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Chemistry Letters Vol.38, No.8 (2009)
Silver-catalyzed Preparation of Oxazolidinones
from Carbon Dioxide and Propargylic Amines
Shunsuke Yoshida, Kosuke Fukui, Satoshi Kikuchi, and Tohru YamadaÃ
Department of Chemistry, Keio University, Hiyoshi, Kohoku-ku, Yokohama 223-8522
(Received May 29, 2009; CL-090526; E-mail: yamada@chem.keio.ac.jp)
The silver-catalyzed carbon dioxide incorporation reaction
Table 1. Examination of various reaction conditions
into various propargylic amines proceeded under mild reaction
conditions to obtain the corresponding oxazolidinone derivatives
in high to excellent yields. The geometry of the C–C double bond
in product was confirmed to be the Z isomer by X-ray analysis.
O
NH2
Cat. AgOAc
DBU
O
NH
Ph
+
CO2
Toluene, rt
Ph
1a
2a
CO2 pressure AgOAc DBU
/MPa
Entrya
Time/h Yield/%b
Carbon dioxide has drawn much attention from the view-
point of global warming as well as one of the most promising al-
ternatives to phosgene.1 The oxazolidinone preparation from
carbon dioxide is also a most attractive synthetic method. Much
effort has been made to develop the chemical fixation of carbon
dioxide into propargylic amines to provide the oxazolidinone de-
rivatives. For example, it was reported that transition-metal salts,
such as copper,2a,2b ruthenium,2c and palladium,2d,2e were em-
ployed as efficient catalysts to promote the reaction of propargyl-
ic amine and carbon dioxide, however, the applicable substrates
in these catalyses were limited to terminal alkynes2a–2e or N-sub-
stituted propargylic amines.2a–2d The super bases, such as MTBD
(7-methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene),3a guanidine de-
rivatives,3b and DBU,3c and an electrochemical method4 were in-
itially attempted to accelerate this reaction, though the N-substi-
tuted propargylic amines3a–3c,4 were required to achieve high
yields of the corresponding oxazolidinone derivatives, or the ter-
minal alkynes4 were only applicable with a decreased reactivity
to afford the product in low yield.3a,3b Recently, supercritical
conditions were applied to the incorporation of carbon dioxide
into propargylic amines in the presence of basic alumina5a or
without any additives.5b Even under these conditions, the sub-
strates to afford the oxazolidinones in high yield were still lim-
ited to the N-substituted terminal alkynes5a or aryl-substituted
internal alkynes.5b In spite of various reports on the incorpora-
tion of carbon dioxide into propargylic amines, practical proce-
dures for the reaction of propargylic amines with internal
alkynes or N-unsubstituted amines have never been achieved.
Also, it should be pointed out that almost all the reported meth-
ods require severe reaction conditions, such as high temperature
and/or high carbon dioxide pressure. In a previous communica-
tion,6 the combined use of a catalytic amount of a silver salt with
DBU was reported to efficiently catalyze CO2 incorporation
under mild reaction conditions into a wide range of propargylic
alcohols to afford the corresponding cyclic carbonates in high to
excellent yields. It was found that this procedure could be suc-
cessfully applied to various terminal and internal N-substituted
and N-unsubstituted propargylic amines to afford the corre-
sponding 1,3-oxazolidin-2-ones in high to excellent yield under
mild reaction conditions. In this communication, we report the
efficient preparation of oxazolidinones from carbon dioxide
and propargylic amines catalyzed by silver salts.
/mol % /mol %
1
2
3
4c
5c,d
1.0
0.1
1.0
0.3
0.1
10
10
10
2
100
100
0
0
0
2
6
46
47
10
95
95
87
91
98
2
aThe reaction was carried out in 1.0 mL of toluene with
0.25 mmol of substrate at room temperature. The reaction pro-
ceeded under homogeneous conditions. bIsolated yield. The
reaction was carried out in 1.5 mL of toluene with 0.50 mmol
c
d
of substrate. The reaction was carried out in DMSO at 25 ꢀC.
alkyne, was first subjected to standard conditions; In the pres-
ence of 10 mol % silver acetate and a stoichiometric amount of
DBU in toluene under 1.0 MPa CO2 pressure at room tempera-
ture, the corresponding oxazolidinone 2a was obtained in 95%
yield after 2 h (Table 1, Entry 1). The reaction proceeded so
smoothly that milder reaction conditions were examined involv-
ing the CO2 pressure, the amount of silver catalyst and DBU in
the reaction of 1a at room temperature (Table 1). Under atmo-
spheric pressure of CO2, the corresponding product was obtained
in an excellent yield (Entry 2). It was found that in the absence of
DBU, the product was obtained in 87% yield (Entry 3). The
mechanism for the present reaction was assumed to be that the
carbamate intermediate derived from propargylic amine with
carbon dioxide successively reacted with the alkyne activated
by the silver catalyst to afford the oxazolidinone via an intramo-
lecular ring-closing reaction. Although DBU base would assist
with the carbamate formation, in equilibrium between the corre-
sponding ammonium carbamate and free propargylic amine un-
der a CO2 atmosphere,5b the amine could work as a base to assist
with the carbamate formation without DBU. In the reaction with
2 mol % silver acetate under a 0.3 MPa CO2 pressure, the prop-
argylic amine 1a was completely consumed to afford the product
in excellent yield (Entry 4). Various solvents were screened ex-
pecting the effective solvation of carbamate intermediate to ac-
celerate the reaction.7 When DMSO was employed as solvent,
the reaction was accelerated dramatically to afford the product
in 98% yield after 10 h even under atmospheric pressure CO2
(Entry 5).
The optimized catalytic system was successfully applied to
various propargylic amines (Table 2). Propargylic amines with
terminal alkyne and N-substituted amines were converted into
1-(Phenylethynyl)cyclohexanamine (1a), a model com-
pound of an N-unsubstituted propargylic amine with an internal
Copyright Ó 2009 The Chemical Society of Japan