Scalable, Durable, and Recyclable Metal-Free Catalysts for Highly Efficient Conversion of CO2 to Cyclic Carbonates

Article abstract of DOI:10.1002/ange.202010651

A series of highly active organoboron catalysts for the coupling of CO₂ and epoxides with the advantages of scalable preparation, thermostability, and recyclability is reported. The metal-free catalysts show high reactivity towards a wide scope of cyclic carbonates (14 examples) and can withstand a high temperature up to 150 °C. Compared with the current metal-free catalytic systems that use mol % catalyst loading, the catalytic capacity of the catalyst described herein can be enhanced by three orders of magnitude (epoxide/cat.=200 000/1, mole ratio) in the presence of a cocatalyst. This feature greatly narrows the gap between metal-free catalysts and state-of-the-art metallic systems. An intramolecular cooperative mechanism is proposed and certified on the basis of investigations on crystal structures, structure–performance relationships, kinetic studies, and key reaction intermediates.

Full text of DOI:10.1002/ange.202010651

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Akzeptierter Artikel
Titel: Scalable, Durable, and Recyclable Metal-Free Catalysts for
Highly-Efficient Conversion of CO2 to Cyclic Carbonates
Autoren: Yao-Yao Zhang, Guan-Wen Yang, Rui Xie, Li Yang, Bo Li,
and Guang-Peng Wu
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Zitierweise: Angew. Chem. Int. Ed. 10.1002/anie.202010651
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Angewandte Chemie
10.1002/ange.202010651
RESEARCH ARTICLE
Scalable, Durable, and Recyclable Metal-Free Catalysts for
Highly-Efficient Conversion of CO to Cyclic Carbonates
2
[a]
[a]
[a]
[a]
[b]
*[a]
Yao-Yao Zhang, Guan-Wen Yang, Rui Xie, Li Yang, Bo Li, and Guang-Peng Wu
[a]
Y.-Y. Zhang, G.-W. Yang, R. Xie, L. Yang, Prof. Dr. G.-P. Wu
MOE Laboratory of Macromolecular Synthesis and Functionalization, Adsorption and Separation Materials and Technologies of Zhejiang Province,
Department of Polymer Science and Engineering
Zhejiang University
Zhe Da Road 38, Hangzhou 310027, China
E-mail: gpwu@zju.edu.cn
[b]
Prof. B. Li
College of Material, Chemistry and Chemical Engineering
Hangzhou Normal University
Yuhangtang Road 2318, Hangzhou 311121, China
Supporting information for this article is given via a link at the end of the document.
O
Abstract: This article communicates a series of highly active
organoboron catalysts for coupling of CO and epoxides with merits
R2
O
O
Cat.
O
O
2
Cat.
+
O
CO2
O
R1
R
2
R2
of scalable preparation, thermostability, and recyclability. The metal-
free catalysts show high reactivity towards a wide scope of cyclic
carbonates (14 examples) and can withstand a high temperature up
n
R1
R1
Cyclic carbonate
Polycarbonate
o
to 150 C. Compared with most of the current metal-free catalytic
2
Scheme 1. The coupling reaction of CO and epoxide.
systems involved using mol% catalyst loading, the catalytic capacity
of our catalyst can be enhanced by three orders of magnitude
(
epoxide/cat. = 200,000/1, mole ratio) in the presence of cocatalyst,
Recently, metal-free catalytic systems gain momentum due
to their versatile chemical structure, simplified preparation
which greatly narrows the gap between the metal-free catalysts and
state-of-the-art metallic systems. An intramolecular cooperative
mechanism is proposed and certified on the basis of investigations
on crystal structures, structure-performance relationship, kinetic
studies, and key reaction intermediates.
[
9]
procedure, and avoidance of metal removal. A number of
[
10]
catalysts, including imidazolium salts,
quaternary ammonium
[11]
[12]
[13]
salts,
phosphonium salts,
amidine-based,
and hydrogen bond donor-based
carbene-
[
14]
[15]
based,
catalytic systems
have been widely investigated for coupling CO
2
with epoxides.
On the other hand, even though the heterogeneous catalytic
[
16]
Introduction
systems can be easily separated from products for reuse, the
investigation of homogeneous compounds is necessary in
clarification of structure−performance relationship, which is
beneficial to the insightful understanding of the reaction
mechanism and the subsequent design of more advanced
catalytic systems. Given that a vast number of organic systems
have been venturing into this field, several representative
systems are selected and presented here. In 2017, Kleij et al.
described a squaramides/quaternary ammonium salt catalytic
The runaway greenhouse effect and the resultant
dangerous climate change urgently require efficient techniques
for chemical transformation of CO
2
into value-added products.
Among many efforts on catalytic transformation of this abundant
[1]
2
C1 feedstock, the coupling of CO and epoxides into cyclic and
polymeric carbonates stands out as a great choice because of
its atom-economic merit and high carbon fixation (Scheme 1).
[15b]
system.
With the assistant of the squaramides in activating
Furthermore, compared to the hottest redox transformation CO
into methanol, syngas, and light alkenes that inevitably involved
2
epoxide, the catalyst displays quite high activity for conversion of
o
hexene epoxide with epoxide/catalyst ratio of 50/1 under 80 C,
[
2]
-1
with harsh conditions, the coupling of CO
2
and epoxides into
10 bar [turnover frequency (TOF) = 85 h ]. The same group also
cyclic and polymeric carbonates could be performed at rather
mild reaction conditions, even at normal temperature and
reported that usage of cavitand-based polyphenols combining
with tetrabutylammonium iodide (TBAI) shows an outstanding
reactivity towards cyclic hexene carbonate with a TOF value of
[3]
pressure in the presence of an efficient catalyst. Currently,
considerable attentions are being paid to the CO -based
-1 [15j]
488 h .
Salophen ligand could effectively transform epoxides into cyclic
carbonates at 120 °C and bar CO pressure with
epoxide/catalyst ratio of 100/1, giving a 94 % conversion of 3-
North and coworkers demonstrated that the halid-free
2
polycarbonates, but the cyclic counterparts have been proved to
be more useful, which are widely utilized in polar aprotic
5
2
[
4]
[5]
solvents, electrolytes for lithium-ion battery, raw materials in
[
6]
[15h]
aromatic polycarbonate,
and pharmaceutical intermediates
Scheme 1). In the past decades, numerous robust metallic
catalysts have been developed for cyclization of CO and
Despite the fact that significant advances were
phenoxypropylene oxide in 3.5 h.
Besides, one more
[
7]
(
challenging task for these organic catalysts is how to effectively
transform epoxides to cyclic carbonates at ambient condition, an
admirable environmental-friendly and carbon-balance approach
2
[
3, 8]
epoxides.
[
3e]
achieved, the time-consuming multistep synthesis of these
organometallic catalysts remains key challenge, such as salen
and porphyrin metal complexes.
for CO
2
conversion.
Facing this challenge, Zhang recently
to
epoxide under mild condition with a TOF up to 2.6 h at catalyst
reported a series of protic ionic liquids for cycloaddition of CO
2
-
1
[
13b]
loading of 6 mol% using epichlorohydrin as the substrate.
Lu
and Zhou described kinds of phosphorus ylide CO adducts that
2
1
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Angewandte Chemie
10.1002/ange.202010651
RESEARCH ARTICLE
were able to effectively catalyze the conversion of terminal
performance relationship of our catalyst was systematically
investigated by tuning the electronic and steric properties on the
boron center (electrophilic site) and the quaternary ammonium
salt (nucleophilic site), as well as the distance between
electrophilic and nucleophilic centers. Lastly, a detailed catalytic
cycle was proposed on the basis of the kinetic studies and key
intermediates in the coupling reaction.
epoxides and CO
2
into cyclic carbonates under ambient
-1
conditions, and an unprecedented TOF value (3.6 h ) of
propylene oxide (PO) was achieved at a PO/catalyst ratio of
[
12g]
20/1.
Despite the fact that the activity for these metal-free
system cannot compete with the state-of-the-art metallic
compounds, these well-defined chemical structures and the
insightful mechanistic investigations shed light on further metal-
free catalyst design.
Results and Discussion
(
A) Synthetic procedure for catalysts 1a ~ 1c
Synthesis and characterization of
Me
Me
N
H
X
9-BBN
nBu
B
nBu
organoboron catalysts. The synthetic
procedure of our bifunctional organoboron
catalysts 1a, 1b, and 1c are presented in
Figure 1A, wherein two-step synthetic
process with a nearly quantitative yield is
N
N
B
+
Me
Me
X
N
X
X
syn-addition
1
a: X=Cl；1b: X=Br；1c: X=I
(B) Single-crystal structures
[
17,18]
applied.
The reaction of amine and
N1
N1
B1
N1
alkyl halide (or alkenyl halide) generated
allyl-containing quaternary ammonium
salts, which sequentially produced the
target catalysts 1a, 1b, and 1c through
B1
B1
Br1
O1
O1
I1
Cl1
1
a
1b∙DMF
1c∙DMF
the
borabicyclo[3.3.1]nonane (9-BBN) via
regioselective anti-Markovnikov syn-
hydroboration
reaction
with


Broad substrate scope (14 examples)
 Wide operating conditions (1 ~ 20 bar, 25 ~ 150
o
C)
High reactivity (TOF of PO = 11,050 h-1)  Thermostability and reusability (Negligible loss after 5 runs)
addition. Benefited from the simple
operation and nearly quantitative yield, the
catalysts can be prepared on >1 kg scale
Figure 1. (A) Synthetic procedure of organoboron catalysts 1a~1c. (B) Single-crystal structures of
catalysts 1a, 1b∙DMF, and 1c∙DMF at the 50 % probability level. All hydrogen atoms were omitted for
clarity. The data for the X-ray crystallographic structure have been deposited in the Cambridge as exemplified by 1c in Figure S1.
Crystallographic Data Center under accession numbers CCDC: 2001552, 2001553, and 2001554.
The prepared catalysts were well
characterized using single-crystal X-ray
diffraction
(SC-XRD)
and
nuclear
More recently, we communicated a series of organoboron
catalysts for copolymerization of CO and internal epoxides [e.g.,
cyclohexene oxide (CHO)] with merits of unprecedented
magnetic resonance (NMR) (Supporting information), and the
crystal structures of 1a, 1b, and 1c are shown in Figure 1B. As
observed from the stable solid structure of 1a, the Cl
coordinates to the boron atom with a Cl B distance of 2.054(3)
2
–
[
17]
reactivity and scalable preparation.
CO -polymer/g catalyst was achieved, which is the highest
record achieved to date in all of homogeneous organic and
metallic catalysts for the CHO/CO coupling reaction.
An efficiency of 5.0 kg
…
2
Å due to the electron-deficient property of boron center and the
–
strong nucleophilicity of Cl . Being different with 1a that is easy
2
to crystallize in non-coordinating solvents (chloroform/hexane), it
was rather difficult to get suitable crystals of 1b and 1c from the
same solvents. The cultivation of crystals of 1b and 1c were
N,N-dimethylformamide (DMF) solution,
respectively. These crystal structures were provided in Figure
Interestingly, by adding one more alkylborane center into the
catalyst structure, the binuclear boron compounds show
unprecedented activity for ring-opening polymerization of
conducted in
a
[
18]
epoxides.
In addition, the synthetic procedure for these
catalysts is extremely simple and efficient, the scalable
preparation of these catalysts could be performed on a kilogram
scale with nearly quantitative yield via two steps using
commercially available stocks.
To broaden the range of applications for these metal-free
catalysts, herein, we show our efforts on coupling of epoxides
1
B, wherein one DMF molecule coordinated to the boron atom in
…
1b and 1c with O B distances of 1.623(4) and 1.615(4) Å,
respectively. In the solid structures of 1b and 1c, the anions are
jointly trapped by the ammonium cation (N ) and boron center
+
–…
–…
(
B), as observed by the Br B and the Br N distances of 6.018
–…
–…
and 5.404 Å, as well as the I B and the I N distances of 6.186
and 5.388 Å, respectively. Since the Br and I have lager radii
and better leaving ability of I in compared with the Cl in 1a, we
can anticipate that the anions of Br and I in 1b and 1c would
dynamically shuttle between the electron-deficient boron center
and electropositive N in non-coordinating solvent respectively;
and this dynamic intramolecular structure has laid a foundation
for the high activity of our catalysts for coupling of epoxides and
CO
–
–
2
with CO into various cyclic carbonates that are widely utilized in
fine chemicals and polymer precursors. We found these
catalysts show comparable reactivity to the metal-based
catalysts towards a wide scope of cyclic carbonates and can
–
–
–
–
o
+
withstand a high temperature up to 150 C. Compared with other
metal-free catalysts reported so far, the catalyst 1c achieves the
highest activity for coupling of PO/CO
2
both at optimized
-
1
o
condition (TOF = 11,050 h , 150 C, and 10 bar) and ambient
2
, which is discussed in follows.
-1
o
condition (TOF = 4.0 h , 25 C, and 1 bar), rendering our
organoboron catalyst to be one of the most efficient metal-free
catalytic systems. Catalyst recycling studies showed negligible
loss in catalytic activity after five batch catalytic runs; while over
Catalytic performance. The screening experimental
results for the CO
Table 1, and the results in entries 1-3 reveal that the nucleophilic
2
/PO coupling reaction are summarized in
99% product selectivity was remained. The structure-
2
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Angewandte Chemie
10.1002/ange.202010651
RESEARCH ARTICLE
–
halide anion on catalyst significantly impacts the performance for
cyclic carbonate formation. 1c bearing I is the most effective
be well explained by the following two facts. First, I has the
–
–
–
–
largest radius compared with Cl and Br , and, in this case, the I
is hard to coordinate to the electron-defect boron center, thereby
-
1
–
-1
catalyst, as witnessed by TOF values of 136 h (1a, Cl ), 192 h
–
-1
–
(
1b, Br ), and 257 h (1c, I ) towards propylene carbonate (PC)
the epoxide substrate would be more likely to be activated by
o
[15b,19]
under 80 C and 20 bar of CO
This observation was very different from the reactivity trend in
the coupling of CHO with CO , where the polymerization activity
2
with catalyst/PO ratio of 1/2,000.
complexation with the Lewis acidic boron center.
The
–
second factor is the good leaving ability of I , which could
contribute to facilitating the formation of the thermally stable
2
–
–
– [17]
–
[8j]
order is Br > Cl > I . The high activity for 1c bearing I could
cyclic carbonate via backbiting reaction.
[
a]
Table 1. Results for the coupling of PO and CO
2
catalyzed by organoboron catalysts.
O
Catalyst
+
O
CO
2
O
O
PO
PC
Catalyst
Cat./PO
(mol ratio)
Pressure
(bar)
Temperature
Time
(h)
Conversion
TOF
(h )
Selectivity
(%)
Entry
o
[b]
-1
[c]
(mol %)
( C)
(%)
1
2
3
4
5
6
7
8
9
1a
1b
1c
1c
1c
1c
1c
1c
1c
1c
1c
1c
1c
1c
1c
1c
1/2,000
20
20
20
20
20
20
20
5
80
80
2
2
2
2
2
2
2
2
2
2
2
1
1
1
16
8
13.6
19.2
25.7
28.5
36.0
44.6
43.4
43.6
43.9
44.3
32.6
44.2
22.1
10.9
>99.9
32.0
136
192
257
285
360
446
434
436
439
443
815
> 99
> 99
> 99
> 99
> 99
> 99
> 99
> 99
> 99
> 99
> 99
> 99
> 99
> 99
> 99
1/2,000
1/2,000
1/2,000
1/2,000
1/2,000
1/2,000
1/2,000
1/2,000
1/2,000
1/5,000
1/10,000
1/50,000
1/100,000
1/500
80
90
100
120
150
120
120
120
120
120
150
150
25
10
15
20
10
10
10
20
10
1
1
[
[
[
d]
d]
d]
[e]
1
1
1
2
3
4
4420
[f]
11050
[g]
10900
31.2
4.0
15
[
h]
16
1/100
1
25
> 99
1
[
a] All the reactions were carried out in 50 mL autoclaves in bulk. [b] The conversion of the PO was average of three runs. [c] The selectivity was determined by H
NMR. [d] 120 equivalents of TBAI to 1c was used as the cocatalyst. In consideration of the catalytic ability of TBAI, the TOF values were respectively corrected as
-1
-1
-1
[
2
e] 2360 h , [f] 6950 h , and [g] 6900 h by eliminating the contribution of TBAI alone. [h] Using CO balloon.
o
Owing to the high activity of complex 1c, we selected it as
the catalysis with 1c/PO=1/100,000 at 150 C (entry 14), the
-1
the model catalyst to probe the effects of reaction pressure and
temperature on the formation of cyclic carbonate with a 1c/PO
ratio of 1/2,000. An increase in the reaction temperature from 80
catalyst remains highly active as the TOF value (10,900 h )
showing negligible decrease at such a diluted concentration of
1c. Compared with most of the current metal-free systems
involved using mol% catalyst loading, the catalytic ability of our
catalyst was enhanced by three orders of magnitude (1c/PO =
1/100,000, mole ratio). These performances greatly narrow the
gap between the metal-free catalysts and state-of-the-art
metallic systems, and in some cases show comparability to
metallic compounds (the comparison of 1c with selected
outstanding catalysts was provided in Scheme S1 and Table
o
to 90, 100, and 120 C resulted in a significant improvement in
-1
the catalytic activity and the TOFs increased from 257 h to 285
-
1
-1
-1
h , 360 h , and 446 h , respectively (Table 1, entries 3–6). The
o
catalyst can stand high temperature up to 150 C without evident
loss in activity (Table 1, entry 7). The unconspicuous
o
improvement at 150 C could be attributed to the temperature-
determined balance of the PO substrate in gas phase and liquid
[
8b,8k,8o,8p,8q]
phase. Then the effect of CO
2
pressure was investigated at 120
C, and the results indicated that the conversion of PO varied a
little with the increase of CO pressure from 5 to 20 bar (Table 1,
entries 6, and 8–10). To our delight, even the catalyst loading
S4).
o
As above mentioned, the catalysts that are efficient under
mild conditions are highly desirable. Consequently, we initially
assessed the performance of 1c at PO/1c ratio of 500/1 at room
2
-1
o
reduced to 0.02 mol% (entry 11), TOF value achieved 815 h .
temperature (25 C) under 20 bar CO
2
pressure and found that
We further reduced the 1c loading to 1/10,000 and 1/50,000
the substrate could be completely transformed into the cyclic
carbonate in 16 h (Table 1, entry 15). This result drives us to
perform the reaction at the most challenging conditions, i.e., at
room temperature and ordinary pressure (1 bar). It was a delight
to find that even with [PO]/[1c] = 100/1, catalyst 1c holds high
(
1c/PO mole ratio, entries 12 and 13), with the assistance of
TBAI, the coupling reaction can proceed smoothly and provided
-
1
a reactivity of TOF up to 11,050 h . To the best of our
knowledge, the highest record value of the metal-free catalyst-
mediated coupling of PO and CO
2
to date. When carrying out
2
reactivity for coupling of PO/CO , giving an admirable TOF value
3
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Angewandte Chemie
10.1002/ange.202010651
RESEARCH ARTICLE
-
1
of 4.0 h (Table 1, entry 16), as far as we know, the highest
activity among all the homogeneous metal-free catalytic systems
under a condition of normal pressure and temperature. The
detailed comparison of 1c with the other reported high-active
metal-free catalytic systems is provided in Scheme S2 and Table
S5.
Increasing the linker length between the boron center and
the ammonium cation has a negative effect on the coupling
reaction (see catalysts 1c and 1f~1h, Figure 2). When the linker
length increases from three to six methylene linkages, the TOF
-1
-1
value dramatically decreased from 446 h (by 1c) to 343 h (by
-1
-1
1f), 294 h (by 1g), and 199 h (by 1h). Notably, this result was
quite different with the copolymerization of CHO and CO
wherein the best performance was observed for a catalyst with a
Reusability of 1c was also investigated in viewpoint of green
chemistry. The coupling reaction of PO/CO
PO with [PO]/[1c] = 250/1 at 25 C and 20 bar of CO for 8 h;
2
,
2
was first run in neat
o
[17]
2
linker of pentamethylene.
We tentatively attributed this
then the catalyst was recycled after distillation of the liquid
product (100 C, 3 torr). Benefit from its heat-resistant merit, the
catalyst could be recycled at least 5 times. The results of five
reaction cycles are shown in Figure S2, where no significant loss
in reactivity was noted.
different trend to the steric demands on both of the reaction
substrates (CHO vs PO) and reaction processes (polymerization
vs cyclization).
The electronic and steric effects on nucleophilic motif were
investigated by changing the substituents on the N atom (Figure
o
2, 1c and 1i~1k). We found that the larger steric hindrance
groups on N atom can help to improve the reactivity of the
coupling reaction. For example, compared to 1c containing
n
N
Bu I
B
n
+
-1
2
Me BuN with a TOF value of 446 h , catalysts 1i bearing
1
d
n
+
n
+
-1
nBu
TOF = 264 h
nBu
3 3
Bu N group and 1j possessing Hex N group gave higher
TOFs of 471 and 475 h , respectively. This improvement of the
reactivity can be attributed to the weakened interaction between
B
N
I
O B
O
N
I
-1
1
c
1e
-1
TOF = 446 h
-1
TOF = 106 h
+
–
–
the N and I ; in this context, the nucleophilic attack of I to the
epoxide was reinforced, thereby the reactivity was promoted.
This hypothesis was experimentally supported by the
observation that the catalyst 1k contains aromatic pyridinium
and displayed a compromised catalytic activity of a TOF of 370
Boron center
R3 R4
B
NnBu
B
N
I
I
B
1
k
I
-1
1f
TOF = 370 h
R7
B
N
-1
-1
–
TOF = 343 h
h
because of the enhanced electrostatic interaction between I
6
N
R
R
n
5
and the pyridinium cation. All these results manifested that the
electrophilic site, nucleophilic center, and linker length in the
bifunctional catalysts are of vital importance in tuning the
Hex
Hex
n
NnBu
B
N
nHex
I
I
B
1g
1j
-1
2
catalytic activity in cycloaddition of CO with epoxides.
TOF = 294 h
-1
TOF = 475 h
n
Bu
n
nBu I
N
Bu
B
N
I
(A)
(B)
n
B
Real-time IR
Bu
Catalyst order
1i
1h
TOF = 199 h
-
1
-1
TOF = 471 h
Figure 2. Systematical study on structure-activity relationships of the
bifunctional catalysts. The TOF values of these catalysts are determined at
o
1
20 C and 20 bar of CO
2
in 2 h with [PO]/[1c] = 2,000/1.
(C)
(D)
Order in PO
Order in CO2
Structure optimization. Based on the inspiring catalytic
results shown above and the block-based design of our
catalysts, we were setting out to systematically study the
structure-activity relationships by manipulating the steric and
electronic substituents on the B and N atoms, as well as the
linker length between the boron center and the ammonium
o
cation (Figure 2). The conversion of PO at 120 C and 20 bar of
CO
benchmark.
At first, we studied the electronic and steric effects on the
boron atom (Figure 2, 1c~1e). Compared with 1c bearing BBN
2
in 2 h with 0.05 mol% catalyst loading was employed as the
Figure 3. Kinetic measurements. (A) IR spectra monitoring formation of PC
catalyzed by 1c. Reaction orders in (B) catalyst, (C) PO, and (D) CO . The
concentration of PC was calculated from the absorbance using the calibration
curve (Figures S3 and S4).
2
-1
structure (TOF = 446 h ), catalyst 1d bearing more steric but
similar electrophilic cyclohexyl groups (BCy ) shows retarded
2
Kinetic studies. To gather in-depth comprehension on the
catalytic mechanism, kinetic measurements were first conducted
to determine the order of each component using the model
-1
reactivity with a TOF value of 264 h ; when using catalyst 1e
that possesses the less steric and electrophilic 4,4,5,5-
tetramethyl-1,2,2-dioxyl substituent (BPin),
a
much lower
2
PO/CO coupling reaction under 1c. Real-time IR spectroscopy
-1
catalytic rate was observed (TOF = 106 h ). These results
demonstrate that both the electronic and steric properties have
great influence on the reactivity.
was used to monitor the reaction process based on the signal of
-1
the generated carbonyl group νC=O at 1,805 cm of PC. A
representative three-dimensional plot of IR spectrum was
4
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Angewandte Chemie
10.1002/ange.202010651
RESEARCH ARTICLE
provided in Figure 3A, which shows an emergence and growth
of the carbonyl stretch of PC. No characteristic signal of
polycarbonate at 1,750 cm appeared in each reaction. This
experimental phenomenon excludes the possibility of the
formation of PC through the depolymerization of the resultant
immediately inserts into the boron-alkoxide bond to form a
boron-carbonate species III. With the assistant of the
electropositive quaternary ammonium, the intramolecular
nucleophilic attack of carbonate on its methylene carbon
produces the five-membered PC IV. After release of PC, the
catalyst is regenerated and the catalysis continues in the same
way.
-1
[
17]
2
polymers that usually occurs in the metal-catalyzed CO /PO
[
20]
coupling reactions.
A first-order kinetic dependence on 1c was observed by
To support the proposed catalytic process, the
intermediates at each step were identified and shown in Figure
o
varying 1c concentrations at 25 C in neat PO and 20 bar of CO
2
[
21]
11
(
Figures 3B and S5). The first-order in 1c reveals that an
4. First of all, 1c was characterized by B NMR spectrum, where
intramolecular cooperating catalysis governed the coupling
reaction of CO and PO. Secondly, a first-order dependence on
PO] was also observed at 0.2 mol% catalyst loading by
maintaining the CO at 20 bar (Figure 3C). Then, we found that
the coupling rate is independent in the range of 3 to 10 bar of
CO pressure, as evidenced by the initial rate of PC formation
was almost constant with varied CO pressure (Figure 3D). For
accuracy, each experiment was carried out for three runs (Figure
D). The low conversions at 1-3 bar of CO pressure could be
attributed to the limited CO diffusion rate. The first order in [PO]
and zero order in CO pressure demonstrated that the ring-
opening of the epoxide is the rate-determining step rather than
the CO insertion in the coupling reaction. The total activation
a broad singlet at 87.58 ppm with a full width at half max (fwhm)
of 841 Hz was observed (Figure 4a), which was approximately
0.53 ppm upfield shifted compared with the corresponding signal
in n-hexyl-9-BBN (88.11 ppm, fwhm = 290 Hz, Figure S10). This
observation confirmed the presence of a weak interaction
2
[
2
–
2
between I and boron center in 1c, as demonstrated at the
beginning part of solid structure analysis.
2
Given the intermediate (I) in scheme 2 could not be directly
detected because PO was ring-opened during the NMR
measurement. In this context, 2-butene oxide was used due to
its similar electronic effect with PO and its low reactivity under
3
2
2
2
11
ambient temperature. The corresponding B NMR spectrum in
Figure 4b shows a singlet at 86.58 ppm with a fwhm of 817 Hz,
a higher field shift of 1.00 ppm than that of 1c (at 87.58 ppm),
which indicates an existence of the weak coordination of the
epoxide to the boron center.
2
energy was determined as 12.11 kcal/mol using Arrhenius
equation (Figure S6), and such low activation energy also
demonstrates the rationality of the high performance for our
catalyst.
Me
Bu
n
O
B
N
Me
Me
Bu
Me
I
O
O
n
(a)
B
N
O
I
Me
Bu
Me
(1c)
n
Me
B
N
I
Me
Bu
Me
n
B
N
Bu
Me
n
B
N
I
O
I
O
(b)
O
O
(IV)
O
Me
Bu
Me
(I)
n
B
N
I
O
Me
Bu
Me
Me
Bu
Me
(c)
n
n
B
N
B
N
I
Me
O
O
O
n
I
CO2
B
N
Bu
O
(II)
Me
O
I
(III)
O
O
Coulombic interaction
Coordination interaction
(d)
Scheme 2. The proposed mechanistic cycle for the coupling of PO with CO
based on 1c.
2
100
80
60
40
20
0
-20
Chemical shift (ppm)
11
Figure 4. B NMR spectra of catalytic intermediates. (a) 1c, (b) 1c/2-butene
Proposed mechanism. A catalytic cycle (Scheme 2) was
proposed for 1c-catalyzed CO /PO coupling reaction on the
oxide (1/10 mole ratio), (c) 1c/PO (1/10 mole ratio), and (d) 1c/PC (1/10 mole
o
2
ratio) in CD
2
Cl
, 192 MHz, 25 C.
2
basis of the above kinetic studies and a deuterium labelling
experiment (Figures S7-S9). As demonstrated in Scheme 2, the
–
I in 1c is co-trapped by a weak coordination with boron center
The direct evidence for the ring-opening of PO by 1c
1c/PO = 1/10, mole ratio) was provided in Figure 4c, wherein a
strong and sharp boron nucleus resonance at -1.32 ppm (fwhm
and
a Coulombic interaction with the ammonium cation.
(
–
Benefiting from the weak interaction of I with boron center, the
added PO can easily coordinate to the electrophilic boron center
and afford an intermediate I. Subsequently, the coordinated PO
=
317 Hz) was clearly observed, which could verify that the
[
22]
intermediate of II involved a tetra-coordinated boron structure.
–
undergoes a nucleophilic attack by the trapped I at the less
The nucleophilic attack of 1c at less steric site of PO was
confirmed by the detection of the corresponding resultant
2
steric site and generates intermediate II. Then, a CO molecule
5
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Angewandte Chemie
10.1002/ange.202010651
RESEARCH ARTICLE
iodoalcohol (Figure S11).
99 % selectivity to cyclic carbonates (2l, 2m, and 2n). These
efforts on broadening the substrate scope provide the
generalization of our catalysts.
We were devoted to isolating and identifying the alkylboron-
carbonate complex III shown in Scheme 2. However, our attempt
to isolate the intermediate III was unsuccessful, because the
cyclization reaction (i.e., from II to IV) was too fast even at low
reaction temperature. Therefore, we were trying to characterize
another boron-carbonate intermediate that could be regarded as
an analogue to III but could not undergo cyclization reaction,
thereby could be monitored by IR. In this context, we initially
prepared a compound II’ (Figure S12 in Supporting information)
to simulate the intermediate II. Compared with the intermediate
O
O
O
O
O
O
O
O
O
Et
Bu
2b
1c]/[Epo.] = 1/1,000,
Conv: 98 %, Sel: >99 % Conv: 96 %, Sel: >99 % Conv: 98 %, Sel: >99 %
2c
=
2d
=
[
6 h 1c
]/[Epo.] 1/1,000,
[
6 h 1c
]/[Epo.] 1/1,000,
6 h
[
yield:
O
95 %
yield:
O
94 %
yield:
O
95 %
–
–
II (I is the counterion), the II’ has a MeO as the counterion. In
–
this regard, a boron-carbonate species (III’) with a MeOCOO
O
O
O
O
O
O
Cl
OBu
anion could be formed when the II’ came in contact with CO
expected, the generation of the III’ was immediately formed after
addition of CO to lI’. This process was monitored by Real-time
2
. As
2f
[a]
2g
2e
1c]/[Epo.] = 1/5,000,
8 h
1c
]/[Epo.]
= 1/500, 6 h 1c
= 1/500, 6 h
[
[
[
]/[Epo.]
2
Conv: 98 %; Sel: >99 % Conv: 97 %; Sel: >99 % Conv: 96 %, Sel: >99 %
–
IR spectroscopy (Figure S13), because the formed MeOCOO
anion could not undergo cyclization reaction. This control
experiment in part support the intermediate of III in Scheme 2.
The evidence for the last intermediate (IV) proposed in the
catalytic cycle in Scheme 2 was provided in Figure 4d. A weak
interaction of PC with 1c (10/1, mole ratio) was detected,
wherein a broad resonance (fwhm = 897 Hz) at 85.76 ppm was
observed. This weak interaction between the PC and catalyst
should be beneficial for the regeneration of catalyst 1c, thereby
guaranteeing the smooth progress of the coupling reaction. In
short, the identification of these intermediates provides solid
yield: 95 %
O
yield: 93 %
O
yield: 93 %
O
O
O
O
O
O
O
O
O
O
OH
2h
1c]/[Epo.] = 1/5,000,
2i
2j
[
8 h
[
1c
]/[Epo.]
= 1/500, 6 h 1c
[
]/[Epo.]
= 1/500, 6 h
Conv: 99 %, Sel: >99 % Conv: 98 %, Sel: >99 % Conv: 98 %, Sel: >99 %
yield:
95 %
yield:
95 %
yield:
95 %
O
O
2
k
O
O
1c
=
1/500, 6 h
O
O
O
O
[
]/[Epo.]
Conv: 99%, Sel:
yield: 97 %
>
99 %
O
O
11
O
O
O
support for the proposed mechanism; meanwhile the related
NMR investigations give a rational explanation of the high
reactivity of our catalysts for coupling of epoxides and CO
B
O
O
O
O
2
.
2l
2m
2n[b]
Catalytic activity towards other epoxides. To test the
scope of viable substrates for this reaction, various epoxides
were investigated for the cyclization reaction with CO , wherein
2
[
1c
]/[Epo.]
= 1/500, 24 h [1c]/[Epo.] = 1/500, 24 h 1c
]/[Epo.]
[
= 1/500, 24 h
Conv: 96%, Sel: >99 % Conv: 78%, Sel: >99 % Conv: 98 %, Sel: >99 %
yield: 95 %
yield:
73 %
yield: 97 %
high reactivity and >99% product selectivity were observed for
all the test substrates (Figure 5). Firstly, 1,2-epoxyalkanes with
varying chain lengths (PO, ethylene oxide, butylene oxide, and
hexene oxide) were effectively transformed to corresponding
Figure 5. Synthesis of cyclic carbonates from various epoxides and CO
2
by
using 1c. General conditions: neat epoxide, 2.0 g of substrate, 120 °C, 20 bar
o
2
initial CO pressure. [a] The reaction was conducted at 150 C. [b] The
o
reaction was conducted at 90 C Epo., epoxide. The pure cyclic carbonate
product was isolated by a column chromatography.
cyclic carbonates (2a
ratio of 1,000/1) with high reactivity regardless of their steric
bulk. The cycloaddition of CO and epoxides with electron-
,
2b, 2c, and 2d) at [substrate]/[catalyst]
2
withdrawing substitute were also tested. The result revealed that
when using epichlorohydrin as the substrate, even the catalyst
loading reduced to 1/5,000 (1c/ epichlorohydrin, mole ratio),
admirable activity was obtained, yielding the chloropropene
carbonate (2e) with a conversion of 98% in 8 h. Styrene oxide
was also successfully transformed to styrene carbonate (2f)
though compromised activity was observed, and the nucleophilic
attack at both methine and methylene sites of styrene oxide was
detected (Figure S14). Glycidol ethers could effectively form
cyclic carbonates as evidenced by the use of butyl glycidyl ether,
glycidyl ether, allyl glycidyl ether, and furyl glycidyl ether.
Additionally, catalyst 1c is also tolerant of alkene and hydroxyl
functionalities, as noted by its ability to afford corresponding
cyclic carbonates 2h, 2i and 2j in 6 h with >98% conversions
and >99% product selectivity. Cyclic bicarbonate (2k) was also
effectively produced using resorcinol diglycidyl ether as the
substrate in 6 h, at a catalyst loading of 0.2 mol%. Finally, more
challenging substrates (internal epoxides) were used to test the
catalytic ability. Though the reactivity was retarded, the epoxides
can be successfully transformed into cyclic carbonate with >
Conclusion
In summary, we show our efforts on cycloaddition of
epoxides with CO into various cyclic carbonates by using a kind
2
of scalable organoboron catalysts. The catalysts are highly
active towards a wide scope of cyclic carbonates, and can
withstand a high temperature up to 150 C, rendering our
organoboron catalysts to be one of the most efficient metal-free
catalytic systems to date. Compared with the current metal-free
catalytic systems involved using mol% catalyst loading, the
catalytic capacity of our catalyst was enhanced by three orders
of magnitude (as high as 200,000/1 of epoxide/catalyst, mole
ratio). This performance greatly narrows the gap between the
metal-free catalysts and the state-of-the-art metallic compounds.
The structure-performance relationship of the catalyst was
systematically investigated. On the basis of the kinetic studies
and key reaction intermediates, a detailed catalytic cycle was
rationally proposed. We hope this work may inspire the areas of
o
6
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Angewandte Chemie
10.1002/ange.202010651
RESEARCH ARTICLE
chemical conversions that are mainly based on the application of
nucleophiles/electrophiles or Lewis acids/bases, such as Lewis
pair chemistry.
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Acknowledgements
This work was supported by the National Natural Science
Foundation of China (Grants 91956123, 21802030, 51973186,
and 21674090).
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2
Keywords: CO capture • Cyclic carbonate • Metal-free •
Catalysis • Cooperative mechanism
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Angewandte Chemie
10.1002/ange.202010651
RESEARCH ARTICLE
Entry for the Table of Contents
A kind of highly active bifunctional organoboron catalyst combined merits of scalable preparation, thermostability, and recyclability
was reported for cyclization of CO and epoxides. An intramolecular cooperative mechanism was proposed and substantiated by
2
investigations on crystal structure of catalysts, structure-performance relationship, kinetic studies, and key reaction intermediates.
9
This article is protected by copyright. All rights reserved.