Tetrabutylphosphonium-Based Ionic Liquid Catalyzed CO2 Transformation at Ambient Conditions: A Case of Synthesis of α-Alkylidene Cyclic Carbonates

Article abstract of DOI:10.1021/acscatal.7b01422

A series of tetrabutylphosphonium ([Bu₄P]⁺)-based ionic liquids (ILs) with multiple-site for CO₂ capture and activation in their anions, which could efficiently catalyze the cyclization reaction of propargylic alcohols with CO₂ at ambient conditions, are reported. Especially, the IL, [Bu₄P]₃[2,4-OPym-5-Ac], which has three interaction sites for attracting CO₂ together with a pK_a1 value of 9.13, exhibited the best performance, affording a series of α-alkylidene cyclic carbonates in moderate to good yields. The mechanism exploration demonstrated that IL served as a bifunctional catalyst with anion simultaneously activating CO₂ via multiple-site cooperative interactions and the C≡C triple bond in propargylic alcohol via inductive effect, thus resulting in the production of α-alkylidene cyclic carbonates. (Chemical Equation Presented).

Full text of DOI:10.1021/acscatal.7b01422

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Article
Tetrabutylphosphonium-Based Ionic Liquid Catalyzed CO2 Transformation at
Ambient Conditions: A Case of Synthesis of #-Alkylidene Cyclic Carbonates
Yunyan Wu, Yanfei Zhao, Ruipeng Li, Bo Yu, Yu Chen, Xinwei Liu, Cailing Wu, Xiaoying Luo, and Zhimin Liu
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b01422 • Publication Date (Web): 07 Aug 2017
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TetrabutylphosphoniumꢀBased Ionic Liquid Catalyzed CO2 Transforꢀ
mation at Ambient Conditions: A Case of Synthesis of αꢀAlkylidene
Cyclic Carbonates
Yunyan Wu,‡a,b Yanfei Zhao,‡a Ruipeng Li,a Bo Yu,a Yu Chen,a,b Xinwei Liu,a,b Cailing Wu,a,b Xiaoy-
ing Luo,a,b and Zhimin Liu*a, b
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a
Beijing National Laboratory for Molecular Sciences, Key Laboratory of Colloid, Interface and Thermodynamics,
CAS Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of
Sciences, Beijing 100190, China.
b University of Chinese Academy of Sciences, Beijing 100049, China.
KEYWORDS: Tetrabutylphosphonium-based ionic liquids, multiple-site, CO2 transformation, ambient conditions, α-
alkylidene cyclic carbonates
ABSTRACT: A series of tetrabutylphosphonium ([Bu4P]+)-based ionic liquids (ILs) with multiple-site for CO2 capture and
activation in their anions are reported, which could efficiently catalyze the cyclization reaction of propargylic alcohols
with CO2 at ambient conditions. Especially, the IL, [Bu4P]3[2,4-OPym-5-Ac], which has three interaction sites for attract-
ing CO2 together with a pKa1 value of 9.13, exhibited the best performance, affording a series of α-alkylidene cyclic car-
bonates in moderate to good yields. The mechanism exploration demonstrated that IL served as a bifunctional catalyst
with anion simultaneously activating CO2 via multiple-site cooperative interactions and the C≡C triple bond in propar-
gylic alcohol via inductive effect, thus resulting in the production of α-alkylidene cyclic carbonates.
As an abundant, nontoxic and renewable C1 building
block, the transformation of CO2 into value-added chemi-
cals and fuels such as methanol,1 formic acid,2 cyclic car-
bonates3 and others4 is both environmentally and practi-
cally attractive, and has been widely investigated. Howev-
er, CO2 is a thermodynamically stable molecule, thus its
transformation under mild conditions, especially at met-
al-free and ambient conditions is challenging. To date,
many efforts have been dedicated to the transformation
of CO2 under mild conditions with focus on developing
efficient catalysts.3,5 For example, metal-free catalysts in-
cluding ionic liquids,5d-5f N-heterocyclic carbenes5g-5h and
frustrated Lewis pairs5i-5j have been presented to be very
effective for the CO2 conversion. Although some progress
has been made, well designed catalytic system that is ca-
pable of activating CO2 and further catalysing its trans-
formation at metal-free and ambient conditions is still
highly desirable.
tion conditions. For example, the protic IL, [DBU][TFE],
that could capture equimolar CO2, was proved to be ca-
pable of catalyzing CO2 transformation under ambient
conditions in the synthesis of quinazo-line-2,4(1H,3H)-
dione from CO2 and 2-aminobenzonitriles.11 Multiple-site
ILs, [P4444][2-OP], possessing two kinds of interacting site
with CO2, were demonstrated to be efficient for catalysing
the cycloaddition reactions of atmospheric CO2 with
epoxides at room temperature under metal- and halogen-
free conditions.12
α-Alkylidene cyclic carbonates are an important class of
heterocyclic frameworks in natural products with poten-
tial bioactivities and also versatile intermediates in organ-
ic synthesis and polymer chemistry.13 The atom-economic
reaction between propargylic alcohols and CO2 is an at-
tractive route to α-alkylidene cyclic carbonates. Various
metal
catalysts,
including,
[(n-C4H9)4N]2WO4,14
ZnI2/Et3N,15
AgOAc/DBU,16
AgOAc/(n-C7H15)4NBr,17
Ionic liquids (ILs), composed entirely of cations and
anions, can be engineered specifically via the careful de-
sign and selection of novel component ions, which there-
fore have displayed promising applications in many areas,
especially in catalysis and gas absorption.6 To date, a se-
ries of CO2-reactive site-containing ILs, including azolate-
,7 phenolate-,8 amino acid-containing anion-9 and pyri-
dine-containing anion-based ILs,10 have been presented as
efficient CO2 adsorbent via forming carbamates and/or
carbonates. These ILs could activate CO2 via cooperative
effects from their cation and anion, thus further resulting
in the chemical transformation of CO2 under mild reac-
Ag2WO4/Ph3P,18
[(PPh3)2Ag]2CO3,19
AgO-
Ac/[P66614][DEIm],20 AgNPs/SMR21 have been proved to be
efficient for this reaction at low temperature and pressure.
Recently, organo-based metal-free catalysts, including N-
heterocyclic carbenes (NHCs),22 alkoxide-functionalized
imidazolium betaine,23 phosphorus ylide (P-ylide) CO2
adducts24 and ILs25 have emerged as powerful catalysts,
which realized the reaction under mild reaction condi-
tions. However, only Ag-based catalysts have realized this
transformation at room temperature and atmospheric
pressure.19,21
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In this work, we report a series of tetrabutylphosphoni-
NMR analysis (Figure S17-S24). Depended on the interac-
um ([Bu4P]+)-based ILs with multiple-site for CO2 capture
and activation in their anions as shown in Scheme 1,
which could efficiently catalyze the cyclization reaction of
propargylic alcohols with CO2 under ambient conditions.
Especially, the IL, [Bu4P]3[2,4-OPym-5-Ac], which has
three interaction sites to attract CO2 together with a pKa1
value of 9.13, was found to be very efficient for this kind of
reaction, affording a series of α-alkylidene cyclic car-
bonates in moderate to good yields. The mechanism ex-
ploration demonstrated that the cooperative interaction
from multiple-interaction sites in the IL anion simultane-
ously activated CO2 and the C≡C triple bond in propar-
gylic alcohols, thus resulting in the production of α-
alkylidene cyclic carbonates.
tion sites in their anions, these ILs showed different abil-
ity to capture CO2. For example, [Bu4P]3[2,4-OPym-5-Ac]
could attract CO2 molecules via three interaction sites in
the anion, and the uptake of CO2 could reach up to 1.46
mol per molar IL at ambient conditions. As shown in the
13C NMR spectrum of the IL exposed to CO2 at ambient
conditions (Figure 1 and S17), two new signals appeared at
δ = 165.9, 158.5 ppm, which were attributed to the carbon-
yl carbon atoms of carbonates from the multiple O elec-
tronegative sites in anion and CO2. This suggests that this
IL could efficiently capture CO2 chemically, which may
result in the activation of CO2. IR and 31P NMR analysis
were also performed to identify the as-formed poly-
carbonates. A new band around 1603 cm-1 appeared in
each IR spectrum of the IL exposed to CO2 for desired
time (Figure S25), which was ascribed to the asymmetrical
stretching vibration of carboxylate ion in O-CO2
interaction, suggesting the formation of poly-carbonates
from the IL and CO2. Moreover, the band became more
intense as prolonging the absorption time, indicating that
the CO2 uptakes by the IL increased with the time. Both
signals in 31P NMR spectrum showed slight shifts after the
IL exposed to CO2 (Figure S26), probably resulting from
the influence of the formation of poly-carbonates in anion.
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Scheme 1. The anion and cation structures in as-
designed multiple active-site ILs and ILs for control
experiment.
The [Bu4P]+-based ILs with different interaction sites in
anions as shown in Scheme 1 were synthesized via depro-
tonation of weak proton donors, including 2,4-
dihydroxypyrimidine-5-carboxylic acid (2,4-OPym-5-Ac),
Figure 1. 13C NMR spectrum of pure [Bu4P]3[2,4-OPym-
5-Ac] and the as-formed intermediate following the
exposure of [Bu4P]3[2,4-OPym-5-Ac] to CO2 (0.1 MPa;
[D6]-DMSO, 0.6 mL, 298 K).
2,4-dihydroxybenzoic
acid
(2,4-OB-Ac),
2,6-
dihydroxypyridine-4-carboxylic acid (2,6-OPy-4-Ac), 4,6-
dihydroxypyrimidine(4,6-OPym), 2-hydroxypyrimidine-5-
carboxylic acid (2-OPym-5-Ac), 2-hydroxypyridine (2-OP),
phenol (PhO) and imidazole (Im), with tetrabu-
tylphosphonium hydroxide ([Bu4P][OH]) (see Supporting
Information). Most of these ILs were in the liquid states
at room temperature. NMR analysis confirmed their for-
mation (Figure S1-S11), and TG analysis showed that they
are thermally stable from 445 K to 511 K (Figure S12-16).
The resultant ILs could efficiently capture CO2 at room
temperature and atmospheric pressure, confirmed by the
All the resultant ILs were examined for catalyzing the
reaction of 2-methyl-3-butyn-2-ol (1a) with CO2, and the
results are shown in Table 1. It was indicated that the
[Bu4P]+-based ILs with interaction sites in anions for CO2
capture were effective for catalysing this reaction under
ambient conditions, showing different activities depend-
ing on their chemical structures. [Bu4P]3[2,4-OPym-5-Ac]
showed the best performance, producing α-alkylidene
cyclic carbonate 2a in 79% yield within 20 h at room tem-
perature and atmospheric pressure (Table 1, entry 1), and
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increasing CO2 pressure to 2 MPa resulted in enhanced
ble 1, entry 11), implying the IL cation also played an im-
portant role in this reaction. In comparation with
[Bu4P]3[2,4-OPym-5-Ac], [Et4P]3[2,4-OPym-5-Ac] and
[P66614]3[2,4-OPym-5-Ac] afforded lower product yields
(Table 1, entries 1, 12 and 13), following the order:
[P66614]3[2,4-OPym-5-Ac] (8%) < [Et4P]3[2,4-OPym-5-Ac]
(20%) < [Bu4P]3[2,4-OPym-5-Ac] (79%). These results
indicate that the catalytic activities of these ILs were sig-
nificantly affected by the carbon chains of the IL cation,
and C4 chain was found to be the best.
product yield up to 91% within shorter time of 5 h (Table 1,
entry 2). The IL maintained its original structure after the
1
reaction, confirmed by H NMR analysis (Figure S27). In
contrast, the other two ILs, [Bu4P]3[2,4-OB-Ac] with three
interaction sites and [Bu4P]3[2,6-OPy-4-Ac] with four in-
teraction sites in their anions, displayed lower activity for
this reaction though they also could efficiently attract
CO2 with high capacity (Table 1, entries 3 and 4). This
implies that the interaction between the IL and CO2 was
not the only one factor to influence the activity of the ILs.
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Table 1. Reaction of 2-methyl-3-butyn-2-ol with CO2
catalyzed by different IL catalysts.a
Though the ILs having two or one reactive sites in their
anions were also effective for the reaction, they showed
lower activity (Table 1, entries 5-9), suggesting that the
interaction sites to bind CO2 in the anion considerably
influenced the activity of the ILs. The reaction did not
occur in the presence of [Bu4P][Br] (Table 1, entry 10),
probably because this IL could not capture CO2. This in-
dicates that the capture and activation of CO2 by the IL
via the interaction site in the anion was crucial to the re-
action. From the above results, it also can be observed
that the ILs with the same interaction sites in their anions
showed different activities, implying that the inherent
properties of the ILs played the key role in catalysing the
reaction.
Entry
Catalysts
pKa1
9.13
Yield %b
79
1
2
[Bu4P]3[2,4-OPym-5-Ac]
[Bu4P]3[2,4-OPym-5-Ac]
9.13
9.97
5.45
8.49
5.45
12.32
16.19
18.30
-
91
66
15
3
[Bu4P]3[2,4-OB-Ac]
Given that the resultant ILs are basic, the pKa1 values of
the anions in tert-butyl alcohol (t-BuOH) were computed
by density functional theory (DFT) calculation6c,20 and
listed in Table 1. In the view of the relationship between
the product yield and pKa1 value of IL anion, we postulat-
ed that the proper basicity also plays an important role in
the catalytic reaction process. For the ILs with three in-
teraction sites in their anions, [Bu4P]3[2,4-OPym-5-Ac]
with a pKa1 value of 9.13 showed the highest product yield
(79%; Table 1, entry 1), while [Bu4P]3[2,4-OB-Ac] with a
pKa1 value of 9.97 gave a slightly lower yield of 66 % (Ta-
ble 1, entry 3). Notably, [Bu4P]3[2,6-OPy-4-Ac] with a pKa1
value of 5.45 gave a very low yield of 15% (Table 1, entry 4),
although it could attract CO2 with an uptake of 3.26 mol
per molar IL. These results indicate that the basicity of
the ILs was an important factor to influence the catalytic
activity of the ILs. The IL with strong basicity led to side
reactions, while the IL with weak basicity exhibited poor
catalytic activity. Therefore, the ILs with higher or lower
pKa1 values showed poor performances, and only the ILs
with appropriate pKa1 value displayed the high efficiency.
The similar phenomenon was also observed in the
base/AgOAc catalytic system.20 From the above findings,
it can be deduced that increasing electronegative sites
with proper pKa value of anion is a facile way to improve
fixation and transformation of CO2. The regularity in pKa
value found here may provide an instruction to design
basic ILs for alkaline-catalyzed reactions. In addition,
[Bu4N]3[2,4-OPym-5-Ac], with the same anion as
[Bu4P]3[2,4-OPym-5-Ac], gave inferior product yield (Ta-
4
[Bu4P]3[2,6-OPy-4-Ac]
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33
40
27
10
0
5
[Bu4P]2[4,6-OPym]
[Bu4P]2[2-OPym-5-Ac]
[Bu4P][2-OP]
6
7
8
[Bu4P][PhO]
[Bu4P][Im]
9
10
11
12
13
[Bu4P][Br]
-
13
[Bu4N]3[2,4-OPym-5-Ac]
-
20
8
[Et4P][2,4-OPym-5-Ac]
-
[P66614][2,4-OPym-5-Ac]
a Reaction conditions: 2-methyl-3-butyn-2-ol (2 mmol), cata-
lyst (0.2 mmol), CO2 (0.1 MPa), 303 K, 20 h; b Yields were
determined by 1H NMR spectroscopy using N,N-
dimethylformamide as an internal standard; c 2MPa, 5h.
Based on the above results, [Bu4P]3[2,4-OPym-5-Ac]
was applied to various terminal propargylic alcohols re-
acting with CO2 at ambient conditions, and the results are
listed in Table 2. It was indicated that this IL allowed all
the reactions to proceed smoothly, affording the corre-
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sponding α-alkylidene cyclic carbonates (2a-2g) in good
examined by NMR and FTIR analysis. As stated in the CO2
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to excellent yields under the experimental conditions.
Especially, excellent yields could be achieved even within
short reaction time (e.g., 5h) at 2 MPa. Notably, propar-
gylic alcohols with bulky isobutyl groups reacted rapidly
with high product yields in the presence of this IL catalyst
(2d and 2f). The better performance of [Bu4P]3[2,4-OPym-
5-Ac] for the substrates with big steric hindrance may be
ascribed to the unique interaction way of the anion of the
IL with the reactants in the reaction process. In contrast,
propargylic alcohols with electron-withdrawing groups
(vinyl) exhibited inferior reactivity especially under ambi-
ent conditions (2h), whereas relatively high CO2 pressure
(e.g., 3 MPa) resulted in improved product yields. Propar-
gylic alcohols with cyclohexyl and phenyl substituents
showed lower activity (2i and 2j), probably due to the ste-
ric hindrance and ring strain. Also, [Bu4P]3[2,4-OPym-5-
Ac] worked well to the substrates with a phenyl group or
pyridyl group at the terminal position (2k and 2l). All the
above results demonstrate that [Bu4P]3[2,4-OPym-5-Ac]
was an efficient metal-free catalyst for propargylic alco-
hols with CO2 to produce α-alkylidene cyclic carbonates
at ambient conditions.
adsorption experiment by the IL, CO2 was captured by the
anion [2,4-OPym-5-Ac]3- to form poly-carbonates, which
was also detected when the CO2 absorption was carried
out in t-BuOH solution of the IL (Figure S28). These
results suggest that the poly-carbonates from the IL and
CO2 may be key intermediates in the reaction process of
propargylic alcohols with CO2. To confirm this, 1a was
mixed with this poly-carbonates at room temperature,
and 2a was detected after 5h accompanied with the
disappearance of poly-carbonates confirmed by the 13C
NMR analysis (Figure S29), thus suggesting that poly-
carbonates were indeed the key intermediate in the
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1
formation of the final product. In the H NMR spectrum
1
of the mixture of [Bu4P]3[2,4-OPym-5-Ac] and 1a, the H
signal of O-H in 1a broadened obviously and shifted from
5.30 to 5.82 ppm (Figure S30), indicating the hydrogen-
bonding interaction between the anion of IL and OH. In
13
the C NMR spectrum of the IL and 1a mixture (Figure 2,
S31, S32), the terminal carbon signal of the triple bond in
1a shifted upfield from 70.3 ppm to 68.9 ppm and the
internal carbon signal showed a downfield shift from 88.6
ppm to 89.4 ppm. In the FTIR spectrum of the mixture,
the band assigned to the alkyne -C ≡ C- stretching
vibration showed a shift from 2118 cm-1 to 2108 cm-1
(Figure S33). However, no obvious changes were observed
in the corresponding 31P NMR spectrum (Figure S34).
From the these findings, it can be deduced that the anion
of the IL activated the -C≡C- in 1a. It is reasonable that
the O-H bond in 1a was activated by the anion [2,4-
OPym-5-Ac] via hydrogen bonding, resulting in the
electron redistribution of alkynyl carbon, thus indirectly
activating the triple bond via inductive effect, which may
facilitate the poly-carbonate intermediate to nucleophilic
addition on the triple bond. The above results indicate
the anion of IL played a key role in activating CO2 and
substrates (e.g., 1a) in the reaction process.
Table 2. Synthesis of various α-alkylidene cyclic car-
bonates.a
c
b
a
a
a
b
c
Reaction conditions: Substrate (2 mmol), catalyst (0.2
mmol), CO2 (0.1 MPa), 303 K, 20 h, Yields were determined
1
by H NMR spectroscopy using N,N-dimethylformamide as
an internal standard; b CO2 (2 MPa), 5h; c CO2 (3 MPa), 20 h;
d CO2 (2 MPa), 20 h; e catalyst (0.4 mmol), CH3CN (2 mL),
20h.
a
c
Figure 2. 13C NMR spectrum of pure 2-methyl-3-butyn-
2-ol and the mixture of [Bu4P]3[2,4-OPym-5-Ac] and
[Bu4N]3[2,4-OPym-5-Ac] (0.1 mmol) and 2-methyl-3-
To explore the reaction mechanism, the interactions
between [Bu4P]3[2,4-OPym-5-Ac] and CO2 or 1a were
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Supporting Information. General experimental methods,
butyn-2-ol (10 mmol) (298 K, [D6]-DMSO in capillary
3
4
5
yields determination and characterization of the products.
“This material is available free of charge via the Internet at
http://pubs.acs.org.”
as an internal standard).
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AUTHOR INFORMATION
8
9
Corresponding Author
*E-mail: liuzm@iccas.ac.cn
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Author Contributions
‡These authors contributed equally.
ACKNOWLEDGMENT
The authors thank the National Natural Science Foundation
of China (21403252, 21533011, 21503239), and the Chinese
Academy of Sciences (QYZDY-SSW-SLH013).
REFERENCES
(1) Schneidewind, J.; Adam, R.; Baumann, W.; Jackstell, R.; Beller, M.
Angew. Chem. Int. Ed. 2017, 56, 1890-1893.
(2) Rohmann, K.; Kothe, J.; Haenel, M. W.; Englert, U.; Hölscher, M.;
Leitner, W. Angew. Chem. Int. Ed. 2016, 55, 8966-8969.
Scheme 2. Possible reaction pathway
(3) Wang, X. C.; Zhou, Y.; Guo, Z. J.; Chen, G. J.; Li, J.; Shi, Y. M.; Liu,
Y. Q.; Wang, J. Chem. Sci. 2015, 6, 6916-6924.
On the basis of the experimental results and previous
reports,15-25 a possible machanism was proposed, as shown
in Scheme 2. In the presence of IL, CO2 is captured and
activated by the anion of IL to form poly-carbonate
intermediate a, meanwhile the O-H bond in propargylic
alcohol is activated by the anion [2,4-OPym-5-Ac]3- via
hydrogen bonding, thus further activating the triple bond
in propargylic alcohol via inductive effect. Then, the
intermediate a adds to the triple bond of propargylic
alcohol via nucleophilic attack, forming intermediate b,
followed by hydrogen migration from the hydroxy group
of the alcohol to produce intermediate c. Finally, the
alkoxide anion of intermediate c attacks the carbonyl
carbon atom to form the α-alkylidene cyclic carbonate,
with the release of the IL.
In summary, a series of [Bu4P]+-based ILs with mul-
tiple-site for CO2 capture and activation in their anions
were presented, which could efficiently catalyze the cy-
clization reaction of propargylic alcohols with CO2 at am-
bient conditions. The IL, [Bu4P]3[2,4-OPym-5-Ac], which
has three interaction sites for attracting CO2 together
with a pKa1 value of 9.13, exhibited highly efficient effi-
ciency, affording a series of α-alkylidene cyclic carbonates
in moderate to good yields. The mechanism exploration
demonstrated that the IL served as a bifunctional catalyst
with anion simultaneously activating CO2 via multiple-
site cooperative interactions and the C≡C triple bond in
propargylic alcohol substrate via inductive effect, thus
resulting in the production of α-alkylidene cyclic car-
bonates. This kind of ILs may find promising applications
in CO2 transformation under mild conditions.
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