Catalyst-free process for the synthesis of 5-aryl-2-oxazolidinones via cycloaddition reaction of aziridines and carbon dioxide

Article abstract of DOI:10.1055/s-0030-1258510

A simple approach for facile synthesis of 5-aryl-2-oxazolidinones in excellent regioselectivity from aziridines under compressed CO₂ conditions was developed in the absence of any catalyst and organic solvent. The reaction outcome was found to be tuned by subtly adjusting CO₂ pressure. The adduct formed in situ of aziridine and CO₂ is assumed to act as a catalyst in this reaction, which was also studied by means of in situ FT-IR technique.

Full text of DOI:10.1055/s-0030-1258510

LETTER
2159
Catalyst-Free Process for the Synthesis of 5-Aryl-2-Oxazolidinones
via Cycloaddition Reaction of Aziridines and Carbon Dioxide
S
ynthesis of 5-Ary
i
l-2-O
x
a
a
zolidinone
o
s
-Yong Dou, Liang-Nian He,* Zhen-Zhen Yang, Jing-Lun Wang
State Key Laboratory and Institute of Elemento-Organic Chemistry, Nankai University, Tianjin 300071, P. R. of China
Fax +86(22)23504216; E-mail: heln@nankai.edu.cn
Received 2 March 2010
and zirconyl chloride3k as effective, recyclable catalysts
for this reaction to selectively synthesize 5-aryl-2-oxazo-
lidinones without any organic solvent and additional addi-
tive. Furthermore, product separation from catalyst and
Abstract: A simple approach for facile synthesis of 5-aryl-2-
oxazolidinones in excellent regioselectivity from aziridines under
compressed CO2 conditions was developed in the absence of any
catalyst and organic solvent. The reaction outcome was found to be
tuned by subtly adjusting CO2 pressure. The adduct formed in situ catalyst recycling is also an important issue to be ad-
of aziridine and CO2 is assumed to act as a catalyst in this reaction,
which was also studied by means of in situ FT-IR technique.
dressed. Thanks to unique features such as low cost, avail-
ability, tunable solvent properties, and easy separation,
Key words: carbon dioxide, aziridine, catalyst-free, 5-aryl-2-
oxazolidinones
supercritical carbon dioxide has been widely used as envi-
ronmentally benign reaction medium and clean extractant.
Moreover, it is worthy to be pointed out that properties of
CO2 could be easily tunable by means of pressure or tem-
Oxazolidinones are important five-membered hetero- perature.9 Herein, we would like to introduce a simple and
cyclic compounds in organic chemistry and medicinal straightforward approach for selective synthesis of 5-aryl-
chemistry, which have been widely used as chiral auxilia- 2-oxazolidinones 2 via carboxylation of aziridines 1 with
ries, intermediates in organic synthesis, and building CO2 by elaborately tuning pressure of CO2 or reaction
blocks for biologically active pharmaceutical agents.1 temperature in the absence of any catalyst (Scheme 1).
With the awareness of environment pollution and sustain-
able development, great efforts have been devoted to syn-
thesizing 2-oxazolidinones using CO2 as a raw material in
recent years, as a alternative to toxic, corrosive phosgene
and carbon monoxide route.2 Currently, there are mainly
four accesses to 2-oxazolidinones starting from CO2: (1)
cycloaddition of aziridines with CO2;3 (2) carboxylative
cyclization of propargylamines with CO2;4 (3) reaction of
propargylic alcohols, primary amines, and CO2;5 and (4)
cyclocondensation of 1,2-amino alcohols with CO2.6 Re-
cently, Li and co-workers demonstrated that oxazolidino-
nes can be synthesized through the copper-catalyzed
coupling of aldehydes, amines, terminal alkynes, and
CO2.7 As a nitrogen analogue of epoxides, aziridines have
high ring strain and could readily react with CO2 to afford
2-oxazolidinones. A variety of homogeneous catalysts in-
cluding (salen)-Cr(III)/DMAP,3a phenol/DMAP,3b alkali
metal halide,3c–f tetraalkylammonium halide system,3f
iodine,3g,h have been developed for the carboxylation of
aziridines. And electrochemical procedure is an alterna-
tive for this transformation.3i However, hazardous organic
solvents, catalysts, and/or co-catalysts are generally re-
quired to achieve high catalytic efficiency.
O
O
Et
N
catalyst-free
Et
Et
+
+
CO2
O
N
N
O
Ph
Ph
Ph
1a
2a
3a
Scheme 1 Carboxylation of aziridine with CO2 without any catalyst
We firstly examined the reaction of 1-ethyl-2-phenylazi-
ridine (1a) and CO2 at 100 °C and 8.0 MPa. As shown in
Table 1, a low yield (29%) of 3-ethyl-5-phenyl-2-oxoazo-
lidinone (2a) was obtained after 10 hours (Table 1, entry
1). When the temperature rose up to 120 °C, the yield of
2a was increased to 63% (entry 2), but further rising tem-
perature did not give markedly improvement in 2a yield
(entry 3). On the other hand, CO2 pressure would be an-
other important factor for this reaction. Indeed, 2a yield
increased as CO2 pressure rising from 3.5 to 9 MPa (en-
tries 4, 7–9), and reached the maximum value of 68% at 9
MPa. A further increase in CO2 pressure resulted in a drop
in the yield (entry 9). This is reasonable because near-
critical CO2 conditions may facilitate the formation of the
zwitterionic adduct of 1 with CO2,4f thereby leading to im-
provement of the reaction, whereas higher pressure may
suppress the interaction between the aziridine and CO2 be-
cause of CO2 diluting effect, thus resulting in a lower ac-
tivity.10 A prolonged reaction time gave full conversion
with excellent 2a yield as high as 89% (entry 10). The
only byproducts were found to be slightly amounts of 1,4-
diethyl-2,5-diphenylpiperazine and 1,4-diethyl-2,3-
diphenylpiperazine, that is, dimers of 2a. In addition,
In this context, Kayaki’s group made a significant ad-
vance and found copolymerization of secondary aliphatic
aziridines and CO2 under supercritical conditions to give
poly(urethane amine)s as main products.8 We also devel-
oped a quaternary ammonium salt functionalized PEG3j
SYNLETT 2010, No. 14, pp 2159–2163
x
x
.x
x
.2
0
1
0
Advanced online publication: 22.07.2010
DOI: 10.1055/s-0030-1258510; Art ID: W03310ST
© Georg Thieme Verlag Stuttgart · New York

2160
X.-Y. Dou et al.
LETTER
Table 1 Carboxylation of 1-Ethyl-2-phenylaziridine (1a) with CO2 in the Absence of Catalysta
Entry
Temp (°C)
100
Time (h)
10
Pressure (Mpa)
Conv. (%)b
Yield of 2a (%)b Regioselectivity 2a/3a (%)b
1
2
8.0
8.0
8.0
3.5
3.5
3.5
5.5
9.0
10.0
9.0
48
73
29
63
67
27
69
53
52
68
54
89
96:4
95:5
95:5
95:5
98:2
97:3
97:3
95:5
95:5
95:5
120
10
3
140
10
77
4
120
10
37
5c
6d
7
120
10
75
120
10
70
120
10
70
8
120
10
76
9
120
10
68
10
120
24
>99
a All the reactions were run with 1a (147 mg, 1 mmol).
b Determined by GC using biphenyl as an internal standard.
c Triethylamine (14 mol%) was added.
d Diethylamine (14 mol%) was added.
diethylamine and triethylamine can promote the reaction secondary amine’s positive effect on the reaction (entries
(entries 5, 6 vs. 4).
5 and 6, Table 1). When diethyl amine was added to the
reaction of 1a with CO2, the characteristic absorption
peaks of the carbamate salt at 1669 cm–1 and the oxalidi-
none 2a at 1766 cm–1, were changing (Figure 1, B) as the
reaction running. Notably, absorption of the carbonyl
group was migrated from 1688 cm–1 (the carbamate salt)
to 1762 cm–1 (the product 2b) when 1b was used as the
substrate (Figure 1, C), presumably implying the adduct
in situ formed from aziridine with CO2 could act as a cat-
alyst. As a result, the reaction worked well without addi-
tional catalyst.
To examine the generality of this approach, we conducted
the reaction under the identical reaction conditions, and
the results are summarized in Table 2.11,12
It is found that the aziridines bearing different alkyl
groups at the nitrogen atom (Table 2, entries 1–7) showed
good performance, while 1b and 1c displayed a relatively
low activity being ascribed to formation of
oligomers3j,8a,b,13 which were detected by ESI-MS. Owing
to steric hindrance, the substrates with branched alkyl
groups at the nitrogen atom exhibited lower performance
(entry 8). The aziridines with electron-withdrawing or
electron-donating groups on the aryl ring gave excellent
results (entries 9, 10). It is worth mentioning that all of the
examples showed high chemo- and regioselectivity (from
84:16 to 97:3) towards 5-substituted product. When alter-
ing R1 to an alkyl-like group, 4-substituted oxazolidinone
could be obtained as a main product with high regioselec-
tivity (2k/3k = 3:97), being in agreement with the previ-
ous results (entry 10).3a,j–k Whereas only the starting
material was recovered and no product was detected in the
case of R2 being aryl or electron-withdrawing group (en-
tries 12, 13).
Based on the experimental results and previous reports, a
hypothetic reaction mechanism for this catalyst-free pro-
cess was proposed as depicted in Scheme 2, which is anal-
ogous to the LiI-catalyzed version.3c,d It mainly comprises
three steps: firstly, coordinating of CO2 with aziridine to
generate the zwitterionic adduct in situ which was detect-
ed by in situ FT-IR under CO2 pressure (step I); then ring
opening through a nucleophilic attack by the partially an-
ionic oxygen of the other adduct, assisted by the pseudo
carbocations scattered on the three-membered ring (step
II); and final cyclization via an intramolecular nucleo-
philic attack leading to the product as well as regeneration
of the adduct (step III). The coordination of CO2 to aziri-
dine is the rate-dominating step, which can explain the R1
and R2 group effect on the activity and selectivity. The
branched substituted group at N atom makes the forma-
tion of the adduct (step I) more difficult and thus shows
lower activity. Furthermore, the aryl or electron-
withdrawing group would cause the coordination between
aziridine and CO2 impossible presumably due to the low
electron density at the N atom, being inconsistent with the
experimental results (entries 11, 12, Table 2). There exist
two possible cycles (A or B) in this catalytic cycle de-
To gain a deep insight into this reaction, in situ FT-IR
spectroscopy was employed to identify the possible inter-
mediates during the reaction. As previously reported,14
amines can react with CO2 to form the carbamate salts, be-
ing considered as an analogue of the adduct formed by
aziridine and CO2. As shown in Figure 1 (A), the absorp-
tion intensity of asymmetric n(C=O) vibrations (1658
cm–1) gradually increased with proceeding the reaction of
CO2 with diethyl amine, indicating the formation of the
ammonium carbamate.15 That could account for tertiary or
Synlett 2010, No. 14, 2159–2163 © Thieme Stuttgart · New York

LETTER
Synthesis of 5-Aryl-2-Oxazolidinones
2161
Table 2 Substrate Scopea
O
O
R2
catalyst-free
120 °C, 9 MPa, 24 h
R2
R2
N
O
N
N
CO2
O
+
+
R1
R1
R1
1
2
3
1a R1 = Ph, R2 = Et
1b R1 = Ph, R2 = H
1c R1 = Ph, R2 = Me
1d R1 = Ph, R2 = n-Pr
1e R1 = Ph, R2 = n-Bu
1h R1 = Ph, R2 = Cy
1i R1 = 4-ClC6H4, R2 = Et
1j R1 = 4-MeC6H4, R2 = Et
1k R1 = CH2Cl, R2 = Bn
1l R1 = Ph, R2 = Ph
1f R1 = Ph, R2 = i-Amyl 1m R1 = Ph, R2 = Ts
1g R1 = Ph, R2 = Bn
Entry
Substrate Conv.
(%)b
Isolated yield
(%)c
Regioselecti-
vity 2/3 (%)d
1
2
1a
1b
1c
1d
1e
1f
>99
>99
100
100
>99
100
100
79
89
69
79
86
91
93
97
66
88
87
94
0
95:5
91:9
88:12
95:5
94:6
96:4
94:6
86:14
97:3
97:3
3:97
–
3
4
5
6
7
1g
1h
1i
8
9
100
100
100
0
10
11
12
13
1j
1k
1l
1m
0
0
–
a Reaction conditions: substrate (1 mmol), 120 °C, 9 MPa, 24 h.
b Determined by GC.
c The total yield of 2 and 3.
d Molar ratio of 2 to 3, determined by GC.
pending on the nature of R1. When R1 is aryl group, cycle
A would be favorable, leading to preferential formation of
2a; while if R1 is alkyl, cycle B could be predominant,
thus resulting in dominantly generating 3a. On the other
hand, CO2 pressure effect also supports our hypothesis
(Table 1).
Figure 1 Results of in situ IR spectroscopy monitoring at various
reaction time (min). Reagents and conditions: A. Et2NH (5 mmol),
25 °C, 4 MPa; B. 1a (5 mmol), Et2NH (45 mol%), 120 °C, 4 MPa; C.
1b (5 mmol), 120 °C, 9 MPa. IR data: 1658, 1669, and 1688 cm–1 cor-
respond to absorption of carbonyl group in the carbamate salt; 1766
and 1762 cm–1 can be the absorption of carbonyl group in oxazoli-
dines; 2340 cm–1 is the absorption of free CO2.
In order to gain further insight into the above mechanism,
we also examined the reaction of (S)-1-butyl-2-phenyl-
aziridine [(S)-1e] with CO2, as shown in Scheme 3, that
there is a double inversion of stereochemistry at the chiral
carbon center, which is attacked, to form (S)-2e. The reac-
tion afforded (S)-2e in 85% yield and (S)-3e in 5% yield
with retention of stereochemistry, implying the reaction
does not involve the chiral center to generate (S)-3e.
oxazolidinones were obtained in high chemo- and regio-
selectivity. Notably, the chemoselectivity can be con-
trolled by tuning CO2 pressure. The detailed reaction
mechanism and further extension of this protocol are un-
der investigation in our laboratory.
In summary, we developed an efficient, simple, and self-
catalytic process for selective synthesis of 5-aryl-2-
oxazolidinones from aziridine and CO2, which requires
neither organic solvent nor catalyst. A variety of 5-aryl-2-
Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synlett.
Synlett 2010, No. 14, 2159–2163 © Thieme Stuttgart · New York

2162
X.-Y. Dou et al.
LETTER
O
O
R2
O
N
R2
N
R2
O
N
3
R1
R1
R1
CO2
(III)
(III)
O
O
(I)
R2
R2
O
N
O
O
N
O
N
R1
R1
O
R2
R1
O
A
B
O
O
O
R1 = Alk
R1 = Ar
(II)
O
R2
(II)
R2
O
N
O
N
R1
R1
O
O
O
O
O
O
R2
O
N
O
R1
Scheme 2 The proposed mechanism
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O
O
n-Bu
catalyst-free
N
n-Bu
n-Bu
O
N
N
O
CO2
+
+
120 °C, 9 MPa, 24 h
Ph
Ph
Ph
(S)-2e
(S)-3e
(S)-1e
Scheme 3 Carboxylation of (S)-1-butyl-2-phenylaziridine
Acknowledgment
Financial support from the National Natural Science Foundation of
China (Nos. 20672054, 20872073) and the 111 project (B06005),
and the Committee of Science and Technology of Tianjin is grate-
fully acknowledged.
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Synthesis of 5-Aryl-2-Oxazolidinones
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In a typical reaction, the carboxylation of aziridine with CO2
was carried out in a 25 mL stainless steel autoclave.
Aziridine (1 mmol) was charged into the reactor at r.t. CO2
was introduced into the autoclave, and then the mixture was
stirred at predetermined temperature for 20 min to reach the
equilibration. The pressure was then adjusted to the desired
pressure, and the mixture was stirred continuously. When
the reaction finished, the reactor was cooled in ice-water and
CO2 was ejected slowly. An aliquot of sample was taken
from the resultant mixture and dissolved in dry CH2Cl2 for
GC analysis. GC analyses were performed on Shimadzu GC-
(15) Formation of Et2NH with CO2 identified by 1H NMR: Kong,
D. L.; He, L. N.; Wang, J. Q. Synlett 2010, 1276.
Synlett 2010, No. 14, 2159–2163 © Thieme Stuttgart · New York