Br?nsted acidic ionic liquids catalyzed the preparation of 13-aryl-5H-dibenzo[b,i]xanthene-5,7,12,14(13H)-tetraones and 3,4-dihydro-1H-benzo[b]xanthene-1,6,11(2H,12H)-triones

Article abstract of DOI:10.1007/s11164-013-1043-1

An environmentally green method for the synthesis of 13-aryl-5H-dibenzo[b, i]xanthene-5,7,12,14(13H)-tetraone and 3,4-dihydro-1H-benzo[b]xanthene-1,6, 11(2H,12H)-trione derivatives using Br?nsted acidic ionic liquids, 3-methyl-1-sulfonic acid imidazolium chloride, 1-H-3-methyl-imidazolium hydrogensulfate, triethylamine-bonded sulfonic acid, and triethylammonium hydrogensulfate as reusable catalysts under thermal solvent-free conditions is described.

Full text of DOI:10.1007/s11164-013-1043-1

Res Chem Intermed
DOI 10.1007/s11164-013-1043-1
Brønsted acidic ionic liquids catalyzed the preparation
of 13-aryl-5H-dibenzo[b,i]xanthene-5,7,12,14
(13H)-tetraones and 3,4-dihydro-1H-benzo[b]
xanthene-1,6,11(2H,12H)-triones
•
Hamid Reza Shaterian Marziyeh Sedghipour
Ebrahim Mollashahi
•
Received: 13 December 2012 / Accepted: 11 January 2013
Ó Springer Science+Business Media Dordrecht 2013
Abstract An environmentally green method for the synthesis of 13-aryl-5H-
dibenzo[b,i]xanthene-5,7,12,14(13H)-tetraone and 3,4-dihydro-1H-benzo[b]xan-
thene-1,6,11(2H,12H)-trione derivatives using Brønsted acidic ionic liquids,
3-methyl-1-sulfonic acid imidazolium chloride, 1-H-3-methyl-imidazolium hydrog-
ensulfate, triethylamine-bonded sulfonic acid, and triethylammonium hydrogensul-
fate as reusable catalysts under thermal solvent-free conditions is described.
Keywords Ionic liquids ꢀ Aldehydes ꢀ Green chemistry ꢀ Domino reactions ꢀ
Catalyst
Introduction
The development of cleaner technologies is a major subject in green chemistry [1].
Among the several aspects of green chemistry, the reduction or replacement of
volatile organic solvents from the reaction medium is of greatest concern [2]. Ionic
liquids (ILs) have received considerable interest as ecofriendly solvents, catalysts,
and reagents in the context of green synthesis because of their unique properties
such as low volatility, nonflammability, high thermal stability, negligible vapor
pressure, and ability to dissolve a wide range of materials [3]. 2-Hydroxy-1,
4-naphthoquinone (HNQ; Lawsone) is the principal natural dye (1.0–1.4 %) in the
leaves of henna, Lawsonia inermis. Henna has been used for more than 4,000 years,
not only as a hair dye but also as a body paint and tattoo dye. Today, semi-
permanent hair dyes containing henna as well as its pure dye ingredient, HNQ, are
widely used and have become increasingly popular due to their natural origin [4, 5].
H. R. Shaterian (&) ꢀ M. Sedghipour ꢀ E. Mollashahi
Department of Chemistry, Faculty of Sciences, University of Sistan and Baluchestan,
PO Box 98135-674, Zahedan, Iran
e-mail: hrshaterian@chem.usb.ac.ir
123

H. R. Shaterian et al.
Naphthoquinones have been the subject of much interest for a number of years
due to their various biological activities, their industrial applications, and their
potential as intermediates in the synthesis of heterocycles [6]. Xanthenes and
benzoxanthenes have been reported to possess diverse biological and therapeutic
properties, such as antibacterial [7], antiviral [8], and anti-inflammatory activity [9],
as well as photodynamic therapy [10]. The other useful applications of this
heterocycles are as dyes [11], fluorescent materials for visualization of biomolecules
[12], and in laser technologies [13]. Some procedures have been developed for
the synthesis of aryl-5H-dibenzo[b,i] xanthene-5,7,12,14 (13H)-tetraone and
3,4-dihydro-1H-benzo[b]xanthene-1,6,11(2H,12H)-triones such as p-toluene sul-
fonic acid (p-TSA) [14, 15], pyrrolidonium hydrogensulfate [16], (4-sulfobu-
tyl)tris(4-sulfophenyl)phosphonium hydrogensulfate [16]. In continuation our
research on ILs as catalysts [17]), we would like to report new applications of
the ILs, 3-methyl-1-sulfonic acid imidazolium chloride {[Msim]Cl}, 1-H-3-methyl-
imidazolium hydrogensulfate [Hmim]HSO4-, triethylamine-bonded sulfonic acid
{[Et3N–SO3H]Cl}, and triethylammonium hydrogensulfate [Et3NH][HSO4] as
catalysts (Fig. 1).
These reusable catalysts were applied in the present research for the preparation
of (1) 13-aryl-5H-dibenzo[b,i]xanthene-5,7,12,14(13H)-tetraones from 2-hydroxy-
1,4-naphthoquinone, and arylaldehydes, and (2) 3,4-dihydro-1H-benzo[b]xanthene-
1,6,11(2H,12H)-triones from 2-hydroxy-1,4-naphthoquinone, arylaldehydes and
dimedone under thermal solvent-free conditions (Scheme 1).
Experimental
Materials and methods
All reagents were purchased from Merck and Aldrich and used without further
purification. 3-Methyl-1-sulfonic acid imidazolium chloride {[Msim]Cl} [18], 1-H-
3-methyl-imidazolium hydrogensulfate [Hmim][HSO4] [19], triethylamine-bonded
sulfonic acid {[Et3N–SO3H]Cl} [20], and triethylammonium hydrogensulfate
[Et3NH][HSO4] [21] were prepared according to the reported procedure. All yields
refer to isolated products after purification. The NMR spectra were recorded on a
Fig. 1 The structures of
3-methyl-1-sulfonic acid
imidazolium chloride
N
N
Cl
H
HSO4
N
N
SO3H
H3C
{[Msim]Cl}, 1-H-3-methyl-
imidazolium hydrogensulfate
[Hmim]HSO4-, triethylamine-
bonded sulfonic acid {[Et3
N–SO3H]Cl}, and
[Hmim]HSO4
[Msim]Cl
+
N
H
+
triethylammonium
hydrogensulfate [Et3NH][HSO4]
HSO4
Cl
N
SO3H
[Et3N-SO3H]Cl
[Et3NH][HSO4]
123

Brønsted acidic ionic liquids catalyzed the preparation
O
O
O
Ar
O
O
O
OH
O
Ionic Liquids (Catalyst)
2
Solvent-Free , 80oC
Ar
O
H
O
13-aryl-5H-dibenzo[b,i]xanthene-5,7,12,14(13H)-tetraones
O
Ar
O
O
O
OH
Ionic Liquids (Catalyst)
Solvent-Free ,100oC
O
Ar
H
O
O
O
3,4-dihydro-1H-benzo[b]xanthene-1,6,11(2H,12H)-triones
Ionic Liquids: 3-methyl-1-sulfonic acid imidazolium chloride,1-H-3-methyl-imidazolium hydrogensulfate,
triethylamine-bonded sulfonic acid, triethylammonium hydrogensulfate
Scheme 1 The preparation of 13-aryl-5H-dibenzo[b,i]xanthene-5,7,12,14(13H)-tetraones and 3,4-dihydro-
1H-benzo[b]xanthene-1,6,11(2H,12H)-triones
Bruker Avance DPX 300 MHz instrument. The spectra were measured in DMSO-d6
relative to TMS (0.00 ppm). Melting points were determined in open capillaries
with a BUCHI 510 melting point apparatus. IR spectra were recorded on a JASCO
FT-IR 460 plus spectrophotometer. TLC was performed on silica–gel Poly Gram
SIL G/UV 254 plates.
General procedure for preparation of 13-aryl-5H-dibenzo[b,i]xanthene-
5,7,12,14(13H)-tetraones
A mixture of aromatic aldehydes (1 mmol), 2-hydroxy-1,4-naphthoquinone (2 mmol)
and ILs 3-methyl-1-sulfonic acid imidazolium chloride {[Msim]Cl} (13 mol%,
-
0.13 mmol, 0.026 g), 1-H-3-methyl-imidazolium hydrogensulfate [Hmim]HSO4
(11 mol%, 0.11 mmol, 0.018 g), triethylamine-bonded sulfonic acid {[Et3N–
SO3H]Cl} (13 mol%, 0.13 mmol, 0.028 g) and triethylammonium hydrogensulfate
[Et3NH][HSO4] (9 mol%, 0.09 mmol, 0.018 g) was heated at 80 °C. The progress of
the reaction was monitored by TLC. After completion of the reaction, water was added
to the mixture and the solid crude product was filtered off. The pure product was
obtained by recrystallization from the hot EtOH. For recycling the catalyst, after
washing solid products with water completely, the water containing ionic liquid (IL is
soluble in water) was evaporated under reduced pressure and IL was recovered and
reused.
Selected spectroscopic data for two known products are given below.
13-Phenyl-5H-dibenzo[b,i]xanthene-5,7,12,14(13H)-tetraone (Table 2, entry 1)
1
Red powder, mp: 304–306 °C; H NMR (300 MHz, DMSO-d6): d = 5.09 (1H, s,
CH), 7.16–8.08 (13H, m, arom.) ppm; IR (KBr) (tmax, cm-1): 3,035, 1,660, 1,569.
123

H. R. Shaterian et al.
13-(4-Bromophenyl)-5H-dibenzo[b,i]xanthene-5,7,12,14(13H)-tetraone
(Table 2, entry 5)
1
Yellow powder, mp: 334–336 °C; H NMR (300 MHz, DMSO-d6): d = 5.17 (1H,
s, CH), 7.43–8.16 (12H, m, arom.) ppm; IR (KBr) (tmax, cm-1): 3,088, 1,663, 1,657,
1,608.
General procedure for preparation of 3,4-dihydro-1H-benzo[b]xanthene-
1,6,11(2H,12H)-triones
A mixture of aromatic aldehydes (1 mmol), 2-hydroxy-1,4-naphthoquinone (1 mmol)
and dimedone (1 mmol) and ILs 3-methyl-1-sulfonic acid imidazolium chloride
{[Msim]Cl} (13 mol%, 0.13 mmol, 0.026 g), 1-H-3-methyl-imidazolium hydrogen-
sulfate [Hmim]HSO4- (12 mol%, 0.12 mmol, 0.020 g), triethylamine-bonded sulfonic
acid {[Et3N–SO3H]Cl} (11 mol%, 0.11 mmol, 0.024 g) and triethylammonium
hydrogensulfate [Et3NH][HSO4] (13 mol%, 0.13 mmol, 0.026 g) was heated at
100 °C. The progress of the reaction was monitored by TLC. After completion of
the reaction, water was added to the mixture and the catalyst was filtered off. The pure
product was obtained by recrystallization from the hot EtOH. For recycling the catalyst,
after completely washing the solid products with water, the water containing ionic
liquid (IL is soluble in water) was evaporated under reduced pressure and IL was
recovered and reused.
Selected spectroscopic data for two known products are given below.
12-Phenyl-3,3-dimethyl-3,4-dihydro-1H-benzo[b]xanthene-1,6,11
(2H,12H)-trione (Table 4, entry 1)
1
Orange powder, mp: 264–266 °C; H NMR (300 MHz, DMSO-d6): d = 0.94 (3H,
s, CH3), 1.07 (3H, s, CH3), 2.15 and 2.31 (2H, AB system, J = 16.2 Hz, CH2), 2.67
(2H, s, CH2), 4.88 (1H, s, CH), 7.10–7.15 (1H, m, H–Ar), 7.21–7.32 (2H, m, H–Ar),
7.43–7.46 (2H, m, H–Ar). 7.80–7.91 (3H, m, H–Ar) 7.99–8.07 (1H, m, H–Ar) ppm;
IR (KBr) (tmax, cm-1): 2,926, 1,678, 1,606.
12-(4-Bromophenyl)-3,3-dimethyl-3,4-dihydro-1H-benzo[b]xanthenes-
1,6,11(2H,12H)-trione (Table 4, entry 5)
Yellow powder.; mp: 267–269 °C. 1H NMR (300 MHz, DMSO-d6): d = 0.94 (3H,
s, CH3),1.07 (3H, s, CH3), 2.15 and 2.31 (2H, AB system, J = 15.9 Hz, CH2), 2.67
(2H, s, CH2), 4.88 (1H, s, CH), 7.107.12(2H, d, J = 8.4 Hz, H–Ar), 7.42–7.45 (2H,
m, H–Ar), 7.83–7.90 (3H, m, H–Ar), 8.03–8.06 (1H, m, H–Ar) ppm; IR (KBr)
(tmax, cm-1): 2,957, 1,660, 1,616, 1,579.
123

Brønsted acidic ionic liquids catalyzed the preparation
Results and discussion
To choose optimum conditions, first we tried to prepare 13-(4-chlorophenyl)-5H-
dibenzo[b,i]xanthene-5,7,12,14(13H)-tetraone from the reaction of 4-chlorobenzal-
dehyde (1 mmol) and 2-hydroxy-1,4-naphthoquinone (2 mmol) as a model under
solvent-free conditions at different temperatures (60, 70, 80 °C) in the presence of A:
3-methyl-1-sulfonic acid imidazolium chloride {[Msim]Cl} (29, 14, 20, 10, 13,
15 mmol%), B: 1-H-3-methyl-imidazolium hydrogensulfate [Hmim]HSO4- (28, 10,
15, 9, 11, 14 mmol%), C: triethylamine-bonded sulfonic acid {[Et3N–SO3H]Cl} (29,
15, 20, 9, 13, 16 mmol%), D: triethylammonium hydrogensulfate [Et3NH][HSO4]
(29, 12, 18, 5, 9, 13 mmol%). The amount of the catalysts with highest yield and short
reaction time were chosen as A: 13, B: 11, C: 13, and D: 9 mol% under thermal (80 °C)
solvent-free conditions (Table 1).
Using these optimized reaction conditions, the scope and efficiency of this
procedure was explored for the synthesis of a wide variety of substituted 13-aryl-
5H-dibenzo[b,i]xanthene-5,7,12,14(13H)-tetraones derivatives. Interestingly, a vari-
ety of aldehydes including substituted arylaldehydes participated well in this
reaction and gave the desired product in good to excellent yields (Table 2). As seen
from Table 2, both aromatic aldehydes carrying electron-donating or electron-
withdrawing substituents act well in these reaction conditions. Aliphatic aldehydes
such as n-heptanal and n-octanal were intact in this reaction.
In continuation of this work, we tried to synthesize 3,4-dihydro-1H-benzo[b]xan-
thene-1,6,11(2H,12H)-triones. To choose optimum conditions, first we tried to prepare
12-(4-chlorophenyl)-3,3-dimethyl-3,4-dihydro-2H-benzo[b]xanthene-1,6,11(12H)-tri-
one from the reaction of 4-chlorobenzaldehyde (1 mmol), 2-hydroxy-1,4-naphthoqui-
none (1 mmol) and dimedone (1 mmol) as a model under solvent-free conditions at
different temperatures (80, 90, 100 °C) in the presence of A: 3-methyl-1-sulfonic acid
imidazolium chloride {[Msim]Cl} (30, 11, 22, 8, 13, 15 mmol%), B: 1-H-3-methyl-
-
imidazolium hydrogensulfate [Hmim]HSO4 (29, 10, 20, 10, 12, 16 mmol%), C:
triethylamine-bonded sulfonic acid {[Et3N–SO3H]Cl} (28, 11, 15, 10, 11, 13 mmol%),
Table 1 Optimization the amount of the ionic liquids as catalyst A: {[Msim]Cl}, B: [Hmim]HSO4-, C:
{[Et3N–SO3H]Cl}, D: [Et3NH][HSO4] at different temperature under solvent-free conditions
Entry Catalyst (mol%)
Temperature Time (min)
(°C)
Yield (%)a
A
B
C
D
A
B
C
–
D
–
A
B
C
D
1
2
3
4
5
6
a
29 28 29 29 60
14 10 15 12 70
20 15 20 18 70
–
–
Trace Trace Trace Trace
20 18 17 20 69
14 12 12 13 79
70
79
85
95
95
68
76
83
98
97
62
71
87
95
96
10
9
9
5
9
80
80
10
5
8
4
4
10
4
7
4
3
87
97
98
13 11 13
15 14 16 13 80
3
3
Yields refer to the isolated pure products. The preparation of 13-(4-chlorophenyl)-5H-
dibenzo[b,i]xanthene-5,7,12,14(13H)-tetraone from the reaction of 4-chlorobenzaldehyde (1 mmol) and
2-hydroxy-1,4-naphthoquinone (2 mmol) was studied
123

H. R. Shaterian et al.
Table 2 The preparation of 13-aryl-5H-dibenzo[b,i]xanthene-5,7,12,14(13H)-tetraones using A:
{[Msim]Cl} (13 mol%), B: [Hmim]HSO4- (11 mol%), C: {[Et3N–SO3H]Cl} (13 mol%), D: [Et3NH][HSO4]
(9 mol%) under Thermal (80 °C) solvent-free conditions
Entry
ArCHO
Time (min)
Yield (%)a
References to the
known products
A
B
C
D
A
B
C
D
1
2
3
4
5
6
7
a
C6H5CHO
7
5
6
7
4
5
5
7
4
6
7
4
5
4
6
5
5
8
5
6
5
8
4
7
9
4
4
6
96
95
90
88
96
89
95
94
92
91
90
94
91
93
95
98
93
90
95
90
95
98
93
90
89
94
90
96
[14], [15], [16], [22]
[14], [15], [16], [22]
[14], [15], [16], [22]
[14], [15], [16], [22]
[14], [15], [16], [22]
[14], [15], [16], [22]
[14], [15], [16], [22]
4-ClC6H4CHO
4-NO2C6H4CHO
4-CH3C6H4CHO
4-BrC6H4CHO
4-FC6H4CHO
3-NO2C6H4CHO
Based on the reaction of arylaldehydes (1 mmol) and 2-hydroxy-1,4-naphthoquinone (2 mmol), Yields
refer to the isolated pure products. All known products were compared with authentic samples in the
literature
D: triethylammonium hydrogensulfate [Et3NH][HSO4] (30, 10, 20, 7, 13, 16 mmol%).
The best result with the highest yield and short reaction time was obtained at 100 °C and
catalytic amounts of A: 13, B: 12, C: 11, D: 13 mol% of the ILs as catalyst (Table 3).
Using these optimized reaction conditions, the scope and efficiency of these
procedures were explored for the synthesis of a wide variety of substituted 3,
4-dihydro-1H-benzo[b]xanthene-1,6,11(2H,12H)-triones (Table 4). Aliphatic alde-
hydes such as n-heptanal and n-octanal were intact in this reaction.
The proposed mechanism is described according to the literature [16] for
preparation of 13-aryl-5H-dibenzo[b,i]xanthene-5,7,12,14(13H)-tetraones in the
presence of Brønsted acidic IL such as 3-methyl-1-sulfonic acid imidazolium
chloride (Scheme 2). Brønsted acidic IL with hydrogen bonding can activate the
carbonyl groups of aldehyde (1) to decrease the energy of transition state. Then,
nucleophilic attack of 2-hydroxy-1,4-naphthoquinone to the (1) causes to the
Table 3 Optimization the amount of the ionic liquids as catalyst A: {[Msim]Cl}, B: [Hmim]HSO4-, C:
{[Et3N–SO3H]Cl}, D: [Et3NH][HSO4] for preparation of 12-(4-chlorophenyl)-3,3-dimethyl-3,4-dihydro-
2H-benzo[b]xanthene-1,6,11(12H)-trione at different temperature under solvent-free conditions
Entry Catalyst (mol%)
Temperature (oC) Time (min)
Yield (%)a
A
B
C
D
A
B
C
D
–
A
B
C
D
1
2
3
4
5
6
a
30 29 28 30 80
11 10 11 10 90
22 20 15 20 90
–
–
–
Trace Trace Trace Trace
15 20 12 13 68
14 17 10 12 73
67
70
80
92
92
65
70
85
95
96
63
69
80
94
95
8
10 10
7
100
9
5
4
10
7
8
4
4
9
6
4
81
93
93
13 12 11 13 100
15 16 13 16 100
5
Based on the reaction of 4-chlorobenzaldehyde (1 mmol), 2-hydroxy-1,4-naphthoquinone (1 mmol)
and dimedone (1 mmol). Yields refer to the isolated pure product
123

Brønsted acidic ionic liquids catalyzed the preparation
Table 4 The preparation of 3,4-dihydro-1H-benzo[b]xanthene-1,6,11(2H,12H)-triones using A:
{[Msim]Cl} (13 mol%), B: [Hmim][HSO4] (12 mol%), C: {[Et3N–SO3H]Cl} (11 mol%), D: [Et3NH]
[HSO4] (13 mol%) under thermal (100 °C) solvent-free conditions
Entry
Aldehydes
Time (min)
Yield (%)a
References to
the known products
A
B
C
D
A
B
C
D
1
2
3
4
5
6
7
8
9
a
C6H5CHO
5
7
7
7
7
4
9
7
5
7
9
6
94
93
86
90
90
96
91
90
92
93
92
90
91
89
92
92
89
93
95
95
91
90
91
97
90
92
89
97
94
96
87
93
95
92
87
93
[16], [23]
[16], [23]
[16], [23]
[16], [23]
[16], [23]
[16], [23]
[16], [23]
[16], [23]
[16], [23]
4-ClC6H4CHO
4-NO2C6H4CHO
4-CH3C6H4CHO
4-BrC6H4CHO
4-FC6H4CHO
3-NO2C6H4CHO
3-BrC6H4CHO
3-ClC6H4CHO
5
4
6
6
8
7
9
10
5
12
10
13
4
8
7
9
9
7
10
10
6
13
8
12
Based on the reaction of arylaldehydes (1 mmol), 2-hydroxy-1,4-naphthoquinone (1 mmol) and
dimedone (1 mmol). Yields refer to the isolated pure products. All know products were compared with
authentic samples in the literature
formation of intermediate (3) which followed by the protonation and nucleophilic
attack of 2-hydroxy-1,4-naphthoquinone giving the intermediate (5). Their
subsequent intramolecular cyclization and dehydration give the 13-aryl-5H-
dibenzo[b,i]xanthene-5,7,12,14(13H)-tetraones (6).
The suggested cyclic catalytic mechanism for the preparation of 3,4-dihydro-1H-
benzo[b]xanthene-1,6,11(2H,12H)-triones is explained in Scheme 3. Brønsted
acidic ILs act as catalyst. Initially, the Knoevenagel condensation of arylaldehydes
(7) with dimedone (8) provide 2-arylidene-5,5-dimethylcyclohexane-1,3-dione (9)
in the presence of the catalyst. Then, nucleophilic attack of 2-hydroxy-1,4-
naphthoquinone to the activated of (9) causes the formation of the intermediate (11)
which followed intramolecular cyclization and dehydration to give 3,4-dihydro-1H-
benzo[b]xanthene-1,6,11(2H,12H)-trione (Scheme 3).
We also investigated the recycling of the catalysts under solvent-free conditions
for the synthesis of 13-aryl-5H-dibenzo[b,i]xanthene-5,7,12,14(13H)-tetraone
(benzaldehyde and 2-hydroxy-1,4-naphthoquinone, Table 2, entry 1) and 3,4-
dihydro-1H-benzo[b]xanthene-1,6,11(2H,12H)-trione (benzaldehyde, 2-hydroxy-
1,4-naphthoquinone, and dimedone; Table 4, entry 1) using 3-methyl-1-sulfonic
acid imidazolium chloride {[Msim]Cl}, 1-H-3-methyl-imidazolium hydrogensulfate
[Hmim]HSO4-, triethylamine-bonded sulfonic acid {[Et3N–SO3H]Cl}, and trieth-
ylammonium hydrogensulfate [Et3NH][HSO4] as catalysts. After completion of the
reaction, the reaction mixture was cooled to room temperature, and the IL was
dissolved in water. For recycling the catalyst, after completely washing the solid
products with water, the water containing ionic liquid (IL is soluble in water) was
evaporated under reduced pressure and IL was recovered and reused. The recovered
catalysts were reused five times without any loss of their activities (Figs. 2, 3).
To show the merit of the present work in comparison with the reported results in
the literature, we compared the results of 3-methyl-1-sulfonic acid imidazolium
123

H. R. Shaterian et al.
O
O
Ar
O
O
+
H2O
O
N
N
Cl
SO3H
(6)
N
N
Ar
H
(1)
O
O
S
O
O
Ar
O
O
H
Cl
O
OH
HO
O
O
(5)
OH
O
Ar
O
O
OH
(2)
O
OH
Ar
O
O
OH
OH2
(2)
(3)
OH
O
(4)
Scheme 2 The proposed mechanism for the preparation of 13-aryl-5H-dibenzo[b,i]xanthene-
5,7,12,14(13H)-tetraones
-
chloride {[Msim]Cl}, 1-H-3-methyl-imidazolium hydrogensulfate [Hmim]HSO4
,
triethylamine-bonded sulfonic acid {[Et3N–SO3H]Cl}, and triethylammonium
hydrogensulfate [Et3NH][HSO4] as catalysts with p-TSA [14, 15], trifluoroacetic
acid (TFA) with 1,1,3,3-N,N,N0,N0-tetramethylguanidinium trifluoroacetate(TMGT) IL
[24], 2-pyrrolidonium hydrogensulfate [16], and (4-sulfobutyl)tris(4-sulfophenyl)phos-
phonium hydrogensulfate [16] in the synthesis 13-phenyl-5H-dibenzo[b,i]xanthene-
5,7,12,14(13H)-tetraone from benzaldehyde, 2-hydroxy-1,4-naphthoquinone. As
shown in Table 5, the present ILs can act as suitable catalysts with respect to reaction
times and yields of the products.
Conclusions
We have developed a green and straightforward protocol for the synthesis
of 13-aryl-5H-dibenzo[b,i]xanthene-5,7,12,14(13H)-tetraones and 3,4-dihydro-
1H-benzo[b]xanthene-1,6,11(2H,12H)-trione using ILs 3-methyl-1-sulfonic acid
imidazolium chloride {[Msim]Cl}, 1-H-3-methyl-imidazolium hydrogensulfate
[Hmim]HSO4-, triethylamine-bonded sulfonic acid {[Et3N–SO3H]Cl}, and trieth-
ylammonium hydrogensulfate [Et3NH][HSO4] as efficient catalysts under thermal
solvent-free conditions. This procedure provides several advantages such as cleaner
123

Brønsted acidic ionic liquids catalyzed the preparation
O
O
Ar
O
+
H2O
O
N
N
Cl
SO3H
+
ArCHO
(7)
(12)
O
O
Product
ionic liquid (catalyst)
(8)
H2O
O
O
Ar
O
O
O
OH
OH
(11)
Ar
(9)
O
OH
O
(10)
Scheme 3 The proposed mechanism for the preparation of 3,4-dihydro-1H-benzo[b]xanthene-1,6,11(2H,
12H)-triones
Fig. 2 Reusability of the ionic liquids in the preparation of 13-phenyl-5H-dibenzo[b,i]xanthene-
5,7,12,14(13H)-tetraone
reactions, easier work-up, reduced reaction times, reusable catalyst, and an eco-
friendly promising strategy. This methodology offers significant improvements with
regard to the scope of this transformation, simplicity in operation, and green aspects
by avoiding expensive or corrosive catalysts.
123

H. R. Shaterian et al.
Fig. 3 Reusability of the ionic liquids in the preparation of 12-phenyl-3,4-dihydro-1H-benzo[b]xanthene-
1,6,11(2H,12H)-trione
Table 5 Comparison the results of 3-methyl-1-sulfonic acid imidazolium chloride {[Msim]Cl}, 1-H-3-
-
methyl-imidazolium hydrogensulfate [Hmim]HSO4 , triethylamine-bonded sulfonic acid {[Et3N–
SO3H]Cl}, and triethylammonium hydrogensulfate [Et3NH][HSO4] as catalysts with other catalysts in the
literature
Entry Catalyst
Catalyst amount Conditions
(mol%)
Time
Yeild
(%)
[Ref.]
1
2
3
4
5
p-TSA
58 mol%
58 mol%
Solvent-free/
100 °C
Solvent-free/
3 h
7 h
81 [14]
74 [15]
p-TSA
80 °C
75 °C
TMGT/TFA
126 mol%/
152 mol%
1–1.2 h 91 [24]
30 min 90 [16]
10 min 95 [16]
2-Pyrrolidonium hydrogen
sulfate
40 mol%
Solvent-free/
100 °C
Solvent-free/
(4-Sulfobutyl)tris(4-
sulfophenyl)phosphonium
hydrogen sulfate
10 mol%
100 °C
6
7
8
9
[Msim]Cl
13 mol%
11 mol%
13 mol%
9 mol%
Solvent-free/
80 °C
Solvent-free/
7 min
7 min
6 min
8 min
96
94
95
98
-
[Hmim]HSO4
80 °C
Solvent-free/
{[Et3N–SO3H]Cl}
[Et3NH][HSO4]
80 °C
Solvent-free/
80 °C
Acknowledgment We are grateful to the University of Sistan and Baluchestan Research Council for the
partial support of this research.
123

Brønsted acidic ionic liquids catalyzed the preparation
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