Cycloaddition of epoxide and CO2 to cyclic carbonate catalyzed by VO(IV) porphyrin

Article abstract of DOI:10.1002/aoc.3278

The cycloaddition of epoxide and CO₂ to synthesize cyclic carbonate catalyzed by VO(IV) porphyrin was achieved under 1.4 MPa at 150°C. The effects of reaction temperature, time, CO₂ pressure, co-catalyst and porphyrin framework were investigated. The catalytic results showed that moderate to high yields of cyclic carbonates were obtained under the optimal reaction conditions.

Full text of DOI:10.1002/aoc.3278

Full Paper
Received: 11 September 2014
Revised: 17 December 2014
Accepted: 19 December 2014
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI 10.1002/aoc.3278
Cycloaddition of epoxide and CO2 to cyclic
carbonate catalyzed by VO(IV) porphyrin
Dongsheng Bai*, Zhiyong Zhang, Guangjian Wang and Fang Ma
The cycloaddition of epoxide and CO2 to synthesize cyclic carbonate catalyzed by VO(IV) porphyrin was achieved under 1.4 MPa at
150°C. The effects of reaction temperature, time, CO2 pressure, co-catalyst and porphyrin framework were investigated. The cat-
alytic results showed that moderate to high yields of cyclic carbonates were obtained under the optimal reaction conditions.
Copyright © 2015 John Wiley & Sons, Ltd.
Additional supporting information may be found in the online version of this article at the publisher’s web-site.
Keywords: epoxide; carbon dioxide; cycloaddition; cyclic carbonate; metalloporphyrin
Aesar and used directly. n-Bu4NF, n-Bu4NCl, n-Bu4NBr, n-Bu4NI and
PPh3 were purchased from Sinopharm Chemical Reagent Co. Ltd.
Introduction
Carbon dioxide (CO2) chemistry has received much attention in
recent decades because CO2 is the most abundant and inexpen-
sive carbon source and the most significant greenhouse gas.[1–4]
Chemical fixation of CO2 into organic compounds has great sig-
nificance for the environment and chemistry. One of the most
efficient strategies is the cycloaddition of epoxides and CO2 to
synthesize cyclic carbonates (Scheme 1). This is an atomically
economic and green reaction. Cyclic carbonates are valuable or-
ganic synthetic intermediates, aprotic polar solvents and poly-
merization reaction monomers.[5–8] Accordingly, various catalytic
systems with different yields and turnover frequencies (TOFs)
have been developed for the coupling reaction of CO2 and ep-
oxides, including metal salts,[9–12] metallic complexes,[13–18] ionic
liquids,[19–23] metal–organic frameworks,[24–26] organocatalysts[27]
and N-heterocyclic carbenes.[28–30]
Phenyltrimethylammonium tribromide (PTAT) and n-Bu4PBr were
obtained from Aladdin.
General Procedure of Coupling Reaction of Epoxides and CO2
All cycloaddition reactions were performed in a 50 ml stainless steel
autoclave equipped with a magnetic stir bar. The autoclave reactor
was successively charged with VO(TRPP) (0.05 mmol), co-catalyst
(0.1 mmol) and epoxides (100 mmol). The reactor was pressurized
with CO2 to 0.8–1.6 MPa and then heated and stirred at the desired
temperature. When the pressure of CO2 fell to 0.1–0.4 MPa, the
reactor was cooled quickly to 0°C in ice water and then slowly
vented. The remaining mixture was distilled under reduced
pressure or purified using column chromatography to afford cyclic
carbonate product.
Porphyrin ligands are widely used due to their planar structure,
electron-rich properties and ability to form complexes.
Metalloporphyrins such as P-450 are easily synthesized when por-
phyrin ligands coordinate with metal ions. Therefore, VCl3 and
other Lewis acids have been studied as catalysts for cyclic carbon-
ate synthesis.[31–37] As we know, VO(IV) ions and their correspond-
ing complexes have been studied for their use as catalysts for
organic synthesis.[38] For example, Coletti and co-workers reported
butylene carbonate synthesis catalyzed by VO(IV) complexes under
a CO2 pressure of 1.0 MPa at 45°C.[39]
Considering the above, in order to expand the types of VO(IV)
complexes that can be used as catalysts for cycloaddition of epoxides
and CO2 to synthesize cyclic carbonates, in the work reported herein,
several oxygenated vanadium porphyrins (VO(TRPP); Scheme 2) were
synthesized, and their catalytic activities were studied.
Preparation of VO(TRPP) Complexes
VO(TRPP) were synthesized, purified and characterized using a
modified literature procedure.[40] In a typical reaction, freshly dis-
tilled ortho-substituted benzaldehyde (2.0 ml, 19.6 mmol) was dis-
solved in 100 ml of argon-purged DMF. Then, concentrated
hydrochloric acid (1.0 ml) was added followed by dropwise addition
of freshly distilled pyrrole (1.4 ml, 19.6 mmol). The mixture was
stirred under argon for 1 h and then 2.5 equiv. of VOSO4 was added.
The final reaction mixture was refluxed for 8 h in air. The solvent
was removed under reduced pressure and the crude product was
washed three times with acidic water and dried. The product was
purified using flash chromatography over neutral aluminium oxide
using chloroform as eluent.
Experimental
*
Correspondence to: Dongsheng Bai, College of Chemistry and Material
Science, Huaibei Normal University, 235000, Huaibei, Anhui, China. E-mail:
baidsh06@lzu.edu.cn
Materials
Propylene oxide (PO) was distilled from CaH2. Other epoxides and
N,N-dimethylaminopyridine (DMAP) were purchased from Alfa
College of Chemistry and Material Science, Huaibei Normal University, 235000,
Huaibei, Anhui, China
Appl. Organometal. Chem. (2015)
Copyright © 2015 John Wiley & Sons, Ltd.

D. Bai et al.
Scheme 1. Synthesis of cyclic carbonates from CO2 and epoxides.
Figure 1. Effect of temperature on yield of PC.
Scheme 2. Structure of VO(TRPP).
Complex 1a. 1H NMR (CDCl3, δ, ppm): 7.83–7.84 (d, 12H, 3, 4-H of
C6H5), 8.13–8.17 (m, 8H, 2-H of C6H5), 9.13 (s, 8H, 8-H of pyrrole). 13
C
NMR (CDCl3, δ, ppm): 121.70 (meso-C), 126.78 (C-3), 127.93 (C-4),
132.69 (C-8), 135.01 (C-2), 141.65 (C-1), 149.40 (C-7). UV–visible
(CH2Cl2): λmax = 402, 422, 556, 595 nm. Elemental analysis: calcd
for C44H28N4OV (%): C, 77.76; H, 4.15; N, 8.24. Found (%): C, 77.74;
H, 4.19; N, 8.22.
Complex 1b. H NMR (CDCl3, δ, ppm): 7.85 (s, 8H, 8-H of pyrrole),
8.17–8.40 (m, 4H, 4, 6-H of C6H5), 8.63 ppm (s, 8H, 3, 5-H of C6H5).
13C NMR (CDCl3, δ, ppm): 116.39 (meso-C), 123.71 (C-8), 129.04
(C-3), 130.13 (C-4), 136.97 (C-5), 138.10 (C-1), 149.24 (C-6), 152.50
(C-7), 161.31 (C-2). Elemental analysis: calcd for C44H24N8O9V (%):
C, 61.48; H, 2.81; N, 13.03. Found (%): C, 61.46; H, 2.85; N, 13.02.
Figure 2. Effect of reaction time on yield of PC.
1
Complex 1c. H NMR (CDCl3, 300 MHz, δ, ppm): 3.45 (s, 12H,
exceeds 99%, which suggests the coupling reaction of PO and
CO2 is completed within 5 h.
H–OCH3), 7.31 (s, 8H, 8-H of pyrrole), 7.74 (s, 4H, 6-H of C6H5),
8.00 ppm (s, 4H, 4-H of C6H5), 8.72 ppm (s, 4H, 3, 5-H of C6H5).
13C NMR (CDCl3, δ, ppm): 53.57 (C–OCH3), 120.89 (C-3), 122.28
(meso-C), 126.51 (C-1), 131.88 (C-8), 134.73 (C-5), 135.82 (C-4),
136.91 (C-6), 143.58 (C-7), 150.27 (C-2). Elemental analysis: calcd
for C48H36N4O5V (%): C, 72.09; H, 4.54; N, 7.01. Found (%): C,
72.08; H, 4.57; N, 7.00.
Effect of Co-catalyst and Porphyrin Framework on
Synthesis of PC
Co-catalyst of Lewis base plays an important role in this coupling
reaction.[41] The catalytic activity of various co-catalysts was stud-
ied. It is evident from Table 1 that n-Bu4NBr shows superior ac-
tivity among the co-catalysts investigated because BrÀ is both a
better nucleophilic group and leaving group. PC yield increases
up to 99% at 150°C under a CO2 pressure of 1.4 MPa within
5 h catalyzed by 1a/n-Bu4NBr (Table 1, entry 4). Although DMAP
and PPh3 as organic bases can catalyze the cycloaddition PO
and CO2, the PC yield is lower than for any quaternary ammo-
nium salt or tetrabutylphosphonium bromide (Table 1, entries 9
and 10 versus 1–8). The yield of PC decreases whether the
amount of n-Bu4NBr is decreased or increased, and the best
1a/n-Bu4NBr mole ratio is 1:2 for this cycloaddition (Table 1,
entries 3–5). The order of catalytic activity of co-catalysts is
n-Bu4NBr > n-Bu4NCl > n-Bu4PBr > n-Bu4NI > PTAT > n-Bu4NF.
Oxygenated vanadium porphyrins with various frameworks were
also investigated. As evident from Table 1, ortho-substituted groups
in porphyrin framework strongly influence the coupling reaction
(Table 1, entries 4, 11 and 12). When the porphyrin framework bears
an electron-withdrawing group (–NO2) or electron-donating group
(–OCH3) the tendency is to give a decreased yield. This phenome-
non can be explained by the steric effect of ortho-substituted
groups in metalloporphyrins. When 1a is used without co-catalyst,
Results and Discussion
Effect of Temperature on Propylene Carbonate (PC) Synthesis
Catalyzed by 1a
The crucial role played by reaction temperature in PC synthesis from
PO and CO2 was investigated first. As shown in Fig. 1, there is only a
62% yield of PC obtained at 120°C under a CO2 pressure of 1.4 MPa
within 5 h, but the PC yield increases to 99% at 150°C. However, a fur-
ther increase in the reaction temperature causes a slight decrease of
PC yield due to the decomposition of n-Bu4NBr (Ti = 114.7°C, TMax
=
118.7°C). This hints that 150°C is the optimal temperature for the
coupling reaction of PO and CO2 catalyzed by 1a/n-Bu4NBr.
Effect of Reaction Time on PC Synthesis Catalyzed by 1a
The PC yield as a function of reaction time is shown in Fig. 2. The
results indicate that this cycloaddition reaction proceeds rapidly
within the first 2 h, and there is a yield of 48% within 1 h and
79% within 2 h. For a longer reaction time of 5 h, the yield of PC
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Copyright © 2015 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. (2015)

Cyclic carbonate synthesis catalyzed by metalloporphyrin
Table 1. Influence of co-catalyst and porphyrin framework on PC yielda
Table 2. Cycloaddition of CO2 to various epoxides catalyzed by
1a/n-Bu4NBra
Entry
Catalyst/co-catalyst (ratio)
Yield (%)b
TOF (hÀ1
)
c
Entry
R
Time (h)
Yield (%)b
1
1a/2n-Bu4NF
1a/2n-Bu4NCl
1a/1n-Bu4NBr
1a/2n-Bu4NBr
1a/4n-Bu4NBr
1a/2n-Bu4NI
1a/2n-Bu4PBr
1a/2PTAT
56
95
224
380
320
396
388
336
356
292
68
1
CH3
5
8
99
76
96
71
93
22
2
2
CH3CH2
ClCH2
3
80
3
4
4
>99
97
4c
5c
6
Ph
12
4.5
12
5
PhOCH2
CH3(CH2)9
6
84
7
89
8
73
aReaction conditions: 100 mmol epoxide, 0.05 mmol 1a, 0.1 mmol
n-Bu4NBr, 1.4 MPa CO2 pressure, 150°C.
9
1a/2DMAP
17
bIsolated yield.
10
11
12
13
14
15d
1a/2PPh3
13
52
cCH2Cl2 (3 ml) was used as diluent.
1b/2n-Bu4NBr
1c/2n-Bu4NBr
1a
49
196
232
—
58
—
n-Bu4NBr
6
12
Cycloaddition Reaction of Various Epoxides and CO2 Catalyzed
VO(acac)2/1n-Bu4NI
100
14
by 1a/n-Bu4NBr
aReaction conditions: 100 mmol PO, 0.05 mmol VO(TRPP), 0.1 mmol co-
catalyst, 1.4 MPa CO2 pressure, 5 h, 150°C.
bIsolated yield.
Under the optimal reaction conditions, various terminal-substituted
epoxides were used and reacted with CO2 to synthesize cyclic car-
bonates in order to extend the scope of reactants. It is evident from
Table 2 that the yields of propylene carbonate (Table 2, entry 1),
chloropropene carbonate (Table 2, entry 3) and phenoxylpropylene
carbonate (Table 2, entry 5) exceed 93% within 5 h. Epichlorohydrin
with a chlorine functional group having larger steric hindrance than
PO with a methyl group is also a highly active epoxide due to the
good electron-withdrawing capability of chlorine (Table 2, entry 1
versus entry 3). The good conversions of epichlorohydrin and phe-
nylglycidyl ether can also be explained by their good electron-
withdrawing capability, allowing for an easier nucleophilic attack
of the epoxide ring carbon atoms (Table 2, entries 3 and 5).[42] How-
ever, when butene oxide and dodecylene oxide are used as sub-
strate, yields of only 76% of butene carbonate and 22% of
dodecylene carbonate are obtained in longer reaction time due
to the low reactivity of electron-rich substrate (Table 2, entries 2
and 6). The yield of styrene carbonate is only 71% due to the steric
hindrance effect of the phenyl group (Table 2, entry 4).
cMoles of PC produced per mole of catalyst per hour.
dReported by Coletti et al.[39] Reaction conditions: 0.4 M butylene
oxide, 0.4 M mesitylene, 0.01 M VO(acac)2, 0.01 M n-Bu4NI, 5 ml
solvent (2-butanone), 1.0 MPa, 18 h, 45°C.
there is no PC obtained (Table 1, entry 13). However, there is a 6%
yield of PC when only n-Bu4NBr is used as catalyst (Table 1, entry
14). Although there was a 100% yield of butylene carbonate when
catalyzed by VO(acac)2/n-Bu4NBr reported by Coletti et al., the
TOF value was very low (Table 1, entry 15).[39]
Effect of CO2 Pressure on PC Synthesis Catalyzed by 1a
The effect of CO2 pressure on PC yield was studied under the opti-
mized reaction conditions mentioned above. It can be seen from
Fig. 3 that the yield of PC greatly depends on CO2 pressure. The
PC yield increases as pressure increases from 0.8 to 1.4 MPa and
reaches a maximum (99%) at 1.4 MPa. However, a further increase
in CO2 pressure causes a slight decrease of PC yield because the
concentration of PO decreases along with an increased amount of
dissolved CO2 at higher pressure. This indicates that 1.4 MPa is
the optimal CO2 pressure for the coupling reaction of PO and CO2
to form PC catalyzed by 1a/n-Bu4NBr.
Conclusions
In summary, we expanded and developed several high-valent va-
nadium porphyrins as catalysts for the synthesis of cyclic carbon-
ates from epoxides and CO2 with high TOF. The yields of cyclic
carbonates were moderate to excellent under a CO2 pressure of
1.4 MPa at 150°C catalyzed by VO(TPP)/n-Bu4NBr.
Acknowledgements
We are grateful for the financial support of the Natural Science
Foundation of Anhui Province (1208085QB36), Anhui University
Natural Science Foundation (KJ2012A256) and National Natural
Science Foundation of China (21303065).
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