Fixation of CO2 into Cyclic Carbonates by Halogen-Bonding Catalysis

Article abstract of DOI:10.1002/cssc.202002525

Halogen bonding, parallel to hydrogen bonding, was introduced into the catalytic cycloaddition of carbon dioxide into epoxide (CCE) reactions. A series of halogen-bond donor (XBD) catalysts of N-iodopyridinium halide featured with N?I bond were synthesized and evaluated in CCE reactions. The optimal XBD catalyst, 4-(dimethylamino)-N-iodopyridinium bromide ([DMAPI]Br), under screened conditions at 100 °C, ambient pressure, and 1 mol % catalyst loading, realized 93 % conversion of styrene oxide into cyclic carbonate in 6 h. The substrate scope was successfully extended with excellent yields (mostly ≥93 %) and quantitative selectivity (more than 99 %). ¹H NMR spectroscopy of the catalyst [DMAPI]Br on substrate epoxide certified that the N?I bond directly coordinated with the epoxide oxygen. A plausible mechanism of halogen-bonding catalysis was proposed, in which the DMAPI cation functioned as halogen-bond donor to activate the epoxide, and the counter anion bromide attacked the methylene carbon to initiate the ring-opening of the epoxide. CCE reactions promoted by N-iodopyridinium halide, exemplify a first case of halogen-bonding catalysis in epoxide activation and CO₂ transformation.

Full text of DOI:10.1002/cssc.202002525

Chemistry–Sustainability–Energy–Materials
Accepted Article
Title: Fixation of CO2 into cyclic carbonates by halogen bonding
catalysis
Authors: Rui Yan, Kai Chen, Zhenjiang Li, Yuanyuan Qu, Luoyu Gao,
Haoying Tong, Yongqiang Li, Jie Li, Yongzhu Hu, and Kai
Guo
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To be cited as: ChemSusChem 10.1002/cssc.202002525
Link to VoR: https://doi.org/10.1002/cssc.202002525
0
1/2020

ChemSusChem
10.1002/cssc.202002525
Fixation of CO into cyclic carbonates by halogen bonding catalysis
2
Rui Yan, Kai Chen, Zhenjiang Li, Yuanyuan Qu, Luoyu Gao, Haoying Tong, Yongqiang Li, Jie Li,
Yongzhu Hu, Kai Guo*
State Key Laboratory of Materials-Oriented Chemical Engineering, College of Biotechnology
and Pharmaceutical Engineering, Nanjing Tech University, 30 Puzhu Road South, Nanjing
2
11816, China.
Abstract
Halogen bonding, parallel to hydrogen bonding, was introduced into the catalytic
cycloaddition of carbon dioxide into epoxide (CCE) reactions. A series of halogen bond donor
(XBD) catalysts of N-iodopyridinium halide featured with N‒I bond were synthesized and
evaluated in CCE reactions. The optimal XBD catalyst, 4-(dimethylamino)-N-iodopyridinium
bromide ([DMAPI]Br), under screened conditions at 100 °C, ambient pressure, and 1 mol %
catalyst loading, realized 93% conversion of styrene oxide into cyclic carbonate in 6 h. The
substrate scope was successfully extended with excellent yields (mostly 93% up) and
1
quantitative selectivity (more than 99%). H NMR titrations of the catalyst [DMAPI]Br on
substrate epoxide certified that N‒I bond directly coordinated with the epoxide oxygen.
Plausible mechanism of halogen bonding catalysis was proposed, in which DMAPI cation
functioned as halogen bond donor activated the epoxide and the counter anion bromide
attacked the methylene carbon to initiate the ring-opening of the epoxide. CCE reactions
promoted by N-iodopyridinium halide exemplified a first case of halogen-bonding catalysis in
epoxide activation and CO transformation.
2
KEYWORDS: carbon dioxide, cyclic carbonate, epoxide, halogen bonding, organocatalysis
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10.1002/cssc.202002525
Table of Contents entry
CO
2
fixation by halogen bonding catalysis: Halogen bond is introduced into the catalytic
into epoxide (CCE) reactions. N-iodopyridinium halide salts are used as
cycloaddition of CO
2
cationic halogen bond donor catalyst to activate the epoxides under ambient pressure to
produce quantitative yields. We aim to exploit a supplementary approach to classical hydrogen
bond to promote CCE reactions.
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10.1002/cssc.202002525
Introduction
The rapid increase of the level of carbon dioxide (CO
2
) in the atmosphere caused by
industrialization, particularly the intensive exploitation of fossil fuels, has led to environmental
[
1]
problems such as global warming, rising of sea levels, and extreme weather events. Chemical
conversions of carbon dioxide into valuable products is one of the most promising strategy to
this issue.[2] Albeit photosynthesis is the predominant pathway of carbon circulation, it is
[
3]
prospective that incorporation of carbon dioxide as a renewable carbon source into value-
[
4]
added chemicals with the assistance of efficient catalysis can be a supplementary approach
to recycle CO from oil and gas consumptions. Energy intensive process is required in chemical
2
[
5]
fixation of carbon dioxide mainly because the carbon in CO is in its most oxidized state. One
2
feasible approach to overcome the high energy barrier was to choose high-energy reactants
to compensate the thermodynamic barrier and utilize efficient catalyst to reduce the
activation energy.[6] In this regard, the cycloaddition of CO
with epoxide (CCE) reaction to
2
afford cyclic organic carbonates was a viable and atom-efficient strategy. These cyclic
carbonates can be used as aprotic polar solvents,[7] electrolyte for lithium batteries,[8]
monomers for polycarbonates[9] and synthon of polyurethanes.[10] So far, a series of metal
complex catalysts[11] and organocatalysts[12] have been explored and successfully applied in
CCE reactions. Noncovalent interaction, predominately hydrogen bonding, is popular in
organocatalysis in the last decade.[13]
The halogen bond (XB), a hydrophobic and soft analogue to hydrogen bond,[14] is defined
by IUPAC in 2013 as a kind of noncovalent interaction between an electrophilic region of
polarized halogen substituent X and a Lewis basic moiety.[15] The molecule providing halogen
atom is usually the acceptor of electrons, which is called “halogen bond donor” (XBD). Another
molecule is “halogen bond acceptor”[16] acting as the donor of electrons. Disclosed by
previously theoretical and experimental studies, the distribution of the electrostatic energy on
surface of halogen substituents is anisotropic,[17] resulting in high electron density
perpendicular to R‒X bond and an electropositive region in elongation of R‒X bond which is
called “σ-hole” afterwards.[18] Unique features deriving from “σ-hole” has been gradually
observed for halogen bonding, that is, high directionality (approximately 180°),[19] strength
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10.1002/cssc.202002525
tunability (in an order of I > Br > Cl > F),[20] hydrophobicity, and donor atom dimensions[21]
Figure 1). Although XB was known as electrophilic centers for 150 years,[22] systematic
(
investigations were performed in the last two decades,[23] most of which were focus on
supramolecular chemistry[24] and crystal engineering.[25] Since the first attempt to introduce
XB into Hantzsch ester reduction was taken by Bolm in 2008,[26] researchers turned attention
to the application of halogen bonding catalysis in organic synthesis as a complement to
hydrogen bonding,[27] and fortunately achieved success in Diels-Alder reaction,[28] Nazarov
cyclisation reaction,[29] Friedel-Crafts reaction[30] and so on. Distinctive hypotheses were
proposed to explain the activation of carbon-bromine bonds[31] and carbonyl group by halogen
bonding. Typically, DFT calculation studies by Huber et al. in 2017 verified the transition state
of Michael reaction catalyzed by C‒I moiety of halogen bond donor catalyst, indicating that
halogen bonding activated oxygen of the carbonyl.[32] To our best knowledge, halogen bonding
has not been exploited in CO
fixation.[33]
2
Figure 1. Hydrogen bond VS Halogen bond. LB = Lewis base, X = halogen.
CCE reactions were well promoted by hydrogen bonding catalysis. Both binary[34] (Scheme
1
a) and one-component[35] (Scheme 1b) H-bonding organocatalysts were designed, in which
H-bond donor activated the epoxide and the nucleophilic anion, usually halide anion
accompanied by an onium counterion, attacked the methylene carbon to trigger the ring-
opening of the epoxide. The parallelism of halogen bonding to the well appreciated hydrogen
bonding inspired us to exploit the possibility of X-bonding in catalytic transformation of CO
CCE reactions.
2
in
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Scheme 1. (a) Binary component system of phenolic compounds combined with nBu NI by
4
Kleij.[34] (b) Bifunctional ammonium salts featured with an alcohol moiety and halide anion by
Werner.[35b] (c) Our proposal.
Herein, we propose a kind of halogen bond donor (XBD) consisted of N-iodopyridinium
−
cation and halide anion (X ) as one-component organocatalyst (Scheme 1c), and disclose
halogen bonding catalysis in CCE reactions for the first time.
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Results and Discussion
Design of the N-iodopyridinium X-bond donor catalysts and evaluation of the catalytic
performances in CCE reactions.
Reconciling with the polarizability of halogen atoms, halogen bond donor (XBD) R‒X
exhibits more Lewis acidity, i.e., stronger halogen bond, in an order X = Cl < Br < I.[36] For most
situations where strong halogen bonding was needed, iodine atom was employed as XBD atom.
R group was structurally diversified, however, classical XBDs were featured with C‒I bond.
Initially neutral XBD emerged as an example of haloperfluoroalkanes[26] and polyfluorinated
aryl iodides.[37] Cationic XBDs were subsequently designed to form stronger halogen bonding,
the most representative of which were a series of imidazolium-based XBDs.[38] Nevertheless,
there are few researches so far on N‒I type XBDs in organocatalysis. Symmetric [N‒I‒N]+
halonium complexes were synthesized, in which iodine accepted electrons in both lobes of
empty p-orbital forming a structure of three-center halogen bond of halonium ions. This kind
of halonium complexes were usually employed as a mild iodine transfer reagent as an example
+
[39]
of [bis(pyridine)iodine(I)] tertrafluoroborate.
To our best knowledge, cationic XBD
characterized with N‒I bond has not been exploited in organocatalysis although N‒I X-bonding
was well documented in crystal engineering.[40] The polarity of N‒I bond is larger than that of
C‒I bond and the electro negativity of nitrogen will enhance the electron deficiency in the
elongation of N‒I bond and thus form stronger halogen bond.
N-Iodopyridinium halide was therefore identified and chosen as XBD to catalyze CCE
reactions. Prototype N-iodopyridinium chloride and N-iodopyridinium bromide were prepared
for their easy syntheses and stabilities at high temperatures. Under conditions of 120 °C and
ambient pressure, the experiments were conducted using styrene oxide (SO) as reference
substrate in the presence of 1 mol % of the catalysts, the conversion of which were 42% and
6
0% respectively (Table 1, entries 5 and 15). In view of electron effect, steric hindrance effect,
and halide anion, an array of N-iodopyridinium halide catalysts (Figure 2, 1‒14) were
synthesized by reactions of substituted pyridines and iodine chloride or iodine bromide,[41] and
their catalytic activities as halogen bond donors were evaluated. The obtained results are
shown in Table 1. Control experiment without loading of any catalysts was carried out under
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10.1002/cssc.202002525
the same conditions with the result of none production of carbonate as expected (Table 1,
entry 1).
Figure 2. N-iodopyridinium halide catalysts with different groups were synthesized as
candidate catalysts used in CCE reactions.
The conversions of SO using catalysts substituted by electron-donating groups (EDGs) in
para-position of nitrogen atom (catalysts 2‒5) are apparently higher than that of catalyst 1
except for catalyst 3, the conversion of which was 31% (Table 1, entries 6‒9). Conversely,
electron-withdrawing substituents (catalysts 8‒10) resulted in sharp decrease of conversions,
best of which was only 26% (Table 1, entries 12‒14). These phenomena were quite different
from those observed in ionic hydrogen bonding. The electron effect of substituents on N-
iodopyridinium aromatic ring in influencing the N‒I halogen bonding abilities was not simply
judged by the dipoles of N‒I, which was straightforwardly predicted by the electron-donating
or electron-withdrawing effect of the substitutional groups. Halogen bonding by its nature was
event of multipole moment that exhibited anisotropy electron density distributions, where the
electrophilic region, the σ-hole, was 180° to the N‒I bond. EDGs on the 4-position of N-
iodopyridinium made the N‒I stronger XBD[42] and showed shorter N‒I···E distances,[43] which
suggested EDGs permitted better XBD catalyst.
Catalysts 11‒14 were also synthesized for the purpose of making a comparison of two
kinds of halide anions. Results showed that 4-(dimethylamino)-N-iodopyridinium bromide
([DMAPI]Br) exhibited the highest conversion, up to 94%, (Table 1, entry 17) higher than
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10.1002/cssc.202002525
classical bromide salt tetrabutylammonium bromide (TBAB) (Table 1, entry 19). The difference
in conversions between chloride anion and bromide anion was marginal, albeit the bromine
salts of catalyst 2 was not synthesized successfully (Table 1, entries 15‒18). Attempts to
synthesize N-iodopyridinium iodide catalysts failed for its lability under room temperature,
which was consistent with previous literature.[44]
Catalysts 6 and 7 with one and two ortho-methyl substitutions were designed to
differentiate the influence of the steric hindrance. When a methyl group was attached to the
ortho-position of nitrogen atom (catalyst 6), the conversion decreased sharply down to 19%
(Table 1, entry 10); no conversion was detected when catalyst 7 with two methyl groups was
used (Table 1, entry 11). It is unfortunate that catalysts with longer and bulky alkyl groups on
the two ortho-positions of nitrogen atom were not prepared successfully probably because
the iodine cation was too huge to be accommodated. However, we can still conclude that steric
hindrance has a negative impact on the catalytic efficiency of halogen bond donor.
Table 1. CCE reactions of styrene oxide (SO) with carbon dioxide to obtain styrene carbonate
[
a]
(SC) catalyzed by catalysts 1-14.
Selectivity
Entry
Catalyst
t [h]
Yield [%][b]
[
%][b]
1
2
3
4
5
6
7
8
9
-
4
4
4
4
4
4
4
4
4
4
4
4
4
4
-
-
DMAP
IBr
26
0
99
0
I
2
0
0
1
2
42
68
31
66
77
19
0
99
99
99
99
99
99
0
3
4
5
1
1
1
1
1
0
1
2
3
4
6
7
8
26
0
99
0
9
10
4
99
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10.1002/cssc.202002525
1
1
1
1
1
2
2
5
6
7
8
9
0
1
11
12
4
4
4
4
4
4
4
60
30
94
67
69
0
99
99
99
99
99
0
13
14
TBAB
NaBr
EPB
45
99
1
[
a] Reaction conditions: SO (10 mmol), CO
2
(1 atm), catalyst (1 mol %), 120 °C; [b] determined by H
NMR with CDCl as solvent.
3
Screening of reaction conditions.
DMAPI]Br (catalyst 13) was finally chosen as the optimal catalyst utilized in the screening
[
of reaction conditions. Efforts to lower the reaction temperature and catalyst loading under
ambient pressure were taken to test the reactivity of the catalyst and determine the optimal
reaction conditions (Table 2). Lowering the temperatures from 120 °C to 60 °C, catalytic
reactivity reached a maximum at the temperature of 100 °C (Table 2, entry 2). When the
temperature was higher or lower than 100 °C, the conversion decreased slightly to 88% or
sharply to 5% respectively (Table 2, entries 1, 3 and 4). It is visibly suggested that 100 °C is the
preferred temperature for the catalyst and that higher or lower ones received decreasing
yields.
At 100 °C, a monotonous decrease of conversion from 94% to 22% was observed by
lowering the catalyst loading from 2% to 0.1% (Table 2, entries 1 and 5‒7). It is worth noting
that conversion of SO increased just slightly from 93% to 94% in 6 h when catalyst loading was
increased from 1% to 2% (Table 2, entries 1 and 7). In overall consideration of cost and practical
conversions, 1 mol % catalyst loading and 100 °C (Figure 3) were finally decided as optimal
conditions which subsequent researches were based on.
Table 2. Screening of reaction conditions of CCE reactions of styrene oxide (SO) and carbon
[
a]
dioxide to obtain styrene carbonate (SC) catalyzed by [DMAPI]Br.
Catalyst loading
Selectivity
Entry
T [°C]
t [h]
Yield [%][b]
[mol %]
[%][b]
1
2
3
4
1.0
120
100
80
6
6
6
6
88
93
47
5
99
1.0
99
1.0
99
1.0
60
99
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10.1002/cssc.202002525
5
6
7
0.1
0.5
2
100
100
100
6
6
6
22
73
94
99
99
99
aReaction conditions: SO (10 mmol), CO
(1 atm), [DMAPI]Br; [b] determined by H NMR with CDCl as
1
2
3
solvent.
Figure 3. Screening of reaction temperature (a) and catalyst loading (b).
Substrate scope studies
Under the optimum conditions, efforts to extend the substrate scope of the protocol were
taken, the results of which are presented in Table 3. Obviously, most of the epoxides were
transformed into their corresponding cyclic carbonates with nearly full conversion ranging
from 93% to 99% with 99% selectivity. A steric demanding epoxide 2-[(2-
methylphenoxy)methyl]oxirane (Table 3, entry 9) and a spiro-epoxy oxindole (Table 3, entry
1
0) exhibited moderate conversions (50% up) due to their steric hindrance. Internal epoxides
cyclohexene oxide and cis-stilbene oxide were tested that resulted no conversion (Table 3,
entries 11 and 12). Pure cyclic carbonates were obtained simply by column chromatography
1
13
and the structures were verified by H NMR and C NMR.
Table 3. Extension of the scope of epoxide substrates with carbon dioxide catalyzed by
[
DMAPI]Br under optimum conditions.[a]
Selectivity
Entry
1
Substrate
Product
Yield [%][b]
[
%][b]
96
99
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10.1002/cssc.202002525
2
3
99
94
99
99
4
75
99
5
6
96
96
99
99
7
8
93
93
99
99
9
58
50
99
99
1
0
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10.1002/cssc.202002525
1
1
1
2
0
0
0
0
[
[
a] Reaction conditions: epoxide substrates (10 mmol), CO
2
(1 atm), 1 mol % of [DMAPI]Br, 100 °C, 5h;
1
13
b] determined by H NMR and C NMR with CDCl
3
as solvent.
Proposed mechanism for CCE reactions catalyzed by halogen bond donors.
Halogen anion played a crucial role of nucleophilic attack during the process of ring-
opening of the epoxide, which is generally known. However, inorganic bromide salt NaBr as
catalyst cannot promote CCE reaction (Table 1, entry 20) demonstrating that cationic moiety
1
is equally important to catalytic procedure. H NMR titration experiments were performed to
figure out the role that cationic moiety of halogen bond donor catalyst played in CCE reactions
(as shown in Figure 4). The ratios of catalyst and SO were changed from 0/1 to 2/1. Small
changes in the chemical shifts of the NMR were observed. Gradually increasing the amount of
the substrate SO, chemical shift of the hydrogen on the carbon adjacent to the nitrogen atom
of pyridine ring ranged from 8.173 to 8.201 ppm. Meanwhile, the methine proton on SO
exhibited the same trend from 3.908 to 3.911 ppm. These phenomena were ascribed to the
delocalization of electron densities by halogen bonding between iodine atom in catalyst
[DMAPI]Br and oxygen atom on SO. When the ratio of catalyst and SO came up to 2/1, the
substrate was saturated with catalyst and the chemical shift of hydrogen atom in two positions
did not change anymore. Alpha proton of the pyridinium, nevertheless, may coordinate with
the epoxide by hydrogen bond. Comparison experiment where N-ethyl pyridinium bromide
(EPB) was employed to catalyze CCE reactions for its easy accessibility and ignorable hydrogen
bond of ethyl group was performed to obtain 45% conversion of SO (Table 1, entry 21) while
the iodopyridinium bromide promoted the 60% conversion (Table 1, entry 15). These results
indicated that the existence of halogen atom, further, the halogen bonding, promoted CCE
reactions.
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1
Figure 4. The chemical shift of hydrogen atom next to nitrogen atom in [DMAPI]Br in the H
NMR spectra (DMSO-d
DMAPI]Br with SO; [DMAPI]Br/SO rations: (1) 0/1, (2) 0.1/1, (3) 0.2/1, (4) 0.5/1, (5) 1/1, (6)
/1.
6
) (left) and the methine proton of SO (right) observed by titrations of
[
2
A series of experiments were carried out to exclude the possibility of the decomposition
of the catalyst, where [DMAPI]Br was possibly collapsed into DMAP and IBr or elemental iodine.
However, experiments employed IBr or I as catalyst demonstrated no conversion of SO (Table
, entries 3 and 4) and merely 26% conversion was obtained when using DMAP as catalyst
2
1
under the same conditions by 4 hours (Table 1, entry 2). As a matter of fact, the salt [DMAPI]Br
resulted in a sharp increase of conversion, up to 94% (Table 1, entry 17). Therefore, DMAP, IBr
or I , individually, were not able to promote the reactions. It was the salt composed of DMAP
2
and IBr in [DMAPI]Br structure that worked in the transformation of SO into cyclic carbonate.
Additionally, thermogravimetric analysis was conducted to find only 2.905% loss of mass when
the temperature was heated to 100.01 °C (See Supporting Information, S21). The weight loss
was attributed to the residual water absorbed; however, no appreciable weight loss was
observed at temperatures higher than 120 °C, which was the up limit of the CCE reactions.
These data exclude the possibility of the decomposition of the [DMAPI]Br catalyst in the
reactions. Evidence of X-ray structural analysis of single crystal of catalyst 4-(trifluoromethyl)-
N-iodopyridinium chloride (catalyst 9) also validated the existence of N-I covalent bond (See
Supporting Information, S21).
With a view to previous reports and above-mentioned experimental data, a probable
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ChemSusChem
10.1002/cssc.202002525
mechanism that contributed to the formation of carbonates from SO was proposed. The
positive charge enhanced N‒I halogen bond of the iodopyridinium[45] was a strong X-bond
donor that will coordinated with the electron rich oxygen of the epoxide by strong X-bonding.
Subsequently, the epoxide was activated, in which the polarity of C‒O bond of the epoxide
increased and the electron density of carbon atom decreased (Scheme 2, step A). Nucleophilic
bromide anion attacked the electron deficient methylene carbon of SO, the ensued ring-
opening of the epoxide formed alkoxide which was stabilized by the halogen bond donor
(Scheme 2, step B). Further insertion of a carbon dioxide between the alkoxide and the XBD
produced carbonate anion (Scheme 2, step C). Finally, ring-closure by intramolecular attack of
carbonate anion on the proximity C–Br carbon expelled bromide anion and released the
product cyclic carbonate. Concurrently, catalyst [DMAPI]Br was regenerated and took part in
the following catalytic cycles (Scheme 2, step D).
Scheme 2. Possible mechanism for the formation of cyclic carbonates from epoxides and
carbon dioxide by halogen bonding catalysis.
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Conclusion
Halogen bonding catalysis was introduced into the cycloaddition of CO
2
into epoxides to
produce cyclic carbonates. N-Iodopyridinium halide, a kind of cationic halogen bond donor
based upon N‒I bond, was chosen as catalyst for its electron deficiency enhanced by positively
charged pyridinium nitrogen. A series of halogen bond donor catalysts were synthesized to
assess the influences of electron and steric effects of the pyridiniums, and the nucleophilicity
of the halide counter anions. 4-Dimethylamino-N-iodopyridinium bromide ([DMAPI]Br) was
evaluated as the optimal catalyst in CCE reactions. Epoxides with variety of substituents were
transformed into the corresponding cyclic carbonates successfully with excellent conversions
and quantitative selectivities under the optimal conditions of 100 °C, ambient pressure, and 1
mol % catalyst loading. The catalytic process was proposed where the N‒I moiety of the
pyridinium formed halogen bonding with the epoxide to activate the substrate and the halide
1
anion attacked the carbon of methylene to initiate ring-opening. H NMR titration experiments
of [DMAPI]Br on styrene oxide verified the activation of the substrate by halogen bond donor
catalyst. Further investigations of N‒X and C‒X type halogen bond donor catalyst will be
carried out to gain more knowledge about halogen bonding catalysis.
Experimental section
Materials. Epoxides were purchased from Alfa Aesar. Carbon dioxide with a purity of
9
9.999% was commercially available from Nanjing Shangyuan Co. Pyridines with different
substituents, ICl and IBr were purchased from Energy Chemical. All reagents were utilized
without any further purification.
1
13
Characterizations. H NMR and C NMR spectra were obtained on a Brucker ARX-250
spectrometer operating at 400 MHz in deuterated solvent at ambient temperature. The
1
13
structures of pure catalysts and obtained cyclic carbonates were verified by H NMR and C
NMR spectra with CDCl as solvent. Conversions and selectivities of epoxides were determined
3
1
by H NMR.
General Procedure for the Synthesis of Catalysts 1-14. Catalysts were synthesized by the
reaction of substituted pyridines with interhalides ICl or IBr. For instance, during the formation
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10.1002/cssc.202002525
of 4-dimethylamino-N-iodopyridinium bromide ([DMAPI]Br), the solution of IBr (0.47 mL, 10
mmol, 1.0 eq) in 20 mL of dry CH Cl was slowly added in a solution of dimethylamionpyridine
2
2
(1.22 g, 10 mmol, 1.0 eq) in 5 mL of dry CH
2
Cl in two hours. The obtained reddish-brown
2
solution was stirred under room temperature for another hour. Crude product was
concentrated by rotary evaporation and then was washed using anhydrous ethanol for three
times. Pure catalyst was obtained after drying for few hours. The procedures of other catalysts
were similar to that of [DMAPI]Br.
Representative Experimental Procedure for the Cycloaddition of Epoxide and CO to
2
Cyclic Carbonate. The cycloaddition reaction of carbon dioxide into epoxides (CCE) reactions
were operated in a Schlenk flask. The inert gas used was argon. After Schlenk operations,
catalyst and epoxide were added into the flask in order in the case of argon. After the argon in
the flask was replaced by carbon dioxide, the Schlenk flask was put into the oil bath pan with
a carbon dioxide balloon. In a typical reaction procedure, the reactor was charged with
[
DMAPI]Br (33 mg, 0.1 mmol, 0.01 eq) and styrene oxide (1.2 mL, 10 mmol, 1 eq) at 100 °C
with a carbon dioxide balloon. The conversions of cyclic carbonate obtained were determined
1
by H NMR spectra with CDCl
3
as solvent. Pure carbonates were obtained by column
chromatography with eluent of petroleum ether (PE) and ethyl acetate (EA) (5/1). The
1
13
structures of carbonates were verified by H NMR and C NMR with CDCl as solvent.
3
AUTHOR INFORMATION
Corresponding Author
*Email: guok@njtech.edu.cn; zjli@njtech.edu.cn.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This work was supported by the National Key Research and Development Program of China
2017YFC1104802), the National Natural Science Foundation of China (U1463201, 21522604),
(
the Natural Science Foundation of Jiangsu Province, China (BK20150031), the Jiangsu
Synergetic Innovation Center for Advanced Bio-Manufacture, the Jiangsu National Synergetic
Innovation Center for Advanced Materials (SICAM), the project funded by the Priority
This article is protected by copyright. All rights reserved.

ChemSusChem
10.1002/cssc.202002525
Academic Program Development of Jiangsu Higher Education Institutions (PAPD), the Top-
Notch Academic Programs Project of Jiangsu Higher Education Institutions (TAPP).
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