Published on the web September 1, 2012
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A Superior New Route to Methyl Phosphonate-based Ionic Liquids
Carmen Froschauer,*1,2 Herbert Sixta,3 Hedda K. Weber,2 Gerhard Laus,1 Volker Kahlenberg,4 and Herwig Schottenberger1
1Faculty of Chemistry and Pharmacy, University of Innsbruck, Innrain 80-82, 6020 Innsbruck, Austria
2Competence Centre of Wood Composites and Wood Chemistry K-Plus, Altenberger Str. 69, 4040 Linz, Austria
3Department of Forest Products Technology, Aalto University, Vuorimiehentie 1, FI-00076 Aalto, Finland
4Institute of Mineralogy and Petrography, University of Innsbruck, Innrain 52, 6020 Innsbruck, Austria
(Received June 5, 2012; CL-120481; E-mail: Carmen.Froschauer@uibk.ac.at)
In the search for new ionic liquids (ILs) for biomass
processing, analytical chemistry, or modern nonvolatile fire
retarding agents, alkyl phosphonate-based ILs represent very
promising candidates. A very practical synthesis of such ILs,
which is superior to the conventional quaternization of
1-alkylimidazoles by dimethyl phosphite (DMP), was devel-
oped. 1,3-Dialkylimidazolium halide salts serve as convenient
starting materials, and various 1,3-dialkylimidazolium methyl
phosphonates are thus accessible by simple, fast, and solvent-
free procedures with DMP, where the anion is methylated.
(Figure 1b). This type of reaction was first reported in 2005 for
the respective ion metathesis of cetyltrimethylammonium
bromide using a large excess of diethyl phosphite under reflux
in toluene.8 An alternative synthesis was reported, also starting
with a bromide salt and exchanging the anion first to hydroxide
and then to the desired phosphonate.1 These procedures were not
suitable for our purpose due to the long reaction time and the
necessary removal of excess alkylating agent and solvent.
Therefore, we developed a convenient synthesis, which is fast,
simple, solvent-free, and provides access to a wide range of
different derivatives. Halide impurities are considered accept-
able to a certain extent (<3 wt %), all the more so since chloride-
based ILs are excellent solvents for biocomposites on their own
(despite other drawbacks).9
For the practical synthesis, neat 1,3-dialkylimidazolium
halide10 (60 mmol) was placed in a flask, and a small excess
(1.01 equiv) of DMP was added. The suspension was heated to
80 °C and stirred for 2 h. The methyl halide generated was
allowed to escape and collected using a cold trap. The volatiles
were removed, and the product was dried in vacuum at 80 °C for
another 4 h. Insoluble starting material, such as 1-propargyl-2,3-
dimethylimidazolium bromide (propargyl means 2-propynyl),
was dissolved in a minimum amount of methanol. This was also
the only case giving a solid product; all other reactions resulted
in room-temperature ILs.
Ionic liquids (ILs) have been frequently investigated for the
exploitation of renewable raw materials such as biomass because
of their ability to dissolve the usually insoluble lignocellulose.
In this context, our attention was attracted to alkyl phosphonate-
based ILs, which were reported to be excellent for use in high-
performance ionic liquid chromatography and extraction of
polysaccharides from bran.1-3 The primary aim of our research
was the synthesis of new alkyl phosphonate-based imidazolium
salts for cellulose dissolution. The synthesis should be practical
for large-scale industrial use; a technical grade of the compound
is accepted in favor of a reasonably priced product. Two
different synthetic routes for these compounds are known, but
both seemed to be too time-consuming and tedious for bulk
syntheses. In the first approach, a dialkyl phosphite is used for
the quaternization of 1-alkylimidazoles to form 1,3-dialkylimi-
dazolium alkyl phosphonate-based ILs4 (Figure 1a), our favored
class of ILs. However, processes of this type are normally very
slow (up to 48 h).5-7 In addition, the alkylation is limited by the
availability of the respective dialkyl phosphites. Therefore
we envisaged an alternative synthetic route with dialkylimida-
zolium halides as starting materials, whereupon the halide anion
is removed after alkylation by dimethyl phosphite (DMP)
The products were analyzed by 1H and 13C NMR spec-
troscopy, a discrepancy was revealed between the expected and
found integration of the H atoms of the methyl phosphonate
CH3-group, which showed less than three H atoms. The
assumption of residual halide in the anion composition due to
incomplete reaction was confirmed by ion-exchange chromatog-
raphy (IEC). The results of the IEC confirmed the expected
halide contents determined by calculation from H-integration
values in 1H NMR spectra. The results are summarized in
Table 1. Chlorides (Entries 1-4) showed an average of 92 mol %
conversion, whereas bromides (Entries 5-7) and iodides
(Entries 8 and 9) gave only 30-40 mol % of the desired methyl
phosphonates. This remarkable performance of the chloride salts
can be ascribed to the high vapor pressure of the escaping CH3Cl
(490 kPa, 20 °C) and the resulting continuous removal of this
1
by-product. In comparison, H NMR spectra of products from
classical quaternization of 1-alkylimidazoles with DMP did not
show these too low H integration values.
In the case of 1-propargyl-2,3-dimethylimidazolium methyl
phosphonate, the coexistence of the starting bromide could be
confirmed by X-ray analysis of single crystals, which were
spotted in the solid product.10,11 By using a large excess of DMP
(4 equiv) and stirring overnight at 80 °C, the conversion of
1-allyl-3-benzylimidazolium bromide (Entry 5) could be slight-
Figure 1. a) Existing procedure: quaternization of alkylimida-
zoles;4 b) novel, generalized synthesis: alkylation of the halide
counter ion of dialkylimidazolium salts with DMP.
Chem. Lett. 2012, 41, 945-946
© 2012 The Chemical Society of Japan
Froschauer, Carmen
