Electronic Tuning of Zinc Porphyrin Catalysts for the Conversion of Epoxides and Carbon Dioxide into Cyclic Carbonates

Article abstract of DOI:10.1002/cctc.201601690

Bifunctional Zn^II porphyrin catalysts bearing tributylammonium bromide groups at the meta positions of the meso-phenyl groups showed high catalytic activity for the synthesis of cyclic carbonates from epoxides and CO₂. Herein, several substituents with different electronic properties were introduced at the para positions. Introduction of electron-donating groups decreased the catalytic activity, whereas the introduction of electron-withdrawing groups increased the activity. The substituents modulated the Lewis acidity of the zinc center, which was confirmed by binding constant experiments. The bifunctional catalyst bearing cyano groups showed the highest catalytic activity with a turnover frequency of up to 42 000 h^?1.

Full text of DOI:10.1002/cctc.201601690

Accepted Article
Title: Electronic Tuning of Zinc Porphyrin Catalysts for the Conversion
of Epoxides and CO₂ into Cyclic Carbonates
Authors: Chihiro Maeda, Sota Sasaki, and Tadashi Ema
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ChemCatChem
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COMMUNICATION
Electronic Tuning of Zinc Porphyrin Catalysts for the Conversion
of Epoxides and CO into Cyclic Carbonates
2
Chihiro Maeda,* Sota Sasaki, and Tadashi Ema*
Abstract: Bifunctional Zn(II) porphyrin catalysts bearing
tributylammonium bromide groups at the meta positions of the meso-
phenyl groups showed high catalytic activity for the synthesis of
however unclear because the alkoxy group can be electron
donating and withdrawing, respectively, because of the
resonance and inductive effects. We envisioned that the
electronic effect of the para substituent on catalytic activity could
be clarified by introducing various para substituents into simple
Zn(II) tetraphenylporphyrin and examining a two-component
2
cyclic carbonates from epoxides and CO . Here several substituents
with different electronics were introduced at the para positions.
Introduction of an electron-donating group decreased the catalytic
activity, while that of electron-withdrawing groups increased the
activity. The substituents modulated the Lewis acidity of the zinc
center, which was confirmed by binding constant experiments. The
bifunctional catalyst bearing the cyano groups showed the highest
catalytic system composed of
5 and tetrabutylammonium
bromide (TBAB). Here we report that the cyano group was the
most effective substituent and that bifunctional porphyrin 3c with
the cyano groups at the para positions showed the highest
–
1
–1
catalytic activities up to a TOF of 42,000 h .
catalytic activities up to a TOF of 42,000 h .
O
Chemical fixation of carbon dioxide (CO
2
) as a C1 feedstock into
a: R = Bu
b: R = Oct
c: R = C12H25 g: R = CH2OPh
d: R = Ph
e: R = CH Cl
f: R = CH2OEt
2
O
catalyst
useful organic compounds has become remarkably important in
+
CO2
O
O
[
1,2]
green and sustainable chemistry.
carbonates from epoxides and CO is one of the most valuable
CO fixations, exhibiting high atom efficiency (Scheme 1).
Synthesis of cyclic
R
R
2
2
1
[
3–8]
2
Cyclic carbonates can be practically used as raw materials for
polycarbonates, as electrolytes in lithium-ion secondary batteries,
and as fuel additives. To date, various catalysts including metal
Scheme 1. Synthesis of cyclic carbonates from epoxides and CO2.
[
4–6]
[7]
R2
R2
complexes
and organocatalysts have been developed to
). The former
R1
R1
R
R
activate the substrates (epoxides and/or CO
2
activates the substrates via coordination interactions whereas
the latter often activates the substrates via hydrogen bonds.
We have employed a porphyrin framework because the
R2
R2
N
Zn
N
N
Zn
N
N
N
N
N
introduction
of
various
metal
ions
and
peripheral
functionalization can modulate the catalytic activities, and
because it is rigid and robust. We have developed bifunctional
metalloporphyrin catalysts with quaternary ammonium bromide
groups at the meta positions of the meso-phenyl groups such as
R2
R1
R2
R1
R
R
R2
R2
[
6]
Zn(II) porphyrin 3a (Figure 1). The metal center and bromide
anions in 3a cooperatively activate epoxides as Lewis acid and
nucleophiles, respectively, and 3a showed high catalytic
activities (turnover number (TON) = 240,000 and turnover
R2 = O(CH ) N Bu Br
+
–
5a: R = H
5b: R = Cl
5c: R = Br
5d: R = CF3
5e: R = CN
2
6
3
a: R1 = H
1
3
3b: R = Br
1
3
c: R = CN
1
–
1
[6e]
3d: R = OMe
frequency (TOF) = 31,500 h ).
that the initial epoxide-ring opening is the rate-determining
DFT calculations indicated
1
+
–
5f: R = OMe
4
: R = O(CH ) N Bu Br
2 6 3
[
6c,e]
step.
Figure 1. Structures of the porphyrin catalysts.
To further improve the catalytic activities, we prepared
porphyrin 4 bearing twelve nucleophiles at both meta and para
positions. Unfortunately, the catalytic activity of 4 was found to
be lower than that of 3a despite more nucleophiles, probably
because the electron-donating alkoxy groups at the para
To investigate the substituent effects on the catalytic activity,
we initially employed the two-component catalytic system
composed of Zn(II) tetraarylporphyrins 5a–f and TBAB (Table 1).
Catalyst 5 (0.01 mol%), TBAB as a co-catalyst (0.08 mol%), and
[
6e]
positions decreased the Lewis acidity of the zinc center of 4.
The electronic effect of the alkoxy group at the para position is
1
,2-epoxyhexane (1a) was heated at 120 °C for 1 h in a
stainless-autoclave under CO (initial pressure of 1.7 MPa). As
2
compared with 5a, 5b–e having electron-withdrawing groups
produced cyclic carbonate 2a in higher yields, while methoxy-
substituted 5f showed lower catalytic activity than 5a. These
results strongly suggest that these para substituents modulate
the Lewis acidity of the metal center.
[
a]
Dr. C. Maeda, S. Sasaki, Prof. T. Ema
Division of Applied Chemistry, Graduate School of Natural Science
and Technology, Okayama University, Tsushima, Okayama 700-
8
530, Japan
E-mail: cmaeda@okayama-u.ac.jp
ema@cc.okayama-u.ac.jp
Table 1. Screening of the catalysts 5a–f for the synthesis of cyclic carbonate
Supporting information for this article is given via a link at the end of
the document.
[
a]
2
a from epoxide 1a.
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ChemCatChem
10.1002/cctc.201601690
COMMUNICATION
[
b]
investigations of the reaction conditions and the protection of the
hydroxy groups of 6b. We finally obtained 6c in moderate yield
by heating 6b with CuCN at 200 °C. Alkylation of 6c followed by
the acid-catalyzed condensation of 7c with pyrrole produced
porphyrin 8c. Zinc insertion and the subsequent reaction of 9c
with tributylamine afforded catalyst 3c (For the synthesis of 3b
and 3d, see the Supporting Information).
Entry
Catalyst
5a
R
Yield [%]
TOF
1
2
3
4
5
6
H
29
59
65
69
73
16
2,900
5,900
6,500
6,900
7,300
1,600
5b
Cl
Br
CF
5c
5d
3
1. pyrrole
TFA, BF
2. DDQ
5e
CN
Br
CN
CN
Br(CH
2
)
6
Br
3 2
•OEt
HO
OH
HO
OH
R
R
CuCN
NMP
K
2
CO
3
5f
OMe
DMF
CH
2
Cl
2
2
6
00 °C
0%
50 °C
48%
45%
CHO
CHO
CHO
[
a] Reaction conditions: 1a (10 mmol), 5 (0.01 mol%), TBAB (0.08 mol%),
6b
6c
2 6
7c: R = O(CH ) Br
CN
CO (1.7 MPa), 120 °C, 1 h, in a 30 mL autoclave. [b] Yields were
determined by H NMR spectroscopy using 2-methoxynaphthalene as an
internal standard.
2
1
R
R
R
R
R
R
N
N
N
NC
M
CN
N
To evaluate the effect of the substituents on the Lewis
acidity of the metal center in the ground state, the binding
8c: M = 2H, R = O(CH
2 6
) Br
Zn(OAc)
5%
Bu N, CHCl
97%
2
, CHCl
3
, MeOH
9
constants (K
Table 2). Zinc porphyrins formed 1:1 complexes with a ligand,
as monitored by UV/Vis titration. The K values were calculated
by the nonlinear least-squares method (see the Supporting
a
) of catalysts 5a–f for epoxide 1a were determined
9c: M = Zn, R = O(CH ) Br
2
6
R
R
3
3
, CH CN, 70 °C
3
(
+
–
CN
2 6 3
3c: M = Zn, R = O(CH ) N Bu Br
a
Scheme 2. Synthesis of 3c.
[
9,10]
Information).
The K
a
values increased in the order: 5f < 5a <
< CN). 5b–e having
5
b < 5c < 5d < 5e (OMe < H < Cl < Br < CF
3
electron-withdrawing groups showed larger association
constants for epoxide 1a than 5a while methoxy-substituted 5f
showed the smallest value. These results are in good agreement
with the catalytic activities of 5a–f (Table 1). It should be noted
that the para-substituents remote from the coordination site are
The CO fixation reactions were carried out at 120 °C for 1 h
2
by using 0.001 mol% 3a–d (Table 3). Catalysts 3b and 3c
having electron-withdrawing groups clearly showed higher
catalytic activity than 3a. This is due to the enhanced Lewis
acidity in 3b and 3c. The substituent effects in the bifunctional
so effective for the tuning of the Lewis acidity of the metal center. catalysts are modest as compared with those of the two-
component catalytic systems (Table 1) although the bifunctional
catalysts are more active than the two-component catalysts. We
therefore investigated the catalytic activities of 3a–d in more
detail. The reaction time was fixed at 1 h, and the temperature
dependency was examined in the range of 120–180 °C (Figure
Table 2. Binding constants of zinc(II) porphyrins for epoxide 3a
–
1
[a]
[b]
Entry
Zn(II) porphyrin
K
a
(M
)
ΔG° (kcal/mol)
2
). The yields of cyclic carbonate 2a increased with an increase
in temperature, and the best yields were recorded at 150–
60 °C. The yields dropped at 180 °C in all cases probably
because the catalysts decomposed at such high
temperature. It is worthy of note that 3c recorded the highest
1
2
3
4
5
6
5a
5b
5c
5d
5e
5f
13.4
16.7
17.9
21.6
23.8
9.80
–1.51
–1.64
–1.68
–1.79
–1.85
–1.33
1
a
[
11]
–
1
yields at all temperatures and maximum TOF value of 42,000 h
[12,13]
at 150–160 °C.
catalyst.
Thus, 3c was found to be the most active
Table 3. Screening of the catalysts 3a–d for the synthesis of cyclic
carbonate 2a from epoxide 1a.
[
a] In CH
2
Cl
2
at 20 °C. [b] Free energy calculated from –RTlnK
a
.
[a]
As shown above, the substituent effects were clearly
[
b]
Entry
Catalyst
3a
R
Yield [%]
TOF
demonstrated in the two-component catalytic system so that we
then applied the substituent effects to the bifunctional system.
We synthesized 3b and 3c bearing the bromo and cyano groups,
respectively. Methoxy-substituted 3d was also synthesized for
comparison. Scheme 2 shows the synthetic scheme for 3c,
which was the most difficult to establish. To introduce the cyano
groups, we attempted various reactions of 4-bromo-3,5-
dihydroxybenzaldehyde (6b) or its derivatives, including the
1
2
3
4
H
13
19
21
13
13,000
19,000
21,000
13,000
3b
Br
3c
CN
OMe
3d
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COMMUNICATION
1
(
1.7 MPa), 20 °C. [d] Yields were determined by H NMR spectroscopy using
[
1
a] Reaction conditions: 1a (10 mmol), 3 (0.001 mol%), CO
20 °C, 1 h, in a 30 mL autoclave. [b] Yields were determined by H NMR
spectroscopy using 2-methoxynaphthalene as an internal standard.
2
(1.7 MPa),
2-methoxynaphthalene as an internal standard. [e] Reaction conditions: 1 (5
mmol), 3c (0.04 mol%), CO (0.1 MPa, balloon), 20 °C.
1
2
In summary, we have synthesized bifunctional Zn(II)
porphyrin catalysts having several substituents. Electronic
substituent effects have been elucidated by catalytic activity
tests and binding constant experiments. Electron-withdrawing
groups at the para positions were found to enhance the Lewis
acidity of the metal center, and bifunctional Zn(II) porphyrin 3c
with the cyano groups showed the highest catalytic activity with
50
40
30
20
10
0
3a
3
3
b
c
3d
–
1
a TOF of 42,000 h . Further investigation on the development of
more active catalysts is currently underway in our laboratory.
Acknowledgements
1
20 130 140 150 160 170 180
This work was supported by JSPS KAKENHI Grant Number
JP16H01030 in Precisely Designed Catalysts with Customized
Scaffolding. We thank Prof. H. Yorimitsu (Kyoto University) for
mass measurements.
temp / °C
Figure 2. Effect of temperature on the synthesis of cyclic carbonate 2a.
Reaction conditions: 1a (1.0 g, 10 mmol), 3 (0.001 mol%), CO (1.7 MPa), 1 h.
2
Keywords: Carbon dioxide fixation · Porphyrinoids ·
Homogeneous catalysts · Epoxides · Cyclic carbonates
Finally, we investigated the substrate scope of the catalyst
3
c (Table 4, entries 1–7). By using 0.005 mol% 3c, various
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catalyst 3c.
2
with
[
a]
Entry
Epoxide
1a
R
Time [h]
Yield [%]
93
TON
[
[
[
[
[
[
[
[
[
b]
b]
b]
b]
b]
b]
b]
c]
e]
1
2
3
4
5
6
7
8
9
Bu
Oct
5
18,600
19,600
19,000
19,600
19,000
18,800
18,000
1,550
1b
5
98
1c
C
12
H
25
9
95
1d
Ph
CH
CH
CH
Bu
Bu
9
98
1e
2
2
2
Cl
12
12
12
48
48
95
[
[
3]
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60
1,500
[
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2
2
(
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COMMUNICATION
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[13] The substituent effect was also observed at 40–100 °C (see the
Supporting Information). 3c recorded the highest yields even at these
lower temperatures.
[
7]
2
013, 135, 5348; b) Y.-B. Wang, D.-S. Sun, H. Zhou, W.-Z. Zhang, X.-B.
[14] The time course experiments supported that the reaction rates depend
on the substituents of the epoxides (see the Supporting Information).
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ChemCatChem
10.1002/cctc.201601690
COMMUNICATION
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COMMUNICATION
Bifunctional Zn(II) porphyrin catalysts
bearing several substituents were
synthesized for the synthesis of cyclic
CN
C. Maeda,* S. Sasaki, T. Ema*
R
R
O
+
CO
2
Bu
Page No. – Page No.
carbonates from epoxides and CO
2
.
R
R
R
R
N
N
N
N
Electronic Tuning of Zinc Porphyrin
Catalysts for the Conversion of
Introduction of electron-withdrawing
groups enhanced the Lewis acidity of
the zinc centers to increase the
NC
Zn
CN TOF = 42,000 h–1
Epoxides and CO
Carbonates
2
into Cyclic
O
O
O
catalytic activity. The bifunctional
catalyst bearing cyano groups showed
the highest activities with a TOF of
R
R
CN
2 6 3
R = O(CH )
N+Bu Br–
Bu
–
1
4
2,000 h .
This article is protected by copyright. All rights reserved.