Zirconyl chloride: an efficient recyclable catalyst for synthesis of 5-aryl-2-oxazolidinones from aziridines and CO2 under solvent-free conditions

Article abstract of DOI:10.1016/j.tet.2009.05.034

Zirconyl chloride was found to be an efficient catalyst for the cycloaddition reaction of aziridines with CO₂, thus leading to the preferential formation of 5-aryl-2-oxazolidinones under solvent-free conditions. The methodology could be extended to various substituted aziridines with high conversion and chemo-, regio-, and stereoselectivity. Furthermore, the catalyst could be reused over five times without significant loss in activity. Interestingly, the recovered catalyst showed higher activity in comparison with the fresh catalyst, presumably due to its morphological variation. The use of this cheap and moisture stable catalyst make this protocol practical, environmentally benign, and economically attractive.

Full text of DOI:10.1016/j.tet.2009.05.034

Tetrahedron 65 (2009) 6204–6210
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Zirconyl chloride: an efficient recyclable catalyst for synthesis of 5-aryl-2-
oxazolidinones from aziridines and CO2 under solvent-free conditions
*
*
Ying Wu, Liang-Nian He , Ya Du, Jin-Quan Wang, Cheng-Xia Miao, Wei Li
Institute of Elemento-Organic Chemistry, State Key Laboratory of Elemento-Organic Chemistry, Nankai University, Tianjin 300071, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 2 April 2009
Received in revised form 9 May 2009
Accepted 15 May 2009
Available online 21 May 2009
Zirconyl chloride was found to be an efficient catalyst for the cycloaddition reaction of aziridines with
CO2, thus leading to the preferential formation of 5-aryl-2-oxazolidinones under solvent-free conditions.
The methodology could be extended to various substituted aziridines with high conversion and chemo-,
regio-, and stereoselectivity. Furthermore, the catalyst could be reused over five times without significant
loss in activity. Interestingly, the recovered catalyst showed higher activity in comparison with the fresh
catalyst, presumably due to its morphological variation. The use of this cheap and moisture stable cat-
alyst make this protocol practical, environmentally benign, and economically attractive.
Ó 2009 Elsevier Ltd. All rights reserved.
Keywords:
Zirconyl chloride
5-Aryl-2-oxazolidinone
Aziridine
Carbon dioxide
1. Introduction
ZrOCl2$8H2O has drawn much attention because of its water-
stability, low toxicity, and commercial availablity,8 especially wide
Given the importance of oxazolidinones in medicinal chemistry1
and synthetic chemistry,2,3 a growing effort has been devoted to
developing new efficient methodology for synthesis of oxazolidi-
nones. Currently, there are mainly three synthetic strategies from
C1 resource: (i) carbonylation of amino alcohols using phosgene,
CO, etc.;4 (ii) reaction of propargylamines or propargylic alcohols
with CO2;5 and (iii) insertion of CO2 into the aziridines moiety.6 The
methods (ii) and (iii) utilizing abundant, renewable and nontoxic
CO2 as a feedstock7 are promising from a green chemistry per-
spective. In this respect, numerous homogeneous catalysts have
been developed for the cycloaddition reaction of aziridines and
CO2, such as dual-component system viz. SalenCr(III)/DMAP6a or
Phenol/DMAP,6b alkali metal halide6c–e or tetraalkyl-ammonium
halide system.6e Particularly, iodine was extremely active for this
reaction even under supercritical CO2 conditions.6f,g Nonetheless,
toxic organic solvents and co-catalysts are generally required to
achieve high yields, along with toilsome purification of product and
a limited substrate scope in the most of above-mentioned cases. In
this context, developing more environmentally benign heteroge-
neous catalysts for regio-selective synthesis of 5-substituted-2-
oxazolidinones will be more desirable.
utilization as a catalyst in organic reactions, such as oxidation of
alcohols,9 nitration of phenolic compounds,10 acylation of alcohols,
phenols, amines and thiols,11 esterification of carboxylic acids and
alcohols,12 Michael addition of amines and indoles to
a,b-un-
saturated ketones,13 Biginelli reaction,14 Mannich-type reactions,15
synthesis of 2-aliphatic aryloxazolines, benzimidazole, benzothia-
zoles, and bis-oxazolines.16 Recently, zirconyl chloride was also
proven to be highly effective for the synthesis of
b
-acetamido ke-
tones,17 enaminones and enamino esters,18
a
-aminophosphonates,19
homoallylic alcohols or amines.20 Furthermore, ZrOCl2$8H2O is
regarded to be an ionic cluster of [Zr4(OH)8(H2O)16]Cl8$12H2O; and
whereby the zirconium cation cluster [Zr4(OH)8(H2O)16]
8þ is usually
thought to be active species for the Lewis acid-catalyzed re-
actions.12–20
We recently reported a quaternary ammonium bromide co-
valently bound to polyethylene glycol was an efficient and re-
cyclable homogeneous catalyst for the synthesis of 5-substituted
oxazolidinones from carbon dioxide.6k However, organic solvents
are required for the separation of products and catalysts. As our
continuing effort on developing efficient approaches for fixing CO2
into oxazolidinones, we herein would like to report the use of zir-
conyl chloride as an effective and recyclable catalyst for the cyclo-
addition of CO2 to aziridines to afford 5-aryl-2-oxazolidinones
under mild conditions without the need of any additive as depicted
in Scheme 1. Moreover, this methodology was successfully applied
to the synthesis of a variety of 5-substituted oxazolidinones with
excellent yields and regio- and stereoselectivities.
* Corresponding authors. Tel./fax: þ86 22 2350 4216 (L.-N.H.); tel./fax: þ86 22
2350 8662 (W.Li).
E-mail addresses: heln@nankai.edu.cn (L.-N. He), weili@nankai.edu.cn (W. Li).
0040-4020/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2009.05.034

Y. Wu et al. / Tetrahedron 65 (2009) 6204–6210
6205
O
of ZrOCl2$8H2O as a catalyst. It is worth mentioning that the by-
products could be detected as oligomers of homopolymerization of
aziridines and copolymerization aziridines/CO2 as reported in the
literature.6i,j
O
R1
N
R1
N
R1
+
Cat.
O
O
N
+
CO2
Solvent-free
R2
R2
R2
1
2
3
The commonly used zirconium (IV) compounds were also ex-
amined for this purpose. It was found that Zr(SO4)2$4H2O, ZrO-
SO4$4H2O, and ZrO(NO3)2$2H2O displayed relatively low catalytic
activity in comparison with ZrOCl2$8H2O under the otherwise
identical reaction conditions (entries 17–19). Those findings pre-
sumably imply that the coexistence of Lewis acidic sites, i.e., zir-
conium(IV) cation and Lewis basic species viz. chloride anion in
zirconyl chloride would be crucial for promoting this reaction.12a
The influence of temperature was studied as shown in Figure 1.
It was obvious that the catalytic activity and yield of 2a were both
sensitive to reaction temperature. The catalytic activity of
ZrOCl2$8H2O showed slight alteration from 293 K to 313 K, then
increased sharply with the temperature increasing from 313 K to
373 K. However, a slight decrease of 2a yield and increase of 3a
yield was found from 373 K to 413 K, probably due to that higher
temperature could presumably accelerate the nucleophilic ring-
opening reaction at the methylene position, as shown in the pro-
posed mechanism (Scheme 3), thus leading to increasing in the
amount of 3a. Accordingly, the appropriate reaction temperature
would be 373 K.
Scheme 1. Synthesis of 2-oxazolidinones from aziridines and CO2.
2. Results and discussion
In the preliminary study, we first screened those commercially
available Lewis acids and organic bases (Table 1, entries 3–13) for
cycloaddition of 1-ethyl-2-phenylaziridine 1a and CO2, and carried
out reactions at 313 K for 4 h in the presence of 8 MPa CO2. The
desired product (2a) was scarcely obtained without the use of any
catalyst (entry 1); and CH2Cl2 gave rise to a trace amounts of 2a and
4aþ5a. Either Lewis acids (entries 5, 6) or bases (entries 8–13)
displayed poor catalytic activity, or inactive toward the reaction.
Fortuitously, ZrOCl2$8H2O under solvent-free conditions (entry 7)
was found to show comparable catalytic activity with one of the
6f
most active catalysts like I2 (entry 4), even higher than LiBr6d by
employing CH2Cl2 as a solvent (entry 3).
Table 1
Cycloaddition reaction of CO2 to aziridine into oxazolidinonesa
Et
N
Et
N
Conversion of 1a
O
O
Et
N
Ph
Ph
100
Cat.
Et
N
Et
N
O
O
+
CO2
+
+
+
N
Ph Ph
N
Ph
Ph
Ph
Et
Et
80
1a
3a
5a
4a
2a
Yield of 2a
60
Entry Catalyst (mol %)
Solvent T (K) P (MPa) t (h) Conv. (%) Yieldb (%)
2a 3a 4aþ5a
40
1
d
d
313
8
8
8
8
8
8
8
8
8
8
8
8
8
6
6
6
6
6
6
4
4
4
4
4
4
4
4
4
4
4
4
4
2
2
2
2
2
2
5
d
d
d
1
1
2
d
CH2Cl2 313
CH2Cl2 313
CH2Cl2 313
CH2Cl2 313
CH2Cl2 313
10
35
98
58
1
96
6
8
36
11
d
d
97
100
100
70
97
99
1
2
11
9
11
d
4
Yield of 3a
20
0
3
LiBr (20)
31
30
22
1
41
3
4
I2 (20)
1
1
d
5
AlCl3 (20)
6
ZnCl2 (20)
290
310
330
350
Temperature (K)
370
390
410
7
8
9
ZrOCl2$8H2O (20)
Morpholine (20)
DMAPc (20)
DBNd (20)
d
d
d
d
d
313
313
313
313
313
2
1
d
1
1
d
d
1
d
d
6
6
5
16
3
11
Figure 1. Influence of temperature on the reaction outcome. Reaction conditions: 1a
(1 mmol, 0.147 g), catalyst (ZrOCl2$8H2O, 0.0161 g, 5 mol %), 8 MPa, 4 h.
10
11
12
13
14
15
16
17
18
19
0.4 d
DBUe (20)
d
d
d
58
70
80
30
53
30
d
d
d
3
6
6
1
1
1
DABCOf (20)
HMTAg (20)
ZrOCl2$8H2O (0.1)
ZrOCl2$8H2O (1)
ZrOCl2$8H2O (5)
Zr(SO4) 2$4H2O (5)
ZrOSO4$4H2O (5)
ZrO(NO3) 2$2H2O (5)
CH2Cl2 313
CH2Cl2 313
As well-known, a significant drawback associated with using
CO2 as a reagent or reaction medium in organic synthesis is the
potential dangers associated with operating at high temperatures
and pressures. As easily seen from Figure 2, pressure has great
d
d
d
d
d
d
373
373
373
373
373
373
100
a
All the reactions were carried out using 1a (0.147 g, 1 mmol).
Determined by GC using an internal standard technique.
DMAP: 4-dimethylamino-pyridine.
DBN: 1,5-diazabicyclo(4,3,0)non-5-ene.
DBU: 1,8-diazabicyclo[5,4,0]-undec-7-ene.
DABCO: 1,4-diazabicyclo[2,2,2]octane.
b
c
d
e
f
80
2a
60
40
g
HMTA: hexamethyl phosphoric triamide.
Subsequently, the influence of the catalyst amount was evalu-
ated by performing the reactions at 373 K and 6 MPa of CO2 for 2 h.
As shown in Table 1, 2a yield was 58% when 0.1 mol % catalyst was
used; and hereby increased to 70% as catalyst loading went to
1 mol % (entries 14, 15). Notably, further increasing the catalyst
quantity to 5 mol %, the reaction gave quantitative conversion with
80% yield of 2a, and 6% of 3a, alongside with small amounts of 1,4-
diethyl-2,5-diphenyl-piperazine 4a and 1,4-diethyl-2,3-diphenyl-
piperazine 5a (entry 16). In other words, 2-oxazolidinone 2a was
formed in high chemo- and regio-selectivity by employing 5 mol %
20
3a
0
0
2
4
6 10
Pressure (MPa)
8
12
14
16
Figure 2. Yield versus CO2 pressure. Reaction conditions: 1a (1 mmol, 0.147 g), catalyst
(ZrOCl2$8H2O, 0.0161 g, 5 mol %), 373 K, 4 h.

6206
Y. Wu et al. / Tetrahedron 65 (2009) 6204–6210
influence on the reaction outcome with variation of CO2 pressure
from 0.1 to 1 MPa. We are glad to find that the reaction performed
smoothly at low pressures (1 MPa). Specifically, 57% yield of 2a was
obtained even at 0.6 MPa of CO2. The 2a yield was slightly changed
from 1 to 12 MPa, but sharply decreased by further increasing in
CO2 pressure up to 15 MPa. Excessive CO2 pressure may cause a low
concentration of aziridine in the vicinity of the catalyst, thus
resulting in a low reaction rate. On the other hand, too high CO2
pressure may retard the interaction between the aziridine and the
catalyst, whereby also cause a low yield of 2a. Moreover, the phase
behavior6k,21 of the reaction visually inspected through a sapphire
window attached to the autoclave revealed that the catalyst existed
as a solid during the reaction.
ZrOCl2·8H2O
Recovered ZrOCl2
0
10
20
30
40
50
60
70
80
2θ (degree)
Furthermore, the influence of reaction time on the reaction was
also examined, and the results were shown in Figure 3. The reaction
of 1a was almost finished in 10 min. Indeed, 2 h is needed for the
reaction to give the best yield of 2a.
Figure 4. XRD patterns of the catalysts (fresh catalyst and the recovered zirconyl
chloride).
The FTIR spectra were recorded between 400 cmꢁ1 and
4000 cmꢁ1 (Fig. 5). The peaks at 1640 and 574 cmꢁ1 are ascribed to
typical absorption of ZrOCl2$8H2O. However, the peak shifts (from
1640 to 1510 cmꢁ1, from 574 to 492 cmꢁ1) and the new peaks (1084
and 700 cmꢁ1) were found in the IR of the recovered zirconyl
chloride as an indication of Zr coordinating with aziridine. In the
region of 3000–3500 cmꢁ1, the broad adsorption was observed at
ca. 3400 cmꢁ1 for both fresh catalyst and the recovered zirconyl
chloride.
100
80
2a
60
40
20
3a
8
0
Recovered ZrOCl2
6
0
1
2
Time (h)
3
4
4
2
0
ZrOCl2·8H2O
Figure 3. Dependence of the yield on reaction time. Reaction conditions: 1a (1 mmol,
0.147 g), catalyst (ZrOCl2$8H2O, 0.0161 g, 5 mol %), 373 K, 6 MPa.
Another advantage of this approach could be related to the
heterogeneous catalytic process under solvent-free conditions. The
catalyst can be easily recovered from the reaction mixture simply
by filtration and reused for the next run after washing with CH2Cl2
and drying at 333 K. As shown in Table 2, the catalyst can be reused
at least five times without significant loss in catalytic activity.
Therefore, the recyclability of catalyst makes the process econom-
ically and potentially viable for commercial applications. In-
terestingly, the yield of 2a for the fresh catalyst was much lower
than that in the subsequent run (run 1 vs 2–5) under otherwise
identical conditions.
3900
3400
2900
2400
1900
Wavenumbers (cm-1)
1400
900
400
Figure 5. FTIR spectrum of fresh catalyst and the recovered zirconyl chloride.
Pyridine is a useful basic probe molecule to distinguish the
acidic nature and the number of acidic sites. Figure 6 shows the IR
spectra of fresh catalyst and the recovered zirconyl chloride after Py
adsorption. Upon adding pyridine to the fresh catalyst, the band of
1403–1484 cmꢁ1 was observed, which generally is regarded as an
indication22 of Lewis acidic sites.
Table 2
Recyclability of the catalysta
However, there is scarcely any peak near 1535–1550 cmꢁ1
,
Run
Conv. (%)
Yield (%)
showing the absence of Brønsted acid sites. By contrast, the
amount23 of Lewis acidic sites (1411–1484 cmꢁ1) for the recovered
2a
3a
4aþ5a
1
2
3
4
5
>99
>99
>99
>99
>99
80
94
91
89
90
6
1
3
4
4
5
4
5
5
4
Recovered ZrOCl2
a
Reaction conditions: 1a (5 mmol, 0.735 g), ZrOCl2$8H2O (5 mol %, 0.0805 g), the
recovered zirconyl chloride (5 mol %, 0.0445 g), 373 K, 6 MPa, 2 h.
1535-1550
ZrOCl2·8H2O
In order to well understanding the present catalysis, both fresh
catalyst and recovered catalyst were characterized using XRD, IR,
and pyridine adsorption IR. The XRD patterns of the samples are
1460
shown in the 2q
of 3ꢀ–80ꢀ region (Fig. 4). The fresh catalyst was
4000
3500
3000
2500
Wavenumber (cm-1)
2000
1500
1000
found to be a mixture of ZrOCl2$8H2O and ZrOCl2$4H2O, being in
good agreement with standard spectra, revealing slight loss of
water content of ZrOCl2$8H2O. The XRD pattern also shows the
recovered zirconyl chloride could be a typical amorphous material.
Figure 6. FTIR spectra of fresh catalyst and the recovered zirconyl chloride after pyr-
idine adsorption.

Y. Wu et al. / Tetrahedron 65 (2009) 6204–6210
6207
Table 3
Cycloaddition reaction of CO2 to aziridinesa
Entry
Substrate
Major product
Time (h)
Conv. (%)a
Yieldb (%)
Regioselectivityc (%)
93:7
O
N
Ph
O
N
1
2
99
86
Ph
2a
1a
O
H
N
2
3
O
NH
1
2
100
99
59
93
92:8
92:8
Ph
Ph
1b
2b
Bun
O
Bun
N
O
N
Ph
Ph
1c
2c
Bui
N
O
O
Bui
4d
2.5
97
92
93:7
O
N
N
Ph
1d
But
Ph
2d
But
N
O
5
19
2
91
98
71
97
89
99:1
93:7
87:13
Ph
Ph
2e
1e
Ph
O
N
Ph
6
O
N
Ph
Ph
2f
1f
Cyclopropyl
N
O
Cyclopropyl
7e
3
100
O
N
Ph
Ph
2g
1g
Cyclohexyl
O
Cyclohexyl
N
O
N
8
9
3
2
98
98
97
97
99:1
98:2
Ph
Ph
2h
1h
Cyclohexyl
O
Cyclohexyl
O
N
N
4-MeC6H4
4-MeC6H4
1i
2i
Cyclohexyl
O
Cyclohexyl
N
O
N
10
11
2
53
52
96:4
4-ClC6H4
4-ClC6H4
2j
1j
Ph
df
24
100
d
d
N
Ph
1k
O
Ph
Ph
N
O
N
12
2
99
97
3:97
Cl
Cl
1l
3l
a
Reaction conditions: 1 (2 mmol), catalyst (ZrOCl2$8H2O, 5 mol %), 6 MPa, 373 K. The conversions were determined by GC.
b
c
d
e
f
The total yield of (2þ3).
Molar ratio of 2 to 3.
The isolated yield of (2dþ3d) is 89%.
The isolated yield of (2gþ3g) is 85%.
1,2,4,5-Tetraphenylpiperazine and 1,2,3,4-tetraphenylpiperazine were detected by LC–MS.

6208
Y. Wu et al. / Tetrahedron 65 (2009) 6204–6210
O
zirconyl chloride could be 2.8 times of fresh catalyst. On the other
hand, the amount of Brønsted acid sites (1535–1550 cmꢁ1 22
also
O
R1
)
R1
N
O
R1
N
increases for the recovered zirconyl chloride compared with fresh
catalyst. A possible explanation may be that amorphous state en-
larges the surface area24 in the case of the recovered zirconyl
chloride. Indeed, the recovered catalyst showed higher activity in
comparison with the fresh catalyst (Table 2, run 1 vs 2–5).
O
N
Zr2+
2Cl
Zr2+
2Cl
R2
R2
3
R2
2
1
The generality and utility of this approach with a variety of
aziridines (1a–l) were evaluated under the identical reaction con-
ditions. As shown in Table 3, a wide set of oxazolidinones were
selectively formed in good yields. Especially, aziridines (1a, 1c)
bearing alkyl groups at the nitrogen atom proceeded smoothly
(entries 1, 3). The 2-phenylaziridine 1b (R1¼H) displayed a rela-
tively low chemoselectivity probably due to the formation of self-
oligomers detected by GC–MS (entry 2). Increasing steric hindrance
of N-substituted group R1 led to a lower activity (entries 3–5), and
a longer time is required to obtain the satisfactory result (entry 5).
In this study, the regioselectivity can be also enhanced from 87:13
(2/3) to 99:1 (entries 1–8) with variation of alkyl substituent at the
nitrogen atom. On the other hand, an electron-donating group on
benzene ring showed higher activity than an electron-withdrawing
group (entry 9 vs 10). However, 1,2,4,5-tetraphenylpiperazine and
1,2,3,4-tetraphenylpiperazine were obtained when both R1 and R2
are phenyl group (entry 11). Concerning regioselectivity, R2 group is
a crucial factor in dominating the selectivity of the reaction.6f If R2 is
an aryl group, producing 2 is favored (entries 1–10); whereas if R2 is
an alkyl group, the main product is 3. Indeed, the 4-substituted
oxazolidinone 3l was preferentially produced in a molar ratio of
3:97 (2l to 3l) when R2 at the carbon atom is an alkyl group (entry
12), which would be explained by the proposed reaction mecha-
nism as outlined in Scheme 3.
O
O
Zr2+
Cl
Zr2+
Cl
R1
R1
Zr2+
O
O
N
N
2Cl
R1
R2
N
R2
Cl
Cl
R2
A
b
a
Zr2+
Cl
Zr2+
Cl
CO2
R1
CO2
R1
N
N
R2
R2
Cl
Cl
C
B
Cl
Cl
H
O
OH2
H2O
H2O
H2O
OH2
Zr2+
OH
OH2
Cl
OH2
OH
Cl
Zr2+
O
H2O
H
Cl
H
O
HO
OH2
Cl
OH2
Zr2+
OH2
HO
H2O
H2O
8+
Zr2+
2Cl
[Zr (OH) (H O)
4 16
]
2Cl
=
=
8
2
Zr2+
OH2
O
Cl
H2O
H
OH2
Cl
Scheme 3. A plausible reaction mechanism.
Moreover, the reaction of a chiral aziridine (S)-1c with CO2
catalyzed by 5 mol % of ZrOCl2$8H2O (Scheme 2) gave the desired
products, i.e., (S)-2c and (S)-3c with retention of stereochemistry.25
3. Conclusions
O
O
Bu-n
We developed an efficient, simple, and environmentally friendly
process for the synthesis of 5-aryl-2-oxazolidinones by using zir-
conyl chloride as a solid catalyst from aziridines and CO2 without
any solvent and additive. Furthermore, the catalyst could be easily
separated by filtration and reused for at least five times. The pro-
tocol presented herein offers salient advantages and features: (1) it
requires no organic solvent; (2) the catalyst is very effective under
mild conditions; (3) the catalyst is moisture stable, cheap and low
toxic, easily handling, and readily available reagent; (4) excellent
yields, regio-, and stereoselectivities toward the target products
were attained; (5) simple workup procedure; (6) the utility of this
method was proven as evidenced from synthesizing various 5-aryl-
2-oxazolidinones.
.
n-Bu
Bu-n
ZrOCl2 8H2O (5 mol%)
N
O
O
N
N
+
+
CO2
373 K, 6 MPa, 2 h
Ph
Ph
Ph
(S)-1c
(S)-2c
Scheme 2. Carboxylation of (S)-1c into oxazolidinones.
(S)-3c
Based on the above results, a plausible mechanism is proposed
to go through a Lewis acid-base bifunctional pathway, as depicted
in Scheme 3.
8þ
in the crystal26 of zirconyl
Cationic cluster [Zr4(OH)8(H2O)16
]
chloride has a strong coordinating ability toward aziridines, and
whereby generates an intermediate A through a ligand exchange
process.12b Accordingly, the present catalytic cycle includes a Zr-
promoted ring-opening of the aziridine through two different
pathways (a and b) mainly depending on the nature of R2 group
when alkyl substitution at the N-position, following by CO2 in-
sertion, and subsequent cyclization via an intramolecular nucleo-
philic attack leading to oxazolidinones and regeneration of the
catalyst. This proposed mechanism could also account for effect of
the R2 substituent on the selective formation of 2 or 3. As deduced
from Scheme 3, if R2 is an aryl group, the intermediate B would be
more stable than C, and thus 2 would be predominantly formed; in
contrast, if R2 is an alkyl group, C would be favored, which in turn
results in dominantly producing 3. Notably, the stereochemistry for
this reaction (Scheme 2) could also support the above mechanism
(Scheme 3); where there is a double inversion of stereochemistry at
the chiral carbon center, which is attacked, to produce (S)-2c or the
reaction does not involve the chiral carbon center to generate
(S)-3c.
4. Experimental
4.1. Caution
Experiments using compressed gases CO2 are potentially haz-
ardous and must only be carried out by using the appropriate
equipment and under rigorous safety precautions.
4.2. Materials
Aziridines were synthesized according to the published pro-
cedures.27 Carbon dioxide with a purity of 99.99% was commer-
cially available. The other organic and inorganic compounds from
Tianjin Guangfu Fine Chemical Research Institute were used
without further purification except for the solvents, which were
distilled by the known method prior to use.

Y. Wu et al. / Tetrahedron 65 (2009) 6204–6210
6209
4.3. Characterization
NMR (75 MHz, CDCl3)
d
12.1, 36.8, 59.3, 69.7, 126.9, 129.0, 129.2,
137.8, 158.1; ESI-MS calcd for C11H13NO2 191.09, found 192.29
(MþH)þ, 214.38 (MþNa)þ, 405.01 (2MþNa)þ. HRMS calcd for
C11H13NO2 (MþH)þ 192.1019, found 192.1015.
The X-ray diffraction was measured at room temperature using
a Rigaku D/max-2500 powder diffractometer, with Cu Ka radiation
(40 kV, 100 mA). The powder samples were mounted on a silicon
plate for X-ray measurement. Pore size distributions, BET surface
areas, and pore volumes were measured by nitrogen adsorption/
desorption using a BELSORP-mini gas sorption analyzer (BEL Japan,
INC). FTIR measurements were performed on a Bruker EQUINOX 55
FTIR instrument. Potassium bromide pellets containing 0.5% of the
catalyst were used in FTIR experiments and 32 scans were accu-
mulated for each spectrum in transmission, at a spectral resolution
of 4 cmꢁ1. The spectrum of dry KBr was taken for background
subtraction. Pyridine adsorption–desorption was monitored by
infrared spectroscopy. Self-supported wafers of 20 mg and 16 mm
diameter were evacuated in situ in an infrared glass vacuum cell
equipped with calcium fluoride windows. The samples were
degassed under high vacuum steady state at 200 ꢀC for 1.5 h. Pyr-
idine adsorption IR spectra were recorded after pyridine adsorption
at room temperature. Pyridine was then desorbed at 200 ꢀC in
dynamic vacuum and spectra were recorded on a Bruker Vector 22
(IR-FT) spectrometer.
4.5.3. 5-Phenyloxazolidin-2-one (2b)
White crystals, mp 85–86 ꢀC. 1H NMR (300 MHz, CDCl3)
d 3.55 (t,
3J¼8.4 Hz, 1H), 3.99 (t, 3J¼8.4 Hz, 1H), 5.62 (t, 3J¼8.4 Hz, 1H), 6.08
(br s, 1H), 7.35–7.43 (m, 5H); 13C NMR (75 MHz, CDCl3)
d 48.2, 77.8,
125.6, 128.9, 138.4, 160.1; ESI-MS calcd for C9H9NO2 163.06, found
164.18 (MþH)þ, 186.28 (MþNa)þ, 349.03 (2MþNa)þ.
4.5.4. 3-Butyl-5-phenyloxazolidin-2-one (2c)
Colorless liquid. 1H NMR (300 MHz, CDCl3)
d
0.94 (t, 3J¼7.2 Hz,
3H), 1.31–1.40 (m, 2H), 1.51–1.58 (m, 2H), 3.23–3.38 (m, 2H), 3.43 (t,
3J¼8.0 Hz, 1H), 3.92 (t, 3J¼8.8 Hz, 1H), 5.49 (t, 3J¼8.0 Hz, 1H), 7.28–
7.42 (m, 5H); 13C NMR (75 MHz, CDCl3)
d 13.4, 19.5, 29.1, 43.6, 51.8,
74.1, 125.2, 128.4, 128.5, 138.7, 157.7; ESI-MS calcd for C13H17NO2
219.13, found 220.34 (MþH)þ, 259.48 (MþK)þ, 461.05 (2MþNa)þ.
4.5.5. 3-Isobutyl-5-phenyloxazolidin-2-one (2d)
White crystals, mp 38–42 ꢀC. 1H NMR (300 MHz, CDCl3)
d 0.91
The products were analyzed by a gas chromatograph (Shimadzu
2014 chromatographer) equipped with a capillary column (RTX-5,
(d, 3J¼4.8 Hz, 3H), 0.93 (d, 3J¼4.8 Hz, 3H), 1.81–1.95 (m, 1H), 3.02–
3.16 (m, 2H), 3.42 (dd, 2J¼8.7 Hz, 3J¼7.5 Hz, 1H), 3.91 (t, 3J¼8.7 Hz,
1H), 5.48 (t, 3J¼8.4 Hz, 1H), 7.32–7.41 (m, 5H); 13C NMR (75 MHz,
30 mꢂ0.25
mm) using a flame ionization detector, and were further
identified by NMR (Bruker 300 or Varian Mercury-Plus 400 spec-
trometer) and ESI-MS (spray voltage 4.8 KV). High-resolution mass
spectrometry was conducted using an Ionspec 7.0T spectrometer by
ESI-FTICR technique. Melting points were measured on an X4 ap-
paratus and uncorrected. The characterization data (1H NMR, 13C
NMR, and ESI-MS) and physical properties are reported below.
CDCl3) d 19.7, 19.8, 26.7, 51.6, 52.6, 74.1, 125.3, 128.5, 128.7, 138.8,
158.0; ESI-MS calcd for C13H17NO2 219.13, found 461.22 (2MþNa)þ,
679.70 (3MþNa)þ. HRMS calcd for C13H17NO2 (MþH)þ 220.1332,
found 220.1339.
4.5.6. 3-tert-Butyl-5-phenyloxazolidin-2-one (2e)
Colorless liquid. 1H NMR (300 MHz, CDCl3)
d 1.41 (s, 9H), 3.45 (t,
4.4. General procedure for the ZrOCl2$8H2O-catalyzed
cycloaddition reaction of CO2 with aziridines
3J¼8.4 Hz, 1H), 3.95 (t, 3J¼8.7 Hz, 1H), 5.36 (t, 3J¼8.1 Hz, 1H), 7.32–
7.41 (m, 5H); 13C NMR (75 MHz, CDCl3)
d 27.3, 50.9, 53.5, 73.4,125.4,
128.5, 128.7, 138.9, 156.6; ESI-MS calcd for C13H17NO2 219.13, found
In a 25 mL autoclave reactor equipped with a magnetic stirrer,
aziridine (1 mmol), catalyst (ZrOCl2$8H2O, 16.1 mg, 0.05 mmol),
and biphenyl (50 mg, an internal standard for GC analysis) were
charged. Then CO2 was introduced into the autoclave. The pressure
was adjusted to 6 MPa at 373 K, and the mixture was stirred for 2 h.
After the reaction was completed, the reactor was cooled in ice-
water and CO2 was ejected slowly. An aliquot of sample was taken
from the resultant mixture for GC analysis. The residue was purified
by column chromatography on silica gel (200–300 mesh, eluting
with 8:1 to 1:1 petroleum ether/ethyl acetate) to provide the de-
sired products. The products were further identified by NMR and
MS as below, being in good agreement with the assigned structures
and consistent with those reported in the literature6a,28 for known
compounds. The NMR charts for the products and the XRD patterns
for fresh ZrOCl2$8H2O and the recovered zirconyl chloride were
given in Supplementary data.
242.46 (MþNa)þ, 259.30 (MþK)þ.
4.5.7. 3-Benzyl-5-phenyloxazolidin-2-one (2f)
White crystals, mp 60–64 ꢀC. 1H NMR (300 MHz, CDCl3)
d 3.28 (t,
3J¼8.4 Hz, 1H), 3.75 (t, 3J¼8.7 Hz, 1H), 4.45 (ABq, JAB¼15.0 Hz,
DnAB¼36.0 Hz, 2H), 5.43 (t, 3J¼8.1 Hz, 1H), 7.27–7.35 (m, 10H); 13C
NMR (75 MHz, CDCl3)
d 48.1, 51.3, 74.3, 125.3, 127.8, 127.9, 128.6,
128.7, 135.5, 138.5, 157.8; ESI-MS calcd for C16H15NO2 253.11, found
276.44 (MþNa)þ, 781.66 (3MþNa)þ.
4.5.8. 3-Cyclopropyl-5-phenyloxazolidin-2-one (2g)
White crystals, mp 52–55 ꢀC. 1H NMR (300 MHz, CDCl3)
d 0.75
(s, 4H), 2.55–2.59 (m, 1H), 3.43 (t, 3J¼8.1 Hz, 1H), 3.88 (t, 3J¼8.7 Hz,
1H), 5.42 (t, 3J¼8.1 Hz, 1H), 7.28–7.37 (m, 5H); 13C NMR (75 MHz,
CDCl3) d 5.4, 5.8, 25.7, 53.3, 74.3,125.4,128.6,128.7,138.5,157.9; ESI-
MS calcd for C12H13NO2 203.09, found 429.27 (2MþNa)þ, 631.80
(3MþNa)þ.
4.5. Characterization data
4.5.9. 3-Cyclohexyl-5-phenyloxazolidin-2-one (2h)
4.5.1. 3-Ethyl-5-phenyloxazolidin-2-one (2a)
White crystals, mp 92–93 ꢀC. 1H NMR (300 MHz, CDCl3)
d 1.0–
Colorless liquid. 1H NMR (300 MHz, CDCl3)
d
1.17 (t, 3J¼7.2 Hz,
1.8 (m, 10H), 3.38 (t, 3J¼8.4 Hz, 1H), 3.70–3.73 (m, 1H), 3.88 (t,
3H), 3.29–3.45 (m, 3H), 3.92 (t, 3J¼8.7 Hz, 1H), 5.48 (t, 3J¼7.8 Hz,
3J¼8.7 Hz, 1H), 5.45 (t, 3J¼8.4 Hz, 1H), 7.35–7.38 (m, 5H); 13C NMR
1H), 7.34–7.42 (m, 5H); 13C NMR (75 MHz, CDCl3)
d
12.4, 38.8, 51.5,
(75 MHz, CDCl3) d 25.1, 25.2, 29.9, 30.3, 48.1, 52.4, 74.4, 125.3, 128.5,
74.2, 125.4, 128.6, 128.8, 138.8, 157.5; ESI-MS calcd for C11H13NO2
191.09, found 192.29 (MþH)þ, 214.38 (MþNa)þ, 405.01 (2MþNa)þ.
HRMS calcd for C11H13NO2 (MþH)þ 192.1019, found 192.1015.
128.7, 138.9, 157.0; ESI-MS calcd for C15H19NO2 245.14, found 246.27
(MþH)þ, 757.70 (3MþNa)þ.
4.5.10. 3-Cyclohexyl-5-p-tolyloxazolidin-2-one (2i)
4.5.2. 3-Ethyl-4-phenyloxazolidin-2-one (3a)
White crystals, mp 89–91 ꢀC. 1H NMR (300 MHz, CDCl3)
d 1.03–
Colorless liquid. 1H NMR (400 MHz, CDCl3)
d
1.05 (t, 3J¼7.2 Hz,
1.86 (m, 10H), 2.36 (s, 3H), 3.38 (t, 3J¼8.0 Hz, 1H), 3.71–3.75 (m, 1H),
3H), 2.79–2.88 (m, 1H), 3.48–3.57 (m, 1H), 4.10 (t, 3J¼8.0 Hz, 1H),
3.85 (t, 3J¼8.7 Hz, 1H), 5.43 (t, 3J¼8.0 Hz, 1H), 7.17–7.26 (m, 4H); 13
C
4.62 (t, 3J¼8.8 Hz, 1H), 4.81 (t, 3J¼7.2 Hz, 1H), 7.30–7.44 (m, 5H); 13
C
NMR (75 MHz, CDCl3) d 21.1, 25.2, 25.3, 25.4, 30.1, 30.5, 48.3, 52.5,

6210
Y. Wu et al. / Tetrahedron 65 (2009) 6204–6210
3. As representative examples using oxazolidinones as intermediates, see: (a)
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N. Phosphorus Sulfur Silicon Relat. Elem. 1988, 38, 137–148.
74.5, 125.5, 129.5, 136.0, 138.6, 157.3; ESI-MS calcd for C16H21NO2
259.16, found 260.02 (MþH)þ, 799.55 (3MþNa)þ. HRMS calcd for
C16H21NO2 (MþH)þ 260.1645, found 260.1652.
4.5.11. 5-(4-Chlorophenyl)-3-cyclohexyloxazolidin-2-one (2j)
White crystals, mp 94–96 ꢀC. 1H NMR (300 MHz, CDCl3)
d 1.05–
1.83 (m, 10H), 3.34 (t, 3J¼8.0 Hz, 1H), 3.69–3.76 (m, 1H), 3.89 (t,
3J¼8.7 Hz, 1H), 5.44 (t, 3J¼8.0 Hz, 1H), 7.27–7.38 (m, 4H); 13C NMR
(75 MHz, CDCl3) d 25.2, 25.3, 30.0, 30.4, 48.2, 52.6, 73.8,126.8,129.0,
134.5, 137.6, 156.8; APCI-MS calcd for C15H18ClNO2 279.10, found
839.62 (3MþH)þ, 859.60 (3MþNa)þ. HRMS calcd for C15H18ClNO2
(MþH)þ 280.1099, found 280.1101.
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4.5.12. 3-Benzyl-4-(cholomethyl)oxazolidin-2-one (3l)
Colorless liquid. 1H NMR (300 MHz, CDCl3)
d 3.52 (d, 2H), 3.86–
3.94 (m, 1H), 4.17 (d, 2J¼15.3 Hz, 1H), 4.24 (t, 3J¼8.9 Hz, 1H), 4.23 (q,
3J¼9.0 Hz, 3J¼5.3 Hz, 1H), 4.82 (d, 2J¼15.3 Hz, 1H), 7.29–7.37 (m,
5H); 13C NMR (75 MHz, CDCl3)
d 43.4, 46.5, 54.8, 65.3, 128.1, 128.3,
129.0, 135.5, 158.0; ESI-MS calcd for C11H12NO2Cl 225.67, found
472.90 (2MþNa)þ. HRMS calcd for C11H12NO2Cl (MþNa)þ
248.0449, found 248.0454.
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4.5.13. 1,4-Diethyl-2,5-diphenyl-piperazine (4a)
White crystals, mp 116–119 ꢀC. 1H NMR (400 MHz, CDCl3)
d 0.91
(t, 3J¼7.2 Hz, 6H), 1.99–2.05 (m, 2H), 2.30 (t, 3J¼10.8 Hz, 2H), 2.54–
2.62 (m, 2H), 3.08 (dd, 2J¼11.6 Hz, 3J¼2.4 Hz, 2H), 3.45 (dd,
3J¼2.0 Hz, 2J¼12.0 Hz, 2H), 7.29–7.43 (m, 10H); LC–MS calcd for
C20H26N2 294.21, found 295.35 (MþH)þ. HRMS calcd for C20H26N2
(MþH)þ 295.2169, found 295.2164.
4.5.14. 1,4-Diethyl-2,3-diphenyl-piperazine (5a)
Colorless liquid. 1H NMR (400 MHz, CDCl3)
d
1.01 (t, 3J¼7.2 Hz,
6H), 2.17–2.26 (m, 2H), 2.33–2.26 (m, 2H), 2.65–2.69 (m, 2H), 2.95–
2.99 (q, 3J¼6.0 Hz, 2H), 3.73 (s, 2H), 7.27–7.38 (m, 6H), 7.69–7.71 (d,
3J¼7.2 Hz, 4H); LC–MS calcd for C20H26N2 294.21, found 295.31
(MþH)þ. HRMS calcd for C20H26N2 (MþH)þ 295.2169, found
295.2167.
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Acknowledgements
18. Zhang, Z. H.; Li, T. S.; Li, J. J. Catal. Commun. 2007, 8, 1615–1620.
19. Bhagat, S.; Chakraborti, A. K. J. Org. Chem. 2008, 73, 6029–6032.
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Financial support from the National Natural Science Foundation
of China (Grant Nos. 20421202, 20672054, and 20872073) and the
111 project (B06005) and Tianjin Natural Science Foundation is
gratefully acknowledged. We also thank the reviewers for their
valuable suggestions regarding revision.
23. The amount of Lewis acid sites is defined as the corresponding peak area di-
vided by the sample weight.
Supplementary data
24. Based on the BET measurement, pore channel was not found in the fresh
catalyst; while the recovered one could show a small pore structure with the
Supplementary data associated with this article can be found in
the online version, at doi:10.1016/j.tet.2009.05.034.
surface area of 3 m2 gꢁ1), pore volume of 0.0368 cm3 ꢁ1, and average pore
g
diameter (24.79 nm), being well agreement with the results of catalytic
activity.
25. (S) -1c was synthesized according to a literature procedure: Calet, S.; Urso, F.;
Alper, H. J. Am. Chem. Soc. 1989, 111, 931–934 The reaction of (S)-1c with CO2 in
the presence of 5 mol % catalyst affords (S)-2c in 99.8% ee with 86% yield and
(S)-3c in 99.9% ee with 7% yield.
26. The zirconyl group consists of a complex cation in which four zirconium atoms
are at the corners of a slightly distorted square, and are linked along each edge
of the square by two OH groups. Each zirconium is bound to four water mole-
cules. There are no zirconium–halogen bonds in this structure. Clearfield, A.;
Vaughan, P. A. Acta Crystallogr. 1956, 9, 555–558.
27. (a) Capriati, V.; Florio, S.; Luisi, R.; Musio, B. Org. Lett. 2005, 7, 3749–3752; (b)
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Calet, S.; Urso, F.; Alper, H. J. Am. Chem. Soc. 1989, 111, 931–934.
28. (a) Bergmann, E. D.; Goldschomidt, Z. J. Med. Chem. 1968, 11, 1121–1125; (b)
Herweh, J. E. J. Org. Chem. 1968, 33, 4029–4033; (c) Das, J. Synth. Commun. 1988,
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2. As representative examples using oxazolidinones as chiral auxiliaries, see: (a)
Prashad, M.; Liu, Y. G.; Kim, H. Y.; Repic, O.; Blacklock, T. J. Tetrahedron: Asym-
metry 1999, 10, 3479–3482; (b) Gawley, R. E.; Campagna, S. A.; Santiago, M.;
Ren, T. Tetrahedron: Asymmetry 2002, 13, 29–36; (c) Phoon, C. W.; Abell, C.
Tetrahedron Lett. 1998, 39, 2655–2658.