BmimOAc ionic liquid: A highly efficient catalyst for synthesis of 3-aryl-2-oxazolidinones by direct condensation of 2-(arylamino) alcohols with diethyl carbonate

Article abstract of DOI:10.1016/j.molcata.2015.07.034

An efficient convenient procedure for the synthesis of 3-aryl-2-oxazolidinones from 2-(arylamino) alcohols and diethyl carbonate (DEC) catalyzed by ionic liquids is described. The effects of reaction time, amount of catalyst and temperature were investigated. Excellent yields of products were obtained under the optimized reaction conditions, when using BmimOAc as a catalyst. An intermediate ethyl 2-(phenyl amino) ethyl carbonate was isolated and characterized. ¹H NMR spectroscopy and DFT calculations indicated that BmimOAc cooperatively activate the substrates through hydrogen bonding with its anion and cation sites. According to these results, a possible reaction mechanism was discussed.

Full text of DOI:10.1016/j.molcata.2015.07.034

Accepted Manuscript
Title: BmimOAc Ionic Liquid: A Highly Efficient Catalyst for
Synthesis of 3-Aryl-2-oxazolidinones by Direct Condensation
of 2-(Arylamino) alcohols with Diethyl Carbonate
Author: Elnazeer H.M. Elageed Binshen Wang Yongya
Zhang Shi Wu Guohua Gao
PII:
S1381-1169(15)30045-5
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.molcata.2015.07.034
MOLCAA 9585
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Journal of Molecular Catalysis A: Chemical
Received date:
Revised date:
Accepted date:
27-4-2015
28-6-2015
29-7-2015
Please cite this article as: Elnazeer H.M.Elageed, Binshen Wang, Yongya Zhang,
Shi Wu, Guohua Gao, BmimOAc Ionic Liquid: A Highly Efficient Catalyst for
Synthesis of 3-Aryl-2-oxazolidinones by Direct Condensation of 2-(Arylamino)
alcohols with Diethyl Carbonate, Journal of Molecular Catalysis A: Chemical
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BmimOAc Ionic Liquid: A Highly Efficient Catalyst for Synthesis of 3-
Aryl-2-oxazolidinones by Direct Condensation of 2-(Arylamino) alcohols
with Diethyl Carbonate
Elnazeer H. M. Elageed, Binshen Wang, Yongya Zhang, Shi Wu,
Guohua Gao*
*
Shanghai Key Laboratory of Green Chemistry and Chemical Processes,
Department of Chemistry
East China Normal University
North Zhongshan Road 3663, Shanghai, 200062, China
Corresponding author:
Prof. Guohua Gao
Corresponding Address:
Shanghai Key Laboratory of Green Chemistry and Chemical Processes,
Department of Chemistry
East China Normal University
North Zhongshan Road 3663, Shanghai, 200062, China
Fax: (+) 86-21 62233323
E-mail: ghgao@chem.ecnu.edu.cn
1

Graphical abstract
Highlights


BmimOAc ionic liquid effectively catalyzes synthesis of 3-aryl-2-oxazolidinones.
Anion and cation of BmimOAc cooperatively activate substrates by hydrogen
bonds.

The ethyl 2-(phenyl amino) ethyl carbonate is the key intermediate in the reaction.
Abstract
An efficient convenient procedure for the synthesis of 3-aryl-2-
oxazolidinones from 2-(arylamino) alcohols and diethyl carbonate (DEC)
catalyzed by ionic liquids is described. The effects of reaction time, amount
of catalyst and temperature were investigated. Excellent yields of products
were obtained under the optimized reaction conditions, when using
BmimOAc as a catalyst. An intermediate ethyl 2-(phenyl amino) ethyl
1
carbonate was isolated and characterized. H NMR spectroscopy and DFT
calculations indicated that BmimOAc cooperatively activate the substrates
through hydrogen bonding with its anion and cation sites. According to these
results, a possible reaction mechanism was discussed.
Keywords: Ionic liquids; Cooperative; Catalysis; Mechanism;
Oxazolidinones.
2

1
. Introduction
3-Aryl-2-oxazolidinones are important materials in field of medicine and
organic synthesis. In recent decades they have attracted much attention due
to their bioactivity. They have been used as a new class of antibiotics, HIV-1
protease inhibitors [1-5], fungicides [6] and synthetic reagents [7].
Conventionally, 3-aryl-2-oxazolidinones are mostly synthesized using
phosgene or its derivatives with amino alcohols catalyzed by strong organic
bases [8, 9]. However phosgene and its derivatives are highly toxic reagents
and the strong bases limit the use of substrates containing different
functional groups. The alternative synthetic approaches via insertion of
carbon dioxide molecule into aziridine moiety [10-14], the reaction of amino
alcohols with carbon dioxide [15-18], and the reaction of propargylic
alcohols, amines with carbon dioxide which are usually catalyzed by copper
salts, palladium or tributylphosphine are also reported [19-22]. However, the
use of these approaches is required relatively drastic conditions (5.0 MPa of
0
CO pressure, 110-180 C and prolonged reaction time 9-20 h) and the
2
catalysts are highly toxic or expensive.
In the course of our studies on the utilization of ionic liquids as catalysts,
we reported approaches for the synthesis of 3-aryl-2-oxazolidinones via
reaction of cyclic carbonates with aromatic amines, and the straightforward
conversion of CO using ethylene oxide and aromatic amines in the presence
2
of catalytic amount of ionic liquids [23-26]. Although these approaches are
regarded as green, they still suffer from relative long reaction time, high
reaction temperature or large amount of catalyst. To overcome these
drawbacks, herein, in this paper we wish to report the success in synthesis of
3
-aryl-2-oxazolidinones in high yields by effective, convenient and eco-
friendly approach via direct condensation of 2-(arylamino) alcohols with
3

‘
greener reagent’ diethyl carbonate (DEC) in the presence of catalytic
amount of BmimOAc (Scheme 1).
Scheme 1. Synthesis of 3-aryl-2-oxazolidinones by reaction of 2-
(
arylamino) ethanol with DEC.
2. Experimental
2.1. General
Diethyl carbonate (DEC) was supplied by Sinopharm. 2-(Phenylamino)
ethanol was obtained from TCI. All ionic liquids were supplied by Center of
Green Chemistry and Catalysis, LICP and CAS. The other reagents were
commercial reagents of AR grade and used without further purification. The
2
-(arylamino) alcohols were synthesized by the reaction of aryl amines with
ethylene carbonate (EC) in the presence of BmimBr ionic liquid as catalysts
see supplementary data). GC analysis was performed by using a Shimadzu
GC-14B equipped with capillary column DM-
701(30m×0.32mm×0.25μm) equipped with a flame ionization detector.
(
a
1
The NMR spectra were recorded on Bruker Ascend 400 and Bruker
DRX500 with tetramethylsilane as the internal standard. The DFT
calculations were carried out by using the B3LYP functional with the 6-311g
basis set as implemented in Gaussian 09 program package. Vibrational
frequency calculations, from which the zero-point energies were derived,
have been performed for each optimized structure at the same level to
identify the natures of all the stationary points.
2.2. Typical procedure for the synthesis of 3-aryl-2-oxazolidinones
In a typical procedure, the reaction of 2-(arylamino) ethanol with DEC was
carried out in a 5 mL round bottomed flask. 2-(Arylamino) ethanol (2.0
4

mmol), DEC (10 mmol, 1.1813g) and BmimOAC (5%, 0.1 mmol, 0.0198g)
0
were mixed together and stirred at 130 C for 1 h under nitrogen atmosphere.
The reaction mixture was analyzed by GC with n-dodecane as the internal
standard. The pure products were obtained by chromatography on silica gel
and structurally characterized by NMR.
2.2.1. 3-Phenyl-2-oxazolidinone (1a)
1
H NMR (400 MHz, CDCl , TMS) δ 7.53-7.55 (m, 2H), 7.36-7.39 (m, 2H),
3
1
3
7
.14-7.15 (m, 1H), 4.44-4.49 (m, 2H), 4.01-4.07 (m, 2H); C NMR (100
MHz, CDCl , TMS) δ 155.4, 138.4, 129.2, 124.2, 118.4, 61.4, 45.3.
3
2.2.2. Ethyl 2-(phenyl amino) ethyl carbonate (2a)
1
H NMR (400 MHz, CDCl , TMS) δ 7.21-7.16(m, 2H), 6.73(t, J= 7.31Hz,
3
1H), 6.62(t, J= 8.07Hz, 2H), 4.32(t, J=5.41Hz, 2H), 4.22(m, 2H), 3.97(s,
1
3
1
H), 3.43(t, J=5.4Hz, 2H), 1.31(t, J= 7.1Hz, 3H); C NMR (100 MHz,
CDCl , TMS) δ 155.2, 147.5, 129.3, 117.9, 112.9, 66.2, 64.2, 42.8, 14.2.
3
2.2.3. 3-(4-Methoxy-phenyl)-oxazolidin-2-one (1b)
1
H NMR (400 MHz, CDCl , TMS) δ 7.43 (d, J = 9.2 Hz, 2H), 6.91 (d, J =
3
9.2 Hz, 2H), 4.46 (t, J = 8.0 Hz, 2H), 4.01 (t, J = 8.0 Hz, 2H), 3.78 (s, 3H);
1
3
C NMR (100 MHz, CDCl , TMS) δ 156.5, 155.7, 131.6, 120.4, 114.4,
3
61.4, 55.6, 45.8.
2.2.4. 3-(4-Ethoxy-phenyl)-oxazolidin-2-one (1c)
1
H NMR (400 MHz, CDCl , TMS) δ 7.40-7.42 (m, 2H), 6.88-6.90 (m, 2H),
3
1
3
4
.42-4.46 (m, 2H), 3.98-4.04 (m, 4H), 1.40 (t, J = 7.0 Hz, 3H); C NMR
(
100 MHz, CDCl , TMS) δ 155.9, 155.7, 131.4, 120.4, 115.0, 63.8, 61.4,
3
45.8, 14.9.
2.2.5. 3-(4-Bromo-phenyl)-oxazolidin-2-one (1d)
5

1
H NMR (400 MHz, CDCl , TMS) δ 7.41-7.48 (m, 4H), 4.45-4.49 (m, 2H),
3
13
3
.99-4.03 (m, 2H); C NMR (100 MHz, CDCl , TMS) δ 155.1, 137.5,
3
132.1, 119.8, 117.0, 61.4, 45.1.
2.2.6. 3-(4-Chloro-phenyl)-oxazolidin-2-one (1e)
1
H NMR (400 MHz, CDCl , TMS) δ 7.47-7.49 (m, 2H), 7.31-7.34 (m, 2H),
3
1
3
4
.46-4.50 (m, 2H), 4.00-4.04 (m, 2H); C NMR (100 MHz, CDCl , TMS) δ
3
1
55.2, 137.0, 129.4, 129.2, 119.5, 61.4, 45.2.
.2.7. 3-(3-Chloro-phenyl)-oxazolidin-2-one (1f)
2
1
H NMR (CDCl ; TMS) δ 7.59 (s, 1H), 7.42-7.7.58 (m, 1H), 7.29 (t, J = 8.3
3
1
3
Hz, 1H), 7.09-7.13 (m, 1H), 4.46-4.50 (m, 2H), 4.01-4.05 (m, 2H); C NMR
(
100 MHz, CDCl , TMS) δ 154.9, 139.5, 134.8, 130.1, 124.0, 118.2, 116.1,
3
61.3, 45.0.
2.2.8. 3-Naphthalen-1-yl-oxazolidin-2-one (1g)
1
H NMR (400 MHz, CDCl , TMS) δ 7.81-7.88 (m, 3H), 7.42-7.55 (m, 4H),
3
1
3
4
1
6
3
3
.53-4.57 (m, 2H), 3.98-4.02 (m, 2H); C NMR (100 MHz, CDCl , TMS) δ
3
57.5, 134.6, 134.1, 129.9, 128.7, 128.6, 127.0, 126.5, 125.7, 124.6, 122.4,
2.6, 49.1.
. Results and discussion
.1. Reaction of 2-(phenylamino) ethanol with DEC catalyzed by
BmimOAC
-Butyl-3-methylimidazolium acetate (BmimOAc) as one of the
1
imidazolium-based ionic liquids is commonly known as a reaction media
and catalyst [27-30]. It has been used as a catalyst for the reaction of N-
heterocycles with dimethyl carbonate [31], and the reaction of cyclic
carbonates with indole [32]. According to this basis, a preliminary
experiment was conducted to identify the products from the reaction of 2-
(
phenylamino) ethanol with DEC. Two products were detected in the
6

reaction mixture, isolated by chromatography on silica gel and structurally
characterized by NMR. They were confirmed to be 3-phenyl-2-
oxazolidinone (1a) and ethyl 2-(phenyl amino) ethyl carbonate (2a) (Scheme
2). To optimize the reaction parameters, the effects of reaction time, amount
of catalyst and temperature were investigated.
Scheme 2. Reaction of 2-(phenylamino) ethanol with DEC catalyzed by
BmimOAc.
3.1.1 The effect of reaction time
The reaction of 2-(phenylamino) ethanol with DEC was carried out in
different time range of 20-60 min. The effect of reaction time on the
conversion of 2-(phenylamino) ethanol and the yields of 2a and 1a are
shown in Fig. 1. From this figure, it can be seen that the reaction goes very
fast. The conversion of 2-(phenylamino) ethanol reached to 98% in 30 min
and 100% in 40 min. In the meanwhile, there are two products 2a and 1a
were obtained. Aleš Růžička and his co-workers [33] reported similar
observation during their studies on transesterification of 2-(phenylamino)
ethanol with DMC using C, N-chelated organotin (IV) compounds as
catalysts. The yield of 2a increased first and reached the highest yield of
66% at 30 min, and then decreased smoothly until it disappeared from the
reaction mixture in 1 h. Meanwhile the yield of 1a was increased by
increasing reaction time till reached 100% at 1 h. These results showed that
2a probably the intermediates of 1a. In order to prove this assumption, 2a
was used as starting material under the catalytic amount of BmimOAc, and
7

0
the sole product of 1a was obtained in 130 C in 1h with >99% yield
(
Scheme 3). Additionally similar compounds of 2a were detected in the
reaction of 1-phenylethanol with DMC [34].
Scheme 3. Reaction of ethyl 2-(phenyl amino) ethyl carbonate in the
presence of DEC catalyzed by BmimOAc.
1
00
100
80
60
40
20
0
8
6
4
2
0
0
0
0
0
20
30
40
50
60
Reaction time (min)
Fig 1. Effect of reaction time. Reaction conditions: 2-(phenylamino)
0
ethanol: 2 mmol, DEC: 10 mmol, BmimOAc: 0.1 mmol, 130 C. (□) Yield of
2a; (■) Yield of 1a; (◊) Conversion of 2-(phenylamino) ethanol.
3.1.2 The effect of catalyst amount
The effect of catalyst amount was shown in Fig. 2. With the increase of the
amount of catalyst from 1 to 5%, the conversion of 2-(phenylamino) ethanol
8

increased and reached to 100 % with 3% of BmimOAc. The yield of
intermediate 2a increased firstly followed by decreasing till it disappeared
from reaction mixture, while the yield of 1a increased dramatically. The
highest yield of 1a was obtained when using 4% of BmimOAc then remains
steady with increase of the amount catalyst.
1
00
100
80
60
40
20
0
8
6
4
2
0
0
0
0
0
1
2
3
4
5
Amount of catalyst (mol%)
Fig 2. Effect of catalyst amount. Reaction conditions: 2-(phenylamino)
0
ethanol: 2 mmol, DEC: 10 mmol, 130 C, 1h. (□) Yield of 2a; (■) Yield of
1a; (◊) Conversion of 2-(phenylamino) ethanol.
3.1.3 The effect of reaction temperature
The reaction of 2-(phenylamino) ethanol and DEC was carried out in the
0
temperature range of 90-130 C. The effect of reaction temperature on the
conversion of 2-(phenylamino) ethanol and the yields of 2a and 1a were
shown in Fig. 3. It is appears that this reaction is highly sensitive to
temperature. The conversion of 2-(phenylamino) ethanol reached 100% at
0
1
10 C, followed by staying at this point with increase of reaction
9

0
0
temperature. From 90 C to 100 C, the yield of 2a was increased rapidly.
Further increase in temperature lead to drop of 2a yield. On the other hand
0
the yield of 1a increased smoothly with increase of temperature from 90 C
0
to 100 C, and then it increased dramatically when the temperature reached
0
1
30 C.
1
00
100
80
60
40
20
0
8
6
4
2
0
0
0
0
0
9
0
100
110
120
130
0
Reaction Temp. ( C)
Fig 3. Effect of reaction temperature. Reaction conditions: 2-(phenylamino)
ethanol: 2 mmol, DEC: 10 mmol, BmimOAC: 0.1 mmol, 1h. (□) Yield of
2a; (■) Yield of 1a; (◊) Conversion of 2-(phenylamino) ethanol.
3.2. Reaction of various 2-(arylamino) alcohols with DEC catalyzed by
BmimOAc
To investigate the generalities of this procedure, the reaction of various 2-
(
arylamino) alcohols with DEC was carried out. The results were listed in
Table 1. Under the identity reaction conditions, the 2-(arylamino) alcohols
with either electron-donating substituents (such as alkoxy) or electron-
withdrawing substituents (such as halides) could be converted to
10

corresponding products in almost quantitative yields (entries 1-5).
Additionally, 2-(naphthalen-1-ylamino) ethanol also gave 3-(naphthalen-1-
yl) oxazolidin-2-one (1g) in excellent yield (entry 6). However
cyclohexylamino alcohols and benzyl amino alcohols, which were
possessing stronger electonegativity, are relatively inactive (entries 7 and 8).
Table 1. Reaction of 2-(arylamino) alcohols with DEC catalyzed by
a
BmimOAc.
11

3.3. Reaction of 2-(phenylamino) ethanol with DEC catalyzed by various
ionic liquids.
Table 2. Reaction of 2-(phenylamino) ethanol with DEC catalyzed by
a
various ionic liquids.
b
Entry
Ionic liquid
Yield (%)
12

2
0
a
1a
>99
2
21
0
9
0
0
1
2
3
4
5
6
7
8
9
BmimOAc
BmmimOAc
BmimCl
BmmimCl
BmimBr
38
70
26
68
17
0
BmmimBr
BmimBF
BmmimBF
4
4
0
14
6
2
0
0
0
0
0
HOEmimCl
HOEmmimCl
TBAA
1
0
1
2
1
1
None
0
a
Reaction conditions: 2-(phenylamino) ethanol: 2
mmol, DEC: 10 mmol, Ionic liquid: 0.1 mmol,
30 C, 1 h.
o
1
b
GC Yield.
To investigate the effect of different ionic liquids on the reaction of 2-
(
phenylamino) ethanol with DEC, a series of ionic liquids have been used as
catalysts. The results were shown in Table 2. BmimOAc gave 1a in a high
yield (>99%) (entry 1). BmimCl, BmimBr catalyze the reaction to give both
2a in relatively high yields 70, 68%, and 1a in the yields of 21, 9% (entries
3
, 5). BmimBF does not show any catalytic activity (entry 7). These results
4
shows that the higher yield of 1a is provided by anions of ionic liquids with
stronger hydrogen bond basicity[35], which follows the order of
OAc>Cl>Br>BF . BmmimOAc, BmmimCl, and BmmimBr, which contain a
4
methyl group in the C-2 of imidazolium ring, give lower yields of both 2a
and 1a in the reaction than their corresponding Bmim based ionic liquids
(
entries 2, 4 and 6). It demonstrates that the C-2 proton of imidazolium ring
of ionic liquids also plays an important role in catalyzing the reaction. The
cooperative activation phenomenon of ionic liquids upon interaction with
substrates was well investigated by Chakraborti and his co-workers [36-40].
It is clear that in this reaction both the anions and cations of ionic liquids
13

play essential role by cooperatively activation of the hydroxyl proton of 2-
phenylamino) ethanol and the carbonyl group of DEC, similar results were
observed by our group in the reaction of aniline with ethylene carbonate
(
[
(
(
23].
In
addition,
1-hydroxyethyl-3-methylimidazolium
chloride
HOEmimCl) and 1-hydroxyethyl-2, 3-dimethyl imidazolium chloride
HOEmmimCl) gave only 2a in low yields 14, 6% (entries 9, 10), showing
that hydroxyethyl group on imidazolium decreases the catalytic activity.
Upon treating the reaction with tetrabutylammonium acetate (TBAA) as
catalyst, 2a was only obtained in 2% yield. On the other hand, when the
reaction was treated without any catalyst, no conversion is observed (entries
11, 12).
3.4. Reaction mechanism studies
1
To investigate the reaction mechanism, HNMR spectroscopy and DFT
calculation were used to determine the interaction between ionic liquid and
substrates. DEC and 2-(phenylamino) ethanol were used as a models.
1
HNMR titration based on the addition aliquots DEC and 2-(phenylamino)
ethanol to BmimOAc was carried out. Figure 4 shows the shifting on
1
HNMR spectra of C-2 proton of imidazolium upon interaction with DEC.
When 1 eq. of DEC was added to BmimOAc, the C-2 proton of imidazolium
cation shifted upfield from δ=10.51 to δ=10.43. This might be result from
the change of the hydrogen bond donor-acceptor pair. In BmimOAc, the C-2
proton of the imidazolium cation acted as a hydrogen bond donor to interact
with the acetate counter ion which, in turn, acted as a hydrogen bond
acceptor. After stepwise addition of DEC into BmimOAc, the intramolecular
hydrogen bond interaction within acetate and C-2 proton was weakened, and
an intermolecular hydrogen bond interaction between imidazolium and DEC
emerged. These results implied that the carbonyl group of DEC could be
14

activated by the C-2 proton of imidazolium. It is clear that the C-2 proton of
imidazolium ring shifted step by step upon addition of DEC due to its
interaction with carbonyl group of DEC. Similar results were reported by us
and others [24-25, 41]. On the addition of 10 eq. of DEC the spectra of C-2
proton underwent significant shift from δ=10.51 to δ =9.97. This indicated
that stronger interaction between C-2 proton of imidazolium and carbonyl
group of DEC will occur when using excess amount of DEC. Meanwhile
when 10 eq. of 2-(phenylamino) ethanol was added to the mixture of DEC
and BmimOAC the dramatically shifting in spectra of C-2 proton from
δ=9.97 to δ=8.50 was observed. In competition environment both
imidazolium and 2-(phenylamino) ethanol acted as a hydrogen bond donors
to interact with acetate anion. Therefore upon addition of 2-(phenylamino)
ethanol the acetate anion goes to interact with it, leading to the C-2 proton of
imidazolium cation free to interact with carbonyl group of DEC, resulting in
extremely upfield shifting to δ=8.50. This demonstrated that 2-
(
phenylamino) ethanol could be activated by the acetate anion through
hydrogen bonding. These NMR studies have illustrated that BmimOAc
could cooperatively activate 2-(phenylamino) ethanol and DEC by hydrogen
bonding.
15

1
Fig 4. Chemical shift in the H NMR spectra of C-2 proton of BmimOAc.
Solvent: CD Cl. (a) BmimOAc (b) BmimOAc: DEC = 1:1 (c) 1:2 (d) 1:3 (e)
3
1:5 (f) 1:10 (mol) respectively, (g) BmimOAc: DEC: 2-(phenylamino)
ethanol = 1:10:10 (mol)
The interactions between BmimOAc and the substrates were also studied by
DFT calculation. The optimized geometrical structures of 2-(phenylamino)
ethanol, DEC and the complex of 2-(phenylamino) ethanol, DEC with
BmimOAc were simulated at the B3LYP/6-311G level (Figure 5). The
length of O(1)-----H(1) bond in 2-(phenylamino) ethanol was elongated from
0.9717 to 1.0199 Å after complexation with BmimOAc, and the hydrogen
bond H(1)-----O(3) was formed with a bond length of 1.5802 Å. This
demonstrates that 2-(phenylamino) ethanol was activated by hydrogen
16

bonding with acetate anion. The bond length of the C(1)-----O(2) in DEC
was elongated from 1.2342 to 1.2416 Å after complexation with BmimOAc,
and hydrogen bond H(2)-----O(2) was formed with a bond length of 2.4029
Å. These results indicate that DEC is activated through hydrogen bonding
between carbonyl carbon and C-2 proton of imidazolium ring of ionic liquid.
The DFT calculations indicated that the cation and anion of BmimOAc
cooperatively activated the substrates via formation of hydrogen bonds.
Fig 5. DFT optimized geometry of (a) 2-(phenylamino) ethanol, (b) DEC
and (c) the complex of 2-(phenylamino) ethanol, DEC with BmimOAc.
According to these results a reaction mechanism was proposed as shown in
Scheme 4. Firstly, the C-2 proton of imidazolium cation activates the
carbonyl group of DEC through the formation of hydrogen bond (C=O-----
H). The activation of carbonyl group by such kind of interaction was also
17

reported in Diels-Alder and acetylation reaction [42, 43]. In the same time
the acetate anion activates 2-(phenylamino) ethanol by hydrogen bond at the
hydrogen atom of hydroxyl group. Secondly, oxygen atom of hydroxyl
group nucleophilic attacks the activated carbonyl group resulting in forming
intermediate 2a. Thirdly, the 2a molecule is subjected to further interaction
with C-2 proton of imidazolium cation which activates the carbonyl group
and acetate anion which activates the amine group. Last, the activated 2a
species undergoes intramolecular cyclization to form final product 3-phenyl-
2
-oxazolidinone. And BmimOAc is recycled to complete the reaction circle.
Scheme 4. The proposed reaction mechanism of 2-(phenylamino) ethanol
with DEC catalyzed by BmimOAc.
4. Conclusion
In summery we have described a simple and environmentally benign
procedure for the synthesis of 3-aryl-2-oxazolidinones by the reaction of 2-
(
arylamino) alcohols with DEC in the presence of ionic liquids. Under mild
18

reaction conditions, BmimOAc can catalyze the generation of various 3-
aryl-2-oxazolidinones in excellent yields. The high catalytic activity of
BmimOAc is attributed to its strong hydrogen bond basicity anion and
1
hydrogen atom in C-2 of imidazolium ring. DFT calculation and H NMR
spectroscopy indicated that both anion and cation of ionic liquid
cooperatively activated the substrates through hydrogen bonding. Beside,
ethyl 2-(phenyl amino) ethyl carbonate is considered as the key intermediate
in the reaction.
Acknowledgments
We thank the National Natural Science Foundation of China (21273078) and
Shanghai Leading Academic Discipline Project (project number B409) for
financial support.
Appendix A. Supplementary data
References
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