Potassium phosphate-catalyzed one-pot synthesis of 3-aryl-2-oxazolidinones from epoxides, amines, and atmospheric carbon dioxide

Article abstract of DOI:10.1039/c6gc02934e

Potassium phosphate was found to be a highly active catalyst in the three-component cycloaddition of amines, epoxides, and carbon dioxide in DMF at ambient temperature to form 3-aryl-2-oxazolidinones. Atmospheric CO₂ and a broad range of amines were employed in this transformation. Aryl isocyanate and 1,2-aminoalcohol were generated in situ as key intermediates. This one-pot reaction is applicable to a variety of terminal epoxides and amines. The key advantages are high yields, simple work-up, inexpensive catalysts, and a practical ten gram-scale synthesis.

Full text of DOI:10.1039/c6gc02934e

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ARTICLE
Potassium Phosphate-Catalyzed One-pot Synthesis of 3-Aryl-2-
oxazolidinones from Epoxides, Amines, and an Atmospheric
Carbon Dioxide
Received 00th January 20xx,
Accepted 00th January 20xx
Ue Ryung Seo and Young Keun Chung*
DOI: 10.1039/x0xx00000x
www.rsc.org/
Potassium phosphate was found to be a highly active catalyst in the three-component cycloaddition of amine, epoxide,
and carbon dioxide in DMF at ambient temperature to form 3-aryl-2-oxazolidinones. Atmospheric CO2 and a broad range
of amines were employed in this transformation. Aryl isocyanate and 1,2-aminoalcohol were generated in situ as key
intermediates. This one-pot reaction is applicable to a variety of terminal epoxides and amines. The key advantages are
high yields, simple work-up, inexpensive catalyst, and a practical ten gram-scale synthesis.
the presence of some precious metal catalysts. Recently Gao
et al.7 reported a three-component reaction using binary ionic
liquids as catalysts under 25 atm of CO2 (Scheme 1, c). They
Introduction
Atmospheric levels of CO2 have been rapidly increased since
the industrial revolution. Carbon dioxide is now considered a
primary cause of global warming. Therefore, efficient methods
that convert carbon dioxide into valuable chemicals have
attracted much attention. Carbon dioxide is consumed as a
reactant in the production of urea, with a smaller fraction used
to produce methanol. Recently, intensive research on CO2 has
been carried out.1
proposed amino alcohols and cyclic carbonates as reaction
intermediates. Some reported the synthesis of oxazolidinones
from propargylamines under atmospheric carbon dioxide
(Scheme 1, d).8 Therefore, developing (or identifying) a new
route to oxazolidinone structures using readily available
starting materials in the absence of toxic and/or precious
metal catalysts under mild reaction conditions is a synthetic
challenge (Scheme 1, e).
Oxazolidinones are five-membered heterocyclic compounds
that contain nitrogen and oxygen. Owing to their utility in
organic synthesis and pharmaceutical, and agricultural
industries, oxazolidinone preparation has been well studied.2
The conventional synthetic methods use phosgene or
isocyanate as carbonyl precursors (Scheme 1, a and b).3
However, the toxicity of these starting materials is not
compatible with green chemistry. Alternative synthetic
methods that use CO2 or cyclic carbonates as carbonyl
precursors in the presence of metal salts have also been
reported.4,5 However, most of the known reactions involving
the use of carbon dioxide as a CO surrogate are conducted
under high pressure of carbon dioxide and/or in the presence
of some precious metal catalysts.6 Among the reported
synthetic methods, the three-component cycloaddition of CO2,
epoxide, and primary amines affording 2-oxazolidinones,7 are
the most interesting and promising synthetic method with
regard to developing CO2 as carbon source. Most of the known
reactions involving the use of carbon dioxide as a CO surrogate
are conducted under high pressure of carbon dioxide and/or in
Previous work
O
R
NH
OH
R
N
(a) [3a]
COCl2
O
+
+
O
O
O
cat.
R2NCO
R2
R2
(b) [3h]
Bu4NI, 80 o
C
N
O
N
O
+
+
R1
cat. = rare earth metal complex
R1
R1
O
O
binary ionic liquids
CO2
O
+
+
ArNH2
Ar
HO
OH
140 o
C
(c) [7a]
N
O
N
(2.5 MPa)
NHR4
R3
R2
[DBUH][HMIm]
60 o
R4
R3
R2
R1
CO2
+
O
(d) [8e]
C
( 0.1 MPa)
R1
This work
O
O
O
K3PO4
130 o
R2
R2
N
R2NH2
+
CO2
+
N
O
O
+
(e)
C
R1
( 0.1 MPa)
R1
R1
- One pot, metal free and mild conditions
a.Department of Chemistry, College of Natural Sciences, Seoul National University,
Seoul 08826, Korea. E-mail: ykchung@snu.ac.kr
K3PO4
- Cheap, readily available substrate and catalyst
- Excellent yield with high regioselectivity
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
Scheme 1. Syntheses of Oxazolidiones
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Recently, K3PO4 has been widely used as a catalyst, additive, giving 3,5-disubstituted oxazolidinone (
and base in organic reactions.9-11 It has many advantages such and 3,4−isomer (
as being non-toxic, inexpensive, soluble in organic solvents,
and is readily available. Furthermore, there is no potential aniline was conducted in the presence of (salcen)CrIII and
A
) as the major product
B
) as the minor product. For example, the
A:
B
ratio was 1.75:1 when the reaction of phenyl isocyanate with
residual metal contamination in products. Among many useful PPh3O.14 Moreover,
A and B were formed in ratios of 1:1.9-2.5
K3PO4-catalyzed reactions, the carboxylation of amines12 in the presence of bimetallic aluminium salen or (salen)VV/
attracted our attention. However, the reaction conditions tetrabutylammonium bromide catalysts.15 Thus, a relatively
◦
(NMP as solvent, 170 C, 5 MPa CO2) were too harsh to be high regioselectivity was observed in the presence of K3PO4.
applicable to standard laboratory facilities. This exciting result Bases K2CO3, DBU, and KOH similarly exhibited high activities
prompted us to investigate the use of readily available and and high regioselectivities (3.2:1 - 4.9:1) (entries 1, 5, and 6).
mildly basic K3PO4 in different carboxylation reactions. Herein Cs2CO3 also showed good reactivity (74%) with a high A: B ratio
we wish to report our recent results using K3PO4 as a catalyst (3.1:1). However, surprisingly poor activity was observed when
in the synthesis of oxazolidinones from epoxides, amines, and using Na3PO4 or K2HPO4 (entries 8-9). The necessity of base
atmospheric carbon dioxide. Aryl isocyanate and 1,2- catalysts in the reaction was confirmed when no reaction was
aminoalcohol were used as key intermediates. Thus, an eco- observed in their absence (entry 10). The effect of various
friendly catalytic system without transition metal catalyst and solvents on the reaction system was also examined (entries 7
toxic reagents has been developed in this study. As far as we and 11-13). DMSO and DMA were quite efficient (entries 11
are aware, this is the first base-metal catalyzed incorporation and 13), while toluene was proven to be inefficient for this
of atmospheric CO2 to oxazolidinones.
transformation (entry 12). When the reaction temperature
was lowered, no improvement in yield was observed (entry
14). DMF gave the best result and was used in further studies.
Decreasing the amount of styrene oxide or K3PO4 led to a
decrease in reaction yield (entries 15 and 16). Reducing the
Results and discussion
The K3PO4-catalyzed synthesis of 3-aryl-2-oxazolidinones was
discovered in a study on the use of polymer catalysts for the
synthesis of oxazolidinones (eq 1). Aniline and styrene oxide
were chosen as starting materials for reaction condition
optimization. Initially, the reaction was carried out under the
reaction conditions (1 mol% polymer catalyst, 1 mol% ZnBr2 as
the Lewis acid, 2mol% DBU as the base, DMF as the solvent, at
reaction time to 16 h resulted in a lower yield of 86% (A: B =
71%: 15%) (entry 17). Therefore, the optimum reaction
conditions were established as follows: 20 mol% K3PO4, 1
mmol amine, 5 mmol aryl epoxide, 2.0 mL DMF, 1 atm CO2
(balloon), 130°C, and 19 h.
Table 1. Screening the reaction conditions a
o
80 C for 10 h) adopted from a previous work on the poly(4-
vinylimidazolium)s/DBU-ZnBr2-catalyzed
carbon dioxide to epoxide. 13
cycloaddition
of
Entry
Solvent
Base (mol%)
Isolated yield (%)
3aa-A
61
6
3aa-B
19
2
Total
80
8
1
2
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMSO
Toluene
DMA
DMF
DMF
DMF
DMF
K2CO3 (20)
Na2CO3 (20)
Cs2CO3 (20)
KHCO3
While optimizing the conditions, we found that the reaction
occurred in the presence of potassium phosphate. Therefore,
we attempted to optimize the reaction conditions in the
presence of a base catalyst. First, conditions such as a reaction
temperature, solvent, and base catalyst, were optimized to
obtain the maximum oxazolidinone yields (Table 1).
3
57
13
69
74
76
3
17
6
74
19
84
89
98
6
4
5
DBU (20)
KOH(20)
15
15
22
3
6
7
K3PO4 (20)
Na3PO4 (20)
K2HPO4 (20)
-
8
Initially, the following reaction conditions were used: 20 mol%
o
base catalyst in DMF at 130 C for 19 h. To improve the yield,
9
2
3
5
10
11
12
13
14b
15c
16
17d
-
-
-
K3PO4 (20)
K3PO4 (20)
K3PO4 (20)
K3PO420)
K3PO4 (20)
K3PO4 (15)
K3PO4 (15)
68
-
18
-
86
-
different bases, including K3PO4, K2CO3, Na2CO3, Cs2CO3,
KHCO3, DBU, KOH, Na3PO4, and K2HPO4 were used (entries 1-9).
The reaction yield was highly sensitive to the base. However,
catalyst basicity was not the sole factor in determining its
activity. Among bases, the best yield was observed for K3PO4
(entry 7). In the presence of K3PO4, 3,5-diphenyl-1,3-
71
61
69
75
71
16
14
14
16
15
87
75
83
91
86
oxazolidin-2-one
one (3aa-B) were isolated in 76% and 22% yield, respectively.
Thus, the overall yield was 98% with an ratio of 3.5: 1. In
(3aa-A) and 3,4-diphenyl-1,3-oxazolidin-2-
a
Reaction conditions: Base in with 2 mL of solvent, 5 mmol of styrene oxide, 1
b
mmol ofaniline at specified temperature under atmospheric carbon dioxide. At
120 oC. c The ratio of 1a:2a = 4:1. d For 16 h
A: B
reactions of a monosubstituted epoxide, such as styrene oxide
with amines, there is regioselectivitiy problem. In general,
epoxide ring-opening occurs at the less hindered carbon atom,
With the optimized reaction conditions established, we
explored the scope of this transformation (Table 2). The nature
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of the aryl and aliphatic epoxides were explored first. Aryl amines bearing electron-donating (3ah
epoxides (1b 1d) bearing a para-halo group (F, Cl, and Br) were and electron-withdrawing substituents
excellent substrates, giving near-quantitative yields with high exception of para-nitroaniline (78%) were all tolerated in the
ratios (4.4:1 − 6.6:1). Aryl epoxides (1e 1g) with meta- or reaction. Steric hindrance was to have a dramatic effect in the
ortho- halo group (Br and Cl) were still excellent substrates (96 cases of 2-bromoaniline 2f), 3,5-dimethylaniline 2k),
− 99%) and gave high ratio (5.4:1 − 11.4:1). When an naphthalen-1-amine (2m). When 2-bromoaniline (2f) was used
electron donating group, such as methyl or tert-butyl, was as an amine source under the optimized reaction conditions,
introduced to the aryl group (1h and 1i), the yield decreased. oxazolidinone derivative 3af was isolated in 38% with an
In particular, when a tert-butyl group was introduced at the ratio of 3.2:1. However, lengthening the reaction time from 19
para- position (1i), the yield decreased to 72% with an to 27 h gave an improved yield of 86%, with an ratio of
ratio of 2.8: 1. Thus, the yield was highly dependent on the 3.8:1. When 3,5-dimethylaniline (2k) or naphthalen-1-amine
electronic effects of the aryl epoxide. The reaction was not 2m) were used as amines, the corresponding oxazolidinones
-
3aj, and 3al, 93-98%)
-
(3ab-3ae, 92-95%)
A
:
B
-
(
(
A: B
A: B
A
:
B
A: B
(
restricted to aryl epoxides. However, high yields of were isolated in 80% (3ak, 4.0:1) and 85% (3am, 3.6:1) yields,
oxazolidinones were not observed for aliphatic epoxides, respectively. Aliphatic amines, such as benzyl amine (2n) and
presumably due to the low reactivity in the reaction with octyl amine (2o), were good substrates, resulting in the
carbon dioxide to form cyclic carbonates (see SI). Aliphatic corresponding oxazolidinones in 92% (3an) and 85% (3ao
epoxides with a long chain alkyl group (1j 1k) afforded a yields, respectively. Interestingly, excellent regioselectivity was
single regioisomer in 45% and 38% yields, respectively. The observed for p-nitroaniline (2g), benzyl amine (2n), and octyl
)
–
regioselectivity might be due to a steric reason.
amine (2o), with only the A isomer produced.
Table 2. Cycloaddition of CO2 with terminal epoxides and aniline a
Table 3. Cycloaddition of CO2 with various amines and styrene oxide a
a
Reaction conditions: 43 mg of K3PO4 in 1 mL of DMF, 5 mmol of styrene oxide
derivatives, 1 mmol of aniline in 1 mL of DMF (total 2 mL of DMF) at 130 oC under
atmospheric carbon dioxide. The data in parentheses are the yields of A and B,
respectively.
a Reaction conditions: 43 mg of K3PO4 in 1 mL of DMF, 5 mmol of styrene oxide, 1
mmol of amines with additional 1 mL of DMF(total 3 mL of EMF) at 130 oC under
atmospheric carbon dioxide. The data in parentheses are the yields of A and B,
respectively. b For 27h
The reaction scope for aniline substrates was explored next
(Table 3). Various amines bearing either electron-donating
(Me, OMe, Bu, OPh) or electron-withdrawing (Cl, Br, I, NO2)
meta- or para-substituents were also tested. In general,
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In addition, the reaction could be conducted on a 10-g-scale
Journal Name
(10.0 g (108 mmol) of aniline, 93% yield, with an
(eq 2).
A: B = 77: 16)
To determine the reactivity of 1,2-aminoalcohol under the
optimized reaction conditions, it was reacted with styrene
carbonate in the presence of K3PO4, which afforded 3,5-
diphenyl-1,3-oxazolidin-2-one quantitatively (eq 7).18
Scaling-up the procedure did not result in lower yields. Thus,
notable features of the current method include: (i) a low cost
catalyst, (ii) mild reaction conditions, (iii) simple operations,
(iv) high efficiency, and (v) scalability.
We monitored the reaction intermediate generated in the
synthesis of 3-aryl-2-oxazolidinones from styrene epoxide,
Two possible paths for the synthesis of 3-aryl-2-oxazolidinones
(3) from CO2, aniline (2a) and styrene oxide (1a) could be
proposed based on the formation of isocyanate from CO2 and
amine as well as the formation of amino alcohol from epoxide
and amine. To determine whether phenyl isocyanate was
generated under these reaction conditions, two experiments
were conducted (eqs. 3 and 4). When aniline was reacted with
carbon dioxide, no reaction was observed (eq 3), which
confirmed that no phenylcarbamic acid was generated during
the reaction. When aniline was reacted with styrene
carbonate, phenyl isocyanate, oxazolidinones16 3aa-A and 3aa-
aniline, and atmospheric carbon dioxide (Figures S1–S10).19
Initially, a rapid consumption of styrene oxide and formation
of styrene carbonate was observed. As time passed, the
concentrations of phenyl isocyanate and amino alcohol slowly
increased and then decreased. Phenyl isocyanate appeared for
a longer period of the reaction time than 1,2-aminoalcohol.
According
to
GC-Mass
analysis,
1-phenyl-2-
B, and 1-phenylethan-1,2-diol were formed in 3%, 7%, 2%, and
(phenylamino)ethan-1-ol was formed as a single regio-isomer
and its reaction with styrene carbonate would lead to the
formation of 3,5-diphenyl-1,3-oxazolidin-2-one. Base on the
above observation, we proposed phenyl isocyanate and amino
alcohol as reaction intermediates under our reaction
conditions. Gao et al.7 proposed 1,2-amino alcohol as an
intermediate, which reacted with another carbonate to
produce oxazolidinone and diol. However, we envisaged two
pathways for our reaction: a pathway based on the formation
13% yields, respectively, after 30 min (eq 4). As expected, no
reaction was observed without K3PO4, and neither 1,2-
aminoalcohol nor hydroxyurethane17 was detected after 30
min. However, with a reaction time of 6 h, the formation of
1,2-aminoalcohol was observed (vide infra).
of phenyl isocyanate and
a pathway based on 1,2-
aminoalcohol. Based on our observations, we predicted that
K3PO4 might be a suitable catalyst for our reactions, because
3-
both K+ and PO4 ions have an effect on some key-step
reactions. Potassium is oxophilic, the central K+ ion would form
a strong coordinate bond with oxygen atoms in epoxide and
3-
cyclic carbonate, and the PO4 counteranion is sufficiently
basic.20 A plausible reaction pathway was proposed (Scheme 2;
for detailed discussion of the reaction mechanism, see
Supporting Information).4h,21
The following reactions, in eqs 5 and 6, were also studied.
When phenyl isocyanate was reacted with styrene oxide in the
presence or absence of carbon dioxide, the expected
oxazolidinone was isolated in 97% (A: B = 4.4:1). The result
obtained in the presence of carbon dioxide was almost
identical to that obtained for the reaction of aniline and
styrene oxide with carbon dioxide.
We studied whether the reaction of aniline with styrene oxide
would generate 1,2-aminoalcohol in the presence of K3PO4
under a nitrogen atmosphere. Only 11 % of 1,2-aminoalcohol
was formed and 80 % of aniline remained intact (eq 6).
Scheme 2. Plausible reaction mechanism
4 | J. Name., 2012, 00, 1-3
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Soc. Rev. 2009, 38, 89–99; d) W. Wang, S. Wang, X. Ma, J.
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Braunstein, D. Matt, D. Nobel, Chem. Rev. 1988, 88, 747–
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Conclusions
A potassium phosphate-catalyzed effective, scalable synthesis
of oxazolidinones from amines, aryl epoxides, and carbon
dioxide under mild conditions has been developed. These
transformations employ CO2 at atmospheric pressure and
provide a streamlined access to useful heterocycle 3-aryl-2-
oxazolidinones. The developed catalytic system is eco-friendly,
requiring no transition metal catalyst or toxic reagents, such as
phosgene or isocyanate. Further applications of this
methodology are currently being explored and will be reported
in due course.
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Experimental section
General procedure for 2-oxazolidiones
Reactions were performed in a tube schlenk equipped with a
stirring bar and capped with a rubber cap and the followings
were placed in the tube in order: 20 mol% of K3PO4 (43 mg, 0.2
mmol), 5 equiv. of styrene oxide (0.6 g, 5 mmol), 1 equiv. of
aniline (93 μL, 1 mmol), and 2mL of DMF. While they were
mixing together, the tube was charged with CO2 by a balloon
for 15 seconds. The mixture was stirred at 130 oC for 19 h
under CO2 (using a balloon). The color of the reaction mixture
changed from light yellow to dark brown. After the reaction,
the mixture was concentrated under reduced pressures.
Purification by flash chromatography on silica gel with n-
hexane and ethyl acetate afforded oxazolidiones. The products
2
3
were characterized by H NMR, 13C NMR, IR, and HRMS, and
1
their melting points were measured.
10 Gram-Scale Experiment
K3PO4 (20 mol%, 4.6 g), styrene oxide (540 mmol, 62 mL),
aniline (108 mmol, 10.05 g), and DMF (100 mL) were place in a
flame-dried two-necked 500 mL schlenk flask. Before the
reaction flask was put to an oil bath, it was purged by CO2 for
30 seconds at room temperature. The reaction mixture was
o
then heated at 130 C for 19 h. To provide CO2 smoothly and
continually, the CO2 balloon was recharged in every 3 h with a
18G needle. After the reaction went completion, the solvent
was removed under reduced pressures. The crude product was
purified by flash chromatography with hexane and ethyl
acetate to afford products (total 93 % isolated yield).
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Acknowledgements
This work was supported by a National Research Foundation of
Korea (NRF) grant funded by the Korean government
(2014R1A5A1011165 and 2007-0093864). URS is a recipient of
BK21 plus Fellowship.
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6 | J. Name., 2012, 00, 1-3
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