Pentafluorophenylammonium triflate: A highly efficient catalyst for the synthesis of quinoxaline derivatives in water

Article abstract of DOI:10.1016/j.crci.2013.11.009

A simple, inexpensive, environmentally friendly and efficient route for the rapid and efficient synthesis of quinoxaline derivatives using pentafluorophenylammonium triflate (PFPAT) as a catalyst is described. Various quinoxaline derivatives were synthesi

Full text of DOI:10.1016/j.crci.2013.11.009

C. R. Chimie 17 (2014) 1023–1027
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Full paper/M e´ moire
Pentafluorophenylammonium triflate: A highly efficient
catalyst for the synthesis of quinoxaline derivatives in water
Samad Khaksar *, Hanieh Radpeyma
Department of Chemistry, Ayatollah Amoli Branch, Islamic Azad University, Amol, Iran
A R T I C L E I N F O
A B S T R A C T
Article history:
A simple, inexpensive, environmentally friendly and efficient route for the rapid and
efficient synthesis of quinoxaline derivatives using pentafluorophenylammonium triflate
Received 24 August 2013
Accepted after revision 20 November 2013
Available online 15 July 2014
(PFPAT) as a catalyst is described. Various quinoxaline derivatives were synthesized in
good to excellent yields. The preparation of PFPAT catalyst from simple and readily
available starting materials makes this method more affordable.
Keywords:
ß 2013 Acad e´ mie des sciences. Published by Elsevier Masson SAS. All rights reserved.
Quinoxaline
Organocatalyst
Heterocyclic
Cyclisation
1
. Introduction
agents and many other applications are well documented
[13–18]. In particular, quinoxaline drugs, such as clofazi-
The use of water as a solvent for organic reactions has
mine, echinomycin, leromycin and actinomycin are some
of the leading ones on the market with diverse functio-
nalization around the quinoxaline motif [19–24]. In view
of the great importance of quinoxaline derivatives, in
recent years, efforts have been made in developing new
methodologies for the synthesis of these compounds [25].
The condensation of aryl 1,2-diamines with 1,2-dicarbonyl
compounds represents an important existing procedure
for the synthesis of 2,3-disubstituted quinoxalines.
been important since the pioneering studies of Diels–Alder
reactions by Breslow [1]. There has been increasing
recognition that organic reactions can proceed well in
aqueous media and offer advantages over those occurring
in organic solvents [2]. Water has interesting physiochem-
ical properties, which include high thermal capacity,
hydrophobic association, high polarity, and hydrogen-
bonding ability [3–7]. Due to these properties, the use of
water as a reaction medium for clean catalytic transforma-
tions would have profound effects on reaction rates and
product selectivity. Quinoxaline derivatives have attracted
much attention owing to their biological activities. They
Generally, this condensation has been carried out under
reflux conditions in ethanol or acetic acid [26]. In recent
years, several new efficient methods have been developed,
including the use of
liquids [28], molecular iodine [21], heteropolyacid [29],
cellulose sulfuric acid [30], Zn[( )proline] [31], hypervalent
b-cyclodextrin (b-CD) [27], ionic
display
a broad range of biological, medicinal, and
pharmacological properties and are constituents of anti-
viral, antibacterial, anti-inflammatory, and antiprotozoal
drugs and as kinase inhibitors [8–12]. Their utility as dyes,
electroluminescent materials, organic semiconductors,
cavitands, chemically controllable switches, DNA cleaving
L
iodine(III) sulfonate in PEG [32], polyaniline-sulfate salt
[33], DABCO [34], CAN [35], fluorinated alcohols [36] and
(NH
4
)
6
Mo
7
O
24Á4 H
2
O [37]. These methods show varying
degree of successes as well as limitations, such as harsh
reaction conditions, expensive and detrimental metal
reagents, tedious work-up, low product yields, long
reaction times, and co-occurrence of several side products.
Therefore, a simple, efficient method for quinoxaline
synthesis remains an attractive goal. Recently Zhang
*
Corresponding author.
E-mail addresses: S.khaksar@iauamol.ac.ir,
samadkhaksar@yahoo.com (S. Khaksar).
1
631-0748/$ – see front matter ß 2013 Acad e´ mie des sciences. Published by Elsevier Masson SAS. All rights reserved.
http://dx.doi.org/10.1016/j.crci.2013.11.009

1
024
S. Khaksar, H. Radpeyma / C. R. Chimie 17 (2014) 1023–1027
Table 1
et al. have found that gallium(III) triflate can catalyze
successfully the reactions of phenylene-1,2-diamines and
Effects of the amount of catalyst PFPAT used and of the solvent on the
formation of 3.
1
,2-diketones under mild conditions [38]. This method was
Entry
PFPAT amount
Condition/
solvent
Time
suitable for a wide range of substrates, and especially for
functionalized phenylene-1,2-diamines. Yadav et al. have
reported a one-pot synthesis of phenylene-1,2-diamine
derivatives by cyclocondensation of arene-1,2-diamines
with 1,2-dicarbonyls using a catalytic amount of bis-
muth(III) triflate [39]. The use of water as a solvent makes
this procedure attractive and environmentally benign.
However, realizing the fact that metal-free organocatalysis
has drawn considerable interest from chemists, and that
metal-free homogeneous catalysis is advantageous for
designing suitable drugs devoid of any metal content, it
would be desirable to develop this reaction using metal-
free Lewis acid catalysts [40].
(mol%)
(h)/yield
1
2
3
4
5
6
7
8
9
0
5
rt/H
rt/H
rt/H
2
O
2
O
2
O
6/30
5/80
1/95
5/10
5/20
5/60
5/10
10/10
1/95
10
10
10
10
10
10
20
2 2
rt/CH Cl
rt/THF
rt/ethanol
rt/toluene
rt/diethyl ether
2
rt/H O
product 3 in 95% yield (Table 1, entry 3). Increasing either
the amount of catalyst and/or prolonging the reaction time
did not improve the yield (Table 1, entry 9), while
reducing these factors led to a reduction in the product
yield (Table 1, entry 2). Further studies confirmed that
10 mol% of PFPAT was optimum for this reaction and gave
a product in 95% yield in just 1 h (Table 1, entry 3). The
reaction was also examined in solvents such as toluene,
Catalysis with small organic molecules, where an
inorganic element is not part of the active principle, has
become a highly dynamic area in chemical research [41].
Such catalysts have several important advantages, since
they are usually robust, highly reactive, eco-friendly,
inexpensive, readily available, and non-toxic [42–44]. As
an inexpensive and commercially available organocatalyst,
+
–
pentafluorophenylammonium triflate (C
6
F
5
N H
3
*TfO ;
2 2
THF, CH Cl , ethanol, and diethyl ether. When the reaction
PFPAT) has received increasing attention as a water-
tolerant Brønsted acid catalyst for organic synthesis
demonstrating highly chemo-, regio- and stereoselective
results [45–47]. Due to the current challenges for
developing environmentally benign synthetic processes
and in continuation of our interest in the application of
new organocatalysts for various organic transformations
was carried out in the presence of PFPAT in water, the
expected product (3a) was obtained in high yield (95%) and
with better reaction times compared with other organic
solvents (Table 1, entries 3–8). With non-polar diethyl
ether and toluene, the desired adduct was not produced.
Moreover, when the reaction was carried out in water, the
solid product was separated at the end of the reaction.
At the beginning of the reaction, the reagents itself
were dissolved completely in the medium to form a
homogeneous mixture (Fig. 1a), but near the completion of
the reaction, the system became a suspension, and the
product precipitated at the end of the reaction (Fig. 1b).
The products were obtained through simple filtering,
and recrystallized from hot ethanol to afford pure
products. The corresponding functionalized quinoxa-
lines-scaffolds 3a, shown in Table 1 and confirmed by
NMR measurements, were obtained in good yield (95%).
Further experiments revealed that a similar procedure
is applicable for the preparation of a wide range of
[
48–53], we report an efficient route for the synthesis of
quinoxaline derivatives using pentafluorophenylammo-
nium triflate (PFPAT) as a catalyst (Scheme 1).
2
. Results and discussion
In order to optimize the reaction conditions, we chose
the condensation of the reaction of benzil (1 mmol) with o-
phenylenediamine catalyzed by PFPAT under different
conditions both in the absence and in the presence of
PFPAT; the results are given in Table 1. It is noteworthy
that in the absence of the catalyst, the reaction was rather
sluggish in water and resulted in very low yields (20–35%)
of quinoxalines, even after long reaction times (Table 1,
entry 1). Then, the effects of temperature, the amount of
catalyst, and the reaction time on the yield of the product
were examined. Reaction at room temperature in water in
the presence of 30 mg (10 mol%) PFPAT afforded the
Ph
Ph
O
O
NH2
NH2
Ph
Ph
N
N
PFPAT (10 mol %)
+
R
H O, r.t, 1 h
2
R
1
2
F
F
3a-p
H
N
-
+
PFPAT: CF SO
H
3
F
3
H
F
F
Scheme 1. Synthesis of 2,3-disubstituted quinoxalines derivatives in
Fig. 1. (Color online.) (a) Homogeneous mixture during the reaction, and
water.
(b) at the end of the reaction; the product has precipitated.

S. Khaksar, H. Radpeyma / C. R. Chimie 17 (2014) 1023–1027
1025
compounds analogous to adduct 3 (Table 2). In order to
evaluate the efficiency of this methodology, various arene-
Similarly, several 1,2-dicarbonyl compounds, such as furil,
1,2-di(4-chlorophenyl)ethanedione, phenylglyoxal, ninhy-
drin and acenaphthene-1,2-quinone also reacted rapidly
with 1,2-diamines to produce a variety of quinoxaline
derivatives (Table 2, entries 8–19). In all cases, the
reactions proceeded rapidly at room temperature with
1
,2-diamines, such as mono- and disubstituted amines,
reacted efficiently with 1,2-dicarbonyl compounds to
give the corresponding 2,3-disubstituted quinoxalines
(
Table 2). The results displayed in Table 2 show that
1
electron-donating groups at the phenyl ring of 1,2-diamine
favored the formation of product (Table 2, entries 2 and 3)
to give quantitative yields. In contrast, electron-with-
drawing groups, such as chloro- and bromo- gave slightly
high efficiency. The products were characterized by H and
1
3
C NMR, IR spectroscopic data and also by comparison
with authentic samples. This method not only affords the
products in excellent yields, but also avoids the problems
associated with catalyst cost, handling, safety, and pollu-
tion. One of the major advantages of this protocol is the
isolation and purification of the products, which have been
lower yields (85–87%) (Table 1, entries
4 and 5).
Interestingly, 2,3-diaminopyridine underwent smooth
coupling with benzil to afford the corresponding pyr-
ido[2,3-b]pyrazine 3f in 80% yield (Table 2, entry 6).
Table 2
Quinoxaline derivatives from the reaction of 1,2-diamines and 1,2-diketones catalyzed by PFPAT.
Entry
1
Dicarbonyls
Diamines
Product
Yield (%)
95
Entry
11
Dicarbonyls
Diamines
Product
Yield (%)
90
3a
3k
NH2
NH2
O
O
O
O
O
O
NH2
NH2
O
O
2
3
4
5
3b
3c
3d
3e
95
92
85
87
12
13
14
15
3l
95
85
80
90
Me
NH2
NH2
Me
NH2
NH2
O
O
Me
Me
NH2
NH2
3m
3n
3o
O
Cl
NH2
O
O
NH2
Cl
Br
NH2
NH2
O
NH2
O
O
N
NH2
NH2
NH2
NH2
Cl
O
O
O
O
NH2
Cl
Cl
6
7
3f
80
80
16
17
3p
3q
85
90
NH2
NH
Me
NH2
NH2
O
O
O
O
N
2
Cl
3g
NH2
NH2
NH2
NH2
O
O
O
H O.H
O
2
8
9
3h
3i
95
95
90
18
19
3r
3s
85
90
O
O
NH2
NH2
NH2
NH2
O
O
N
N
O
O
H O.H
2
Me
Cl
NH2
NH2
O
O
O
NH2
NH
O.H O
2
O
2
O
O
1
0
3j
O
O
NH2
NH2
O
O

1
026
S. Khaksar, H. Radpeyma / C. R. Chimie 17 (2014) 1023–1027
Table 3
Comparison the results of the synthesis of 2,3-diphenylquinoxaline using different catalysts.
Entry
Catalyst
Time
Yield (%)
Temperature (8C)
Reference
1
2
3
4
5
6
montmorillonite K10
2.5 h
35 min
24 h
100
95
98
80
92
99
95
rt
[54]
I
2
/DMSO
rt
[20]
2+
Zn -K10-clay (clayzic)
cellulose sulfuric acid
Keggin type heteropolyacids
MeOH/AcOH
rt
[55]
2.30 h
1 h
rt
[30]
rt
[29]
5 min
1 h
160
rt
[56]
1
2
PFPAT
This work
H
N
PFPAT
.
.
HO
R
R
R
O
O
H N
2
R
R
N
N
+
R
HO
-
2H O
2
H N
N
H
2
PFPAT
Scheme 2. Proposed mechanism for the synthesis of quinoxaline.
achieved by simple filtration and crystallization of the
crude products.
ꢀ
ꢀ
ꢀ
ꢀ
the product is easy to isolate/purify;
there are no side reactions;
handling and processing are cheap and simple;
quinoxaline derivatives are produced through an envir-
onmentally benign process.
The efficiency of various catalysts in the synthesis of
quinoxalines derivatives has been compared in Table 3.
The best yield and the shorter reaction time are attributed
to the high efficiency of the PFPAT organocatalyst.
A plausible reaction mechanism and the role of PFPAT
are depicted in Scheme 2. In this process, PFPAT acts as
Brønsted acid and plays a significant role in increasing the
electrophilic character of the electrophiles (Scheme 2).
The recovered PFPAT from the aqueous layer was
reused in three subsequent runs with a gradual decrease in
activity. For example, the treatment of benzil (1) with o-
phenylenediamine (2) in the presence of 10 mol% PFPAT in
water (4 mL) for 60 min gave the desired product 3a in 95%,
4. Experimental
4.1. Typical experimental procedure
A mixture of 1,2-dicarbonyl compounds (1 mmol), aryl
1,2-diamines (1 mmol) dissolved in 4 mL water, and PFPAT
(10 mol%) was stirred for the appropriate reaction time. The
reaction was monitored by TLC. After completion of the
reaction (monitored by TLC), the resultant was cooled with
ice-salt bath, filtered and washed with ethanol and purified
by recrystallization from hot ethanolto afford pure products
3a–p, and the filtrate containing PFPAT could be directly
used by adding the reactants. After three recycles, the
catalytic activity of PFPAT remained unchanged. The
productswerecharacterizedbycomparisonoftheirphysical
and spectral data with those of authentic samples. Spectro-
scopic data for the selected examples are shown below.
Spectroscopic data for selected examples as follows.
9
0%, and 88% yields over three cycles.
We believe that the procedure is simple, convenient
and does not require any aqueous work-up, thereby
avoiding the generation of waste, and may contribute to
the area of green chemistry.
3
. Conclusions
In conclusion, a simple and highly efficient method for
the synthesis of quinoxaline derivatives through the
reaction of aryl 1,2-diamines with 1,2-dicarbonyl com-
pounds at room temperature catalyzed by pentafluoro-
phenylammonium triflate in water at room temperature
has been proposed in this work. In contrast to the existing
methods using potentially hazardous catalysts/additives,
the present method offers the following competitive
advantages:
4.1.1. 2,3-Diphenylquinoxaline
White solid; mp 126–127 8C; IR (KBr): 3051, 1630, 1528,
À1
1
1348, 772 cm
3
; H NMR (CDCl , 400 MHz): d = 7.33–7.41
(m, 6H), 7.53–7.57 (m, 4H), 7.76–7.83 (m, 2H), 8.20–8.23(m,
2H); C NMR (100 MHz, CDCl ): d = 128.2, 128.9, 129.2,
3
129.8, 129.9, 139.1, 141.2, 153.4 (Table 1, entry 1).
1
3
ꢀ
ꢀ
PFPAT is easy to prepare from commercially available
pentafluoroaniline and triflic acid;
the reaction proceeds with short reaction time;
4.1.2. 6-Methyl-2,3-diphenyl-quinoxaline
White solid; mp 118–120 8C; IR (KBr): 3052, 1640,
À1
1
1532, 1344, 775 cm
; H NMR (CDCl , 400 MHz): d = 2.6
3

S. Khaksar, H. Radpeyma / C. R. Chimie 17 (2014) 1023–1027
1027
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13
.96 (s, 1H); 8.05 (d, J = 8.5 Hz, 1H); C NMR (100 MHz,
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39.2, 139.7, 140.4, 141.3, 152.5, 153.3 (Table 1, entry 2).
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d
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1
1
530, 1340 cm ; H NMR (400 MHz, CDCl ): d = 7.34–7.45
3
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d
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311.
13
m, 1H), 9.20 (d, J = 4.8 Hz, 1H); C NMR (100 MHz, CDCl
3
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= 125.6, 128.5, 128.8, 129.7, 129.8, 130.2, 130.6,
36.6,138.5, 138.9, 150.2, 154.4, 155.1, 156.7 (Table 1,
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[
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4
.1.4. 2-Furan-3-yl-3-furan-2-yl-quinoxaline
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Light yellow solid; mp 134 À 136 8C; IR (KBr): 3427,
À1
1
[23] T.M. Potewar, S.A. Ingale, K.V. Srinivasan, Synth. Commun. 38 (2008)
601.
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2
CDCl
983, 1605, 1567, 1442, 879, 743 cm ; H NMR (400 MHz,
3
3
): d = 6.68–6.53 (m, 4H), 7.68–7.63 (m, 2H), 7.80–7.73
[
(
dd, J = 3.1 Hz, J = 6.8 Hz, 2H), 8.18–8.11 (dd, J = 3.5 Hz,
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13
J = 6.3 Hz, 2H); C NMR (125 MHz, CDCl
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3
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Light yellow solid; mp 116 À 118 8C; IR (KBr): 3422,
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À1
1
2
980, 1600, 1567, 1444 cm ; H NMR (400 MHz, CDCl ):
3
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= 2.57 (s, 3H), 6.68–6.65 (m, 4H), 7.59–7.54 (dd, J = 1.8 Hz,
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1
J = 8.6 Hz, 1H), 7.64–7.60 (m, 2H), 7.93–7.88 (s, 1H), 8.04–
[
13
7
2
1
3
.99 (d, J = 8.6 Hz, 1H). C NMR (100 MHz, CDCl ): d = 21.6,
(
1.9, 111.9, 112.6, 112.8, 117.2, 118.9, 124.6, 127.9, 128.6,
32.8, 141.2, 150.9 (Table 1, entry 9).
(
[
(
4
.1.6. Acenaphtho[1,2-b]quinoxaline
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White solid; mp 242–245 8C. IR (KBr): 3443, 3047, 2922,
À1
1
2
361, 1614, 1481 cm
.
3
H NMR (400 MHz, CDCl ):
d
= 7.70–7.75 (m, 2 H), 7.98 (t, J = 7.7 Hz, 2 H), 8.15 (d,
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13
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1
28.9, 129.6, 130.1, 133.2, 142.5, 145.3 (Table 1, entry 11).
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