A novel one-pot synthesis of oxazolidinones through direct introduction of CO2 into allylamine derivatives

Article abstract of DOI:10.1016/j.tetlet.2014.01.029

1,3-Oxazolindin-2-one derivatives were obtained for the first time through carboxylative cyclization of allylamines in the absence of any metal or base catalyst. An electron-withdrawing substituent on the allylic double bond is crucial for the reaction success. Allylamines react with CO₂ in MeCN/MeOH mixture and in scCO₂ giving satisfactory results.

Full text of DOI:10.1016/j.tetlet.2014.01.029

Tetrahedron Letters 55 (2014) 1379–1383
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
A novel one-pot synthesis of oxazolidinones through direct
introduction of CO2 into allylamine derivatives
⇑
Laura Soldi, Chiara Massera, Mirco Costa, Nicola Della Ca’
Dipartimento di Chimica and CIRCC, Parco Area delle Scienze 17/A, I-43124 Parma, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
1,3-Oxazolindin-2-one derivatives were obtained for the first time through carboxylative cyclization of
allylamines in the absence of any metal or base catalyst. An electron-withdrawing substituent on the
allylic double bond is crucial for the reaction success. Allylamines react with CO2 in MeCN/MeOH mixture
and in scCO2 giving satisfactory results.
Received 21 November 2013
Revised 20 December 2013
Accepted 8 January 2014
Available online 15 January 2014
Ó 2014 Elsevier Ltd. All rights reserved.
Keywords:
Oxazolidinone
Allylamine
Carbon dioxide fixation
One-pot synthesis
Supercritical fluids
The efficient reconversion of carbon dioxide into organic
molecules is currently one of the most important targets in syn-
thetic organic chemistry. Since CO2 is an ubiquitously available
raw material, its utilization in the production of fine-chemicals,
fuels, and materials is of considerable interest.1 Environmentally
impacting processes, such as those concerning the synthesis of
organic carbonates, methanol, or some pharmaceutical specialities,
could be replaced by more friendly methodologies based on the
use of CO2.
The widespread use of oxazolidin-2-ones as chiral auxiliaries
and antimicrobials since their first report in 19812 induced many
researchers to look for new ways for their preparation avoiding
the traditional and more direct ones between amino alcohols and
either phosgene or diethyl carbonate. Over the years attractive
synthetic methodologies based on the use of carbon oxides have
been developed. Thus, oxazolidin-2-ones were obtained in
excellent yields with high catalytic efficiencies by direct PdI2/KI-
catalyzed oxidative carbonylation of the readily available
2-amino-1-alkynols.3 CO2 has been also employed as source for
oxazolidinone formation. In fact the direct addition of CO2 to
b-amino alcohols at high pressure and temperature to give these
target compounds has been reported for the first time in 1959.4
More recently, many researchers described the efficient conversion
of aziridines into oxazolidinones by CO2 incorporation5 even in the
absence of catalysts and/or under supercritical conditions.6 More-
over, 5-methylene-1,3-oxazolidin-2-ones can be obtained by either
reacting alkynes, primary amines, and CO2 in the presence of
copper(I) catalyst under solvent-free conditions7 or starting from
N-substituted propargylamines and CO2 using ruthenium, copper,
or silver complexes as catalysts.8 Some years ago we reported
the direct introduction of CO2 into secondary acetylenic amines,
via intramolecular cyclization to 5-methylene-1,3-oxazolidin-2-
ones in the absence of metals.9 The reaction is based on the forma-
tion of carbamate salts in the presence of catalytic amounts of
strong organic bases such as pentaalkylguanidines. Subsequently,
secondary propargyl amines were found to react smoothly with
CO2 under supercritical conditions to give 5-methyleneoxazolidi-
nones even in the absence of any metal or base catalyst.10
On the ground of the reported behavior of the N-substituted
propargyl amines with CO2,9,10 we verified the feasibility of incor-
porating CO2 into allylamines in order to obtain oxazolidin-2-ones.
The addition of carbamic acid derivatives to C@C double bonds has
been scarcely investigated. We mention for example the stoichi-
ometric transformation of ammonium carbamates and cyclic diole-
fins bound to a Pd center,11 and a three-component reaction
involving CO2, secondary amines, and vinyl ethers.12 In general
catalysts, such as palladium or guanidine bases, or iodo derivatives,
such as iodine or tBuOI, are mandatory to enable the reaction
between allylamines and CO2.13
We now report a convenient one-pot synthesis of oxazolidin-2-
ones through the direct incorporation of carbon dioxide into allyl-
amine derivatives in the absence of any base or catalyst.
In our initial study allylamines were allowed to react in an auto-
clave under 40 bar of CO2 (measured at room temperature) at 90 °C
for 24 h in MeCN, MeOH, and their mixtures. While allylamines
⇑
Corresponding author. Tel.: +39 0521 905676; fax: +39 0521 905472.
E-mail address: nicola.dellaca@unipr.it (N. Della Ca’).
0040-4039/$ - see front matter Ó 2014 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2014.01.029

1380
L. Soldi et al. / Tetrahedron Letters 55 (2014) 1379–1383
bearing hydrogen, alkyl, or aryl groups on the terminal carbon of
the double bond (R1 = H, alkyl, aryl) failed to react, on the contrary,
very good results were obtained for the first time with an alkoxy-
carbonyl moiety as an activating substituent (EWG). At first the
reaction of substrate 1a with CO2 was considered in order to find
out the appropriate conditions (Table 1). Only moderate yield of
2a was obtained in pure MeCN while even worse in pure MeOH.
Mixtures with different MeCN/MeOH vol/vol ratio were also tested
and, in particular, those with MeCN/MeOH in 1:1 (vol/vol) ratio led
to an excellent result (Table 1, entry 3). In addition, CO2 pressure
affects positively the yield of 2a until 40–50 bar, when a plateau
is reached (Table 1, entries 3 and 6–8).
The aprotic polar solvent MeCN, able to dissolve the carbamate
salt formed from secondary allylamines, carbon dioxide and the
same protonated amine, promotes its dissociation by stabilization
of the ammonium cation and simultaneously frees up its counter-
part (the carbamate anion) promoting the intramolecular nucleo-
philic attack of the double bond. On the other hand, the protic
polar solvent is able to dissolve a larger amount of CO2. The mix-
ture causes a beneficial effect on the process.
in para position, led to better yields of the corresponding oxazolid-
inone derivatives 2h and 2i (Table 2, entries 11 and 12).
The formation of carbamate anions through CO2 addition to a
secondary amine, taking place under widely different conditions
of temperature and CO2 pressure, is the first step of the process.
Its concentration and stability are determined by the nature of
both the counterion and the solvent.15 The tendency of the oxygen
of the carbamate anion to attack the C@C unsaturation leading to
an intramolecular cyclization, is favored by the electrophilic char-
acter of the double bond. Moreover, the substrate reactivity is
influenced by R2 substituents, in fact a more electron withdrawing
group (R2 = MeCHCO2Me, Table 2, entry 4) is less effective than
other alkyl substituents, probably because of the reduced nucleo-
philicity of the nitrogen atom.
At this point we considered the possibility to eliminate the or-
ganic solvent mixture, carrying out the reactions in supercritical
CO2 (scCO2) due to its tunable intrinsic properties such as polarity,
density, and environmental advantages.16 Its behavior can affect
the solubility of organic compounds and, as a consequence, the
reaction course. The dense CO2 medium can facilitate the forma-
tion of carbamic acid intermediates, leading to a marked improve-
ment of the cyclic urethane formation. The increased activity in
scCO2 is not unprecedented. Recently, efficient syntheses of ure-
thanes have been achieved by using scCO2 as reactant and reaction
medium.17 However, the CO2 chemical fixation under scCO2 condi-
tions often needs longer reaction times extending to several hours
the achievement of satisfactory results.16 Preliminary experiments,
carried out with scCO2 as both reagent and reaction medium,
showed that the best yields were obtained at 80–90 bar of CO2
pressure. The reaction of allylamines 1a–i (2.0 mmol) and 12 g of
liquid CO2 was performed into a stainless steel autoclave under
stirring (45 mL) at 110 °C for 48–72 h. The results collected in the
Table 3 show that in scCO2 substrates 1a, 1b, and 1d (Table 3, en-
tries 1, 2, 4 and 5) gave yields slightly better with respect to the
ones obtained in MeCN/MeOH mixtures, while with substrates
1c, 1f–i (Table 3, entries 3 and 7–10) a noticeable increase of the
yields was achieved. Improvement of the reaction performance
may lie in increasing the density of scCO2 and the solvation effect,
eventually leading to a liquid phase able to dissolve the reactants
and ammonium carbamate salts derived from CO2, where the neat
reaction might proceed. As previously reported, no product was
obtained with substrate 1e (Table 3, entry 6).
Allylamines with R1 and R2 of different nature were caused to
react under the previously optimized reaction conditions.14 The re-
sults, reported in Table 2, show that an EWG on the double bond is
crucial for the success of the reaction. Substrates bearing the CO2-
Me moiety on the double bond, such as 1a, 1b, and 1d, gave excel-
lent yields of the corresponding products 2a, 2b, and 2d (Table 2,
entries 1, 2, 5 and 6). A confirmation of the structure of compound
2b (5-(carbomethoxymethyl)-3-cyclohexyloxazolidin-2-one) was
obtained by the X-ray diffraction analysis on a single crystal
(Fig. 1).
The substrate 1c, in spite of the presence of the CO2Me group,
showed a lower reactivity (Table 2, entry 3) even at a higher tem-
perature (Table 2, entry 4) probably owing to the sterically encum-
bered NR2 moiety combined with its lower basicity. On the other
hand, it was proved that substrate 1e did not react under the re-
ported conditions, recovering the unconverted reagent (entry 7).
Only a strong EWG on the double bond was able to promote the
cyclization step and to this end an aryl bearing a nitro group in
ortho or in para position was employed as R1 substituent. The sub-
strate 1f (R1 = 2-nitrophenyl, R2 = tBu) was caused to react under
the same conditions leading to product 2f in low yield (entry 8).
Substrate 1g showed an analogous behavior (entry 9), even at
higher temperature (110 °C, entry 10). The nitro group in ortho po-
sition could interfere with the amine moiety preventing the carba-
mate formation and consequently the cyclization. Carboxylation
reactions carried out with 1h and 1i, bearing a nitro substituent
Substrates 1d, 1g, and 1i, containing a chiral ( ) center in R2
group, reacted with CO2 leading to two diastereoisomers of oxazo-
lidinone derivatives in 1:1 molar ratio (Table 2, entries 3, 5, 9, 10
and 12). The evidence for a diastereoselective control in the
Table 1
Optimization of reaction conditions for the synthesis of 2aa
tBu
Solvent
90 °C
N
MeO 2
C
NHtBu
+
CO2
O
O
MeO2C
2a
1a
Entry
MeCN (ml)
MeOH (ml)
CO2 (bar)
Conversionb of 1a (%)
Yieldb of 2a (%)
1
2
3
4
5
6
7
8
4
3
2
1
—
2
2
2
—
1
2
3
4
2
2
2
40
40
40
40
40
20
30
50
90
92
96
90
87
82
94
96
42
61
92
23c
6c
74
87
91
a
b
c
Reaction and conditions: 1a (2.0 mmol), MeOH/MeCN (4.0 mL, 1/1 vol/vol), CO2 (pressure measured at room temperature), 90 °C, 24 h.
Determined by 1H NMR.
Residual unknown oligomeric material was present in the crude reaction mixture.

L. Soldi et al. / Tetrahedron Letters 55 (2014) 1379–1383
1381
Table 2
Synthesis of N-alkyl-1,3-oxazolidin-2-one derivatives 2 from allylamines 1 and CO2
a
R2
MeOH/MeCN
1:1
N
R1
NHR2
+
CO2
O
90 °C, 24h
Conv.b of 1 (%)
96
R1
O
1a-i
2a-i
Entry
1
Substrate 1
T (°C)
Product 2
Yieldb,c of 2 (%)
tBu
1a
N
O
MeO2C
NHtBu
90
90
2a 92(86)c
2b 95(88)c
O
O
MeO2C
MeO2C
Cy
1b
N
MeO2C
NHCy
2
98
O
H
(L)-1c
Me
Ph
CO2Me
O
MeO2C
N
3
4
90
68
49
2c 35d
2c 44d
MeO2C
NH
Me
H
O
MeO2C
(L)-1c
110
H
1d
Me
O
MeO2C
5
90
93
2d 88e(81)c
N
O
Me
H
NH
Ph
MeO2C
6
7
(R)-1d
90
90
96
0
2d 77e(70)c
2e 0
tBu
1e
N
Ph
NHtBu
NHtBu
NH
O
Ph
O
tBu
1f
N
NO2
O
O
8
90
35
2f 18
NO2
Ph
1g
Me
NO2
N
O
O
9
90
15
18
2g 14
Me
O2N
Ph
10
1g
110
2g 15e
tBu
1h
N
O
O2N
O
11
90
60
2h 51
NH
tBu
NO2
Ph
1i
Me
O2N
N
O
O
12
Me
NH
90
60
2i 31e
Ph
NO2
a
b
c
Reaction conditions: 1 (2.0 mmol), MeOH/MeCN (4.0 mL, 1/1 vol/vol), CO2 (45–60 bar, measured at room temperature), 24 h.
Determined by 1H NMR.
Isolated yield.
A 1.5:1 mixture of two diastereoisomers.
A 1:1 mixture of two diastereoisomers.
d
e

1382
L. Soldi et al. / Tetrahedron Letters 55 (2014) 1379–1383
R2
N
R1
O
R1
2
NHR2
+
CO2
O
1
R1
NH2R2
R2
R2
Figure 1. Single-crystal X-ray diffraction analysis for 2b.
2
N
N
O
O
R1
R1
Scheme 1. Formation pathway for oxazolidinones 2.
O
R1
O
Table 3
+
Synthesis of N-alkyl-1,3-oxazolidin-2-one derivatives 2 from allylamines 1 and CO2 in
NHR2
R2H2N
R1
a
scCO2
Entry
1
Pb (bar)
Time (h)
Conv.c of 1 (%)
Yieldc of 2 (%)
1
2
3
4
5
6
7
8
9
1a
1b
(L)-1c
1d
(R)-1d
1e
1f
1g
1h
1i
90
98
86
92
92
94
90
86
86
88
48
48
72
72
72
72
72
72
72
72
100
100
80
89
94
0
93
91
100
93
2a 98(96)
2b 96(94)
2c 64(60)d
2d 75(71)e
2d 73(69)e
2e 0
2f 83(78)
2g 80(76)e
2h 99(97)
2i 83(78)e
straightforward method for the green synthesis of substituted five
membered cyclic urethanes useful as intermediates in many re-
search fields.
Acknowledgments
This work was supported by MIUR and Parma University. The
NMR instrumentation was provided by Centro Interdipartimentale
‘Giuseppe Casnati’ of Parma University.
10
a
b
c
Reaction conditions: 1 (2.0 mmol), liquid CO2 (12.0 g), 24 h, 110 °C.
Pressure at reaction temperature.
Determined by 1H NMR (isolated yield in brackets).
A 1.5:1 mixture of two diastereoisomers.
d
e
Supplementary data
A 1:1 mixture of two diastereoisomers.
Supplementary data (experimental details and the characteriza-
tion data for starting materials 1a–i and products 2a–i) associated
with this article can be found, in the online version, at http://
dx.doi.org/10.1016/j.tetlet.2014.01.029.
synthesis could be provided starting from (L)-1c and (R)-1d ob-
tained using the corresponding chiral amines, such as (L)-alanine
methyl ester and (R)-methylbenzyl amine, respectively. In both
cases two diastereoisomers were obtained and the relative quanti-
ties were determined by gas chromatographic and 1H NMR analy-
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L. Soldi et al. / Tetrahedron Letters 55 (2014) 1379–1383
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