Ionic liquid[DBUH][BO2]: an excellent catalyst for chemical fixation of CO2under mild conditions

Article abstract of DOI:10.1039/d0nj04631k

The green basic IL[DBUH][BO₂]was easily synthesized for the first time and used to catalyze the cycloaddition of CO₂and epoxides under solvent- and halogen-free conditions at atmospheric pressure and room temperature with high target product yields. Moreover, this IL could be easily recovered and reused at least five times without activity loss. The basic anion and the cation with N-H had excellent synergistic catalytic effects on promoting these reactions. Particularly, this basic IL, the anion of which had a strong nucleophilic ability, was also very effective for the reactions of CO₂and 2-aminobenzonitriles into quinazoline-2,4(1H,3H)-diones, giving the desired products in high yields at atmospheric pressure and room temperature.

Full text of DOI:10.1039/d0nj04631k

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Ionic liquid [DBUH][BO2]: an excellent catalyst for
chemical fixation of CO2 under mild conditions†
Cite this: New J. Chem., 2021,
45, 4611
Zheng Wang,a Da Li,b Shangqing Chen,a Jiayin Hu, *a Yanxi Gong,a Yafei Guoa
ac
and Tianlong Deng
*
The green basic IL [DBUH][BO2] was easily synthesized for the first time and used to catalyze the
cycloaddition of CO2 and epoxides under solvent- and halogen-free conditions at atmospheric pressure
and room temperature with high target product yields. Moreover, this IL could be easily recovered and
reused at least five times without activity loss. The basic anion and the cation with N–H had excellent
synergistic catalytic effects on promoting these reactions. Particularly, this basic IL, the anion of
Received 17th September 2020,
Accepted 18th January 2021
which had
a strong nucleophilic ability, was also very effective for the reactions of CO2 and
DOI: 10.1039/d0nj04631k
2-aminobenzonitriles into quinazoline-2,4(1H,3H)-diones, giving the desired products in high yields at
rsc.li/njc
atmospheric pressure and room temperature.
reaction. Although some of the aforementioned ILs showed good
catalytic activity, the reaction could proceed at high CO2 pressure
Introduction
Compared with toxic phosgene and carbon monoxide, chemical and/or high reaction temperature. In this way, a dual-ionic liquid
fixation of the abundant, nontoxic, easily available but inert system [TMGH+][ÀO2MMIm+][BrÀ] was reported for use in the
greenhouse gas carbon dioxide (CO2) into high value-added cycloaddition of CO2 and epoxides at atmospheric pressure and
chemicals has important significance and long-term potential low reaction temperature,21 and the halide ion in this IL was an
in both academia and industry,1–3 especially at room temperature integral part in the catalytic process. Recently, a halogen-free basic
and atmospheric pressure of CO2.4–6 Among various CO2-related IL [P4444][2-OP] was developed to catalyze these reactions at room
reactions, the transformation of CO2 and epoxides to temperature efficiently,22 but the synthetic process of this IL was
five-membered cyclic carbonates through the formation of a complex. Therefore, the exploration of an efficient, green and
new C–O bond (Scheme 1) occupies an important position,7,8 easy-to-prepare IL for this class of reactions at atmospheric
not only because the products are important green polar aprotic pressure and room temperature under halogen- and solvent-free
solvents and intermediates in the fields of chemistry, electricity conditions is still highly desirable. Borates including metaborate
and medicine,9,10 but also because this reaction has high atom (BO2À), diborate (B2O42À), triborate (B3O5À), tetraborate (B4O72À),
efficiency without any by-product.11
pentaborate (B5O8À) and hexaborate (B6O102À) are widely distrib-
Ionic liquids (ILs), which have some unusual properties uted in salt lakes, and research studies are mainly concentrated
including negligible vapor pressure, nonflammability, and on the extraction of borate.23 However, we recently found that the
designability,12 are one kind of excellent catalyst for the reactions borate anion could efficiently catalyze the reaction of CO2 and
of CO2 and epoxides.13 For example, homogeneous and hetero- 2-aminobenzonitrile for its strong basicity.24 Compared with other
geneous quaternary ammonium-ILs,14 functionalized ILs borates, BO2À has stronger basicity and a smaller size, indicating
including hydroxyl-ILs,15,16 urea derivative based ILs,17 amino its better catalytic activity for the chemical fixation of CO2.
acid-based ILs18,19 and Lewis basic ILs20 are proposed for this 1,8-Diazabicyclo[5.4.0]undec-7-ene (DBU) was the superbase and
was used to easily synthesize various ILs by regulating the anion
via neutralization.25 Recently, the [DBUH] based IL has already
a Tianjin Key Laboratory of Brine Chemical Engineering and Resource
Eco-utilization, College of Chemical Engineering and Materials Science,
Tianjin University of Science and Technology, Tianjin 300457, China.
E-mail: hujiayin@tust.edu.cn, tldeng@tust.edu.cn; Tel: +86-22-60602963
b Tianjin Colouroad Coatings & Chemicals Co., Ltd., Tianjin, 300457, China.
E-mail: elita80s@163.com
c College of Marine and Environmental Science, Tianjin University of Science and
Technology, Tianjin 300457, China
Scheme 1 The green route for the synthesis of cyclic carbonates using
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
CO2 and epoxides.
d0nj04631k
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been used to catalyze this kind of reaction. For example, Hou et al. Catalytic reaction
developed halogen-free [DBUH]3NbO5 and used it for the
synthesis of cyclic carbonates from epoxides and CO2 at 130 1C
and 3.0 MPa with high activity and selectivity.26
As an example, the procedure using epichlorohydrin as the
substrate is described, and those for other substrates were
similar. Epichlorohydrin (2 mmol, 0.1850 g) and [DBUH][BO2]
(1.5 mmol, 0.2941 g) were loaded into a 22 mL stainless-steel
batch reactor equipped with a magnetic stirrer. The air in the
reactor was removed by blowing CO2 into the reactor. Then the
pressure of CO2 was kept at 0.1 MPa using a balloon with CO2.
The reactor was placed in a constant temperature water bath
and the reaction mixture was stirred for a desired time. After
the reaction, the reactor was placed in ice water for 20 minutes
and the reaction mixture was passed through a plug of silica
gel. The crude mixture was purified via silica gel column
chromatography (EtOAc : petroleum ether = 1 : 15) to obtain
the desired cyclic carbonate 4-chloromethyl-[1,3]dioxolan-2-one.
Herein, the green and halogen-free basic IL [DBUH][BO2]
was easily synthesized for the first time and used to catalyze the
cycloaddition of CO2 and epoxides without any solvents at
atmospheric pressure and room temperature with high target
product yields. Moreover, this IL could be easily recovered and
reused at least five times without activity loss. Particularly, this
basic IL could also effectively catalyze the reactions of CO2 and
2-aminobenzonitriles into quinazoline-2,4(1H,3H)-diones at
atmospheric pressure and room temperature with high yields
of the desired products.
Typical procedure for the synthesis of quinazoline-2,4(1H,3H)-
diones
Experimental
Chemicals
As an example, the procedure using 2-aminobenzonitrile as the
substrate is described, and those for other substrates were
similar. In a typical experiment, 2-aminobenzonitrile (2 mmol,
0.2643 g), toluene (1 mL) and [DBUH][BO2] (1.5 mmol, 0.2941 g)
were loaded into a 22 mL stainless-steel batch reactor equipped
with a magnetic stirrer. The air in the reactor was removed by
blowing CO2 through the reactor. Then the pressure of CO2 was
kept at 0.1 MPa using a balloon with CO2. The reactor was
placed in a 30 1C oil bath for the desired time. After the
reaction, 25 mL water was added into the reactor and the mixture
was centrifuged to precipitate the product. Then the product was
washed with tert-butyl methyl ether (3 Â 15 mL) and ethanol
(3 Â 15 mL), and then dried at 95 1C for 3 h to obtain the target
product.
CO2 was supplied by Tianjin Feilin Gases Co., Ltd with a purity
of 99.99%. 1,8-Diazabicyclo[5.4.0]undec-7-ene (DBU) (98%) and
2-amino-5-bromobenzonitrile (99%) were purchased from
J&K Scientific Ltd. 2-Amino-5-chlorobenzonitrile (98%) and
2-aminobenzonitrile (98%) were obtained from 3A Chemicals
Ltd. Boric acid (99%), metaboric acid (99%) and glacial acetic
acid (99%) were purchased from Shanghai Aladdin Bio-Chem
Technology Co., Ltd. Epichlorohydrin (99%), butylene oxide
(98%) and 3-bromo-1,2-epoxypropane (97%) were purchased
from Tokyo Chemical Inorganic Co., Ltd. 1,2-Epoxy-3-phenoxy-
propane (99%) was purchased from InnoChem Co., Ltd. N,N-
Dimethylformamide (AR), dimethyl sulfoxide (AR), tert-butyl
methyl ether (AR), ethanol (AR) and diethyl ether (AR) were
purchased from Shanghai Macklin Biochemical Co., Ltd. Dimethyl
sulfoxide-d6 (99.9%) was supplied by Cambridge Isotope
Laboratories, Inc. 1-Butyl-3-methylimidazolium nitrate ([Bmim][NO3]),
1-butyl-3-methylimidazolium dicyanamide ([Bmim][N(CN)2])
and 1-butyl-3-methylimidazolium acetate ([Bmim][OAc]) were
supplied by Lanzhou Yulu Fine Chemical Co., Ltd. All the
chemicals were used without further purification.
Catalyst recyclability
Epichlorohydrin (2 mmol, 0.1850 g) and [DBUH][BO2]
(1.5 mmol, 0.2941 g) were loaded into a 22 mL stainless-steel
batch reactor equipped with a magnetic stirrer. The air in the
reactor was removed by blowing CO2 into the reactor. Then the
pressure of CO2 was kept at 0.1 MPa using a balloon with CO2.
The reactor was placed in a 30 1C water bath and the reaction
mixture was stirred for 6 h. After the reaction, the product was
extracted by diethyl ether (3 Â 2 mL) and the yield was
Apparatus
Standard column chromatography was performed on 20–40 mm
silica gels using flash column chromatography. NMR spectra
were recorded on a Bruker Fourier 400 MHz NMR spectrometer
(400 MHz for 1H and 100 MHz for 13C). The elemental analysis
was carried out with a Flash EA 1112 elemental analyzer.
1
determined via H NMR using DMF as the internal standard;
in particular, the extraction experiments were conducted under
a CO2 atmosphere and at 0 1C. The IL separated was used
directly for the next run after being dried under vacuum at
60 1C for 24 h.
Synthesis of [DBUH][BO2]
The IL [DBUH][BO2] was synthesized via neutralization. In the
experiment, methanol (10 mL), DBU (10 mmol, 1.5224 g) and
HBO2 (10 mmol, 0.4382 g) were loaded into a 50 mL flask in a
Results and discussion
Ionic liquid [DBUH][BO2]
30 1C water bath and the neutralization reaction was allowed to The IL [DBUH][BO2][DBUH][BO2][DBUH][BO2] was synthesized
proceed for 24 h. Then, the solvent was removed by rotary via neutralization of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU)
evaporation under vacuum at 70 1C, and finally the IL was and metaboric acid (HBO2), as shown in Scheme 2. The
obtained. This IL was a colourless liquid at room temperature. formation of this superbase-derived IL [DBUH][BO2] was shown
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a
Table 1 IL-catalyzed reaction of epichlorohydrin (1a) and CO2
Entry
IL
Yieldb (%)
Scheme 2 Preparation of the IL [DBUH][BO2].
1
2
3
4
5
6
7
8c
—
0
o5
o5
42
68
54
[Bmim][NO3]
[Bmim][N(CN)2]
[Bmim][OAc]
[DBUH][OAc]
[DBUH][H2BO3]
[DBUH][BO2]
[DBUH][BO2]
via FT-IR spectroscopy (Fig. 1). The broad absorption peak
at 3435 cmÀ1 corresponded to the stretching vibration peak of
N–H, while the stretching vibration peak of O–H at 3200 cmÀ1
belonging to HBO2 disappeared,27 suggesting the formation of
[DBUH]+. Furthermore, the 1H NMR peaks of [DBUH][BO2] agreed
well with those of [DBUH]+ in the aprotic IL [DBUH][MIm],21
which further proved the formation of [DBUH][BO2]. The 1H and
13C NMR spectra of the IL [DBUH][BO2] are presented in the ESI.†
In addition, the elemental analysis of the IL [DBUH][BO2] for C,
H and N was carried out (calcd: C 55.14, H 8.74, N 14.29; found: C
55.02, H 8.82, N 14.13).
91
94
a
The typical reaction conditions were as follows until otherwise stated:
2 mmol of 1a (0.1850 g), 1.5 mmol of the IL, 0.1 MPa CO2, 30 1C, 4 h.
b
Yields were determined via 1H NMR spectroscopy using DMF as the
c
internal standard. The reaction time was 6 h.
anion, the catalytic activity of the DBU-based IL was better than
that of the imidazolium-based IL, indicating that [DBUH]+ also
had a positive role in this reaction. Therefore, the DBU-based ILs
1,8-diazabicyclo[5.4.0]-7-undecenium borate ([DBUH][H2BO3]) and
1,8-diazabicyclo[5.4.0]-7-undecenium metaborate [DBUH][BO2],
the anions of which had stronger basicity, were used for this
reaction. Interestingly, both ILs showed catalytic activities for this
reaction, but the activities were very different with the yields of the
target product 2a being 54% and 91% (entries 6 and 7), respectively.
This may be because [H2BO3]À was so big that too much steric
hindrance reduced the activity, while [BO2]À with strong basicity
and an appropriate size showed excellent activity for this reaction
(entries 7 and 8).
Screening of catalysts
Firstly, the catalytic activities of various halogen-free ILs were
investigated using epichlorohydrin (1a) as the substrate at room
temperature and 0.1 MPa CO2 within 4 h, and the results are given
in Table 1. Clearly, the reaction did not proceed in the absence of
any of the catalysts (Table 1, entry 1). For imidazolium-based ILs
(entries 2–4), the neutral ILs 1-butyl-3-methylimidazolium nitrate
([Bmim][NO3]) and 1-butyl-3-methylimidazolium dicyanamide
([Bmim][N(CN)2]) had little catalytic effects on this reaction, while
the basic IL 1-butyl-3-methylimidazolium acetate ([Bmim][OAc])
could catalyze the reaction, with the yield of the target product
4-chloromethyl-[1,3]dioxolan-2-one (2a) being 42%, suggesting
that the basic anion was crucial for the catalytic activity. With
this in mind, the basic DBU-based IL was used for this reaction,
and it was found that the IL 1,8-diazabicyclo[5.4.0]-7-undecenium
acetate ([DBUH][OAc]) showed good catalytic activity and gave
product 2a in 68% yield (entry 5), which was much higher than
that of [Bmim][OAc]. This result showed that, with the same basic
Effect of reaction conditions
Subsequently, the effects of different [DBUH][BO2] amounts on
the yield of 2a were studied (Fig. 2a), and the results showed
that a 75% molar ratio of [DBUH][BO2] was enough for the
reaction. The effect of reaction time on the yield of 2a with a
25% molar ratio of [DBUH][BO2][DBUH][BO2] was also tested
and the results are shown in Fig. S3 (ESI†). The results
indicated that, with 25% [DBUH][BO2], this reaction could also
proceed smoothly, while a much longer reaction time was
needed. Therefore, given the recyclability of the IL and the
reaction time, we chose this high IL/substrate ratio (75%) as the
optimal condition. Fig. 2b illustrates the dependence of yield
on the reaction time. It was found that the yield of 2a increased
with increasing reaction time and could reach 94% in 6 h.
Thus, the optimal reaction conditions were obtained (Table 1,
entry 8). It can be seen that this easily prepared halogen-free
IL is a very excellent catalyst considering that the reaction
proceeded without any solvent under mild conditions within
a short time.
The reusability of the catalyst
Because the IL can be separated by adding diethyl ether after
Fig. 1 The FT-IR spectrum of the IL [DBUH][BO2].
the reaction, the reusability of [DBUH][BO2] was tested, and the
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possible mechanism was proposed, which is presented in
Scheme 3. Firstly, the [BO2]À caused a nucleophilic attack on
the C atom of CO2 to form the carbamate anion. Because of the
strong basicity and nucleophilic ability of [BO2]À, this step
proceeded smoothly, and this is why the reactions could
proceed at atmospheric pressure of CO2. It was reported that
the N–H bond and halide anion had a synergistic activation
effect on epoxides to promote its ring-opening.21,22,27,28
Compared with the halide anion, the OÀ of the carbamate
anion also had nucleophilic ability. Therefore, in the next step,
the OÀ of the carbamate anion caused a nucleophilic attack on
the less sterically hindered C atom of the epoxide, and at the
same time the H atom of the N–H bond belonging to [DBUH]+
interacted with the O of the epoxide through hydrogen bonding
to promote opening of the epoxide ring. Then, the OÀ of the
epoxide that was stabilized by the N–H of [DBUH]+ caused a
nucleophilic attack on the C atom of the activated CO2, leading
to the formation of cyclic carbonate through intramolecular
ring-closure, and the catalyst was regenerated.
Fig. 2 (a) Dependence of the yield on the amount of [DBUH][BO2] in 6 h.
Reaction conditions: 1a (2 mmol, 0.1850 g), CO2 (0.1 MPa), 30 1C and
different [DBUH][BO2] contents; (b) the effect of reaction time on the yield
of 2a with 75% [DBUH][BO2]. Reaction conditions: 1a (2 mmol, 0.1850 g),
CO2 (0.1 MPa), 30 1C and different reaction times. The yield was deter-
mined via 1H NMR spectroscopy using DMF as the internal standard.
results are shown in Fig. 3. The catalytic activity of the IL did
not show any obvious change after it was reused five times,
indicating its good recyclability.
In addition, comparison of the catalytic performance
between the IL in this work and the reported halogen-free ILs
for the reaction of epichlorohydrin and CO2 is shown in Table
S1 (ESI†). Compared with [DBUH]3NbO5, the catalytic activity of
the IL [DBUH][BO2] was much higher. It was also found that the
catalytic activity of the IL [DBUH][BO2] was no less than that of
[P4444][2-OP]. Although the amount was much higher,
[DBUH][BO2] was easy to prepare and recycle.
Reaction with 2-aminobenzonitriles
According to the possible mechanism mentioned above, the H
atom and the basic [BO2]À from the IL [DBUH][BO2] were the
key factors that influenced its catalytic activity. Coincidentally,
for the reactions of CO2 and 2-aminobenzonitriles into
quinazoline-2,4(1H,3H)-diones, the strong basicity that could
activate the amino group of 2-aminobenzonitriles was necessary
for the catalyst,27 while the IL [DBUH][BO2] met this condition.
Therefore, in order to verify indirectly the mechanism which we
proposed, and broaden the application range of the IL
[DBUH][BO2], the reaction of CO2 with 2-aminobenzonitrile to
quinazoline-2,4-(1H,3H)-dione catalyzed by [DBUH][BO2] was
conducted at 0.1 MPa CO2 and room temperature, and the
results are shown in Scheme 4. To our delight, the IL
[DBUH][BO2] showed good catalytic activity for this reaction,
Reaction mechanism
The results in Table 1 suggest that the basic anion [BO2]À
having a strong nucleophilic ability played an important role in
the catalytic activity, while the [DBUH]+ cation also exhibited a
catalytic effect. Thus, on the basis of the experimental results of
this work and the related knowledge in the literature,22 we
deduced that the IL [DBUH][BO2] had an excellent synergistic
catalytic effect on the reactions of CO2 and epoxides and a
Fig. 3 Recyclability of [DBUH][BO2]. Reaction conditions: 1a (2 mmol,
0.1850 g), IL (1.5 mmol, 0.2941 g), CO2 (0.1 MPa), 30 1C and 6 h.
Scheme 3 Proposed mechanism for the [DBUH][BO2] catalyzed
cycloaddition of epoxides and CO2.
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Table
2
Reactions of various substrates with CO2 catalyzed by
[DBUH][BO2]a
Entry Substrate
Product
Reaction time (h) Yield (%)
1
2
6
93
Scheme 4 The reaction of CO2 and 2-aminobenzonitrile into
quinazoline-2,4(1H,3H)-dione catalyzed by [DBUH][BO2].
91
96b
24
and the isolated yield of the target product quinazoline-2,4-
(1H,3H)-dione was up to 94% in 6 h.
3
4
72
72
90b
89c
In addition, on the basis of the related knowledge in the
literature,29,30 a possible mechanism for the IL [DBUH][BO2]
catalyzed reaction of CO2 and 2-aminobenzonitriles is shown in
Scheme 5. Firstly, the [BO2]À with strong basicity could attack
both the H atom of the amino group of 2-aminobenzonitrile
and the C atom of CO2. In this way, the electrophilic attack of
CO2 on the amino group is more favorable, and the carbamate
anion, as well as HBO2, was formed. Then, intramolecular
nucleophilic cyclization and subsequently a rearrangement
occur to form a quinazoline anion. Finally, the quinazoline
anion extracts the proton from HBO2 to form the target
product, and the basic IL can be recovered for the second cycle.
5
6
72
6
67c
0
7
8
6
94d
Catalyst generality
Finally, in order to explore the universality of the IL
[DBUH][BO2], the reactions of CO2 with different substituted
epoxides and with different substituted 2-aminobenzonitriles
were examined using [DBUH][BO2] as the catalyst under mild
conditions and the isolated yields of the target products are
summarized in Table 2. For the reactions of CO2 with different
substituted epoxides, the reactivity of the epoxides depended
strongly on the size of the substituted R group instead of the
electronic effects of the substituted R group. When R groups
were Br (2b, 2e, electron-withdrawing group) and the alkoxy
group (2c, 2d, electron-donating group) that were bigger than
Cl, the reactions could require a long time or even higher
reaction temperature to complete (Table 2, entries 2–5).
But when R was a methyl group, the desired product 2f was
12
12
88d
90d
9
a
Reaction conditions: substrate 2 mmol, [DBUH][BO2] 1.5 mmol
b
(0.2941 g), CO2 0.1 MPa, 30 1C. [DBUH][BO2] 0.5 mmol (0.0980 g).
c
d
The reaction temperature was 80 1C. [DBUH][BO2] 6 mmol (1.1764 g),
methylbenzene (1 mL).
not detectable. For the reactions of CO2 with different substituted
2-aminobenzonitriles, the reactivity of the 2-aminobenzonitriles
also depended strongly on the substituted R group, and the
reaction was sensitive to the electronic effect of substituted R.
When R was a halogen with some electron-withdrawing effects, a
much longer time was needed to complete the reaction (2g–2i).
Conclusions
In conclusion, [DBUH][BO2] was synthesized successfully and
utilized as the catalyst to effectively promote the reactions of
CO2 and epoxides under solvent- and halogen-free conditions
at atmospheric pressure and room temperature with high
yields of cyclic carbonates. Compared with other halogen-free
ILs, the IL [DBUH][BO2] can be prepared more easily and has
better catalytic universality. Moreover, this IL could be easily
Scheme 5 Proposed mechanism for the reaction of CO2 and 2-
aminobenzonitriles.
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There are no conflicts to declare.
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Acknowledgements
The authors gratefully acknowledge the partial financial support
from the National Natural Science Foundation of China (21901183
and 21773170) and the Yangtze Scholars and Innovative Research
Team of the Chinese University (IRT_17R81).
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4616 | New J. Chem., 2021, 45, 4611À4616
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