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A. Zicmanis, L. Anteina / Tetrahedron Letters 55 (2014) 2027–2028
Table 1
genuine purities of the investigated ILs. Titrations were possible
because the investigated ILs contain a slightly basic dimethyl phos-
phate anion. Therefore a titrimetric method (quantitative analysis)
was developed in our laboratory for more accurate evaluation of
the purity of the ILs with dimethyl phosphate anions.7 Solutions
of these ILs in glacial acetic acid were titrated with 0.05 M per-
chloric acid, also in glacial acetic acid. Fine titration curves in
potentiometric titration experiments have allowed us to obtain
high accuracy in our measurements and using further only high-
quality ILs (purity P98.5%). The same samples were also analyzed
by HPLC/MS for comparison. Only one substance was discovered in
every examined sample this way. Hence, the proposed titrimetric
method is a much more convenient method for analyses of ILs than
chromatography. Therefore, the described 1,3-dialkylimidazolium
dimethyl phosphates can be considered as readily available and
excellent media for the Knoevenagel condensation reaction re-
ported herein.
Impact of the type of anion present in the ILs on the yield of ethyl 2-cyano-3-phenyl-
propenoate (3) prepared via the Knoevenagel condensation reaction
Entry
ILa
Yield of 3b (%)
1
2
3
4
5
6
[MMIm]DMP
[MMIm]OMs
[MMIm]OTs
[BMIm]DMP
[BMIm]OMs
[BMIm]OTs
98
19
15
99
12
32
a
ILs (6) with 1,3-dimethylimidazolium (MMIm) and 1-butyl-3-methylimidazo-
lium (BMIm) cations and various anions: DMP—dimethyl phosphate,
OMs—mesylate, OTs—tosylate.
b
Yield of the isolated product 3 after 1 h at 90 °C with a molar ratio of benzal-
dehyde/ethyl cyanoacetate/IL = 1:1:1.
Table 2
In conclusion, 1,3-dialkylimidazolium dimethyl phosphates can
be used as reaction media and catalysts for the Knoevenagel con-
densation benzaldehyde and active methylene compound 2. Al-
most quantitative yields of the products can be obtained in these
solvents. The purity of these ionic liquids can be easily monitored
by potentiometric titrations in acetic acid.
Impact of the structure of the cation in ILs with a dimethyl phosphate anion (6) on the
yield of ethyl 2-cyano-3-phenylpropenoate (3) prepared via Knoevenagel
condensation
Entry
IL
R
R1
Yield of 3a (%)
1
2
3
4
5
6
6a
6b
6c
6d
6e
6f
CH3
CH3
n-C4H9
n-C4H9
n-C8H17
n-C8H17
H
CH3
H
CH3
H
CH3
92
91
87
82
87
88
Acknowledgments
We gratefully acknowledge the financial support from the
Latvian Research Council and Mr. E. Pridans and Dr. U. Kalejs
(BAPEKS, Ltd, Latvia) for fruitful discussions.
a
Yield of isolated product 3 at 25 °C after 10 min with a molar ratio of benzal-
dehyde/ethyl cyanoacetate/ionic liquid = 1:1:1.
Supplementary data
The Knoevenagel condensation in 1,3-dialkylimidazolium di-
methyl phosphate ILs can be performed even at room temperature.
Acceptable yields of product 3 (81–92%) were obtained in only
10 min. Thus 1,3-dialkylimidazolium dimethyl phosphates are
superior media for the rapid and easy preparation of unsaturated
compounds in comparison with other ionic liquids.
Supplementary data (full experimental details and characteriza-
tion of the ionic liquids 6a–6f) associated with this article can be
High yields of isolated products, with only negligible differences
between them, were observed for all the examined 1,3-dialkylim-
idazolium-based ILs with dimethyl phosphate anions, regardless of
the length of the carbon atom chain in their cations. ILs with more
hydrophobic cations provided slightly lower yields than those with
more hydrophilic cations. A hydrogen atom at C-2 was preferable
to a methyl group in terms of the yield of the product. The impacts
of the IL cations on the results of the same reactions are very much
less than those resulting from the type of anion (Table 2).
References and notes
4. General procedure for the Knoevenagel condensation reaction. Benzaldehyde (1)
(5 mmol), ethyl cyanoacetate (2) (5 mmol), and 1,3-dimethylimidazolium
dimethyl phosphate (5 mmol) were stirred at 90 °C for 1 h. The mixture was
cooled and extracted with Et2O (4 Â 5 mL). The combined organic layer was
evaporated and the residue crystallized from EtOH. Alkene 3 was obtained as a
colorless product.
The ILs with different alkyl group lengths at N-1 (methyl, butyl,
or octyl), and different substituents at C-2 (H or Me) (Scheme 2)
were prepared according to a slightly modified recently described
method.5,6 Simple alkylation reactions of N-substituted imidazoles
with trimethyl phosphate at 80 °C under argon gave the desired ILs
in 82–99% yields and high purity (P98.5%).
The structures of the ILs (qualitative analysis) were confirmed
from their 1H NMR and mass spectra. The quantitative analysis of
ILs by HPLC has always shown a tendency toward uncertain results
and therefore a more accurate method was needed for the analyses
of high-quality ILs. A highly accurate titrimetric analysis in a
non-aqueous solution was selected for the determination of the
6. General procedure for the synthesis of ionic liquids with dimethyl phosphate anions.
1-Alkyl-imidazole (0.10 mol) and trimethyl phosphate (0.10 mol) were mixed at
room temperature under argon, then heated to 80 °C and stirred at this
temperature for 1 h. MeCN (20 mL) was added and the mixture stirred at 80 °C
for 23 h. The mixture was left at room temperature, and the resulting precipitate
(or oil) was separated, washed twice with MeCN and dried under vacuum
(2 Torr) for 4 h at 80 °C.
7. General procedure for quantitative analysis of ionic liquids with dimethyl phosphate
anions. A sample of an ionic liquid (100–150 mg) was dissolved in glacial AcOH
(50 mL) and titrated with a solution of perchloric acid in glacial AcOH (0.05 mol/
L). The purity (content of the major substance, %) of the sample was calculated.
Zicmanis, Andris
