Synthesis of carbonates from CO2 and epoxides catalyzed by the system of N-heterocyclic carbene, hydrogen bond donor, CrCl2, and tetrabutylammonium bromide

Article abstract of DOI:10.1002/aoc.6347

A three-component catalytic system including pyridine-bridged benzimidazolium salts, CrCl₂, and tetrabutylammonium bromide (TBAB) was developed. Based on the control experiments and spectroscopic measurements, the role of the three components in the catalytic process was clarified, in which benzimidazolium salts were used as N-heterocyclic carbene precursor, a new Cr complex generating from the coordination of CrCl₂ with pyridine nitrogen and pyrazole nitrogen bearing benzimidazolium salts was employed as hydrogen bond donor, TBAB was used as nucleophilic reagent, respectively. Under mild conditions (50°C and 1?bar CO₂), the terminal epoxides displayed high reactivity in the three-component catalytic system. The catalytic system showed also high catalytic activity for the internal epoxides by increasing the temperature and CO₂ pressure and/or prolonging the reaction time.

Full text of DOI:10.1002/aoc.6347

Received: 21 March 2021
DOI: 10.1002/aoc.6347
Revised: 10 June 2021
Accepted: 11 June 2021
F U L L P A P E R
Synthesis of carbonates from CO and epoxides catalyzed by
2
the system of N-heterocyclic carbene, hydrogen bond donor,
CrCl , and tetrabutylammonium bromide
2
Kuikui Zhang
| Zhenbang Liu
| Ning Liu
School of Chemistry and Chemical
Engineering, Key Laboratory for Green
Processing of Chemical Engineering of
Xinjiang Bingtuan, Shihezi University,
Shihezi, Xinjiang, China
Abstract
three-component
A
catalytic
system
including
pyridine-bridged
benzimidazolium salts, CrCl , and tetrabutylammonium bromide (TBAB) was
2
developed. Based on the control experiments and spectroscopic measurements,
the role of the three components in the catalytic process was clarified, in which
benzimidazolium salts were used as N-heterocyclic carbene precursor, a new
Correspondence
Ning Liu, School of Chemistry and
Chemical Engineering, Key Laboratory for
Green Processing of Chemical
Engineering of Xinjiang Bingtuan, Shihezi
University, North Fourth Road, Shihezi,
Xinjiang 832003, China.
Cr complex generating from the coordination of CrCl with pyridine nitrogen
2
and pyrazole nitrogen bearing benzimidazolium salts was employed as hydro-
gen bond donor, TBAB was used as nucleophilic reagent, respectively. Under
ꢀ
Email: ningliu@shzu.edu.cn
mild conditions (50 C and 1 bar CO
), the terminal epoxides displayed high
2
reactivity in the three-component catalytic system. The catalytic system
showed also high catalytic activity for the internal epoxides by increasing the
Funding information
Open Sharing Fund for the Large-scale
Instruments and Equipments of Shihezi
University; Shihezi Young and Middle-
Aged Leading Scientists Program, Grant/
Award Number: 2019RC01; Xinjiang
Bingtuan Young and Middle-Aged
Leading Scientists Program, Grant/Award
Number: 2020CB027
temperature and CO pressure and/or prolonging the reaction time.
2
K E Y W O R D S
carbon dioxide, cooperative catalysis, cyclic carbonate, cycloaddition, epoxide
[
4]
[5]
[6]
1
| INTRODUCTION
catalysts, such as iron,
manganese,
cobalt,
[
7]
[8]
[9]
[10]
chromium, aluminum, molybdenum, zinc,
and
[
11]
The use of carbon dioxide (CO ) instead of fossil fuels as
magnesium,
have been developed for the cycloaddi-
2
starting material is a sustainable and renewable alterna-
tive for synthesis of chemicals. Therefore, the utiliza-
tion of CO with epoxides, with a focus on achieving
catalytic efficiency under mild reaction conditions. In
2
[1]
tion of CO as a carbon feedstock in organic synthesis
addition,
imidazolium salts,
phosphates,
methods,
organocatalyzed
approaches
including
2
[
12]
[13]
has attracted wide interest. To date, a number of methods
quaternary ammonium salts,
and hydrogen bond donors promoted
have been demonstrated to a sustainable
[2]
[14]
for utilizing CO have been developed. The synthesis of
2
[
3c,15]
cyclic carbonates via the fixation of CO have witnessed a
2
[16]
significant development in recent years due to the
transformation possessing the features of high atomic
alternatives for the synthesis of cyclic carbonates.
Three-component catalytic systems for the synthesis of
cyclic carbonates were summarized in Table 1. Huang
[3]
economy. It is well known that CO was the thermody-
2
[
17]
namically inert and highly kinetically stable molecular;
and Shi
reported an important work for the synthesis
thus, high temperature and high pressure for CO in the
of cyclic carbonates using three-component systems com-
2
synthesis of cyclic carbonates is often required. A wide
range of strategies employed the metal-mediated
posed of NaI/PhOH/Ph P (Table 1, entry 1). Werner
3
[
12c,18]
and co-workers
had developed two kinds of
Appl Organomet Chem. 2021;e6347.
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https://doi.org/10.1002/aoc.6347

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ZHANG ET AL.

ZHANG ET AL.
3 of 15
[
21]
three-component catalytic systems using KI/carbene/
literatures.
These benzimidazolium salts 1a–f using as
crown ether (Table 1, entry 2) and CaI /crown ether/
hydrogen bond donors were evaluated in the cycloaddi-
2
Ph P (Table 1, entry 3) as catalysts, respectively. Very
tion of epoxides with CO (Scheme 1).
3
2
[19]
recently, Wu and Lin
reported a catalytic system using
The reaction of 1,2-hexylene oxide (2c) with CO was
2
Co(NO ) Á6H O/hydrogen bond donor/KBr as catalysts
used as a benchmark reaction to optimize the reaction
conditions employing benzimidazolium salts 1a men-
tioned above as hydrogen bond donors, metal salts as
Lewis acids, and quaternary ammonium salt as nucleo-
phile (Table 2). First, the influence of Lewis acids in cata-
lytic efficiency was investigated (Table 2, entries 1–8),
3
2
2
(
Table 1, entry 4). Although extensively studied, the
fixation of CO three-component systems remains some
2
significant challenges, including performing in mild
reaction temperature and under low pressure for CO2.
In this work, a three-component cooperative cata-
lyzed method for the synthesis of cyclic carbonates had
been developed. We expected that the three-component
catalytic system had the synergistic effect in which the
carboxyl hydrogen bearing Cr complex in situ generating
was used as hydrogen bond donor, TBAB acted as a
and finding CrCl was best effective for the synthesis of
2
cyclic carbonates in 62% yield of 3c (Table 2, entry 4).
The loading amount of CrCl2 was also investigated
(Table 2, entries 4 and 9–11), and 1.5 mol% of CrCl was
2
appropriate in the view of the cost (Table 2, entry 10).
nucleophile, and benzimidazolium salts activated CO by
In the presence of CrCl as Lewis acid, the promoted
2
2
forming N-heterocyclic carbene (NHC)-adducts. The
carboxyl functionalized benzimidazolium salts, used as
NHC precursor, reacted with CO2 to form NHC-CO2
adducts, which required extra electron donors to stabilize
Cr center. Therefore, we designed and synthesized a
pyridine-bridged tridentate ligands base on our research
ability of benzimidazolium salts 1a–f as hydrogen bond
donors was also explored (Table 3, entries 1–6), and
benzimidazolium salt 1a displayed the best promoted
ability (Table 3, entry 1). The leaving ability of halide
À
À
À
anion was in the following order: I > Br > Cl . The
strong leaving ability of iodine anion was beneficial to
generate N-heterocyclic carbene CO2 adducts, so that
benzimidazolium salt 1a bearing iodine anion exhibited
high performance (Table 3, entry 1). The influences of
loading amount of 1a (Table 3, entries 1 and 7–9) were
also evaluated, and it was found that 2 mol% of hydrogen
bond donor 1a was appropriate for achieving 66%
yield (Table 3, entry 7). The major function of pyrazole
and indazole bearing benzimidazolium salts had a role
of ligand for coordinating with Cr center. Therefore,
[20]
in catalysts design and synthesis.
This catalytic system
showed wide substrate scope and functional group toler-
ance under mild reaction conditions (Table 1, entry 5).
2
| RESULTS AND DISCUSSION
A series of carboxyl functionalized benzimidazolium salts
a–f were synthesized based on our previous
1
SCHEME 1 The synthesis for carboxyl functionalized benzimidazolium salts

4
of 15
ZHANG ET AL.
TABLE 2 Optimization of Lewis acid for cyclic carbonate synthesis
[
a]
[b]
[b]
À1
Entry
Lewis acid (mol%)
Yield (%)
Conversion (%)
Selectivity (%)
TON
63
TOF (h
2.6
)
1
2
3
4
5
6
7
8
9
FeCl
2
3
(0.5)
(0.5)
(0.5)
(0.5)
(0.5)
(0.5)
55
52
45
62
52
55
34
35
66
68
69
63
59
52
67
59
59
40
39
71
73
74
99
98
97
99
97
96
95
96
99
99
99
FeCl
59
2.5
CoCl
2
52
2.2
CrCl
CrCl
ZnCl
2
67
2.8
3
59
2.5
2
59
2.5
MnCl
MgCl
2
(0.5)
(0.5)
(1.0)
(1.5)
(2.0)
40
1.7
2
39
1.6
CrCl
CrCl
CrCl
2
2
2
71
3.0
1
1
0
1
73
3.0
74
3.1
ꢀ
Note: Conditions: 1,2-hexylene oxide (5.0 mmol), 1a (0.05 mmol, 1.0 mol%), Lewis acid (as indicated in Table 2), TBAB (0.1 mmol, 2.0 mol%), 50 C, CO
2
balloon (1 bar), 24 h, neat.
a
Yields were determined by column chromatography.
b
Conversion and selectivity were determined by GC. TON and TOF were calculated based on the amount of 1a.
TABLE 3 Optimization of hydrogen bond donor for cyclic carbonate synthesis
[a]
[b]
[b]
À1
Entry
Hydrogen bond donor (mol%)
Yield (%)
Conversion (%)
Selectivity (%)
TON
67
TOF (h
2.8
)
1
2
3
4
5
6
7
8
9
1a (1.0)
1b (1.0)
1c (1.0)
1d (1.0)
1e (1.0)
1f (1.0)
1a (2.0)
1a (3.0)
1a (5.0)
62
52
31
41
35
38
65
66
68
67
60
37
46
39
41
68
71
73
99
97
93
96
95
93
99
99
99
60
2.5
37
1.5
46
1.9
39
1.6
41
1.7
34
1.4
24
1.0
15
0.6
Note: Conditions: 1,2-Hexylene oxide (5.0 mmol), Hydrogen bond donor (as indicated in Table 3), CrCl
2
(0.025 mmol, 0.5 mol%), TBAB (0.1 mmol, 2.0 mol%),
ꢀ
5
0 C, CO
2
balloon (1 bar), 24 h, neat.
a
Yields were determined by column chromatography.
b
Conversion and selectivity were determined by GC. TON and TOF were calculated based on the amount of hydrogen bond donor.

ZHANG ET AL.
5 of 15
TABLE 4 Optimization for cyclic carbonate synthesis
Hydrogen bonddonor Lewis acid Nucleophile
Yield
Conversion
(%)
Selectivity
(%)
TOF
[a]
[b]
[b]
[c]
À1 [c]
Entry (mol%)
(mol%)
(mol%)
(%)
75
83
89
80
69
70
54
39
32
10
9
TON
(h
1.7
1.8
1.9
1.8
1.5
1.6
1.2
0.9
-
)
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1a (2.0)
1a (2.0)
1a (2.0)
1a (2.0)
1a (2.0)
1a (2.0)
1a (2.0)
1a (2.0)
1a (0)
CrCl
CrCl
CrCl
CrCl
CrCl
CrCl
CrCl
CrCl
CrCl
CrCl
CrCl
CrCl
CrCl
CrCl
CrCl
CrCl
CrCl
CrCl
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
(1.5)
(1.5)
(1.5)
(1.5)
(1.5)
(1.5)
(1.5)
(0)
TBAB (3)
TBAB (4)
TBAB (5)
TBAI (5)
TBAC (5)
TBAO (5)
80
86
92
84
73
75
57
42
34
13
14
0
99
99
99
99
99
99
99
99
96
99
99
0
40
43
46
42
37
38
29
21
-
4
TBASO H (5)
TBAB (5)
TBAB (5)
TBAB (0)
TBAB (0)
TBAB (0)
TBAB (5)
TBAB (10)
TBAB (20)
TBAB (5)
TBAB (5)
TBAB (5)
(1.5)
(1.5)
(0)
0
1
2
3
4
5
6
7
8
1a (2.0)
1a (2.0)
1a (0)
7
0.3
0.3
-
7
(1.5)
(0)
0
-
1a (0)
18
42
44
70
75
74
23
44
47
73
77
78
95
96
95
95
96
96
-
-
1a (0)
(1.5)
(1.5)
(1.5)
(1.5)
(1.5)
-
-
1a (0)
-
-
[
[
[
d]
e]
f]
1a (0)
-
-
1a (0)
-
-
1a (0)
-
-
ꢀ
2 2
Note: Conditions: 1,2-Hexylene oxide (5.0 mmol), 1a (as indicated in Table 4), CrCl (as indicated in Table 4), nucleophiles (as indicated in Table 4), 50 C, CO
balloon (1 bar), 24 h, neat.
a
Yields were determined by column chromatography.
b
Conversion and selectivity were determined by GC.
c
TON and TOF were calculated based on the amount of 1a.
d
ꢀ
8
0 C.
e
ꢀ
1
00 C.
f
2
CO balloon (5 bar).
the coordination ability of pyrazole and indazole was
crucial for their catalytic performance. Because the
density of electron cloud of indazole tends to benzene
ring, the electron-donating properties of indazole was
weaker in comparison with pyrazole. Therefore, the
pyrazole ring had a stronger coordination ability than
that of indazole, which was demonstrated in our
entries 1–3) were also evaluated; 5 mol% of TBAB
was appropriate for high conversion, and achieving
89% yield (Table 4, entry 3). Besides TBAB, other
nucleophiles including tetrabutylammonium iodide
(TBAI),
tetrabutylammonium
tetrabutylammonium
acetate
chloride
(TBAO),
(TBAC),
and
tetrabutylammonium hydrogen sulfate (TBASO H)
4
[22]
previously reported literature.
catalytic activity between 1a and 1d or 1b and 1e
Table 3, entries 1 vs. 4 and 2 vs. 5) was caused by the
The difference in the
were explored under the optimized reaction conditions
(Table 4, entries 4–7), and TBAB showed the best
performance for the transformation (Table 4, entry 3).
Generally, there are two factors affecting the nucleo-
philic attack process of the nucleophiles, which are the
nucleophilic ability and the size of the counterions.
The bromide anions had good nucleophilicity and
(
difference in coordination ability between pyrazole and
indazole.
The influences for the loading amount of nucleo-
phile tetrabutylammonium bromide (TBAB) (Table 4,

6
of 15
ZHANG ET AL.
TABLE 5 Scope for epoxides
ꢀ
Note: Conditions unless specified otherwise: epoxides (5.0 mmol), 1a (2 mol %), CrCl
2
(1.5 mol%), TBAB (5 mol%), 50 C, CO
2
balloon (1 bar), 24 h, neat, Yields
were determined by column chromatography. Conversion and selectivity were determined by GC.
a
ꢀ
8
0 C.
b
ꢀ
1
00 C, 5 bar CO
2
.
c
ꢀ
1
00 C, 5 bar CO
2
, TBAB (10 mol %), 36 h.
d
1
13
The ratio of cis:trans and d.r. values were determined by H and C NMR spectroscopy. TON and TOF were calculated based on the amount of 1a.
moderate size, which promoted the nucleophilic attack
to epoxides. The benzimidazolium salt itself contained
halogen anions; however, additional halogen anions
from TBAB are still required, which can be explained
of three catalytic components, three control experi-
ments were performed in the absence of CrCl or 1a
2
or TBAB, respectively. Notably, CrCl , 1a, and TBAB
2
were necessary for high efficiency, because the absence
of any one of them resulted in a remarkable decline
in the yield of 3c (Table 4, entries 8–13 vs. 3). To
prove the effect of 1a, a series of control experiments
in the absence of 1a were performed (Table 4, entries
14–18). When the amount of TBAB increased to 10%
or 20% in the absence of 1a, the yield of the target
product increased slightly (Table 4, entries 14 and
15 vs. 3). The increase of reaction temperature and
[23]
by Cal oꢀ et al.
They had found that the chemical
structure and size of the cations bearing quaternary
ammonium salts had evident influence in the nucleo-
philic attack ability of anions. The results indicated
that the difference of the cations led to different
nucleophilic abilities of the two types of halogen
anions, and the halogen anions from TBAB had higher
nucleophilic abilities than that of the halogen anions
[20b]
on benzimidazolium salts.
To explore the function
pressure
of
CO2
effectively
promoted
this

ZHANG ET AL.
7 of 15
transformation; however, the result was difficult to
achieve the result obtained in the absence of 1a in the
mild conditions (Table 4, entries 16–18 vs. 3). This
result indicated that benzimidazolium salt 1a is
essential in this transformation.
Under the optimized conditions obtained above, the
scope of epoxides was explored (Table 5). First, a wide
range of aliphatic substituted terminal epoxides were
used as substrates, and a series of aliphatic mono-
substituted cyclic carbonates were obtained in high yields
2,2-dimethyloxirane are often challenging substrates. To
our delighted, we found that it was also converted to the
desired product 3 L in 70% yield when reaction tempera-
ꢀ
ture increased to 80 C.
Internal epoxides such as bicyclic epoxides were
particularly challenging substrates and were difficult to
convert them into the desired products because of their
highly steric hindrance. Therefore, the substrates toler-
ance of the cyclohexene oxides was explored in the
established catalyzed system. Three cyclohexene oxides,
including two cyclohexene oxide containing exo- or
internal-cyclic double bonds, could be converted into
(Table 5, 3a-j). The phenyl substituted terminal epoxides
were examined, and producing 3 k in 81% yield. The
SCHEME 2 Study on active sites of catalysts
FIGURE 1 FT-IR spectra for reaction mechanism

8
of 15
ZHANG ET AL.
their cyclic carbonates 3 m-o in good yields by increasing
entry 3) to 33% when the C2 position was methylated,
in which methylation of the C2 position prevented the
interaction between CO2 and active site of the C2
position. Therefore, we inferred that 1a was used as
NHC precursor to activate CO2 through reacted with
reaction temperature, increasing pressure for CO , and/or
2
loading amount of TBAB and prolonging time. The
bio-derived epoxides were high sterically hindered
substrates and had a potential application in polymer
synthesis. For example, the reaction of epoxidized methyl
oleate with CO2 was also converted and a bio-based
carbonate 3p was isolated in 38% yield with a cis:trans
ratio of 26:74.
To clarify the influence of each active sites bearing
benzimidazolium salt 1a, two catalysts were designed
to obtain some mechanistic insights into the reaction
mechanism (Scheme 2). To explore the effect of
carboxyl group bearing 1a, a catalyst 1g was synthe-
sized by the replacing of carboxyl group with propyl
group. In the replacement of 1a with 1g resulted a
significant decline in yield from 89% to 35%
CO to form NHC adducts.
2
In addition, the function of pyridine ring and pyrazole
ring bearing benzimidazolium salt 1a was explored
in Scheme 2. When pyridine ring and pyrazole ring
were removed from imidazolium salt 1a (Scheme 2, eqs
3 and 4), both 1i and 1j showed low performance
compared with that of 1a. We inferred that pyridine
ring and pyrazole ring had played a role of nitrogen
donors to coordinate with CrCl to generate a new Cr
2
complex.
To investigate the interaction between 1a and
1,2-hexylene oxide, several control experiments were
performed using FT-IR method. The absorption peak
(Scheme 2, eq 1 vs. Table 4, entry 3). The result
À1
suggested that the carbonyl group had played an
important role in the catalytic performance.
at 2948 cm was assigned to the stretching vibration
peak of the carboxyl group bearing 1a (Figure 1a,
black curve). When 50 equiv of 1,2-hexylene oxide was
added to 1a, the absorption peak of the carboxyl group
disappeared (Figure 1a, blue curve). These changes
indicated that the hydrogen bond between the carboxyl
group bearing 1a and 1,2-hexylene oxide may occur.
To demonstrate the role of hydrogen atom at C2
position of 1a in reaction process, a catalyst 1h was
synthesized by introducing a methyl group to the C2
position of 1a (Scheme 2, eq 2). The result indicated
that the yield obviously decreased from 89% (Table 4,
1
FIGURE 2
H NMR experiments for the interaction between 1a and epoxide 2c

ZHANG ET AL.
9 of 15
1
FIGURE 3
H NMR experiments for the interaction between 1a and CO
2
When CrCl2 was added to 1,2-hexylene oxide, no
obvious changes were observed (Figure 1a, green
curve), indicating CrCl2 did not act as a Lewis acid.
Therefore, we next explored the interaction between 1a
and CrCl2 (Figure 1b). When CrCl2 was added to
the carboxyl group bearing 1a had not changed
(Figure 2b); however, the addition of 1,2-hexylene
oxide led to the disappearance (Figure 2c). These
changes indicated that the hydrogen bond interaction
between the carboxyl group bearing 1a and
bromide anion of TBAB did not occur; however, the
hydrogen bond interaction between the carboxyl
group bearing 1a and oxygen atom of 1,2-hexylene
oxide was observed. Full NMR spectra are detailed in
Figure S29.
1
1
a, the absorption peaks of the carboxyl group bearing
a disappeared (Figure 1b, red curve). This change
indicated that the carboxyl group bearing 1a coordi-
nated with CrCl to generate a new Cr complex. This
2
result was consistent with control experiments in
Scheme 2 (eqs 3 and 4). Full IR spectra are detailed in
Figures S27 and S28.
To determine whether the NHC-CO2 adducts via
the reaction of C2-H of 1a with were formed, a wide
1
To prove whether the hydrogen bond interaction
between the carboxyl group bearing 1a and bromide
range of control experiments was performed by
H
NMR method (Figure 3). When CO2 and/or TBAB
were added to 1a, the absorption peak of C2-H bearing
1a had not disappeared (Figure 3a–c); however, the
addition of 1,2-hexylene oxide led to the disappearance
[24]
anion of TBAB occurs,
three control experiments
were performed by NMR method (Figure 2). When
TBAB was added to 1a, the absorption peak of

10 of 15
ZHANG ET AL.
1
3
FIGURE 4
C NMR experiments for the interaction between 1a and CO
2
(
Figure 3d,e). These changes indicated that the NHC-
Based on control experiments (Scheme 2) and spectro-
scopic measurements (IR and NMR in Figures 1 and 2)
CO adducts may be formed. In order to confirm the
2
results above, a series of control experiments were
above, we illustrated the role of CrCl , benzimidazolium
salt 1a, and TBAB in the catalytic process. As shown
in Scheme 3, the carboxyl hydrogen bearing
2
13
performed by C NMR method (Figure 4). When CO2
and/or TBAB were added to 1a, a new absorption
peak in 172.27 ppm was observed (Figure 4a–c).
benzimidazolium salt 1a was used as hydrogen bond
[25]
[27]
According to literatures,
we inferred that the
donor,
and TBAB acted as a nucleophile. The C2
carboxyl group bearing 1a reacted with CO to gener-
hydrogen at benzimidazolium salt 1a could react with CO
2
2
[
26,28]
ate acid anhydride intermediates. When 1,2-hexylene
oxide was added to the reaction mixtures above, a new
absorption peak in 166.11 ppm was observed
to form N-heterocyclic carbene adducts,
which has
played a role in activating CO . As shown in Scheme 3, a
2
new Cr complex A generated from the coordination of
(Figure 4d), and simultaneously acid anhydride inter-
benzimidazolium salt 1a with CrCl was used as Lewis
2
mediates disappeared. The results indicated that NHC-
acid through the hydrogen bond interaction between
carboxyl hydrogen and oxygen atom of epoxides.
Epoxides underwent continuous ring-opening reaction
via intramolecular nucleophilic attack of bromide from
[26]
CO adducts
were produced when CO transferred
2
2
from the carboxyl group to C2 position of 1a. The
NHC-CO2 adducts was disappeared when the desired
product 3c was achieved (Figure 4e). Full NMR spectra
are detailed in Figures S30 and S31.
TBAB, CO insertion reaction, and ring-closure reaction to
2
form target products and release catalytic active species.

ZHANG ET AL.
11 of 15
SCHEME 3 Proposed mechanism for the synthesis of cyclic carbonates
3
| CONCLUSIONS
bond interaction between carboxyl hydrogen and oxygen
atoms with epoxides. The benzimidazolium salts could
In conclusion, an efficient catalytic system was presented
to prepare cyclic carbonates through the cycloaddition
of epoxides with CO2, which was composed
activate CO via the formation of N-heterocyclic carbene
adducts.
2
of benzimidazolium salts, CrCl , and TBAB. The three-
2
component catalytic system showed wide scope of termi-
nal epoxides, as well as internal epoxides with excellent
functional group tolerance in high products yield. The
role of each component in the catalytic process was
explored in the control experiments and spectroscopic
measurements. The results indicated that CrCl2
coordinated with benzimidazolium salts to form a new
Cr complex as Lewis acids, in which the carboxyl group
bearing new Cr complex enhanced the nucleophilic
attack of TBAB to open the ring of epoxides by hydrogen
4 | EXPERIMENTAL
4.1 | Analytical methods
All solvents and reagents were purchased from commer-
cial sources used without additional purification. NMR
spectra were recorded on a Bruke Avance III HD
400 spectrometer using TMS as internal standard
1
13
(400 MHz for H NMR and 100 MHz for C NMR).
High-resolution mass spectroscopy (HRMS) data were

12 of 15
ZHANG ET AL.
collected on a Bruker solanX 70 FT-MS and Agilent 6224
TOF mass spectrometer. FT-IR spectra were recorded on
Tianjin Gangdong FTIR-650. All products were isolated
by flash chromatography on a silica gel (300–400 mesh)
column.
4.2.2 | Benzimidazolium salt 1b
Purification by flash chromatography (dichloromethane:
ꢀ
ꢀ
methanol = 10:1): a yellow solid, mp 196.7 C to 201.2 C;
1
H NMR (400 MHz, DMSO-d ): δ 10.79 (s, 1H), 8.78
6
(
1
d, J = 2.4 Hz, 1H), 8.52–8.50 (m, 1H), 8.43 (t, J = 8.0 Hz,
H), 8.29–8.27 (m, 1H), 8.13 (d, J = 8.0 Hz, 1H), 8.06
4
.2 | General procedure for the synthesis
(d, J = 7.6 Hz, 1H), 7.94 (s, 1H), 7.83–7.77 (m, 2H), 6.70
(t, J = 1.6 Hz, 1H), 4.85 (t, J = 6.8 Hz, 2H), 3.11
of benzimidazolium salts
13
(
t, J = 6.8 Hz, 2H). C NMR (100 MHz, DMSO-d ): δ
6
A mixture of 2,6-dibromopyridine (0.5 mmol), benz-
171.97, 150.10, 145.63, 143.75, 143.30, 143.17, 131.60,
129.06, 128.10, 127.97, 127.19, 115.61, 114.42, 113.96,
imidazole (0.75 mmol), CuI (0.1 mmol), and K CO3
2
(
9
1.5 mmol) in DMSO (2 ml) was stirred for 24 h at
112.59, 109.18, 43.40, 33.32. IR: (KBr) 3340, 2959, 1725,
ꢀ
[21]
À1
0 C under nitrogen atmosphere.
The solvent was
1619, 1550, 1474, 1397, 1245, 1184, 1032, 933, 742 cm
.
concentrated under vacuum, and the product of
-bromo-6-substituent-pyridine was isolated by short
chromatography. Thereafter, 2-bromo-6-substituent-
HRMS: (ESI-TOF), m/z calcd.: 334.1299 for C H N O ;
18 16 5 2
+
2
found: 334.1294 [M À Br] . The NMR spectra and FT-IR
spectra of benzimidazolium salt 1b can be found in
Figures S2 and S33.
pyridine (0.5 mmol), N-heterocycle (0.75 mmol), CuI
(
0.1 mmol), and K CO (1.5 mmol) in DMSO (2 ml)
2 3
ꢀ
was allowed to react 48 h at 120 C under nitrogen
atmosphere. The solvent was removed under reduced
pressure, and the product of pyridine-bridged pincer-
type compound was isolated by short chromatography.
The pyridine-bridged pincer-type compound (0.5 mmol)
and 3-iodopropionic acid (0.7 mmol) were put into
a 25-ml Schlenk tube and added 3-ml acetonitrile
solvent under a nitrogen atmosphere. The reaction was
4.2.3 | Benzimidazolium salt 1c
Purification by flash chromatography (dichloromethane:
ꢀ
ꢀ
methanol = 10:1): a yellow solid, mp 199.1 C to 204.7 C;
1
H NMR (400 MHz, DMSO-d ): δ 10.91 (s, 1H), 8.79
6
(d, J = 2.8 Hz, 1H), 8.53–8.51 (m, 1H), 8.42 (t, J = 8.0 Hz,
1H), 8.30–8.28 (m, 1H), 8.14 (d, J = 8.0 Hz, 1H), 8.07
(d, J = 8.0 Hz, 1H), 7.94 (d, J = 0.8 Hz, 1H), 7.84–7.78
(m, 2H), 6.71 (t, J = 2.0 Hz, 1H), 4.85 (t, J = 6.8 Hz, 2H),
3.17 (t, J = 6.8 Hz, 2H). C NMR (100 MHz, DMSO-d6):
δ 171.63, 150.18, 145.70, 143.78, 143.36, 131.66, 129.14,
128.12, 127.98, 127.21, 115.64, 114.43, 113.99, 112.65,
ꢀ
carried out at 80 C for 24 h. Then, the acetonitrile
was evaporated under reduced pressure, and the
crude products were purified by silica gel column
chromatography using dichloromethane: methanol = 10:1
as eluent.
13
1
1
09.22, 43.13, 33.04. IR: (KBr) 3340, 2989, 1733, 1619,
557, 1474, 1397, 1253, 1192, 1032, 940, 742 cm . HRMS:
À1
4
.2.1 | Benzimidazolium salt 1a
(ESI-TOF), m/z calcd.: 334.1299 for C H N O ; found:
18 16 5 2
+
3
34.1307 [M À Cl] . The NMR spectra and FT-IR spectra
Purification by flash chromatography (dichloromethane:
methanol = 10:1):
of benzimidazolium salt 1c can be found in Figures S3
ꢀ
a
yellow solid, mp 194.3 C to
and S34.
ꢀ
1
2
(
8
(
1
(
01.5 C;
s, 1H), 10.62 (s, 1H), 8.76 (d, J = 2.4 Hz, 1H), 8.54–
.50 (m, 1H), 8.45 (t, J = 8.0 Hz, 1H), 8.31–8.27
m, 1H), 8.16 (d, J = 8.4 Hz, 1H), 8.00 (d, J = 7.6 Hz,
H), 7.96 (d, J = 1.2 Hz, 1H), 7.85–7.80 (m, 2H), 6.72
dd, J = 2.4 Hz, J = 1.6 Hz, 1H), 4.84 (t, J = 7.2 Hz,
H NMR (400 MHz, DMSO-d ): δ 12.73
6
4.2.4 | Benzimidazolium salt 1d
Purification by flash chromatography (dichloromethane:
ꢀ
ꢀ
methanol = 10:1): a black solid, mp 228.3 C to 230.2 C;
13
1
2
H), 3.14 (t, J = 7.2 Hz, 2H). C NMR (100 MHz,
H NMR (400 MHz, DMSO-d ): δ 10.55 (s, 1H), 8.51–8.58
6
DMSO-d ): δ 171.66, 150.20, 145.58, 143.82, 143.38,
(m, 2H), 8.44 (t, J = 8.0 Hz, 1H), 8.36–8.26 (m, 3H), 7.96–
7.89 (m, 2H), 7.84–7.75 (m, 2H), 7.53 (t, J = 6.8 Hz, 1H),
7.39–7.34 (m, 1H), 4.87 (t, J = 6.8 Hz, 2H), 3.13
6
1
1
2
9
43.14, 131.62, 129.17, 128.02, 127.25, 115.58, 114.40,
14.00, 112.73, 109.23, 43.14, 32.85. IR (KBr) 3393,
944, 1710, 1610, 1557, 1466, 1390, 1253, 1192, 1017,
13
(t, J = 6.4 Hz, 2H). C NMR (100 MHz, DMSO-d ): δ
6
À1
33, 780 cm . HRMS: (ESI-TOF), m/z calcd.: 334.1299
171.92, 152.79, 144.92, 143.15, 143.10, 138.87, 138.00,
131.59, 129.47, 128.73, 127.68, 127.28, 126.08, 123.33,
121.68, 115.11, 114.59, 114.47, 113.99, 113.64, 43.31,
33.03. IR: (KBr) 3393, 2989, 1725, 1588, 1557, 1458, 1344,
+
for C H N O ; found: 334.1292 [M À I] . The NMR
18
16 5 2
spectra and FT-IR spectra of benzimidazolium salt 1a
can be found in Figures S1 and S32.

ZHANG ET AL.
13 of 15
À1
1
1
245, 1184, 1108, 1008, 796, 749 cm . HRMS: (ESI-
H NMR (400 MHz, DMSO-d ): δ 10.64 (s, 1H), 8.77
6
TOF), m/z calcd.: 384.1455 for C H N O ; found:
(d, J = 2.4 Hz, 1H), 8.54 (dd, J = 7.2 Hz, J = 2.4 Hz, 1H),
8.44 (t, J = 8.0 Hz, 1H), 8.27 (dd, J = 6.4 Hz, J = 2.4 Hz,
1H), 8.15 (d, J = 8.4 Hz, 1H), 8.02 (d, J = 8.0 Hz, 1H),
7.95 (d, J = 1.6 Hz, 1H), 7.86–7.79 (m, 2H), 6.71
22
18 5 2
+
3
84.1448 [M À I] . The NMR spectra and FT-IR spectra
of benzimidazolium salt 1d can be found in Figures S4
and S35.
(
dd, J = 2.4 Hz, J = 1.6 Hz, 1H), 4.61 (t, J = 7.2 Hz, 2H),
1
3
2.06 (hex, J = 7.2 Hz, 2H), 1.02 (t, J = 7.2 Hz, 3H).
C
4.2.5 | Benzimidazolium salt 1e
NMR (100 MHz, DMSO-d ): δ 150.12, 145.73, 143.69,
6
143.32, 142.58, 131.69, 129.27, 128.04, 127.21, 115.71,
Purification by flash chromatography (dichloromethane:
114.20, 113.99, 112.57, 109.18, 48.79, 22.05, 10.78. IR:
(KBr) 3126, 3071, 1614, 1554, 1470, 1391, 1306, 1246,
1092, 1041, 930, 804, 767 cm . The NMR spectra and
ꢀ
ꢀ
methanol = 10:1): a white solid, mp 242.2 C to 243.3 C;
1
À1
H NMR (400 MHz, DMSO-d ): δ 12.73 (s, 1H), 10.57
6
(s, 1H), 8.61 (t, J = 4.4 Hz, 2H), 8.46 (t, J = 8.0 Hz, 1H),
FT-IR spectra of benzimidazolium salt 1g can be found in
8
.37 (d, J = 7.6 Hz, 1H), 8.31 (t, J = 8.4 Hz, 2H), 7.96
Figures S7 and S38.
(
(
(
(
d, J = 8.0 Hz, 1H), 7.90 (d, J = 7.6 Hz, 1H), 7.85–7.77
m, 2H), 7.56–7.52 (m, 1H), 7.40–7.36 (m, 1H),4.87
13
t, J = 6.8 Hz, 2H), 3.16 (t, J = 6.8 Hz, 2H). C NMR
4.2.8 | Benzimidazolium salt 1h
100 MHz, DMSO-d ): δ 171.77, 152.83, 144.96, 143.17,
6
1
1
1
1
38.91, 138.03, 131.61, 129.51, 128.74, 127.69, 127.28,
26.12, 123.36, 121.72, 115.13, 114.60, 114.48, 114.03,
Purification by flash chromatography (dichloromethane:
ꢀ
methanol = 10:1):
a
yellow solid, mp 199.6 C to
ꢀ
1
13.70, 43.11, 32.81. IR: (KBr) 3393, 2998, 1733, 1596,
203.2 C;
H NMR (400 MHz, DMSO-d6): δ 12.67
À1
557, 1474, 1397, 1360, 1245, 1184, 1092, 749 cm
.
(s, 1H), 8.61–8.52 (m, 1H), 8.45 (t, J = 8.0 Hz, 1H),
8.28–8.26 (m, 1H), 8.23–8.20 (m, 1H), 8.01–7.90
(m, 1H), 7.85–7.79 (m, 1H), 7.76–7.64 (m, 2H), 6.66 (dd,
J = 2.8 Hz, J = 1.6 Hz, 1H), 4.81 (t, J = 6.8 Hz, 2H),
4.05–4.00 (m, 1H), 3.00 (s, 3H), 1.99–1.75 (m, 1H),
HRMS: (ESI-TOF), m/z calcd.: 384.1455 for C H N O ;
22
18 5 2
+
found: 384.1451 [M À Br] . The NMR spectra and FT-IR
spectra of benzimidazolium salt 1e can be found in
Figures S5 and S36.
1
3
1
1
.24–1.15 (m, 1H). C NMR (100 MHz, DMSO-d ): δ
6
71.77, 152.74, 150.95, 143.95, 143.82, 143.31, 130.63,
4
.2.6 | Benzimidazolium salt 1f
130.61, 127.71, 127.12, 126.71, 119.29, 114.10, 113.73,
1
13.35, 109.19, 41.49, 32.42, 12.18. IR: (KBr) 3386, 2936,
À1
Purification by flash chromatography (dichloromethane:
methanol = 10:1): a red solid, mp 194.6 C to 195.7 C; H
NMR (400 MHz, DMSO-d ): δ 12.72 (s, 1H), 10.20 (s, 1H),
1565, 1458, 1390, 1253, 1039, 940, 727, 658 cm
.
ꢀ
ꢀ
1
HRMS: (ESI), m/z calcd.: 348.1455 for C H N O ;
19 18 5 2
+
found: 348.1448 [M À I] . The NMR spectra and FT-IR
spectra of benzimidazolium salt 1h can be found in
Figures S8 and S39.
6
9
.19 (s, 1H), 8.63 (s, 1H), 8.46 (t, J = 8.0 Hz, 1H), 8.28
(
(
(
2
d, J = 8.4 Hz, 1H), 8.17 (d, J = 8.0 Hz, 1H), 8.12
t, J = 2.0 Hz, 1H), 8.01 (d, J = 8.0 Hz, 1H), 7.81
d, J = 8.0 Hz, 1H), 7.48–7.34 (m, 2H), 4.55 (t, J = 6.8 Hz,
13
H), 3.06 (t, J = 6.8 Hz, 2H). C NMR (100 MHz,
4.2.9 | Benzimidazolium salt 1i
DMSO-d ): δ 172.18, 149.20, 145.84, 144.81, 144.37,
6
1
1
3
7
43.17, 136.34, 131.83, 125.09, 124.46, 124.07, 120.62,
19.84, 115.54, 114.17, 112.04, 46.02, 34.05. IR: (KBr)
332, 3012, 1725, 1596, 1540, 1458, 1306, 1245, 1092,
Purification by flash chromatography (dichloromethane:
ꢀ
methanol = 10:1):
a
yellow solid, mp 207.5 C to
ꢀ
1
209.8 C;
H NMR (400 MHz, DMSO-d6): δ 12.69
À1
96, 735 cm . HRMS: (ESI-TOF), m/z calcd.: 334.1299
(s, 1H), 10.49 (s, 1H), 8.80–8.79 (m, 1H), 8.49–8.45
(m, 1H), 8.32–8.24 (m, 2H), 8.06–8.04 (d, J = 8.4 Hz,
1H), 7.82–7.72 (m, 3H), 4.81 (t, J = 6.4 Hz, 2H), 3.12
+
for C H N O ; found: 334.1299 [M À I] . The NMR
18
16 5 2
spectra and FT-IR spectra of benzimidazolium salt 1f can
13
be found in Figures S6 and S37.
(t, J = 6.8 Hz, 2H). C NMR (100 MHz, DMSO-d ): δ
6
1
1
71.67, 149.50, 147.26, 142.78, 140.63, 131.58, 129.40,
27.67, 127.15, 125.17, 117.08, 115.82, 114.27, 43.03,
4.2.7 | Benzimidazolium salt 1g
32.79. IR: (KBr) 3332, 2987, 1744, 1693, 1540, 1475,
À1
1439, 1338, 1261, 1160, 780, 735 cm . The NMR
Purification by flash chromatography (dichloromethane:
methanol = 10:1): a yellow solid, mp 203.2 C to 203.5 C;
spectra and FT-IR spectra of benzimidazolium salt 1i
can be found in Figures S9 and S40.
ꢀ
ꢀ

14 of 15
ZHANG ET AL.
4
.2.10 | Benzimidazolium salt 1j
DATA AVAILABILITY STATEMENT
The data that support the findings of this study are
available in the supporting information of this article.
Purification by flash chromatography (dichloromethane:
methanol = 10:1):
ꢀ
a
yellow solid, mp 152.1 C to
ꢀ
1
1
8
53.4 C; H NMR (400 MHz, DMSO-d ): δ 9.78 (s, 1H),
.13–8.10 (m, 1H), 8.02–8.00 (m, 1H), 7.71–7.66
6
ORCID
Kuikui Zhang https://orcid.org/0000-0003-4320-0603
Zhenbang Liu https://orcid.org/0000-0003-0773-8668
Ning Liu https://orcid.org/0000-0001-7299-0400
(
(
m, 2H), 4.68 (t, J = 6.4 Hz, 2H), 4.09 (s, 3H), 2.99
t, J = 6.4 Hz, 2H). C NMR (100 MHz, DMSO-d ): δ
13
6
1
1
1
7
72.22, 143.61, 132.16, 131.32, 126.99, 126.91, 114.14,
14.07, 43.02, 33.87, 33.45. IR: (KBr) 3332, 3080, 1725,
698, 1558, 1458, 1386, 1353, 1245, 1153, 758,
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[
CONFLICT OF INTEREST
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