Efficient route for oxazolidinone synthesis using heterogeneous biopolymer catalysts from unactivated alkyl aziridine and CO2 under mild conditions

Article abstract of DOI:10.1016/j.apcata.2012.09.031

Biopolymers made of polysaccharide chains are emerging as promising materials for designing efficient, cheap, environmental friendly and recyclable heterogeneous catalysts. In this study, we synthesized a series of covalently functionalized chitosan-alkyl pyridinium halides (CS-RPX, R = ethyl, propyl, butyl, hexyl and X = Cl, Br) and evaluated their potential application as catalysts for the chemical transformation of CO₂ to 4-methyl-2-oxazolidinone using 2-methylaziridine under mild reaction conditions. The catalysts were characterized using different physicochemical methods, including X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), thermo gravimetric analysis (TGA), elemental analysis (EA) and field emission scanning electron microscopy (FE-SEM). ¹H NMR, GC-MS, EA and FT-IR were used to confirm successful oxazolidinone formation. Cycloaddition was found to proceed through the synergistic effect of the hydroxyl and amine groups of chitosan together with the anion. The catalyst was reused five times after the cycloaddition reaction, with a loss of 2-6% in conversion and 1-3% in selectivity per cycle. The effect of different reaction parameters, such as catalyst amount, time, temperature and CO₂ pressure were studied to determine the reaction conditions that resulted in the highest conversion and selectivity.

Full text of DOI:10.1016/j.apcata.2012.09.031

Applied Catalysis A: General 447–448 (2012) 107–114
Contents lists available at SciVerse ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Efficient route for oxazolidinone synthesis using heterogeneous
biopolymer catalysts from unactivated alkyl aziridine and CO under
2
mild conditions
Amal Cherian Kathalikkattil, Jose Tharun, Roshith Roshan, Han-Geul Soek, Dae-Won
∗
Park
School of Chemical and Biomolecular Engineering, Pusan National University, Busan 609-735, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 30 July 2012
Received in revised form 6 September 2012
Accepted 10 September 2012
Available online 8 October 2012
Biopolymers made of polysaccharide chains are emerging as promising materials for designing efficient,
cheap, environmental friendly and recyclable heterogeneous catalysts. In this study, we synthesized a
series of covalently functionalized chitosan-alkyl pyridinium halides (CS-RPX, R = ethyl, propyl, butyl,
hexyl and X = Cl, Br) and evaluated their potential application as catalysts for the chemical transfor-
mation of CO2 to 4-methyl-2-oxazolidinone using 2-methylaziridine under mild reaction conditions. The
catalysts were characterized using different physicochemical methods, including X-ray diffraction (XRD),
Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), thermo gravi-
metric analysis (TGA), elemental analysis (EA) and field emission scanning electron microscopy (FE-SEM).
Keywords:
Chitosan
Pyridinium ionic liquids
Carbondioxide
1
H NMR, GC–MS, EA and FT-IR were used to confirm successful oxazolidinone formation. Cycloaddition
2
4
-Methylaziridine,
-Methyloxazolidin-2-one
was found to proceed through the synergistic effect of the hydroxyl and amine groups of chitosan together
with the anion. The catalyst was reused five times after the cycloaddition reaction, with a loss of 2–6% in
conversion and 1–3% in selectivity per cycle. The effect of different reaction parameters, such as catalyst
amount, time, temperature and CO2 pressure were studied to determine the reaction conditions that
resulted in the highest conversion and selectivity.
©
2012 Elsevier B.V. All rights reserved.
1
. Introduction
approach due to increasing concerns over the development of eco-
friendly processes. Moreover, among the alternative methods for
oxazolidinone synthesis, viz., carbonylation of amino alcohols using
phosgene or CO [21–24] and the reaction of propargylamine or
propargylic alcohol with CO2 [25–27], the former one (using phos-
gene or CO) is considered as less attractive C1 feedstock from a
green prospective in comparison to the cheap, nontoxic and abun-
dant CO2.
Increasing focus on research for the development of feasible
alternative pathways to obtain valuable chemicals using CO as a C1
2
building block is motivated by advantages like its abundance, non-
inflammability, inexpensiveness and non-toxicity [1,2]. Despite the
challenge raised by the kinetic and thermodynamic stability of CO2,
the synthesis of useful products like cyclic and poly carbonates
from CO2 using epoxides in presence of catalyst have been well
progressed and are continuously being updated [3–5]. Recently,
aziridines, the highly ring-strained heterocyclic N-analogues of
epoxides, are being emerged as an attractive substrate for CO2
fixation via [2 + 3] coupling to yield oxazolidinones (Scheme 1)
A few homogeneous catalysts like inorganic salts, complexes
[12,20,28–32], organic compounds, organic salts, ionic liquids
[7–10,33–35], etc. have been used for the cycloaddition of aziridines
with CO . Despite being the most active homogeneous catalyst,
2
ionic liquids are fraught with drawbacks, including the tedious
process required to separate the reaction mixture at the end of
synthesis. More recently, polymer supported ionic liquids [12,36]
were developed in an attempt to overcome these limitations. Even
though these synthetic polymers offer themselves as good plat-
forms for rendering heterogeneity to ionic liquids, they still poses
the limitation of acting only as a scaffold and do not promote
or enhance the reaction. Recently, the role of vicinal hydroxyl
groups in enhancing the cycloaddition of aziridines with CO2 was
demonstrated by the high efficiency of polymer supported diol
functionalized imidazolium ionic liquids [12].
[
6–12]. Oxazolidinones are a class of 5-membered heterocyclic
compounds that find applications as intermediates and chiral aux-
iliaries in organic synthesis and polymer synthesis [13–17], and
biologically active pharmaceutical agents [18,19]. Even though the
development of this process had been stagnant for the past three
decades [20], researchers have more recently revisited using this
∗ Corresponding author. Tel.: +82 51 510 2399; fax: +82 51 512 8563.
E-mail address: dwpark@pusan.ac.kr (D.-W. Park).
0
926-860X/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.apcata.2012.09.031

1
08
A.C. Kathalikkattil et al. / Applied Catalysis A: General 447–448 (2012) 107–114
Scheme 1. Cycloaddition of aziridines with CO2.
In this context, chitosan like biopolymers, which inherently pos-
sesses plenty of hydroxyl groups, could be regarded as efficient and
natural support materials. Chitosan (CS), the second most abundant
polysaccharide found on earth next to cellulose, is a deacetylated
derivative of chitin which occurs in the exoskeleton of shellfish
shell such as crab, lobster and shrimp and in food items such
as grain, yeast, bananas, and mushrooms. Dry pawn waste con-
tains 23% chitin and dry squid contains 15% chitin [38], which
could be extracted using 5% NaOH followed by 30% HCl. Chitosan
stands an attractive biopolymer for catalytic applications due to its
low cost, biocompatibility, biodegradability, and non-toxicity [37].
Thus, chitosan is a much more environmental friendly support than
synthetic polymers. Furthermore, in comparison to cellulose, chi-
tosan is blessed with primary amine group which acts as an adaptor
for easy covalent incorporation of catalytically active sites [39,40].
Sun et al. [41] recently succeeded in covalently tethering imidaz-
olium ionic liquids to chitosan, whereby utilizing the potentials of
hydroxyl and amine groups to act synergistically with the halide ion
in the cycloaddition of epoxides. However, for the cycloaddition of
Scheme 2. Schematic representation of the synthesis of chitosan supported alkyl
pyridinium halides (CS-RPX).
dibromoethane and pyridine. During this preparation step, dibro-
moethane (35 g, 186.3 mmol) was dissolved in 20 mL acetone under
nitrogen atmosphere in a 100 mL two-neck round bottomed flask
at room temperature. Pyridine (5 mL, 62.1 mmol) was then slowly
added and refluxed for 24 h. The precipitate formed was filtered,
washed in acetone for 6 h using a soxhlet extractor and dried under
◦
vacuum at 60 C for 12 h. The second step in this process was
the covalent immobilization of Br-EPBr to chitosan (CS). In this
step, 1.78 g chitosan (10.98 mmol) was added to 30 mL 1-methyl-
◦
2-pyrrolidinone (NMP) and stirred at 50 C for 12 h in a two-neck
flask. Br-EPBr (6.62 g, 24.8 mmol in 10 mL NMP) was added and
◦
then stirred at 80 C for 24 h. Upon completion of the reaction,
aziridines with CO , pyridinium salts have been reported to show
the product was precipitated by its addition to 100 mL of ethanol,
washed 4–5 times with dry ethanol and then rinsed with dry ace-
tone. The sample was dried under vacuum at 80 C for 24 h to
obtain CS-EPBr. The other CS-RPX catalysts, viz., CS-EPCl, CS-EPBr,
CS-PrPBr, CS-BPBr, and CS-HPBr were synthesized using a similar
process. CS-EMImBr [41] was also synthesized and its activity in
the cycloaddition of CO2 with aziridines was compared with the
CS-RPX catalysts.
2
better activity than the imidazolium counterparts [9]. Despite the
high ring strain energy of aziridines, the readily available unacti-
vated aziridines are less susceptible to nucleophilic ring opening
when compared to the aziridines that have been activated by qua-
ternization, N-substitution with electron withdrawing substituents
and Lewis base adduct formation [42]. In this study, we designed
an efficient catalyst system that could facilitate the cycloaddi-
tion of even unactivated aziridine like 2-methylaziridine (MeAz)
under mild conditions. This was accomplished by synthesizing chi-
tosan supported alkyl pyridinium halides (CS-RPX, R = Et, Pr, Bu,
Hex and X = Cl, Br) and engaging them as catalysts for the synthe-
sis of 4-methyl-2-oxazolidinone from 2-methylaziridine and CO2.
The process represents a recyclable, eco-friendly and energy-saving
pathway for the cycloaddition of unactivated aziridines with CO2
under mild conditions.
◦
2.3. Characterization
The catalysts were characterized by X-ray diffraction (XRD), X-
ray photoelectron spectroscopy (XPS), fourier transform infra-red
spectroscopy (FT-IR), elemental analysis (EA), thermogravimetric
analysis (TGA) and field emission scanning electron microscopy
(FE-SEM). The characterizations of the various X-RPX salts (EA, 1
H
NMR) are provided in the supplementary data. The cyclic product
was identified and confirmed using the GC–MS, EA, FT-IR and 1
H
2
. Experimental
NMR techniques. X-ray diffraction (XRD) patterns were obtained on
a Philips PANalytical X’pert PRO Model power diffractometer that
was operated at 40 kV and 30 mA using Ni filtered Cu-K␣ radiation
2
.1. Materials
(ꢀ = 1.5404
A˚ ). The diffractograms were recorded in the 2ꢁ range
Chitosan, 1-methyl-2-pyrrolidinone (99%), 1,2-dichloroethane
◦
(
99.8%), 1,2-dibromoethane (98%), 1,3-dibromopropane (99%), 1,4-
5–55 . XPS analyses were performed using an X-ray photoelectron
spectrometer (Theta Probe AR-XPS System, Thermo Fisher Scien-
tific, U.K.) with monochromatic Al-K␣ radiation (hꢂ = 1486.6 eV).
The FT-IR spectra were obtained on Vertex 80 V Microscopic FT-
dibromobutane (99%), 1,6-dibromohexane (96%), pyridine (≥99%),
acetone (≥99.9%), anhydrous tetrahydrofuran (THF) and anhydrous
ethanol were purchased from Aldrich and used as received. 2-
Methylaziridine (90%) and biphenyl (99.5%) were also obtained
from Aldrich. Carbon dioxide of 99.999% purity was used without
further purification.
−
1
IR/Raman Spectrophotometer at a resolution of 4 cm . Elemental
analyses of the catalysts and oxazolidinone were carried out on
a Vario Micro Cube analyzer. Thermogravimetric analyses (TGA)
were performed with 3.78–6.05 mg of CS-RPX using an SDT-Q600
analyzer (TA Instruments Inc.). The surface features of the materi-
als were observed using an S-4200 field emission scanning electron
microscope (FE-SEM, Hitachi). The 1H NMR spectra of X-RPX and
2.2. Catalyst synthesis
The chitosan supported alkyl pyridinium halide catalysts, CS-
−
−
oxazolidinone were obtained on a Varian 500 MHz using D O
RPX, (RPX represents the ionic liquid, where X = Cl or Br and
R = ethyl (E), propyl (Pr), butyl (B) and hexyl (H)) were synthesized
2
and CD OD respectively as solvents. Gas chromatography/mass
3
spectrometry (GC–MS) analysis of the cycloaddition products was
performed on an Agillent 5975 C GC/MSD analyzer.
[
(
41] in two steps (Scheme 2) as follows (CS-EPBr): First, N-
2-bromoethyl)pyridinium bromide, Br-EPBr, was prepared from

A.C. Kathalikkattil et al. / Applied Catalysis A: General 447–448 (2012) 107–114
109
Table 1
ꢀ
H, NH (d)), 4.51 (t, 1H, CH2 (c )), 3.92–4.02 (m, 2H, CH2 (c)
1
Effect of various CS-RPX catalysts on the synthesis of MeAz.a
and CH (b)), 1.25(d, 3H, CH3 (a)) C, H, N Data: Calculated (%):
C = 47.52; H = 6.98; N = 13.85. Observed (%): C = 47.87; H = 7.18;
−
1
N = 13.85. IR spectral data: (ꢂmax/cm ) 3318(b), 2980(s), 2936(w),
730(s), 1484(m), 1404(m), 1241(s), 1030(m). GC–MS: m/z calcd
for C H7NO : 101.05. Found: m/z 101.1.
1
4
2
3. Results and discussion
The primary role of ionic liquid based catalysts in the cycload-
dition of aziridine with CO2 is to promote the nucleophilic ring
opening of aziridine to yield an intermediate that can undergo
cycloaddition with CO [9,10]. Pinhas et al. [11] previously reported
2
that, generally the cycloaddition of CO2 with 2-aryl aziridines
easily led to the regioselective product 5-aryl-2-oxazolidinones,
while 2-alkyl aziridines yielded 4-alkyl and 5-alkyl isomers in
a ratio of 2:1 ratio. Six membered ring dimers of aziridine,
viz., 2,3- and 2,5-dimethyl piperazines were also reported to be
generated along with the major products. Generally, chitosan func-
tionalization is synthetically achieved by dispersing chitosan in
iso-propanol. However, chitosan has low dispersion in iso-propanol
and remains clumped until the final step. To address this issue, we
found that chitosan could be better dispersed by using1-methyl-2-
pyrrolidinone (NMP) as the solvent for synthesis.
Conv (%)b,c
Yield (%)b
TONd
Entry
Catalyst
a
b
(c + d)
1
2
3
4
5
6
7
8
9
None
0
0
0.1
0
0
0
-
CS
Trace
77.6
98.3
11.0
98.1
97.9
93.9
84.2
99.8
0.0
6.3
5.8
0.0
2.5
12.2
4.5
5.3
9.8
4.8
4.1
9.8
11.5
0.5
5.9
13.5
0.8
EPBre
66.5
88.4
1.2
84.1
85.2
83.5
67.5
93.4
154.6
195.7
27.8
194.6
194.1
194.1
188.0
198.7
CS-EPBr
CS-EPCl
CS-PrPBr
CS-BPBr
CS-HPBr
CS-EMImBr
f
1
0
CS-EPBr
5.7
a
◦
Reaction conditions: Meaz = 28.3 mmol (2 mL, 25 C), CS and CS-RPX = 0.2 mmol,
3.1. Characterization of chitosan supported pyridinium catalysts
◦
PCO2 = 2 MPa, temperature = 100 C, time = 4 h, 600 rpm. THF = 2 mL.
b
Determined by GC.
Internal standard = 0.05 g biphenyl.
Various physicochemical methods such as XRD, XPS, FT-IR, TGA,
EA and FE-SEM were used to characterize the synthesized cata-
lysts. However, due to the structural similarity between the various
CS-RPX catalysts, only the catalytic features of CS-EPBr will be
described in detail. The characteristics of all other CS-RPX catalysts
and their precursors are provided in the supporting data.
c
d
e
Turnover number (TON): moles of aziridine converted per mole of ionic liquid.
.141 mmol of EPBr, equivalent to the loaded EPBr in 0.2 mmol of the synthesized
0
CS-EPBr.
f
◦
Semi-batch reaction conditions: Meaz = 28.3 mmol (2 mL, 25 C), CS-
◦
EPBr = 0.2 mmol, PCO2 = 0.8 MPa (continuous supply), temperature = 100 C,
time = 4 h, 600 rpm, THF = 2 mL.
The successful covalent immobilization of the pyridinium IL
(
EPBr) on the surface of chitosan was confirmed using XPS anal-
2
.4. Cycloaddition of 2-methylaziridine with CO2
ysis by comparing the Br 3d spectrum of EPBr, Br-EPBr and CS-EPBr
(
anion, only appeared to have a Br 3d peak at 65 eV whereas Br-EPBr,
the precursor for the functionalization of chitosan which contained
−
Fig. 1(a)). The parent IL (EPBr) which contained bromine as Br
The synthesis of methyloxazolidinones from 2-methylaziridine
(
MeAz) and CO2 using various CS-RPX catalysts was carried out in
a 60 mL stainless steel reactor equipped with a magnetic stirrer.
In a typical batch operation, an appropriate amount of the CS-RPX
catalyst was charged in the reactor containing 2 mL (28.27 mmol)
of MeAz and 2 mL THF. The reactor was pressurized with CO2 to a
preset pressure at room temperature. The reactor was then heated
to the desired temperature and stirred at 600 rpm. After the reac-
tion was complete, the reactor was cooled in ice and the remaining
CO was vented off carefully. 8 mL of THF was added, and the prod-
uct was filtered and subjected to GC analysis. The catalyst was then
recycled.
−
both Br and CH Br in the 1:1 ratio exhibited two Br 3d peaks
2
with the same intensities at 65 eV and 67.5 eV respectively. The
catalyst material CS-EPBr only exhibited a peak at 65 eV, conclu-
sively demonstrating the successful tethering of the precursor to
the chitosan surface [43]. The Br 3d spectra for the other CS-RPBr
catalysts and the Cl 2p spectra of CS-EPCl produced similar peaks
and are shown in Fig. S1(a) and (b), respectively, in the supporting
data. Further evidence of successful covalent immobilization was
obtained by comparing the N 1s spectra of CS, Br-EPBr and CS-EPBr
2
(
Fig. 1(b)). The N 1s peak of chitosan was at 396.5 eV, which corre-
sponded to the primary amine group, while the N 1s peak of Br-EPBr
for pyridinium N+ was at 399.5 eV [45]. As expected, CS-EPBr con-
tained two major peaks at 397.4 eV and 399.5 eV, which was due to
2
.5. Product separation and analysis
Product analysis was carried out using EA, FT-IR, 1H NMR, and
the secondary amine group and pyridinium N , respectively. The N
+
GC–MS analyzers. The conversion of MeAz was obtained from
gas chromatography (GC, HP 6890, Agilent Technologies) and
the yield was calculated using biphenyl (0.05 g) as an internal
standard. The residue was purified over a silica gel by column
chromatography using hexane/ethyl acetate = 9:1 to 3:7 as eluant
to afford the products. The presence of the expected products 4-
methyl-1,3-oxazolidin-2-one (a), 5-methyl-1,3-oxazolidin-2-one
1s band, corresponding to the unreacted amine groups of chitosan,
was observed as a weak shoulder peak in the primary amine region
at 396.4 eV. The N 1s spectra of various CS-RPX materials and their
X-RPX counterparts are shown in Fig. S2 of the supporting data.
Further characterization of the bulk CS-RPX catalysts were car-
ried out using the X-ray diffraction technique. Two distinct peaks
around 2ꢁ values of 10 and 19 were observed in the XRD pattern of
chitosan. These peaks were attributed to the diffraction at the plane
of the crystal region in the chitosan structure [44]. These peaks,
which resulted from the crystallization capacity of chitosan due
to the extensive intramolecular H-bonding between NH2 and OH
groups in the repeating hexosaminide residues [45], became less
◦
◦
(
b), 2,5-dimethyl-piperazine (c) and 2,3-dimethyl-piperazine (d),
were detected in the crude mixture by GC-MS analysis (Table 1).
The isolated major product 4-methyl-1,3-oxazolidin-2-one (a), a
1
colourless liquid, was further confirmed by H NMR, EA, and
1
FT-IR analysis. H NMR Data (500 MHz, CD OD) ı ppm: 4.80 (s,
3

1
10
A.C. Kathalikkattil et al. / Applied Catalysis A: General 447–448 (2012) 107–114
Fig. 2. XRD patterns of CS and CS-EPBr catalyst.
Fig. 1. Comparison of XPS data of EPBr, Br-EPBr and CS-EPBr using (a) Br3d and (b)
N1s spectra illustrating the successful covalent immobilization of EPBr on chitosan
surface.
Fig. 3. FT-IR spectra of CS and CS-EPBr.
decomposition of the polysaccharide chain in chitosan [40]. CS-
RPX, including CS-EPBr, was synthesized under dry conditions and
showed comparatively less water loss around 100 C (Fig. S5, sup-
intense upon covalent immobilization of pyridinium ILs to chitosan.
Representative patterns comparing the XRD of CS and CS-EPBr are
shown in Fig. 2, while that of the remaining catalysts are depicted
in Fig. S3 of the supporting data. CS-RPX compounds gave weak and
◦
porting data). The decomposition of the polysaccharide chain for all
◦
broad XRD peaks at 2ꢁ = 20 [45].
The FT-IR spectra of CS and CS-EPBr are shown in Fig. 3. The
peaks corresponding to the saccharide structure were found around
−
1
−1
(C O C in glycosidic linkage) and 890 cm
1
[
155–1040 cm
−1
37,46]. The broad peak around 3500 cm was assigned to both
O
H as well as N H stretching vibrations [45,46]. The peak
−1
around 1660 cm
corresponded to the C O stretching, which
was observed in both CS and CS-RPX materials. However, the NH
bending vibration of the primary NH2 group of chitosan, which
−1
−1
appeared at 1600 cm , migrated to 1540 cm due to the forma-
tion of the secondary >N H in CS-EPBr [47]. These findings confirm
the successful covalent immobilization of EPBr on CS-EPBr. The
other CS-RPX catalysts exhibited similar peaks and the results are
shown in Fig. S4 of the supporting data.
The results of the thermal analysis (TGA) of CS and CS-EPBr
◦
are shown in Fig. 4. For CS, the first weight loss around 100 C
resulted from the removal of absorbed water molecules. The fur-
◦
ther weight loss occurring around 300 C was likely due to the
Fig. 4. Thermogravimetric plots (TGA–DTG) of CS and CS-EPBr.

A.C. Kathalikkattil et al. / Applied Catalysis A: General 447–448 (2012) 107–114
111
Table 2
containing CS-EPCl (entry 5) was only 11% and 28, respectively. A
high order of regioselectivity (around 94:6) was achieved using CS-
EPBr, which parallels the high regioselectivity reported by Pinhas
et al. with alkyl aziridines [11]. The turnover number obtained with
equimolar quantities of EPBr (TON = 155, entry 10) clearly supple-
Loading of ionic liquids on various CS-RPX catalysts (obtained from elemental anal-
ysis data).
Entry
Sample
%C
%H
%N
%O
IL loading (mmol g−1)
1
2
3
4
5
6
CS
40.3
6.92
7.31
6.27
6.43
6.82
6.71
7.2
42.88
38.10
33.14
34.65
33.37
34.81
–
CS-EPCl
CS-EPBr
CS-PrPBr
CS-BPBr
CS-HPBr
41.86
37.93
37.10
38.73
38.82
6.18
5.64
5.45
5.24
5.59
1.85
2.13
2.05
1.98
1.76
ments the expected role of synergism played by OH and NH groups
2
of CS-EPBr in catalyzing the reaction.
In contrast, the alkyl chain length of the pyridinium cation
was found to play only a negligible role in the conversion and
selectivity of cycloaddition (entries 4, 6–8). However, CS-EPBr was
the most active catalyst among them, with highest conversion,
yield and TON. To obtain a quantitative information regarding
the difference in activity with the imidazolium and pyridinium
cation, the cycloaddition reactions of aziridine were performed
using a recently reported biopolymer catalyst, CS-EMImBr [41] (CS-
EMImBr is chitosan immobilized imidazolium ionic liquid (entry
9)). In these experiments, all of the CS-RPBr catalysts displayed a
better conversion, selectivity and TON than CS-EMImBr. Based on
these combined findings, CS-EPBr was selected for further studies.
◦
CS-RPX materials occurred at around 260 C, which was in agree-
ment with the assumptions from the XRD analysis regarding the
reduced crystallinity.
Based on the elemental analysis of the various CS-RPX materi-
als (Table 2), the loading of various X-RPX ionic liquids, viz., EPCl,
EPBr, PrPBr, BPBr and HPBr in the CS-RPX were calculated (based on
halides) to be 1.85, 2.13, 2.05, 1.98 and 1.76 mmol per gram of chi-
tosan, respectively. The highest loading was observed for CS-EPBr
(
Table 2, entry 3).
The SEM images of CS-RPX (Fig. S6, supporting data) showed that
3.4. Proposed mechanism of cycloaddition reaction
the material was uniform with a lower crystallinity than pristine
CS, which is in agreement with the surface modification analyses
of CS-RPX.
As per the previous reports, the general mechanism of ionic liq-
uid based catalysis for the cycloaddition of alkylaziridine with CO2
commences with attack by the anion at the least hindered ␤-carbon,
leading to the ring opening of aziridines [9,10,36]. To rationalize
our experimental results of the catalytic activities of CS-RPX cat-
alysts in the cycloaddition of CO2 and MeAz (Scheme 3), we are
herewith proposing a plausible mechanism which also involves
additional intermediates in the pathway. The N-atom of MeAz binds
strongly with the hydroxyl group of chitosan (CS) through both
intramolecular N—H· · ·O and H—O· · ·N hydrogen bonding interac-
tions. This strong H-bonding owes to the NH group of MeAz that
can behave as intramolecular H-bonding donor as well as acceptor
unlike N-substituted aziridines which can act only as a H-bonding
acceptor at the heterocyclic N-atom. The halide ion of the bound
chitosan generates synchronized nucleophilic attack at the least
sterically hindered ␤-carbon atom of MeAz (Scheme 3) paving way
to the formation of the ring-opened intermediate 2 [45]. Parallel
to this, the secondary NH group of the catalyst interacts with CO2
leading to the carbamate salt 3 formation [41]. It is noteworthy
that the potential of such a carbamate salt formed between cata-
lyst and CO2 in synergistically boosting the cycloaddition [41] is
unexplored in the cycloaddition of CO2 and aziridines. In the next
3.2. Characterization of the cyclic product, oxazolidinone
A very strong absorption band at 1730 cm−1was observed in
the FT-IR spectrum of 4-methyl-1,3-oxazolidin-2-one (Fig. S7, sup-
porting data), which was characteristic of the amide linkage in
oxazolidinone and corresponded to the ꢂC O stretching vibrations
[
1
17,20,48,49]. A secondary amine vibration was also observed at
484 cm . In addition, the absorption band at 3318 cm cor-
−
1
−1
responded to the ꢂN H in the oxazolidinone. The other bands
−
−
1
at 2980 and 2936 cm for alkane ( CH2 and CH ) stretch-
3
1
ing vibrations, 1404 cm for alkyl bending vibrations, and 1241
−1
and 1030 cm
for C O and C N stretching vibrations further
confirmed oxazolidinone formation. H NMR analysis in CD OD
1
3
(
Fig. S8, supporting data) also confirmed successful 4-methyl-1,3-
oxazolidin-2-one formation. ı ppm: 4.80 (s, 1H, NH (d)), 4.51 (t,
ꢀ
1
H, CH (c )), 3.92–4.02 (m, 2H, CH (c) and CH (b)), 1.25 (d, 3H,
2
2
CH3 (a)). Product formation was further confirmed by elemental
analysis and GC–MS (Table S1 and Fig. S9, supporting data).
−
3.3. Influence of various chitosan-pyridinium (CS-RPX) catalysts
step, the nucleophilic attack of the N of 2 on carbamate salt 3 at
on oxazolidinone synthesis
its carbonyl centre produces the alkyl urethane anion 4 along with
the regeneration of the catalyst CS-RPX. Further, the ring closure
−
In the absence of a catalyst, the reaction of CO and aziridine did
takes place on the alkyl urethane anion 4 when the O atom attacks
2
not afford any product (entry 1). The biopolymer support material
chitosan resulted in only trace amounts of MeAz conversion and no
the ␤-carbon atom, so that the halide gets eliminated, resulting in
the regeneration of the catalyst and the formation of the product
4-methyl-2-oxazolidinone a. The comparatively disfavored isomer
5-methyl-2-oxazolidinone b is formed upon the nucleophilic attack
of halide on the more substituted carbon (␣-carbon) atom. The two
other side products, piperazines c and d (Table 1) are formed by the
nucleophilic attack of intermediate 2 on the ␤- or ␣-carbon atom
of intermediate 1.
4
-methyl-2-oxazolidinone (a) was produced (entry 2). Catalysis of
the cycloaddition of 2-methyl aziridine (MeAz) with CO2 was con-
ducted in a batch-wise operation in the presence of 0.71 mol% of
the catalyst relative to MeAz at temperatures, CO2 pressures and
◦
reaction times varying from 40–100 C, 0.8 to 2 MPa and 1–5 h at
6
00 rpm (Table 1). Reactions were performed with the ionic liquid
precursor, 1-ethylpyridinium bromide (EPBr, entry 3) and a sim-
ilar heterogeneous catalyst CS-EMImBr (entry 9) [41] in order to
compare the efficiencies of the CS-RPX catalysts.
3.5. Influence of reaction parameters
The effect of molecular structure and composition of the CS-
RPXs on the cycloaddition reactions of CO2 with MeAz are shown
in Table 1 (entries 4–8). The nucleophilicity of the anion associated
with the CS-RPX was found to play a key role in dictating the cata-
lyst activity. Of the two anions, the more nucleophilic bromide ion
of CS-EPBr (entry 4) yielded a conversion of 98% MeAz with a high
TON of 196, whereas the conversion and the TON using chloride ion
The effects of reaction parameters, viz., catalyst amount, reac-
tion time, temperature, and CO2 pressure on the cycloaddition of
MeAz with CO2 were studied using CS-EPBr as the catalyst (Fig. 5).
The conversion rate of MeAz which was low at a catalyst amount
of 0.18 mol% was found to increase when the catalyst amount
was increased up to 0.71 mol%. However, a further increase in the
catalyst amount did not result in any significant increase in the

1
12
A.C. Kathalikkattil et al. / Applied Catalysis A: General 447–448 (2012) 107–114
Scheme 3. Proposed reaction mechanism for the cycloaddition of 2-methylaziridine with CO2.
yield of the major product 4-methyl-2-oxazolidinone (a) (Table 1),
which may be ascribed to the hindrance in mass transfer occurring
between active site and reagent resulting from the low dispersity
of excess catalyst in the reaction mixture [50]. The influence of cat-
alyst amount on the cycloaddition of CO2 with MeAz is shown in
Fig. 5. The 0.71 catalyst mol% is the lowest reported for MeAz–CO2
cycloaddition to date [29,31]. Moreover, this is one of the low-
est reported catalyst mole percentages for heterogeneous catalysts
used in the cycloaddition of aziridines and CO (including activated
2
aziridines).
A study on the effect of reaction time on CS-RPX catalyzed
cycloaddition is depicted in Fig. 6. The conversion increased with an
increase in the reaction time and a 98.3% conversion was obtained
at 4 h. However, there was only a slight increase in the yield of 4-
methyl-2-oxazolidinone (a) (Table 1) at reaction times longer than
◦
4
h. We also investigated the effect of temperature (40–100 C) on
cycloaddition. As illustrated in Fig. 7, temperature had a positive
effect on the yield of oxazolidinone. A steep increase in the MeAz
conversion and yield of a was observed when the temperature was
Fig. 6. Effect of reaction time on MeAz conversion and selectivity of (a).
◦
◦
increased from 40 C to 100 C and the highest conversion of 98.3%
◦
was observed at 100 C. However, a further increase in the tem-
perature did not have a significant effect on the reaction and only
a slight increase in the selectivity of 4-methyl-2-oxazolidinone (a)
Fig. 7. Effect of reaction temperature on MeAz conversion and selectivity of (a).
was observed at higher temperatures, which was presumably due
to the lower solubility of the CO2 gas phase in the system [50].
Thus, the optimum temperature for this process was determined
Fig. 5. Dependence of CS-EPBr catalyst amount on the conversion of MeAz and
selectivity of 4-methyl-2-oxazolidinone (a).
◦
to be 100 C.

A.C. Kathalikkattil et al. / Applied Catalysis A: General 447–448 (2012) 107–114
113
Fig. 8. Effect of CO2 pressure on MeAz conversion and selectivity of (a).
Fig. 9. Catalyst recycle test.
4. Conclusions
The dependence of the cycloaddition reactions on CO2 pressure
was also studied between 0.8 MPa and 2.4 MPa (Fig. 8). Lower CO2
pressures of the order of 0.8 MPa gave low conversions with a selec-
tive production of the side products, piperazines c and d (Table 1).
The formation of low energy chitosan-CO2 complex 3 (Scheme 3)
We initiated biopolymer based material as a cheap, recyclable
and environmentally benign catalyst for the synthesis of oxazolidi-
none by the cycloaddition of aziridine with CO . The pyridinium
2
halide functionalized chitosan based biopolymer catalyst (CS-RPX)
in the present study offered a synergistic mechanism for the
cycloaddition process by means of hydroxyl, amine and halide
anion which in turn eased the successful cycloaddition of an unacti-
has a direct dependence on the available amount of CO , whereas
2
the H-bonded intermediates 1 and 2 (Scheme 3) are independent of
CO2 concentrations. Therefore at low CO2 pressures, the formation
of chitosan-CO2 complex 3 was retarded, whereas the increased
chance for nucleophilic attack of intermediate 2 on the ␤- or ␣-
carbon atom of intermediate 1 lead to the dimerized aziridines,
viz., 2,5- and 2,3-dimethyl piperazines, c and d respectively. At
higher CO2 pressures (1.2–2.4 MPa), where more of intermediate
vated alkyl aziridine with CO under mild reaction conditions and a
2
low catalyst mol percentage of 0.71% with respect to the substrate.
Moreover, CS-RPX catalysts are the first biopolymer based cata-
lysts used in the cycloaddition of aziridines with CO2 and exploits
the H-bonded assisted low energy pathways without performing
a synthetic modification to achieve cooperating functional groups.
3
(Scheme 3) might have been formed, the reaction completes
A high turnover number (TON) was achieved with the catalyst and
with 4-methyl-2-oxazolidinone (a) as the major product. However,
beyond 2 MPa CO2 pressures, there was no significant rise in the
yield, probably due to the effect of dilution by excessive quantities
◦
the optimum reaction conditions were 4 h, 100 C, 2 MPa CO pres-
2
sure and 0.2 mmol catalyst. A further increase in the yield and TON
was obtained even at 0.8 MPa under similar reaction conditions in
a semi-batch operation. In addition, the catalyst could be reused up
to five times by simple filtration without considerable loss in the
activity and yield.
of CO [50]. In order to establish the effect of a continuous supply of
2
CO2 at lower pressures, we performed a reaction at 0.8 MPa using
CS-EPBr under semi-batch conditions (entry 10). It was observed
that the conversion and yield (a) and TON increased to >99%, 93.4
and 199 respectively under these conditions.
Acknowledgements
3
.6. Reusability of the catalyst
This study was supported by the Korean Ministry of Education,
Science and Technology through the National Research Founda-
tion (2012-001507) and the Brain Korea 21 Project. The authors
are grateful to KBSI for performing characterization studies.
It is reported previously that ionic liquids used in the cycload-
dition of aziridines and CO2 have been recycled up to four times
without significant loss of activity or selectivity [10]. However, the
ionic liquid catalyst was reported to be recovered after the prod-
uct was separated out by distillation under reduced pressure from
the reaction mixture, which is both economically and technologi-
cally unviable. In contrast, the heterogeneous catalyst CS-EPBr in
our present study was recovered easily by simple filtration from
the crude mixture after the reaction. The recovered catalyst was
Appendix A. Supplementary data
Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/
j.apcata.2012.09.031.
◦
washed with dichloromethane, dried at 80 C for 24 h and reused
References
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remain active up to the fifth cycle, with a slow decrease in conver-
sion and selectivities during each cycle. The final (fifth) recycle of
the catalyst resulted in a MeAz conversion of 85.3% and selectivity
of 82.7% a.
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