1-Butyl-3-methylimidazolium hydrogen sulfate [bmim]HSO4: An efficient reusable acidic ionic liquid for the synthesis of 1,8-Dioxo- octahydroxanthenes

Article abstract of DOI:10.1002/jccs.200900098

1-Butyl-3-methylimidazolium hydrogen sulfate [bmim]HSO₄ as an acidic ionic liquid was prepared and used as a catalyst for the synthesis of 1,8-dioxo-octahydroxanthenes in excellent yields and short reaction times at 80 °C. The ionic liquid was easily separated fromthe reactionmixture by water extraction and was recycled four times without any loss in activity.

Full text of DOI:10.1002/jccs.200900098

Journal of the Chinese Chemical Society, 2009, 56, 659-665
659
Note
1-Butyl-3-methylimidazolium Hydrogen Sulfate [bmim]HSO4: An Efficient
Reusable Acidic Ionic Liquid for the Synthesis of
1,8-Dioxo-octahydroxanthenes
Khodabakhsh Niknam* and Maryam Damya
Chemistry Department, Faculty of Sciences, Persian Gulf University, Bushehr 75169, Iran
1-Butyl-3-methylimidazolium hydrogen sulfate [bmim]HSO4 as an acidic ionic liquid was prepared
and used as a catalyst for the synthesis of 1,8-dioxo-octahydroxanthenes in excellent yields and short reac-
tion times at 80 °C. The ionic liquid was easily separated from the reaction mixture by water extraction and
was recycled four times without any loss in activity.
Keywords: Acidic ionic liquids; [bmim][HSO4]; Xanthenes; Dimedone; Aldehydes.
INTRODUCTION
apy14 and anti-inflammatory activities.15 There are several
reports in the literature for the synthesis of 1,8-dioxooc-
tahydroxanthene derivatives employing aromatic alde-
hydes and 5,5-dimethyl-1,3-cyclohexanedione; these in-
clude InCl3.4H2O in ionic liquid,16 solid-state condensation
by grinding at room temperature,17 diammonium hydrogen
phosphate,18 p-dodecylbenzenesulfonic acid in water,19
Fe3+-montmorilonite,20 NaHSO4-SiO2 or silica chloride,21
amberlyst-15,22 silica sulfuric acid,23 tetrabutylammonium
hydrogen sulfate,24 trimethylsilylchloride (TMSCl),25 1-
methylimidazolium trifluroacetate,26 and montmorillonite
K-10-supported.27 Each of these methods have their own
advantages but also some of them often suffer from one or
more disadvantages such as prolonged reaction time, te-
dious work-up processes, low yield, lack of easy availabil-
ity/preparation of starting materials, expensive reagents
and hazardous reaction conditions. Hence, a more realistic
catalyzed condensation between active methylene car-
bonyl compound and aldehyde is needed for contemporary
chemical synthesis with less waste and more facile isola-
tion of products, perhaps with reuse of the catalysts as well.
Recently, 1-butyl-3-methylimidazolium hydrogen
sulfate ([bmim]HSO4) as an acidic ionic liquid (Scheme I)
has been used as an efficient IL catalyst for many organic
transformations.6-7,28
Ionic liquids (ILs) are now well known as salts with
melting points below 100 °C, and typically have broad liq-
uid ranges, low vapor pressures, and may be both non-co-
ordinating and highly polar. They are receiving consider-
able global attention because they offer a unique environ-
ment for chemistry, biocatalysts, separation science, mate-
rial synthesis, and electrochemistry.1 These ionic liquids
have several interesting properties such as excellent chemi-
cal and thermal stability, non-volatility, a non-coordinating
nature, good solvating capability, a wide liquid range, and
ease of recycling. Furthermore, their hydrophobicities/hy-
drophilicities can be tuned by appropriate modification of
the cation or anion.1,2 Therefore, room temperature ionic
liquids have found wide use in catalytic and non-catalytic
reactions.3 In addition, the synthesis of task-specific ionic
liquids, which have a functional group in their framework,
may expand the application of ionic liquids in organic
chemistry.4-6 Recently, acidic ionic liquids have been suc-
cessfully used in these reactions due to their unique proper-
ties such as being vapor-less and their reusability.5-9
Xanthenes are an important class of organic com-
pounds that find use as dyes, fluorescent material for visu-
alization of biomolecules and in laser technologies due to
their useful spectroscopic properties.10 Xanthenes have
also received significant attention from many pharmaceuti-
cal and organic chemists essentially because of the broad
spectrum of their biological and pharmaceutical properties
such as their antiviral,11 antibacterial,12 and antinociceptive
activities13 as well as their efficiency in photodynamic ther-
RESULTS AND DISCUSSION
In our continued interest in the development of a
highly expedient methodology29-32 for the synthesis of fine
chemicals and heterocyclic compounds of biological im-

660 J. Chin. Chem. Soc., Vol. 56, No. 3, 2009
Niknam and Damya
Scheme I Preparation of 1-butyl-3-methylimidazolium hydrogen sulfate [bmim][HSO4]
Scheme II Synthesis of 1,8-dioxo-octahydroxanthene using [bmim][HSO4] as catalyst
portance, we report here the synthesis of 1,8-dioxo-octa-
hydroxanthenes in the presence of 1-butyl-3-methylimi-
dazolium hydrogen sulfate ([bmim]HSO4) as an acidic
ionic liquid (Scheme II).
Table 1. Optimization of the reaction conditions on the conden-
sation reaction of dimedone (1 mmol), benzaldehyde (1
mmol) in the presence of IL
Entry
Temperature (°C)
Time (h)
Yield (%)a
We first studied a reaction between 5,5-dimethyl-
1,3-cyclohexanedione (1) and benzaldehyde by screening
the reaction conditions. In order to determine the optimum
conditions, we examined the influence of the reaction tem-
perature, the reaction time, and the amounts of IL (Table 1).
In all reactions the conditions were optimized for a 100%
conversion. It can be seen that the best result was obtained
with 0.1 g of [bmim]HSO4 at 80 ºC (Table 1, entry 5).
After optimizing the conditions, we next examined
the generality of these conditions to other substrates using
several aromatic aldehydes (Scheme II). The results are
summarized in Table 2.
1
2
3
4
5
6
7
8
25
30
20
16
09
05
03
03
04
15
Trace
40
50
75
70
85
80
85
90
85
1000
80
80
41b
a In all reactions the conditions were optimized for a 100%
conversion.
b The reaction was performed without IL.
clohexanedione and benzaldehyde were detected. Investi-
gations proved the successive reuse of the recovered ionic
liquid. It could be seen that no considerable change in ac-
tivity of [bmim]HSO4 was observed even after four consec-
utive runs (Table 2, product 3a).
It can be seen that [bmim]HSO4 as environmentally
benign acidic IL catalyzed the condensation of dimedone
and a wide range of aromatic aldehydes at 80 ºC. As indi-
cated in Table 2, in all cases the reaction gives the products
in good yields and prevents problems which many associ-
ate with solvent use such as cost, handling, safety and pol-
lution.
The practical synthetic efficiency of this reaction was
highlighted by the reaction of terephthaldehyde (4) and
isophthaldehyde (5) with dimedone (1) to give structurally
complex xanthenone derivatives (3k, and 3l) (Scheme III).
An important feature of this method is that the acid-
sensitive functionality present in the molecule remains un-
affected. This fact was amply demonstrated by the reaction
of pyridine-3-caboxaldehyde (6) with dimedone (1), which
gave 9-(pyridine-3-yl)-1,8-dioxo-octahydroxanthene (3m)
in excellent yield (Scheme IV).
To make this method more suitable we examined the
reusability of the [bmim]HSO4 using 5,5-dimethyl-1,3-cy-
clohexanedione and benzaldehyde as model substrates. For
this aim, after completion of the reaction (monitored by
TLC), water was added to the reaction mixture and then the
solid was isolated by filtration. The IL in water could be re-
covered easily by evaporation at 80 ºC in a vacuum. The re-
covered IL was washed with diethyl ether. After evapora-
tion at 80 ºC in a vacuum for 1 h, IL was assessed by 1H
NMR spectroscopy and no traces of 5,5-dimethyl-1,3-cy-
In conclusion, we have developed a novel and effi-
cient method for the synthesis of 1,8-dioxo-octahydroxan-
thenes in high yields employing [bmim]HSO4 as an acidic

Acidic Ionic Liquid Catalized Synthesis of Xanthenes
J. Chin. Chem. Soc., Vol. 56, No. 3, 2009 661

662 J. Chin. Chem. Soc., Vol. 56, No. 3, 2009
Niknam and Damya
IL. The application of an inexpensive, easily available and
reusable IL makes this method simple, clean, practical and
economically viable. In contrast to other acids, storage and
handling of this compound do not need special precautions
and it can be stored on the bench top for weeks without
losing its activity.
Aldrich chemical companies. The products were character-
ized by comparison of their spectral (IR, 1H NMR), TLC
and physical data with those reported in the literature. The
1-butyl-3-methylimidazolium hydrogen sulfate [bmim]
[HSO4] was synthesized according to a reported procedure
for 1-hexyl-3-methylimidazolium hydrogen sulfate ([hmim]
[HSO4]).5,6
EXPERIMENTAL SECTION
General
General procedure for the synthesis of 1,8-dioxo-
octahydroxanthene derivatives
Chemicals were purchased from Fluka, Merck and
To a mixture of an aromatic aldehyde (1 mmol) and

Acidic Ionic Liquid Catalized Synthesis of Xanthenes
J. Chin. Chem. Soc., Vol. 56, No. 3, 2009 663
Scheme III Synthesis of bis(1,8-dioxo-octahydroxanthenes)
Scheme IV Synthesis of 9-(pyridine-3-yl)-1,8-dioxo-octahydroxanthene
5,5-dimethyl-1,3-cyclohexanedione (2 mmol) in a round
bottomed flask, [bmim]HSO4 (0.1 g) was added. The mix-
ture was heated at 80 ºC and the reaction was monitored by
TLC. After completion of the reaction, water was added to
the mixture and then filtered. The solid residue was recrys-
tallized from ethanol.
2H, J = 16.3 Hz), 2.27 (d, 2H, J = 16.3 Hz), 2.50 (s, 4H),
4.74 (s, 1H), 7.21 (d, 2H, J = 8.4 Hz), 7.37 (d, 2H, J = 8.4
Hz). 13C NMR (CDCl3, 125 MHz), d: 27.73, 29.69, 31.98,
32.62, 41.28, 51.12, 115.63, 120.66, 130.60, 131.57,
143.64, 162.82, 196.69.
1
3d: mp 225-227 °C, (Lit.:24 225-227 °C; H NMR
Spectral data
(CDCl3, 500 MHz), d: 1.05 (s, 6H), 1.13 (s, 6H), 2.19 (d,
2H, J = 16.2 Hz), 2.26 (d, 2H, J = 16.2 Hz), 2.48 (s, 4H),
5.03 (s, 1H), 7.09 (dt, 1H, J1 = 7.6 Hz, J2 = 1.6 Hz), 7.19 (dt,
1H, J1 = 7.6 Hz, J2 = 1.1 Hz), 7.26 (dd, 1H, J1 = 7.9 Hz, J2 =
1.0 Hz),), 7.46 (d, 1H, J = 7.3 Hz). 13C NMR (CDCl3, 125
MHz), d: 27.79, 29.69, 32.28, 32.43, 41.25, 51.14, 114.13,
126.74, 128.20, 130.56, 133.34, 133.88, 140.32, 163.37,
196.84.
3a: mp 203-204 °C, (Lit.:24 204-206 °C; H NMR
1
(CDCl3, 500 MHz), d: 1.02 (s, 6H), 1.13 (s, 6H), 2.19 (d,
2H, J = 16.2 Hz), 2.26 (d, 2H, J = 16.2 Hz), 2.50 (s, 4H),
4.78 (s, 1H), 7.12 (t, 1H, J = 7.2 Hz), 7.24 (t, 2H, J = 7.5
Hz), 7.32 (d, 2H, J = 7.6 Hz). 13C NMR (CDCl3, 125 MHz),
d: 27.75, 29.69, 32.26, 32.61, 41.29, 51.18, 116.07, 126.76,
128.45, 128.80, 144.54, 162.70, 196.76.
1
1
3b: mp 230-232 °C, (Lit.:25 230-232 °C; H NMR
3e: mp 184-186 °C, (Lit.:24 182-184 °C; H NMR
(CDCl3, 500 MHz), d: 1.03 (s, 6H), 1.14 (s, 6H), 2.20 (d,
2H, J = 16.3 Hz), 2.27 (d, 2H, J = 16.3 Hz), 2.50 (s, 4H),
4.75 (s, 1H), 7.22 (d, 2H, J = 8.5 Hz), 7.27 (d, 2H, J = 8.5
Hz). 13C NMR (CDCl3, 125 MHz), d: 27.72, 29.68, 31.89,
32.61, 41.28, 51.13, 115.69, 128.63, 130.19, 132.45,
143.13, 162.83, 196.71.
(CDCl3, 500 MHz), d: 0.99 (s, 6H), 1.09 (s, 6H), 2.17 (d,
2H, J = 16.2 Hz), 2.22 (d, 2H, J = 16.2 Hz), 2.46 (s, 4H),
4.71 (s, 1H), 7.06 (dt, 1H, J1 = 9.1 Hz, J2 = 1.5 Hz), 7.13 (t,
1H, J = 7.9 Hz), 7.21 (d, 1H, J = 1.2 Hz), 7.23 (t, 1H, J = 1.3
Hz). 13C NMR (CDCl3, 125 MHz), d: 27.79, 29.62, 32.16,
32.63, 41.27, 51.13, 115.51, 127.05, 127.40, 128.75,
129.65, 134.28, 146.54, 162.99, 196.67.
1
3c: mp 240-241 °C, (Lit.:25 240-242 °C; H NMR
1
(CDCl3, 500 MHz), d: 1.03 (s, 6H), 1.14 (s, 6H), 2.20 (d,
3f: mp 222-223 °C, (Lit.:24 221-223 °C; H NMR

664 J. Chin. Chem. Soc., Vol. 56, No. 3, 2009
Niknam and Damya
(CDCl3, 500 MHz), d: 1.02 (s, 6H), 1.15 (s, 6H), 2.20 (d,
2H, J = 16.3 Hz), 2.29 (d, 2H, J = 16.3 Hz), 2.53 (s, 4H),
4.86 (s, 1H), 7.51 (dd, 2H, J1 = 7.0 Hz, J2 = 1.7 Hz), 8.12
(dd, 2H, J1 = 7.0 Hz, J2 = 1.7 Hz). 13C NMR (CDCl3, 125
MHz), d: 27.70, 29.64, 32.64, 32.79, 41.27, 51.03, 114.96,
123.83, 129.78, 146.92, 151.94, 163.36, 196.63.
83 (10.4); Anal. Calc. C, 77.14; H, 7.44; O, 15.41; Found
C, 76.98; H, 7.29; O, 15.26.
1
3l: mp 238-240 °C, (Lit.:24 236-238 °C; H NMR
(CDCl3, 500 MHz), d: 1.03 (s, 12H), 1.11 (s, 12H), 2.16 (d,
4H, J = 16.2 Hz), 2.21 (d, 4H, J = 16.2 Hz), 2.46 (d, 4H, J =
17.4 Hz), 2.56 (d, 4H, J = 17.4 Hz), 4.72 (s, 2H), 7.07-7.09
(m, 3H), 7.15 (s, 1H). 13C NMR (CDCl3, 125 MHz), d:
28.02, 29.57, 31.76, 32.56, 41.27, 51.27, 116.01, 126.84,
128.18, 128.66, 144.04, 162.72, 196.66.
1
3g: mp 252-254 °C, (Lit.:24 248-249 °C; H NMR
(CDCl3, 500 MHz), d: 0.98 (s, 6H), 1.07 (s, 6H), 2.13 (d,
2H, J = 16.2 Hz), 2.21 (d, 2H, J = 16.2 Hz), 2.45 (s, 4H),
5.51 (s, 1H), 7.21 (dt, 1H, J1 = 7.7 Hz, J2 = 1.4 Hz), 7.34 (d,
1H, J = 7.5 Hz), 7.41 (dt, 1H, J1 = 7.5 Hz, J2 = 1.2 Hz), 7.73
(d, 1H, J = 8.1 Hz). 13C NMR (CDCl3, 125 MHz), d: 28.00,
29.16, 29.36, 32.48, 41.26, 51.04, 114.60, 125.03, 127.59,
131.46, 132.40, 138.46, 150.27, 163.44, 196.73.
3m: mp 184-186 °C; IR (KBr): 3012, 2960, 1608,
1572, 1424, 1364, 1272, 1236, 1124, 1084, 856 (cm-1); 1H
NMR (CDCl3, 500 MHz), d: 1.01 (s, 6H), 1.12 (s, 6H), 2.18
(d, 2H, J = 16.3 Hz), 2.26 (d, 2H, J = 16.3 Hz), 2.50 (s, 4H),
4.73 (s, 1H), 7.16-7.18 (m, 1H), 7.72-7.75 (dt, 1H, J1 = 7.8
Hz, J2 = 1.9 Hz), 8.36 (dd, 1H, J1 = 4.7 Hz, J2= 1.5 Hz),
8.45 (d, 1H, J = 1.9 Hz); 13C NMR (CDCl3, 125 MHz), d:
27.79, 29.58, 32.63, 41.20, 51.03, 115.11, 123.44, 136.96,
140.07, 148.04, 149.90, 163.24, 196.73; MS (m/z): 353
(M+2) (22.2), 352 (M+1) (95.2), 351 (M+) (base peak), 273
(73.8), 217 (19.8), 161 (11.9), 78 (10.3); Anal. Calc. C,
75.19; H, 7.17; N, 3.99; O, 13.66; Found C, 74.99; H, 6.98;
N, 3.77; O, 13.49.
1
3h: mp 170-172 °C, (Lit.:24 170-172 °C; H NMR
(CDCl3, 500 MHz), d: 0.98 (s, 6H), 1.10 (s, 6H), 2.15 (d,
2H, J = 16.3 Hz), 2.24 (d, 2H, J = 16.3 Hz), 2.49 (s, 4H),
4.82 (s, 1H), 7.38 (t, 1H, J = 7.9 Hz), 7.79 (d, 1H, J = 7.7
Hz), 7.96 (dd, 1H, J1 = 8.0 Hz, J2 = 1.9 Hz), 8.02 (t, 1H, J =
1.9 Hz). 13C NMR (CDCl3, 125 MHz), d: 27.72, 29.61,
32.52, 32.66, 41.23, 51.06, 114.96, 122.06, 123.02,
129.21, 136.07, 146.74, 148.73, 163.46, 196.76.
1
1
3i: mp 242-244 °C, (Lit.:24 240-242 °C; H NMR
3n: mp 174-176 °C, (Lit.:24 176-178 °C; H NMR
(CDCl3, 500 MHz), d: 0.98 (s, 6H), 1.08 (s, 6H), 2.15 (d,
2H, J = 16.3 Hz), 2.21 (d, 2H, J = 16.3 Hz), 2.44 (s, 4H),
3.71 (s, 3H), 4.68 (s, 1H), 6.74 (dd, 2H, J1 = 6.8 Hz, J2 = 1.9
Hz), 7.19 (dd, 2H, J1 = 6.8 Hz, J2 = 1.9 Hz). 13C NMR
(CDCl3, 125 MHz), d: 27.76, 29.69, 31.38, 32.61, 41.29,
51.20, 55.52, 113.89, 116.21, 129.73, 136.98, 158.38,
162.48, 196.86.
(CDCl3, 500 MHz), d: 1.16 (s, 12H), 2.31 (d, 2H, J = 16.3
Hz), 2.35 (d, 2H, J = 16.3 Hz), 2.45 (d, 2H, J = 18.7 Hz),
2.50 (d, 2H, J = 17.8 Hz), 4.44 (d, 1H, J = 6.0 Hz), 6.30 (d,
1H, J = 16.0 Hz), 6.36 (dd, 1H, J1 = 16.0 Hz, J2 = 6.0 Hz),
7.17-7.20 (m, 1H), 7.26 (t, 2H, J = 7.5 Hz), 7.31 (dd, 2H, J1
= 7.1 Hz, J2 = 1.4 Hz). 13C NMR (CDCl3, 125 MHz), d:
28.02, 28.32, 29.66, 32.62, 41.38, 51.28, 114.95, 126.77,
127.51, 128.70, 130.84, 131.76, 137.69, 163.44, 196.91.
1
3j: mp 215-217 °C, (Lit.:24 217-218 °C; H NMR
(CDCl3, 500 MHz), d: 0.98 (s, 6H), 1.09 (s, 6H), 2.15 (d,
2H, J = 16.3 Hz), 2.20-2.23 (m, 5H), 2.45 (s, 4H), 4.70 (s,
1H), 7.00 (d, 2H, J = 8.0 Hz), 7.17 (d, 2H, J = 8.0 Hz). 13
ACKNOWLEDGMENT
C
Financial support for this work by the Research
Council of Persian Gulf University, Bushehr, Iran, is grate-
fully acknowledged.
NMR (CDCl3, 125 MHz), d: 21.47, 27.80, 29.69, 31.86,
32.62, 41.30, 51.20, 116.19, 128.66, 129.20, 136.17,
141.63, 162.51, 196.80.
3k: mp > 300 °C (dec.); IR (KBr): 3010, 2950, 2881,
1660, 1620, 1462, 1425, 1365, 1200, 1162, 1003, 808
(cm-1); 1H NMR (CDCl3, 500 MHz), d: 0.87 (s, 12H), 0.99
(s, 12H), 2.07 (d, 4H, J = 16.2 Hz), 2.11 (d, 4H, J = 16.2
Hz), 2.33 (d, 4H, J = 17.6 Hz), 2.39 (d, 4H, J = 17.6 Hz),
4.59 (s, 2H), 6.99 (s, 4H); 13C NMR (CDCl3, 125 MHz), d:
27.80, 29.41, 31.03, 32.57, 41.12, 51.10, 115.91, 128.16,
142.15, 162.93, 196.73; MS (m/z): 624 (M+2) (1.6), 623
(M+1) (8.8), 622 (M+) (19.2), 416 (9.6), 350 (85.6), 349
(base peak), 311 (9.6), 273 (76.0), 217 (31.2), 161 (20.0),
Received December 31, 2008.
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