Ionic liquids as a recyclable reaction medium for the Baylis-Hillman reaction

Article abstract of DOI:10.1016/S0040-4020(01)00316-7

The Baylis-Hillman reaction using 1,4-diazabicyclo[2.2.2]octane (DABCO) has been shown to be 33.6 times faster in the recyclable ionic liquid 1-n-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF₆]) than in acetonitrile. Low yields (14-2

Full text of DOI:10.1016/S0040-4020(01)00316-7

TETRAHEDRON
Pergamon
Tetrahedron 57 02001) 4189±4193
Ionic liquids as a recyclable reaction medium for the
Baylis±Hillman reaction
p
JoaÄo N. Rosa, Carlos A. M. Afonso and Antonio G. Santos
p
Â
  Ã
Departamento de Quõmica, Centro de Quõmica Fina e Biotecnologia, Faculdade de Ciencias e Tecnologia, Universidade Nova de Lisboa,
2825-114 Caparica, Portugal
Received 20 November 2000; revised 8 March2001; accepted 14 March2001
AbstractÐThe Baylis±Hillman reaction using 1,4-diazabicyclo[2.2.2]octane 0DABCO) has been shown to be 33.6 times faster in the
recyclable ionic liquid 1-n-butyl-3-methylimidazolium hexa¯uorophosphate 0[bmim][PF6]) than in acetonitrile. Low yields 014±20%) of
adducts are obtained from aliphatic aldehydes and moderate to high yields 039±72%) from aromatic aldehydes. Recycling and reuse of the
reaction medium was demonstrated. q 2001 Elsevier Science Ltd. All rights reserved.
1. Introduction
that, at least in the case of using 1,4-diazabicyclo[2.2.2]-
octane 0DABCO) as a catalyst, the overall condensation is
reversible.9b Although this reaction has the potential to be
synthetically useful, it suffers from a series of drawbacks;
the major one being its very slow rate even in the absence of
solvent and withhighconcentrations of catalyst 0usually
DABCO).7 A series of efforts have been made to improve
the reaction rate: these include high pressure,10a
microwave10b or ultra-sounds,10c and it has also been
reported,10d though uncon®rmed by other researchers,10e
that low temperatures increase the rate of this reaction.
The use of catalysts other than DABCO, such as DBU,10f
TiCl4,10g Ti0OiPr)4,10h Me2S,10h phosphines,7,10i and calco-
genide10j compounds, or additives 0co-catalysts) including
LiClO4,10k Lewis acids and hydrogen-bond donors such as
alcohols10e has also been reported to markedly increase the
reaction rate. Some examples of chiral catalysts have also
been reported.7c,10l,10m
Room temperature ionic liquids, especially those based
upon the 1-n-alkyl-3-methylimidazolium cation, have
attracted growing interest in the last few years.1 They
offer an alternative and ecologically sound medium
compared to conventional organic liquids as they are non-
volatile, recyclable, non-explosive, thermally robust and
can exhibit an electrochemical window of up to 3.0 V.
They are excellent solvents for carbonyl compounds, alkyl
halides, alcohols and amines and, depending on the anion,
they can be immiscible with water and some organic
solvents, suchas alkanes and dialkyl ethers.
1
Studies on
regioselective alkylation of naphthol and on Diels±Alder
reactions in ionic liquids have suggested reactivity patterns
similar to those observed, respectively, in DMF and in
alcoholic solvents.2 On the other hand, when the ionic
solvent was used as a stationary phase for gas chromato-
graphy, it appeared to have a dual nature; behaving as a non-
polar stationary phase for non-polar analytes but polar for
analytes withproton-donor 0e.g. phenol, carboxylic acids)
or proton-acceptor groups 0e.g. amines).3 These media have
also been applied to non-catalytic1,2a,2c,4 and catalytic
reactions1,2a,5 as well as to selective extraction.6
Although the Baylis±Hillman reaction is often conducted in
The Baylis±Hillman reaction is a totally atom-ef®cient
reaction, since all the atoms from the reagents are
incorporated in the product, thus being an inherently
`green' transformation.7 It also allows the synthesis of
highly functionalized and useful intermediaries in a single
step.7,8 In the proposed mechanism 0Scheme 1) the rate
determing step corresponds to the reaction between the
aldehyde and the enolate 1.9a Further studies also suggested
p
Corresponding authors. Tel 1 351 21 2948358; fax 1 351 21 2948550;
e-mail: cma@dq.fct.unl.pt; ags@dq.fct.unl.pt
Scheme 1.
0040±4020/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4020001)00316-7

4190
J. N. Rosa et al. / Tetrahedron 57 ,2001) 4189±4193
the absence of a solvent, this may not be feasible under
certain circumstances, for instance when the reactants are
not liquid. In these cases an increase in the reaction rate has
been observed bothin water and ¯uorinated solvents. Due to
charge separation in the intermediate steps, polar solvents
suchas THF or acetonitrile favour the reaction.
7,10e
The
expectation that the polar nature of the ionic liquid could
also promote this transformation, prompted us to study
the use of the 1-n-butyl-3-methylimidazolium [bmim]
cation based ionic liquid medium in the Baylis±Hillman
reaction.
2. Results and discussion
In order to compare the reaction rates in ionic liquids with
conventional solvents, the condensation between benzal-
dehyde, and methyl acrylate was carried out in acetonitrile,
[bmim][BF4] and in [bmim][PF6]. The results are shown in
Table 1 and Fig. 1. Using 100 mol% and 20 mol% DABCO
in [bmim][PF6] the reaction was, respectively, 33.6 and 11.1
times faster than in acetonitrile 0with 100 mol% DABCO,
Table 1, Entries 3 and 4) which has been reported as the
optimal conventional solvent.10e The ionic liquid
[bmim][BF4] gave also a reaction 14.1 times faster than in
acetonitrile 0Table 1, Entry 2). In the case of the ionic
solvent [bmim][PF6], the fastest reaction occured at room
temperature, with both higher 0508C) or lower temperatures
038C and 2508C) decreasing the reaction rate 0Table 1,
Entries 4 to 7). This temperature dependence has also
been reported in acetonitrile.10e The reason for the reactivity
increase in ionic solvents in relation to the conventional
organic solvent acetonitrile could be attributed to a shift
in the equilibrium, due to stronger stabilization of the
zwitterionic intermediate 1. Similar argument has recently
been presented to explain the increase of the reaction rate in
the presence of DBU.10f
Figure 1. Effect of the ionic liquid [bmim][PF6] on reaction rates. Reactions
were performed on a 1 mmol scale, in 100 ml solvent using a ratio of 1.1:1
methyl acrylate/benzaldehyde at room temperature.
Seeking for further improvement on the reaction rate using
DABCO 0100 mol%) in [bmim][PF6], several Lewis acids
in 10 mol% to 15 mol% were tested as presented in Fig. 1
and Table 1 0Entries 8 to 17). An increase of the reaction
rate by 1.6 times was observed only in the case of LiClO4.
Other Lewis acids lead to a general decrease 0in the range of
0.4±0.6) which appears to be independent of the hardness
and af®nity of the Lewis acids. This inhibition may result
from a decrease in the amount of DABCO in solution due to
the interference of the Lewis acid with the base DABCO.
Not only the reaction rates, but also the isolated yields
obtained after 24 hin [bmim][PF 6] in the presence of
some selected Lewis acids, were lower than in the absence
of additive 0Table 2, Entries 5 to 9). Despite promoting an
increase of the reaction rate by 1.6 times, LiClO4 was the
only one that produced a lower yield 046%) after 24 h 0Table
2, Entry 9) which can be due to the reversible nature of the
reaction9b promoted by the system DABCO/LiClO4.
Table 1. Relative rates observed for methyl acrylate/benzaldehyde/
DABCO reaction in ionic solvents under various conditionsa
Entry
Solvent
T /8C
Additiveb
Relative ratec
1
2
3
4
5
6
7
8
CH3CN
rt
rt
rt
rt
50
3
250
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
no
no
no
no
no
no
no
1.0
14.1
11.1d
33.6
21.0
6.3
[bmim][BF4]
[bmim][PF6]
[bmim][PF6]
[bmim][PF6]
[bmim][PF6]
[bmim][PF6]
[bmim][PF6]
[bmim][PF6]
[bmim][PF6]
[bmim][PF6]
[bmim][PF6]
[bmim][PF6]
[bmim][PF6]
[bmim][PF6]
[bmim][PF6]
[bmim][PF6]
In order to test the generality of ionic liquids as media for
the Baylis±Hillman reaction, several aldehydes were
allowed to react withacrylates in [bmim][PF ], as shown
1.8
Me2OzBF3
Et2OzBF3
t-BuMeOzBF3
SnCl4
13.1
13.8
12.1
21.4
52.8
18.8
14.4
16.1
21.8
19.5
9
6
10
11
12
13
14
15
16
17
in Table 2. Aliphatic aldehydes gave poor yields 014±20%,
Entries 11 to 13), while moderate to high yields were
obtained witharomatic aldehydes 039±72%, Entries 5, 10
and 16±19), which is in line with what is usually observed
for this reaction.7 In the case of benzaldehyde, the isolated
yield of 2 in [bmim][PF6] was close to that of the reaction
performed in the absence of solvent and considerably higher
than in acetonitrile 0Table 2, Entries 1, 2 and 5). Using
3,4,5-trimethoxybenzaldehyde, the yield of 2 061%) in
[bmim][PF6], in the presence of 50 mol% of DABCO for
48 h, is considerably higher than the one reported 05%) in
the absence of solvent and in the presence of a large excess
of methyl acrylate11 0Table 2, Entries 20 and 21).
e
LiClO4
Zn0OTf)2
Cu0OTf)2
Sc0OTf)3
La0OTf)3
Yb0OTf)3
a
All reactions were carried out on a 1.0 mmol scale, in 100 ml solvent
using a ratio of 1.1:1:1 methyl acrylate/benzaldehyde/DABCO.
10 mol% of additive were used.
Relative rates were measured by comparison against initial rate constants
0t#300 min).
20 mol% DABCO were used.
b
c
d
e
15 mol% of LiClO4 were used.

J. N. Rosa et al. / Tetrahedron 57 ,2001) 4189±4193
Table 2. Preparative Bayllis-Hillman reactions in ionic media
4191
addition of Lewis acids. Nevertheless, the reaction in
ionic liquids was shown to perform well with aromatic alde-
hydes as substrates. The ionic liquid medium [bmim][PF6]
can be recycled after the reaction and reused without
reduction of reaction yields.
Entry
Aldehyde
R0
Solvent
Yield 0%)
3. Experimental section
3.1. General remarks
1
2
3
4
5
6
7
8
PhCHO
PhCHO
PhCHO
PhCHO
PhCHO
PhCHO
PhCHO
PhCHO
Me
Me
Me
Me
Me
Me
Me
Me
Me
t-Bu
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
CH3CN
Neat
Neat
35
68
69a
57
[bmim][BF4]
[bmim][PF6]
[bmim][PF6]
[bmim][PF6]
[bmim][PF6]
[bmim][PF6]
[bmim][PF6]
[bmim][PF6]
[bmim][PF6]
[bmim][PF6]
[bmim][PF6]
[bmim][PF6]
[bmim][PF6]
[bmim][PF6]
[bmim][PF6]
[bmim][PF6]
[bmim][PF6]
[bmim][PF6]
65
57b
61c
33d
46e
42
All glassware was oven dried and cooled in a desiccator
0P2O5 desiccant) prior to use. Commercially supplied
reagents were used as supplied, except for benzaldehyde,
cinnamaldehyde, furfuraldehyde and methyl acrylate, which
were distilled and stored under an argon atmosphere and
protected from light. Acetonitrile was distilled over calcium
hydride powder under argon. 1-n-Butyl-3-methylimidazo-
lium tetra¯uoroborate 0[bmim][BF4]) and 1-n-butyl-3-
methylimidazolium hexa¯uorophosphate 0[bmim][PF6])
were prepared following reported procedures.6b,12
9
PhCHO
PhCHO
10
11
12
13
14
15
16
17
18
19
20
21
CH30CH2)2CHO
0CH3)2CHCHO
c-C6H11CHO
4-ClC6H4CHO
4-MeOC6H4CHO
4-MeC6H4CHO
PhCHˆCHCHO
2-furaldehyde
3,4,5-0MeO)3C6H2CHO
3,4,5-0MeO)3C6H2CHO
3,4,5-0MeO)3C6H2CHO
20
14
20
72
39
61
43
68
1H and 13C NMR spectra were recorded on a BruÈker AMX
400 spectrometer. Chemical shifts are reported down®eld
in parts per million 0ppm) from a tetramethyl silane taken
as the reference. Infrared spectra 0IR) were recorded on a
PerkinElmer FTIR 683 or a Buck Scienti®c M-500
spectrometers as thinly dispersed ®lms 0from dichloro-
methane).
72
61f
40g
All reactions were carried out on a 5.0 mmol scale, in 500 ml solvent using a
ratio of 1.1:1:1 acrylate /aldehyde/DABCO at room temperature for 24 h,
except for entries 14±21 which were performed on 1.0 mmol scale, in
100 ml solvent.
a
Using 11 mol% of DABCO for 3 days.
Using 50 mol% of Cu0OTf)2.
Using 50 mol% of Zn0OTf)2.
Using 50 mol% of Sc0OTf)3.
Using 50 mol% of LiClO4.
Using 50 mol% of DABCO for 48 h.
Using 50 mol% of DABCO for 24 h.
b
c
Gas liquid chromatography 0GLC) was carried out on a
United Technologies Packard 437A gas chromatograph
using N2 as carrier gas and carbowax 20M over chromosorb
101 as stationary phase. Flash chromatography column was
carried out using MN Kieselgel 60M gel 040±63 mm, Art.
815381). All eluents were distilled prior to use. Preparative
and analytical thin layer chromatography 0TLC) was carried
out using MN Kieselgel G/UV254 0Art. 816320) glass
backed plates and MN Alugramw SIL G/ UV254 0Art.
818133), respectively. The plates were visualized using
ultraviolet light 0254 nm).
d
e
f
g
To evaluate the possibility of recycling the ionic liquid used
for the reaction, p-chlorobenzaldehyde, methyl acrylate and
DABCO in [bmim][PF6] were allowed to react in the ionic
solvent for 24 h and then the products were extracted with
ethyl ether. A second amount of aldehyde, acrylate and
DABCO was added and the process was repeated up to 4
times, as shown in Table 3. The increase in the isolated
yields after the ®rst cycle can be attributed to an
accumulation in the amount of DABCO, uncompleted
extracted with diethyl ether.
3.2. General procedure for the Baylis±Hillman reaction
followed by GLC )Table 1 and Fig. 1)
Methyl acrylate 0100 ml, 1,1 mmol) was added to a stirred
mixture of benzaldehyde 0100 ml, 1.0 mmol), 1,4-diazabi-
cyclo[2,2,2]octane 0DABCO) 0112 mg, 1.0 mmol) and
additive 0Lewis acid, 0.1 mmol) in classic solvent or ionic
liquid, at 0±508C under argon atmosphere, and the mixture
was further stirred for at least 24 h at the same temperature.
The course of the reaction was followed by GLC, by taking
5 ml samples from the reaction media at determined time
intervals, and diluting with dichloromethane 01 ml)
followed by injection in the gas chromatograph 0carrier
gas ¯ow: 25 ml/min, oven Tˆ1908C). The convertion of
the reaction for each sample was determined by comparing
the peak areas for the Baylis±Hillman product 2 0tRˆ4.7
min) and for unreacted benzaldehyde 0tRˆ1.2 min) using a
detector response factor of 1.1 relative to mol benzaldehyde/
mol product 2.
In summary, a considerable increase in the reaction rate of
the Baylis±Hillman reaction was observed in ionic liquid
[bmim][PF6] when compared to acetonitrile, the reported
optimal conventional solvent. In this reaction no signi®cant
improvements in reaction yield were observed by further
Table 3. Reuse of the ionic liquid [bmim][PF6]
Cycle
Yield 0%)
Cycle
Yield 0%)
1
2
53
69
3
4
59
76
All cycles were carried out on a 2 mmole scale, in 200 ml [bmim][PF6]
using a ratio of 1.1:1:1 methyl acrylate/p-chlorobenzaldehyde/DABCO at
room temperature for 24 h.

4192
J. N. Rosa et al. / Tetrahedron 57 ,2001) 4189±4193
3.3. General procedure for preparative Baylis±Hillman
reaction )Table 2)
1489, 1438, 1407, 1284, 1195, 1150, 1090, 1042, 1014,
961 cm21. 13C NMR 0100 MHz, CDCl3, 258C): d 51.9,
72.6, 126.4, 128.0, 128.6, 139.8, 141.7, 166.7. Other spec-
tral data were identical to those previously reported.14
Alkyl acrylate 05.5 mmol for Entries 1±13 and 1.1 mmol for
Entries 14±21) was added to a stirred solution of aldehyde
05 mmol for Entries 1±13 and 1 mmol for Entries 14±21),
1,4-diazabicyclo[2,2,2]octane 0DABCO) 0560 mg, 5 mmol
for Entries 1±13 and 112 mg, 1 mmol for Entries 14±21)
and Lewis acid 00.5 mmol, only for Entries 6±9) in 500 ml
0Entries 1, 4±13) or 100 ml of solvent 0usually
[bmim][PF6]), at room temperature. The resulting mixture
was further stirred for 24 h at room temperature under argon
atmosphere. The reaction medium was diluted with
dichloromethane 010 times the volume of solvent used for
the reaction) and the resulting solution was puri®ed on a
TLC plate or ¯ash chromatography column 0silica gel).
3.3.7.
Methyl
3-hydroxy-3-) p-methoxyphenyl)-2-
methylenepropanoate )Table 2, Entry 15). p-Methoxy-
benzaldehyde 0122 ml) and methyl acrylate 0100 ml) were
used in 1 mmol scale. Puri®cation by TLC using n-hexane/
ethyl acetate 3:1 afforded the title compound as a colourless
oil 087 mg, 39%); IR 0neat): 3470, 3000, 2952, 2836, 1720,
1610, 1511, 1439, 1394, 1250, 1174, 1148, 1033, 958 cm21.
13C NMR 0100 MHz, CDCl3, 258C): d 51.8, 55.1, 72.5,
113.8, 125.5, 128.0, 133.6, 142.3, 159.3, 166.9. Other spec-
tral data were identical to those previously reported.15
3.3.8. Methyl 3-hydroxy-3-)p-methylphenyl)-2-methylene-
propanoate )Table 2, Entry 16). p-Methylbenzaldehyde
0118 ml) and methyl acrylate 0100 ml) were used in
1 mmol scale. Puri®cation by TLC using n-hexane/ethyl
acetate 3:1 afforded the title compound as a colourless oil
0119 mg, 61%); IR 0neat): 3447, 3024, 2951, 2922, 1722,
1629, 1512, 1438, 1397, 1275, 1195, 1149, 1040, 957 cm21.
13C NMR 0100 MHz, CDCl3, 258C): d 20.9, 51.8, 73.0,
125.9, 126.6, 129.2, 137.6, 138.5, 142.2, 166.9. Other spec-
tral data were identical to those previously reported.14
3.3.1. Methyl 3-hydroxy-3-phenyl-2-methylenepropano-
ate )Table 2, Entry 5). Benzaldehyde 0500 ml) and methyl
acrylate 0500 ml) were used in a 5 mmol scale. Puri®cation
by ¯ash chromatography using n-hexane/ethyl acetate 9:1
afforded the title compound as a colourless oil 0615 mg,
65%). Spectral data were identical to those previously
reported.10m
3.3.2. t-Butyl 3-hydroxy-3-phenyl-2-methylenepropano-
ate )Table 2, Entry 10). Benzaldehyde 0500 ml) and t-butyl
acrylate 0726 ml) were used in 5 mmol scale. Puri®cation by
¯ash chromatography using n-hexane/ethyl acetate 19:1
afforded the title compound as a colourless oil 0492 mg,
42%). Spectral data were identical to those previously
reported.9b
3.3.9. Methyl 3-hydroxy-5-phenyl-2-methylene-4-penteno-
ate )Table 2, Entry 17). Cinnamaldehyde 0126 ml) and
methyl acrylate 0100 ml) were used in 1 mmol scale. Puri-
®cation by TLC using n-hexane/ethyl acetate 3:1 afforded
the title compound as a colourless oil 094 mg, 43%); Spec-
tral data were identical to those previously reported.10m,15,16
3.3.3. Methyl 3-hydroxy-2-methylenehexanoate )Table 2,
Entry 11). Butyraldehyde 0451 ml) and methyl acrylate
0500 ml) were used in 5 mmol scale. Puri®cation by
¯ash chromatography using n-hexane/ethyl acetate 9:1
afforded the title compound as a colourless oil 0182 mg,
20%). Spectral data were identical to those previously
reported.13
3.3.10. Methyl 3-)2-furyl)-3-hydroxy-2-methylenepro-
panoate )Table 2, Entry 18). Furaldehyde 083 ml) and
methyl acrylate 0100 ml) were used in 1 mmol scale.
Puri®cation by TLC using n-hexane/ethyl acetate 17:3
afforded the title compound as a colourless oil 0124 mg,
68%); IR 0neat): 3440, 3120, 2953, 1721, 1633, 1500,
1439, 1385, 1284, 1197, 1152, 1040, 1012, 955 cm21.
Other spectral data were identical to those previously
reported.15
3.3.4. Methyl 3-hydroxy-4-methyl-2-methylenepentano-
ate )Table 2, Entry 12). Isobutyraldehyde 0456 ml) and
methyl acrylate 0500 ml) were used in 5 mmol scale.
Puri®cation by ¯ash chromatography using n-hexane/ethyl
acetate 9:1 afforded the title compound as a colourless oil
0121 mg, 14%). Spectral data were identical to those
previously reported.10m
3.3.11. Methyl 3-hydroxy-3-)3,4,5-trimethoxyphenyl)-2-
methylenepropanoate )Table 2, Entry 19). 3,4,5-
Trimethoxybenzaldehyde 0196 mg) and methyl acrylate
0100 ml) were used in 1 mmol scale. Puri®cation by TLC
using n-hexane/ethyl acetate 3:2 afforded the title
compound as a colourless oil10b 0203 mg, 72%); IR 0neat):
3487, 2998, 2942, 2838, 1720, 1629, 1593, 1504, 1461,
3.3.5. Methyl 3-cyclohexyl-3-hydroxy-2-methylenepro-
panoate )Table 2, Entry 13). Cyclohexanecarboxaldehyde
0606 ml) and methyl acrylate 0500 ml) were used in 5 mmol
scale. Puri®cation by ¯ash chromatography using n-hexane/
ethyl acetate 9:1 afforded the title compound as a colourless
oil 0198 mg, 20%). Spectral data were identical to those
previously reported.10c,10m
1
1421, 1327, 1233, 1192, 1126, 1056, 1005 cm21. H NMR
0400 MHz, CDCl3, 258C): d 3.57 0s, 3H, CO2CH3), 3.74 0s,
3H, ArOCH3), 3.76 0s, 6H, Ar0OCH3)2), 5.43 0s, 1H,
HOCH), 5.81 0s, 1H, vCH), 6.25 0s, 1H, vCH), 6.52 0s,
2H, ArH). 13C NMR 0100 MHz, CDCl3, 25 8C): d 51.8, 55.9,
60.6, 73.0, 103.6, 126.0, 137.0, 137.4, 142.0, 153.2, 167.0.
3.3.6. Methyl 3-) p-chlorophenyl)-3-hydroxy-2-methylene-
propanoate )Table 2, Entry 14). p-Chlorobenzaldehyde
0141 mg) and methyl acrylate 0100 ml) were used in
1 mmol scale. Puri®cation by TLC using n-hexane/ethyl
acetate 9:1 afforded the title compound as a colourless oil
0163mg, 72%); IR 0neat): 3448, 2952, 1716, 1630, 1596,
3.4. Recycling assays
Methyl acrylate 0200 ml, 2.2 mmol) was added to a stirred
solution of p-chlorobenzaldehyde 0281 mg, 2 mmol) and
DABCO 0224 mg, 2 mmol) in 200 ml of [bmim][PF6], at

J. N. Rosa et al. / Tetrahedron 57 ,2001) 4189±4193
4193
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room temperature. The resulting mixture was further stirred
for 24 hat room temperature. The reaction medium was
extracted with diethyl ether 02£4 ml) for 2£1 hand the
volume of the combined ethereal extracts was reduced to
about 3 ml under reduced pressure. The resulting solution
was applied on a TLC plate 0silica gel) and eluted with
n-hexane/ethyl acetate 9:1. A new portion of reactants and
DABCO was added to the recycled [bmim][PF6], and the
cycle was repeated.
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Basavaiah,
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