Microwave-assisted, rapid cycloaddition of allyl glycidyl ether and CO 2 by employing pyridinium-based ionic liquid catalysts

Article abstract of DOI:10.1016/j.catcom.2014.05.016

This study investigated the use of pyridinium-based ionic liquids (ILs) as an efficient catalyst for the rapid solvent-free microwave-assisted cycloaddition of allyl glycidyl ether (AGE) and CO₂ to yield allyl glycidyl carbonate (AGC) under moderate reaction conditions. The cycloaddition reaction occurred over a short reaction time of 30 s, resulting in a high turnover frequency (TOF) ranging from 200 to 7000 h^{- 1}. The effects of alkyl chain length and anion of pyridinium-based catalysts on the cycloaddition reactivity were studied. The effects of reaction parameters such as the amount of catalyst, microwave power, CO₂ pressure, and reaction time were also investigated.

Full text of DOI:10.1016/j.catcom.2014.05.016

Catalysis Communications 54 (2014) 31–34
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Microwave-assisted, rapid cycloaddition of allyl glycidyl ether and CO by
2
employing pyridinium-based ionic liquid catalysts
a
a
a
a
Jose Tharun , Amal Cherian Kathalikkattil , Roshith Roshan , Dong-Heon Kang ,
b
a,
Hee-Chul Woo , Dae-Won Park ⁎
Division of Chemical Biomolecular Engineering, Pusan National University, Busan 609-735, Republic of Korea
a
b
Department of Chemical Engineering, Pukyong National University, Busan 608-739, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
This study investigated the use of pyridinium-based ionic liquids (ILs) as an efficient catalyst for the rapid
solvent-free microwave-assisted cycloaddition of allyl glycidyl ether (AGE) and CO to yield allyl glycidyl carbon-
2
Received 12 March 2014
Received in revised form 1 May 2014
Accepted 19 May 2014
ate (AGC) under moderate reaction conditions. The cycloaddition reaction occurred over a short reaction time of
−1
3
0 s, resulting in a high turnover frequency (TOF) ranging from 200 to 7000 h . The effects of alkyl chain length
Available online 27 May 2014
and anion of pyridinium-based catalysts on the cycloaddition reactivity were studied. The effects of reaction pa-
rameters such as the amount of catalyst, microwave power, CO
investigated.
2
pressure, and reaction time were also
Keywords:
Microwave
Pyridinium ionic liquids
Cycloaddition
© 2014 Elsevier B.V. All rights reserved.
CO
Allyl glycidyl ether
2
1
. Introduction
CO
2
and epoxide is the presence of a strong nucleophilic anion group,
cycloaddi-
which helps in the ring opening of epoxides for effective CO
2
Conventional methods in chemical synthesis are orders of magni-
tion. Hence, ionic liquids (ILs) possessing strong nucleophilic anions
would be the most appropriate choice as the catalyst for this process
[11]. ILs have advantages such as negligible vapor pressure, excellent
thermal stability, and other special characteristics compared to conven-
tional organic and inorganic solvents. ILs are composed of cation/anion
tude too slow to satisfy the current demand for the manufacture of
new compounds. Chemists have been under growing pressure to devel-
op new methods that are rapid and environmentally friendly. The intro-
duction of microwave flash heating for organic synthesis in recent years
has resulted in dramatically reduced reaction times and cleaner reac-
tions with lesser side products, thereby increasing product yields and
simplifying the course of reactions for combinatorial chemistry [1–4].
For the past two decades, great efforts have been made towards the fix-
ation of carbon dioxide due to the growing concern over the deleterious
effects of greenhouse gases on the environment [5,6]. The synthesis of
(Lewis acid and Lewis base sites) combinations, which activate CO
2
molecules and simultaneously interact with other substrates [11–14].
Over the past decades, numerous strategies have been proposed for
the chemical absorption of CO by employing ILs as a catalyst. Despite
their high catalytic activity, ILs still fall short owing to their inherent
drawbacks such as extensive energy consumption for CO desorption,
2
2
five-membered ring carbonates from epoxide and CO
methodology from a resource utilization standpoint because of the
00% atom economy of the reaction and the wide variety of applications
2
is a promising
low capture efficiency, and slow sorption kinetics. On the other hand,
microwave-heating technology relies on an ionic conduction mecha-
nism. Molecules possessing a high dipole moment can be easily heated
by microwave radiation [1–4]. There exist limited studies on the
1
of cyclic carbonates––such as polar aprotic solvents, precursors of poly-
meric materials, and intermediates in the syntheses of pharmaceuticals
2
microwave-assisted synthesis of cyclic carbonates from CO and epox-
[
6,7].
Until now, a wide variety of catalysts such as Lewis acids, transition-
metal complexes, and organometallic compounds under high CO pres-
ides [15–18]. Similarly, among the various epoxides employed for the
cycloaddition reactions, epoxides bearing ether oxygen in the side
chain showed higher activity in IL associated catalytic systems [15,
2
sure have been employed for the synthesis of cyclic carbonates [8–10].
2
19–27]. Allyl glycidyl ether (AGE) can be copolymerized with CO to
The most important criteria for the synthesis of cyclic carbonates from
form biodegradable aliphatic polycarbonate bearing pendant allyl func-
tionality [28]. And also, studies on pyridinium-based ILs are few com-
pared to those on other types of ILs [19]. Thus, this paper presents a
detailed investigation of solvent-free microwave-assisted cycloaddition
⁎
Corresponding author. Tel.: +82 51 510 2399; fax: +82 51 512 8563.
E-mail address: dwpark@pusan.ac.kr (D.-W. Park).
2
of AGE and CO for the synthesis of allyl glycidyl carbonate (AGC) by
http://dx.doi.org/10.1016/j.catcom.2014.05.016
566-7367/© 2014 Elsevier B.V. All rights reserved.
1

32
J. Tharun et al. / Catalysis Communications 54 (2014) 31–34
employing pyridinium-based ILs as the catalyst (Scheme S1, Supple-
mentary information).
Table 1
Catalyst screening for the cycloaddition of AGE and CO
2
.
Entry
Catalyst
Conversion (%)
Selectivity (%)
TOF (h−1)
2
. Experimental
1
2
3
4
5
6
7
8
9
None
EPyCl
PPyCl
BPyCl
HPyCl
BPyBr
BMPyI
0
79
82
86
78
95
93
10
92
73
0
90
91
96
93
95
87
94
96
89
0
1770
1838
1927
1748
2129
2084
224
2
.1. Materials
1
-Ethylpyridinium chloride (N98%) (EPyCl), 1-propylpyridinium
chloride (N98%) (PPyCl), 1-butylpyridinium chloride (N98%) (BPyCl),
-hexylpyridinium chloride (N98%) (HPyCl), 1-butylpyridinium bro-
mide (N98%) (BPyBr), and 1-butylpyridinium hexafluorophosphate
N98%) (BPyPF ) were purchased from TCI chemicals whereas 1-butyl-
-methylpyridinium iodide (N99%) (BMPyI), 1-butyl-3-methyl-
BPyPF
6
1
BMImBr
TBAB
2062
1636
10
(
4
6
Reaction conditions: AGE = 18.6 mmol, catalyst amount = 1 mmol, PCO2 = 0.96 MPa,
microwave power = 200 W, time = 30 s. TOF: moles of AGE converted per mol of
ionic liquid/h.
imidazolium bromide (N97%) (BMImBr), and tetrabutylammonium
bromide (N98%) (TBAB) were obtained from Aldrich and used without
further purification. Allyl glycidyl ether (AGE) was purchased from
Aldrich and used as received. Carbon dioxide of 99.999% purity was
used without further purification. CH Cl was obtained from SK
2 2
Chemicals, Korea, and used as received.
The anion effect of pyridinium-based ILs was investigated using
6
anions possessing Cl, Br, I, and PF (Table 1). Comparing BPyCl, BPyBr,
and BMPyI (entries 4. 6, 7), catalysts containing I or Br anions
showed high catalytic activities of 93% and 95% with excellent TOF of
084 and 2129 h (respectively) under moderate reaction conditions
0.96 MPa, 200 W, 30 s). This was most likely due to the high nucleophi-
licity and leaving ability of these anions. Compared to I and Br sys-
tems, less nucleophilic anions Cl and PF
lower catalytic activity [24,30]. Various other ILs, such as imidazolium-
based BMImBr and ammonium salt-based TBAB, were also tested
entries 9 and 10) using solvent-free microwave cycloaddition and
they resulted in good activity. The high activity observed for BMImBr
and TBAB confirms the most important criteria for obtaining high cata-
2
lytic activity for cyclic carbonate synthesis from CO and epoxides,
namely, the presence of strong nucleophilic anions with appropriate
alkyl chain length cations. From the above studies and various others
8,24,29,30], bromide or iodide ions with butyl group-containing cat-
ions could serve as the best catalyst for solvent-free cycloaddition of
epoxide and CO under moderate reaction conditions. In most cases,
the TOFs for IL-based cycloaddition of epoxides and CO were lower
2
than 50 h
−
−
−
1
2
(
2
.2. Cycloaddition of epoxides and CO
2
−
−
For the reaction, a multi-mode microwave reactor (KMIC-2KW)
containing a source with a continuously adjustable power setting from
kW to 2 kW (adjusted via a 3-stub tuner) as well as a temperature
−
−
6
(entries 4 and 8) showed
0
controller unit operating at a frequency of 2.450 GHz was used. The
surface temperature of the reactor was measured using an IR tempera-
ture detector. The cut off temperature was kept at 175 °C considering
the subtle stability of Pyrex glass microwave reactor. The synthesis of
AGC from AGE and CO (Scheme S1) using pyridinium based ILs as a cat-
2
alyst was performed with the microwave reactor in a 40 mL Pyrex glass
reactor equipped with a magnetic stirrer. For each typical reaction, ILs
(
[
(
0.1–2.5 mmol) and AGE (18.6 mmol) were charged into the reactor
without a solvent and then purged several times with CO . The reactor
was then pressurized with CO and heated to a desired temperature
2
2
2
by stirring the reaction mixture at 600 rpm. After the completion of re-
action time, cycloaddition was stopped by cooling the reaction mixture
to room temperature and venting the remaining CO . The product was
2
dissolved in dichloromethane and an analysis was carried out using a
gas chromatography/mass spectrometry (GC–MS, Micromass, UK) anal-
ysis. The conversion of epoxides was obtained from gas chromatogra-
phy (GC, HP-5 capillary column, HP 6890, Agilent Technologies, Santa
Clara, CA, USA) data.
−1
[30–34]. To date, the highest TOF obtained for an IL-
−1
based system (HETBA) is in the range of 3000–14,000 h for the syn-
thesis of propylene carbonate from propylene oxide and CO in a
micro-reactor (3.5 MPa, 180 °C, 14 s) [35]. Pyridinium-based ILs have
been employed earlier for the cycloaddition of butyl glycidyl ether and
CO
of 15 h
2
2
to yield butyl glycidyl carbonate, but it resulted in a very low TOF
−1
under conventional reaction conditions of 0.82 MPa at
1
40 °C for 1 h [19]. Utilizing microwave-assisted cycloaddition of
AGE and CO to yield AGC in the presence of pyridinium-based ILs,
2
−
1
3
. Results and discussion
high TOFs ranging from 200 to 7000 h were achieved under moderate
reaction conditions of 0.96 MPa, 200 W, and 30 s. In this study, we
achieved a 450-fold increase in the TOF by using microwaves, which is
much larger than the TOF achieved using conventional synthesis tech-
niques [19].
3
.1. Catalytic activity
The catalytic activity of pyridinium-based ILs was tested using a
microwave-assisted solvent-free cycloaddition reaction using AGE as
the epoxide substrate. The observed results are tabulated in Table 1.
Entry 1 shows the need for an appropriate catalyst because no product
was formed in the absence of a catalyst. When the size of the cations
in the pyridinium IL was increased from ethyl to butyl (entries 2–4),
catalytic activity also increased reaching a maximum of 86% conversion
3.2. Effect of catalyst amount
Since BPyBr showed good catalytic activity, it was used to examine
the effects of reaction parameters (catalyst amount, time, microwave
power, and CO
2
pressure) on the cycloaddition reaction of AGE with
−
1
for BPyCl with a high turn-over frequency (TOF) of 1927 h under a
moderate reaction condition of 0.96 MPa of CO pressure, 200 W of mi-
crowave power, and reaction time of 30 s. The rate-determining step of
the epoxide-CO cycloaddition reaction involved a nucleophilic attack of
CO . Table 2 shows the relationship between the AGE conversion and
2
2
the amount of catalyst. The conversion increased from 31% to 95%
when the amount of catalyst increased from 0.1 mmol to 1.0 mmol,
−
1
−1
2
whereas the TOF decreased from 6947 h to 2129 h (entries 1–3).
This can be explained by the effect of the catalytically active sites in
AGE. The conversion efficiency of the substrate is mainly determined
by the collision frequency between the catalytically active sites and
the substrate. As the number of catalytically active sites increases with
the increase of catalyst concentration under a constant substrate vol-
ume, the collision frequency based on the catalytically active sites
anions on the epoxide ring. Therefore, bulky ILs, having longer distances
between cations and anions, have a higher anion activation capacity
[13]. However, a further increase in cationic size (entry 5) resulted in
a slight reduction of catalytic activity. This could be explained by the ste-
ric hindrance of the bulky ILs, which compensated for the increase of the
anion activation ability by increasing the alkyl chain length [29].

J. Tharun et al. / Catalysis Communications 54 (2014) 31–34
33
Table 2
Effect of catalyst amount on the reactivity of BPyBr for the synthesis of AGC.
Entry
Catalyst amount (mmol)
Conversion (%)
Selectivity (%)
TOF (h−1)
1
2
3
4
5
6
0.1
0.5
1
1.5
2
31
79
95
96
94
93
99
97
95
95
92
90
6947
3541
2129
1434
1053
834
2.5
Reaction conditions: AGE = 18.6 mmol, catalyst = BPyBr, PCO2 = 0.96 MPa, microwave
power = 200 W, time = 30 s. TOF: moles of AGE converted per mol of ionic liquid/h.
decreases. As a result, the number of substrate molecules which need to
be activated per unit time by each active site, is reduced [30]. Further in-
crease in the amount of catalyst did not produce any significant increase
in the conversion most likely owing to the hindrance of the mass trans-
fer between the active site and reagent caused by the low dispersity of
the excess catalyst in the reaction mixture [35].
Fig. 1. Effect of microwave power on the reactivity of BPyBr for the synthesis of AGC. Re-
action conditions: AGE = 18.6 mmol, catalyst amount = 1 mmol, PCO2 = 0.96 MPa,
time = 30 s.
3
.3. Effect of reaction time
The effect of reaction time on the microwave-assisted, solvent-free
cycloaddition of AGE and CO
periments were performed at a microwave power of 200 W, 0.96 MPa
of CO pressure, and 1 mmol of BPyBr. The results indicated that the re-
action proceeded rapidly for 30 s, with 95% conversion and 95% selectiv-
ity. After this period, the reaction rate did not vary significantly. Thus, a
reaction time of 30 s was determined to be the maximum time required
for AGE conversion. Remarkably, the selectivity remained greater than
2
to yield AGC is illustrated in Table 3. Ex-
2
3.5. Effect of CO pressure
2
Fig. 2 shows the effect of CO
cycloaddition of AGE and CO in the presence of BPyBr at 200 W for
30 s. As seen in Fig. 2, the AGE conversion increased from 10% to 74%
when the CO pressure increased from 0.34 MPa to 0.82 MPa. Upon a
further increase in the pressure (0.96 MPa), the conversion increased
to a maximum of 95%. During these catalytic reactions, higher CO pres-
sure can effectively increase the solubility of CO in AGE, enabling the
2
pressure on the microwave-assisted
2
2
9
0% over the entire course of the reaction. The only byproduct obtained
2
during the course of the reaction was 3-allyloxy-1,2-propanediol.
2
reaction equilibrium to shift towards carbonate formation. However, it
has been observed that a too high pressure decreases the conversion
3
.4. Effect of microwave power
2
owing to the effect of dilution by excessive quantities of CO [8].
Fig. 1 shows the effects of microwave power on the BPyBr-catalyzed
cycloaddition of AGE and CO
2
at 0.96 MPa of CO
2
pressure over a power
2
3.6. Cycloaddition of CO with various epoxides
range of 50–500 W for 30 s. The heat generated by microwave irradia-
tion greatly depends on the concentration of the reaction mixture and
the dielectric properties of the material [1–4]. The high catalytic activity
of BPyBr observed over a relatively short reaction time of 30 s arises
from the effective polarization of the catalyst by microwave radiation.
There was a steady increase in AGE conversion as the microwave
power increased from 50 W to 200 W. However, a further increase in
microwave power from 200 W to 500 W resulted in a slight decrease
in the selectivity. It appears that the higher temperatures attained
Various epoxide substrates were subjected to the cycloaddition reac-
tion using BPyBr under the optimized conditions determined as ex-
plained above. The results are summarized in Table 4. Among the
terminal epoxides, AGE (entry 2) showed the highest conversion. It is
also observed in our previous works, that the epoxide substrates
with higher microwave power suppressed the dissolution of CO
2
,
resulting in an insufficient supply of CO molecules available for cyclo-
2
addition to epoxides [15–18]. In order to understand the heat generated
during the microwave assisted BPyBr-catalyzed cycloaddition of AGE
and CO , a graph is plotted (Fig. S1, Supplementary information) show-
2
ing the temperature profiles of the reactor under different microwave
power. The temperature increased rapidly until 30 s and then it in-
creased slowly until a constant value was reached.
Table 3
Effect of reaction time on the reactivity of BPyBr for the synthesis of AGC.
Entry
Time (sec)
Conversion (%)
Selectivity (%)
TOF (h−1)
1
2
3
4
5
10
20
30
60
120
77
83
95
98
99
94
95
95
94
96
5115
2757
2129
1091
553
2
Fig. 2. Effect of CO pressure on the reactivity of BPyBr for the synthesis of AGC. Reaction
Reaction conditions: AGE = 18.6 mmol, BPyBr = 1 mmol, PCO2 = 0.96 MPa, microwave
power = 200 W. TOF: moles of AGE converted per mol of ionic liquid/h.
conditions: AGE = 18.6 mmol, catalyst amount = 1 mmol, microwave power = 200 W,
time = 30 s.

34
J. Tharun et al. / Catalysis Communications 54 (2014) 31–34
Table 4
Cycloaddition of CO
Acknowledgments
2
and various epoxides with BPyBr catalyst.
Entry
1
Epoxide
Cyclic carbonate
Conversion (%)
Selectivity (%)
96
This study was supported by the National Research Foundation of
Korea (NRF 2012-001507, and Global Frontier Program) and the Brain
Korea 21 PLUS Project.
70
2
3
95
82
95
91
Appendix A. Supplementary data
Supplementary data to this article can be found online at http://dx.
doi.org/10.1016/j.catcom.2014.05.016.
4
10
90
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[
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