Catalytic annulation of epoxides with heterocumulenes by the indium-tin system

Article abstract of DOI:10.3390/molecules23040782

In the synthesis of five-membered heterocycles by the annulation of epoxides with heterocumulenes such as carbon dioxide and isocyanates, we developed the indium-tin catalytic system and synthesized various cyclic adducts including novel types products under mild reaction conditions.

Full text of DOI:10.3390/molecules23040782

molecules
Communication
Catalytic Annulation of Epoxides with
Heterocumulenes by the Indium-Tin System
Itaru Suzuki, Akira Imakuni, Akio Baba and Ikuya Shibata *
Research Center for Environmental Preservation, Osaka University, Osaka 565-0871, Japan;
suzuki@epc.osaka-u.ac.jp (I.S.); imakuni@epc.osaka-u.ac.jp (A.I.); baba@chem.eng.osaka-u.ac.jp (A.B.)
*
Correspondence: shibata@epc.osaka-u.ac.jp; Tel.: +81-6-6879-8975
ꢀ
ꢁꢂꢀꢃꢄꢅꢆꢇ
ꢀ
ꢁꢂꢃꢄꢅꢆ
Received: 15 March 2018; Accepted: 28 March 2018; Published: 28 March 2018
Abstract: In the synthesis of five-membered heterocycles by the annulation of epoxides with
heterocumulenes such as carbon dioxide and isocyanates, we developed the indium-tin catalytic
system and synthesized various cyclic adducts including novel types products under mild
reaction conditions.
Keywords: indium; tin; epoxide; carbon dioxide, isocyanate, cyclic carbonate, 2-oxazolidinone
1
. Introduction
Synthesis of five-membered heterocycles by the annulation of epoxides with heterocumulenes
such as carbon dioxide and isocyanates has been intensively studied [
carbon dioxide, its catalytic transformation to useful organic compounds has attracted much
attention [ ]. In particular, fixation of carbon dioxide to cyclic carbonates is one of the most
important processes of high atom efficiency [ ]. Cyclic carbonates are known to be efficient aprotic
polar solvents [10 12], electrolytes in lithium ion batteries [13 14] and materials for producing
polycarbonates [15]. Instead of the products from carbon dioxide, the cyclic adducts of epoxides with
isocyanates such as 2-oxazolidinones [16 17] are important biologically-active compounds [18 22] and
synthetic intermediates such as precursors of amino alcohols and chiral auxiliaries [23 31]. We present
1–3]. For the use of
4–8
9
–
,
,
–
–
here a simple, easily available catalyst, the indium-tin system, and provide the environmentally-benign
process to annulated adducts under mild conditions.
2
. Results
2
.1. Synthesis of Cyclic Carbonates
Until now, various catalysts such as transition metal compounds, ionic liquid, onium salts and
alkali metal salts, etc., have been developed for the reaction of epoxides with carbon dioxide [5,32,33]
.
However, these methods suffer from either the need for co-solvent, the requirement for high
temperature, high CO pressure or expensive catalyst. Indium reagents and catalysts have been
2
applied in modern organic synthesis by their mildness and easy handling character [34]. By using the
indium halide-phosphine complex, we have already developed the reaction of terminal epoxides to
give cyclic carbonates under atmospheric CO pressure at room temperature [35], which indicated
2
that indium halide-based catalysts have efficient catalytic activities. As shown in Table 1, we screened
indium halide catalytic systems in the reaction of epoxide 1a with CO (3.9 MPa) at room temperature.
2
The sole use of InCl did not have catalytic activity (Entry 1). Interestingly, the combination of Bu SnI
2
3
2
with InCl increased the yield of carbonate 2a (Entry 2). The sole use of Bu SnI was not effective at
3
2
2
all (Entry 3). Thus, the InCl -Bu SnI system showed a high catalytic activity. Of particular interest
3
2
2
is that the reaction proceeded well even at room temperature. The choice of acetonitrile as a solvent
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is essential because no reaction proceeded when other solvents such as hexane, benzene, CHCl and
3
THF were used. In acetonitrile, the reaction proceeded very well, and various cyclic carbonates
obtained from epoxides 1b 1f catalyzed by InCl -Bu SnI . Epoxides having aliphatic and aromatic
substituents were reactive to afford the corresponding cyclic carbonates 2b
chemoselectivities were observed because of the mild conditions. Functionalized cyclic carbonates
2 were
–
3
2
2
–
2c (Entries 4 and 5). High
2
d–2f were synthesized from epoxides having halogen and oxygen substitutes (Entries 6–8).
a
Table 1. Synthesis of cyclic carbonates 2 from epoxides 1 with CO at rt .
2
Yield 2 (%) b
trace
Entry
R
Cat.
Time (h)
Product
1
2
3
Me (1a)
InCl3
5
5
5
2
a
InCl -Bu SnI
78
3
2
2
2
2
2
Bu SnI
trace
2
2
4
5
6
Et (1b)
Ph (1c)
InCl -Bu SnI
8
10
5
2b
2c
85
69
82
3
2
InCl -Bu SnI
3
2
CH Cl (1d)
InCl -Bu SnI
2d
2
3
2
CH OPh
2
7
8
InCl -Bu SnI
5
2e
2f
90
68
3
2
2
2
(
1e)
CH OMe
2
InCl -Bu SnI
10
3
2
(1f)
a
InCl3 0.5 mmol, Bu2SnI2 1 mmol, epoxide 1 10 mmol, CO2 3.9 Pa, MeCN 3 mL; b Determined by H-NMR.
1
2
.2. Synthesis of 2-Oxazolidinones
Instead of using carbon dioxide, the cyclic adducts of epoxides with isocyanates are highly in
demand [18
quaternary ammonium salts [43
tetraphenylantimony iodide [48
promoting reactions require relatively severe conditions (over 100 C). The Pd-catalyzed reaction
enabled mild conditions and accomplished an asymmetric reaction; however, in the reaction, only
–31]. To effect the reaction, various catalysts have been used such as lithium halides [36–42],
,
44], phosphonium salt [45], AlCl [46], magnesium halides [47],
3
–
50] and the chromium(Salphen) complex [51]. These catalysts
◦
vinyl-substituted epoxides could be applicable where
π-allyl palladium intermediates should be
generated [52]. We have already reported that the Bu SnI-Ph PO or Ph SbI catalyzes the annulation
3
3
4
of epoxides with aromatic isocyanates to give 3,5-disubstututed-2-oxazolidinones [53
indicated that tin halide-based catalysts would afford efficient catalytic activity. In view of these
backgrounds, we tested the catalytic activity of the Bu SnI -InCl system as shown in Table 2.
–56], which
2
2
3
In the reaction of epoxybutane 1b with tert-BuN=C=O (3a), a quantitative yield of 2-oxazolidinone

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4
a
was obtained (Entry 1). Interestingly, steric hindrance of isocyanates was not a problem to give
2
-oxazolidinones 4. The use of either Bu SnI or InCl was not effective (Entries 2 and 3). Thus, it
2 2 3
was clear that the InCl -Bu SnI catalytic system showed a high activity even for the synthesis of
3
2
2
2
-oxazolidinones 4. Other epoxides such as epichlorohydrin 1d and glycidylic ethers 1e, 1f also reacted
well (Entries 4–6). Of course, primary aliphatic isocyanate 3b and phenyl isocyanate 3c also gave
the desired products 4e and 4f, although the yields were moderate owing to the trimerization of an
isocyanate as a side reaction (Entries 7 and 8) [51,57]. Thus, higher yields of 4a–d in the reaction of
tert-BuN=C=O (3a) were achieved because the trimerization would be depressed by steric hindrance
of 3a. In all cases, regioselective ring opening of epoxides took place at the less substituted site to give
3
,5-disubstituted-2-oxazolidinones 4.
Table 2. Synthesis of 2-oxazolidinones 4 from epoxides 1b–f with isocyanates 3 a.
Yield 3 (%) b
Entry
R1
R2
Conditions
Product
1
Et (1b)
t-Bu (3a)
rt, 10 h
4a
79
2
3
7 c
trace d
◦
4
5
6
7
8
CH Cl (1d)
60 C, 3 h
4b
4c
4d
4e
4f
79
90
99
49
64
2
CH OPh
2
◦
60 C, 3 h
(
1e)
CH OMe
2
◦
60 C, 3 h
(1f)
◦
Et (1b)
Et (1b)
n-Bu (3b)
60 C, 7 h
Ph (3c)
rt, 20 h
a
InCl
3
0.25 mmol, Bu
2
SnI
2
0.5 mmol, epoxide
1
5 mmol, isocyanate
3
5.5 mmol, MeCN 1.5 mL, under nitrogen;
b
1
c
d
Determined by H-NMR; Only Bu2SnI2 was used; Only InCl3 was used.
In the reaction with diphenyl carbodiimide, an analogue of isocyanates, the oxazolidin-2-imine
was obtained in good yield (Scheme 1).
5,

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Scheme 1. Catalytic synthesis of oxazolidin-2-imine 5.
3
. Discussion
As shown in Figure 1, the structure of the Bu SnI -InCl system could be supposed by the
2
2
3
measurement of 1 Sn-NMR spectra in acetonitrile. The addition of equimolar InCl to Bu SnI in
acetonitrile clearly changed the 1 Sn peak from a strong one at
downfield shift indicates that tin species had a positive character by the combination with InCl [58
19
3
2
2
19
−
38 ppm to a broad one at 3 ppm. This
,
59].
3
Figure 1. 119Sn NMR of the tin-indium system.
The catalytic cycle is explained as shown in Scheme 2. By the interaction of tin and indium species,
δ+
δ−
Lewis acidic tin species like Bu SnI [InCl I] would be generated, which activate the epoxide
2
3
ring [60
reagents easily takes place [64
at the less substituted carbon. Next, the tin-oxygen bond in
an adduct . In the case of isocyanates, the addition occurs at the C=N group selectively to give a
stannylcarbamate 69 71]. At the last stage, the Sn-X (X=O, NR) bond in the intermediate attacks
and regenerate
showed single regio isomers because of the regioselective
–
63]. This active bimetallic species is plausible because transmetallation between tin and indium
68]. The ring opening of an epoxide to proceeds regioselectively
is added to heterocumulene to give
–
A
A
B
B
[
–
B
the terminal alkyl iodide [72
the catalyst. The H-NMR of products
,
73] to afford cyclic carbonates
2
and 2-oxazolidinones
4
1
4
ring opening of epoxides. For example, 5-H and 4-H peaks for 4f were found at 4.59 (1H), 4.08–3.66
1
(
2H) ppm, respectively. On the other hand, it has been reported by us that H-NMR for another regio
isomer showed its 5-H and 4-H peaks at 4.65–4.30 (2H), 4.22–4.04 (1H) ppm, respectively [49].

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Scheme 2. A plausible catalytic cycle.
4
. Materials and Methods
4
.1. Analysis
FTIR spectra were recorded as a thin film on a Nicolet IS5 spectrometer (Thermo Electron
1
13
Scientific Instruments LLC, Madison, WI, USA). All H and C-NMR spectra were recorded with a
JEOL JMTC-400/54/SS (400 and 100 MHz, respectively) in deuteriochloroform (CDCl ) containing
3
0
.03% (w/v) of tetramethylsilane as an internal standard. Temperatures shown in schemes or tables
1
were controlled by a constant-temperature oil bath. Yields were determined by H-NMR using
1,1,1,2-tetrachloroethane or 1,1,2,2-tetrachloroethane as an internal standard. Mass spectra were
recorded on a JEOL JMS-DS-303 spectrometer (JEOL Ltd., Tokyo, Japan). Flash column chromatography
was performed by Yamazen YFLC-AI-580 using Hi-Flash Silica gel 2L Hi-Flash Column 20 mL/min
eluted by hexane/EtOAc with the gradation mode changing from 9/1–3/7 depending on R values of
f
each compound. Bulb-to-bulb distillation (Kugelrohr) was accomplished at the oven temperature and
pressure indicated.
Dehydrated acetonitrile (MeCN) was purchased from commercial sources and used as obtained.
Deuterated acetonitrile was also purchased and stored drying over 4 Å molecular sieves. All epoxides,
isocyanates, carbodiimide and InCl were also purchased and used as obtained. Bu SnI was prepared
3
2
2
according to the previous report [74].
4
.2. General Procedure for Synthesis of Cyclic Carbonates 2a–f from Epoxides 1 with CO2
To a 50-mL autoclave, InCl (0.5 mmol), Bu SnI (1.0 mmol) and epoxide (10 mmol) were added
in MeCN (3 mL). The autoclave was flushed with CO (3.9 MPa) and stirred at room temperature
1
3
2
2
2
for 5–10 h. After release of the CO gas, the reaction mixture was quenched with H O (20 mL) and
2
2
extracted with Et O (3
×
20 mL). The collected organic layer was dried over MgSO . After filtration,
4
2
the mixture was concentrated in vacuo. The residue was purified by column chromatography. Further
purification was performed by Kugelrohr distillation to give a pure product 2.
1
4
-Methyl-1.3-dioxolan-2-one (2a). Colorless liquid. The NMR data of H and 13C agreed with the
1
previous report [75]. H-NMR: (270 MHz, CDCl )
δ
4.70 (ddd, J = 6.4, 7.3, 7.8 Hz, 1H, 4-H), 4.38 (dd,
J = 7.8, 8.3 Hz, 1H, 5HH), 3.84 (dd, J = 7.3, 8.3 Hz, 1H, 5HH), 1.29 (d, J = 6.34 Hz, 3H, 4-CH3). C-NMR:
67.5 MHz, CDCl ) δ 155.00 (C-2), 73.55 (C-4), 70.58 (C-5), 19.18 (4-CH ).
3
13
(
3
3

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1
4
-Ethyl-1.3-dioxolan-2-one (2b). Colorless liquid. The NMR data of H and 13C agreed with the previous
1
report [76]. H-NMR: (270 MHz, CDCl ) δ 4.70–4.65 (m, 1H, 4-H), 4.56 (dd, J = 8.3, 8.3 Hz, 1H, 5HH),
.12 (dd, J = 7.8, 7.8 Hz, 1H, 5HH) 1.88–1.68 (m, 2H, 4-CH CH ), 1.03 (t, J = 7.3 Hz, 3H, 4-CH CH ).
3
13
4
C
2
3
2
3
NMR: (67.5 MHz, CDCl ) δ 154.91 (C-2), 77.88 (C-4), 68.83 (C-5), 26.59 (4-CH CH ), 8.20 (4-CH CH ).
3
2
3
2
3
1
4
-Phenyl-1.3-dioxolan-2-one (2c). Colorless liquid. The NMR data of H and 13C agreed with the previous
1
report [75]. H-NMR: (270 MHz, CDCl )
δ
7.44–7.18 (m, 5H, Ph), 5.66 (t, J = 8.1 Hz, 1H, 4-H) 4.78 (dd,
J = 8.3, 8.3 Hz, 1H, 5HH), 4.33 (dd, J = 7.8, 7.8 Hz, 1H, 5HH). C-NMR: (67.5 MHz, CDCl3) δ 154.71
3
13
(C-2), 135.67 (i), 129.62 (m), 129.12 (p), 125.77 (o), 77.92 (C-4), 71.10 (C-5).
1
4
-(Chloromethyl)-1,3-dioxolan-2-one (2d). Colorless liquid. The NMR data of H and 13C agreed with
1
the previous report [75]. H-NMR: (270 MHz, CDCl )
δ
5.10–5.02 (m, 1H, 4-H), 4.63 (dd, J = 8.8, 8.8
3
Hz, 1H, 5HH), 4.41 (dd, J = 5.8, 8.8 Hz, 1H, 5HH), 3.88 (dd, J = 4.4, 12.2 Hz, 1H, CHHCl), 3.76 (dd,
J = 3.4
4
,
12.2 Hz, 1H, CHHCl). 13C-NMR: (67.5 MHz, CDCl3)
4.01 (CH Cl).
δ
154.33 (C-2), 74.33 (C-4), 66.70 (C-5),
2
1
4
-(Phenoxymethyl)-1,3-dioxolan-2-one (2e). Colorless liquid. The NMR data of H and 13C agreed with
1
the previous report [76]. H-NMR: (270 MHz, CDCl )
δ
7.35–6.87 (m, 5H, Ph), 5.08–4.99 (m, 1H, 4-H),
3
4
.62 (dd, J = 8.3, 8.8 Hz, 1H, CHHOPh), 4.54 (dd, J = 6.4, 8.8 Hz, 1H, CHHOPh), 4.25 (dd, J = 3.9,
1
1
0.3 Hz, 1H, 5HH), 4.15 (dd, J = 3.9, 10.3 Hz, 1H, 5HH). 13C-NMR: (67.5 MHz, CDCl3)
δ 157.72 (o),
54.67 (C-2), 129.55 (p), 121.98 (m), 114.59 (m), 74.10 (CH OPh), 66.87 (C-4), 66.23 (C-5).
2
1
4
-(Methoxymethyl)-1,3-dioxolan-2-one (2f). Colorless liquid. The NMR data of H and 13C agreed with
1
the previous report [35]. H-NMR: (270 MHz, CDCl )
δ
4.88–4.80 (m, 1H, 4-H), 4.51 (dd, J = 8.3, 8.8 Hz
,
3
1
H, 5HH), 4.37 (dd, J = 6.3, 8.3 Hz, 1H, 5HH), 3.67 (dd, J = 3.4, 11.2 Hz, 1H, CHHOCH ), 3.54 (dd,
3
13
J = 3.9, 11.2 Hz, 1H, CHHOCH ), 3.43 (s, 3H, OCH ). C-NMR: (67.5 MHz, CDCl3)
δ 154.91 (C-2),
3
3
7
5.01 (C-4), 71.27 (CH OCH ), 65.97 (C-5), 59.34 (OCH ).
2 3 3
4
.3. General Procedure for Synthesis of 2-Oxazolidinones 4 from Epoxides 1b–f with Isocyanates 3 and
Oxazolidin-2-imine 5
To a two-neck 10-mL reaction vessel, InCl (0.25 mmol), Bu SnI (0.50 mmol), epoxide
1 (5 mmol)
3
2
2
and isocyanate
3
(5.5 mmol) were added in MeCN (1.5 mL) under N atmosphere. The reaction mixture
2
◦
was stirred at room temperature or 60 C for 3–20 h. After completion of the reaction, the mixture was
quenched with H O (20 mL) and extracted with Et O (3
×
20 mL). The collected organic layer was
2
2
dried over MgSO . After filtration, the mixture was concentrated in vacuo. The residue was purified
4
by column chromatography. For some cases, further purification was performed by distillation to give
a pure product 4.
◦
−1
.
3
-(tert-Butyl)-5-ethyloxazolidin-2-one (4a). Colorless liquid. bp: 78 C/2 mmHg. IR (neat): 1743.33 cm
+
1
HRMS: (EI+, 70 eV) Calculated (C H NO ) 171.1259 (M ) Found: 171.1257. H-NMR: (270 MHz,
9
17
2
CDCl3) δ 4.30 (ddd, J = 6.4, 7.8, 8.8 Hz, 1H, 5-H), 3.64 (dd, J = 8.3, 8.8 Hz, 1H, 4HH), 3.20 (dd, J = 7.8,
8
.3 Hz, 1H, 4HH), 1.70 (m, 2H, CH CH ), 1.38 (s, 9H, NC(CH ) ), 0.99 (t, J = 7.3 Hz, 3H, CH CH ),
2
3
3 3
2
3
1
3
C-NMR: (67.5 MHz, CDCl3)
δ
159.76 (C-2), 73.21 (C-5), 52.92 (NC(CH ) ), 48.06 (4-C), 27.45 (CH CH ),
3 3
2
3
2
7.11 (NC(CH ) ), 8.55 (CH CH ).
3
3
2
3
◦
-(tert-Butyl)-5-(chloromethyl)oxazolidin-2-one (4b). Colorless liquid. bp: 80 C/2 mmHg. IR (neat):
3
1
−
743.33 cm . HRMS: (EI+, 70 eV) Calculated (C H ClNO ) 191.0713 (M ) Found: 191.0714. H-NMR:
8 14 2
1
+
1
(
2
270 MHz, CDCl3) δ 4.64–4.55 (m, 1H, 5-H), 3.74 (dd, J = 8.8, 8.8 Hz, 1H, 4HH), 3.65 (dd, J = 5.9, 6.4 Hz,
13
H, CH Cl), 3.52 (dd, J = 5.9, 8.8 Hz, 1H, 4HH), 1.40 (s, 9H, NC(CH ) ). C-NMR: (67.5 MHz, CDCl3)
2
3
3
δ 155.62 (C-2), 70.09 (C-5), 53.52 (NC(CH ) ), 46.00 (CH Cl), 44.70 (C-4), 27.16 (NC(CH ) ).
3
3
2
3 3

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◦
-(tert-Butyl)-5-(phenoxymethyl)oxazolidin-2-one (4c). Colorless liquid. bp: 140 C/2 mmHg. IR (neat):
3
1
−
727.91 cm . HRMS: (EI+, 70 eV) Calculated (C H NO ) 249.1365 (M ) Found: 249.1371. H-NMR:
9 17 2
1
+
1
(
8
270 MHz, CDCl3) δ 7.10 (m, 5H, Ph), 4.72 (m, 1H, 5-H), 4.10 (d, J = 2.9 Hz, 2H, OCH ), 3.76 (dd, J = 8.8,
2
13
.8 Hz, 1H, 4HH), 3.59 (dd, J = 5.9, 8.8 Hz, 1H, 4HH), 1.40 (s, 9H, NC(CH ) ). C-NMR: (67.5 MHz,
3 3
CDCl3)
δ 158.09 (C-2), 156.24, 129.53, 121.45, 114.14, 69.57 (OCH ), 67.97 (C-5), 53.45 (NC(CH ) ), 45.55
2 3 3
(
4-C), 27.36 (NC(CH ) ).
3 3
◦
-(tert-Butyl)-5-(methoxymethyl)oxazolidin-2-one (4d). Colorless liquid. bp: 75 C/0.07 mmHg. IR (neat):
3
1
−
743.33 cm . HRMS: (EI+, 70 eV) Calculated (C H NO ) 187.1208 (M ) Found: 187.1212. H-NMR:
9 17 3
1
+
1
(
270 MHz, CDCl3) δ 4.51 (m, 1H, 5-H), 3.64 (dd, J = 8.3, 8.8 Hz, 1H, 4HH), 3.52 (dd, J = 4.9 Hz, 2H,
13
CH O), 3.43 (dd, J = 6.4, 8.3 Hz, 1H, 4HH), 3.40 (s, 3H, OCH ), 1.38 (s, 9H, NC(CH ) ). C-NMR: (67.5
2
3
3 3
MHz, CDCl3) δ 156.30 (C-2), 72.66 (OCH ), 70.38 (C-5), 59.92 (OCH ), 53.13 (NC(CH ) ), 45.14 (4-C),
2 3 3 3
2
7.14 (NC(CH ) ).
3 3
◦
−1
.
3
-Butyl-5-ethyloxazolidin-2-one (4e). Colorless liquid. bp: 85 C/2 mmHg. IR (neat): 1751.05 cm
+
1
HRMS: (EI+, 70 eV) Calculated (C H NO ) 171.1259 (M ) Found: 171.1257. H-NMR: (270 MHz,
9
17
2
CDCl3)
δ 4.72 (m, 1H, 5-H), 3.59 (dd, J = 8.3, 8.3 Hz, 1H, 4HH), 3.25 (m, 2H, NCH ), 3.14 (dd, J =
2
6
.8, 8.3 Hz, 1H, 4HH), 1.83–1.64 (m, 2H, NCH CH ), 1.53 (dt, J = 7.3, 7.3 Hz, 2H, CH CH ), 1.34
2
2
2
3
(
tq, J = 7.3, 7.3 Hz, 2H, NCH CH CH ), 1.00 (t, J = 7.3 Hz, 3H, CH CH ), 0.94 (t, J = 7.3 Hz, 3H,
2 2 2 2 3
13
NCH CH CH CH ). C-NMR: (67.5 MHz, CDCl3)
δ 157.81 (C-2), 74.18 (C-5), 49.06 (C-4), 43.39
2
2
2
3
(
(
NCH ), 29.02 (NCH CH ), 27.70 (CH CH ), 19.46 (NCH CH CH ), 13.34 (NCH CH CH CH ), 8.39
2
2
2
2
3
2
2
2
2
2
2
3
CH CH ).
2
3
1
5
-Ethyl-3-phenyloxazolidin-2-one (4f). Colorless liquid. The NMR data of H and 13C agreed with the
1
previous report [77]. H-NMR: (270 MHz, CDCl )
δ 7.56–7.10 (m, 5H, Ph), 4.59 (ddt, J = 6.4, 7.3, 8.3 Hz,
3
1
7
H, 5-H), 4.08 (dd, J = 8.3, 8.8 Hz, 1H, 4HH), 3.66 (dd, J = 7.3, 8.8 Hz, 1H, 4HH), 1.96–1.71 (dq, J = 6.4,
.3 Hz, 2H, CH CH ), 1.07 (t, J = 7.3 Hz, 3H, CH CH ). C-NMR: (67.5 MHz, CDCl3) δ 154.88 (C-2),
2 3 2 3
13
1
38.29, 128.91, 123.79, 118.06, 74.05 (C-5), 49.94 (C-4), 27.88 (CH CH ), 8.62 (CH CH ).
2 3 2 3
1
5
-Ethyl-N,3-diphenyloxazolidin-2-imine (
5
). Colorless liquid. The NMR data of H and 13C agreed with
1
the previous report [56]. H-NMR: (270 MHz, CDCl )
5
J = 6.8 Hz, 3H, CH CH ). C-NMR: (67.5 MHz, CDCl3)
δ
8.23–6.60 (m, 10H, Ph 2), 4.70–4.15 (m, 1H,
-H), 3.92 (t, J = 8.1 Hz, 4-HH), 3.52 (dd, J = 6.8, 8.1 Hz, 1H, 4HH), 1.95–1.35 (m, 2H, CH CH ), 0.97 (t,
×
3
2 3
13
δ
148.79, 148.05, 140.44, 139.34, 129.03, 124.03,
2
3
1
23.30, 122.82, 119.34, 76.16 (C-5), 50.74 (C-4), 27.44 (CH CH ), 8.78 (CH CH ).
2
3
2
3
4
.4. Observation of Tin-Indium System by 119Sn NMR
To a two-neck 10-mL reaction vessel, InCl (1.0 mmol) and Bu SnI (1.0 mmol) were added in
3
2
2
MeCN-d (1 mL) and stirred at room temperature for several minutes. After transferring of the reaction
3
119
mixture into an NMR test tube and addition of tetramethyl stannane as an internal standard, Sn
NMR was recorded.
5
. Conclusions
A novel type of indium-tin species Bu SnI [InCl I] was revealed to be effective for the catalytic
δ+
δ−
3
2
annulation of epoxides. In the reaction with carbon dioxide, the fixation of CO proceeded well even
2
at room temperature. In the reaction with isocyanates, novel types of 2-oxazolidiones were obtained in
good yields.

Molecules 2018, 23, 782
8 of 11
Acknowledgments: We are grateful for financial support from The Naito Foundation. We also thank the
Instrumental Analysis Center, Faculty of Engineering, Osaka University, for assistance with collecting the
spectral data.
Author Contributions: Ikuya Shibata and Akio Baba conceived and designed the experiments; Akira Imakuni
and Itaru Suzuki performed the experiments; Itaru Suzuki analyzed the data; Akira Imakuni contributed
reagents/materials/analysis tools; Ikuya Shibata wrote the paper.
Conflicts of Interest: The authors declare no conflicts of interest.
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