ChemComm
Cite this: Chem. Commun., 2011, 47, 2973–2975
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COMMUNICATION
Esterification of carboxylate-based ionic liquids with alkyl halidesw
Bin Zhao, Lasse Greiner* and Walter Leitnerac
a
ab
Received 4th November 2010, Accepted 20th December 2010
DOI: 10.1039/c0cc04798h
A
facile reaction of 1-ethyl-3-methylimidazolium acetate
(
[EMIm]Ac) with dichloromethane at room temperature was
observed with esters among the products. This esterification can
be exploited for mild solvent-free esterification with a range of
other carboxylate-based ionic liquids and alkyl halides.
Ionic liquids (IL) are considered as alternative solvents for
1
,2
many reactions. Generally, synthesis protocols of IL often
include the use of volatile solvents which can be readily
removed from the non-volatile IL. Here, chloroalkanes like
dichloromethane (DCM) are often chosen for extraction or as
solvent for IL synthesis. Recently, [EMIm]Ac was recognised
as an excellent solvent for carbohydrates and their oligomers,
Fig. 1 Proposed mechanism of the reaction of [EMIm]Ac with
dichloromethane via 2-(acetoxymethyl)-1-ethyl-3-methylimidazolium
as intermediate.
Alternatively, when [EMIm]Ac is prepared via 1-ethyl-3-
methylimidazolium-2-carboxylate and acetic acid neither
chloride nor other ions are detectable by ion chromatography.
As [EMIm]Ac is highly hygroscopic, the inhibitory effect of
water was tested. With 10 wt% water the reaction between
3
,4
namely cellulose.
purified [EMIM]Ac, we observed a facile reaction between
EMIm]Ac and DCM. The reaction gave acetate esters and
During preparation to obtain highly
[
chloride was liberated. This is noteworthy, as halides such as
chloride is known to influence not only density and viscosity
of the liquids but also to deactivate transition metal-based
[
EMIm]Ac and dichloromethane was below the detection limit
after stirring for 14 hours with AgNO
1
H-NMR over 5 days.
3
–HNO
3
as well as by
1
,2,5,6
catalysts and enzymes already in trace amounts.
Chloroform also reacts with [EMIm]Ac, but comparably
slower, probably due to higher steric hindrance. Only, when
the mixture of [EMIm]Ac with chloroform was heated to
Subsequently, the mixture of [EMIm]Ac and dichloro-
methane was stirred at room temperature. Already, after 5
ꢀ
1
minutes, chloride was detected by addition of 0.1 mol L
1
1
reflux the color changed and the integrals in H-NMR changed
3 3
AgNO –HNO solution. The H-NMR showed deviations
proportion (see ESIw).
from the starting mixture (see ESIw) and indicated a derivative
of the imidazolium ring with changes of both chemical shift
and integrals for the acetate anion. According to Isleyen and
Dogan methylene diacetate can be prepared by tetrabutylam-
With these observations we set out to see whether this could
be used in synthetic applications. Compared to other methods
to synthesize esters the access via alkyl halide and carboxylate
salts, is attractive, especially, as alkyl halides are cost effective
starting materials. So far, its industrial application is limited
7
monium acetate with dichloromethane, so that the signals at
5
.64 ppm and 2.06 ppm can be assigned to methylene diace-
by low conversion, low yield, the solid state, and low solubility
8–16
tate. Analysis by LC-MS gave a peak at m/z = 183.2 in line
with 2-(acetoxymethyl)-1-ethyl-3-methylimidazolium. Based
on these findings the reaction products can be explained by
in organic solvents of metal carboxylate salts.
The ester
synthesis of quaternary ammonium carboxylates with alkyl
halides must be conducted in organic solvents, as these salts
8,17
the intermediate formation of chloromethyl acetate in an S
N
2
are solids.
By using carboxylate-based IL these short-
reaction and subsequent nucleophilic attack of this intermedi-
ate by either acetate or imidazolium (Fig. 1).
comings could be overcome.
Based on the high nucleophilic activity of acetate as a
counterion of imidazolium, we investigated a series of ester
synthesis with alkyl halides and carboxylate-based IL to assess
the scope of the reaction. A range of reactions were carried out
a
Institut fu¨r Technische und Makromolekulare Chemie, RWTH
Aachen University, Worringerweg 1, 52056 Aachen, Germany
DECHEMA Karl-Winnacker-Institut, Theodor-Heuss-Allee 25,
Max-Planck-Institut fu¨r Kohlenforschung, Mu¨lheim an der Ruhr,
Germany
(
Table 1). For the primary alkyl halides, the selectivity towards
b
the corresponding ester was higher than 99% as no byproduct
was detected. At room temperature yield of 1-chlorobutane
was limited (entry 1) but at slightly elevated temperature
quantitative conversion was achieved (entry 2). Addition of
sodium acetate and 10-fold increase of chloro-butane gave the
c
w Electronic supplementary information (ESI) available: See DOI:
0.1039/c0cc04798h
1
This journal is c The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 2973–2975 2973