Optimised microwave-assisted synthesis of methylcarbonate salts: A convenient methodology to prepare intermediates for ionic liquid libraries

Article abstract of DOI:10.1039/b918713h

The reaction of 1-butylpyrrolidine with dimethyl carbonate to yield the ionic liquid precursor, 1-butyl-1-methylpyrrolidinium methylcarbonate, has been investigated under microwave heating conditions and the reaction parameters optimised to achieve 100% yield of the pyrrolidinium salt with no by-products in under 1 h. The reactions of tributylamine, trioctylphosphine, and 1-butylimidazole with dimethyl carbonate under comparable conditions have also been evaluated, yielding the corresponding methylcarbonate salts which can be used as intermediates for the preparation of halide-free ionic liquids without generating any undesirable salt wastes.

Full text of DOI:10.1039/b918713h

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Optimised microwave-assisted synthesis of methylcarbonate salts: a
convenient methodology to prepare intermediates for ionic liquid libraries
John D. Holbrey,*a Robin D. Rogers,a Saloni S. Shuklaa and Cecilia D. Wilfreda,b
Received 10th September 2009, Accepted 26th October 2009
First published as an Advance Article on the web 18th November 2009
DOI: 10.1039/b918713h
The reaction of 1-butylpyrrolidine with dimethyl carbonate to yield the ionic liquid precursor,
1-butyl-1-methylpyrrolidinium methylcarbonate, has been investigated under microwave heating
conditions and the reaction parameters optimised to achieve 100% yield of the pyrrolidinium salt
with no by-products in under 1 h. The reactions of tributylamine, trioctylphosphine, and
1-butylimidazole with dimethyl carbonate under comparable conditions have also been evaluated,
yielding the corresponding methylcarbonate salts which can be used as intermediates for the
preparation of halide-free ionic liquids without generating any undesirable salt wastes.
etc. – all of which are hazardous, highly reactive and toxic
Introduction
reagents – to prepare ionic liquids. For those times when
The most versatile and commonly studied types of ionic liquids,
this approach is not practical, for example when the desired
referred to as aprotic ionic liquids, contain a fully alkylated
alkylating agent is too reactive (e.g. alkylnitrates) or unreactive
organic cation.1 These ionic liquids are generally prepared by
(e.g. simple alkylcarboxylate esters such as ethyl ethanoate), then
initial alkylation of an organic base to yield the cation of choice
a reasonable compromise is the formation of intermediate salts
followed, optionally, by a metathesis step to substitute the anion
in which the anion of choice can be subsequently introduced
formed from the leaving group of the alkylating agent with a
simply, safely, at low cost, and with the least possible generation
more desirable species following the general scheme below:
of waste.
Base + R–X → [Base–R]+ [X]- → [Base–R]+ [Y]- + MX↓
Using these criteria, one of the most appealing reagents is
dimethyl carbonate (DMC).16,17 DMC is a non-toxic compound
Alkylation with haloalkanes, such as chloroethane or bro-
with a versatile and tuneable chemical reactivity, and can func-
tion as either a methoxycarbonylating or as a methylating agent18
depending on the experimental conditions.19 For methylat◦ion
reactions the reaction temperature must typically be >120 C,
i.e. under pressure. DMC has been used as a replacement for
highly toxic and hazardous conventional methylating agents
such as methyl halides, methyl triflate and dimethyl sulfate20
in the synthesis of surfactants, detergents, and phase transfer
reagents from amines and phosphines.21 More recently, ionic
liquids have also been synthesised by methylation with DMC
followed by reaction of the methylcarbonate salts formed with
Brønsted acids, as shown schematically in Fig. 1.22-24
mobutane, has been the method of choice since the earliest
studies on ionic liquids.2 The haloalkanes are relatively cheap,
simple to use, and reaction optimisation methods including
the use of microwave3 or ultrasonic induction,4 high-sheer
mixing5 and microchannel reactor arrays6 have been reported.
However, unless halide or halometallate ionic liquids are the
desired products (for example as solvents for cellulose,7 as
Lewis-acidic catalysts,8 or electrochemical solvents9), then a
subsequent metathesis step to exchange the anion is required.
Anion metathesis is usually achieved through the precipitation
of insoluble silver,10 or other halide salts,11 although the use
of anion exchange columns12 and electrodialysis cells to form
hydroxide solutions13 have also been described. In all cases, salt
wastes are produced as by-products and can leave a legacy of
halide contamination in the final products which can lead to
catalyst poisoning and unforeseen plant corrosion problems.
A better approach is to design the ionic liquid syntheses
so that the anion required in the final product is introduced
as the leaving group of the alkylating agent. A variety of
alkylating agents have been used in this way including alkyl
triflates, trifluoroacetates and sulfonates,14 and alkylsulfates,15
aThe QUILL Research Centre, School of Chemistry and Chemical
Engineering, The Queen’s University of Belfast, Belfast, BT9 5AG,
Northern Ireland, UK. E-mail: j.holbrey@qub.ac.uk
Fig. 1 Schematic of the two-stage ionic liquid synthesis using DMC as
a clean methylating agent to form intermediate methylcarbonate salts
followed by neutralisation with Brønsted acids producing only molar
equivalents of carbon dioxide and methanol as by-products.
bFoundation and Applied Science Department, Universiti Teknologi
PETRONAS, Bandar Seri Iskandar, 31750, Tronoh, Perak, Malaysia
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In contrast to the salt waste generated from the more
traditional metathesis approaches, forming methylcarbonate
salts and then neutralising with a Brønsted acid produces
only carbon dioxide and methanol as by-products which are
facile to remove and yields ionic liquids that are intrinsi-
cally halide-free. As such, the methodology is widely ap-
plicable, although with 1-alkylimidazoles, additional reaction
pathways can yield C(2)- and C(4)-carboxylation, as well as
N-methylation, so that 1,3-dialkylimidazolium-2-carboxylate25
and 1,3-dialkyl-4-carboxylate zwitterions26 are formed in addi-
tion to 1,3-dialkylimidazolium methylcarbonate salts.27 Impor-
tantly, though, the zwitterions still react with acids, liberating
CO2, and so can be used as precursors to ionic liquids.28
Despite the clear interest in the opportunities to use DMC
as a methylating agent to yield ionic liquid precursors, almost
all the available data from the literature relates to unoptimised
batch reactions with time-scales on the order of tens of hours
to days. Here we describe the results of studies into the
optimisation of the reaction of DMC with a range of organic
bases: 1-butylpyrrolidine, tributylamine, trioctylphosphine, and
1-butylimidazole (Fig. 2), in a microwave reactor under auto-
geneous pressure to yield quaternised methylcarbonate salts in
high yield in a conveniently short time-frame.
electrospray MS were conducted by ASEP, Queen’s University
Belfast.
Reaction of 1-butylpyrrolidinium with DMC
The reaction of 1-butylpyrrolidine with dimethyl carbonate in
methanol was screened as a function of temperature (from 130–
170 ◦C), 1-butylpyrrolidine:DMC ratio (from 1 : 1–1 : 4), and re-
action time (t = 0–5 h). In a typical procedure, 1-butylpyrrolidine
(5.66 g, 0.0445 mol), dimethyl carbonate (4.10 g, 0.0445 mol) and
methanol (10 cm3) were placed in a microwave quartz reaction
tube, sealed and heated with magnetic stirring to 130 ◦C.
Under the efficient microwave heating, the internal temper-
ature of the reaction vessel reached the reaction set-points in
3–5 min. and was maintained at temperature
5
◦C using the
power compensation feedback control. After defined times, the
microwave heating was stopped and the reaction vessel cooled to
room temperature quenching the reaction. The reaction mixture
1
was sampled and analysed by H NMR spectroscopy, and the
product distribution and conversion of 1-butylpyrrolidine to
1-butyl-1-methylpyrrolidinium methylcarbonate were deter-
mined as a function of time, initial reagent composition and
temperature from the relative integrals of the N-CH2 signals of
the reagent and product at ~3 ppm.
For optimised reactions (1 : 4 base:DMC, 130 ◦C, 1 h), the
solvent and the excess alkylating agent were removed under
reduced pressure at 50 ◦C and the resulting pale brown ionic
liquid was recrystallised from methanol–ethyl ethanoate at 4 ◦C
as colourless hygroscopic plates, mp 63 ◦C. (Found: C, 54.72; H,
10.79; N, 5.48. [C13H30N][C2H3O3] requires C, 60.80; H, 10.67;
N, 6.45%)†; dH (300 MHz, DMSO-d6) 3.60 (4H, m, NCH2), 3.43
(5H, m, NCH2, NCH3), 3.10 (3H, s, [CH3OCO2]-), 2.18 (4H, m,
NCH2CH2), 1.76 (2H, m, NCH2CH2),1.38 (2H, m, CH2CH3),
0.98 (3H, t, CH2CH3); dC (125 MHz, DMSO-d6) 15◦6.2, 63.5,
53.9, 51.5, 47.3, 25.2, 21.2, 19.4, 12.5. Decomp. 163 C, water
content 0.2157 wt%.
Fig.
2 Reaction scheme for the synthesis of 1-butyl-1-methyl-
pyrrolidinium methylcarbonate from the reaction of 1-butylpyrrolidine
with dimethyl carbonate in methanol, and the structures of the
cations synthesised here: 1-butyl-1-methylpyrrolidinium (1), tributyl-
methylammonium (2), trioctylmethylphosphonium (3), and 1-butyl-3-
methylimidazolium (4).
Reaction of tributylamine with DMC
Our objective was to identify conditions where methylcar-
bonate salts could be rapidly synthesised in high purity, in order
to obtain useful intermediates for the synthesis of ionic liquid
libraries with common cations and a wide variety of anions, with
minimal toxic or hazardous by-product formation.
The reaction of tributylamine with dimethyl carbonate in
methanol was screened as a function of temperature (from 110–
160 ◦C), tributylamine:DMC ratio (from 1 : 2–1 : 4), and reaction
time (t = 0–10 h). In a typical procedure, tributylamine (9.27 g,
0.05 mol), dimethyl carbonate (4.50 g, 0.05 mol) and methanol
(10 cm3) were placed in a microwave quartz reaction tube, sealed
and heated to 130 ◦C. The internal pressure and temperature of
the reaction vessel were monitored. The reaction mixture was
Experimental
1
Dimethyl carbonate, methanol, 1-butylpyrrolidinium, tributy-
lamine, 1-butylimidazole, and trioctylphosphine of AR grade
were purchased from Aldrich and used as received. Reactions
were carried in quartz pressure vessels using a Milestone
Microsynth microwave reactor. Reaction products were char-
acterised by 1H and 13C NMR spectroscopy in DMSO-d6
using a JEOL 300 MHz NMR spectrometer. Melting points
and glass transition temperatures were determined using a TA
Instruments Q2000 DSC. Thermal decomposition temperatures
were determined from the onset to 5 wt% mass loss heating at
sampled and analysed by H NMR spectroscopy, the product
distribution and conversion being determined from the relative
integrals of the N-CH2 signals of the reagent and product at
~3 ppm.
Tributylmethylammonium methylcarbonate was isolated
from the reaction under optimised conditions (1 : 2 base:DMC,
160 ◦C, 10 h) after removing the volatile solvent and excess DMC
under reduced pressure as a brown oil, Tg -59.5 ◦C. (Found: C,
62.01; H, 12.26; N, 5.03. [C13H30N][C2H3O3] requires C, 65.42;
5
◦C min-1 in nitrogen, using a TA Instruments Q5000 TGA.
Water content was measured using a Cou-Lo Aquamax KF
Moisture meter (GRS Scientific). CHN elemental analyses and
† Typical results; it was not possible to obtain stable, reproducible
elemental analyses of these extremely hygroscopic materials.
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H, 12.07; N, 5.09%)†; dH (300 MHz, DMSO-d6) 3.17–3.21 (9H, s
and m, [CH3OCO2]- and NCH2–), 2.97 (3H, s, NCH3), 1.62 (6H,
m, NCH2CH2-), 1.32 (6H, m, -CH2CH3), 0.954 (9H, t, J =
7.9 Hz, CH3); dC (125 MHz, DMSO-d6) 149.8, 60.9, 49.1, 48.0,
23.9, 19.9, 14.0. ESI-MS; +ve mode: 200.24 (100%, [C13H30N]+);
-ve mode: 75.01 (100%, [C2H3O3]-). Decomp. 150 ◦C, water
content 2.14 wt%.
magnetic stirring, varying the reagent ratios between 1 : 1 and
1 : 4 1-butylpyrrolidine:DMC and reaction temperature between
130–170 ◦C using a controlled, programmed heating profile
where the sample was heated to reaction temperature, held at
temperature, and then rapidly cooled to room temperature for
analysis.
The internal reaction temperature was monitored using an
in situ fibre-optic probe placed in the reaction solution and the
temperature maintained using a feedback control to moderate
the power output of the microwave magnetron. In a typical
heating profile, the sample is heated with stirring, over 3–5 min,
from ambient to the defined reaction temperature and then held
for the duration of the reaction. This short initial ramping
of the temperature resulted in an excellent correspondence
between the programmed and actual monitored temperatures
during the initial heating (Fig. 3) with little observed thermal
lag. If an instantaneous temperature jump to (high) reaction
temperatures was programmed, the finite time required for the
power compensation feedback control to kick-in could result
in initial overheating and temperature oscillation. From this
point of view, the presence of methanol as a moderately polar
cosolvent is advantageous in acting as a microwave adsorbant
during the early stages of the reaction, allowing even heat
distribution through the reaction media.
Reaction of trioctylphosphine with DMC
The reaction of trioctylphosphine with dimethyl carbonate in
methanol was performed at 140 ◦C with a 1 : 3 base:DMC
ratio over 5 h. Trioctylphosphine (18.53 g, 0.05 mol), dimethyl
carbonate (12.30 g, 0.15 mol) and methanol (10 cm3) were
loaded under a dry dinitrogen atmosphere into a microwave
quartz reaction tube, sealed and heated to 140 ◦C for 5 h, then
cooled rapidly to room temperature. The internal pressure and
temperature of the reaction vessel were monitored. The reaction
1
mixture was sampled and analysed by H NMR spectroscopy,
showing complete conversion to the product.
1,1,1-Trioctyl-1-methylphosphonium methylcarbonate was
isolated from the reaction mixture as a pale yellow oil, after
removing the volatile solvent and excess DMC under reduced
pressure. dH (300 MHz, DMSO-d6) 3.36 (3H, s, [CH3OCO2]-),
1.89 (6H, m, NCH2-), 1.49 (3H, d, NCH3), 1.10–1.21 (12H, m),
0.92 (24H, m), 0.52 (9H, t, CH3).
Reaction of 1-butylimidazole with DMC
A quartz microwave vessel was charged with 1-butylimidazole
(6.378 g, 0.05 mol), dimethyl carbonate (6.756 g, 0.075 mol) and
methanol (10 cm3). After sealing and◦placing in the microwave
reactor, the vessel was heated to 150 C. The internal pressure
in the reactor increased on heating to 8–12 bar. Reactions were
quenched after 3, 6, 9, and 12 h and sampled for analysis by
1H NMR, using the downfield ring-hydrogen signals of the
heterocycle.
The crude reaction mixture showed the presence of two
products, 1-butyl-3-methylimidazolium methylcarbonate and
1-butyl-3-methylimidazolium-2-carboxylate, identified by 1H
NMR. dH (300 MHz, DMSO-d6) 6.86 (s, 1H, C(4)-H 1-
butylimidazole), 7.15 (s, 1H, C(5)-H 1-butylimidazole), 7.575
(d, 1H, J = 2 Hz, C(4)-H 1-butyl-3-methylimidazolium-
2-carboxylate), 7.61 (s, 1H, C(2)-H 1-butylimidazole), 7.64
(d, 1H, J = 2 Hz, C(5)-H 1-butyl-3-methyl-imidazolium-2-
carboxylate), 7.73 (dd, 1H, C(4)-H, J1 = J2 = 1.5 Hz 1-
Fig. 3 The short initial heating ramp (heating from ambient to final
reaction temperature over the first 0–5 min) in a typical reaction profile
gives excellent thermal control with the internal reaction temperature
(red line) corresponding excellently with the set-point programmed
temperature (black line). During the initial heating step, the internal
reactor pressure (blue line) increases, but then remains constant for the
remainder of the reaction time.
butyl-3-methylimidazolium), 7.8 (dd, 1H, C(5)-H, J1 = J2
=
1.5 Hz, 1-butyl-3-methylimidazolium), 9.392 (s, 1H, C(2)-H
1-butyl-3-methylimidazolium). The major product, 1-butyl-3-
methylimidazolium methylcarbonate, was confirmed by ESI-
MS; +ve mode: 139.1218 (100%, [C8H15N2]+); -ve mode: 75.0082
(100%, [C2H3O3]-).
Set-point and internal temperature and pressure were mon-
itored as the reaction progressed. On heating, the internal
pressure in the vessels increased to a maximum of 8–12 bar.
Reactions were quenched and sampled at times between 0–5 h
for analysis.
Results and discussion
Reactions were followed by 1H NMR spectroscopy. The
conversion of 1-butylpyrrolidine was monitored by comparing
the relative values of the integrated signals from the amine and
product. The percentage conversion of 1-butylpyrrolidine with
reaction time is shown in Fig. 4 at constant 1 : 1 base:DMC
1-Butylpyrrolidine + DMC
DMC and 1-butylpyrrolidine were reacted in methanol in a
Milestone MicroSynth microwave reactor under autogeneous
pressure in a sealed 100 cm3 quartz reaction vessels with
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Table 1 Second-order rate coefficients (k2) for the methylation of
1-butylpyrrolidinium with dimethyl carbonate at 1 : 1 reagent ratios
obtained from the gradients of the linear fit to the data plotted in Fig. 6.
T/◦C
k2/mol-1 s-1
130
140
170
5.08 ¥ 10-3
18.6 ¥ 10-3
35.5 ¥ 10-3
The influence of base:DMC ratio was examined at 130 ◦C
(Fig. 5). The addition of excess methylating agent resulted in
a marked increase on the rate of reaction and also enabled the
reaction to proceed to completion, with no 1-butylpyrrolidine
detectable in the reaction mixtures after 2 h at 1 : 2 base:DMC
and after 1 h at 1 : 4 base:DMC.
The initial rates of reaction at 1 : 1 base:DMC ratio were
investigated by plotting the reciprocal of 1-butylpyrrolidine
content (mol-1) against time for reactions at 130, 140, and
170 ◦C. The data (Fig. 6) show linear dependence of 1/[1-
butylpyrrolidine] with time indicating, as anticipated, overall
second-order rate kinetics for the [A] + [B] → [C] reaction. The
second-order rate coefficients (k2) were determined at the three
temperatures from the slopes of the plots of 1/[conc] vs. time
and are listed in Table 1. It is worth noting that k2 determined
Fig.
4
Conversion of 1-butylpyrrolidine to 1-butyl-1-methyl-
pyrrolidinium methylcarbonate with time at 130 ◦C (᭺), 140 ◦C (ꢀ),
and 170 ◦C (ꢀ) using a 1 : 1 base:DMC ratio.
ratio as a function of temperature and in Fig. 5 as a function of
base:DMC ratio at 130 ◦C.
◦
for the 1 : 1 composition at 130 C (5 ¥ 10-3 mol-1 s-1) is
comparable in magnitude with that reported for the alkylation of
1-methylimidazole with bromohexane (k2 = 1.1 ¥ 10-3 mol-1 s-1)
at 60 ◦C.29
Fig.
5
Conversion of 1-butylpyrrolidine to 1-butyl-1-methyl-
pyrrolidinium methylcarbonate with time using 1 : 1 (᭺), 1 : 2 (ꢀ), and
1 : 4 (ꢁ) base:DMC ratios at 130 ◦C.
Only one product, 1-butyl-1-methylpyrrolidinium methylcar-
bonate, was identified in the reaction mixtures on the basis
1
of H and 13C NMR spectroscopy, and was isolated from the
Fig. 6 Variation of the reciprocal concentration of 1-butylpyrrolidine
(1/[1-butylpyrrolidine]) present in the reaction mixtures as a function
of time at 1 : 1 base:DMC at 130 ◦C (᭺), 140 ◦C (ꢀ), and 170 ◦C
(ꢀ), showing the linear dependence of 1/[conc]t with t indicating initial
second-order rate kinetics.
optimised syntheses (see Experimental section) as an extremely
hygroscopic low-melting crystalline solid after evaporation of
the excess DMC and methanol followed by recrystallisation from
methanol–ethyl ethanoate, and was fully characterised.
Using a 1 : 1 reagent ratio, 80% conversion of 1-butyl-
pyrrolidine to 1-butyl-1-methylpyrrolidinium methylcarbonate
was achieved after 5 h at 130 ◦C (Fig. 4). Increasing the reaction
temperature to 140 ◦C resulted in both a significant increase
in the initial rate of reaction and in the overall conversion
(~98%), although further increments in the reaction temperature
to 170 ◦C gave rise to much smaller changes to either the
rate or final conversion; discolouration of the reaction mixtures
was observed, indicating the start of undesirable decomposition
pathways.
Taking the data points for conversion after 1 h, the tem-
perature dependence of the rate coefficient was fitted using
the Arrhenius equation (k2 = Aexp(-Ea/RT)) and gave a pre-
exponential factor (A) of 1.3 ¥ 10-6 mol-1 s-1 and activation
energy Ea = 64 kJ mol-1 (though we do note that there is a
significant uncertainty implicit in extrapolating from just three
points, and from such a narrow temperature range).
With efficient heat transfer, the reaction of 1-butylpyrrolidine
with dimethyl carbonate was optimised to give complete
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conversion of the amine to quaternary methylcarbonate salt
in <1 h using an excess 1 : 4 ratio of DMC as methylating
agent at 130 ◦C with overall second-order kinetics. Methylation
of 1-butylpyrrolidine was observed with all reagent ratios
from 1 : 1 to 1 : 4 base:DMC at temperatures above 130 ◦C
with only one product, 1-butyl-1-methylpyrrolidinium methyl-
overall conversion of tributylamine to tributylmethylammonium
methylcarbonate was observed, and the maximum yield (~90%)
was achieved in 4–6 h. Examination of the thermal stability
of isolated tributylmethylammonium methylcarbonate by TGA
under dinitrogen showed that the onset of thermal decomposi-
tion at ambient pressure started at around 150 ◦C (most likely via
Hoffmann E2 elimination pathways to generate volatile organic
fragments30), and it is likely that the limiting conversion in the
higher temperature reaction is a result of competing thermal
decomposition. When the methylation reaction was attempted
at higher temperatures, an increase in the reactor pressure was
observed that approached the defined safe limits of operation of
the microwave, and reactions were abandoned. These results,
for the reaction of tributylamine with DMC, are consistent
with data previously reported under conventional heating at
moderate pressures, where yields of 91–96% were recorded using
comparable reagent ratios and temperatures.18
As anticipated from classical kinetics, and from the compara-
tive reaction of 1-butylpyrrolidine, increasing the concentration
of DMC in the reaction mixture gave an increase in the reaction
rate, although this was accompanied by only a small increment
in the overall conversion, from 90 to around 95%. Data for
reactions at 1 : 2, 1 : 3 and 1 : 4 base:DMC ratio at 150 ◦C are
shown in Fig. 8. The results at 1 : 3 and 1 : 4 base:DMC ratio are
effectively identical within experimental error.
1
carbonate, observed by H NMR spectroscopy. Notably, the
addition of a 2–4-fold excess of DMC resulted in the rapid,
complete conversion of 1-butylpyrrolidine to product, from
which the unreacted DMC and methanol solvent could be
easily removed under reduced pressure. The product, 1-butyl-
1-methylpyrrolidinium methylcarbonate, was isolated as a pale
brown low-melting hygroscopic solid. Recrystallisation from
methanol–ethyl ethanoate gave the salt as extremely hygroscopic
colourless plates.
It is worth noting that during similar studies on the synthesis
of quaternary ammonium methylcarbonate surfactants, Earl
et al.18comment that products containing a brown colouration
were obtained when the methylation reactions were performed
in the presence of atmospheric oxygen – presumably due to
formation of an oxidation product – and that colourless or very
pale yellow salts were formed when reactions were performed
in the absence of oxygen. In the reactions performed in this
work, no special precautions were taken to exclude air from the
reaction mixtures.
Reaction of DMC with other organic bases
The reactivity of other organic bases with DMC was explored
under analogous conditions to that used above with 1-butyl-
pyrrolidine. Methylation of tributylamine with DMC was much
slower than for 1-butylpyrrolidine under comparable conditions.
Using a 1 : 2 base:DMC ratio, less than 20% methylation of
◦
tributylamine was observed at 110 C after 7 h. Increasing the
temperature led to increased reactivity and to an increase in the
apparent initial rate and the overall yield, from 77% at 140 ◦C to
90% at 160 ◦C after 10 h (Fig. 7), in contrast to 98% conversion
of 1-butylpyrrolidine at 140 ◦C after 4 h.
Fig.
8
Conversion of tributylamine to tributylmethylammonium
methylcarbonate with time using 1 : 2 (᭺), 1 : 3 (ꢀ), and 1 : 4 (ꢀ)
base:DMC ratios at 150 ◦C.
To examine whether thermal decomposition of the quaternary
ammonium methylcarbonate salt was the limiting factor in the
reactions at elevated temperatures, the reaction of trioctylphos-
phine with DMC at 140 ◦C with a 1 : 3 phosphine:DMC
ratio for 5 h reaction time was performed. It was anticipated
that the greater thermal stability of tetraalkylphosphonium
salts compared to tetraalkylammonium salts should allow
quantitative formation of the phosphonium methylcarbonate
with no thermal degradation. The reaction of a 1 : 3 ratio
of trioctylphosphine:DMC at 140 ◦C for 5 h gave complete
methylation and formation of the trioctylmethylphosphonium
methylcarbonate salt.
Fig.
7
Conversion of tributylamine to tributylmethylammonium
methylcarbonate with time at 140 ◦C (ꢀ), 150 ◦C (ꢀ), and 160 ◦C
(ꢁ) using 1 : 2 base:DMC ratios.
On increasing the temperature further, to 160 ◦C, the rate
of reaction increased, but no further enhancement in the
The reaction of 1-butylimidazole with DMC was conducted
using 1 : 1–1 : 4 ratios of 1-butylimidazole:DMC at 140 ◦C
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and 150 ◦C. The reaction rate was slower than that with
1-butylpyrrolidine though similar to that with tributylamine. In
contrast to the reactions with the saturated amines, where only
one reaction product was observed, with 1-butylimidazole two
products, 1-butyl-3-methylimidazolium methylcarbonate and
1-butyl-3-methylimidazolium-2-carboxylate, were formed dur-
ing the reaction.
At 1 : 1.5 ratio, the conversion of 1-butylimidazole to products
peaked at 92% after 12 h, whereas with increasing the 1-
butylimidazole:DMC ratio further, a slight increase in total
conversion was achieved, to 94% with 1 : 2 ratio and to 97%
at 1 : 4. The rate of conversion increased in each case, so that the
reaction was complete after 12 h at 1 : 1.5, 9 h at 1 : 2, and 6 h
at 1 : 4 1-butylimidazole:DMC. In all the reactions, the ratio
of imidazolium salt to imidazolium-2-carboxylate zwitterion
formation remained consistent at approximately 4 : 1 irrespective
of temperature and composition.
Unusually, the product distribution for the reactions here
of 1-butylimidazole with DMC differs from that previously
reported for reaction of 1-methylimidazole,24,27 in which the
1,3-dimethylimidazolium-2-carboxylate zwitterion was isolated
in up to 85% yield. The reasons for this inversion is not
apparent, but may be the result of greater steric hindrance in
1-butylimidazole compared to 1-methylimidazole, or the forma-
tion of solid 1,3-dimethylimidazolium-2-carboxylate displacing
the product equilibrium.
At 140 ◦C and 1 : 1 1-butylimidazole:DMC ratio, only 35%
conversion of 1-butylimidazole to products was observed after
7 h compared to 98% conversion of 1-butylpyrrolidine after
4 h. When an excess of DMC was used and the temperature
was raised to 150 ◦C, both reactivity and total conversion to
products were increased. Fig. 9 shows the total conversion,
and distribution of the two products with time for reactions at
1 : 1.5, 1 : 2 and 1 : 4 1-butylimidazole:DMC ratio conducted at
150 ◦C.
Compared to 1-butylpyrrolidine, which gave complete con-
version at the same reagent ratios in less than 1 h at 130 ◦C,
both tributylamine and 1-butylimidazole were significantly less
reactive, although reactions could be run to completion in 6 h.
The probable cause for the differences in reactivity of the amines
is most likely steric crowding at the reaction centre, rather than
differences in the pKa of the different bases.
Conclusions
Methylcarbonate salts are versatile halide-free intermediates
for ionic liquid synthesis. Clean, quantitative formation of the
methylcarbonate salts with no waste provides a platform to
access large libraries of ionic liquids around common cations
that are entirely halide-free, with only carbon dioxide and
methanol produced as by-products through simple acid/base
chemistry. This presents a highly desirable alternative to the
conventional salt–salt metatheses usually used for ionic liquid
synthesis.
While enhanced reaction kinetics arising from ‘mi-
crowave effects’ have been largely dispelled following careful
experimentation,31 there is still a clear utility in using microwave
reactors, not least of which is the ability to heat samples
rapidly with good thermal control. This provides a very useful
methodology for screening and development, and, more im-
portantly, with rapid heating to reaction temperature, potential
side-reactions arising with dimethyl carbonate at intermediate
temperatures (for example from the carboxylation pathway) can
be reduced or eliminated.
The microwave batch synthesis of 1-butyl-1-methyl-
pyrrolidinium methylcarbonate has been optimised in a Mile-
stone Microsynth microwave reactor with 100% yield of the
desired product obtained in 1 h using a 1 : 4 base:DMC ratio
at 130 ◦C. For the reaction with tributylamine, 98% conversion
to the desired product could be achieved after 6 h, however
increasing the temperature to >140 ◦C to enhance the reaction
rate initiated thermal decomposition of the ammonium methyl-
carbonate salts, resulting in poorer yields. Under comparable
conditions, the more thermally stable trioctylmethylphospho-
nium methylcarbonate could be obtained quantitatively.
Fig. 9 Change in percentage composition with time for the reaction
of 1-butylimidazole with DMC in 1 : 1.5 (top) 1 : 2 (middle) and 1 : 4
(bottom) base:DMC ratios (total conversion, black; 1-butylimidazole
starting material, blue; 1-butyl-3-methylimidazolium methylcarbonate,
red; 1-butyl-3-methylimidazolium-2-carboxylate, green) at 150 ◦C.
412 | Green Chem., 2010, 12, 407–413
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The reaction of 1-butylimidazole with DMC yielded two
products, the desired imidazolium methylcarbonate and zwit-
terionic imidazolium-2-carboxylate salts, in approximately 4 : 1
ratio irrespective of the conditions used. Interestingly, both
the methylcarbonate salt and zwitterionic carboxylate can be
converted to ionic liquids by reaction with acid.28 Further work
will examine a wider reaction space for the imidazolium salts,
to try to identify conditions where carboxylate formation is
suppressed and the imidazolium methylcarbonate is formed as
the sole product.
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Acknowledgements
The authors wish to thank Petroliam Nasional Berhad
(PETRONAS) for financial support for this project (and par-
ticularly the Management of Petronas Research Sdn Bhd and
Universiti Teknologi Petronas for initiating and establishing
the collaborative program with QUB), Milestone Microwave
Laboratory Systems for the loan of microwave reactors, and
Prof. C. R. Strauss for invaluable discussions.
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