Br?nsted acid ionic liquids catalyzed Friedel-Crafts Alkylations of electron-rich arenes with aldehydes

Article abstract of DOI:10.1016/j.apcata.2014.05.014

Triarylmethanes (TRAMs) and diarylalkanes (DIAAs) are valuable intermediates with wide applications in many fields. TRAMs are usually obtained from the acid-catalysed bisarylation of activated aryl aldehydes. However, the synthesis poses many problems, such as harsh reaction conditions, and the disposal of the excess solvents and/or toxic metal waste. In this study, some functionalized ionic liquids including Br?nsted acid ionic liquids (BAILs) and traditional ionic liquids were designed and synthesized. BAILs catalyzed Friedel-Crafts (F-C) alkylation was applied in this specific reaction for the first time. And the BAILs showed bifunctional properties acting as catalyst and solvent. Research shows that BAILs can be used for catalyzing F-C alkylations of electron-rich arenes with aromatic or aliphatic aldehydes successfully under mild reaction conditions. Furthermore, BAILs containing triflic anion has higher activity than other BAILs and traditional ionic liquids. [HSO ₃-pmim][OTf] gets the highest yields in the presence of 20 mol% of BAILs at 40 °C to give the corresponding TRAMs derivatives. After five cycles, the yields remain about 93-97%. Finally, according to IR spectrum and the experimental validation, the aromatic electrophilic substitution reaction was considered to be the possible catalysis mechanisms.

Full text of DOI:10.1016/j.apcata.2014.05.014

Accepted Manuscript
¨
Title: Bronsted acid ionic liquids catalyzed Friedel-Crafts
Alkylations of electron-rich arenes with aldehydes
Author: Ailing Wang Xueliang Zheng Zhuangzhi Zhao
Changping Li Yingna Cui Xuefang Zheng Jingmei Yin Guang
Yang
PII:
S0926-860X(14)00338-X
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.apcata.2014.05.014
APCATA 14834
To appear in:
Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
5-1-2014
14-5-2014
17-5-2014
Please cite this article as: A. Wang, X. Zheng, Z. Zhao, C. Li, Y. Cui, X. Zheng,
¨
J. Yin, G. Yang, Bronsted acid ionic liquids catalyzed Friedel-Crafts Alkylations
of electron-rich arenes with aldehydes, Applied Catalysis A, General (2014),
http://dx.doi.org/10.1016/j.apcata.2014.05.014
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Brönsted acid ionic liquids catalyzed Friedel-Crafts Alkylations of electron-rich arenes
with aldehydes
Ailing Wang, a Xueliang Zheng, a Zhuangzhi Zhao,a Changping Li,*a Yingna Cui,a Xuefang Zheng,*b Jingmei Yin, a and
Guang Yang a
a Department of Chemical Engineering, Dalian University, Dalian, 116622, PR China
b College of Bioengineering, Dalian University, Dalian, 116622, PR China
Abstract
Triarylmethanes (TRAMs) and diarylalkanes (DIAAs) are valuable intermediates with wide applications in
many fields. TRAMs are usually obtained from the acid-catalysed bisarylation of activated aryl aldehydes.
However, the synthesis poses many problems, such as harsh reaction conditions, and the disposal of the
excess solvents and/or toxic metal waste etc. In this study, some functionalized ionic liquids including
Brönsted acid ionic liquids (BAILs) and traditional ionic liquids were designed and synthesized. BAILs
catalyzed Friedel-Crafts (F-C) alkylation was applied in this specific reaction for the first time. And the
BAILs showed bifountional properties acting as catalyst and solvent. Research shows that BAILs can be used
for catalyzing F-C alkylations of electron-rich arenes with aromatic or aliphatic aldehydes successfully under
mild reaction conditions. Furthermore, BAILs containing triflic anion has higher activity than other BAILs
and traditional ionic liquids. [HSO3-pmim][OTf] gets the highest yields in the presence of 20 mol % of
BAILs at 40 ºC to give the corresponding TRAMs derivatives. After five cycles, the yields remains about
93%～97%. Finally, according to IR spectrum and the experimental validation, the aromatic electrophilic
substitution reaction was considered to be the possible catalysis mechanisms.
Keywords: Triarylmethane; Functionalized ionic liquids; Friedel-Crafts; alkylation; Aldehydes.
1. Introduction
Triarylmethanes (TRAMs) and diarylalkanes (DIAAs) have been extensively applied as key intermediates
in synthesis, medicine, and industrial chemistry [1], including photochromic agents [2], dyes [3], protective
groups [4], and building blocks for dendrimers and NLOs [5,6]. Ring-hydroxylated TRAMs were also
reported to exhibit antioxidant, antitumor, antitubercular, antiviral, antifungal and anti-inflammatory activities
[7].
Page 1 of 16

Due to their interesting chemical properties, synthesis process of TRAMs and DIAAs have received more
and more attention mainly about Friedel-Crafts (F-C) alkylation of electron-rich arenes with aldehydes and
their imines [8-10]. The process involves a multistep F-C alkylation promoted by Lewis or Bronsted acids
such as AuCl3 [11], [Ir(COD)Cl]2-SnCl4 [12], ZnBr2/SiO2 [13], FeCl3/Ac2O [14], Bi2(SO4)3/TMSCl [15],
molecular iodine [16], TfOH [17,18], silica gel-supported NaHSO4 [19], o-benzenedisulfonimide [20],
polystyrene-supported sulfonic acid [21], perfluorinated sulfonic acid resin (Nafion-H) [22], montmorillonite
[23, 24 ] and so on. However these methods encounter with many problems, such as longer reaction time, low
yield of products, harsh reaction conditions, and generation of acidic waste, especially the use of hazardous
and expensive catalysts, and volatile organic solvents. So, the development of green catalysts and non-volatile
solvents can be of great significance.
The emergence of ionic liquids (ILs) has provided new option to solve the above problems. Over the past
few years, ILs have attracted considerable attention as an environmentally benign alternative for organic
synthesis procedures [25-31]. Compared with traditional Brönsted acid catalysts, Brönsted acid ionic liquids
(BAILs) have more advantages such as, non-volatility, non-flammability, good chemical and thermal stability
over a wide temperature range, and good solubility with many organic compounds. Recently, some research
have focused on the synthesis of strong BAILs and applied as dual solvent–catalyst for esterification [32,33],
Fischer indole synthesis [34,35], Mannich Reaction [36], dehydration [37], Beckmann rearrangement [38]
and so on [39-47]. However, to the best of our knowledge, the BAILs were not used as both environmentally
friendly novel catalysts and green solvents to catalyze F-C alkylation. Here, we report BAILs catalyzed F-C
alkylation using electron-rich arenes with aromatic or aliphatic aldehydes to generate either TRAMs or
DIAAs respectively without any solvents, especially under mild reaction conditions.
In this study, a series of functionalized ionic liquids (FILs) including Brönsted acid ionic liquids and
traditional ionic liquids were designed and synthesized shown in Scheme 1. Then the above ILs was applied
to catalyze F-C alkylation reaction of 1,2,4-trimethoxybenzene (1a) with benzaldehyde (2a) , which was
chosen as model reaction to investigate the synthesis of bis(2,4,5-trimethoxyphenyl)phenylmethane (3a).
After optimization, the catalysis process with FILs [HSO3-pmim][OTf] was investigated. Some important
factors, such as system temperature, reaction time and the amount of catalyst, different kinds of aldehydes
and electron-rich arenes were investigated. Moreover, the recycling and reuse of [HSO3-pmim][OTf] was
investigated. Finally, the possible catalysis mechanisms were probed.
Page 2 of 16

2. Experimental
2.1. Materials and methods
Reagents used in the study were purchased as the analytical reagent grade and used without further
purification. TLC analyses were performed using percolated TLC silica gel F254 plates. Melting points were
recorded on X - 4 digital microscopic melting point apparatus. NMR spectra were recorded on an Avance
III-400 (Bruker) NMR spectrometer.
2.2. BAILs Preparation
BAILs were prepared according to reference [46]. A stoichiometric amount of 1,3-propane sultone was
added dropwise to a solution of N-methylimid-azole or triethylamine in acetonitrile at 0 ºC, and then the
mixture was stirred at 50 °C for 3 h. The obtained white powder was filtered, washed with diethyl ether for 3
times and dried in vacuo to afford pure 1-alkyl-3-(alkyl-3-sulfonate) imidazolium zwitterions ( ). The
precursor of BAILs ( ) was characterized by 1H NMR.
After the synthesis of zwitterionic precursor (Ⅰ), a stoichiometric amount of acid aqueous solution was
added into it and stirred at 90 °C for 5 h. The BAILs were obtained by rotary distillation and vacuum drying.
The synthesis routes were summarized in Scheme 1.
CH3CN
HR
+
SO3H
N
N
O
O
SO3
N
N
N
N
S
0°C-50°C
90 °C 5 h
R
O
BAILs
R = HSO4, [HSO3-pmim][HSO4]
R = CF3COO, [HSO3-pmim][CF3COO]
R = TsO, [HSO3-pmim][TsO]
R = CH3SO3, [HSO3-pmim][CH3SO3]
R = CF3SO3, [HSO3-pmim][OTf]
Scheme 1. Synthesis of BAILs
2.3. General procedure for triarylmethanes and diarylalkanes synthesis
A mixture of aldehyde 2 (1.0 mmol), aromatic compound 1 (2.0 mmol) and BAILs (0.2 mmol) was
stirred at 40 °C for the desired time. The reaction was monitored by thin layer chromatography (TLC). After
the completion of the reaction, 1 ml of water was added into the reaction mixture, and the target compound
Page 3 of 16

was obtained though simple filtration. The pure ionic liquids got by drying the filtrate were reused for another
cycle. Synthesis route is listed in Scheme 2.
R2
BAILs
R2
+
R1CHO
R1
R2
TRAMs
DIAAs
or
1
2
Scheme 2. Synthesis route of TRAMs and DIAAs catalyzed by BAILs
3. Results and discussion
3.1 Selection of BAILs
The designable property makes it possible the diversity of functionalized ILs. So the proper chosen of ILs
is very important for the F-C alkylation reaction process. The selection of ILs including traditional ionic
liquids (in Table 1 entries 1-2) and Brönsted acid ionic liquids (BAILs) was probed. The result was shown in
Table 1. Seven types of BAILs were used for catalyzing F-C alkylations of electron-rich
1,2,4-trimethoxybenzene (1a) with benzaldehyde (2a). The corresponding TRAM bis(2,4,5-trimethoxyphenyl)
phenylmethane (3a) at 40°C (Scheme 3) were obtained. There is no other extra solvent added during the
reaction process. Results show that [HSO3-pmim][OTf] was chosen as catalyst, the yields 99% was achieved
in 30 minutes. Furthermore, the reaction can be realized free of traditional volatile organic solvent. However,
traditional ionic liquids were not efficient for the reaction (Table 1, entries 1 and 2). To be noted, it can be
observed that the BAILs containing triflic anion get higher yields than other BAILs. At the same time, it can
be observed that there is no relationship between the catalytic capability and acidic strength, which was also
reported by previous literatures [46,48].
Table 1. Friedel-Crafts Alkylation reaction of 1a and 2a in the presence of ILs
Entry
IL
Time/h
12
Yieldb/%
1
2
3
4
[BMIM][BF4]
0
[BMIM][PF6]
12
0
[HSO3-pmim][OTf]
[HSO3-pmim][CF3COO]
0.5
6
99
86
Page 4 of 16

5
6
7
[HSO3-pmim][CH3SO3]
[HSO3-pmim][HSO4]
[HSO3-pmim][TsO]
8
5
6
83
80
78
a
Reaction conditions: 1,2,4-trimethoxybenzene (1a, 2.00 mmol), benzaldehyde (2a, 1.00 mmol), catalysts
(0.2 mmol), 40 °C, solvent free. b Isolated yield.
3.2 Optimization of the reaction conditions
In this section, the reaction of arene (1a) with aldehyde (2a) was also chosen as typical one for the
synthesis of 3a. [HSO3-pmim][OTf], was chosen as catalyst and solvent. Some factors including the effect of
system temperature, reaction time, ratio of IL (mol%) were investigated in detail (Table 2). Results show that
the reaction cannot realized without addition of [HSO3-pmim][OTf] (Table 2, entry 1). However, 1 mol % of
[HSO3-pmim][OTf] was introduced into the system, the yield of 65% can be got in 5 h (Table 2, entry 2).
With the increase of the amount of IL to 20 mol %, the reaction activity was strengthened and the reaction
time was shortened obviously only in 0.5 h. At the same time, effect of system temperature was shown in
table 2 entries 2, 3, 4, 5 and 6. Results show that higher temperature is not beneficial for the reaction process.
So 40 °C was chosen as operation temperature throughout the investigation. At this temperature, the initial
material was almost completely converted by TLC monitoring and the yield reach up to 99 % (Table 2, entry
9).
OMe
OMe
OMe
OMe
CHO
MeO
[HSO3-pmim][OTf]
+
MeO
OMe
MeO
OMe
1a
2a
3a
Scheme 3 Reaction of 1a with 2a
Table 2. [HSO3-pmim][OTf] catalyzed Friedel-Crafts Alkylation reaction of 1a with 2a in different
conditions
Entry
IL (mol%)
Temp./ °C
Time/h
Yieldb/%
1
2
0
1
rt
rt
24
5
0
65
Page 5 of 16

3
4
5
6
7
8
9
1
1
40
50
60
70
40
40
40
0.5
0.5
0.5
0.5
0.5
0.5
0.5
77
77
76
76
85
91
99
1
1
10
15
20
a
Reaction conditions: 1,2,4-trimethoxybenzene (1a) (2.00 mmol), benzaldehyde (2a) (1.00 mmol),
solvent free ; b Isolated yield.
In order to prove the universality of F-C Alkylation reaction catalyzed by BAILs further, different
substrates (1 and 2, in Scheme 2) were chosen under the optimal conditions, which were shown in Table 3.
Eleven kinds of aldehydes including aromatic aldehyde and aliphatic aldehyde were chosen as electrophil
reagent. There are different functional groups in aromatic ring of aromatic aldehydes including
electron-donating group (Table 3, entries 2-4, and 8) and electron-withdrawing group (Table 3, entries 5-7,
and 9). From results, BAILs can catalyze Friedel-Crafts Alkylation reaction of 1,2,4-Trimethoxybenzene with
all kinds of aromatic aldehydes to get the corresponding symmetric TRAMs 3b-v with high yields. Aromatic
aldehyde including Electron-withdrawing group is beneficial for improving the yield of F-C Alkylation
reaction. Then, different aliphatic aldehydes were chosen as substrates with electron-rich arene (1a) to get the
DIAAs derivatives 3h and 3i (Table 3, entries 10-11). Interestingly, benzaldehyde (Table 3, entry 10) without
any substituent group in aromatic ring achieve high yield about 99%. From the above investigation, it can be
concluded that the Friedel-Crafts Alkylation reaction catalyzed by BAILs is suitable for different aldehydes
including aromatic aldehydes (4-R1CHO or 2-R1CHO) and aliphatic aldehydes. And most of the reaction
yields were more than 90%.
Table 3. [HSO3-pmim][OTf] catalyzed Friedel-Crafts Alkylation reaction of electron-rich arene (1a)
with various aldehydesa
Entry
R1
Product
3a
Time/h
0.5
Yieldb/%
99
1
2
C6H5
4-CH3OC6H4
4
83c
3b
Page 6 of 16

3
4-CH3C6H4
2-OHC6H4
4-NO2C6H4
4-ClC6H4
4-FC6H4
3
90
74c
94
91
92
92
93
99
90c
3c
3dc
3e
3f
4
8
5
0.5
0.5
1
6
7
3g
3u
3v
3h
3ic
8
2-CH3OC6H4
2,6-Cl2C6H3
H
0.5
0.5
0.5
3
9
10
11
C5H11
a
Reaction conditions: 1,2,4-trimethoxybenzene (2.00 mmol), aldehydes (1.00 mmol),
b
c
[HSO3-pmim][OTf] (0.2 mmol), 40 °C, solvent free. Isolated yield. The products were purified by
chromatogram after filtration.
To probe if arenes have influence on F-C Alkylation reaction catalyzed by BAILs, various arenes as
nucleophiles with 4-nitrobenzaldehyde or benzaldehyde were investigated at 40 °C, with 20 mol%
[HSO3-pmim][OTf] which was shown in Table 4. With the decrease of methoxy group in quality (compounds
3a, 3j, 3l), the electron cloud density on the benzene ring of nucleophiles is reduced correspondingly. At the
same time, the activity of Friedel-Crafts Alkylation reaction gradually decrease, which can be reflected in the
end time of reaction in turn (0.5h, 2h, 6h) and in the corresponding yields (99%, 86%, 45%). Moreover, the
catalytic synthesis of compounds (for example, 3e, 3k, 3n) is consistent with the above change rules.
According to the theory of nucleophile reaction, when the aromatic ring of nucleophile is connected with
weaker electron donor group such as methyl (3q) or withdrawing electron group such as nitryl (3r), the
electron cloud density on the aromatic ring of nucleophile is lower. So F-C Alkylation reaction cannot be
catalyzed by BAILs. It can be concluded that not all arenes is suitable for the F-C Alkylation reaction
catalyzed by BAILs. Only electron-rich arenes can be chosen as all kinds of aldehydes.
Table 4. [HSO3-pmim][OTf] catalyzed Friedel-Crafts Alkylation reaction of aromatic aldehydes with
nucleophiles
Page 7 of 16

OH
OH
OMe
OMe
OMe
OMe
OMe
OMe
MeO
MeO
HO
MeO
MeO
MeO
MeO
HO
3l 6 hd, (45%)c
OMe
3j 2 hd, (86%)
3o 7 hd, (63%)
3a 0.5 hd, (99%)
NO2
HO
O2N
MeO
OH
3q 12 hd, (0%)
3r 12 hd, (0%)
3p 6 hd, (40%)c
OMe
OMe
OMe
OMe
MeO
MeO
MeO
MeO
OMe
NO2
MeO
OMe
NO2
MeO
NO2
d
3k 2 hd, (87%)
3n 3 hd, (61%)
3e
0.5 h , (94%)
a Reaction conditions: nucleophiles (2.00 mmol), aromatic aldehydes (1.00 mmol), [HSO3-pmim][OTf] (0.2
mmol), 40 °C, solvent free.
b Isolated yields after column chromatography. c 50 mol % [HSO3-pmim][OTf] was used.
d The time reaction was completed.
3.3 Reuse of [HSO3-pmim][OTf]
Reuse of BAILs was also carried out by using [HSO3-pmim][OTf] (0.2 mmol) to catalyze model reaction
of substitution 1a (2.00 mmol) with 2a (1.00 mmol) at 40 °C. After the completion of the reaction, 1 ml of
water was added into the reaction mixture, and the target compound was got though simple filtration. The
pure ILs was obtained by drying the filtrate. Then the recycled ILs can be reused for another cycles. The
results are summarized in Fig 1.The yields of 93%-99% were obtained during the five recycling cycles. This
result shows that the BAILs based catalyst was stable. It is worth noting that the catalyst was recycled simply
by evaporation of water.
Page 8 of 16

Fig 1. Recycling of [HSO3-pmim][OTf]
a
Reaction conditions: substitution 1a (2.00 mmol), 2a (1.00 mmol), [HSO3-pmim][OTf] (0.2 mmol), 40 °C,
solvent free. b Isolated yield.
3.4 Alkylation mechanism catalyzed by Brönsted acid [HSO3-pmim][OTf]
The possible mechanism is discussed as follows (Fig 2). In general, it is a multistep Friedel–Crafts
alkylation catalyzed by Brönsted acid [HSO3-pmim][OTf]. Aromatic nucleophilic addition reaction was taken
place for benzaldehyde at the very beginning; however, the reaction did not stop at this step. The compound
diarylmethanol(Ⅱ) immediately reacts with a second molecule of the arene-rich compound and TRAMs were
got following a typical aromatic electrophilic substitution mechanism. So, the above mechanism also proves
the greenness of this process because only one molecule of water is produced as by-product during
bisarylation (TRAMs or DIAAs) synthesis process.
Page 9 of 16

O
SO3H
N
N
H
CF3SO3
H
R2
R1
OH
(2)
H
R2
R2
H
R1
R2
R1
R1
OH
(1)
TRAMs
H
OH2
R1
H
R2
H2O
OH
R1
CF3SO3
R2
R1
CF3SO3
SO3
N
N
Fig 2. Mechanism of Friedel-Crafts Alkylation reaction catalyzed by [HSO3-pmim][OTf].
To investigate the mechanism of BAILs catalyzed F-C alkylation reaction, the reaction of arene (1a)
with aldehyde (2a) was chosen as typical substrates to synthesize 3a. As the reaction with BAILs
[HSO3-pmim][OTf] is so quick, which is not suitable for the investigation of synthesis mechanisms. So its
analogues-[HSO3-pmim][HSO4] with lower activity was chosen as substitute to prolong reaction time. During
the synthesis process, the amount of aldehyde (2a) was added excessively. After 5 min or 30 min, samples
were removed from the reaction system, respectively. Then water was added to quench the reaction. The
corresponding organic layer was determined by IR. Results were listed in Fig. 3. In Fig.3, the carbonyl
characteristic peak of 2a is about 1698.3cm-1. And the C-O bond stretching vibration peak of 1a is about 1260
cm-1. From Fig. 3A (5 min) and Fig. 3B (30 min), the relative intensity peak of 1698.3 cm-1 is decreasing
obviously, which indicates that the amount of aldehyde (2a) is decreasing and the reaction was proceeding.
From IR spectrum, there appears no diarylmethanol ( ) found. However, this transient state would never be
avoided during F-C alkylation reaction. In order to verify the reaction may encounter with the above process,
another experiment was carried out (Scheme 4). Arene (1a) and diarylmethanol ( ) were chosen as substrates,
and [HSO3-pmim][HSO4] as the catalyst and solvent. The substrates were converted to 3a completely within
1 min. That is to say, it is a fast reaction. Diarylmethanol ( ) may be emergent in this specific process. But it
is converted into TRAMs quickly. So no Diarylmethanol ( ) can be detected through the IR spectrum. From
the above investigations, the proposed mechanism would obey the above scheme.
Page 10 of 16

Fig 3. IR Spectra of Friedel-Crafts Alkylation reaction mixture (Reaction time: A 5 min, B 30 min)
OMe
OMe
OMe
HO
OMe
OMe
MeO
+
MeO
OMe
MeO
OMe
MeO
OMe
1a
3a
Scheme 4 Reaction of 1a with diarylmethanol (Ⅱ)
4. Conclusion
Series of functionalized BAILs and traditional ILs were designed and synthesized. BAILs were applied for
catalyzing F-C alkylation of electron-rich arenes with aromatic or aliphatic aldehydes successfully. Research
shows that [HSO3-pmim][OTf] as catalyst gets the highest yield. In optimal conditions, the yield of TRAMs
or derivatives can reach up to 97% for one cycle, which is much higher than the other BAILs and traditional
ILs. After five cycles, the catalytic activity of BAILs remains very high ranging from 93% to 97%. Finally,
Page 11 of 16

the possible catalysis mechanisms were investigated. The above study would provide new green catalysts and
solvents for the catalysis synthesis of TRAMs and DIAAs.
Acknowledgments
Sincere thanks should be given to National Natural Science Foundation of China (NSFC Grant NO.
21006007, 21176033 and 21210102025) for financial support of this project.
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Page 14 of 16

R2
BAILs
R2
R1CHO
R1
R2
TRAM
DIAA
s
s or
In this study, some functionalized ionic liquids including Brönsted acid ionic liquids (BAILs) and traditional
ionic liquids were designed and synthesized. BAILs catalyzed Friedel‐Crafts(F‐C) alkylation was applied in this
specific reaction. Research shows that BAILs can be used for catalyzing F‐C alkylations of electron‐rich arenes
with aromatic or aliphatic aldehydes successfully. After five cycles, the yields remains about 93%~97% and
without noticeably decreasing the catalytic activity.
Graphical Abstract
Page 15 of 16

Highlights
•
Brönsted acid ionic liquids (BAILs) were designed and synthesized.
BAILs can be used for catalyzing Friedel‐Crafts alkylations successfully.
[HSO3‐pmim][OTf] show higher catalytic activity.
•
•
•
BAILs can be recycled without noticeably decreasing the catalytic activity
Page 16 of 16