Mg(BF4)2 doped in [BMIm][BF4]: A homogeneous ionic liquid-catalyst for efficient synthesis of 1,8-dioxo-octahydroxanthenes, decahydroacridines and 14-aryl-14H-dibenzo[a,j] xanthenes

Article abstract of DOI:10.1016/j.molcata.2012.07.026

A homogeneous ionic liquid made up of 0.5 mol% Mg(BF₄) ₂ doped in [BMIm]BF₄ was obtained from a mixture of MgCl₂ and [BMIm]Cl subjected to anion exchange with NaBF₄. The ionic liquid efficiently catal

Full text of DOI:10.1016/j.molcata.2012.07.026

Journal of Molecular Catalysis A: Chemical 363–364 (2012) 465–469
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Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Mg(BF4)2 doped in [BMIm][BF4]: A homogeneous ionic liquid-catalyst for
efficient synthesis of 1,8-dioxo-octahydroxanthenes, decahydroacridines and
14-aryl-14H-dibenzo[a,j]xanthenes
Kurosh Rad-Moghadam∗, Seyyedeh Cobra Azimi
Chemistry Department, University of Guilan, 41335-19141 Rasht, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
A homogeneous ionic liquid made up of 0.5 mol% Mg(BF4)2 doped in [BMIm]BF4 was obtained from a
mixture of MgCl2 and [BMIm]Cl subjected to anion exchange with NaBF4. The ionic liquid efficiently
catalyzes the synthesis of 1,8-dioxo-octahydroxanthenes, 14-aryl-14H-dibenzo[a,j]xanthenes, and 1,8-
dioxo-decahydroacridines, giving excellent yields in short periods of time at 80 ◦C. The integrity of the
ionic liquid remains reasonably unchanged when it is separated from the reaction mixture by water
extraction, as it can be recycled several times without any loss of activity in each of the title syntheses.
Magnesium cation, though in low concentration, appeared as an essential element of this ionic liquid
catalysis, perhaps due to its non-coordinated existence giving rise to a promoted Lewis acidic character-
istic. Application of this new homogeneous catalyst system offered the advantages of high yields, short
reaction times, and easy workup procedure compared to the conventional methods of the syntheses.
© 2012 Elsevier B.V. All rights reserved.
Received 4 April 2012
Accepted 23 July 2012
Available online 31 July 2012
Keywords:
Xanthenes
Acridines
Ionic liquid
Magnesium cation
Homogeneous catalyst
1. Introduction
properties and in turn on the properties of their cationic and
anionic constituents. Another aspect of ionic liquids is their poten-
Though ionic liquids belong to the class of strongly polar
solvents capable to dissolve both organic and inorganic materi-
als, they may be designed non-coordinating, in contrast to polar
molecular solvents, to avoid any undesirable binding to metal cat-
alysts in pre-transition states and hence offer great advantages
in terms of catalysis and selectivity [1–4]. In such conditions,
metal catalysts exist relatively uncomplexed and their almost
bare cations are expected to have higher activity than in tra-
ditional polar organic solvents. Application of the ionic liquids
tial to bias reaction equilibriums in favor of products. For example,
hydrophilic ionic liquids tend to facilitate dehydration of interme-
diate products by stabilizing the eliminated water molecules [8].
Owing to these fascinating features associated with easy phase-
separation from reaction mixtures, ionic liquids have achieved an
outstanding position as a new generation of solvents in the area
of organic synthesis. In continuation of our studies focused on
development of ionic liquids as catalysts in synthesis of organic
compounds [9–12], herein we describe the efficiencies of the ionic
liquid composed of Mg(BF4)2 doped in [BMIm][BF4] for promoting
the synthesis of 1,8-dioxo-octahydroxanthenes, 14-aryl-14H-
dibenzo[a,j]xanthenes and 1,8-dioxo-decahydroacridines.
Xanthenes and benzoxanthene derivatives are the parent frame-
works found in a large number of naturally occurring as well as
synthetic products possessing prominent positions in medicinal
chemistry [13,14]. Xanthenediones are likewise special structural
units constituting various natural products [15] and being used as
versatile synthons, because of inherent reactivity of their inbuilt
pyran ring [16]. Consequently, many alternative catalysts such as
p-toluenesulfonic acid [17], p-dodecylbenzenesulphonic acid [18],
triethylbenzylammonium chloride [19], diammoniumhydrogen
phosphate under various conditions [20], sulfonic acid under ultra-
sonic irradiation [21], InCl3/ionic liquid [22], Fe3+–montmorilonite
[23], NaHSO4–SiO2 or silica chloride [24], amberlyst-15 [25], P2O5
−
−
composed of weakly coordinating anions, such as BF4 and PF6
,
together with suitably substituted cations often resulted in a
pronounced chemical reactivity of the dissolved catalysts and
therefore led to improvement of both reactivity and selectivity
[5–7]. Moreover, ionic liquids by virtue of their organic and ionic
nature are potent solvents to exert nearly all kinds of interac-
tions on reacting species including transition states. Usually, these
interactions alternatively give rise to considerable reductions in
activation energies either by stabilization of transition states or
destabilization of reactants. As a result, the catalytic activity of
ionic liquids strongly depends on their chemical and physical
∗
Corresponding author.
E-mail address: radmm880@gmail.com (K. Rad-Moghadam).
1381-1169/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molcata.2012.07.026

466
K. Rad-Moghadam, S.C. Azimi / Journal of Molecular Catalysis A: Chemical 363–364 (2012) 465–469
Table 1
Optimization of reaction conditions for the model reactants, 5,5-dimethyl-1,3-cyclohexanedione and 4-chlorobenzaldehyde.
Cl
O
O
O
O
H
Ionic liquid or neat
+
O
Cl
O
1
2
3
Entry
Ionic liquida
Temperature (◦C)
Time
12 h
Yield (%)b
1
2
3
4
5
6
7
8
[Bmim][BF4]–0.5 mol% Mg(BF4)2
[Bmim][BF4]–0.5 mol% Mg(BF4)2
[Bmim][BF4]–0.5 mol% Mg(BF4)2
[Bmim][BF4]–0.5 mol% Mg(BF4)2
[Bmim][BF4]–0.5 mol% Mg(BF4)2
[Bmim][BF4]–0.5 mol% Mg(BF4)2
[Bmim][BF4]–0.5 mol% Mg(BF4)2
[Bmim][BF4]–0.5 mol% Mg(BF4)2
[Bmim][BF4]
20
30
40
50
60
70
80
90
80
80
80
80
Trace
15
25
42
68
80
87
87
–
10 h
7 h
3 h
1.5 h
40 min
15 min
15 min
12 h
12 h
4 h
9
10
11
12
[Bmim][Cl]
–
51
45
[Bmim][BF4]–0.5 mol% MgCl2
Neatc
12 h
a
The reactions were carried out in the presence of 1 mL of the ionic liquid.
Isolated yields.
The reaction was run under neat condition.
b
c
or InCl3 [26], montmorilonite [27], and silica sulfuric acid [28] have
been used for the synthesis of xanthenedione derivatives. How-
ever, each of the methods have their own merit some methods
suffer from long reaction times, low yields, use of large quantities
of volatile organic solvents, harsh reaction conditions, and tedious
workup. Therefore, development of an alternative efficient and ver-
satile method for synthesis of these valuable compounds could
be so worthwhile. 1,4-Dihydropyridines represent an important
class of compounds which are found in many sorts of biologically
active compounds such as vasodilators, bronchodilators, anti-
atherosclerotics, antitumors, geroprotectives, hepatoprotective
agents [29], and calcium channel blockers [30,31]. 1,8-Dioxo-
decahydroacridines with inbuilt 1,4-dihydropyridine nucleus have
shown very high lasing efficiencies [32] and hence used as pho-
toinitiators [33]. As a consequence of these interesting properties,
a large library of acridinediones has been synthesized by the reac-
tion of aldehydes with 2 equivalents of 1,3-cyclohexanedione or
5,5-dimethyl-1,3-cyclohexanedione and appropriate amines via
various methods [34–38].
in favor of increased entropy and without receiving significant
solvating interactions with the ionic liquid. The new ion–ion elec-
trostatic bondings created on dissolution of Mg2+ ions in [BMIm]BF4
are rather weak forces assuming to be insufficient to completely
cover the lattice energy of the initial magnesium salt. It was ini-
tially difficult to expect that such a low-concentration of Mg2+ ions
could impart any considerable catalysis activity to the ionic liquid.
However, the ionic solution appeared as a very efficient catalyst
when applied to three defined acid-demanded trial reactions. Cer-
tainly, the catalysis activity is attributed to the dissolved Lewis
acidic Mg2+ ions, as the reactions have not remarkably proceeded
in the Mg2+-free ionic liquid, [BMIm]BF4. Activity of the catalyst
was first evaluated for synthesis of xanthene-1,8-diones by exam-
ining its impact on the rate and yield of the model reaction between
5,5-dimethyl-1,3-cyclohexanedione 1 and 4-chlorobenzaldehyde.
To determine the optimum condition and also to discerning the
main element of the catalysis, the reaction was carried out at vari-
able temperatures and alternatively in closely related ionic liquids
(Table 1). It could be seen that the best result was obtained with
1 mL of [BMIm][BF4]–0.5 mol% Mg(BF4)2 at 80 ◦C (Table 1, entry 7).
Monitoring by TLC has shown no detectable progress of the reaction
in the ionic liquids [BMIm]BF4 and [BMIm]Cl. These observations
are addressing the catalysis interactions to Mg2+ ion and not owing
to strictly ionic nature of the catalyst. Interestingly, the model reac-
tion gave low yield in even long time when it was run at 80 ◦C in
the ionic liquid [BMIm][BF4]–0.5 mol% MgCl2 being made simply
by dissolving 0.5 mol% MgCl2 in [BMIm][BF4]. In this ionic solution,
the Mg2+ ions are assumed to be relatively tethered by coordination
with chloride ions. This finding clearly lends considerable credence
to hypothesis that Mg2+ ion gains an enhanced catalytic activity in
the non-coordinating medium of [BMIm]BF4.
2. Results and discussion
Magnesium constitutes the central element of several fre-
quently used efficient catalysts and involves, at normal oxidation
state, in bioactivities of living organisms implying its bio-friendlier
consistency. Despite of these fascinating features and several recent
reports on utility of magnesium compounds as catalysts [39–41], so
far the catalysis capacity of magnesium cation has not been exam-
ined in ionic-liquid medium. It seems reasonable to expect that
Mg2+ cations dissolved in less coordinative ionic medium would
have intensed chemical activity. To deal with this idea, we required
a homogeneous solution of Mg2+ in an ionic liquid composed of a
bulky organic cation and an almost non-coordinating anion. In this
regard, a room temperature ionic liquid was prepared via saturation
of [BMIm]BF4 with Mg(BF4)2. Atomic absorption analysis of this liq-
uid has shown that it is 0.5 mol% in concentration of dissolved Mg2+
ion. This low saturated concentration of Mg2+ is not more surpris-
ing by considering that it is a hard cation being dissolved mostly
After optimizing the conditions, the scope of method was suc-
cessfully testified by using a wide range of aromatic aldehydes at
80 ◦C. The results, summarized in Table 2, delineate the efficiencies
of the method in terms of fairly high yields of the products and
short reaction times.
To compare the efficiencies of this ionic liquid system with the
previously reported catalysts in synthesis of xanthenes, we have

K. Rad-Moghadam, S.C. Azimi / Journal of Molecular Catalysis A: Chemical 363–364 (2012) 465–469
467
Table 2
Synthesis of 1,8-dioxo-octahydroxanthenes by the reaction of 5,5-dimethyl-1,3-cyclohexanedione and aromatic aldehydes in [BMIm][BF4]–0.5 mol% Mg(BF4)2 at 80 ◦C.
O
O
Ar
O
[BMIm][BF4]- Mg(BF4)2
80°C
+
Ar-CHO
O
O
1
2a-k
3a-k
Producta
Ar
Time (min)
Yield (%)b
3a
3b
3c
3d
3e
3f
3g
3h
3i
C6H5
2-ClC6H4
4-ClC6H4
30
15
15
15
15
30
30
30
30
30
30
97
92
86
87
86
92
82
95
91
81
97
3-NO2C6H4
4-NO2C6H4
4-FC6H4
4-HOC6H4
4-H3CC6H4
4-H3COC6H4
4-N(CH3)2C6H4
Cinnamaldehyde
3j
3k
a
Isolated yield.
Refs. [18,23].
b
gathered the results obtained by using this ionic liquid and those
reported on application of some other catalysts to the synthesis of
9-(2-chlorophenyl)-1,8-dioxo-octahydroxanthene 3b in Table 3. As
is shown in Table 3, effecting the reaction in the ionic liquid appre-
ciably improves the synthesis in several aspects featured by some
other methods, for example higher temperatures, longer reaction
times, lower yields, and limitation of the substrate applicability.
The reaction goes to complete in shorter time and affords high yield
when is run in the [BMIm]BF4–0.5 mol% Mg-(BF4)2 at 80 ◦C.
Further evidences on efficiency of this catalyst were
obtained by application of the ionic system to the synthesis
of dibenzo[a,j]xanthenes using the pseudo-three-component
reaction between 2-naphthol and various aldehydes at 80 ◦C. As
is shown in Table 4, the reactivity of 2-naphthol with aldehydes
in the ionic liquid spans a considerable range, affording a variety
of 14-substituted-14H-dibenzo[a,j]xanthenes. It is noteworthy
that the presence of electron-donating and electron-accepting
as well as steric demanded substituents on the reacting alde-
hydes does not affect the overall yield and rate of the reactions
sensibly.
However specifying the exact catalysis events through the for-
mation of xanthenes awaits for further investigations, a reasonable
interpretation for catalytic actions of the ionic liquid based on a
more accepted mechanism of the reactions [25,26] was depicted in
Scheme 1. The xanthene synthesis presumably initiates by nucle-
ophilic addition of the substrate phenol 4 or the enol 1 on the
reactant aldehyde activated by magnesium cation. The resulting
adduct 6 undergoes dehydration to give the key enone interme-
diate 7 which is likely activated by magnesium cation to follow a
Michael type addition onto the second molecule of 1 or 4. The later
reaction would produce the bis-phenol or alternatively the bis-enol
intermediate 8 ensuing a magnesium cation mediated dehydration
to afford the xanthenes 3 or 5.
Encouraged by these results, we aimed to evaluate the catalytic
method for the reaction of 5,5-dimethyl-1,3-cyclohexanedione
1, an aldehyde, and NH4OAc in 1 mL of [BMIm][BF4]–0.5 mol%
Mg(BF4)2 at 80 ◦C to afford 1,8-dioxo-decahydroacridine deriva-
tives 9a–j. Again the reactions went to completion within short
periods of time to give fairly high yields of the products. Here
also the aromatic aldehydes containing electron-donating or
electron-withdrawing groups afforded fairly high yields of 1,8-
dioxo-decahydroacridines (Table 5). All the products are known
compounds, and their identity were verified by IR and 1H NMR
spectroscopic analysis as well as comparison of their physical data
with those of authentic samples.
Workup procedure is so simple for all the syntheses and includes
addition of water at the end of reaction, filtration, and finally
recrystallization of the products from ethanol. The ionic liquid was
removed simply by dissolution in water added to the reaction mix-
ture and then recovered by evaporation of water at 80 ◦C under
reduced pressure. The catalytic activity of the ionic liquid system
remained evidently unaltered even after five successive retrieval
and when reused in the same synthesis (Table 6).
3. Experimental
All the solvents and reagents were purchased from Fluka or
Merck chemical companies. Melting points were measured on an
Electrothermal apparatus and are uncorrected. IR spectra were
obtained in KBr discs on a Shimadzu IR-470 spectrometer. 1H NMR
spectra were recorded on a BRUKER DRX-300 AVANCE spectrome-
ter at 300.13 MHz.
Table 3
Synthesis of 9-(2-chlorophenyl)-1,8-dioxo-octahydroxanthene 3b using [BMIm][BF4]–0.5 mol% Mg(BF4)2 and comparison with some other reported catalysts.
Entry
Catalyst/solvent/conditions
Time (min)
Yield (%)
References
1
2
3
4
5
6
7
NaHSO4·SiO2/CH3CN/reflux
390
360
360
300
360
60
90
92
92
90
90
85
92
[24]
[24]
[23]
[25]
[18]
[27]
This work
Silica chloride/CH3CN/reflux
Fe3+-montmorillonite/solvent-free/100 ◦
Amberlyst-15/CH3CN/reflux
C
C
p-Dodecylbenezenesulfonic/H2O/reflux
Montmorillonite K10/solvent-free/120 ◦
[BMIm]BF4–0.5 mol% Mg(BF4)2/neat/80 ◦
C
15

468
K. Rad-Moghadam, S.C. Azimi / Journal of Molecular Catalysis A: Chemical 363–364 (2012) 465–469
Table 4
Synthesis of 14-aryl-14H-dibenzo[a,j]xanthenes catalyzed by [BMIm][BF4]–0.5 mol% Mg(BF4)2 at 80 ◦C.
Ar
OH
[BMIm][BF4]- Mg(BF4)2
+
Ar-CHO
80°C
O
4
2a-l
5a-l
Producta
Ar
Time (min)
Yield (%)b
5a
5b
5c
5d
5e
5f
5g
5h
5i
C6H5
2-ClC6H4
3-ClC6H4
4-ClC6H4
3-NO2C6H4
4-NO2C6H4
4-FC6H4
4-HOC6H4
4-H3CC6H4
4-H3COC6H4
2,4-(Cl)2C6H3
15
15
15
15
30
30
30
30
30
30
30
30
95
98
86
94
98
98
84
98
95
91
85
95
5j
5k
5l
Cinnamaldehyde
a
All the products were characterized by comparison of their spectroscopic and physical data with those reported in literature.
Refs. [17,25,26].
b
O
Mg2+
Ar
Ar
H
2
O
3 or 5
Mg2+
O
1 or 4
H2O-Mg2+
Ar
O
O
H
Ar
H
H2O-Mg2+
Ar
Ar
OH
Mg2+
1 or 4
Mg2+
O
OH
OHHO
8
Mg2+
2 Mg2+
7
6
Scheme 1. A plausible mechanism for synthesis of xanthenes mediated by catalysis of magnesium cation.
Table 5
Synthesis of 1,8-dioxo-octahydroacridines by the reaction of 5,5-dimethyl-1,3-cyclohexanedione, aromatic aldehydes, and NH4OAc in the ionic liquid [BMIm][BF4]–0.5 mol%
Mg(BF4)2 at 80 ◦C.
O
O
Ar
O
[BMIm][BF4]- Mg(BF4)2
80°C
+
NH4OAc
+
Ar-CHO
O
N
H
9a-j
1
2a-j
Producta
Ar
C6H5
2-ClC6H4
4-ClC6H4
3-NO2C6H4
4-NO2C6H4
4-BrC6H4
4-HOC6H4
4-H3CC6H4
4-H3COC6H4
4-N(CH3)2C6H4
Time (min)
Yield (%)b
9a
9b
9c
9d
9e
9f
9g
9h
9i
15
15
15
15
30
30
15
30
30
30
82
98
95
98
85
78
96
81
82
80
9j
a
Isolated yield.
Refs. [33–38].
b

K. Rad-Moghadam, S.C. Azimi / Journal of Molecular Catalysis A: Chemical 363–364 (2012) 465–469
469
Table 6
for the synthesis of 1,8-dioxo-octahydroxanthenes, 14-aryl-14H-
dibenzo[a,j]xanthenes, and 1,8-dioxo-decahydroacridines. The
ionic liquid system tolerates the presence of various functional
groups on the aldehydes used in this investigation and have the
advantages of being inexpensive, easily available, and reusable. It
is a dilute homogeneous metal catalyst having the capacity to dis-
solve the reactants and tolerating air and moisture due to stability
of its ionic constituents. Application of this catalyst offered a sim-
ple, clean, and cost effective method for the synthesis of the title
compounds. It is important to note that in the absence of the ionic
liquid, the yields of reactions were traced even at 80 ◦C and longer
reaction time.
The effect of [BMIm][BF4]–0.5 mol% Mg(BF4)2 recycling on the xanthene yield.
Run
Cycle
Yield (%)a
1
2
3
4
5
Fresh
86
85
85
84
83
1
2
3
4
Reaction conditions: 5,5-dimethyl-1,3-cyclohexanedione 1, 2.0 mmol; 4-
chlorobenzaldehyde 2c, 1.0 mmol; 1 mL of [BMIm][BF4]–0.5 mol% Mg(BF4)2
at 80 ◦C.
a
Isolated yields.
3.1. Synthesis of the ionic liquid [Bmim][BF4]–0.5 mol% Mg(BF4)2
Acknowledgment
1-Methylimidazole (5.1 g, 62.1 mmol) was added to 32 mL of 1-
chlorobutane. The mixture was heated to reflux for 24 h and then
cooled to room temperature. The obtained oily product was sepa-
rated from reaction mixture by decanting and washed with EtOAc
(2× 20 mL). Solvent of the collected organic phase was removed
under reduced pressure to give 1-butyl-3-methylimidazolium
chloride ([BMIm]Cl). In second step, to a solution of [BMIm]Cl (7.0 g,
40.1 mmol) and MgCl2 (1.0 g, 10.5 mmol) diluted with dry ace-
tone (50 mL) was added NaBF4 (8.7 g, 79.2 mmol). The mixture was
stirred for 48 h at room temperature to get a solution and then
to afford precipitation of NaCl. After separation of the precipitates
(mainly NaCl associated with excess Mg2+ salts) the solvent of the
filtered solution was removed under reduced pressure whereupon
the ionic liquid [BMIm] BF4 doped with Mg(BF4)2 was obtained as
a yellow oil. Atomic absorption analysis of the obtained ionic liquid
has revealed a concentration of 0.6 mg/mL of Mg2+ being equal to
0.5 mol% of Mg2+ in the ionic liquid.
We gratefully acknowledge the financial support from the
Research Council of University of Guilan.
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1,8-dioxo-octahydroxanthenes and
14-aryl-14H-dibenzo[a,j]xanthene derivatives
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To a mixture of an aromatic aldehyde (1 mmol) and 5,5-
dimethyl-1,3-cyclohexanedione or 2-naphthol (2 mmol) in a round
bottom flask was added 1 mL of [BMIm][BF4]–0.5 mol% Mg(BF4)2.
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3.3. General procedure for the synthesis of
1,8-dioxo-decahydroacridine derivatives
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solution of an aromatic aldehyde (1 mmol) and 5,5-dimethyl-1,3-
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4. Conclusion
An enhanced catalytic activity was observed for the mag-
nesium salt of a less coordinative anion, Mg(BF4)2, dissolved
in the ionic liquid [BMIm]BF4. Based on this finding, the
ionic liquid [BMIm]BF4 doped with 0.5 mol% Mg(BF4)2 was pre-
pared and employed in development of an expedient method