Tuning the basicity of ionic liquids for efficient synthesis of alkylidene carbonates from CO2 at atmospheric pressure

Article abstract of DOI:10.1039/c6cc02853e

A strategy to achieve the efficient synthesis of alkylidene carbonates from CO₂ at atmospheric pressure by tuning the basicity of ionic liquids was developed. Excellent yields were obtained due to basic ionic liquids' dual roles both as absorbents and as activators. The reaction mechanism was investigated through a combination of NMR spectroscopy, controlled experiments and quantum calculations, indicating the importance of a moderate basicity.

Full text of DOI:10.1039/c6cc02853e

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DOI: 10.1039/C6CC02853E
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Tuning the Basicity of Ionic Liquids for Efficient Synthesis of
Alkylidene Carbonates from CO2 at Atmospheric Pressure
Kaihong Chen,a Guiling Shi,a Rina Dao,a Ke Mei,a Xiuyuan Zhou,a Haoran Li,a and Congmin Wang*ab
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
A strategy to achieve efficient synthesis of alkylidene carbonates reaction,7 and so on. Weak basicity would lead to low activity, while
from CO2 at atmospheric pressure by tuning the basicity of ionic strong basicity would result in some side reactions such as
liquids was developed. Excellent yields were obtained due to basic polymerization or even the alteration of the reaction route.8
ionic liquids’ dual roles both as absorbents and as activators. The Meanwhile, it was reported that reaction rate had a linear
reaction mechanism was investigated through a combination of relationship with catalyst acidity in the Brønsted acid catalysis.9 Is
NMR spectra, controlled experiments and quantum calculations, there similar relationship in Brønsted base-catalyzed reactions,
indicating the importance of a moderate basicity.
especially in CCU processes? Can we develop a new and versatile
method to tune the basicity for improving these base-catalyzed
reactions?
As the suspect for global warming and an abundant C1 resource,
CO2 capture and utilization (CCU) has attracted more and more
attention.1 In industry, as a traditional absorbent for CO2, aqueous
amine solution bears many disadvantages, such as high energy
demand, corrosion and solvent loss.2 Ionic liquids (ILs) offer a new
opportunity for developing some novel systems that are capable of
efficiently and reversibly capturing CO2 because of their unique
properties.3 Numbers of researches have reported that ILs can be
an ideal absorbent for CO2 through tuning their structures and
properties.4 In 2002, Davis et al. reported the first example of CO2
chemisorption by amino-functionalized IL, which could absorb 0.5
mol CO2 per mol IL under ambient pressure.4a Subsequently, other
anion-functionalized ILs were developed for improving CO2
capture.4 However, CO2 chemisorption often needs high absorption
enthalpy, leading to high energy requirement for regeneration and
difficult desorption. On the other hand, further energy input (high
CO2 pressure and high temperature) or stoichiometric amounts of
base is often needed in CO2 utilization, because CO2 is the most
Cyclic carbonates is a hot topic in organic synthesis, which can
be used as electrolytes in Li ion batteries, organic solvents and the
key intermediates in pharmaceutical synthesis.10 Recently, numbers
of attentions were paid to CO2 fixation into cyclic carbonates.11
Synthesis of alkylidene carbonates from CO2 and propargylic
alcohols is a challenging way to reach cyclic carbonates, since this
process embraces some side reactions.12 Numerous catalysts have
been developed for this process,13 however, most of them need a
high CO2 pressure, because these systems represent weak ability in
CO2 capture and activation. How can we develop a new method for
efficient synthesis of alkylidene carbonates under mild condition?
Herein, we report a new method for highly efficient synthesis of
alkylidene carbonates from CO2 at atmospheric pressure under mild
condition. The essence of our strategy is to tune the basicity of ILs
in the reaction, where basic ILs acts both as absorbents and as
activators. The results show that both high conversion and good
selectivity can be achieved at room temperature through finely
tuning the basicity of ILs. NMR spectra, quantum calculation and
experiment demonstrate that moderate basicity is very important
to this reaction.
The effect of the basicity on this reaction was investigated,
which was shown in Table 1. At first, the reaction did not occur
without base or with a weak base, Et3N (Table 1, entry 1 and 2).
When the superbase was used, the yield of 51% was achieved by
DBU (Table 1, entry 3). Clearly, this reaction was significantly
affected by the strength of base, because the pKa in acetonitrile for
Et3N and DBU are 18.8 and 24.3, respectively.14 In this point, the
traditional superbase was replaced by some basic ILs, because their
structures and basicity can be tuned conveniently (Fig. 1). To our
delight, a full conversion and higher yield were got when a strong
oxidized state of carbon and thermodynamic stabilized.1,
3a
Therefore, the energy requirement could be reduced if CO2
absorption and CO2 fixation could be combined through a base-
catalyzed reaction, rather than releasing CO2 under heating or high
vacuum.
Meanwhile, there are numerous of base-catalyzed reactions,
like aldol reaction,5 Morita-Baylis-Hillman reaction,6 Rauhut-Currier
a.Department of Chemistry, ZJU-NHU United R&D Center, Zhejiang University,
Hangzhou 310027, China. E-mail: chewcm@zju.edu.cn; Fax: +86-571-8795-1895.
b.Key Laboratory of Biomass Chemical Engineering of Ministry of Education,
Zhejiang University, Hangzhou 310027, China.
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
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DOI: 10.1039/C6CC02853E
Fig. 1 The structures of the cation and the anions in basic ionic liquids.
Fig.2 The relationship between conversion (
pKa.
○) and yield (■) with calculated
basic ILs [P66614][Im] was used as base (Table 1, entry 4). What’s
more, moderate basic ILs [P66614][DEIm] improved the yield to 91%
(Table 1, entry 8). However, only half of 1a was converted to 2a
when weak basic ILs, [P66614][Tetz], was used (Table 1, entry 10).
In order to make it more visualized, pKa values of these anions
in tBuOH were calculated by DFT calculation.15 When we take a look
at the basicity of these ILs, these outcomes are interesting. At first,
strong basicity meant full conversion, while the weaker basicity, like
[P66614][Tetz], resulted in a lower conversion. However the stronger
basicity did not mean a higher yield, for example, only a yield of
68% was achieved when [P66614][Im] was used. It was interesting
that the selectivity increased from 68% to 91% when the pKa
reduced from 18.3 to 7.6. However, the selectivity decreased a little
while the pKa further reduced to 5.0.
Figure 2 shows the relationship between conversion and yield
with calculated pKa. Clearly, the higher the basicity is, the higher
the conversion is. Almost full conversion can be achieved within 5h
while the pKa value of the anion is higher than 7.6. As for the anions
with lower basicity, like [3-CN-Triz] and [Tetz], the reaction is not
complete, indicating the poor catalytic activity. Nevertheless, the
basicity exhibits a different influence on the yield. The increasing
basicity will reduce the yield when the pKa is higher than 7.6. In
contrast, the selectivity descends a little while the pKa reduce from
7.6.
Various silver and copper catalysts were investigated, where the
AgOAc displayed a better catalytic reactivity (Table S1). The effect
of the temperature and the amount of the catalyst on this reaction
was also investigated (Table S2). It was surprised that the yield
decreased significantly when the temperature increased from 30 to
o
40 C. The yield of 91% can be achieved while only 1% AgOAc was
used.
Clearly, high conversion and good selectivity for the synthesis of
alkylidene carbonates from atmosphere CO2 under mild condition
could be achieved through tuning the basicity of ILs. Why the
basicity of ILs exhibited such a significant role in this reaction? Why
weak basicity led to low activity, while strong basicity resulted in
poor selectivity? Furthermore, it was very surprised that the
conversion reduced with increasing the temperature (Table S2).
To begin with, the role of ILs in this reaction should be figured
out. Hence serials of controlled experiments were performed. At
first, NMR spectroscopic experiments revealed that hydrogen bond
were formed between the bases and 1a, where the OH signal
became broad after adding bases (Fig.3). Therefore, hydrogen
abstraction is not the prerequisite for this reaction, while the
hydrogen bond is the way to start this reaction.
It is well-known that azole-functionalized ILs can absorb CO2
under mild condition.4g To gain more information about the effect
of CO2 absorption capacity on this reaction, the reaction under 0.1
bar CO2 was performed. As shown in Table 1 entry 3 and 6,
[P66614][DEIm] could give a yield of 73%, however, for DBU, only a
low yield of 2% was detected. It was known that DBU could react
with CO2, but the capacity was weaker than that by anion-
functionalized ILs.4g,16 Furthermore, to verify this result,
[P66614][DEIm]-CO2 complex was added into the reaction, which was
formed after the capture of CO2. As seen in Table 1 entry 11, the
conversion and selectivity were 73% and 87% after 5 h, respectively.
The results indicated that this reaction carried out well with
[P66614][DEIm]-CO2. It should be mentioned that the basicity of Et3N
is stronger than [P66614][Tetz]. Clearly, CO2 absorption is significant
in this reaction, which is more important when the pressure of CO2
decreased.17 Therefore, it is understandable that a longer time is
needed to ensure 1a convert fully at 40 oC, because the CO2 capture
process will be hindered by high temperature.
Table 1 The effect of different basic ionic liquids on the synthesis of alkylidene
carbonates from atmospheric CO2.[a]
Calculated
Conversion
[%][b]
Entry
Base
Yield [%][b]
pKa[15]
1
2
-
-
-
<1
<1
<1
Et3N
<1
3
DBU
-
>99
>99
>99
>99
>99
>99
68
51(2)
68
4
[P66614][Im]
18.3
13.5
11.7
9.3
7.6
6.0
5.0
-
5
[P66614][Triz]
[P66614][4-CHO-Im]
[P66614][CO2Me-Triz]
[P66614][DEIm]
[P66614][3-CN-Triz]
[P66614][Tetz]
[P66614][DEIm]-CO2
76
6
82
7
86
8
91(73)
58
9
10
11
46
73[c]
40
64
Moreover, DFT calculations were used to investigate the role of
the basicity (Fig.4 and ESI).13 As shown in Figure 4a, calculated CO2
absorption enthalpy for [DEIm] was -4.5 kcal/mol in tBuOH, which
suggests the weak reaction between [DEIm] and CO2.4b,18 In
addition, hydrogen abstraction by [DEIm] required Gibbs free
[a]Reaction conditions: 1a (2 mmol), AgOAc (0.2 mmol), base (0.4 mmol) and CO2 (1 bar) for 5
h at 30 oC. [b] Determined by GC using internal standard method, the yield in bracket was
under 0.1 bar CO2 for 48h at 30 oC.[c] [P66614][DEIm] with captured CO2 was used instead of
bubbling CO2.
2 | J. Name., 2012, 00, 1-3
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vanished when 1a was mixed with DBU [Fig.3b]. NMRVsiepweAcrttricolescOonpliniec
DOI: 10.1039/C6CC02853E
experiments revealed that the H in OH was more likely to be
activated by a strong base. Furthermore, CO2 capture process also
played a key role in this reaction. With strong basicity, the CO2
absorption capacity for [P66614][Im] was equimolar.4b However,
[P66614][Tetz] shows a low CO2 absorption capacity, which was 0.08
mol CO2 per mol IL.4b In this point, poor catalytic activity for weak
basicity can be understood by considering both weak H activation
and low CO2 absorption capacity.
In addition, it was well-known that terminal alkyne canreact
with silver to form silver acetylide in the presence ofbase, leading to
other side reactions such aspolymerization.19 In this regard, Gibbs
free energies for abstraction the H from terminal alkyne the with
different anions were calculated. Gibbs free energy to abstract the
H from terminal alkyne for [Im] was 17.3 kcal/mol, while that for
[DEIm] and [Tetz] was 31.9 kcal/mol and 35.4 kcal/mol, respectively.
Clearly, the lower Gibbs free energy for [Im] led to poorer
selectivity at room temperature.20
The utility and generality of this reaction was also investigated
from atmospheric CO2 under mild condition using [P66614][DEIm]
(Table 2). As it shown, all of these derivatives could have a good to
excellent yield. Meanwhile, it needed longer time to make sure the
raw material conversetotally, when there was a large substituent
group. For example, with an ethyl group, 2b was obtained in a yield
of 91% within 35 h (Table 2, entry 2). Treatment of 1c or 1d with
CO2 afforded the corresponding cyclic carbonate with 98% and 97%
yield, respectively (Table 2, entry 3 and 4). As for the propargylic
alcohol with cyclopropyl group (1f), it could also afford a yield of
78% (Table 2, entry 5). Furthermore, the reaction at 10 mmol
scale 1a was investigated (Scheme S2). As can be seen, 85 % of
2a was obtained after 12 h. The reusability of the catalyst
system was also investigated, which was shown in Fig. S2. It
was found that this system could be recycled for more than 5
times without significantly loss of catalytic activity.
Fig.3 1H NMR spectrum of (a) 1a (0.2 mmol) with (b) DBU (0.04 mmol), (c)
[P66614][Im] (0.04 mmol), (d) [P66614][DEIm] (0.04 mmol), and (e) [P66614][Tetz]
(0.04 mmol) in DMSO-d6 (0.55mL).
energy of 8.1kcal/mol [Fig.4b], which was agreeable with NMR
results, where [P66614][DEIm] had poor basicity to abstract the H.
When CO2 was taken into account, however, this case became more
favorable and the Gibbs free energy reduced to 6.8 kcal/mol (Fig.4c).
It was believed that CO2-complex can nucleophilic attack triple bond
13j
in 1a to generate 2a,13i,
so the intermediate of IIa was also
calculated (Fig.4d). As shown, the nucleophilic attackof [DEIm]-CO2
needs Gibbs free energy of 28.1 kcal/mol, which was too high for
the reaction to occur at room temperature.
On the basis of controlled experiments and quantum
calculations, a plausible mechanism was proposed (Scheme 1).
Firstly, 1a was activated by [DCIm] through Hydrogen bond and Ia
was formed, at the same time, [DEIm] absorbed CO2 to form
[DEIm]- CO2. Then, CO2 in [DEIm]- CO2 was abstracted by Ia to give
Ib and [DEIm]. Finally, 2a was generated throughintramolecular
ring-closing reaction in Ib and [DEIm] was recycled.
With this proposed mechanism, the effect of the basicity on this
reaction can be analyzed step by step. Firstly, the influence of the
basicity on H activation and CO2 absorption was investigated. It was
understandable that the stronger basicity the base had, the H in OH
was easier to be activated. The OH signal in 1H NMR was still a peak
when a weak basic ILs [P66614][Tetz] was used [Fig.3e]. However, this
signal became broad when [P66614][Tetz] was replaced by the
stronger basic ILs [P66614][Im] or [P66614][DEIm] [Fig.3c and 3d].
What’s more, due to the high basicity of DBU, the OH signal was
In summary, we developed a new strategy to achieve efficient
synthesis of alkylidene carbonates from CO2 at atmospheric
pressure under mild condition by tuning the basicity of ILs. Through
a
combination of NMR spectra, controlled experiments and
quantum calculations, it was found that a moderate basicity was
significant to this reaction. The strong basicity would lead to side
reaction, while the weak base exhibited poor catalytic activity.
Furthermore, excellent yields were achieved when other
Fig.4 Optimized structures for (a) [DEIm]- CO2, ∆H = -4.5 kcal/mol; (b)
hydrogen abstraction by *DEIm+, ∆G = 8.1 kcal/mol; (c) Ib, ∆G = 6.8 kcal/mol;
(d) IIa, the nucleophilic attack of [DEIm]- CO2, ∆G = 28.1 kcal/mol.
Scheme
1 The plausible mechanism for the synthesis of alkylidene
carbonates from CO2 and propargylic alcohol using [P66614] [DEIm] as the
base.
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Table 2. The synthesis of alkylidene carbonates fromatmospheric CO2 using
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[P66614][DEIm].[a]
DOI: 10.1039/C6CC02853E
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2
3
4
5
1a
1b
1c
1d
1f
Me
Me
Me
Me
Et
30
35
48
45
50
89 (85)
91
Ph
98
-(CH2)5-
-(CH2)4-
97
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78
[a] Reaction conditions: substrate (2 mmol), AgOAc (0.02 mmol), [P66614][DEIm] (0.2mmol)
and CO2 (1 bar) at 20 oC. [b] Determined by NMRusing internal standard method, yield in
bracket was isolated yield.
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propargylic alcohols were used. We believe that this strategy
through tuning the basicity of ILs provides a new insight to CO2
capture and utilization. The method developed by this work can be
also extended to other base-catalyzed reactions.
We acknowledge the support of the National Key Basic
Research Program of China (2015CB251401), the National Natural
Science Foundation of China (21176205, 21322602), the Research
Fund for the Doctoral Program of Higher Education of China
(20120101110042), and the Fundamental Research Funds of the
Central Universities.
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than the value calculated at vacuum.
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