
‘DABCO–AcOH–H2O’. Although there are several methods that afford good yields
123

A. G. Mulik et al.
Table 2 Synthesis of 1H-pyrazolo[1,2-b]phthalazine-dione derivatives in the presence of A:
DABCO:CH3COOH:H2O (1:1:3) and B: [Mim]Ac PILs as catalyst under solvent-free conditions
Entry
Aldehydes
Time (min)
Yield (%)c
Melting point reported. m.p. (°C)/
Lit. m.p (°C) [Ref]
A
5
B
Aa
Bb
1
4a
4b
4c
10
95
90
276/(276–278) [25]
260–262/(259–261) [25]
266/(266–267) [26]
CHO
2
3
5
5
15
15
94
90
92
87
CHO
Cl
CHO
Cl
4
CHO
4d
8
15
87
85
269–270/(270–272) [25]
Cl
5
6
CHO
NO2
4e
4f
5
5
10
10
94
92
90
88
265/(265–266) [26]
CHO
268–270/(269–271) [27]
NO2
7
8
CHO
4g
4h
5
8
10
10
95
90
90
86
231/(230–232) [27]
NO2
263–265/(263–265) [26]
CHO
F
9
CHO
4i
4j
5
5
8
90
95
87
92
212 The product was
prepared for first time
OH
10
10
230–232 The product
was prepared for first time
CHO
N
Reaction at 1 mmol of phthalhydrazide, malononitrile and aryl aldehydes at RT (27 °C)
a
Synthesis using DABCO:AcOH:H2O in proportions of 1:1:3, respectively
b
Synthesis using 1-methylimidazole:AcOH in proportions of 1:1, respectively
c
Isolated yields
123

Protic ionic liquids: a lucid, rational tool for synthesis…
DABCO PILs
[Mim]Ac
100
80
60
40
20
0
Yields (%)
1
2
3
Reacꢀon Cycles
Fig. 3 Recyclability of DABCO:AcOH:H2O and 1-methylimidazolium acetate [Mim]Ac as catalyst
under solvent-free condition using a model reaction of phthalhydrazide, benzaldehyde and malononitrile
Table 3 Synthesis of quinoxaline using DABCO salts with weak acids as catalyst under solvent-free
conditions
Entry
Catalyst
Temperature (°C)
Time (min)
Yield (%)a
1
2
3
4
5
DABCO–AcOH–3H2O
R.T.
80 °C
R.T.
R.T.
R.T.
2 h
20
85
50
00
00
DABCO–AcOH–3H2O
20 min
1 h
DABCO–PhCOOH–3H2O
DABCO–Cinnamic acid–3H2O
DABCO–Hippuric acid–3H2O
2 h
2 h
Reaction carried out at 1 mmol of each component, i.e., orthophenylene diamine and benzil
a
Isolated yield
N
O
N H
N H
2
2
DABCO:AcOH:H
80
O (1:1:3)
2
0
+
C
R
N
O
7a-7l
C H
3
6a-6d
5a-5d
O
N H
N H
N H
N H
N H
2
2
2
2
2
O
2 N
H
3 C
N H
2
C l
O
5d
5b
5c
6b
C H
3
O
O
O
O
O
O
6c
6d
Scheme 2 Synthesis of quinoxaline derivatives using DABCO:AcOH:H2O (1:1:3) as a recyclable
solvent–catalyst system
123

A. G. Mulik et al.
Table 4 Synthesis of quinoxaline derivatives using ionic liquid DABCO:AcOH:3H2O as catalyst at
80 °C under solvent-free conditions
Melting Point
Time Yieldb
Reported. m.p.
(0C)/Lit. m.p (0C)
[Ref]
Entry
1,2-diamine
1,2-diketone
Product
(min)
(%)
N
N
NH2
NH2
O
O
1
20
90
126/126-127 [25]
7a
N
NH2
NH2
O
O
120-121/122-123
[30]
2
3
Cl
30
20
80
88
N
Cl
7b
N
NH2
O
O
O2N
187/187 [30]
N
NH2
O2N
7c
CH 3
CH3
N
N
O
NH2
NH2
4
5
20
25
86
87
145/142-143 [29]
O
CH3
CH3
7d
CH3
CH3
N
N
O
O
NH2
NH2
136/137 [25]
H3
C
H3C
CH3
CH3
7e
N
NH2
NH2
O
O
O
O
O
6
7
35
40
80
83
131/131-132 [30]
O
N
7f
N
N
NH2
O
O
O
O
O
122/122 [30]
Cl
O
Cl
NH2
7g
N
N
O
NH2
NH2
O
O
O
8
30
45
82
79
119/119-120 [30]
O
O
H3C
H3C
7h
N
O
NH2
NH2
O
O
O
O
174-175/174-176
[30]
O
9
N
O2N
O2N
7i
123

Protic ionic liquids: a lucid, rational tool for synthesis…
Table 4 continued
Melting Point
Reported. m.p.
(0C)/Lit. m.p (0C)
[Ref]
Time Yieldb
(min) (%)
Entry
1,2-diamine
1,2-diketone
Product
N
N
NH2
NH2
O
O
10
25
90
244/246 [31]
7j
N
N
NH2
O
O
240-242/240-242
[32]
11
12
40
20
82
82
O2N
NH2
O2N
7k
N
N
NH2
NH2
O
O
218/218-219 [32]
H3C
H3C
7l
Reaction carried out at 1 mmol of Each component i.e. 1,2-diamine and 1,2-diketone
a
Isolated yield
of quinoxaline, some of them suffer from drawbacks such as volatile organic
solvents, unsatisfactory product yields, critical product isolation procedures,
expensive and detrimental metal precursors, and harsh reaction conditions, which
limit their use under the aspect of environmentally benign processes [29–36].
Initially, o-phenylenediamine and benzil was reacted at room temperature using
DABCO acetic acid salt, but this was found to be time consuming and sluggish
(Table 3, entry 1). Thus, it was decided to elevate the temperature and we carried
out the same reaction at 80 °C, and to our surprise a solid white mass of product was
obtained after 20 min. In addition, we also tried the protocol with various salts of
DABCO with weak acids such as benzoic acid, cinnamic and hippuric acid, but to
no benefit regarding yield and time (Table 3, entries 2–5).
Here again, we investigated the substrate scope by reacting various 1,2-diamine
with substituted 1,2-diketones (Scheme 2), and the results are listed in Table 4. All
the derivatives were obtained in excellent yields, which depicts our protocol as
versatile and convenient for the synthesis of a wide array of quinoxaline derivatives.
We can’t explain the exact mechanism, but a probable mechanism for the
formation of quinoxaline and phthalazinedione derivatives has been shown in Fig. 4.
Furthermore, using the same ionic liquid DABCO:AcOH:H2O (1:1:3), one-pot,
three-component synthesis of various benzopyran derivatives was performed
(Scheme 3). The reaction proceeds smoothly, affording various derivatives in good
to excellent yields (Table 5).
123

A. G. Mulik et al.
N
-
-
CH CO
CH CO
3
N
3
2
2
N +
N +
H
H
CN
CN
O
O
NH
2
O
+
H
Ar
NH
2
H
N +
-H
O
H
2
-
CH CO
N
H
N
3
2
O H
O H
NC
NC
O
O
N
H
O
O
N H
Ar
2
Ar
+
NH
NH
N
N
CN
N
N
O
O
NH
N
N
CN
H
O
N
NH
N
Ar
CN
Ar
O
Fig. 4 Suggested mechanism for synthesis of 1H-pyrazolo[1,2-b]phthalazinediones and quinoxalines
using DABCO PIL
O
O
DABCO:AcOH:H2O
RT
CN
CN
CN
+
Ar-CHO
+
O
O
NH2
10
9
8
11a-g
Scheme 3 Synthesis of tetrahydrobenzo[b]pyran derivatives using DABCO:AcOH:H2O (1:1:3) as a
recyclable solvent–catalyst system
From the results, it is clearly seen that use of PIL as a catalyst offers a sustainable
and efficient alternative when the protocol involves hydrophobic substrates like
phthalhydrazide and 1,2-diketones.
In conclusion, via this report we suggest that many useful organic structural
motifs can be synthesized using a single solvent–catalyst system like PILs. We have
successfully demonstrated the use of protic DABCO and imidazolium-based ILs as
solvent–catalyst systems for synthesis of phthalazinediones, quinoxalines and
benzopyrans. This novel methodology is endowed with fascinating aspects, such as
fast conversions within the shortest reaction time, a convenient workup procedure
including mere filtration, averting the use of tedious column chromatography,
improved yields and high purity of desired products. With these results in hand, we
can say PILs act as an efficient dual solvent–catalyst system for various organic
transformations.
These inexpensive and readily prepared PILs will provide new opportunities for
the study of green synthesis, and green solvents and catalysts, which will upsurge
the catalytic tools of the current scientific community.
123

Protic ionic liquids: a lucid, rational tool for synthesis…
Table 5 Synthesis of tetrahydrobenzo[b]pyran derivatives using ionic liquid DABCO:AcOH:3H2O as a
catalyst at room temperature
Entry
Aldehydes
Yielda
Time (min)
Melting point reported. m.p.
(°C)/l m.p (°C) [Ref]
1
2
3
4
5
6
7
C6H5
11a
11b
11c
11d
11e
11f
90
91
90
87
85
84
86
10
8
232/233–234 [37]
4-OCH3C6H4
4-CH3C6H4
4-NO2C6H4
4-OHC6H4
3-NO2C6H4
4-ClC6H4
196-198/197–199 [38]
210/209–211 [38]
8
15
15
15
15
174-176/175–176 [38]
215/214-215 [39]
201-203/201–205 [38]
212-214/215–218 [37]
11g
Reaction carried out at 1 mmol of each component
a
Isolated yield
Acknowledgments We are thankful to the Department of Chemistry, Shivaji University, Kolhapur for
providing IR, 1H and 13C NMR Spectral analytical facilities and Balwant College, Vita for providing the
laboratory.
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