DBU and DBU-Derived Ionic Liquid Synergistic Catalysts for the Conversion of Carbon Dioxide/Carbon Disulfide to 3-Aryl-2-oxazolidinones/[1,3]Dithiolan-2-ylidenephenyl- Amine

Article abstract of DOI:10.1002/cctc.201500928

An intermolecular synergistic catalytic combination of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) and a DBU-derived bromide ionic liquid has been developed for the conversion of CO₂, epoxides, and amines under metal- And solvent-free conditions. Various 3-aryl-2-oxazolidinones are produced in moderate to excellent yields within a short reaction time. NMR spectroscopy and DFT calculations demonstrate that DBU as a hydrogen bond acceptor and the ionic liquid as a hydrogen bond donor activate the substrates cooperatively by inducing hydrogen bonds to promote the reaction effectively. Based on these results, a possible reaction mechanism on the synergistic catalysis of DBU and the ionic liquid is proposed. In addition, the reaction of CS₂, ethylene oxide, and aniline catalyzed by the combination of DBU and the DBU-derived ionic liquid also proceeds smoothly, which opens a hitherto unreported route to [1,3]dithiolan-2-ylidenephenylamine in a straightforward way.

Full text of DOI:10.1002/cctc.201500928

DOI: 10.1002/cctc.201500928
Full Papers
DBU and DBU-Derived Ionic Liquid Synergistic Catalysts
for the Conversion of Carbon Dioxide/Carbon Disulfide to
3-Aryl-2-oxazolidinones/[1,3]Dithiolan-2-ylidenephenyl-
amine**
Binshen Wang, Zhoujie Luo, Elnazeer H. M. Elageed, Shi Wu, Yongya Zhang, Xiaopei Wu,
Fei Xia,* Guirong Zhang, and Guohua Gao*[a]
An intermolecular synergistic catalytic combination of 1,8-dia-
zabicyclo[5.4.0]undec-7-ene (DBU) and a DBU-derived bromide
ionic liquid has been developed for the conversion of CO2, ep-
oxides, and amines under metal- and solvent-free conditions.
Various 3-aryl-2-oxazolidinones are produced in moderate to
excellent yields within a short reaction time. NMR spectroscopy
and DFT calculations demonstrate that DBU as a hydrogen
bond acceptor and the ionic liquid as a hydrogen bond donor
activate the substrates cooperatively by inducing hydrogen
bonds to promote the reaction effectively. Based on these re-
sults, a possible reaction mechanism on the synergistic cataly-
sis of DBU and the ionic liquid is proposed. In addition, the re-
action of CS2, ethylene oxide, and aniline catalyzed by the
combination of DBU and the DBU-derived ionic liquid also pro-
ceeds smoothly, which opens a hitherto unreported route to
[1,3]dithiolan-2-ylidenephenylamine in a straightforward way.
Introduction
The atmospheric CO2 level has been rising since the start of
the industrial revolution because of the combustion of fossil
fuels. CO2 is a major greenhouse gas that causes climate
change.[1] It is also an abundant, nontoxic, nonflammable, and
renewable carbon source.[2] As a result of environmental and
economic concerns, intensive research has been conducted
with CO2 as a feedstock in recent years.[3]
mass,[13] triphenylphosphine and phenol,[14] and ionic liquids;[15]
and ammonium salts combined with alcoholic/phenolic com-
pounds[16] and metal complexes[17] are suitable for the catalytic
chemical fixation of CO2. However, these intermolecular syner-
gistic catalysts are mostly limited to the production of cyclic
carbonates.
3-Aryl-2-oxazolidinones are important heterocyclic com-
pounds because of their bioactivity. They exhibit the selective
and reversible inhibition of monoamine oxidase A and potent
activity against Gram-positive bacterial pathogens, which in-
clude methicillin-resistant Staphylococcus aureas (MRSA) and
vancomycin-resistant Enterococci (VRE).[18] The conventional
conversions to 3-aryl-2-oxazolidinones are through the reaction
of phosgene with 2-alkamine[19] and the reaction of isocyanate
with epoxide,[20] which are considered nongreen as toxic start-
ing materials are used. Alternative synthetic routes through
the reaction of aryl halides with oxazolidin-2-ones,[18a,21] the re-
action of aryl iodides with amino alcohol carbamates,[22] and
the reaction of CO2 with amino alcohols have also been report-
ed.[23] However, most of these routes rely on metal catalysts
that are toxic or expensive. He et al. and others have reported
the synthesis of 3-benzyl-2-oxazolidinones by the reaction of
CO2 with aziridines.[24] Additionally, ionic liquids have been
used to catalyze the preparation of 3-aryl-2-oxazolidinones by
the reaction of aromatic amines with cyclic carbonates,[25] and
the reaction of 2-(arylamino) alcohols with diethyl carbonate
which have been reported by us.[26]
As an efficient method for the conversion of inert CO2 to val-
uable chemicals, the synergistic catalysis that activates CO2
and other substrates simultaneously has drawn much atten-
tion. Particularly, compared to intramolecular synergistic cata-
lysts,[4] intermolecular synergistic catalysts are readily available,
inexpensive, and avoid tedious preparation processes. Various
combined intermolecular synergistic catalysts have been devel-
oped for the conversion of CO2, such as organic bases com-
bined with cellulose,[5] N-bromosuccinimide (NBS),[6] and poly-
ethylene glycol.[7] In addition, organometallic complexes com-
bined with organic bases,[8] CO2 adducts of N-heterocyclic car-
benes,[9] ammonium salts,[8a,10] and polyether-potassium salt
complexes;[11] metal salts combined with formic acid,[12] bio-
[a] B. Wang, Z. Luo, E. H. M. Elageed, S. Wu, Y. Zhang, X. Wu, Prof. Dr. F. Xia,
G. Zhang, Prof. Dr. G. Gao
Shanghai Key Laboratory of Green Chemistry and Chemical Processes
Department of Chemistry
East China Normal University
North Zhongshan Road 3663, Shanghai (P.R. China)
E-mail: fxia@chem.ecnu.edu.cn
Recently, we developed a new approach for the one-pot
conversion of CO2, ethylene oxide (EO), and amines to 3-aryl-2-
oxazolidinones in the presence of ionic liquids as catalysts.[27]
Although the approach is regarded as green, it still suffers
ghgao@chem.ecnu.edu.cn
[**] DBU=1,8-Diazabicyclo[5.4.0]undec-7-ene.
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201500928.
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from harsh reaction conditions (extended reaction time 9 h),
and the reaction mechanism is still unclear. In continuation of
this work, herein, we present a study on the utilization of inter-
molecular synergistic catalysis of readily available organic
bases and organic base-derived ionic liquids to effectively syn-
thesize various 3-aryl-2-oxazolidinones by the reaction of CO2,
epoxides, and amines. NMR spectroscopy and DFT calculations
were used to investigate the reaction mechanism. Moreover,
a hitherto unreported reaction of CS2, EO, and aniline also pro-
ceeded with the use of the combination of the organic base
and ionic liquid as catalysts, which produced [1,3]dithiolan-2-
ylidenephenylamine in a moderate yield (Scheme 1).
nopyridine (DMAP; aqueous pKa =9.2) or N-methylimidazole
(MIm; aqueous pKa =7.1) provided the product in yields of 53
and 44%, respectively (Table 1, entries 4 and 7).[7,28] Further-
more, their derived ionic liquids, HDMAPBr and HMImBr, gave
1a in low yields of 8 and 5%, respectively (Table 1, entries 5
and 8). However, both the combination of DMAP with
HDMAPBr and that of MIm with HMImBr exhibited distinctly
higher catalytic activities than the base or ionic liquid alone,
which gave the product in yields of 85 and 77% respectively
(Table 1, entries 6 and 9). These results showed the synergistic
catalytic effects of the three organic bases with their corre-
sponding ionic liquids to accelerate the reaction.
Effect of different organic bases and bromides
The combinations of different organic bases with HDBUBr
were screened for the conversion of CO2 to 1a, and the results
are summarized in Table 2. In addition to the bicyclic amidine
DBU, which gave an excellent yield (Table 1, entry 3), cyclic
DMAP and MIm also provided 1a in high yields of 89 and 84%
(Table 2, entries 1 and 2). The tertiary amine triethylamine
(TEA) afforded 1a in a yield of 82% (Table 2, entry 3). Diethyla-
mine (DEA) and imidazole (Im), which have secondary amino
groups in their structures, produced 1a in moderate yields of
65 and 63%, respectively (Table 2, entries 4 and 5). If pyridine
(Py) was used as the organic base, the product was obtained
in a yield of 47% (Table 2, entry 6). 1,4-Diazabicyclo[2.2.2]oc-
tane (DABCO), the catalytic activity of which might be affected
by a high steric hindrance in part, provided 1a in a yield of
42% (Table 2, entry 7). Furthermore, triethanolamine (TEOA), di-
ethanolamine (DEOA), and monoethanolamine (MEOA), which
all bear hydroxyl functional groups, gave 1a in low yields of
16–23% (Table 2, entries 8–10). The different organic bases em-
ployed showed quite strong effects on the reaction, which
probably results from their different alkalinity, steric hindrance,
and functional groups.[5]
Scheme 1. Conversion of CO2/CS2, epoxides, and amines to 3-aryl-2-oxazoli-
dinones/[1,3]dithiolan-2-ylidene aryl amine.
Results and Discussion
Synergistic catalysis of organic bases and organic base-de-
rived ionic liquids
To investigate synergistic catalytic effects, several typical organ-
ic bases with different alkalinities and their corresponding
ionic liquids were applied as catalysts for the reaction of CO2,
EO, and aniline to synthesize 3-phenyl-2-oxazolidinone (1a;
Table 1). Specially, bromide ionic liquids were used because
bromide was found to play an important role in reactions that
involve CO2 and epoxide because of its good nucleophilicity
and leaving ability.[10b–e] If we used the single superbase diaza-
bicyclo[5.4.0]undec-7-ene (DBU; aqueous pKa =11.5) alone,
a moderate yield of 54% was obtained after 1 h at 1408C. The
use of only the DBU-derived ionic liquid HDBUBr afforded the
desired product in a poor yield of 5%. Notably, the combina-
tion of DBU with HDBUBr gave an excellent yield of 93%
within 1 h (Table 1, entries 1–3). Additionally, only dimethylami-
In the presence of the optimal organic base DBU, the effect
of different bromides on the reaction was investigated
(Table 3). Ionic liquid 1-butyl-3-methylimidazolium bromide
(BmimBr) and tetrabutylammonium bromide (Bu4NBr) gave 1a
Table 2. Conversion of CO2 to 1a catalyzed by various organic bases
Table 1. The synergistic catalytic effect of organic base and ionic liquid
for the conversion of CO2 to 1a.[a]
combined with HDBUBr.[a]
Entry
Organic
base
Yield[b]
[%]
Entry
Organic
base
Ionic
liquid
Yield[b]
[%]
1
2
3
4
5
6
7
8
9
10
DMAP
MIm
TEA
DEA
Im
89
84
82
65
63
47
42
19
23
16
1
2
3
4
5
6
7
8
9
DBU
–
DBU
DMAP
–
DMAP
MIm
–
–
54
5
93
53
8
85
44
5
HDBUBr
HDBUBr
–
HDMAPBr
HDMAPBr
–
Py
DABCO
TEOA
DEOA
MEOA
HMImBr
HMImBr
MIm
77
[a] Reaction conditions: aniline (2 mmol), EO (2 mL), CO2 (2.5 MPa), organ-
[a] Reaction conditions: aniline (2 mmol), EO (2 mL), CO2 (2.5 MPa), organ-
ic base (0.4 mmol), ionic liquid (0.1 mmol), 1408C, 1 h. [b] GC yield of 1a.
ic base (0.4 mmol), HDBUBr (0.1 mmol), 1408C, 1 h. [b] GC yield of 1a.
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Table 3. Conversion of CO2 to 1a catalyzed by various bromides com-
bined with DBU.[a]
Table 4. Optimization of reaction parameters for the conversion of CO2
to 1a.[a]
Entry
Bromide
Yield[b]
[%]
Entry
DBU+HDBUBr
[mmol]
T
[8C]
t
[h]
P
Yield[b]
[%]
[MPa]
1
2
3[c]
4[c,d]
5[c]
6[c]
BmimBr
Bu4NBr
HBr
90
87
78
86
74
70
1
2
3
4
5
6
7
8
0.4+0.1
0.3+0.1
0.4+0.05
0.4+0.1
0.4+0.1
0.4+0.1
0.4+0.1
0.4+0.1
0.4+0.1
0.4+0.1
0.2+0.05
0.4+0.1
0.4+0.1
0.4+0.1
140
140
140
130
120
110
100
130
130
130
130
80
1
1
1
1
1
1
1
0.5
1
1
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.0
1.5
2.5
2.5
2.5
1.0
93
86
85
93
82
56
36
84
77
69
57
30
51
67
HBr
1-bromobutane
NBS
[a] Reaction conditions: aniline (2 mmol), EO (2 mL), CO2 (2.5 MPa), DBU
(0.4 mmol), bromide (0.1 mmol), 1408C, 1 h. [b] GC yield of 1a. [c] DBU
(0.5 mmol). [d] With 0.5 g 4 Š molecular sieves.
9
10
11
12
13
14
12
12
12
12
100
130
in yields of 90 and 87%, respectively, which were slightly lower
than that of HDBUBr (93%; Table 1, entry 3). This could be at-
tributed to the better activation of HDBU+ toward substrates.
If HBr (40% aqueous solution) was applied as the bromide, the
reaction provided 1a in 78% yield. In comparison, 0.5 g 4 Š
molecular sieves was added to minimize the possible prohibi-
tion of water, and then the yield of 1a increased to 86%
(Table 3, entries 3 and 4). The high yields afforded by HBr
might be caused by the in situ generation of HDBUBr by the
neutralization of HBr and DBU. The use of 1-bromobutane,
which might react with DBU to generate ionic liquid Bu-DBUBr
in situ,[29] provided 1a in a yield of 74% (Table 3, entry 5).
Moreover, NBS also gave a higher yield of 1a (70%; Table 3,
entry 6) than if no bromide was used (54%; Table 1, entry 1).
All applied bromides gave the desired product 1a in good to
high yields, which shows that it exhibits a moderate influence
on the reaction.
[a] Reaction conditions: aniline (2 mmol), EO (2 mL). [b] GC yield of 1a.
generalities of the synergistic catalytic combination (Table 5).
With the use of the combination of DBU and HDBUBr as cata-
lysts at 1308C in 1 h, aromatic amines with either electron-
withdrawing substituents (such as halides) or electron-donat-
ing substituents (such as alkyl or alkoxy groups) could be con-
verted to the corresponding 3-aryl-2-oxazolidinones 1b–g in
moderate to excellent yields (Table 5, entries 1–6). Naphthalen-
1-ylamine provided the desired product 1h in 85% yield
(Table 5, entry 7). However, aliphatic amines that possess
a higher electronegativity gave the corresponding products in
lower yields, for example, cyclohexylamine gave 1i in 27%
yield (Table 5, entry 8). Furthermore, propylene oxide could
also be employed as epoxide to synthesize 5-methyl-3-aryl-2-
oxazolidinones 1j and 1k in high yields under slightly harsh
reaction conditions (1608C, 9 h; Table 5, entries 9 and 10).
Optimization of reaction conditions
To optimize the other reaction parameters, the reaction of CO2,
EO, and aniline catalyzed by the combination of DBU and
HDBUBr was performed with different catalyst amounts, reac-
tion temperatures, reaction times, and CO2 pressures (Table 4).
A decrease of the amount of DBU or HDBUBr led to a decrease
in the yield of 1a from 93 to 86 and 85%, respectively (Table 4,
entries 1–3). If reaction temperature was reduced to 1308C, the
yield of 1a remained at 93%. However, a further reduction of
the reaction temperature to 120, 110, or 1008C led to 1a in
lower yields of 82, 56, or 36%, respectively (Table 4, entries 4–
7). A decrease in the reaction time to 0.5 h led to a reduced
yield of 84% (Table 4, entry 8). If the CO2 pressure was de-
creased to 2.0 or 1.5 MPa, the yields of 1a were 77 and 69%,
respectively (Table 4, entries 9 and 10). In addition, under an
extended reaction time of 12 h along with lower catalyst
amounts, reaction temperatures, or CO2 pressures, the reaction
only afforded 1a in moderate yields (Table 4, entries 11–14).
Reaction mechanism studies by NMR spectroscopy and DFT
calculations
In our previous study,[27] 2-(phenylamino)ethanol was revealed
to be the key intermediate in the one-pot conversion of CO2,
EO, and aniline to 1a. To further rationalize the catalytic role of
DBU and HDBUBr in the reaction, the interactions between the
catalysts and the substrates were studied by NMR spectrosco-
py and DFT calculations.
1
Firstly, the interactions were investigated by H and 13C NMR
spectroscopy. Upon the addition of 1 equivalent of EO, the res-
onance of the NÀH1 proton of HDBUBr shifted downfield from
d=10.005 to 10.019 ppm (Figure 1a and b), which indicated
the formation of a hydrogen bond between HDBUBr and EO.
This suggested that HDBUBr could activate EO as a hydrogen
bond donor. An upfield shift of the resonance of C1 of EO from
d=39.85 to 39.53 ppm was observed after the addition of
1 equivalent of HDBUBr (Figure 1c and d). In contrast, Jerome
et al. reported that the carbon signal of EO underwent a down-
field shift upon the addition of neutral alcohols as hydrogen
bond donors.[16a] The difference in shift implied that besides
Generalities of the synergistic catalytic combination
Under the optimized reaction parameters, the reactions of CO2,
different epoxides, and amines were performed to explore the
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Table 5. Conversion of CO2 to various 2-oxazolidinones catalyzed synerg-
istically by DBU and HDBUBr.[a]
Entry
Amine
Product
Yield[b]
[%]
1
2
>99
94
3
85
4
5
6
88
69
50
1
Figure 1. Partial H NMR spectra of a) only HDBUBr (0.1 mmol) and b) the
complex of HDBUBr (0.1 mmol) with EO (0.1 mmol), and partial 13C NMR
spectra of c) only EO (0.5 mmol) and d) the complex of EO (0.5 mmol) with
HDBUBr (0.5 mmol) in CD3CN (0.5 mL).
7
85
27
90
90
8
9[c]
10[c]
[a] Reaction conditions: amine (2 mmol), EO (2 mL), CO2 (2.5 MPa), DBU
(0.4 mmol), HDBUBr (0.1 mmol), 1308C, 1 h. [b] GC yield of 2-oxazolidi-
none. [c] Propylene oxide (40 mmol), 1608C, 9 h.
the hydrogen bond interaction between HDBU+ and EO, BrÀ
also interacted with the carbon atom of EO, which increased
the electron density around the carbon atom to result in the
upfield shift of the resonance of C1. The signal of the NÀH2
proton of aniline in the mixture with 1 equivalent of DBU un-
derwent a downfield shift from d=4.070 to 4.139 ppm and
peak broadening, which suggests that aniline could be activat-
ed by DBU as a hydrogen bond acceptor (Figure 2a and b).
Furthermore, in the presence of 1 equivalent of DBU, the reso-
nance of the OÀH3 proton of 2-(phenylamino)ethanol shifted
downfield dramatically from d=2.796 to 3.923 ppm and
became much broader, and the signal of the NÀH4 proton of
2-(phenylamino)ethanol shifted downfield slightly from d=
4.401 to 4.525 ppm (Figure 2c and d). This demonstrated that
DBU could activate the hydroxyl group and NH group of 2-
(phenylamino)ethanol through hydrogen bonds, especially on
the hydroxyl group.
1
Figure 2. Partial H NMR spectra of a) only aniline (0.1 mmol), b) the complex
of aniline (0.1 mmol) with DBU (0.1 mmol), c) only 2-(phenylamino)ethanol
(0.1 mmol), and d) the complex of 2-(phenylamino)ethanol (0.1 mmol) with
DBU (0.1 mmol) in CD3CN (0.5 mL).
bond length of N1ÀH1 of HDBUBr was elongated from 1.012 to
1.018 Š, and the hydrogen bond between H1 of the hydrogen
bond donor HDBUBr and O1 of EO was formed with a bond
length of 1.910 Š (Figure 3). Meanwhile, the bond length of
C1ÀO1 of EO was elongated from 1.418 to 1.428 Š, which thus
facilitated the ring opening of EO. In the complex of DBU with
Subsequently, the interactions between the catalysts and
the substrates were investigated by DFT calculations at the
M06-2x/6-31+G* level. In the complex of HDBUBr with EO, the
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Figure 3. DFT optimized structures of a) HDBUBr, b) EO, and (c) the complex
of HDBUBr with EO. The bond lengths are in Š.
aniline, the hydrogen bond between N3 of the hydrogen bond
acceptor DBU and H2 of aniline was formed with a bond
length of 2.043 Š (Figure 4), and the bond length of N2ÀH2 of
aniline was elongated from 1.012 to 1.025 Š, which increased
the nucleophilicity of N2. In the complex of DBU with 2-(phe-
nylamino)ethanol, two hydrogen bonds between N3 of DBU
and 2-(phenylamino)ethanol were formed, in which the hydro-
gen bond between N3 and H3 of the hydroxyl group (1.885 Š)
was shorter than that between N3 and H4 of the NH group
(2.104 Š). Accordingly, the bond length of O2ÀH3 of the hydrox-
yl group was elongated from 0.967 to 0.984 Š and the bond
length of N4ÀH4 of the NH group was elongated from 1.009 to
1.018 Š, which showed that the hydroxyl group underwent
a stronger activation from DBU than the NH group.
Scheme 2. Proposed reaction mechanism for the synthesis of 1a.
one-pot conversion of CO2, EO, and aniline to 1a catalyzed
synergistically by DBU and HDBUBr (Scheme 2). Additionally,
a more detailed DFT calculation of the reaction pathways and
energy profiles was performed for the one-pot reaction
(Figure 5). In Circle 1, EO is activated through a hydrogen bond
interaction with the proton of HDBUBr and attacked nucleo-
philically by the bromide anion via the transition state Ts-C1-1,
which has a remarkable energy barrier of 38.0 kcalmolÀ1, to
result in the formation of ring-opened Int-C1-1. Then, the nu-
cleophilic attack of Int-C1-1 towards the carbon atom in CO2
via Ts-C1-2 affords an alkyl carbonate salt Int-C1-2. An intramo-
lecular ring-closure of Int-C1-2 via Ts-C1-3 gives the ethylene
carbonate (Pro-C1). In parallel, aniline is activated by DBU
through a hydrogen bond interaction in Circle 2. Thereafter,
the activated aniline attacks the carbon atom of 2-bromoetha-
nol in Int-C1-1 nucleophilically via the transition state Ts-C2-2
to give 2-(phenylamino)ethanol (Pro-C2) with an energy release
of 21.4 kcalmolÀ1. Finally, in Circle 3, the hydroxyl group of 2-
(phenylamino)ethanol is activated by DBU and then attacks
the carbonyl group of ethylene carbonate nucleophilically to
produce a salt Int-C3-1, which has an energy of 14.8 kcalmolÀ1.
A subsequent intramolecular proton transfer via Ts-C3-2 leads
to a carbonate Int-C3-2.[26] In the intermediate Int-C3-2, the NH
group forms the hydrogen bond interaction with DBU. By
a subsequent intramolecular nucleophilic attack of a nitrogen
atom to a carbonyl group via Ts-C3-3, the desired product 1a
and a byproduct ethylene glycol are produced.
The NMR spectroscopy and DFT calculation results indicated
consistently that DBU and HDBUBr could activate the sub-
strates cooperatively by inducing hydrogen bonds, which cata-
lyzed the reaction synergistically. Based on these results and
previous studies, a possible mechanism is proposed for the
Conversion of CS2 to [1,3]dithiolan-2-ylidenephenylamine
catalyzed by DBU and HDBUBr
To take full advantage of the catalytic effect, the combination
of DBU and HDBUBr was then employed to catalyze the reac-
tion of CS2, EO, and aniline, which produced [1,3]dithiolan-2-yli-
denephenylamine (2a) as the main product, rather than 3-
Figure 4. DFT optimized structures of a) aniline, b) the complex of aniline
with DBU, c) 2-(phenylamino)ethanol, and d) the complex of 2-(phenylami-
no)ethanol with DBU. The bond lengths are in Š.
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Figure 5. Calculated reaction pathways for the proposed catalytic cycles shown in Scheme 2, which includes the crucial transition states, intermediates, and
products. a) The black and red curves denote the pathways of Circle 1 and Circle 2, respectively. b) The blue curve denotes the pathways of Circle 3.
Table 6. Conversion of CS2, EO, and aniline to 2a catalyzed by DBU and
phenyl-2-thiazolidinethione or 3-phenyl-2-oxazolidinethione.
[1,3]Dithiolan-2-ylidene aryl amines, as bioactive cyclic dithio-
carbonimidates, are effective anti-inflammatory analgesics with
low ulcerogenicity.[30] Traditional methods for the synthesis of
[1,3]dithiolan-2-ylidene aryl amines rely on the use of toxic iso-
thiocyanates.[31] An alternative method established by the reac-
tion of aromatic amines, CS2, and dibromoethane has also
been reported.[32] However, it suffers from a tedious and waste-
ful multistep operational procedure. Recently, the preparation
of [1,3]dithiolan-2-ylidene aryl amines by the reaction of ethyl-
ene trithiocarbonate and aromatic amines has also been devel-
oped.[25b] To the best of our knowledge, this is the first report
on the direct preparation of 2a from CS2, EO, and aniline in
a one-pot reaction (Table 6).
HDBUBr.[a]
Entry
T
[8C]
t
[h]
DBU+HDBUBr
[mmol]
Yield[b]
[%]
1
2
3
4
5
6
7
8
140
150
140
130
140
140
140
140
6
6
9
6
3
6
6
6
0.4+0.1
0.4+0.1
0.4+0.1
0.4+0.1
0.4+0.1
0.4+0.05
0.3+0.1
0.2+0.1
56
53
54
44
44
48
56
46
[a] Reaction conditions: aniline (1 mmol), EO (0.5 mL), CS2 (20 mmol).
In the presence of 0.4 mmol DBU and 0.1 mmol HDBUBr as
catalysts, the reaction proceeded smoothly to afford 2a in
56% yield at 1408C within 6 h (Table 6, entry 1). The increase
of the reaction temperature or reaction time did not improve
the yields. However, the decrease of the reaction temperature
or reaction time resulted in lower yields of 2a (Table 6, en-
tries 2–5). With a decrease of the amount of HDBUBr from 0.1
to 0. 05 mmol, the yield of 2a was reduced to 48%. If we de-
creased the amount of DBU from 0.4 to 0.3 mmol, the yield re-
[b] GC yield of 2a.
mained at 56%. However, a further decrease in the amount of
DBU to 0.2 mmol gave 2a in a lower yield of 46% (Table 6, en-
tries 6–8).
In the reaction of CS2, EO, and aniline, ethylene trithiocar-
bonate, which could be formed from 1,3-oxathiolane-2-
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thione,[33] was also detected. Hence, a reaction course was pro-
posed for the generation of 2a (Scheme 3). Firstly, the cycload-
dition of EO and CS2 affords 1,3-oxathiolane-2-thione. The sub-
sequent cleavage of the CÀS bond within the five-membered
ring and cyclization gives thiirane. The thiirane undergoes fur-
ther reaction with CS2 to produce ethylene trithiocarbonate.[33]
Finally, the reaction between ethylene trithiocarbonate and
aniline provides the desired product 2a.[25b]
31+G* level with the Gaussian09 program package. The frequency
analyses were performed at the same level based on the optimized
gaseous structures to identify the minima and transition states on
potential surfaces. The solvent effect on the solute was evaluated
using the polarizable continuum model (PCM) in Gaussian 09. All
the calculated Gibbs free energies DGsol were evaluated under the
experimental conditions T=413 K and p=25 bar. More details
about the optimized structures are provided in the Supporting In-
formation.
General procedure for the synthesis of organic base-derived
bromide ionic liquids
The organic base-derived bromide ionic liquids were synthesized
by neutralization. DBU/DMAP/MIm (25.0 mmol) was added into
a 25 mL round-bottomed flask equipped with a magnetic stirrer.
Hydrobromic acid (40% aqueous solution, 5.057 g, 25.0 mmol) was
added dropwise at <58C with cooling in an ice bath. The reaction
mixture was stirred at RT for 24 h. The desired bromide ionic liquid
was obtained after the aqueous phase was evaporated under
vacuum at 658C for 10 h.
Scheme 3. Proposed reaction course for the synthesis of 2a.
Conclusions
HDBUBr: Isolated yield=99%. 1H NMR (400 MHz, D2O) d=3.56–
3.62 (m, 4H), 3.37 (t, J=5.8 Hz, 2H), 2.66–2.68 (m, 2H), 2.03–2.09
(m, 2H), 1.73–1.77 ppm (m, 6H); 13C NMR (100 MHz, CDCl3, TMS)
d=166.2, 54.7, 48.9, 38.0, 32.5, 29.0, 26.8, 24.0, 19.5 ppm.
Combinations of organic bases with organic base-derived ionic
liquids are presented as simple and efficient intermolecular
synergetic catalysts for the chemical fixation of CO2 with epox-
ides and amines under metal- and solvent-free conditions. The
combination of an organic base and an ionic liquid exhibits
distinctly a higher catalytic activity for the generation of 3-
phenyl-2-oxazolidinone than each of them alone. We used the
combination of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) and
the DBU-derived bromide ionic liquid (HDBUBr) as catalysts to
produce a series of 3-aryl-2-oxazolidinones in moderate to ex-
cellent yields within a short reaction time. NMR spectroscopy
and DFT calculations reveal that DBU and HDBUBr can activate
substrates cooperatively through hydrogen bonds. According-
ly, a reaction mechanism is proposed that exhibits the intermo-
lecular synergetic catalytic effect of DBU and HDBUBr for the
one-pot reaction. Furthermore, with the use of CS2, ethylene
oxide, and aniline as starting materials, a direct protocol for
the synthesis of [1,3]dithiolan-2-ylidenephenylamine has also
been developed in the presence of the combination of DBU
and HDBUBr as catalysts.
HDMAPBr: Isolated yield=94%. 1H NMR (400 MHz, D2O) d=7.99
(d, J=7.6 Hz, 2H), 6.85 (d, J=7.6 Hz, 2H), 3.18 ppm (s, 6H);
13C NMR (100 MHz, D2O) d=157.5, 138.2, 106.8, 39.4 ppm.
1
HMImBr: Isolated yield >99%. H NMR (400 MHz, D2O) d=8.69 (s,
1H), 7.46 (s, 2H), 3.95 ppm (s, 3H); 13C NMR (100 MHz, D2O) d=
135.0, 123.0, 119.5, 35.6 ppm.
Typical procedure for the conversion of CO2 to 1a catalyzed
synergistically by DBU and HDBUBr
Typically, DBU (0.061 g, 0.4 mmol), HDBUBr (0.023 g, 0.1 mmol), ani-
line (0.186 g, 2.0 mmol), and EO (2 mL, 40 mmol) were added to
a stainless-steel autoclave reactor with an inner volume of 50 mL.
The reactor was pressurized with approximately 1 MPa of CO2 pres-
sure at ambient temperature. Subsequently, the reactor was
heated to 1308C, and the CO2 pressure was adjusted to 2.5 MPa
for 1 h. After the completion of the reaction, the autoclave was
cooled to RT and the remaining CO2 was vented slowly. Chloroform
(7 mL) was added to the reaction mixture. The organic phase was
washed with water (3”7 mL) to remove ionic liquid and then ana-
lyzed by GC with biphenyl as the internal standard. Product 1 was
purified by chromatography on silica gel and characterized struc-
turally by NMR spectroscopy.
Experimental Section
General
CO2 with a purity of 99.995% was commercially available. BmimBr
was supplied by the Centre for Green Chemistry and Catalysis,
LICP, CAS. NBS was supplied by Fluorochem. 2-(Phenylamino)etha-
nol was supplied by TCI. The other compounds were supplied by
Sinopharm. All chemicals were used without further purification.
1
3-Phenyl-2-oxazolidinone (1a): H NMR (400 MHz, CDCl3, TMS) d=
7.53–7.55 (m, 2H), 7.36–7.39 (m, 2H), 7.14–7.15 (m, 1H), 4.44–4.49
(m, 2H), 4.01–4.07 ppm (m, 2H); 13C NMR (100 MHz, CDCl3, TMS)
d=155.4, 138.4, 129.2, 124.2, 118.4, 61.4, 45.3 ppm. HRMS (EI): m/z:
calcd for C9H9NO2: 163.0633 [M+]; found 163.0630.
GC analysis was performed by using a Shimadzu GC-14B equipped
with a capillary column DM-1701 (60 m”0.32 mm”0.25 mm) and
a flame ionization detector. The NMR spectra were recorded by
using Bruker Ascend400 and Bruker DRX500 instruments with tet-
ramethylsilane as the internal standard at 298 K. HRMS spectra
were recorded by using a Waters, GCT Premier with electron ioni-
zation (EI). The DFT calculations were performed at the M06-2x/6-
3-(4-Bromophenyl)oxazolidin-2-one (1b): 1H NMR (400 MHz,
CDCl3, TMS) d=7.41–7.48 (m, 4H), 4.45–4.49 (m, 2H), 3.99–
4.03 ppm (m, 2H); 13C NMR (100 MHz, CDCl3, TMS) d=155.1, 137.5,
132.1, 119.8, 117.0, 61.4, 45.1 ppm; HRMS (EI): m/z: calcd for
C9H8BrNO2: 240.9738 [M+]; found 240.9740.
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3-(4-Chlorophenyl)oxazolidin-2-one (1c): 1H NMR (400 MHz,
CDCl3, TMS) d=7.47–7.49 (m, 2H), 7.31–7.34 (m, 2H), 4.46–4.50 (m,
2H), 4.00–4.04 ppm (m, 2H); 13C NMR (100 MHz, CDCl3, TMS) d=
155.2, 137.0, 129.4, 129.2, 119.5, 61.4, 45.2 ppm: HRMS (EI): m/z:
calcd for C9H8ClNO2: 197.0244 [M+]; found 197.0241.
20 mmol), and EO (0.51 mL, 10 mmol) were mixed together and
heated to 1408C for 6 h. After the completion of the reaction,
chloroform (7 mL) was added to the reaction mixture. The organic
phase was washed with water (3”7 mL) to remove ionic liquid,
and then analyzed by GC with biphenyl as the internal standard.
Product 2a was purified by chromatography on silica gel and char-
acterized structurally by NMR spectroscopy.
3-(3,5-Dichlorophenyl)oxazolidin-2-one (1d): 1H NMR (400 MHz,
CDCl3, TMS) d=7.47 (s, 2H), 7.10 (s, 1H), 4.49 (t, J=8.0 Hz, 2H),
4.01 ppm (t, J=8.0 Hz, 2H); 13C NMR (100 MHz, CDCl3, TMS) d=
154.7, 140.2, 135.5, 123.9, 116.3, 61.4, 45.0 ppm; HRMS (EI): m/z:
calcd for C9H7Cl2NO2: 230.9854 [M+]; found 230.9855.
[1,3]Dithiolan-2-ylidenephenylamine (2a): 1H NMR (400 MHz,
CDCl3, TMS) d=7.33 (t, J=7.8 Hz, 2H), 7.11 (t, J=7.4 Hz, 1H), 6.96
(d, J=6.4 Hz, 2H), 3.47–3.56 ppm (m, 4H); 13C NMR (100 MHz,
CDCl3, TMS) d=171.4, 152.3, 129.0, 124.5, 120.1, 37.9, 35.1 ppm;
HRMS (EI): m/z: calcd for C9H9NS2 195.0176 [M+]; found 195.0173.
3-(4-Ethoxyphenyl)oxazolidin-2-one (1e): 1H NMR (400 MHz,
CDCl3, TMS) d=7.40–7.42 (m, 2H), 6.88–6.90 (m, 2H), 4.42–4.46 (m,
2H), 3.98–4.04 (m, 4H), 1.40 ppm (t, J=7.0 Hz, 3H); 13C NMR
(100 MHz, CDCl3, TMS) d=155.9, 155.7, 131.4, 120.4, 115.0, 63.8,
61.4, 45.8, 14.9 ppm; HRMS (EI): m/z: calcd for C11H13NO3: 207.0895
[M+]; found 207.0892.
Acknowledgements
We thank the National Natural Science Foundation of China
(grant number 21273078, 21473056, 21573072), Shanghai Natu-
ral Science Foundation (14ZR1411800) and Shanghai Leading
Academic Discipline Project (project number B409) for financial
support.
3-(4-Methoxyphenyl)oxazolidin-2-one (1 f): 1H NMR (400 MHz,
CDCl3, TMS) d=7.43 (d, J=9.2 Hz, 2H), 6.91 (d, J=9.2 Hz, 2H), 4.46
(t, J=8.0 Hz, 2H), 4.01 (t, J=8.0 Hz, 2H), 3.78 ppm (s, 3H); 13C NMR
(100 MHz, CDCl3, TMS) d=156.5, 155.7, 131.6, 120.4, 114.4, 61.4,
55.6, 45.8 ppm; HRMS (EI): m/z: calcd for C10H11NO3: 193.0739 [M+];
found 193.0741.
1
3-p-Tolyloxazolidin-2-one (1g): H NMR (400 MHz, CDCl3, TMS) d=
Keywords: carbon dioxide fixation · cooperative effects ·
7.39 (d, J=8.4 Hz, 2H), 7.16 (d, J=8.4 Hz, 2H), 4.40- 4.44 (m, 2H),
3.96–4.00 (m, 2H), 2.31 ppm (s, 3H); 13C NMR (100 MHz, CDCl3,
TMS) d=155.4, 135.8, 133.7, 129.6, 118.4, 61.3, 45.3, 20.8 ppm:
HRMS (EI): m/z: calcd for C10H11NO2: 177.0790 [M+]; found
177.0791.
density functional calculations
mechanisms
· ionic liquids · reaction
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3-Naphthalen-1-yloxazolidin-2-one (1h): H NMR (400 MHz, CDCl3,
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Published online on January 21, 2016
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