Bronsted acid ionic liquid as a solvent-conserving catalyst for organic reactions

Article abstract of DOI:10.1002/cssc.201402220

A sulfonyl-containing ammonium-based Bronsted acid ionic liquid was prepared and used as a liquid heterogeneous catalyst for organic reactions. The unique macroscopic phase heterogeneity of the IL in the reaction system not only ensures an excellent catalytic activity of the IL catalyst but also avoids the use of organic reaction solvents. The catalyst system is applicable for a wide range of reactions.

Full text of DOI:10.1002/cssc.201402220

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DOI: 10.1002/cssc.201402220
Brønsted Acid Ionic Liquid as a Solvent-Conserving
Catalyst for Organic Reactions
Amir Taheri,[a] Xiaojuan Pan,[a] Changhui Liu,[a] and Yanlong Gu*[a, b]
A sulfonyl-containing ammonium-based Brønsted acid ionic
liquid was prepared and used as a liquid heterogeneous cata-
lyst for organic reactions. The unique macroscopic phase het-
erogeneity of the IL in the reaction system not only ensures an
excellent catalytic activity of the IL catalyst but also avoids the
use of organic reaction solvents. The catalyst system is applica-
ble for a wide range of reactions.
Figure 1. Schematic representation of acid catalyst systems.
Nearly 20 million tons of volatile organic compounds (VOCs)
are released into the atmosphere each year through the activi-
ties of chemical industry[1] and are linked to global climate
change and human illness. In order to reduce the emissions of
VOCs, a lot of research has been devoted to the development
of green solvents in the past two decades.[2] From the view-
point of conserving materials, lessening energy consumption,
and reducing reaction volume, solvent-free conditions are pre-
ferred to facilitate reactions of industrial significance. Although
some reactions have been successfully performed under sol-
vent-free conditions,[3] the use of organic solvents is still man-
datory for most of the organic reactions. The main obstacles of
eliminating solvents from these reactions stem from some piv-
otal functions of solvents, which cannot be fully attained with-
out their use. Of all these functions, stabilization of the reac-
tion intermediate ranks among the most important.
that enables the IL catalyst to show a macroscopic phase het-
erogeneity in the reaction system. This idea, if realized, might
allow many reactions to be performed under solvent-free con-
ditions. In this paper, a sulfonyl-containing ammonium-based
Brønsted acid IL was synthesized that displayed an outstand-
ing catalytic activity under solvent-free conditions for many or-
ganic reactions. Intriguingly, both substrate and product were
immiscible with the IL, and the reaction proceeded under bi-
phasic conditions, in which the Brønsted acid IL worked as
a liquid heterogeneous catalyst (Figure 1, right).
The Brønsted acid ILs were prepared through a procedure
depicted in Figure 2. Initially, divinyl sulfone was treated with
an aliphatic amine in methanol at 608C, and a double Michael
addition reaction provided sulfonyl-containing tertiary amines,
3a and 3b. The following quaternization of these amines with
1,3-propanesulfonate formed 4a and 4b. The structure of 4a
was confirmed by single crystal X-ray diffraction.[8] Finally, treat-
ment of these zwitterionic salts with triflic acid (TfOH) at 808C
produced the desired ILs, 1a and 1b. Both 1a and 1b are vis-
cous liquids at room temperature. With these ILs in hand, we
then started to examine their catalytic activity in organic reac-
tions. In order to compare the efficiency of these ILs with that
of non-sulfonyl ILs, two other Brønsted acid ILs, 1c and 1d,
were also prepared (Figure 2).
The use of Brønsted acid ionic liquids (ILs) as catalysts in or-
ganic reactions has gained considerable attention in the past
decade.[4] When conventional ILs are used as catalysts, primari-
ly to facilitate catalyst recycling, reactions often proceeded
under solvent-free conditions (Figure 1, center).[5] In these
cases, a mixture composed of substrate, product, and IL played
the role of solvent. However, because the carbocation inter-
mediate that was generated during the reaction may not be
stabilized sufficiently in this mixture,[6] the full catalytic activity
of the IL was, sometimes, not achieved.[7] We envisaged that
this problem might be solved by a judicious structural design
The newly synthesized Brønsted acidic ILs were then utilized
in some acid-catalyzed reactions that are associated with a car-
bocation intermediate. Firstly, arylmethylation of phenylacety-
lene 6a with benzhydrol 5a was examined, which has been re-
[a] A. Taheri, X. Pan, C. Liu, Prof. Y. Gu
Key Laboratory for Large-Format Battery, Materials and System
Ministry of Education, School of Chemistry and Chemical Engineering
Huazhong University of Science and Technology
Wuhan, 430074 (PR China)
Fax: (+86)027-87-54-45-32
E-mail: klgyl@hust.edu.cn
[9]
alized by means of two catalytic systems, TfOH/Fe(OTf)3 and
Cu(OTf)2.[10] However, the reactions have to be performed in or-
ganic solvents. We found that, under solvent-free conditions,
TfOH is ineffective for this reaction (Table 1, entry 1). A moder-
ate yield could be obtained in dichloroethane (DCE) (entry 2).
Previously, 77% of yield was obtained by adding a catalytic
amount of Fe(OTf)3 into the TfOH/DCE system (entry 3).[9] With
the hope of developing a solvent-free system, 1a was then
used as catalyst. To our delight, the yield of 7a reached 95%
[b] Prof. Y. Gu
State Key Laboratory for Oxo Synthesis and Selective Oxidation
Lanzhou Institute of Chemical Physics
Chinese Academy of Science
Lanzhou, 730000 (PR China)
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cssc.201402220.
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catalytic activity.[8] Quite recently,
some ILs were proved to be
labile under harsh conditions.[12]
In particular, after a long time of
irradiation with ultrasound or
microwave, acidic species, such
as SO2 and SO3, could be detect-
ed in the pyrolysis products of
sulfur-containing
ILs;
these
acidic species can, if produced,
potentially influence the reaction
outcome. To this end, the reac-
tion was stopped after 1 hour,
and 1a was neutralized with an
aqueous solution of sodium hy-
Figure 2. Sulfonyl-containing Brønsted acid ILs used in this work.
Table 1. Arylmethylation of phenylacetylene with benzhydrol.[a]
droxide (1.0n). No precipitate
was observed after adding
AgNO3 (aq.) into the system, in-
dicating that sulfur oxides were not formed in our system.
Interestingly, when 1a was used as catalyst, two phases
could be observed clearly even at high temperature (1008C,
Figure 3a).[13] The upper phase was composed mainly of organ-
ic compounds, and only tiny amount of 1a could be detected
1
by H NMR. In order to check where the reaction takes place,
Entry
Catalyst and some specified conditions
Yield [%]
the upper organic phase was separated after being heated for
30 min of reaction. Half of the solution was directly subjected
to isolation. The remainder was stirred at 1008C for 2 h. Finally,
almost the same yield was obtained (34% vs 35%). This implies
that 1a dissolved in the organic phase is not responsible for
the progress of the reaction. We have also measured the
volume of the ionic phase at 1008C. It was found that it by in-
creased approximately 6% after adding organic substrates.[8]
All these results imply that, during the reaction, the IL phase
absorbs organic substrates without itself becoming significant-
ly dissolved into the organic phase.[14] For this reason, we
speculated that the reaction might take place either in the IL
phase or at the interface between the ionic and organic
phases. The sulfonyl fragments in the ionic phase might play
a key role in enhancing the catalytic activity of 1a. In order to
verify this, a neutral salt, 1e, which also contains a sulfonyl
group, was added to the 1d-catalysed reaction (5 mol%). In
this case, yield of 7a increased to 71% from 37% (entry 8). Be-
cause 1e alone cannot initiate the reaction (entry 9), a synergis-
tic effect between sulfonic and sulfonyl groups in the ionic
phase could be responsible for the improvement in yield.
As water was produced during the reaction, we were inter-
ested in (i) where was the generated water? and (ii) how the
water affected the reaction? By means of analysis with Karl-
1
2
3
4
5
6
7
8
TfOH, 24 h
TfOH in DCE, reflux, 24 h
TfOH/Fe(OTf)3 in DCE, reflux, 24 h
1a
1b
1c
1d
1d/1e
trace
52
77[b]
95, 89[g]
90
25
37
71
9
10
11[f]
1e
0
1a with water
1a
79,[c] 63,[d] 8[e]
94
[a] 5a, 2.5 mmol, 6a, 2.5 mmol, catalyst, 0.125 mmol, 1008C, 2 h. [b] Liter-
ature result in Ref. [9]. [c] 0.5 mmol water. [d] 1.0 mmol water.
[e] 2.0 mmol water. [f] 1a was reused for the seventh time. [g] The reac-
tion was performed at a scale of 30 mmol.
within 2 h at 1008C (entry 4). IL 1b also exhibited excellent
performance, and 7a could be obtained in 90% yield (entry 5).
In a sharp contrast to these good results, only 25% of yield
was obtained with the conventional Forbes’s IL 1c under the
identical conditions (entry 6). When 1d, the sulfonyl-free con-
gener of 1b, was used as catalyst, only 37% of 7a was ob-
tained (entry 7). On the basis of these results, we can conclude
that the existence of sulfonyl groups in the ILs 1a and 1b sig-
nificantly influences their catalytic activities.
In order to shed light on the intensification effect
of the sulfonyl group on the catalytic activity of 1a,
we scrutinized the acid strength of the catalysts by
Hammett method.[11] It was found that the acid
strength of TfOH is greater than that of the others.
However, no significant difference on their acid
strength was observed between 1a and Forbes’s IL
Figure 3. Photos of different IL systems and a schematic representation of the catalytic
1c, which ruled out the effect of acid strength on the reaction. Photos are taken at the end of the reaction, 1008C: (a) 1a; (b) 1c; (c) 1d.
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Fischer titration, it was found that the generated water exists
predominantly in the IL phase during the reaction. Fortunately,
in the model reaction, the 1a catalyst is quite robust in the
presence of a small amount of water. This can be verified by
the control experiments in Table 1 (entry 11).
Because 1a is not soluble in non-polar organic solvent, it
can be easily recovered at the end of the reaction. A reuse ex-
periment demonstrated that 1a can be used at least 8 times
without significant loss of its activity (Table 1, entry 12).[15] The
reaction can also be effectively scaled up with similar efficien-
cy. For example, the reaction of 5a (30 mmol) with 6a
(30 mmol) gave the corresponding arylmethylation product 7a
in 89% yield (7.15 g, Table 1, entry 4). We also probed the
scope of 1a-catalyzed arylmethylation reaction with respect to
both the alcohol and the alkyne components. As shown in
Table 2, benzhydrols with different substituents smoothly re-
acted with phenylacetylene, producing the corresponding
products in generally excellent yields (entries 1–7). Carveol can
also be used as substrate to react with 6a over 1a catalyst
Figure 4. Cyclotrimerization of acetophenone.
yields under identical conditions. These results indicated that
sulfonyl group in 1a indeed played a key role in limiting the
dissolution of acetophenone into the IL phase, ensuring thus
a macroscopic phase heterogeneity of the reaction system.
TfOH and toluenesulfonic acid (PTSA) are also ineffective for
this reaction. In literature, when PTSA was used as catalyst, a re-
action time of 10 h and a temperature of 1308C were needed
in order to obtain a comparable yield with that of 1a.[16b] Many
acetophenone derivatives were applicable in this system.[8] Fur-
thermore, in this reaction, 1a can be reused at least 11 times
without appreciable loss of its activity.
Table 2. Arylmethylation of terminal alkynes catalyzed by 1a.[a]
Entry
R1
R2
R3
Product
Yield [%]
Electrophilic reaction of aldehyde with 1,1-diphenylethylene
has been sporadically investigated by using acid as catalyst.[17]
However, the reported yields are rather low (<50%) except on
using some unusual substrates that have high reactivity, such
as para-vinylanilines. Quite recently, Huang et al reported an
efficient catalyst, Fe(OTf)3/PTSA; however, the reaction was per-
formed in a mixed solvent composed of toluene and metha-
nol.[18] Because the catalytic activity of the sulfonyl-containing
Brønsted acid IL was not significantly affected by the presence
of a small amount of water, we then investigated the reaction
of benzaldehyde and 1,1-diphenylethanol 10a, which was con-
sidered as an alternative to 1,1-diphenylethylene (less expen-
sive), with these ILs. A biphasic system can be visually ob-
served (see Figure 5, picture a) during the reaction. The desired
product, 11 a, could be obtained in 96% of yield after 45 min
of reaction with 1a. Interestingly, because 11 a is a solid and
insoluble in the ionic phase, at the end of the reaction, the
crude product solidified on top of the catalyst (picture (c)). This
avoids the use of silica column chromatography for the prod-
uct isolation. With Forbes’s IL 1c and some other commonly
used acid catalysts, the reaction barely proceeded under the
identical conditions. Compared with Huang’s catalyst (12 h re-
action time is needed),[18] our IL catalyst not only avoids the
use of solvent, but also permits the reaction finishing within
a very short time. Substrate scope of this system also proved
to be excellent.[8]
1
2
3
4
5
6
7
8
p-OMe
p-OMe
p-OMe
p-Cl
m-Br
H
p-OMe
H
H
H
H
H
H
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
p-EtC6H4
p-PrnC6H4
p-BunC6H4
p-PnnC6H4
p-FC6H4
p-MeOC6H4
7b
7c
7d
7e
7 f
7g
7h
7i
7j
7k
7l
7m
7n
7o
86
93
88
87
86
85
84
95
95
96
94
89
92
93
o-FC6H4
o-ClC6H4
p-ClC6H4
o-MeC6H4
2-thienyl
(CH2)2CH
C6H5
C6H5
C6H5
C6H5
C6H5
9
10
11
12
13
14
C6H5
5b+6a
[a] Alcohol: 2.5 mmol, alkyne: 2.5 mmol, 1a: 0.125 mmol, 1008C, 2 h.
(entry 14). The scope of the reaction with respect to alkyne
was also found to be excellent (entries 8–13).
In the above-mentioned reactions, a macroscopic phase het-
erogeneity of the sulfonyl-containing Brønsted acid IL is the
key to ensuring an excellent activity of 1a. However, a limita-
tion may appear in utilizing these ILs for a reaction with polar
substrates, which tended to dissolve the catalyst, and conse-
quently diminished the catalytic activity. To this end, these ILs
were then applied in the cyclotrimerization of acetophe-
none,[16] which is a moderately polar substance. As shown in
Figure 4, more than 90% yields were obtained within 3 h
when 1a and 1b were used as catalysts. In these cases, the re-
action also proceeded under biphasic conditions, whereas the
use of 1c and 1d as catalysts resulted in homogenization of
the reaction mixture, and 9a was obtained only in moderate
Inspired by the reaction in Figure 5, we envisioned that
a tandem dehydration/Michael addition reaction of 10a and
methyl vinyl ketone 12a might be possible. Although the nu-
cleophilicity of styrene has been well explored before, the use
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sulfonyl group endowed these ILs a unique macro-
scopic phase heterogeneity in the reaction system,
thus ensuring outstanding catalytic activities of the
ILs. These catalytic systems are applicable in a wide
range of acid-catalyzed reactions. In particular, a hith-
erto unreported reaction of 1,1-diphenylethanol and
a,b-unsaturated ketone was realized by using these
IL catalysts.
Acknowledgements
Financial support from National Natural Science Foun-
dation of China (21173089 and 21373093), the Cooper-
ative Innovation Center of Hubei Province and Chutian
Scholar Program of the Hubei Provincial Government
are greatly appreciated. The authors are also grateful
for all the staff members in the Analytical and Testing
Center of HUST. This work is also supported by the Fundamental
Research Funds for the Central Universities of China
(2014ZZGH019).
Figure 5. Reaction of benzaldehyde with 10a (Photos are taken at 808C, (a) 5 min;
(b) 15 min; (c) at the end of the reaction).
of styrene as Michael donor has not been reported yet. We
found that in the presence of 5 mol% of 1a, the expected
product 13a was obtained in 95% yield under solvent-free bi-
phasic conditions.[8] (Table 3, entry 1). In a sharp contrast to
this good result, poor yields were obtained with 1c, triflic acid,
H2SO4, and Sc(OTf)3 under the identical conditions (entries 2–
7). A tertiary alcohol, 10b, also participated readily in this reac-
tion (entry 8). Other a,b-unsaturated ketones, such as 12b to
12 f, can also be used as Michael acceptors (entries 9–14).
In summary, sulfonyl-containing ammonium-based Brønsted
acid ILs were prepared and used as liquid heterogeneous cata-
lysts for organic reactions under solvent-free conditions. The
Keywords: acid catalysis
chemistry · ionic liquid
·
biphasic catalysis
·
green
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Entry
Catalyst
Substrate
Product
Yield [%]
1
2
3
4
5
6
7
8
1a
1c
10a+12a
10a+12a
10a+12a
10a+12a
10a+12a
10a+12a
10b+12a
10a+12b
10a+12c
10a+12d
10b+12d
10a+12e
10a+12 f
13a
13a
13a
13a
13a
13a
13b
13c
13d
13e
13 f
13g
13h
94, 93[b]
26
24
15
17
13
96
94
96
94
91
93
TfOH
H2SO4
PTSA
Sc(OTf)3
1a
1a
1a
1a
1a
9
10
11
12
13
1a
1a
80
[a] 10a: 2.5 mmol, a,b-unsaturated ketone: 2.5 mmol, catalyst,
0.125 mmol, 708C, 0.5 h. [b] 1a was reused for the fifth time.
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[8] See the Supporting Information.
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[13] A reaction video at 1008C is available in the Supporting Information, in
which a phase separation can be clearly seen after stirring is stopped.
[14] Such responses to aromatics by the imidazolium-based ILs have been
reported and ascribed to the formation of liquid clathrates: J. D. Hol-
brey, W. M. Reichert, M. Nieuwenhuyzen, O. Sheppard, C. Hardacre, R. D.
Rogers, Chem. Commun. 2003, 476–477.
Received: March 24, 2014
[15] The structure of the recovered IL 1a did not change as evidenced by
1H and 13C NMR analysis (see ESI).
Published online on && &&, 0000
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A. Taheri, X. Pan, C. Liu, Y. Gu*
Brains not Brønsted: Sulfonyl-contain-
ing ammonium-based Brønsted acid
ionic liquids (ILs) are prepared and used
as liquid heterogeneous catalysts for or-
ganic reactions under solvent-free con-
ditions. The sulfonyl group endowed
these ILs with unique macroscopic
phase heterogeneity in the reaction
system, ensuring outstanding catalytic
activities of the ILs. These catalytic sys-
tems are applicable in a wide range of
acid-catalyzed reactions.
&& – &&
Brønsted Acid Ionic Liquid as
a Solvent-Conserving Catalyst for
Organic Reactions
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemSusChem 0000, 00, 1 – 5
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6
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