Efficient Aluminum Catalysts for the Chemical Conversion of CO2 into Cyclic Carbonates at Room Temperature and Atmospheric CO2 Pressure

Article abstract of DOI:10.1002/cssc.201901661

A series of dimeric aluminum compounds [Al(OCMe₂CH₂N(R)CH₂X)]₂ [X=pyridin-2-yl, R=H (Pyr^H); X= pyridin-2-yl, R=Me (Pyr^Me); X=furan-2-yl, R=H (Fur^H); X= furan-2-yl, R=Me (Fur^Me); X=thiophen-2-yl, R=H (Thio^H); X= thiophen-2-yl, R=Me (Thio^Me)] containing heterocyclic pendant group attached to the nitrogen catalyze the coupling of CO₂ with epoxides under ambient conditions. In a comparison of their catalytic activities with those of aluminum complexes without pendant groups at N [X=H, R=H (H^H); X=H, R=Me (H^Me)] or with non-heterocyclic pendant groups [X=CH₂CH₂OMe, R=H (OMe^H); X=CH₂CH₂NMe₂, R=H (NMe2^H); X=CH₂CH₂NMe₂, R=Me (NMe2^Me)], complexes containing heterocycles, in conjunction with (nBu)₄NBr as a cocatalyst, show higher catalytic activities for the synthesis of cyclic carbonates under the same ambient conditions. The best catalyst system for this reaction is Pyr^H/(nBu)₄NBr system, which gives a turnover number of 99 and a turnover frequency of 4.1 h^?1, making it 14- and 20-times more effective than H^H/(nBu)₄NBr and H^Me/(nBu)₄NBr, respectively. Although there are no direct interactions between the aluminum and the heteroatoms in the heterocyclic pendants, electronic effects combined with the increased local concentration of CO₂ around the active centers influences the catalytic activity in the coupling of CO₂ with epoxides. In addition, Pyr^H/(nBu)₄NBr shows broad epoxide substrate scope and seven terminal epoxides and two internal epoxides undergo the designed reaction.

Full text of DOI:10.1002/cssc.201901661

Accepted Article
Title: Efficient Aluminum Catalysts for the Chemical Conversion of CO2
into Cyclic Carbonates at Room Temperature and Atmospheric
CO2 Pressure
Authors: Yoseph Kim, Kyunglim Hyun, Duseong Ahn, Ran Kim, Myung
Hwan Park, and Youngjo Kim
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: ChemSusChem 10.1002/cssc.201901661
Link to VoR: http://dx.doi.org/10.1002/cssc.201901661
A Journal of
www.chemsuschem.org

10.1002/cssc.201901661
ChemSusChem
1
Efficient Aluminum Catalysts for the Chemical Conversion of CO2 into Cyclic
Carbonates at Room Temperature and Atmospheric CO2 Pressure
2
3
4
5
Yoseph Kim, Kyunglim Hyun, Duseong Ahn, Ran Kim, Myung Hwan Park,* and Youngjo Kim*
6
Y. Kim, K. Hyun, D. Ahn, R. Kim, Prof. Dr. Y. Kim
7
Department of Chemistry and BK21+ Program Research Team, Chungbuk National
University, Cheongju, Chungbuk 28644, Republic of Korea
8
9
E–mail: ykim@chungbuk.ac.kr
10
11
12
13
14
15
16
17
18
19
Prof. Dr. M. H. Park
Department of Chemistry Education, Chungbuk National University, Cheongju, Chungbuk
28644, Republic of Korea
E–mail: mhpark98@chungbuk.ac.kr
K. Hyun and D. Ahn contributed equally to this work.
Supporting information and the ORCID identification number(s) for the author(s) of this
article can be found under: https://doi.org/10.1002/cssc.201901661
.
1
This article is protected by copyright. All rights reserved.

10.1002/cssc.201901661
ChemSusChem
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
Abstract: We have synthesized dimeric aluminum compounds [Al(OCMe2CH2N(R)CH2X)]2 (X =
pyridin–2–yl, R = H (PyrH); X = pyridin–2–yl, R = Me (PyrMe); X = furan–2–yl, R = H (FurH); X
= furan–2–yl, R = Me (FurMe); X = thiophen–2–yl, R = H (ThioH); and X = thiophen–2–yl, R =
Me (ThioMe)) containing heterocyclic pendant group attached to the nitrogen. These complexes
were used to catalyze the coupling of CO2 with epoxides under ambient conditions. A comparison
of their catalytic activities with those of aluminum complexes without N–functionalized pendants
(X = H, R = H (HH); X = H, R = Me (HMe)) or with non–heterocyclic pendants (X = –CH2CH2OMe,
R = H (OMeH); X = –CH2CH2NMe2, R = H (NMe2H); and X = –CH2CH2NMe2, R = Me (NMe2Me))
revealed that aluminum complexes containing heterocycles, in conjunction with (n–Bu)4NBr as a
cocatalyst, showed higher catalytic activities than other complexes for the synthesis of cyclic
carbonates under the same ambient conditions. The best catalytic system for this reaction was the
PyrH/(n–Bu)4NBr system, which showed a TON of 99 and a TOF of 4.1 h-1, making it 14–fold and
20–fold more effective than HH/(n–Bu)4NBr and HMe/(n–Bu)4NBr, respectively. Although no direct
interactions between the aluminum and the heteroatoms in the heterocyclic pendants, its electronic
effects combined with the increased local concentration of CO2 around the active centers influences
the catalytic activity in the coupling of CO2 with epoxides. In addition, PyrH/(n–Bu)4NBr showed
the broad epoxide substrate scope, and seven terminal epoxides and two internal epoxides
underwent the designed reaction.
2
This article is protected by copyright. All rights reserved.

10.1002/cssc.201901661
ChemSusChem
38
Introduction
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
The increase in the atmospheric concentration of carbon dioxide (CO2) has caused many
environmental problems, such as global warming due to its greenhouse effects, and reducing this
concentration has become a global issue that must be addressed to achieve a sustainable society.
Since CO2 is safe, inexpensive, abundant, and renewable, it is a good candidate as a C1 synthon for
various chemical reactions; the chemical transformation of CO2 into industrially valuable
compounds, such as urea, methanol, cyclic carbonates, poly(alkylene carbonate), and sodium 2–
hydroxybenzoate, is a active field of research in CO2 fixation and green chemistry.[1] Within this
field, the most active area of research into CO2 transformations is the coupling of CO2 with epoxides
to produce the corresponding cyclic carbonates, which can be used as aprotic solvents, electrolytes
for lithium–ion batteries, monomers for polymerizations, and pharmaceutical intermediates.[2] To
date, many catalytic systems, including metal–based catalysts and organocatalysts, for the synthesis
of cyclic carbonates from CO2 and epoxides have been developed.[3] Even though the synthesis of
a cyclic carbonate from CO2 and an epoxide is a thermodynamically favorable process, the
industrial-scale syntheses of cyclic carbonates require high reaction temperatures and high CO2
pressures to complete this reaction, and thus overall, this process consumes more energy that it
releases and it is associated with additional, indirect CO2 emissions.[4] Therefore, it is necessary to
develop efficient catalytic systems for the synthesis of cyclic carbonates from CO2 and epoxides
that are effective under ambient temperature and pressure.
Several catalytic systems, including metal–based catalysts and organocatalysts for converting
CO2 into cyclic carbonates at room temperature and 1 bar CO2, have been reported.[3,5] In particular,
aluminum, the most abundant metal in the earth’s crust (the metals are in the order Al, Fe, Ca, Na,
K, Mg, and Ti), is a very attractive metal for this study due to its low cost, low toxicity and high
Lewis acidity. However, only a few examples of efficient non–toxic catalysts with aluminum metal
centers capable of operating at ambient temperature and 1 bar CO2 are known (see Figure 1).[6] As
3
This article is protected by copyright. All rights reserved.

10.1002/cssc.201901661
ChemSusChem
63
64
65
66
67
68
69
70
71
shown in Figure 1, styrene oxide is usually used as the model substrate for coupling reactions with
CO2 because it has moderate reactivity relative to other epoxides along with lower toxicity and a
higher boiling point.[1a] However, non–toxic Al–based catalysts require catalyst loading (> 2.5
mol%) and a long reaction time (24 h) to reach a high conversion to styrene carbonate under ambient
conditions. Their catalytic activities are in the range of TON = 8.4–39.2 and TOF = 0.63–3.73 h–1,[6]
which are slightly higher than organocatalysts, but organocatalysts are among the least active
catalysts for the synthesis of cyclic carbonates under ambient conditions.[5] Thus, the development
of new, more active Al–based catalytic systems that allow lower catalyst loading (< 1.0 mol%) and
a high TON (> 100) under ambient conditions is needed.
4
This article is protected by copyright. All rights reserved.

10.1002/cssc.201901661
ChemSusChem
72
73
Figure 1. Representative Al–based catalysts for the synthesis of styrene carbonate at ambient
74
75
76
77
78
79
80
81
temperature and 1 bar CO2
Recently, we reported dimeric aluminum compounds containing ethanolateamine ligands with
simple heteroatom sidechains, such as OMe and NMe2, as catalysts for the coupling of CO2 with
propylene oxide in the presence of (n–Bu)4NI with 0.1 mol% catalyst loading and 10 bar CO2 at
70 °C.[7] We have demonstrated that the activities of the complexes are highly dependent on the
nature of the substituent on the nitrogen atom; thus, their activities increased as the nucleophilicity
of the sidechain increased (NMe2 > OMe). These aluminum complexes also have a C2 axis at the
centroid of the plane of the Al2O2 ring, and we thought that these complexes would be good
5
This article is protected by copyright. All rights reserved.

10.1002/cssc.201901661
ChemSusChem
82
83
candidates for efficient catalysts in the coupling of CO2 with epoxides under ambient conditions if
a basic residue could be introduced to increase the local concentration of CO2 around the active
center. Thus, we chose dimeric aluminum compounds with ethanolateamine ligands with pyridine
or furyl groups instead of NMe2 or OMe as model catalysts in this study. Some studies have shown
that catalysts with pyridine or furyl heterocyclic side chains have improved catalytic activities and
lifetimes and that the pyridine moieties could increase the local concentration of CO2 around active
centers. For example, a titanium bis(amidinate) catalyst with a pyridine sidechain showed a higher
activity in the synthesis of isotactic polypropylene than the corresponding catalyst with NMe2
arms.[8] In addition, chromium complexes containing bis(phosphanyl)amine with a donor group on
the nitrogen of the ligand are more effective for ethylene trimerization and tetramerization and have
longer catalytic lifetimes than those without a donor group on the nitrogen of the ligand.[9]
Furthermore, pyridinemethanol, which can bind CO2 to the N atom of pyridine, can be used as an
organocatalyst for the coupling of epichlorohydrin with CO2; however, benzyl alcohol did not show
any catalytic activity under the same conditions.[10]
84
85
86
87
88
89
90
91
92
93
94
95
96
Herein, we report the synthesis of C2–symmetric aluminum complexes chelated by substituted
ethanolateamine bearing heterocyclic pendant moieties such as pyridin–2–yl (Pyr), furan–2–yl
(Fur), and thiophen–2–yl (Thio) attached to the nitrogen atom (as shown in Figure 2) and their use
as catalysts for coupling CO2 and epoxides at ambient temperature and 1 bar CO2. Their catalytic
activities were then compared with those of the analogous species containing simple dimethylamino
(NMe2) and methoxy (OMe) pendants attached to the nitrogen and with no sidechains (H) under
the same reaction conditions.
97
98
99
100
101
102
103
6
This article is protected by copyright. All rights reserved.

10.1002/cssc.201901661
ChemSusChem
104
105
Figure 2. Dimeric aluminum complexes were used as catalysts for the synthesis of cyclic
carbonates in this study.
106
107
108
Results and Discussion
Synthesis and Characterization
109
110
Scheme 1. Synthetic routes for aluminum complexes. Each aluminum complex (XR) is
111
112
113
114
115
116
abbreviated by using substituents X and R.
The free ligands, HOCMe2CH2N(R)CH2X, were synthesized according to previously reported
procedures[7] by the reaction of RNHCH2X with isobutylene oxide in almost quantitative yields.
The alcoholysis of AlMe3 is a useful synthetic route for accessing dimeric aluminum compounds
[Al(OCMe2CH2N(R)CH2X)]2. In all cases, the aluminum complexes were prepared under an inert
atmosphere. As depicted in Scheme 1, the direct complexation of N–substituted amino–2–
7
This article is protected by copyright. All rights reserved.

10.1002/cssc.201901661
ChemSusChem
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
methylpropan–2–ol with AlMe3 in toluene proceeds rapidly with the evolution of methane gas to
give the eleven aluminum compounds in high yields (75–90%). The reaction temperature (0 °C)
and time (12 h) were optimized to obtain the maximum yield of the aluminum compounds. The
crude compounds were purified by washing with n–hexane, and analytically pure aluminum
compounds were obtained as colorless crystals after recrystallization from toluene. They were not
stable in air but were thermally stable even at 130 °C. Especially, aluminum complexes PyrH and
PyrMe with a heterocyclic pendant attached to the nitrogen are much more thermally stable than HH
and HMe, which are the analogous aluminum complexes with no heterocyclic ring (See Supporting
Information). As given in the Supporting Information, all the compounds were characterized by 1H
13
and C NMR spectroscopy and by elemental analysis, and the structure of ThioH was confirmed
by single–crystal X–ray crystallography.
The 1H and 13C NMR spectra were consistent with the expected structures, and all the chemical
1
shifts of the protons and carbon atoms were in the expected ranges. The H NMR spectra of
aluminum compounds [Al(OCMe2CH2N(R)CH2X)]2 did not differ significantly from those of free
ligands HOCMe2CH2N(R)CH2X. However, a strongly shielded resonance for Al–Me at
approximately –1.0 ppm in the 1H NMR spectrum and at approximately –6.0 ppm in the 13C NMR
spectrum confirmed the successful complexation of the ligand with the aluminum center.
Interestingly, all the Al–Me resonances in XMe complexes showed greater downfield shifts than
those of the Al–Me resonances of the corresponding XH complexes. Unlike complexes XMe, the –
OCMe2CH2NHCH2– methylene protons in XH complexes were appeared as doublets due to the
presence of a proton on the nitrogen atom.
8
This article is protected by copyright. All rights reserved.

10.1002/cssc.201901661
ChemSusChem
138
139
Figure 3. X–ray structure of ThioH (50% thermal ellipsoids). For clarity, all H atoms are omitted
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
except for H atom attached to the nitrogen.
The molecular structure of ThioH was determined by single–crystal X–ray diffraction analysis.
X–ray-quality crystals were obtained from an NMR tube containing ThioH in CDCl3 at room
temperature. The crystallographic data and structure refinement details are summarized in Table S1
(Supporting Information), and the molecular structure is shown in Figure 3. As shown in Figure 3,
pseudo–C2 symmetric ThioH contains a four–membered Al2O2 ring with a C2 axis at the center of
the ring. The two bridging O atoms link the two AlMe2 moieties, and consequently, ThioH is dimeric
with two five–coordinate Al centers joined through two bridged oxygen atoms, O1 and O1ʹ. Each
of the aluminum centers in ThioH is bound to one N, two O, and two C atoms of methyl groups. In
particular, Figure 3 shows that the thiophene moiety is oriented away from the aluminum center and
does not coordinate to the metal. Furthermore, there is no direct interaction between the two
aluminum atoms. All Al–O, Al–N, and Al–C bond lengths are similar to those found in related
pentacoordinate aluminum complexes.[11] The distortion of the coordination around the 5–
coordinate Al metal center can be determined by the trigonality parameter, τ.[12] Since the largest
bond angle (α) and the second largest angle (β) around the Al center are ∠N1–Al–O1´ [155.67(29)o]
155
and ∠O1–Al–C2 [120.51(13)o], respectively, the trigonality parameter (τ = [α – β]/60) for ThioH
156
157
was calculated to be 0.59. Thus, the two aluminum centers in ThioH have intermediate geometries
between trigonal bipyramidal and square pyramidal.
158
9
This article is protected by copyright. All rights reserved.

10.1002/cssc.201901661
ChemSusChem
159
Synthesis of cyclic carbonates
160
161
Scheme 2. Synthesis of styrene carbonate (2a) from styrene oxide (1a) by using aluminum
162
163
164
165
166
167
168
catalysts.
As shown in Scheme 2, the synthesis of styrene carbonate (2a) from styrene oxide (1a) and CO2
at 25 °C and 1 bar CO2 without solvent for 24 h using 1 mol% (n–Bu)4NBr and 1 mol% catalyst
was chosen as the model reaction to screen the aluminum complexes given in Scheme 1. Each
reaction was analyzed by 1H NMR spectroscopy to determine the conversion of 1a into 2a. 1a was
easily converted into 2a with high selectivity (> 99%) without any polymerized byproducts. The
results are summarized in Table 1.
169
Table 1. Catalyst screening for the model reaction between CO2 and styrene oxide (1a).[a]
Entry
Catalyst
Conversion[b] (%)
TON[c]
99
90
76
32
32
27
31
20
23
7
TOF[d] (h–1)
4.1
1
2
3
4
5
6
7
8
9
PyrH
99
90
76
32
32
27
31
20
23
7
PyrMe
FurH
3.8
3.2
1.3
1.3
1.1
1.3
0.83
0.96
0.29
0.21
FurMe
ThioH
ThioMe
NMe2H
NMe2Me
OMeH
HH
10
11
HMe
5
5
170
171
172
173
[a] Reaction conditions: 1a (10 mmol), catalyst (0.1 mmol, 1.0 mol%), (n–Bu)4NBr (0.1 mmol, 1.0
mol%), r.t., 1 bar CO2 (balloon), 24 h. [b] Conversion was determined by 1H NMR spectroscopy of
an aliquot of the reaction mixture after 24 h (see the Supporting Information). [c] Turnover number
= (mol of 1a consumed)/(mol of catalyst used). [d] Turnover frequency = TON/h.
174
175
176
177
178
The eleven synthesized aluminum complexes showed various catalytic activities for the synthesis
of 2a under ambient conditions. Among them, PyrH gave almost complete conversion of 1a into 2a
at 1 atm of CO2 and 25 °C after 24 h. Complex PyrH in the presence of (n–Bu)4NBr showed the
highest activity with a TON of 99 and TOF of 4.1 h–1 (Table 1, entry 1). Under the same conditions,
PyrMe, which contained a sterically hindered methyl group instead of a hydrogen atom on the
10
This article is protected by copyright. All rights reserved.

10.1002/cssc.201901661
ChemSusChem
179
180
181
182
183
184
185
186
187
188
189
nitrogen, showed a TON of 90 and a TOF of 3.8 h–1, which are lower than those of PyrH (Table 1,
entry 2). Similar trends were observed between FurH and FurMe (Table 1, entries 3 and 4) and
between ThioH and ThioMe (Table 1, entries 5 and 6). Interestingly, the difference in activities
between FurH and FurMe is much larger than that between PyrH and PyrMe and that between ThioH
and ThioMe. Based on the heterocyclic pendant group attached to a nitrogen atom, we found that the
activity decreases in the order pyridine > furan > thiophene (Table 1, entries 1, 3, and 5 and entries
2, 4, and 6). The best catalyst, PyrH, showed an activity 3.2 times higher than that of NMe2H with
dimethylamino pendant groups (Table 1, entry 7), 4.3 times higher than that of OMeH with methoxy
pendant groups (entry 9) and 14.1 times higher than that of HH without N–substituents (entry 10).
Similarly, PyrH was 4.9 times more active than NMe2Me (Table 1, entry 8) and 19.8 times more
active than HMe (entry 11).
190
Table 2. Cocatalyst screening for the coupling of CO2 with styrene oxide (1a) using PyrH.[a]
Entry
Cocatalyst
(n–Bu)4NCl
(n–Bu)4NBr
(n–Bu)4NI
(n–Bu)4PBr
PPNCl
Conversion[b] (%)
TON[c]
18
99
77
34
23
25
3
TOF[d] (h–1)
0.75
4.1
1
2
3
4
5
6
7
8
18
99
77
34
23
25
3
3.2
1.4
0.96
1.0
0.13
0.042
PPNBr
DMAP
MTBD
1
1
191
192
193
194
[a] Reaction conditions: 1a (10 mmol), PyrH (0.1 mmol, 1.0 mol%), cocatalyst (0.1 mmol, 1.0
mol%), r.t., 1 bar CO2 (balloon), 24 h. [b] Conversion was determined by 1H NMR spectroscopy of
an aliquot of the reaction mixture after 24 h (see the Supporting Information). [c] Turnover number
= (mol of 1a consumed)/(mol of PyrH used). [d] Turnover frequency = TON/h.
195
196
197
198
199
200
The reaction was further optimized and the effect of the cocatalyst on the coupling of CO2 with
styrene oxide (1a) was investigated by using the most active catalyst (PyrH) in conjunction with six
different cocatalysts, namely, (n–Bu)4NCl, (n–Bu)4NBr, (n–Bu)4NI, (n–Bu)4PBr, PPNCl, and
PPNBr (Table 2). As shown in entry 2 of Table 2, (n–Bu)4NBr as a cocatalyst resulted in better
activity than the other five compounds. The catalytic activity decreased in the order of Br > I > Cl
for the tetrabutylammonium salts at 25 °C and 1 bar CO2 (Table 2, entries 1–3). These results are in
11
This article is protected by copyright. All rights reserved.

10.1002/cssc.201901661
ChemSusChem
201
202
203
204
205
206
207
208
209
good agreement with previous works,[6a,c–f] and our catalytic system also needed a balance between
nucleophilicity and leaving-group ability. Phosphonium–based bromides such as (n–Bu)4PBr was
not as effective in this reaction as the corresponding ammonium–based cocatalyst such as (n–
Bu)4NBr (Table 2, entries 2 and 4). Sterically hindered phosphonium salts gave activities similar to
that of (n–Bu)4PBr (Table 2, entries 4–6). Non–halide cocatalysts such as dimethylaminopyridine
(DMAP) (Table 2, entry 7) and 1,3,4,6,7,8-Hexahydro-1-methyl-2H-pyrimido[1,2-a]pyrimidine
(MTBD) (entry 8) gave the trace amount of 1b under the same condition. Since (n–Bu)4NBr in
conjunction with PyrH showed the highest activity, this pair was selected as the optimal cocatalyst
system.
210
Table 3. Synthesis of styrene carbonate (2a) using PyrH and (n–Bu)4NBr.[a]
PyrH
[mol%]
1.0
(n–Bu)4NBr
Conversion[b]
TOF[d]
[h–1]
0.63
0.29
4.1
Entry
TON[c]
[mol%]
[%]
15
7
99
42
31
48
8
1
2
3
4
5
6
7
8
0
1.0
1.0
0.5
0.5
1.0
0.1
0.05
15
7
0
1.0
1.0
0.5
0.5
0.1
0.05
99
42
62
96
80
80
1.8
2.6
4.0
3.3
4
3.3
211
212
213
214
[a] Reaction conditions: 1a (10 mmol), 25 °C, 1 bar CO2 (balloon), and 24 h. [b] Conversion was
1
determined by H NMR spectroscopy of an aliquot of the reaction mixture after 24 h (see the
Supporting Information). [c] Turnover number = (mol of 1a consumed)/(mol of PyrH used). [d]
Turnover frequency = TON/h.
215
216
217
218
219
220
221
222
The effect of reducing the catalyst loading in the coupling reaction of styrene oxide (1a) with 1
bar pressure of CO2 at 25 °C with the best catalyst system (PyrH/(n–Bu)4NBr) was investigated, and
the results are shown in Table 3. Control experiments (Table 3, entries 1 and 2) showed that neither
(n–Bu)4NBr nor PyrH alone displayed significant catalytic activity with 15% and 7% conversion in
the absence of the other catalyst component under the same reaction conditions, and this is a further
demonstration of the synergistic effect of using PyrH with (n–Bu)4NBr. When 1.0 mol% PyrH and
1.0 mol% (n–Bu)4NBr loading was used at 25 °C, the reaction was complete (99%) within 24 h
(Table 3, entry 3). In addition, when the PyrH loading was reduced from 1.0 mol% to 0.5 mol% to
12
This article is protected by copyright. All rights reserved.

10.1002/cssc.201901661
ChemSusChem
223
224
225
226
227
228
0.1 mol% to 0.05 mol% while keeping a PyrH to (n–Bu)4NBr ratio of 1:1, the yield of styrene
carbonate gradually decreased from 99% to 31% to 8% to 4%, respectively (Table 3, entries 3, 5, 7,
and 8). A 2:1 ratio of PyrH to (n–Bu)4NBr showed a half reduction in the yield of 2a (Table 3, entry
4). Increasing the ratio of catalyst to cocatalyst from 1:1 to 1:2 increased the yield of styrene
carbonate (Table 3, entries 5 and 6). At least 1 mol% PyrH and 1 mol% (n–Bu)4NBr should be used
when screening epoxides at 25 °C and 1 bar CO2 for 24 h.
229
Table 4. Synthesis of 2a–n from epoxides 1a–n using PyrH and (n–Bu)4NBr.[a]
230
Entry
substrate
Conversion[c] [%]
Yield[d] [%]
TON[e]
TOF[f] [h–1]
1
2
3
4
5
6
7
8
9
1a
1b
1c
1d
1e
1f
1g
1h
1i
1j
1k
1l
1m
1n
99
99
99
99
67
56
98
94
94
99
99
99
99
95
97
93
99
98
65
53
95
92
90
98
97
96
97
91
99
99
99
99
67
56
98
94
94
99
99
99
99
95
4.1
4.1
4.1
4.1
2.8
2.3
4.1
3.9
3.9
8.3
8.3
4.1
4.1
4.0
10[b]
11[b]
12
13
14
231
232
233
234
235
[a] Reaction conditions: epoxides (10 mmol), PyrH (0.1 mmol, 1.0 mol%), (n–Bu)4NBr (0.1 mmol,
1.0 mol%), 25 °C, 1 bar CO2 (balloon), and 24 h. [b] 12h. [c] Conversion was determined by H
1
NMR spectroscopy of an aliquot of the reaction mixture after 24 h (see the Supporting Information).
[d] isolated yield. [e] Turnover number = (mol of 1a consumed)/(mol of PyrH used). [f] Turnover
frequency = TON/h.
236
237
We next investigated the substrate scope using 1.0 mol% PyrH and 1.0 mol% (n–Bu)4NBr at 25 °C
and 1 bar CO2 for 24 h, and the results are shown in Table 4. The tested substrates were fourteen
13
This article is protected by copyright. All rights reserved.

10.1002/cssc.201901661
ChemSusChem
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
terminal epoxides, namely, styrene oxide (1a), propylene oxide (1b), 1,2–epoxybutane (1c), 1,2–
epoxyhexane (1d), 1,2–epoxydecane (1e), 1,2–epoxydodecane (1f), 1,2–epoxy–3–methoxypropane
(1g), 1,2–epoxy–3–phenoxypropane (1h), tert–butyl glycidyl ether (1i), epichlorohydrin (1j),
epibromohydrin (1k), 2-(4-fluorophenyl)oxirane (1l), 2-(4-chlorophenyl)oxirane (1m), and 2-(4-
bromophenyl)oxirane (1n). In all cases except for 1e and 1f, excellent isolated yields (> 90%) and
conversions (> 94%) of these substrates into the corresponding cyclic carbonates were obtained.
The fact that epoxides 1a–1d showed complete conversion within 24 h means that the chain length
and steric hindrance from the substituents on the epoxides did not affect the catalytic activity (Table
4, entries 1–4). However, long and straight octyl and decyl substituents on the epoxides reduced the
conversion (Table 4, entries 5 and 6). Although PyrH/(n–Bu)4NBr still showed high activity for 1e–
1g, the presence of a heteroatom in the substituent on the epoxide and steric hindrance from the
substituents resulted in a slight decrease in activity because heteroatoms in the epoxide substrate
may bind to the aluminum center (Table 4, entries 7–9). As expected, very reactive epichlorohydrin
(1j) and epibromohydrin (1k) showed the complete conversion within 12 h (Table 4, entries 10 and
11). In addition, other epoxides containing halogens and aromatic halogens showed the excellent
catalytic activities (Table 4, entries 10–14).
14
This article is protected by copyright. All rights reserved.

10.1002/cssc.201901661
ChemSusChem
O
PyrH
1.0 mol%
O
R3
1.0 mol% (n-Bu)4NBr
CO2
O
O
+
R2
R1
R3
75 oC, 36 h
1 bar
(balloon)
1o-t
R1
R2
2o-t
O
O
O
O
O
O
O
O
O
O
2p
2o
2q
conversion = 81 %
Yield = 76 %
conversion = 96%
Yield = 92%
conversion = 81%
Yield = 80%
selectivity = 99%
TON = 81, TOF = 2.25 h-1
selectivity = 99%
TON = 96, TOF = 2.67 h-1
selectivity = 99%
TON = 81, TOF = 2.25 h-1
cis/trans = 99:1
cis/trans = 99:1
cis/trans = 99:1
O
O
O
O
O
O
O
O
O
Ph
Ph
2s
2t
conversion = 85%
Yield = 79%
conversion = 27%
Yield = 25%
2r
trace
selectivity = 99%
TON = 85, TOF = 2.36 h-1
selectivity = 100 %
TON = 27, TOF = 0.75 h-1
cis/trans = 0:100
cis/trans = 0:100
254
255
256
Figure 4. Synthesis of cyclic carbonates 2o–t from epoxides 1o–t and CO2 catalyzed by PyrH and
(n–Bu)4NBr.
257
258
259
260
261
262
263
264
We also investigated the synthesis of more challenging cyclic carbonates using internal epoxides
such as cis–cyclopentene oxide (1o), cis–3,4–epoxytetrahydrofuran (1p), cis–cyclohexene oxide
(1q), cis–cyclooctene oxide (1r), trans–2,3–butylene oxide (1s), and trans–stilbene oxide(1t) to
elucidate the mechanism of this reaction, and the results are shown in Figure 4. The coupling
reactions were performed at 75 °C and 1 bar CO2 over 36 h using 1.0 mol% PyrH/(n–Bu)4NBr.
Internal epoxides 1o–t were all converted to the corresponding cyclic carbonates without any
polymeric side products (selectivity > 99%). Epoxide 1o, with a five–membered ring, was more
reactive than epoxides with heteroatom (1p) and six–membered ring (2q). Eight–membered ring
15
This article is protected by copyright. All rights reserved.

10.1002/cssc.201901661
ChemSusChem
265
266
267
268
269
270
271
272
273
system (1r) did not show any catalytic activity. The steric hindrance of the substituents from methyl
(1s) to phenyl (1t) resulted in a dramatic decrease in activity. We found the retention of the
stereochemistry of all starting epoxides were observed. As expected, the cyclic carbonates obtained
from 1,2–disubstituted epoxides showed configurational retention from two consecutive SN2
reactions. This means that two stereochemical inversions occur at the carbon atom of the epoxide
during the reaction. Thus, a plausible mechanism for the synthesis of the cyclic carbonates from
epoxides and CO2 by using PyrH in the presence of (n–Bu)4NBr as a cocatalyst is shown in Figure
5. This mechanism is similar to the mechanism previously proposed for the synthesis of cyclic
carbonates using other catalyst systems.
274
275
-0.917 ppm
-0.934 ppm
276
277
(a)
(b)
278
279
280
Figure 5. (a) A plausible mechanism for the synthesis of cyclic carbonates from epoxides and CO2
by using PyrH in the presence of (n–Bu)4NBr. (b) The chemical shift of Al–Me in PyrH in the 1H
NMR spectrum before (down) and after (up) adding 2 equivalents of styrene oxide in CDCl3.
16
This article is protected by copyright. All rights reserved.

10.1002/cssc.201901661
ChemSusChem
281
282
283
284
285
286
287
288
289
The dimeric five–coordinate complex PyrH becomes monomeric four–coordinate intermediate I,
and then the epoxide attaches to the Al atom of intermediate I, and resulting five–coordinate
complex II is activated by the bromide anion of the cocatalyst. The nucleophilic ring opening of the
epoxide gives carbonato species III, which is converted into coordinated Al complex IV by the
insertion of CO2. Finally, cyclic carbonate as the final product and regenerated intermediate I are
1
produced. The H NMR spectra of PyrH with and without styrene oxide clearly confirm the
existence of intermediate I. A clear downfield shift in the signal of Al–Me of PyrH from –0.934 ppm
to –0.917 ppm was observed, indicating that the epoxide binds to the Al metal center to generate
intermediate I.
290
291
CONCLUSION
In conclusion, we developed one of the most effective aluminum catalysts to date for the
292
293
294
295
296
297
298
299
300
301
302
303
304
305
generation of cyclic carbonates via the coupling of epoxides and CO2 under ambient temperature
and CO2 pressure. Aluminum complexes XR (X = Pyr, Fur, Thio; R = H, Me) chelated by
substituted ethanolateamine ligands bearing heterocyclic pendant moieties such as pyridin–2–yl
(Pyr), furan–2–yl (Fur), and thiophen–2–yl (Thio) on the nitrogen atom were rationally designed.
Among these complexes, the solid-state structure of ThioH was determined by single–crystal X–ray
diffraction analysis, and pseudo–C2 symmetric dimeric ThioH contains a four–membered Al2O2 ring
with a C2 axis at the center of the ring. All the aluminum complexes were used as catalysts for the
coupling of CO2 with epoxides, and among the eleven catalysts, complex PyrH showed the highest
activity for the synthesis of styrene carbonate with a TON of 99 and TOF of 4.1 h–1 at room
temperature and 1 bar CO2. The best catalyst, PyrH, showed 3.2 times higher activity than NMe2H
with dimethylamino pendant groups, 4.3 times higher activity than OMeH with methoxy pendant
groups and 14.1 times higher activity than HH without nitrogen substituents with fixed catalyst and
(n–Bu)4NBr loading (both at 1.0 mol%), a reaction temperature of 25 °C, 1 bar CO2, and a reaction
time of 24 h. In addition, the PyrH/(n–Bu)4NBr catalytic system was highly effective for forming
17
This article is protected by copyright. All rights reserved.

10.1002/cssc.201901661
ChemSusChem
306
307
308
cyclic carbonates with a wide range of terminal and internal epoxides. In particular, the cyclic
carbonates obtained from 1,2–disubstituted epoxides showed configurational retention indicative of
two consecutive SN2 reactions.
309
310
EXPERIMENTAL SECTION
General Considerations. All manipulations were performed under a dry, inert atmosphere in a
311
312
313
314
315
316
317
318
319
glovebox or with a dual–manifold Schlenk line using the appropriate techniques for conducting air-
sensitive reactions under a nitrogen atmosphere.[13] Nitrogen was deoxygenated with an activated
Cu catalyst and dried with Drierite.[14] All chemicals were purchased from commercial sources
(purity > 95%) and used as received unless otherwise indicated. Toluene and n–hexane were purified
by a Grubbs solvent purification system under a nitrogen atmosphere and stored over activated
molecular sieves (4 Å).[15] Carbon dioxide (99.999%) was used as received without further
purification. All epoxides were purified by treatment with calcium hydride to remove residual water.
CDCl3 was dried with activated molecular sieves and used after vacuum transfer to a Schlenk tube
equipped with a J. Young valve.
320
321
322
323
324
Measurements. 1H NMR and 13C NMR spectra were recorded at ambient temperature with a Bruker
DPX–500 MHz NMR spectrometer with standard parameters. All chemical shifts are reported in δ
1
13
units referenced to the residual CDCl3 (δ 7.24 for H NMR; δ 77.00 for C NMR). Elemental
analyses were performed with an EA 1110–FISONS analyzer. High–resolution mass spectrometry
(HRMS) data were acquired on a high–resolution Q–TOF mass spectrometer (ionization mode: ESI).
325
326
327
328
329
Synthesis.
methylmethanamine,[17]
(dimethylamino)ethyl)(methyl)amino)–2–methylpropan–2–ol,[18]
Ligands
N–methyl–1–(pyridin–2–yl)methanamine,[16]
1–(furan–2–yl)–N–
N–methyl–1–(thiophen–2–yl)methanamine,[17]
1–((2–
and
2–methyl–1–
[20]
(methylamino)propan–2–ol[19] and aluminum complexes NMe2H,[7] OMeH,[7] and HMe
were
prepared by literature procedures.
18
This article is protected by copyright. All rights reserved.

10.1002/cssc.201901661
ChemSusChem
330
331
332
333
334
335
336
337
338
339
Synthesis of 2–methyl–1–((pyridin–2–ylmethyl)amino)propan–2–ol. Isobutylene oxide (0.72 g,
10 mmol) and pyridin–2–ylmethaneamine (1.08 g, 10 mmol) were added to a 25-mL screw cap vial
containing a stirring bar. The vial was tightly sealed with Teflon tape and paraffin film. The mixture
was maintained at room temperature overnight and was then heated for 3 days at 55 °C. The removal
of the volatile compounds at reduced pressure gave the desired product as a colorless oil (80%, 1.44
g). 1H NMR (CDCl3): δ 8.47 (m, 1H, pyridine–H), 7.57 (m, 1H, pyridine–H), 7.20 (m, 1H, pyridine–
H), 7.08 (m, 1H, pyridine–H), 3.88 (s, 2H, –CMe2CH2NHCH2–), 2.51 (s, 2H, –CMe2CH2NHCH2–),
1.11 (s, 6H, –CMe2CH2NHCH2–). 13C NMR (CDCl3): δ 159.6, 149.1, 136.4, 122.1, 121.9 (pyridine),
69.28 (–CMe2CH2NHCH2–), 59.92 (–CMe2CH2NHCH2–), 55.52 (–CMe2CH2NHCH2–), 27.27 (–
CMe2CH2NHCH2–). HRMS m/z calcd for [C10H16N2O + H] 181.1341. Found: 181.1335.
340
341
342
343
344
345
346
347
348
Synthesis of 1–((furan–2–ylmethyl)amino)–2–methylpropan–2–ol. In a manner analogous to
that used in the synthesis of 2–methyl–1–((pyridin–2–ylmethyl)amino)propan–2–ol, 1–((furan–2–
ylmethyl)amino)–2–methylpropan–2–ol was prepared from furan–2–ylmethanamine (0.97 g, 10
1
mmol) and isobutylene oxide (0.72 g, 10 mmol) in a yield of 83% (1.40 g). H NMR (CDCl3): δ
7.26 (q, 1H, J = 1.0 Hz, furan–H), 6.21 (q, 1H, J = 2.0 Hz, furan–H), 6.07 (dd, 1H, J1 = 4.0 Hz, J2
= 0.5 Hz, furan–H), 3.72 (s, 2H, –CMe2CH2NHCH2–), 2.44 (s, 2H, –CMe2CH2NHCH2–), 1.08 (s,
6H, –CMe2CH2NHCH2–). 13C NMR (CDCl3): δ 153.9, 141.5, 109.9, 106.5 (furan), 69.18 (–
CMe2CH2NHCH2–), 59.33 (–CMe2CH2NHCH2–), 46.58 (–CMe2CH2NHCH2–), 27.13 (–
CMe2CH2NHCH2–). HRMS m/z calcd for [C9H15NO2 + H] 170.1181. Found: 170.1176.
349
350
351
352
353
354
Synthesis of 2–methyl–1–((thiophen–2–ylmethyl)amino)propan–2–ol. In a manner analogous to
that used in the synthesis of 2–methyl–1–((pyridin–2–ylmethyl)amino)propan–2–ol, 2–methyl–1–
((thiophen–2–ylmethyl)amino)propan–2–ol was prepared from thiophen–2–ylmethanamine (1.13 g,
10 mmol) and isobutylene oxide (0.72 g, 10 mmol) in a yield of 83% (1.54 g). 1H NMR (CDCl3): δ
7.19 (dd, 1H, J1 = 6.0 Hz, J2 = 1.5 Hz, thiophene–H), 6.93 (m, 1H, thiophene–H), 6.90 (m, 1H,
thiophene–H), 4.02 (d, 2H, J = 1.0 Hz,–CMe2CH2NHCH2–), 2.57 (s, 2H, –CMe2CH2NHCH2–),
19
This article is protected by copyright. All rights reserved.

10.1002/cssc.201901661
ChemSusChem
13
355
356
357
1.16 (s, 6H, –CMe2CH2NHCH2–). C NMR (CDCl3): δ 144.2, 126.5, 124.6, 124.3 (thiophene),
69.30 (–CMe2CH2NHCH2–), 59.34 (–CMe2CH2NHCH2–), 48.87 (–CMe2CH2NHCH2–), 27.25 (–
CMe2CH2NHCH2–). HRMS m/z calcd for [C9H15NOS + H] 186.0953. Found: 186.0948.
358
359
360
361
362
363
364
365
366
367
Synthesis of 2–methyl–1–(methyl(pyridin–2–ylmethyl)amino)propan–2–ol. In a manner
analogous to that used in the synthesis of 2–methyl–1–((pyridin–2–ylmethyl)amino)propan–2–ol,
2–methyl–1–(methyl(pyridin–2–ylmethyl)amino)propan–2–ol was prepared from N–methyl–1–
(pyridin–2–yl)methanamine (1.22 g, 10 mmol) and isobutylene oxide (0.72 g, 10 mmol) in a yield
of 90% (1.75 g). 1H NMR (CDCl3): δ 8.51 (m, 1H, pyridine–H), 7.62 (m, 1H, pyridine–H), 7.28 (m,
1H, pyridine–H), 7.12 (m, 1H, pyridine–H), 3.80 (s, 2H, –CMe2CH2NMeCH2–), 2.50 (s, 2H, –
CMe2CH2NMeCH2–), 2.39 (s, 3H, –CMe2CH2NMeCH2–), 1.14 (s, 6H, –CMe2CH2NMeCH2–). 13C
NMR (CDCl3): δ 159.5, 149.1, 136.5, 122.6, 122.1 (pyridine), 70.28 (–CMe2CH2NMeCH2–), 68.47
(–CMe2CH2NMeCH2–), 65.51 (–CMe2CH2NMeCH2–), 45.61 (–CMe2CH2NMeCH2–), 27.84 (–
CMe2CH2NMeCH2–). HRMS m/z calcd for [C11H18N2O + H] 195.1497. Found: 195.1492.
368
369
370
371
372
373
374
375
376
377
Synthesis of 1–((furan–2–ylmethyl)(methyl)amino)–2–methylpropan–2–ol. In a manner
analogous to that used in the synthesis of 2–methyl–1–((pyridin–2–ylmethyl)amino)propan–2–ol,
1–((furan–2–ylmethyl)(methyl)amino)–2–methylpropan–2–ol was prepared from 1–(furan–2–yl)–
N–methylmethanamine (1.11 g, 10 mmol) and isobutylene oxide (0.72 g, 10 mmol) in a yield of 94%
(1.72 g). 1H NMR (CDCl3): δ 7.35 (q, 1H, J = 1.0 Hz, furan–H), 6.29 (m, 1H, furan–H), 6.17 (m,
1H, furan–H), 3.65 (s, 2H, –CMe2CH2NMeCH2–), 3.20 (s, 1H, HOCMe2CH2NMeCH2–), 2.42 (s,
2H, –CMe2CH2NMeCH2–), 2.38 (s, 3H, –CMe2CH2NMeCH2–), 1.15 (s, 6H, –CMe2CH2NMeCH2–).
13C NMR (CDCl3): δ 152.9, 142.2, 110.1, 108.5 (furan), 70.12 (–CMe2CH2NMeCH2–), 67.18 (–
CMe2CH2NMeCH2–), 56.18 (–CMe2CH2NMeCH2–), 45.26 (–CMe2CH2NHCH2–), 27.97 (–
CMe2CH2NHCH2–). HRMS m/z calcd for [C10H17NO2 + H] 184.1338. Found: 184.1332.
378
Synthesis of 2–methyl–1–(methyl(thiophen–2–ylmethyl)amino)propan–2–ol. In a manner
20
This article is protected by copyright. All rights reserved.

10.1002/cssc.201901661
ChemSusChem
379
380
381
382
383
384
385
386
387
analogous to that used in the synthesis of 2–methyl–1–((pyridin–2–ylmethyl)amino)propan–2–ol,
2–methyl–1–(methyl(thiophen–2–ylmethyl)amino)propan–2–ol was prepared from N–methyl–1–
(thiophen–2–yl)methanamine (1.27 g, 10 mmol) and isobutylene oxide (0.72 g, 10 mmol) in a yield
of 95% (1.89 g). 1H NMR (CDCl3): δ 7.20 (m, 1H, thiophene–H), 6.93 (m, 1H, thiophene–H), 6.87
(m, 1H, thiophene–H), 3.83 (s, 2H, –CMe2CH2NMeCH2–), 2.44 (s, 2H, CMe2CH2NMeCH2), 2.36
13
(s, 3H, –CMe2CH2NMeCH2–), 1.17 (s, 6H, –CMe2CH2NMeCH2–). C NMR (CDCl3): δ 142.6,
126.4, 125.7, 124.7 (thiophene), 70.03 (–CMe2CH2NMeCH2–), 67.43 (–CMe2CH2NMeCH2–),
58.60 (–CMe2CH2NMeCH2–), 44.68 (–CMe2CH2NHCH2–), 27.79 (–CMe2CH2NHCH2–). HRMS
m/z calcd for [C10H17NOS + H] 200.1109. Found: 200.1104.
388
389
390
391
392
393
394
395
396
397
398
399
Synthesis of [Me2Al(OCMe2CH2NHCH2(C5H4N))]2 (PyrH). AlMe3 (1.0 mL of 2.0 M solution in
toluene, 2.0 mmol) was added to a stirred solution of 2–methyl–1–((pyridin–2–
ylmethyl)amino)propan–2–ol (0.36 g, 2.0 mmol) in toluene at 0 °C. The mixture was warmed to
room temperature and stirred overnight. The residue, which was obtained by removing the solvent
under vacuum, was washed with n–hexane and recrystallized from toluene. The desired product,
PyrH, was isolated as colorless crystals after the solution had been stored at –20 °C in a freezer for
a few days (80%, 0.47 g). 1H NMR (CDCl3): δ 8.56 (m, 2H, pyridine–H), 7.62 (m, 2H, pyridine–
H), 7.17 (m, 4H, pyridine–H), 3.80 (d, J = 6.5 Hz, 4H, pyridine–CH2N–), 2.37 (d, 4H, J = 8.6 Hz,
13
–CH2CMe2–), 1.24 (s, 12H, –CMe2–), –0.93 (s, 12H, Al–Me). C NMR (CDCl3): δ 157.3, 149.7,
136.5, 122.8, 122.3 (pyridine), 70.36 (–CMe2O–), 59.83 (pyridine–CH2N–), 52.38 (–CH2CMe2–),
27.64 (–CMe2O–), –5.98 (Al–Me). Anal. Calcd for C24H42N4O2Al2: C, 61.00; H, 8.96; N, 11.86.
Found: C, 60.97; H, 8.94; N, 12.05.
400
401
402
403
Synthesis of [Me2Al(OCMe2CH2N(Me)CH2(C5H4N))]2 (PyrMe). In a manner analogous to that
used in the synthesis of PyrH, desired product PyrMe was prepared as colorless crystals from a
solution of AlMe3 (1.0 mL of 2.0 M solution in toluene, 2.0 mmol) and 2–methyl–1–
(methyl(pyridin–2–ylmethyl)amino)propan–2–ol (0.39 g, 2.0 mmol) in toluene in a yield of 75%
21
This article is protected by copyright. All rights reserved.

10.1002/cssc.201901661
ChemSusChem
404
405
406
407
408
409
(0.38 g). 1H NMR (CDCl3): δ 8.49 (d, 2H, J = 4.3 Hz, pyridine–H), 7.56 (m, 2H, pyridine–H), 7.23
(d, 2H, J = 7.7 Hz, pyridine–H), 7.10 (m, 2H, pyridine–H), 3.77 (s, 4H, pyridine–CH2N–), 2.59 (s,
13
4H, –CH2CMe2–), 2.19 (s, 6H, N–Me), 1.23 (s, 12H, –CMe2–), –0.87 (s, 12H, Al–Me). C NMR
(CDCl3): δ 156.9, 149.2, 136.2, 124.5, 122.3 (pyridine), 72.98 (–CMe2O–), 67.20 (pyridine–CH2N–),
64.41 (–CH2CMe2–), 43.89 (N–Me), 30.68 (–CMe2O–), –5.95 (Al–Me). Anal. Calcd for
C26H46N4O2Al2: C, 62.38; H, 9.26; N, 11.19. Found: C, Found: C, 62.37; H, 9.32; N, 11.07.
410
411
412
413
414
415
416
417
418
Synthesis of [Me2Al(OCMe2CH2NHCH2(C4H3O))]2 (FurH). In a manner analogous to that used
in the synthesis of PyrH, desired product FurH was prepared as colorless crystals from a solution of
AlMe3 (1.0 mL of 2.0 M solution in toluene, 2.0 mmol) and 1–((furan–2–ylmethyl)amino)–2–
methylpropan–2–ol (0.34 g, 2.0 mmol) in toluene in a yield of 87% (0.39 g). 1H NMR (CDCl3): δ
7.37 (m, 2H, furan–H), 6.32 (m, 2H, furan–H), 6.19 (d, 2H, J = 3.2 Hz, furan–H), 3.72 (d, 4H, J =
7 Hz, furan–CH2N–), 2.40 (d, 4H, J = 8.8 Hz, –CH2CMe2–), 2.09 (t, 2H, J = 7.5 Hz, –NH–), 1.21
(s, 12H, –CMe2–), –0.96 (s, 12H, –Al–Me). 13C NMR (CDCl3): δ 151.8, 142.5, 110.3, 108.3 (furan),
70.39 (–CMe2O–), 59.42 (furan–CH2NH–), 44.16 (–CH2CMe2–), 27.63 (–CMe2O–), –6.16 (Al–Me).
Anal. Calcd for C22H40N2O4Al2: C, 58.65; H, 8.95; N 6.22. Found: C, 58.42; H, 9.08; N, 6.17.
419
420
421
422
423
424
425
426
427
428
Synthesis of [Me2Al(OCMe2CH2N(Me)CH2(C4H3O))]2 (FurMe). In a manner analogous to that
used in the synthesis of PyrH, desired product FurMe was prepared as colorless crystals from a
solution of AlMe3 (1.0 mL of 2.0 M solution in toluene, 2.0 mmol) and 1–((furan–2–
ylmethyl)(methyl)amino)–2–methylpropan–2–ol (0.37 g, 2.0 mmol) in toluene in a yield of 90%
(0.43 g). 1H NMR (CDCl3): δ 7.39 (m, 2H, furan–H), 6.34 (m, 2H, furan–H), 6.23 (d, 2H, J = 3.1
Hz, furan–H), 3.73 (s, 4H, furan–CH2N–), 2.51 (s, 4H, –CH2CMe2–), 2.24 (s, 6H, N–Me), 1.32 (s,
13
12H, –CMe2–), –0.83 (s, 12H, Al–Me). C NMR (CDCl3): δ 150.0, 142.5, 110.6, 110.2 (furan),
70.83 (–CMe2O–), 65.90 (furan–CH2N–), 53.48 (–CH2CMe2–), 43.27 (N–Me), 31.58 (–CMe2O–),
–6.22 (Al–Me). Anal. Calcd for C24H44N2O4Al2: C, 60.23; H, 9.27; N, 5.85. Found: C, 60.34; H,
9.13; N, 5.81.
22
This article is protected by copyright. All rights reserved.

10.1002/cssc.201901661
ChemSusChem
429
430
431
432
433
434
435
436
437
438
Synthesis of [Me2Al(OCMe2CH2NHCH2(C4H3S))]2 (ThioH). In a manner analogous to that used
in the synthesis of PyrH, desired product ThioH was prepared as colorless crystals from a solution
of AlMe3 (1.0 mL of 2.0 M solution in toluene, 2.0 mmol) and 2–methyl–1–((thiophen–2–
1
ylmethyl)amino)propan–2–ol (0.37 g, 2.0 mmol) in toluene in a yield of 89% (0.43 g). H NMR
(CDCl3): δ 7.24 (m, thiophene–H), 6.98 (m, 2H, thiophene–H), 6.92 (d, 2H, J = 2.8 Hz, thiophene–
H), 3.92 (d, 4H, J = 6.9 Hz, thiophene–CH2N–), 2.47 (d, 4H, J = 8.8 Hz, –CH2CMe2–), 1.82 (d, 2H,
13
J = 7.3 Hz, –NH–), 1.22 (s, 12H, –CMe2–), –0.91 (s, 12H, Al–Me). C NMR (CDCl3): δ 140.9,
127.2, 126.7, 125.4 (thiophene), 70.46 (–CMe2O–), 59.19 (thiophene–CH2N–), 46.01 (–CH2CMe2–),
27.65 (–CMe2O–), –5.91 (Al–Me). Anal. Calcd for C22H40N2O2S2Al2: C, 54.75; H, 8.35; N, 5.80.
Found: C, 54.61; H, 8.51; N, 5.84.
439
440
441
442
443
444
445
446
447
448
Synthesis of [Me2Al(OCMe2CH2N(Me)CH2(C4H3S))]2 (ThioMe). In a manner analogous to that
used in the synthesis of PyrH, desired product ThioMe was prepared as colorless crystals from a
solution of AlMe3 (1.0 mL of 2.0 M solution in toluene, 2.0 mmol) and 2–methyl–1–
(methyl(thiophen–2–ylmethyl)amino)propan–2–ol (0.40 g, 2.0 mmol) in toluene in a yield of 88%
(0.45 g). 1H NMR (CDCl3, ppm): δ 7.26 (m, thiophene–H), 6.98 (m, 2H, thiophene–H), 6.91 (d, 2H,
J = 3.0 Hz, thiophene–H), 3.94 (s, 4H, thiophene–CH2N–), 2.54 (s, 4H, –CH2CMe2–), 2.27 (s, 6H,
–NMe–), 1.34 (s, 12H, –CMe2–), –0.79 (s, 12H, Al–Me). 13C NMR (CDCl3): δ 136.9, 128.6, 126.7,
125.7 (thiophene), 71.92 (–CMe2O–), 66.11 (thiophene–NMe–), 56.23 (thiophene–CH2N–), 43.14
(–CH2CMe2–), 31.20 (–CMe2O–), –6.02 (Al–Me). Anal. Calcd for C24H44N2O2S2Al2: C, 56.44; H,
8.68; N, 5.49. Found: C, 56.17; H, 8.68; N, 5.37.
449
450
451
452
453
Synthesis of [Me2Al(OCMe2CH2N(Me)CH2CH2NMe2)]2 (NMe2Me). In a manner analogous to
that used in the synthesis of PyrH, desired product NMe2Me was prepared as colorless crystals from
a solution of AlMe3 (1.0 mL of 2.0 M solution in toluene, 2.0 mmol) and 1–((2–
(dimethylamino)ethyl)(methyl)amino)–2–methylpropan–2–ol (0.35 g, 2.0 mmol) in toluene in a
1
yield of 85% (0.39 g). H NMR (CDCl3): δ 2.68 (t, 4H, J = 7.9 Hz, Me2NCH2–) 2.49 (s, 4H, –
23
This article is protected by copyright. All rights reserved.

10.1002/cssc.201901661
ChemSusChem
454
455
456
457
458
CH2CMe2–), 2.43 (t, 4H, J = 5.5 Hz, Me2NCH2CH2–), 2.31 (s, 6H, –NMe–), 1.29 (s, 12H, –CMe2–),
13
–0.87 (s, 12H, Al–Me). C NMR (CDCl3): δ 71.39 (–NCH2CMe2–), 68.07 (–NCH2CH2NMe2),
56.67 (–NCH2CH2NMe2), 54.28 (–NCH2CMe2–), 45.92 (–NMe2), 43.77 (–NMe–), 31.27 (–
NCH2CMe2–), −5.91 (Al–Me). Anal. Calcd for C22H54N4O2Al2: C, 57.36; H, 11.82; N, 12.16. Found:
C, 57.30; H, 11.97; N, 12.01.
459
460
461
462
463
464
465
466
Synthesis of [Me2Al(OCMe2CH2NHMe)]2 (HH). In a manner analogous to that used in the
synthesis of PyrH, desired product HH was prepared as colorless crystals from a solution of AlMe3
(1.0 mL of 2.0 M solution in toluene, 2.0 mmol) and 1–((2–methoxyethyl)(methyl)amino)–2–
1
methylpropan–2–ol in toluene (0.21 g, 2.0 mmol) in toluene in a yield of 78% (0.25 g). H NMR
(CDCl3): δ 2.42 (d, 4H, J = 8.9 Hz,–CH2N–), 2.35 (d, 6H, J = 6.2 Hz, N–Me), 1.75 (t, 2H, J = 6.4
Hz, N–H), 1.23 (s, 12H, –CMe2–), –1.01 (s, 12H, Al–Me). 13C NMR (CDCl3): δ 70.06 (–CMe2O–),
62.45 (–CH2N–), 34.37 (N–Me), 27.68 (–CMe2O–), –4.97 (Al–Me). Anal. Calcd for C14H36N2O2Al2:
C, 52.81; H, 11.40; N, 8.80. Found: C, 52.66; H, 11.51; N, 8.62.
467
468
469
470
471
472
473
General procedure for catalyst screening under ambient conditions. Styrene oxide (1a, 10
mmol), catalyst (0.1 mmol), and (n–Bu)4NBr (32 mg, 0.1 mmol) were charged in a 20-mL round
bottomed flask with a magnetic stirring bar in a glovebox. A rubber balloon containing
approximately 2 L of CO2 was connected to the flask, and then the reaction vessel was well sealed
with Parafilm®. The reaction vessel was stirred at 25 °C for 24 h. After the desired time was reached,
an aliquot of the reaction mixture was transferred to an NMR cell, and the conversion of styrene
oxide (1a) into styrene carbonate (2a) was determined by 1H NMR spectroscopy.
474
475
476
477
General procedure for the synthesis of terminal cyclic carbonates 2a–2g under ambient
conditions. Epoxides 1a–1g (10 mmol), PyrH (47 mg, 0.1 mmol), and (n–Bu)4NBr (32 mg, 0.1
mmol) were charged in a 20 mL round bottomed flask with a magnetic stirring bar in glovebox. A
rubber balloon containing approximately 2 L CO2 was connected to the flask, and then the reaction
24
This article is protected by copyright. All rights reserved.

10.1002/cssc.201901661
ChemSusChem
478
479
480
481
vessel was well sealed with Parafilm®. The reaction vessel was stirred at 25 °C for 24 h. After the
desired time was reached, an aliquot of the reaction mixture was transferred to an NMR cell, and
1
the conversion of epoxides (1a–1g) into cyclic carbonate (2a–2g) was determined by H NMR
spectroscopy. Cyclic carbonates 2a–2g were purified by column chromatography.
482
483
484
485
486
487
488
General procedure for the synthesis of 2h and 2i at 1 bar pressure. Epoxides 1h–1i (10 mmol),
PyrH (47 mg, 0.1 mmol), and (n–Bu)4NBr (32 mg, 0.1 mmol) were charged in a 20 mL round
bottomed flask with a magnetic stirring bar in glovebox. A rubber balloon containing approximately
2 L CO2 was connected to the flask, and then the reaction vessel was well sealed with Parafilm®.
The reaction vessel was stirred at 75 °C for 36 h. After the desired reaction time, an aliquot of the
reaction mixture was transferred to an NMR tube, and the conversion of the epoxide (1h–1i) into
the cyclic carbonate (2h–2i) was determined by 1H NMR spectroscopy.
489
490
491
492
493
494
495
496
497
498
499
X–ray crystallographic structure determination. The crystallographic measurements were
performed at 100(2) K for ThioH using a Bruker Apex II diffractometer with Mo Kα (λ = 0.71073
Å) radiation. Specimens of suitable quality and size were selected, mounted, and centered on the
X–ray beam using a video camera. The structures were solved by direct methods and refined by
full–matrix least–squares methods using the SHELXTL program package with anisotropic thermal
parameters for all non–hydrogen atoms, resulting in the X–ray crystallographic data of ThioH in
CIF format. Final refinement based on the reflections (I > 2σ(I)) converged at R1 = 0.0423, wR2 =
0.1103, and GOF = 1.079. Further details and selected bond lengths and angles are given in Tables
S1and S2 (see the Supporting Information). CCDC 1893339 (ThioH) contains the supplementary
crystallographic data for this paper. These data can be obtained free of charge from the Cambridge
Crystallographic Data Centre.
500
Acknowledgments
25
This article is protected by copyright. All rights reserved.

10.1002/cssc.201901661
ChemSusChem
501
502
503
504
This work was financially supported by the National Research Foundation of Korea (NRF) grant
funded by the Korea government (No. 2018R1A2B6004037) and the Korea Institute of Energy
Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry and Energy
(MOTIE) of the Republic of Korea (No. 20172010202000).
505
506
Conflict of interest
The authors declare no competing financial interests.
507
508
Keywords: aluminum catalysts · cyclic carbonates · room temperature · atmospheric CO2 pressure
· epoxides
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
[1] a) F. D. Bobbink, A. P. van Muyden, P. J. Dyson, Chem. Commun. 2019, 55, 1360–1373; b) Q.–
W. Song, Z.–H. Zhou, L.–N. He, Green. Chem. 2017, 19, 3707–3728.
[2] Q. Liu, L. Wu, R. Jackstell, M. Beller, Nature Comm. 2015, 6, 5933.
[3] Selected recent reviews: a) H. Büttner, L. Longwitz, J. Steinbauer, C. Wulf, T. Werner, Top. Curr.
Chem. 2017, 375, 50; b) M. Alves, B. Grignard, R. Mereau, C. Jerome, T. Tassaing, C. Detrembleur,
Catal. Sci. Technol. 2017, 7, 2651–2684; c) M. Cokoja, M. E. Wilhelm, M. H. Anthofer, W. A.
Herrmann, F. E. Kühn, ChemSusChem 2015, 8, 2436–2454; d) G. Fiorani, W. Guo, A. W. Kleij,
Green Chem. 2015, 17, 1375–1389; e) C. Martín, G. Fiorani, A. W. Kleij, ACS Catal. 2015, 5, 1353–
1370; f) J. W. Comerford, I. D. V. Ingram, M. North, X. Wu, Green Chem. 2015, 17, 1966–1987.
[4] M. Mikkelsen, M. Jørgensen, F. C. Krebs, Energy Environ. Sci. 2010, 3, 43–81.
[5] selected reviews a) R. R. Shaikh, S. Pornpraprom, V. D'Elia, ACS Catal. 2018, 8, 419–450; b) F.
D. Monica, A. Buonerba, C. Capacchione, Adv. Synth. Catal. 2019, 361, 265–282.
[6] a) M. North, S. C. Z. Quek, N. E. Pridmore, A. C. Whitwood, X. Wu, ACS Catal. 2015, 5, 3398–
3402; b) Y. R. Yepes, C. Quintero, D. O. Meléndez, C. G. Daniliuc, J. Martínez, R. S. Rojas,
Organometallics 2019, 38, 469–478; c) J. Martínez, J. A. Castro–Osma, C. Alonso–Moreno, A.
26
This article is protected by copyright. All rights reserved.

10.1002/cssc.201901661
ChemSusChem
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
Rodríguez–Diéguez, M. North, A. Otero, A. Lara–Sánchez, ChemSusChem 2017, 10, 1175–1185;
d) J. Martínez, J. A. Castro–Osma, A. Earlam, C. Alonso–Moreno, A. Otero, A. Lara–Sánchez, M.
North, A. Rodríguez–Diéguez, Chem. Eur. J. 2015, 21, 9850–9862; e) W. Clegg, R. W. Harrington,
M. North, R. Pasquale, Chem. Eur. J. 2010, 16, 6828–6843; f) J. A. Castro–Osma, C. Alonso–
Moreno, A. Lara–Sánchez, J. Martínez, M. North, A. Otero, Catal. Sci. Technol. 2014, 4, 1674–
1684; g) M. A. Gaoan, F. de la Cruz–Martínez, J. Fernández–Baeza, L. F. Sánchez–Barba, C.
Alonso–Moreno, A. M. Rodríguez, A. Rodríguez–Diéguez, J. A. Castro–Osma, A. Otero, A. Lara–
Sánchez, Dalton Trans. 2019, 48, 4218–4227; h) D. O. Meléndez, A. Lara–Sánchez, J. Martínez, X.
Wu, A. Otero, J. A. Castro–Osma, M. North, R. S. Rojas, ChemCatChem. 2018, 10, 2271–2277.
[7] S. H. Kim, D. Ahn, M. J. Go, M. H. Park, M. Kim, J. Lee, Y. Kim, Organometallics 2014, 33,
2770–2775.
[8] T. Elkin, M. Botoshansky, R. M. Waymouth, M. S. Eisen, Organometallics 2014, 33, 840–843.
[9] P. R. Elowe, C. McCann, P. G. Pringle, S. K. Spitzmesser, J. E. Bercaw, Organometallics 2006,
25, 5255–5260.
[10] L. Wang, G. Zhang, K. Kodama, T. Hirose, Green Chem. 2016, 18, 1229–1233.
[11] W. H. Woo, K. Hyun, Y. Kim, J. Y. Ryu, J. Lee, M. Kim, M. H. Park, Y. Kim, Eur. J. Inorg.
Chem. 2017, 5372–5378.
[12] A. W. Addison, T. N. Rao, J. Reedjik, J. van Rijn, G. C. Verschoor, J. Chem. Soc., Dalton Trans.
1984, 1349–1356.
[13] a) D. F. Shriver, M. A. Drezdzon, The Manipulation of Air–Sensitive Compounds, Wiley, New
York, 2nd edn, 1986; b) G. S. Girolami, T. B. Rauchfuss, R. J. Angelici, Synthesis and Technique in
Inorganic Chemistry, University Science Books, Sausalito, 3rd edn, 1999.
[14] W. L. F. Armarego, C. L. L. Chai, Purification of Laboratory Chemicals, Elsevier, New York,
6th edn, 2009.
[15] A. B. Pangborn, M. A. Giardello, R. H. Grubbs, R. K. Rosen, F. J. Timmers, Organometallics
1996, 15, 1518–1520.
27
This article is protected by copyright. All rights reserved.

10.1002/cssc.201901661
ChemSusChem
550
551
552
553
554
555
556
557
[16] M. C. Joshi, J. Okombo, S. Nsumiwa, J. Ndove, D. Taylor, L. Wiesner, R. Hunter, K. Chibale,
T. J. Egan, J. Med. Chem. 2017, 60, 10245–10256.
[17] Y. Ren, Y. Su, L. Sun, S. He, L. Meng, D. Liao, X. Liu, Y. Ma, C. Liu, S. Li, H. Ruan, X. Lei,
X. Wang, Z. Zhang, J. Med. Chem. 2017, 60, 972–986.
[18] S. M. George, B. K. Park, C. G. Kim, T.–M. Chung, Eur. J. Inorg. Chem. 2014, 2002–2010.
[19] B. Glowacki, R. Pallach, M. Lutter, F. Roesler, H. Alnasr, C. Thomas, D. Schollmeyer, K.
Jurkschat, Chem. Eur. J. 2018, 24, 19266–19279.
[20] S. H. Kim, S. Y. Han, J. H. Kim, Y. Y. Kang, J. Lee, Y. Kim, Eur. J. Inorg. Chem. 2015, 2323–
2329.
558
559
28
This article is protected by copyright. All rights reserved.

10.1002/cssc.201901661
ChemSusChem
560
561
562
Table of Contents (TOC)
563
29
This article is protected by copyright. All rights reserved.