Synthesis of 1,3-dialkylimidazolium-2-carboxylates by direct carboxylation of 1,3-dialkylimidazolium chlorides with CO2

Article abstract of DOI:10.1016/j.tetlet.2006.06.106

1,3-Dialkylimidazolium-2-carboxylates 1a and 1b are obtained in good to excellent yield and selectivity by carboxylation of the corresponding 1,3-dialkylimidazolium chloride salts with a CO₂/Na₂CO₃ system at temperatures ranging from 80 to 135 °C. The effect of temperature and reaction time on the yield and the selectivity of the carboxylation products has been studied. Coupling the CO₂-based synthesis of 1,3-dialkylimidazolium-2-carboxylates with the transcarboxylation reaction described earlier [Tommasi, I.; Sorrentino, F.; Tetrahedron Lett., 2005, 46, 2141] allows us to set up a new synthetic procedure for the synthesis of organic carboxylates and alkylcarbonate anions avoiding the use of strong bases.

Full text of DOI:10.1016/j.tetlet.2006.06.106

Tetrahedron Letters 47 (2006) 6453–6456
Synthesis of 1,3-dialkylimidazolium-2-carboxylates by
direct carboxylation of 1,3-dialkylimidazolium chlorides with CO2
*
Immacolata Tommasi and Fabiana Sorrentino
Department of Chemistry, University of Bari, Campus Universitario, via Orabona 4, 70126 Bari, Italy
Received 23 January 2006; revised 19 June 2006; accepted 21 June 2006
Abstract—1,3-Dialkylimidazolium-2-carboxylates 1a and 1b are obtained in good to excellent yield and selectivity by carboxylation
of the corresponding 1,3-dialkylimidazolium chloride salts with a CO
The effect of temperature and reaction time on the yield and the selectivity of the carboxylation products has been studied. Coupling
the CO -based synthesis of 1,3-dialkylimidazolium-2-carboxylates with the transcarboxylation reaction described earlier [Tommasi,
2 2 3
/Na CO system at temperatures ranging from 80 to 135 °C.
2
I.; Sorrentino, F.; Tetrahedron Lett., 2005, 46, 2141] allows us to set up a new synthetic procedure for the synthesis of organic car-
boxylates and alkylcarbonate anions avoiding the use of strong bases.
Ó 2006 Published by Elsevier Ltd.
1. Introduction
ð2Þ
We have recently published the utilisation of imidazolium
carboxylates of formula 1,3-dimethyl-imidazolium-2-car-
boxylate (1a) and 1-butyl, 3-methyl-imidazolium-2-car-
boxylate (1b) as CO carriers in a transcarboxylation of
2
acetophenone and methanol for the synthesis, in high
In this reaction, DMC acts as an alkylating/carboxylating
agent. With the aim of setting up a new synthetic proce-
yield and 100% selectivity, of benzoylacetate and mono-
methylcarbonate anions (Eq. 1).1
dure for compounds 1a and 1b using CO we set out to
2
investigate direct carboxylation reactions of the corre-
sponding 1,3-dialkylimidazolium chlorides with CO2.
Here we report the synthesis of 1,3-dialkylimidazolium-
2
-carboxylates 1a and 1b by carboxylation of the corre-
sponding imidazolium chloride salt with a CO /Na CO
3
2
2
system at temperatures ranging from 80 to 135 °C. The
synthesis of the 1,3-dialkyl-imidazolium-2-carboxylates
using CO and the subsequent transfer of the CO -moiety
ð1Þ
2
2
to C–H active compounds and alcohols represents a new
synthetic strategy for obtaining, in high yields, organic
carboxylates and alkylcarbonates avoiding the use of
strong bases.
Previously, we reported the synthesis of 1,3-dimethyl-imi-
2. Synthesis of 1,3-dialkylimidazolium-2-carboxylates
dazolium-2-carboxylate (1a) from 1-methyl-imidazole
2
,3
and dimethylcarbonate according to Eq. 2.
The synthesis and full characterisation of the imidazo-
lium carboxylates of formula 1,3-diisopropyl-4.5-
4
dimethyl-imidazolium-2-carboxylate 3 (Eq. 3) and 1,
5
*
Corresponding author. Tel./fax: +39 080 5442083; e-mail: tommasi@
chimica.uniba.it
3-bis(2,6-diisopropylphenyl)imidazolium-2-carboxylate
4 (Scheme 1) has been recently reported in the literature.
0040-4039/$ - see front matter Ó 2006 Published by Elsevier Ltd.
doi:10.1016/j.tetlet.2006.06.106

6454
I. Tommasi, F. Sorrentino / Tetrahedron Letters 47 (2006) 6453–6456
ð4Þ
The reaction proceeds in high yield under strictly anhy-
drous conditions. Table 1 illustrates the dependence of
the yield and the selectivity of the carboxylation prod-
ucts on the reaction conditions. The results show that
the direct carboxylation can be carried out at relatively
moderate temperatures (80 °C) in good yield without
the formation of significant amounts of 4- and 5-carbox-
ylate isomers (Scheme 2) that are formed at higher
temperatures.
Scheme 1.
These carboxylates were synthesised by different proce-
dures. Kuhn et al. has reported the synthesis of 1,
4
3
-diisopropyl-4.5-dimethyl-imidazolium-2-carboxylate 3
from the corresponding carbene and CO as shown in
Eq. 3
2
Moreover, optimal conditions for the synthesis of 1a
were obtained carrying out the reaction at 110 °C for
3
6 h (entry 2). Under these conditions, the 1,3-dimethyl-
imidazolium-2-carboxylate is formed in high yield
92%) and selectivity (91%). Carrying out the reaction
(
ð3Þ
at 135 °C for 8 h causes the formation of the 4- and 5-
carboxylate isomers in a relatively high yield (entries 3
9
and 6).
5
while Louie and co-workers has reported the synthesis of
1
3
. Recycle of the imidazolium cation
,3-dimesitylimidazolium-2-carboxylate (4a) and 1,3-
bis(2,6-diisopropylphenyl)imidazolium-2-carboxylate
4b) either by condensation of the corresponding carbene
with CO (Scheme 1, route a) or by reaction of the 1,3-
dimesitylimidazolium and 1,3-bis(2,6-diisopropylphe-
1
As we have reported in a previous letter the 1,3-dialkyl-
imidazolium salt obtained as a product of the trans-
carboxylation reaction (2a and 2b, Eq. 1) can be
recovered easily and quantitatively from the reaction
mixture. Due to the stability of the dialkylimidazolium
cation under the reaction conditions, we set out to inves-
(
2
t
nyl)imidazolium salts with KO Bu under an atmosphere
of CO (Scheme 1, route b).
2
The synthesis of carboxylates 1a and 1b by procedures
described in Eq. 3 and Scheme 1 is made difficult as
t
the reaction of the imidazolium salts with KO Bu at
room temperature caused dimerisation of the corre-
sponding imidazole-2-ylidenes. As a matter of fact, the
isolation of 1-butyl, 3-methyl-imidazol-2-ylidene re-
6
quires more drastic conditions as reported by Seddon.
We have shown that 1,3-dialkylimidazolium-2-carboxy-
7
8
lates 1a and 1b can be synthesised from 1,3-dialkylim-
idazolium chlorides and CO through a Kolbe–Schmitt-
2
type reaction carboxylating the imidazolium salts with
Na CO /CO in DMF according to Eq. 4.
2
3
2
Scheme 2.
Table 1. Effect of temperature and reaction time on the yield and selectivity of the carboxylation reaction of 1,3-dimethylimizolium chloride (MMIM
Cl) and 1-butyl, 3-methyl-imidazolium chloride (BMIM Cl) salts
a
Entry Compound
Conditions
Yield (%)
Selectivity (%)
1,3-Dialkyl-
Temperature (°C) Time (h)
1,3-Dialkyl-
imidazolium-2-COO
1,3-Dialkyl-
imidazolium-5-COO
ꢀ
ꢀ
ꢀ
imidazolium-4-COO
1
2
3
4
5
6
MMIM Cl
MMIM Cl
MMIM Cl
BMIM Cl
BMIM Cl
BMIM Cl
80
110
135
80
110
135
24
36
8
24
36
8
65
92
87
63
82
71
100
91
68
100
82
60
—
9
32
—
10
22
—
—
—
—
8
18
a
Total conversion with respect to 1,3-dialkylimidazolium chlorides.

I. Tommasi, F. Sorrentino / Tetrahedron Letters 47 (2006) 6453–6456
6455
Acknowledgements
Financial support from University of Bari (Fondi di
Ateneo 2002-2003) is gratefully acknowledged.
References and notes
1
. Tommasi, I.; Sorrentino, F. Tetrahedron Lett. 2005, 46,
141.
2
2
. Holbrey, J. D.; Reichert, W. M.; Tkatchenko, I.; Bouajila,
E.; Walter, O.; Tommasi, I.; Rogers, R. D. Chem.
Commun. 2003, 28.
Scheme 3.
tigate the recycling of the cation in a new carboxylation
reaction. Attempts to carboxylate the imidazolium cat-
3. Aresta, M.; Tkatchenko, I.; Tommasi, I. In Ionic Liquids
as Green Solvents: Progress and Prospects; Rogers, R. D.,
Seddon, K. R., Eds.; ACS Symposium Series; ACS:
Washington DC, 2003; Vol. 856, p 93.
ꢀ
ꢀ
ꢀ
ion starting from its BF4 , PF6 , and BPh4 salts (these
1
anions were used in the transcarboxylation reaction,
4
5
6
. Kuhn, N.; Steimann, M.; Weyers, G. Z. Naturforsch.
999, 54b, 427.
. Duong, H. A.; Tekavec, T. N.; Arif, A. M.; Louie, J.
Chem. Commun. 2004, 112.
. K. R. Seddon has reported the synthesis and isolation of
the 1-butyl,3-methyl-imidazol-2-ylidene by heating the
corresponding imidazolium halide with strong bases under
reduced pressure to separate the product, Earle J. M.,
Seddon R. K., U.S. Patent 0186803, 2003.
Eq. 1) afforded a very low yield of the carboxylate prod-
uct (<5%). Good results were obtained in the carboxyl-
ation reaction of 1,3-dialkylimidazolium chlorides
1
(
results shown in Table 1) and, in particular, 1,3-di-
methylimidazolium-2-carboxylate was obtained with a
higher yield and selectivity (entry 2, Table 1). In order
to obtain 1,3-dialkylimidazolium chlorides, as products
+
+
of the transcarboxylation reaction, Na or K chlorides
had to be used which show a very low solubility in most
organic solvents. We solved this problem by carrying
7
. Synthesis of 1,3-dimethylimidazolium-2-carboxylate (1a).
5
.21 g (0.039 mol) of 1,3-dimethylimidazolium chloride
and 4.17 g (0.39 mol) of dry Na CO were placed in a
2
3
out the transcarboxylation reaction using NaBPh and
4
glass reactor of a magnetically stirred stainless steel auto-
clave under nitrogen atmosphere. 30 mL of dry DMF
was added to the mixture. After closing, the autoclave was
recovering 1,3-dialkylimidazolium chloride from the
reaction mixture by anion metathesis from 1,3-dialkyl-
1
0
imidazolium tetraphenylborate. It is possible, thus,
to set up a two-step synthetic procedure for the synthesis
of organic carboxylates and alkylcarbonates using CO2
with recycling of the 1,3-dimethylimidazolium cation
2
pressurised by CO up to 50 bar, and the mixture was
heated to 110 °C for 36 h under continuous stirring. The
reaction was then stopped by cooling and depressurising
the autoclave. The reaction mixture was transferred into a
1
00 mL Schlenk tube previously purged with nitrogen.
(
Scheme 3).
The hot suspension (100 °C) was rapidly filtered under a
N atmosphere. By removing the solvent under reduced
2
The literature reports several examples of carboxylation
with CO2 of active methylene compounds, including
acetophenone, by using a variety of catalysts such as me-
pressure, a white solid was obtained (5.03 g, 92% yield)
fully characterised as a mixture of 1,3 dimethylimidazo-
lium-2-carboxylate (91% of the mixture) and 1,3 dimethy-
limidazolium-4-carboxylate (9% of the mixture).
Characterisation of 1,3-dimethylimidazolium-2-carboxyl-
ate. Anal. Calcd for C H N O : C, 51.42; H, 5.75; N,
1
1,12
13
tal phenoxides (MOPh, M = alkali metals
or Zn ),
i
14
La(OPr )
complexes and organic bases like guani-
3
dine,1 DBU,
5
16
diphenylcarbodiimide,
17
anilides.
18
6
8
2
2
These synthetic methodologies are usually affected by
relatively low yields in benzoylacetate or require an ex-
19.99. Found: C, 51.68; H, 5.77; N, 19.77; IR (Nujol,
KBr): 3152 (w), 3104 (s), 1665 (s), 1514 (m), 1247
(
m), 1193 (w), 1103 (m) 1039 (w), 821 (m), 796 (s), 721
cess of the catalyst for substrate quantitative conver-
ꢀ
1 1
sion.11 Here we present a new synthetic strategy for
(m) cm
;
H NMR (D O, 500 MHz): d 3.87 (s, 6H,
2
1
3
N–CH ), 7.32 (s, 2H, C4–H and C5–H); C NMR (D O,
3
2
the synthesis of benzoylacetate in high yield avoiding
the use of strong bases.
1
1
25 MHz): d 36.64 (s, N–CH
39.43 (s, C2), 158.19 (s, C(O)O ). The 1,3-dimethylimid-
3
), 122.93 (s, C4 and C5),
ꢀ
azolium-4-carboxylate isomer was identified by compar-
ison of its spectroscopic data with those of authentic
samples obtained as described elsewere (Tommasi et al.,
3
4. Conclusions
Fischer J., Siegel W., Bomm V., Fischer M., Mundinger
1
We describe a new easy synthesis of imidazolium
K., U.S. Patent 6 175 019, 2001). H NMR (D
O,
2
carboxylates 1a and 1b by the direct carboxylation of
500 MHz): d 3.77 (s, 3H, N1–CH
3
), 3.90 (s, 3H, N3–
), 7.68 (s, 1H, C5–H); C NMR (D O, 125 MHz): d
5.54 (s, N1–CH ), 36.46 (s, N3–CH ), 126.8 (s, C5),
130.52 (s, C4), 137.78 (t, C2, JC–D = 33 Hz), 163.23 (s,
C(O)O–).
. Synthesis of 1-butyl, 3-methylimidazolium-2-carboxylate
1b). 1-Butyl, 3-methyl imidazolium chloride (6.32 g,
0.036 mol) and 3.83 g (0.036 mol) of dry Na CO were
1
3
CH
3
2
dialkylimidazolium chlorides with CO . Coupling this
2
3
3
3
carboxylation reaction to the transcarboxylation reac-
1
1
tion of acetophenone or methanol previously reported
–
(
Eq. 1) provides a new synthetic strategy for the synthe-
8
sis of organic carboxylates and alkyl carbonates with
(
CO avoiding the use of strong bases. The dimethyl-
2
2
3
imidazolium chloride (acting as a ‘catalyst’ in the syn-
thetic process) can be effectively recycled from the
reaction mixture by anion metathesis with NaCl.
placed, under nitrogen atmosphere, in a glass reactor of a
magnetically stirred stainless steel autoclave and to this
was added 30 mL of dry DMF. After closing, the

6456
I. Tommasi, F. Sorrentino / Tetrahedron Letters 47 (2006) 6453–6456
2 2 2 3
autoclave, pressurised by CO up to 50 bar, and the 36.06 (s, N3–CH ), 49.57 (s, N1–CH –CH –CH –CH ),
1
2
3
mixture was heated to 110 °C for 36 h. The reaction was
stopped by cooling and depressurising the autoclave. The
reaction mixture was transferred into a 100 mL Schlenk
tube previously purged with nitrogen and filtered at room
temperature. After evaporation of the solvent under
125.26 (s, C5), 131.38 (s, C4), 137.53 (t, C2,
J
C–D
=
ꢀ
32 Hz), 163.03 (s, –C(O)O ).
Spectroscopic characterisation of 1-butyl, 3-methyl imidazo-
1
lium-5-carboxylate. H NMR (D
(m, 3H, N1–CH –CH –CH –CH ), 1.17 (m, 2H, N1–CH
CH –CH –CH ), 1.73 (m, 2H, N1–CH –CH –CH –CH ),
2
O, 500 MHz): d 0.74
2
2
2
3
2
–
reduced pressure, and recrystallisation from CH CN
3
2
2
3
2
2
2
3
(
4 °C), 5.4 g of a light yellow solid was obtained (82%
4.00 (s, 3H, N3–CH
CH ), 7.72 (s, 1H, C4–H); C NMR (D
13.09 (s, N1–CH –CH –CH –CH ), 19.16 (s, N1–CH –
3
), 4.25 (t, 2H, N1–CH
2
–CH
2 2
–CH –
1
3
yield) fully characterised as a mixture of 1-butyl,3-methy-
limidazolium-2-carboxylate (82% of the mixture), 1-
butyl,3-methylimidazolium-4-carboxylate (10% of the
mixture) and 1-butyl,3-methylimidazolium-5-carboxylate
3
2
O, 125 MHz):
2
2
2
3
2
CH
2 2
–CH –CH
3
), 32.35 (s, N1–CH
2
–CH
2
2
–CH –CH
–CH
3
),
36.14 (s, N3–CH
3
), 49.67 (s, N1–CH
2
2
–CH –
2
(8% of the mixture).
Characterisation of 1-butyl, 3-methylimidazolium-2-carbox-
CH
3
), 126.66 (s, C4), 130.78 (s, C5), 142.26 (t, C2,
1
ꢀ
JC–D = 39 Hz), 163.09(s, –C(O)O ).
ylate (1b). Elemental analysis of the mixture: Anal. Calcd
10. Recycling of 1,3-dimethylimidazolium chloride. The proce-
dure for the synthesis of sodium benzoylacetate from 1,3-
for C
9.73; H, 7.78; N, 15.18; IR (Nujol, KBr): 3145 (w), 3088
s), 1662 (s), 1506 (m), 1323 (s) 1215 (w), 1172 (m), 1070
9 14 2 2
H N O : C, 59.32; H, 7.74; N, 15.37. Found: C,
5
dimethylimidazolium-2-carboxylate and NaBPh
4
is
1
(
(
5
described in a previous letter. Following this procedure
1,3-dimethylimidazolium tetraphenylborate salt was
obtained (3.24 g, 7.78 mmol) which was treated with NaCl
(0.482 g, 8.25 mmol, 6% excess) and extracted with a
water/THF biphasic system (20 mL of water and 75 mL of
THF). After separation of the two phases 1,3-dimethy-
limidazolium chloride was recovered from the aqueous
phase by precipitation with acetone followed by filtration
and drying in vacuo (0.94 g, 7.1 mmol, 91% yield).
ꢀ1
1
s), 883 (m), 794 (s), 661 (m) cm
00 MHz): d 0.63 (m, 3H, N–CH –CH
m, 2H, N–CH –CH –CH –CH ), 1.55 (m, 2H, N–CH –
;
H NMR (D
2
O,
2
2
–CH –CH ), 1.02
2
3
(
2
2
2
3
2
CH –CH –CH ), 3.81 (m, 3H, N–CH ), 4.25 (t, 2H, N–
2
2
3
3
3
CH
1
2
–CH
2
–CH
2
–CH
3
), 7.34 3(d, 1H, C4–H,
H–H
J =
13
.8 Hz), 7.39 (d, 1H, C5–H, JH–H = 1.8 Hz); C NMR
(
D O, 125 MHz): 13.08 (s, N–CH –CH –CH –CH ),
2
2
2
2
3
1
CH
CH
9.16 (s, N–CH
2
–CH
2
–CH
2
–CH
3
), 32.27 (s, N–CH
), 49.57 (s, N–CH
2
-
2
2
–CH
–CH
2
–CH
–CH
3
), 36.88 (s, N–CH
3
2
–
4
NaBPh was recovered from the organic phase by remov-
ing THF under reduced pressure and filtering the residual
water suspension. After washing with diethyl ether and
2
3
), 122.4 (s, C4), 123.39 (s, C5), 140.29 (s,
ꢀ
C2), 158.15 (s, –C(O)O ).
9
. A mixture of 1-butyl, 3-methylimidazolium-4-carboxylate
and 1-butyl, 3-methylimidazolium-5-carboxylate was
obtained carrying out the carboxylation reaction at
drying in vacuo, 2.27 g of NaBPh
was recovered.
11. As examples see: Bottaccio G., Chiusoli G. P., Alneri E.,
Marchi M., Lana G., U.S. Patent 4,032, 555, 1977;
Bottaccio, G.; Chiusoli, G. P.; Felicioli, M. G.; Tosi, E.
Gazz. Chim. Ital. 1976, 46, 831.
4
(6.63 mmol, 85% yield)
1
35 °C for 8 h. The product mixture obtained as described
in note 8 was reacted with a stoichiometric amount of
3
4
HBF as reported elsewhere. After decarboxylation of 1-
butyl, 3-methyl-imidazolium-2-carboxylate the pH was
corrected to neutrality. A mixture of 1-butyl, 3-methyl-
imidazolium-4-carboxylate (65%) and 1-butyl, 3-methyl-
imidazolium-5-carboxylate (35%) was obtained by
12. Patmore E. L., Siegart W. R., Chafetz H., U.S. Patent
3,692,826, 1972.
13. Kunert, M.; Brauer, M.; Klobes, O.; Gorls, H.; Dinjus, E.;
Anders, E. Eur. J. Inorg. Chem. 2000, 1803.
14. Abe, H.; Inoue, S. J. Chem. Soc., Chem. Commun. 1994,
1197.
15. Patmore E. L., U.S. Patent 3,694,496, 1972.
16. Mori, H. Bull. Chem. Soc. Jpn. 1988, 61, 435.
17. Chiba, K.; Tagaya, H.; Karasu, M.; Ono, T.;
Hashimoto, K.; Moriwaki, Y. Bull. Chem. Soc. Jpn.
1991, 64, 966.
recrystallisation from CH CN.
3
Spectroscopic characterisation of 1-butyl, 3-methyl imidazo-
1
lium-4-carboxylate: H NMR (D
2
O, 500 MHz): d 0.74
), 1.17 (m, 2H, N1–CH
CH –CH –CH ), 1.73 (m, 2H, N1–CH –CH –CH –CH ),
(
m, 3H, N1–CH
2
–CH
2
–CH
2
–CH
3
2
–
2
2
3
2
2
2
3
3
.95 (s, 3H, N3–CH
CH ), 7.78 (s, 1H, C5–H); C NMR (D
3.09 (s, N1–CH –CH –CH –CH ), 19.16 (s, N1–CH –
3
), 4.42 (t, 2H, N1–CH
2
–CH
2 2
–CH –
1
3
3
2
O, 125 MHz):
1
18. Chiba, K.; Tagaya, H.; Karasu, M.; Ishizuka, M.;
Toshiyuki, Sugo Bull. Chem. Soc. Jpn. 1994, 67, 452.
2
2
2
3
2
CH –CH –CH ), 31.55 (s, N1–CH –CH –CH –CH ),
2
2
3
2
2
2
3