Preparation of Lewis acid ionic liquids for one-pot synthesis of benzofuranol from pyrocatechol and 3-chloro-2-methylpropene

Article abstract of DOI:10.1515/chempap-2015-0145

Several Lewis acid ionic liquids (LAILs) with different acidic scales were synthesised and used as catalysts for the synthesis of benzofuranol by condensation of pyrocatechol and 3-chloro-2-methylpropene in one pot. The catalytic activity of these ionic liquids was correlated with their Lewis acidity. Low to moderate conversion with excellent selectivity to benzofuranol was obtained in the presence of the appropriate LAILs. Compared to the two-step synthetic method currently used in industry, a higher yield plateau (81.1 %) of benzofuranol was achieved in the presence of [BMIm][AlCl₄] IL as catalyst at 418 K after 4 h. Furthermore, the catalyst is readily separated from the resultant products via decantation and could be reused after treatment in vacuum.

Full text of DOI:10.1515/chempap-2015-0145

Chemical Papers 69 (10) 1361–1366 (2015)
DOI: 10.1515/chempap-2015-0145
ORIGINAL PAPER
Preparation of Lewis acid ionic liquids for one-pot synthesis
of benzofuranol from pyrocatechol and 3-chloro-2-methylpropene
b
a
a
a
aHan-Cheng Zhou*, Xiu-Lei Li, Juan-Li Liu, Cheng Peng, Bin Zhang,
cJin Chen, Qiong Su, Lan Wu, You-Zhu Yuan*
a
a
c
aKey Laboratory for Utility of Environment-Friendly Composite Materials and Biomass in University of Gansu Province,
College of Chemical Engineering, Northwest University for Nationalities, Lanzhou 730030, China
bSchool of Physical Science and Technology, Lanzhou University, Lanzhou 730000, China
cState Key Laboratory of Physical Chemistry of Solid Surfaces and National Engineering Laboratory for Green Chemical
Productions of Alcohols-Ethers-Esters, College of Chemistry and Chemical Engineering, Xiamen University,
Xiamen 361005, China
Received 15 December 2014; Revised 8 April 2015; Accepted 22 April 2015
Several Lewis acid ionic liquids (LAILs) with different acidic scales were synthesised and used
as catalysts for the synthesis of benzofuranol by condensation of pyrocatechol and 3-chloro-2-
methylpropene in one pot. The catalytic activity of these ionic liquids was correlated with their
Lewis acidity. Low to moderate conversion with excellent selectivity to benzofuranol was obtained
in the presence of the appropriate LAILs. Compared to the two-step synthetic method currently
used in industry, a higher yield plateau (81.1 %) of benzofuranol was achieved in the presence of
[BMIm][AlCl4] IL as catalyst at 418 K after 4 h. Furthermore, the catalyst is readily separated from
the resultant products via decantation and could be reused after treatment in vacuum.
c
ꢀ 2015 Institute of Chemistry, Slovak Academy of Sciences
Keywords: benzofuranol, condensation, Lewis acid ionic liquid, acidity
Introduction
and anti-inflammatory activities (Yang et al., 2006)
and pterocarpans, which contain a benzofuran moi-
ety, exhibit antiviral and antibacterial properties (Seo
et al., 2011). Apart from its application in drugs, ben-
zofuranol is also the direct source of some agrochemi-
cals such as arbofuran, carbosulphan and furathiocab
(Tamura et al., 2003). Hence benzofuranol plays an
important role in the manufacture of drugs and agro-
chemicals. The current pathway for the production
of benzofuranol in industry is shown in Fig. 1; this
pathway entails two separate steps. In the first step,
the etherification of pyrocatechol (I ) and 3-chloro-3-
methylpropene (II ) is performed in the presence of
alkali metal carbonate or hydroxide as catalysts to
form 2-(2-methylallyloxy)phenol (III ). Subsequently,
benzofuranol (V ) is prepared by the consecutive re-
arrangement and cyclisation of III catalysed by acidic
Benzofuranol (2,2-dimethyl-2,3-dihydrobenzofu-
ran-7-ol) and its derivatives, which are important in-
termediates of drugs and agrochemicals, have been
extensively applied in pharmaceutical synthesis and
pesticide production (Engler et al., 1993; Kossakowski
et al., 2006, 2009). For example, 4,6-di-tert-butyl-2,2-
dipentyl-2,3-dihydrobenzofuran-5-ol, an important
derivative of benzofuranol, which performs a more ef-
fective inhibition of the peroxidation of low-density
lipoprotein in vivo than natural antioxidants such as
vitamin E, has been used as an antiatherogenic agent
(Murakata & Kimura, 2010). Similarly, as the deriva-
tives of benzofuranol, spirocyclicbenzofuranones are
the main components of antimycotics such as griseo-
fulvin and aromatase inhibitors due to their herbicidal
*Corresponding author, e-mail: hanchengzhou@hotmail.com; yzyuan@xmu.edu.cn
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H. C. Zhou et al./Chemical Papers 69 (10) 1361–1366 (2015)
Fig. 1. Route for synthesis of benzofuranol (V ) from pyrocatechol (I) and 3-chloro-2-methylpropene (II).
Fig. 2. Structures of ionic liquids synthesised.
substances such as ZnCl2, SiO2-Al2O3 or γ-Al2O3 in
the second step. Clearly, this process possesses some
drawbacks. For example, a large amount of salts is
produced as a by-product in the firststep. The sep-
aration of these salts prolongs the production time
and inevitably increases the overall cost. In addition,
there are problems with corrosion and the tedious sep-
aration of the product-catalyst caused by using acidic
substances as catalysts in the second step. Accord-
ingly, the development of an efficient and eco-friendly
method for the synthesis of V is highly desirable.
Ionic liquids (ILs), owing to their negligible vapour
pressure, high thermal stability, wide electrochemistry
window and selective solubility, have gained recogni-
tion as environmentally benign media and catalysts
and have been reported in a number of applications
(Handy, 2011; Welton, 1999; Greaves & Drummond,
2008). Lewis acid ILs (LAILs), an interesting subset of
ILs, have been successfully applied in several organic
reactions (MacFarlance et al., 2006). For instance, in
2002, Harjani et al. (2002) reported that 1-butyl-3-
methylimidazolium chloroaluminate LAIL functioned
well as catalyst and solvent in the Knoevenagel con-
densation of benzaldehyde and substituted benzalde-
hydes with diethyl malonate to afford benzylidene-
malonate, whereas it was not a suitable solvent for
the linear dimerisation of 1-butene in the presence of a
nickel complex as catalyst (Chauvin et al., 1997). Zhao
et al. (2007) described 1-ethyl-3-methylimidazolium
chlorochrominate IL as an efficient catalyst in con-
verting sugars to 5-hydroxymethylfurfural. Recently,
Yue et al. (2013) used [BMIm][ZnCl3] LAIL as cata-
lyst for the degradation of poly(ethylene terephtha-
late); the conversion achieved 100 % and the yield
of the corresponding product was 83.8 %. Also, Sri-
vastava et al. found that the bi-functional acidic
IL [MImC3SO3H][ZnCl3] is an effective catalyst for
the Beckmann rearrangement of acetophenoneoxime
(Kore & Srivastava, 2013). Clearly, in comparison with
inorganic acid catalysts, the use of LAILs in organic
synthesis not only enhances the yields, but reduces the
pollution and also simplifies the product separation.
In effect, Lewis acids play an important role as
catalysts in the second step of the process for pro-
ducing benzofuranol (Fig. 1). However, to the best of
our knowledge, to date there have been few attempts
to use LAILs as catalysts for the synthesis of V. The
present work sought an efficient and environmentally
friendly synthesis of V from I and II in one pot using
LAILs as catalysts, with the highest plateau achieved
in the presence of [BMIm][AlCl4] IL.
Experimental
Pyrocatechol (99 mass %) and 3-chloro-2-methyl-
propene (99 mass %) were acquired from Alfa Ae-
sar (China), 1-methylimidazole (AR), 1-chlorobutane
(AR), acetonitrile (AR), anhydrous aluminium chlo-
ride (AR) and other chemicals (AR) were pur-
chased from Sinopharm Chemical Reagent (China).
1-Methylimidazole and 1-chlorobutane were double-
distilled immediately prior to use. All other chemicals
were used as received.
General preparation of Lewis acid ILs
These LAILs were synthesised following the pro-
cedures described in the literature (Fig. 2) (Chau-
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H. C. Zhou et al./Chemical Papers 69 (10) 1361–1366 (2015)
1363
vin et al., 1997; Harjani et al., 2002; Zhao et al.,
2007; Olivier-Bourbigou et al., 2010; Yue et al., 2013).
terised by IR. Fig. 3A shows that two IR bands at
2250 cm−1 and 2291 cm−1, due to C—N stretch-
—
—
1
The structures were confirmed by H and 13C NMR
ing vibrations, can be observed in pure acetoni-
studies and thermal gravimetric analysis (TGA). The
NMR spectroscopy was performed on a Bru¨ker AV-400
Fourier transform NMR spectrometer using an inner
capillary filled with CDCl3. Chemical shifts were re-
ported in parts per million (ppm, δ). Thermal gravi-
metric analysis (TGA) was performed using asimulta-
neous thermal analysis-STA 409EP. The thermal de-
composition temperature (Td) was recorded with 10 %
of mass loss of LAILs with a scan-rate of 10 K min−1
under N2 atmosphere.
trile (curve a). After different LAILs were added
in acetonitrile, the IR bands of C—N shifted to
—
—
the higher wavenumbers due to the strong interac-
tion between acetonitrile and LAILs. The stronger
the acidity, the stronger the interaction and the
larger blue shift induced. Accordingly, the relative
acid strength of these LAILs has the following or-
der, from the right: [BMIm][Al2Cl7]; [BMIm][AlCl4];
[BMIm][ZnCl3];
[BMIm][SnCl3];
[BMIm][FeCl4];
[BMIm][CuCl2] (Chen et al., 2010). Clearly, the an-
ions play a crucial role in determining the acidity of
these LAILs. The change in the anions leads to the
great difference in acidity.
Acidity measurement
Measurement of the acidic scale of LAILs was con-
ducted on a Jasco 610 instrument using KBr technique
following the procedures previously reported in the lit-
erature (Yang & Kou, 2004; Chen et al., 2010). The
specific band stretches of IR spectra recorded the rel-
ative acidities of these LAILs.
Catalytic test of LAILs
Prior to investigating the performance of differ-
ent LAILs in the catalysed condensation of I and II,
[BMIm][AlCl4] IL was selected due to its immiscibility
with toluene and higher apparent viscosity, as catalyst
in the biphasic catalysis to optimise the reaction con-
ditions including the molar ratio of reactants, reaction
temperature, timeand mediumprofile.The results are
listed in Table 1. It can be seen that the higher tem-
perature promptedthe conversion of I, but too high a
temperature decreased the selectivity of V (Table 1,
entries 1–3 and 4). For instance, a 76.2 % conversion
of I with 95.6 % selectivity to V could be obtained
with a 5 % molar ratio of the catalyst to substrates
at 418 K for 4 h (Table 1, entry 3) whereas, when the
reaction temperature was increased to 433 K, a higher
conversion of I (86.0 %) was obtained at the cost of the
selectivity to V (Table1, entry 4). Furthermore, better
results/yields were obtained by increasing the ratio of
either II or [BMIm][AlCl4] IL to the totalsubstrates.
For example, a 79.4 % conversion of I accompanied
with 99.1 % selectivity to V (Table 1, entry 5) was
obtained as a result of the enhancement of the ratio
of II. In comparison with the results for entry 5, there
is no significant effect to further enhance the ratio of
II (Table 1, entry 6). Similarly, the highest yield of V
(81.1 %) was achieved by increasing the catalysts-to-
substrate molar ratio from 5 % to 8 % (Table 1, entries
5 and 7). Further increase in the amount of catalysts
led to a reduction in the selectivity of V, although the
conversion ratio of Iwas increased slightly (Table 1,
entry 8). It is also worthnoting that no significant in-
crease either in the conversion of I or selectivity to V
occurred as a result of the reaction time being pro-
longed from 4 h to 6 h or even 8 h (Table 1, entries 7,
9 and 10). Hence, optimisation of the reaction condi-
tions for the[BMIm][AlCl4] catalyst revealed that the-
maximum yield of V was obtained at 418 K for 4 h,
with a molar ratio of I : II : ILs = 50 : 60 : 8, and the
isolated yield was 76.2 %.
Synthesis ofbenzofuranol
3.0 mmolof pyrocatechol, calculated amounts of
3-chloro-2-methylpropene and ILs were added to a
100 mL Parr reactor with 23 mL of toluene. The reac-
tion mixture was heated to 418 K and held constant
for the desired time. The resultant mixture formed two
layers after cooling to ambient temperature, and the
upper phase was separated and analysed using a cap-
illary GC (SGE BPX5: 30 m × 0.25 mm × 0.25 m)
equipped with a flame ionisation detector and o-xylene
as an internal standard. The toluene in the separated
upper phase was removed by distillation and the yield
of benzofuranol was obtained.
Results and discussion
Characterisation of LAILs
The LAILs were synthesised with yields of 93–
98 %. Most of them were miscible with polarorganic
solvents such as butane-1-ol, acetonitrile and alcohol,
but insoluble or partly soluble in non-polar organic
solvents such as benzene and diethyl ether. The sin-
gle decomposition peak in the TGA profiles (see S3
in Supporting Information) implies that the synthe-
sised LAILs were not mixed compounds, which ac-
cords with the NMR data (see S4–S5 in Supporting
Information). The thermal stability of these LAILs
was characterised by the thermal decomposition tem-
perature (Td), ranging from 238◦C to 410◦C.
Acidity of LAILs
The relative acidity of these LAILs was charac-
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H. C. Zhou et al./Chemical Papers 69 (10) 1361–1366 (2015)
Table 1. Synthesis of (V ) from (I) and (II) catalysed by [BMIm][AlCl4] under different conditionsa
Selectivity/%c
Entry
I : II : IL (molar ratio)
Temperature/K
Time/h
Conversion/%c
III
V
Othersb
1
2
3
4
5
6
7
8
9
50 : 50 : 5
50 : 50 : 5
50 : 50 : 5
50 : 50 : 5
50 : 60 : 5
50 : 80 : 5
50 : 60 : 8
50 : 60 : 10
50 : 60 : 8
50 : 60 : 8
373
393
418
433
418
418
418
418
418
418
4
4
4
4
4
4
4
4
6
8
6.0
25.1
76.2
86.0
79.4
80.1
82.4
82.9
83.2
84.0
99.1
92.6
1.3
0
0.6
0.3
0
0
0
0
0.7
5.8
0.2
1.6
3.1
8.6
0.3
0.3
1.1
1.9
0.8
0.9
95.6
91.4
99.1
99.4
98.9
98.1
99.2
99.1
10
a) Reaction conditions: 3.0 mmol (0.33 g) of I, calculated amounts of II and catalyst in 23 mL toluene under different conditions;
b) others include IV, VI and polymer of III; c) data from GC.
Table 2. Synthesis of (V ) from (I) and (II) catalysed by different acidic catalystsa
Selectivity/%d
Entry
Catalyst
Conversion/%d
III
V
Othersc
1
2
3
4
5
6
7
8
9
[BMIm][Cl]
[BMIm][CuCl2]
[BMIm][FeCl4]
[BMIm][SnCl3]
[BMIm][ZnCl3]
[MMIm][AlCl4]
[BMIm][Al2Cl7]
[CP][CH3SO3]b
AlCl3
0
53.2
52.5
76.1
77.6
81.9
82.7
5.8
0
13.2
11.5
5.1
3.8
0.3
0
0
83.5
85.6
94.5
95.1
98.8
96.1
91.4
93.7
0
3.3
2.9
1.4
1.1
0.9
3.9
0.7
0.2
7.9
3.9
9.2
a) Reaction conditions: 3.0 mmol (0.33 g) of I, 3.6 mmol (0.33 g) of II and 0.24 mmol catalyst in 23 mL toluene at 418 K for 4 h;
b) CP = caprolactam; c) others include IV, VI and polymer of III; d)data from GC.
Table 2 compares the performance of different
acidic ILs as catalysts for the one-pot synthesis of V
from I and II in toluene as the medium under the same
conditions. It can be seen that these catalysts exhib-
ited low to high catalytic activities in the condensa-
tion of I and II to produceV in onepot. Combined
with the acidity order of these LAILs given above, it
was evident that the relative acid strength affected
the catalytic performance in the process of the syn-
thesis of V (Fig. 3B). For example, in the presence
of pure [BMIm][Cl] IL, the relative acid strength of
which was at the weakest level out of the catalysts
investigated, there was no significant conversion of I
(Table 2, entry 1). The catalysts with Lewis acidity, ei-
ther [BMIm][CuCl2] or [BMIm][FeCl4], exhibited im-
proved activity, affording conversions of I of 53.2 %
and 52.5 %, respectively (Table 2, entries 2 and 3).
When the reaction for the synthesis of V was catalysed
by [BMIm][SnCl3] or [BMIm][ZnCl3] IL, the conver-
sion of I attained 76.1 % (Table 2, entry 4) and 77.6 %
(Table 2, entry 5), respectively, and the selectivity to
V was at an equal level because of their similar rela-
tive acid strengths. For comparison, the Brønsted acid
IL [CP][CH3SO3] and AlCl3 were also tested in this
reaction. The conversion of I was lower than 10.0 %
although the selectivity to V was maintained > 90.0 %
(Table 2, entries 8 and 9). Furthermore, the catalytic
performance of the homologues of [BMIm][AlCl4] and
[BMIm][Al2Cl7], which have higher relative acidity
than the LAILs referred to above, were tested under
the same conditions. A lower selectivity of V (96.1 %)
was also obtained in the presence of [BMIm][Al2Cl7]
IL as catalyst, although the conversion of I increased
from 82.4 % to 82.7 % when the [BMIm][AlCl4] was re-
placed by the stronger acidic [BMIm][Al2Cl7] IL (Ta-
ble 1, entry 7 and Table 2, entry 8). Similarly, the
[MMIm][AlCl4] was chosen to test the influence of the
LAIL’s cation on the catalytic performance. Clearly,
the influence is negligible provided that the anions are
same. Accordingly, the [BMIm][AlCl4] catalyst exhib-
ited the best catalytic activity, giving a conversion of
82.4 % of I accompanied with a 98.9 % selectivity to
V at 418 K for 4 h, out of the tested ILs. This is better
than the highest yield obtained in the two-step syn-
thesis of benzofuranol currently adopted in industry,
as detailed previously (Fig. 1) (Wang et al., 2011).
It is also worthnoting the negative correlation of the
yield of intermediate III and the relative acid strength
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H. C. Zhou et al./Chemical Papers 69 (10) 1361–1366 (2015)
1365
Fig. 3. FT-IR spectra of pure acetonitrile (a), [BMIm][CuCl2] + acetonitrile (b), [BMIm][FeCl4] + acetonitrile (c), [BMIm][SnCl3]
+ acetonitrile (d), [BMIm][ZnCl3] + acetonitrile (e), [BMIm][AlCl4] + acetonitrile (f) (A). Relationship of blue shift
wavenumber and yield of V (B).
bottom residues from the Parr reactor were collected
and treated in vacuum (667–1340 Pa) for reuse.The
reusability of the [BMIm][AlCl4] catalyst is shown in
Fig. 4. It can be seen that there was no obvious loss
of catalytic activity in one-pot synthesis of benzofura-
nol from pyrocatechol and 3-chloro-2-methylpropene
after five consecutive recycling procedures provided
that the catalyst was treated simply in vacuum af-
ter each recycling procedure. The conversion of I re-
mained > 80.0 % and the selectivity to Vremained at
> 95.0 %. The results indicate that LAILs having an
aluminium chlorate anion are a class of robust and
reusable catalysts for the direct synthesis of benzofu-
ranol from I and II. In comparison with Lewis acid
Fig. 4. Recycling of [BMIm][AlCl4] IL in one-pot synthesis of
catalysts, these ILs are not only readily separated and
V from I and III. Reaction conditions: 3.0 mmol (0.33 g)
can be reused directly without obvious decrease in cat-
of I, 3.6 mmol (0.33 g) of II and 0.24 mmol of catalyst
alytic activity, but they also cause little corrosion to
in 23 mL toluene at 418 K for 4 h. Data from GC.
the equipment and have a negligible effect on the en-
vironment.
Legend: conversion (full) and selectivity (patterned).
of catalysts. This may be explained by the fact that
the cyclisation is the rate-controlling step catalysed by
Lewis acid catalysts in the synthesis of V (Mathewet
al., 2005). The above results revealed that the one-
pot synthesis of benzofuranol was largely dependent
on the acidity of LAILs.
Conclusions
One-pot synthesis of V from I and II was achieved
using LAILs as catalysts. In comparison with the
two-step industrial process for the production of V,
this method is more economical and environmentally
friendly, and the yield of V is slightly higher than in
the current industrial process. Furthermore, the cat-
alytic performance of these acidic ILs is closely related
to their acidity. The IL [BMIm][AlCl4] catalyst exhib-
ited a markedly higher activity for the V formation
Reusability of [BMIm][AlCl4] IL
In order to evaluate the reusability, after the upper
phase was separated from the reaction mixture, the
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H. C. Zhou et al./Chemical Papers 69 (10) 1361–1366 (2015)
than the other acidic ILs tested; an 82.4 % conver-
sion of I accompanied with selectivity to V as high as
98.9 % was obtained when the reaction was carried out
in a sealed reactor at 418 K for 4 h. In addition, the
use of LAILs as catalysts reduces environmental pollu-
tion and corrosion of equipment. Nonetheless, acidic
ILs with the proper ratio of Lewis/Brønsted acidic
sites may be the optimal candidates for catalysing the
synthesis of V from I and II in one-pot. Further inves-
tigation of this issue is currently under way.
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Supplementary data associated with this article
(Preparation of Lewis acid ionic liquids for one-pot
synthesis of benzofuranol from pyrocatechol and 3-
chloro-2-methylpropene) can be found in the online
version of this paper (DOI: 10.1515/chempap-2015-
0145).
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