Amino acids/superbases as eco-friendly catalyst system for the synthesis of cyclic carbonates under metal-free and halide-free conditions

Article abstract of DOI:10.1080/00397911.2017.1339802

An eco-friendly and efficient binary catalyst system of superbases and amino acids was developed for the synthesis of cyclic carbonates from epoxides and CO₂ under metal-free and halide-free conditions. Among the various amino acids and superbases systems tested, the L-histidine/1,8-diazabicyclo[5.4.0]-undec-7-ene (DBU) system achieved the highest conversion of propylene oxide and selectivity of propylene carbonate. The effect of various reaction parameters was evaluated. A possible catalyst mechanism for L-histidine synergized with DBU in the ring opening of epoxide and DBU introduced CO₂ activation. The process herein represents a green, simple, and cost-effective route for the chemical fixation of CO₂ into cyclic carbonates.

Full text of DOI:10.1080/00397911.2017.1339802

Synthetic Communications
An International Journal for Rapid Communication of Synthetic Organic
Chemistry
ISSN: 0039-7911 (Print) 1532-2432 (Online) Journal homepage: http://www.tandfonline.com/loi/lsyc20
Amino Acids/Superbases as Eco-Friendly Catalyst
System for the Synthesis of Cyclic Carbonates
Under Metal-Free and Halide-Free Conditions
Yaqiong Qi, Weiguo Cheng, Fei Xu, Shengli Chen & Suojiang Zhang
To cite this article: Yaqiong Qi, Weiguo Cheng, Fei Xu, Shengli Chen & Suojiang Zhang
(
2017): Amino Acids/Superbases as Eco-Friendly Catalyst System for the Synthesis of Cyclic
Carbonates Under Metal-Free and Halide-Free Conditions, Synthetic Communications, DOI:
0.1080/00397911.2017.1339802
1
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Date: 19 December 2017, At: 03:54

Amino Acids/Superbases as Eco-Friendly Catalyst System for the Synthesis of Cyclic
Carbonates under Metal-Free and Halide-Free Conditions
1
,2
2
2
1
2
Yaqiong Qi , Weiguo Cheng , Fei Xu , Shengli Chen , Suojiang Zhang
1
State Key Laboratory of Heavy Oil Processing and Department of Chemical Engineering,
China University of Petroleum, Beijing, PR China
2
Beijing Key Laboratory of Ionic Liquids Clean Process, State Key Laboratory of
Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of
Sciences, Beijing, PR China
Corresponding author: Shengli Chen, E-mail: slchen@cup.edu.cn
Corresponding author: Suojiang Zhang, E-mail: sjzhang@home.ipe.ac.cn
Abstract
An eco-friendly and efficient binary catalyst system of superbases and amino acids was
developed for the synthesis of cyclic carbonates from epoxides and CO under metal-free
2
and halide-free conditions. Among the various amino acids and superbases systems tested,
the -histidine/DBU system achieved the highest conversion of propylene oxide and
L
selectivity of propylene carbonate. The effect of various reaction parameters were
evaluated. A possible catalyst mechanism for -histidine synergized with DBU in the ring
L
opening of epoxide and DBU introduced CO activation. The process herein represents a
2
green, simple and cost-effective route for the chemical fixation of CO into cyclic
2
carbonates.
1

Graphical Abstract
INTRODUCTION
As an attractive C1 feedstock, carbon dioxide (CO ) is renewable, abundant, inexpensive,
2
nontoxic and nonflammable, so the fixation of CO into useful chemical products has
2
1
a, b
attracted much attention in recent year.
Insertion of CO into epoxides to synthesize
2
2
a, b, c
cyclic carbonates is one of the effectual means of CO fixation.
The cyclic carbonates
2
could be widely used in many kinds of materials, such as raw materials for synthesizing
3
a, b, c
polycarbonates, electrolytes for batteries, fuel additives and aprotic solvents.
4
a, b
A variety of catalysts have been used for this coupling reaction, including metal oxides,
5
6
4a, 6
metal salen complexes, alkali metal halides, N-heterocyclic carbonates,
ionic
7a, b
8a, b
9
10
liquids,
functional organic polymers,
porphyrin, organic bases, etc. Most of the
catalysts above contain metal cations or halide anions, which have toxicity,
certain corrosion resistance and environmentally hazardous problems. It is necessary to
explore a metal-free and halide-free catalyst system for this cycloaddition under relative
mild conditions.
2

In recent years, superbases have attracted much attention because they can realize CO2
1
0
capture and activation. They provide excellent CO absorption and chemical activation
2
with high CO capacity and low desorption energy. They can be used to catalyze the
2
1
0c
11
cycloaddition reaction of epoxides and CO . In recent reports, saccharide, cellulose
2
1
0g, 12
and metal compounds
were added to the superbases catalytic system to improve the
catalytic efficiency. However, the catalyst systems used were still undesirable in either
catalytic activity or environmental protection. Thus, exploration of good catalytic activity
and nontoxic superbases catalyst system for the cycloaddition of CO with epoxides into
2
cyclic carbonates is still needed. Amino acids (AA) are natural, abundant, non-toxic
biological chemical substances, they are widely used in functional material, catalysts,
13a, b
synthesis of peptide intermediates and chiral solvents.
As a natural hydrogen bond
donors, amino acids have amino group (e.g. -NH , -NH) and strong hydrogen bonding
2
1
4
function groups (e.g. -COOH or -OH), He et al. reported that amino acid could activate
the epoxide through hydrogen bonding.
In this work, we use the system of amino acids and superbases to catalyze the cycloadditon
of epoxides and CO . Based on the ability of superbases for CO absorption and activation
2
2
and the strong hydrogen bonding in amino acids, the system of amino acids and superbases
may have better catalytic activity than other metal-free and halide-free catalytic systems
3

which were mentioned above. In order to test this thought, We have investigated different
combination of eight kinds of amino acids and twelve species of organic bases and
optimized reaction parameters of cycloaddition. -Histidine/DBU system achieved the best
L
o
catalytic activity at 120 C, 2 MPa (CO pressure), 2 h.
2
RESULTS AND DISCUSSION
A variety of amino acids and organic bases were selected to explore the catalytic activity
for cycloaddition of epoxides and CO to cyclic carbonates. The amino acids were glycine,
2
L-threonine, -proline, -tyrosine, -histidine, -lysine, -aspartic acid and -trytoplan. The
L
L
L
L
L
L
bases were 1,8-diazabicyclo[5.4.0]-undec-7-ene (DBU), monoethanolamine (MEOA),
diethanolamine (DEOA), triethanolamine (TEOA), triethylamine (TEA),
7
1
-methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene (MTBD) diethylamine (DEA),
,4-diazabicyclo[2.2.2]octane (DABCO), 1,5,7-tri-azabicyclo[4.4.0]dec-5-ene (TBD),
N,N-dimethylaminopyridine (DMAP), pyridine (Py), and tetramethylethylenediamine
TMEDA).
(
First of all, The amino acids were tested combining with DBU to study the catalytic
property of different amino acids by the synthesis of propylene carbonate (PC) from
o
propylene oxide (PO) and CO . All the reactions were proceeded at 120 C, 2 MPa for 2 h
2
4

under solvent free conditions. As is shown in the Table 1, DBU reveals a PO conversion of
6% under 2 MPa of CO pressure (entry 1). The high catalytic activity of DBU might own
8
2
to its CO activation ability and nucleophilic capability. Amino acids resulted in a PO
2
conversion lower than 20% when it used alone under the employed reaction conditions. In
comparison, the system of amino acids and DBU achieved higher catalytic activity than
DBU or amino acids, so it indicated that the synergistic action of amino acids and DBU
accelerate the reaction rate. In the presence of DBU, the activity order for the amino acids
is -histidine ( -his) > -threonine ( -thr), -aspartic acid ( -asp), -tryptophan ( -try) >
L
L
L
L
L
L
L
L
glycine (gly) > -proline ( -pro), -tyrosine ( -tyr) > -lysine ( -lys). -Histidine combined
L
L
L
L
L
L
L
with DBU achieved the highest PO conversion of 97% with selectivity for PC in excess of
9% under the same condition. Therefore, -histidine was used for the further research.
9
L
In order to know the reasons why the addition of amino acids achieved better catalytic
activity, we explored the interaction of -histidine and epoxides, which were confirmed by
L
IR (Scheme 3). Because of the high volatility of PO, We explore the interaction of
+
L-histidine with other epoxides. The characteristic absorbance of NH region of the
3
-1
zwitterionic -histidine is 3015.2 cm without epoxides. However, the interaction between
L
+
+
NH region of the zwitterionic -histidine and 2,2-dimethyloxirane leads to a shift of NH
3
3
L
-1
-1
region from 3015.2 cm to 3016.9 cm (Scheme 3A). Moreover, the interaction between
+
NH region of the zwitterionic -histidine with styrene oxide and 1,2-epoxyheane were
3
L
5

+
also investigated (Scheme 3B and Scheme 3C). The signal absorbance of NH region of
3
-1
-1
the zwitterionic -histidine shifts downfield from 3015.2 cm to 3031.2 cm in Scheme 3B.
L
+
The absorbance of NH region of the zwitterionic -histidine shifts downfield from 3015.2
3
L
-1
-1
cm to 3017.4 cm in Scheme 3C. Those also suggest stronger interaction between
+
epoxides and the NH region of the zwitterionic -histidine.
3
L
Different organic bases were examined on the cycloadditon reaction of PO and CO in the
2
presence of -histidine, the corresponding results were shown in Table 2. From Table 2, we
L
can find that the organic bases exhibited different performance, DBU achieved higher
catalytic activity than others. When the DBU was replaced by Py, an unsatisfactory result
was obtained with only a 76% PO conversion.
According to the report, the organic base which has a lower pK value would lead to a less
a
15
negative Gibbs free energy of the reaction of CO2. Hence, the activation ability of the
superbases to CO bases on the established pK might increase in the order of TEOA <
2
a
DABCO < DEOA < TMEDA < MEOA < DEA < Py < DMAP < TEA < DBU < MTBD <
TBD. While, Table 2 indicates that the actually activity order of the superbases has no
direct effect with the pK , the increasing order is Py < MEOA < TEA < DEA < TBD <
a
TEDMA < MTBD < DEOA, TEOA < DABCO < DMAP < DBU. The possible reason for
this phenomenon is that the synergistic effect of hydroxyl groups in -histidine and the
L
6

steric hindrance of superbases make a bigger difference in the catalytic activity. According
to the discussions above, -histidine/DBU catalyst system was selected for the further
L
survey research. .
Subsequently, the influence of reaction conditions on the fixation of CO to PO was studied.
2
Table 3 shows the effect of the molar ratio of DBU to -histidine by regularly changing the
L
amount of DBU at a fixed weight of -histidine (2% molar ratio to PO). It can be seen that
L
the catalyst activity increased apparently with the increasing molar ratio of DBU to
L-histidine from 1:2 to 5:1. However, it did not bring about any increase when the molar
ratio is higher than 5:1. It is because the concentration of catalyst is high enough to catalyze
the reaction. According to the result discussed above, -histidine/DBU catalytic system
L
shows highest catalytic activity at a molar ratio of 5:1 (DBU to -histidine).
L
Subsequently, we investigated the effects of other parameters on the catalytic activity of
L-histidine/DBU. Figure 1 shows that the temperature also has a great influence on the PO
o
conversion and PC selectivity in the range of 80-140 C. A low conversion of PO was
o
o
o
obtained at 80 C, the catalytic activity increased sharply from 80 C to 110 C, the
o
conversion of PO reached highest at 120 C with 97% PO conversion and PC selectivity
o
more than 99%. When the reaction temperature was higher than 120 C, the conversion of
7

PO kept stable and selectivity of PC decreased. It may be because the high temperature
o
promotes the formation of by-products. 120 C could be the optimal temperature, which
was used for further survey research.
Next, the dependence of PO conversion and PC selectivity on the reaction pressure was
also evaluated (Figure 2). The CO pressure also has a large influence on the PO
2
o
conversion at 120 C with the increasing pressure between 1.0 MPa to 3.0 MPa. Variation
of CO pressure from 1.0 MPa to 2.0 MPa induced a sustaining increased from 88% to 97%.
2
However, with the increasing of CO pressure beyond 2.0 MPa, the PO conversion keeps
2
almost unchanged, the selectivity of PC decreased. The reason for this phenomenon may
be that the increasing concentration of CO lead to lower PO concentration around
2
1
6
catalyst.
The reaction time was also found to play an important role in the
L-histidine/DBU-catalyzed cycloaddtion reaction of epoxides and CO , which was
2
demonstrated in Figure 3. The conversion of PO achieved 85% in 0.5 h and it approached
to 97% in 2 h, thereafter, the conversion of PO kept stable after 2 h. The reason for this
phenomenon may be that the reaction has reached a stable state.
8

In order to evaluate the binary catalyst system of different epoxides, various epoxides were
explored in the present of DBU and -histidine. The results were summarized in Table 4. It
L
can be found that the catalyst system is available to the monosubstituted and disubstituted
epoxides. Among the monosubstituted terminal epoxides (1a-1f), the conversion of
epoxides were decreased gradually in the order of epoxides ethylene oxide (1a) >
propylene oxide (1b) > butyloxirane (1d) > epichlorohydrin (1c) > cyclohexene oxide (1e).
Disubstituted epoxides showed (1f, 1h) lower activity than mono-substituted epoxides
(1a-1e). Compared to other epoxides, 1,2-epoxyethylbenzene (16h) needed a much longer
reaction time to get a high conversion (98%).
Possible Mechanism For The Cycloaddtion Of Epoxide With CO2
1
6
According to the previous reports and experiment results obtained, the possible
mechanism for the cycloadditon of CO and epoxide by -histidine/DBU system was
2
L
+
proposed in Scheme 5. In step 1, the oxygen of epoxide ring was activated by the NH3
region of zwitterionic -histidine, at the same time, the DBU made a nucleophilic attack on
L
the less sterically hindered carbon atom of the epoxide ring, then the ring-opened
immediate 1 would be formed. In parallel, another DBU might interact with the eletrophilic
carbon atom of the CO molecule to provide a carbon Salt 2. Subsequently, the immediate 1
2
made the nucleophilic attack on the carbon Salt 2, a new alkyl carbonate Compound 3
9

would be formed. In the final step, the cyclic carbonate was acquired and the catalyst is
regenerated by intramolecular ring closure. In this reaction, -histidine and DBU had a
L
synergistic influence on the ring opening of epoxide. Meanwhile, DBU had a significant
effect on the fixation of CO . Therefore, this catalyst system of -histidine and DBU played
2
L
a synergetic catalysis role in making the reaction proceed smoothly.
CONCLUSIONS
In summary, an efficient binary catalyst system of superbases and amino acids was
developed for the synthesis of cyclic carbonates from epoxides and CO under metal-free
2
and halide-free conditions. Among the various amino acids and superbases, the
L-histidine/DBU system had the highest activity and exhibit the maximum 97% PO
conversion and 99% PC selectivity. The binary catalyst system was found to apply to other
epoxides. A possible catalyst mechanism for -histidine synergized with DBU in the ring
L
opening of epoxide and the activation of CO induced by DBU was proposed. The catalyst
2
system of -histidine and DBU played a synergetic catalysis role in making the reaction
L
proceed smoothly. The result of this work furnished an instance of the application of
biomaterials combine with superbases as potential material in organic synthesis and
catalysis.
1
0

EXPERIMENTAL
Synthesis of PC from PO and CO using the DBU/L-histidine catalyst system was
2
performed in a 25 mL pressure reactor equipped with a magnetic stirrer, electrical heating
mantle and automatic temperature control system. For each typical batch operation, PO (1
mL, ~14 mmol) and DBU (10% mol) -histidine (2% mol) were charged into the reactor.
L
Subsequently, CO was charged into the reactor, the pressure was adjusted to 2.0 MPa at
2
o
1
20 C. The reactor was maintained for 2 h, and the pressure and temperature were kept
constant during the reaction. After the reaction time closed, cycloaddition was stopped by
cooling the reaction mixture to ambient temperature, and the excess CO was vented.
2
1
The products were identified by 6280 GC and 6890-5973B GC-MS. H NMR was carried
out at room temperature on a Bruker400MHz NMR spectrometer using CDCl as solvent.
3
ESI-MS analysis were performed on a Bruker time of flight mass spectrometer micro
TOF-Q II using an electrospray ionization (ESI) souce. NMR data and HRMS date of the
1
4
-methyl-1, 3-dioxolan-2-one (2b) was provided as follows: H NMR (400 MHz, CDCl ),
3
δ (ppm):1.42-1.43 (d, J=6.0 Hz, 3H), 3.98 (t, J=8.8 Hz, 1H), 4.51 (t, J=8.0 Hz, 1H),
1
3
4
.79-4.86 (m, 1H); C NMR (100.4 MHz, CDCl ), δ (ppm): 19.15, 70.53, 73.48, 154.95
3
+
(
C=O). HRMS (ESI): m/z calcd for C H O Na [M+Na] 125.0784, found 125.0271.
4
6
3
SUPPORTING INFORMATION
11

1
13
Supporting Information: Full experimental detail, H and C NMR spectra and HRMS
date of cyclic carbonates, IR spectra of amino acids with and without PO. This material can
be found via the “Supporting Information” section of this article’s webpage.
ACKNOWLEDGMENTS
Support of this work from the National Key Projects for Fundamental Research and
Development of China (No. 2016YFB0600903), National Natural Sciences Foundation of
China (No. 21676274), Key Research Program of Frontier Sciences，CAS
(QYZDY-SSW-JSC011) and Fund of State Key Laboratory of Multiphase Complex
Systems, IPE, CAS (No. MPCS-2015-A-05) is gratefully acknowledged.
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1
6

a
Table 1. Coupling of PO with CO by different catalysts.
2
b
Entry Base AA
Results
PO conversion/% PC selectivity/%
1
2
3
4
5
6
7
8
9
1
1
DBU /
86
99
/
/
/
L-His 18
L-Thr 4
85
81
L-Asp Trace
Trace
99
DBU L-Thr 96
DBU Gly 95
99
DBU L-Pro 94
DBU L-Tyr 94
DBU L-His 97
DBU L-Lys 86
DBU L-Asp 96
DBU L-Trp 96
99
99
99
0
1
2
99
99
1
99
a
Reaction conditions: PO (1 mL), DBU (10% mol), Amino Acid (2% mol), CO pressure
2
o
(
2.0 MPa), 2 h, 120 C.
b
Determined by GC through using area normalization method.
1
7

a
Table 2. The effect of bases on the PO conversion and PC selectivity.
b
Entry Catalyst
Results
PO conversion/% PC selectivity/%
1
2
3
4
5
6
7
8
9
1
1
TBD/ -His
90
93
97
85
96
76
89
81
99
99
99
99
95
97
99
99
99
99
99
99
L
MTBD/ -His
L
DBU/ -His
L
TEA/ -His
L
DMAP/ -His
L
Py/ -His
L
DEA/ -His
L
MEOA/ -His
L
TMEDA/ -His 91
L
0
1
2
DEOA/ -His
93
L
DABCO/ -His 95
L
1
TEOA/ -His
93
L
a
Reaction conditions: PO (1 mL), base (10% mol), -histidine (2% mol), CO pressure (2.0
L
2
o
MPa), 2 h, 120 C.
b
Determined by GC through using area normalization method.
1
8

a
Table 3. Effect of DBU amount on fixation of CO with PO into PC.
2
b
Entry DBU/ -His Results
L
PO conversion/% PC selectivity/%
1
2
3
4
1:2
1:1
3:1
5:1
7:1
78
90
96
97
97
99
99
99
99
99
5
a
o
Reaction conditions: PO (1 mL), -histidine (2% mol), CO pressure (2.0 MPa), 120 C, 2
L
2
h
b
Determined by GC through using area normalization method.
1
9

Table 4. Cycloaddition of CO and various epoxides with -histidine/DBU catalyst system.
2
L
a
b
Entry
Epoxide
Product
Results
Conversion/%
Selectivity/%
99
O
O
c
c
1
99
1
a
O
O
2
a
2
O
O
2
3
97
88
99
99
O
1
b
O
b
O
O
CH Cl
2
1c
O
O
O
O
CH Cl
2
2c
O
O
O
4
5
6
93
85
87
99
99
Bu
1
1
d
O
Bu 2d
O
Ph
e
O
Ph 2e
O
O
99
99
O
1
f
O
2
f
O
O
d
d
7
98
O
O
1
g
2
g
a
Reaction conditions: epoxide (1 mL), DBU (10% mol), -histidine (2% mol), CO
L
2
o
pressure (2.0 MPa), 2 h, 120 C.
2
0

b
c
d
Determined by GC through using area normalization method. 1 h. 16 h.
2
1

Scheme 1. Cycloadditon of PO and CO using -histidine/DBU
2
L
2
2

Scheme 2. Chemical structure of amino acids used in this work and their designations.
2
3

Scheme 3. The IR spectrum of -histidine with and without epoxides.
L
2
4

Scheme 4. Chemical structures of organic bases used in this work. The data in parentheses
is pK value.
a
2
5

Scheme 5. The proposed mechanism for the -histidine/DBU catalyzed reaction.
L
2
6

Figure 1. Dependence of PO conversion and PC selectivity on reaction temperature.
Reaction conditions: PO (1 mL), -histidine (2% mol), DBU (10% mol), CO pressure (2.0
L
2
MPa), 2 h.
2
7

Figure 2. Dependence of PO conversion and PC selectivity on reaction pressure. Reaction
o
conditions: PO (1 mL), -histidine (2% mol), DBU (10% mol), 2 h, 120 C.
L
2
8

Figure 3. Dependence of PO conversion and PC selectivity on reaction time. Reaction
conditions: PO (1 mL), -histidine (2% mol), DBU (10% mol), CO pressure (2.0 MPa),
L
2
o
1
20 C.
2
9