Novel Br?nsted acidic deep eutectic solvent as reaction media for esterification of carboxylic acid with alcohols

Article abstract of DOI:10.1016/j.tetlet.2012.07.063

New halogen-free Br?nsted acidic deep eutectic solvents (DES) have been prepared by mixing new quaternary ammonium methanesulfonate salts with p-toluenesulfonic acid (PTSA). They have been used as dual solvent-catalyst for esterification of several carboxylic acids with different alcohols with a reagent molar ratio of 1:1. The method is mild, safe, and simple. Ease of recovery and reusability of DES with high activity makes this method efficient and eco-friendly. The tunability of DES properties, attained by changes in the cation, was performed in order to achieve various esters in good yields.

Full text of DOI:10.1016/j.tetlet.2012.07.063

Tetrahedron Letters 53 (2012) 5151–5155
Contents lists available at SciVerse ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Novel Brønsted acidic deep eutectic solvent as reaction media
for esterification of carboxylic acid with alcohols
Valerio De Santi a, Fabio Cardellini a, Lucia Brinchi b, Raimondo Germani a,
⇑
a CEMIN, Centro di Eccellenza Materiali Innovativi Nanostrutturati, Dipartimento di Chimica, Università di Perugia, via Elce di Sotto 8, Perugia I-06123, Italy
b CRB, Centro di Ricerca sulle Biomasse, Dipartimento di Ingegneria Industriale, via G Duranti, Perugia I-06125, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
New halogen-free Brønsted acidic deep eutectic solvents (DES) have been prepared by mixing new qua-
ternary ammonium methanesulfonate salts with p-toluenesulfonic acid (PTSA). They have been used as
dual solvent-catalyst for esterification of several carboxylic acids with different alcohols with a reagent
molar ratio of 1:1. The method is mild, safe, and simple. Ease of recovery and reusability of DES with high
activity makes this method efficient and eco-friendly. The tunability of DES properties, attained by
changes in the cation, was performed in order to achieve various esters in good yields.
Ó 2012 Elsevier Ltd. All rights reserved.
Received 6 March 2012
Revised 11 July 2012
Accepted 13 July 2012
Available online 20 July 2012
Keywords:
Deep eutectic solvent
Brønsted acid
p-Toluenesulfonic acid
Fischer esterification
Ionic liquids
Synthesis of organic esters has a main role in organic synthesis
from its infancy, due to the utility of esters as products or interme-
diates in diverse fields, both in the laboratory and in industry.1
Most of the recent research work dealing with esterification has
been associated with green chemistry: replacement of conven-
tional organic solvents by new reaction media such as, for example,
water or ionic liquids is often studied, as shown in the review of
Otera and Nishikido.1 Condensation between carboxylic acids and
alcohols catalyzed by p-toluenesulfonic acid (PTSA) takes place
smoothly in imidazolium ionic liquids, but only when using dried
IL.2 A new approach for esterification has involved the use of newly
synthesized Brønsted acid ILs as dual solvent-catalysts.3–5 A high
temperature (110 °C) has been often used, and a few of the ILs con-
tain halogen atom which may cause serious concerns.6 Moreover,
the preparation of ILs often requires the use of organic solvents,
and heat supply,5,7 and cost can be high.
Deep eutectic solvents have recently appeared in the horizon as
a new generation of ionic solvents of low cost; some authors refer
to them as ‘advanced ILs’.8 They are generally formed by the mix-
ture of hydrogen bond donor (HBD) molecules with halide salts,
which produces liquids with physical and solvent properties com-
parable to ILs.9,10 DES are easy to prepare in a pure state from low
cost starting materials, and they should be tunable without organic
synthesis of new compounds. The principle of ‘creating’ DES by
complexing a halide salt was first demonstrated by Abbott for
mixtures of quaternary ammonium salts with a range of amides9
and later extended to a wide variety of other HBD such as acids,
amines, and alcohols, including glycerol.11 Systems based on cho-
line chloride (ChCl) have been shown to be good solvents for chem-
ical and biochemical transformations12,13 and also to have practical
use in several applications, such as metal oxide processing,14 elec-
tropolishing,15 and extraction of glycerol from biodiesel.16 Prasad
and coworkers have recently used DES formed by ChCl + ZnCl2 as
a Lewis acid catalyst as well as reaction medium to prepare long
chain wax esters in high yields, although they use high tempera-
ture (110 °C) with need of heat supply and the system contains
several halogen atoms, and also metal atom, which can cause
concerns.17
Here we report a novel class of DES that is designed to be a
strong Brønsted acid. It is without any halogen or metal atom,
and it is formed upon mixing a quaternary ammonium salt bearing
methanesulfonate as the counterion, with p-toluenesulfonic acid
(PTSA). Trimethylcyclohexyl ammonium methanesulfonate (TCy-
AMsO), trimethylbenzyl ammonium methanesulfonate (TBn-
AMsO), trimethyloctyl ammonium methanesulfonate (TOAMsO),
and trimethylcyclohexyl ammonium p-toluenesulfonate (TCyA-
Tos), shown in Scheme 1, are the quaternary ammonium salts that
were used.
⇑
Corresponding author. Tel.: +39 075 5855632; fax: +39 075 5855560.
E-mail address: germani@unipg.it (R. Germani).
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.07.063

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V. De Santi et al. / Tetrahedron Letters 53 (2012) 5151–5155
CH3
-
-
CH3SO3
CH3
CH3
CH3SO3
N
N
CH3
CH3
CH3
TCyAMsO
TBnAMsO
-
SO3
CH3
CH3
CH3
-
CH3SO3
N
CH3
CH3
CH3
N
CH3
TOAMsO
TCyATos
CH3
Scheme 1. Trimethylcyclohexyl ammonium methanesulfonate (TCyAMsO), trimethylbenzyl ammonium methanesulfonate (TBnAMsO), trimethyloctyl ammonium
methanesulfonate (TOAMsO), trimethylcyclohexyl ammonium p-toluenesulfonate (TCyATos).
They were prepared with a procedure that avoids several draw-
backs often encountered in organic synthesis. Our procedure is
similar to the preparation of published 1,3-dialkylimidazolium
methanesulfonate18,19 and gives high yield of product in short time
in only one step, without any byproduct and without the need of
energy supply since the reaction itself – exothermic - provides
the necessary heat.
80 °C for our systems showed no significant change in viscosity
with variations of the cation, but a relatively higher value was
measured for TCyATos, indicating an effect of the anion. This can
probably be related to the presence of an aromatic moiety in the
anion of TCyATos, and its interaction with the same aromatic moi-
ety of PTSA. For TBnAMsO, a liquid at room temperature, viscosity
was measured over a range of temperatures and the profile is re-
ported in Figure 2. The significant change of viscosity with temper-
ature is also consistent with the data in the literature for DES.9,10,22
Herein we show the use of our new DES as the dual solvent-cat-
alyst in an efficient, simple, and clean method for esterification of
carboxylic acids with alcohol, under mild conditions, with
facile isolation of products and reuse of the reaction medium§
(Scheme 2).
Table 1 shows the results of optimized conditions for the reac-
tion of lauric acid with methanol, chosen as a model reaction (Ta-
ble 1, entry 1). The ester methyl laurate was obtained
quantitatively (97% by g.c.) in 2 h at 60 °C with the ionic eutectic
solvent TCyAMsO-PTSA used as a minor component in the reaction
mixture, that is, DES/acid = 1/3.3. In the absence of DES, and using a
solution of PTSA in water as the reaction medium, the product
yield was only 16% (Table 1, entry 3). On the other hand, the sys-
tem DES formed by ChCl (choline chloride) and ZnCl2, which was
successful for long chain wax esters at 110 °C, gave no product un-
der our milder conditions (Table 1, entry 2). The product was easily
isolated: in fact DES and water remained in the lower phase while
the ester product remained in the upper liquid phase and it was
simply pipetted off using a Pasteur pipette. This phase separation
shifts the equilibrium toward product formation in DES.
DES were simply prepared from TCyAMsO, TBnAMsO, TOAMsO
or TCyATos, and p-toluenesulfonic acid monohydrate,à and were
used immediately after, without any treatment. The mixtures of
TCyAMsO, TBnAMsO, TOAMsO, and TCyATos, with PTSA form
eutectics at a molar ratio of ammonium salt/PTSA of 1. The melting
points of some mixtures are reported in Figure 1. A significant
depression of the melting point is observed
Viscosities of the eutectics were measured;20 in fact viscosity is
an important property of ionic liquids (and DES) and a relatively
low viscosity is considered to be a distinctive property between io-
nic liquids and molten salts, which are highly viscous.21
Values of viscosity,
g, at 80 °C are 95, 75, 65, and 170 cP for,
respectively, TCyAMsO, TBnAMsO, TOAMsO, and TCyATos, with
PTSA at a molar ratio of ammonium salt/PTSA of 1. These values
are relatively low, considering that generally the viscosity values
were found to cover the range 50–5000 cP for DES and for ionic liq-
uids.10 These preliminary data relating to values of viscosity at
TCyAMsO was prepared from N,N-dimethylcyclohexylamine and methyl meth-
anesulfonate, with ethyl acetate as the solvent. The reaction is exothermic and the
mixture reached reflux without external energy supply. After cooling to r.t. a white
solid formed, that was washed twice with diethyl ether, and dried in vacuum to give a
white crystalline solid, with m.p. 224–225 °C. Yield is 95%. 1H NMR (200 MHz, CD3OD)
d = 1.1–2.3 (m, 10 H, 5 CH2); 2.69 (s, 3 H, CH3SO3À); 3.06 (s, 9 H, 3 CH3–N+); 3.37 (m, 1
H, R-CH-N+). TOAMsO was prepared similarly from N,N-dimethyloctylamine. A white
crystalline solid, with m.p. 178–180 °C was obtained in 94% yield. 1H NMR (200 MHz,
CD3OD) d = 0.88 (t, 3 H, R-CH3); 1.33 (m, 10 H, 5 CH2); 1.78 (m, 2 H, CH2-C-N+); 2.69 (s,
3 H, CH3SO3-); 3.11 (s, 9 H, 3 CH3-N+); 3.31 (m, 2 H, R-CH2-N+). TBnAMsO was
prepared from dimethylbenzylamine and methyl methanesulfonate; the white solid,
with m.p. 222–223 °C, was formed in a yield of 93%. 1H NMR (200 MHz, CD3OD):
d = 2.69 (s, 3 H, CH3SO3-); 3.10 (s, 9 H, 3 CH3-N+); 4.52 (s, 2H, CH2-Ar); 7.56 (s, 5H, Ar).
TCyATos was prepared similarly from N,N-dimethylcyclohexylamine and methyl p-
toluenesulfonate. The white solid with m.p. 241–243 °C was obtained in a yield of
92%. 1H NMR (200 MHz, CD3OD) d = 1.1–2.3 (m, 10 H, 5 CH2); 2.36 (s, 3H, CH3Ar); 3.06
(s, 9H, 3CH3); 3.37 (m, 1H, CH-N+); 7.23 (d, 2H, Ar); 7.70 (d, 2H, Ar).
The procedure was tested with several other acids, also shown in
Table 1. Yields reported are based on g.c. analysis but yields by
weight were also evaluated in some cases, and were comparable
with the g.c. data. Data in Table 1 show how the reaction becomes
more difficult as the acid chain length increases (Table 1, entries 5,
§
DES was used right after its preparation. Equimolar amounts (4.5 mmol) of acid
and alcohol were added, and the resulting mixture, was heated to 60 °C (or 80 °C if
specified) and magnetically stirred for the specified amount of time. Initially the
reaction mixture is homogeneous and fluid, and then a heterogeneous system formed
as reaction proceeded, due to insolubility of the esters produced in the DES. For the
g.c. analysis further elaboration was as follows. At the end of the reaction,
tbutylbenzene was added, as the internal standard, to the mixture, which was then
extracted with diethyl ether. Organic layer was washed with NaHCO3, dried over
Na2SO4 and analyzed by g.c.
à
Equimolar amounts of quaternary ammonium salt (1.5 mmol) and p-toluenesul-
fonic acid monohydrate (Sigma–Aldrich, 98,5+% used as received) were mixed in a
screw-capped 3 ml vial. The mixture was magnetically stirred and heated to 60 °C
until a clear colourless liquid was obtained (about 10 min).

V. De Santi et al. / Tetrahedron Letters 53 (2012) 5151–5155
5153
250
200
150
100
50
Anyway the use of TCyAMsO-PTSA gave generally good yields
(>75%) in 2 h and at 60 °C, as shown in Table 1, entries 11, 12,
13. For secondary alcohols the Fischer reaction is generally more
difficult due to steric hindrance23 and yields were no higher than
ca. 50% for the reaction of lauric acid with cyclohexanol or isopro-
panol (Table 1, entries 14, 17). In these cases we attempted to
accelerate the reaction and improve the yield both increasing the
reaction time and rising the temperature. We got yields >75% in
6 h and at 80 °C, as shown in entries 16 and 19 of Table 1. We tried
to extend the method to hindered acids, and tried the reaction of n-
octanol with isobutyric acid and trimethylacetic acid, using the
reaction with acetic acid for comparison. We got good yields also
with hindered acids in 2 h with a temperature of 80 °C (Table 1, en-
tries 20–22).
The Fischer esterification rate for reaction of benzoic acid with
methanol is known to be lower than reaction of aliphatic acid,
due to electronic effects.23 In fact it gave a yield of only 40% in TCy-
AMsO-PTSA in 2 h at 60 °C. The yield was 55% raising the tempera-
ture to 80 °C, as shown in Table 2, entries 1, 2. As for esterification of
lauric acid, also for benzoic acid in the absence of DES, and using a
solution of PTSA in water as the reaction medium, the product yield
was low, only 22% (Table 2, entry 3). The use of the longer alcohol
n-octanol gave even worse yields (Table 2, entry 8, 9), as expected
because methanol is generally the most reactive alcohol. Attempts
to improve yields of the reaction of benzoic acid were carried out.
The use of longer reaction times at 80 °C gave interesting results.
In the case of reaction with methanol, data in Table 2 show the exis-
tence of an optimum reaction time, that is at 4 h, (yield 69%, Table 2,
entry 4), because for both shorter and longer reaction time the yield
decreased (Table 2, entries 2, 5, 6). On the other hand, for n-octanol
the yield monotonically increased with reaction time, and the best
yield obtained is 86% at 17 h (Table 2, entries 9–12). Experiments
carried out to verify the extent of the reverse reactions showed that
in the case of methyl benzoate the hydrolysis significantly occurred.
In fact if methyl benzoate was added to a system containing DES
(formed by TCyMsO/PTSA) and water in equimolar amounts with
the ester (water is a Fischer esterification product) the methyl ester
content decreased by 15% in 2 h.
For reaction of benzoic acid with both methanol and n-octanol
yields were improved using DES formed by ammonium salts bear-
ing an aromatic moiety. For the reaction with n-octanol, the use of
DES formed by TBnAMsO gave a good yield of octyl benzoate, 72%,
at 80 °C in 4 h (Table 2, entry 13), and the use of DES formed by
TCyATos was even better, leading to an yield of 88% at 80 °C in
4 h (Table 2, entry 14). Also in this case, we can see how useful
the tunability of DES is. The use of DES formed by TCyATos was also
tried to improve the reaction of benzoic acid with methanol, and
the yield does not vary significantly with respect to reaction in
TCyAOMs (Table 2, entry 7). No attempt of yield improvement,
with variation of reaction times, was investigated.
The method was extended to the reaction in which both the
acid and the alcohol are solids, and the reaction of benzoic acid
and 2-(4-hydroxyphenyl)-ethanol was carried out. The reaction
medium TCyAMsO-PTSA worked well, became homogeneous and
limpid at 80 °C and the product formed. Anyway the yield (deter-
mined by weight) is not high (Table 2, entry 15) although 17 h of
reaction time were used. Furthermore, in this case the use of DES
bearing an aromatic moiety, TCyATos-PTSA, led to a decreased
yield (Table 2, entry 16), meaning that the effect of changes in
the DES structure on the reaction yield is dependent on the reagent
structures. The method was also extended to aliphatic acid bearing
an aromatic moiety. Reaction of phenylacetic acid with methanol
was carried out. Results were rather good with TCyAMsO-PTSA
(Table 2, entry 17), and were further improved using TCyATos-
PTSA with a 82% yield (Table 2, entry 18).
0
0.0
0.2
0.4
0.6
0.8
1.0
x, PTSA molar fraction
Figure 1. Variations in melting points with composition for mixture of the various
ammonium salts with PTSA: TCyAMsO (.); TOAMsO (s); TCyATos (h); TBnAMsO
(j).
1600
1400
1200
1000
800
600
400
200
0
30
40
50
60
70
80
Temperature, ºC
Figure 2. Variations in viscosity with temperature for TBnAMsO/PTSA 1:1. The line
is drawn to guide the eye.
O
O
O
R1-OH
H2O
R
R
R1
OH
Scheme 2. Esterification reaction.
7). Attempts to increase the yield were carried out: an increase in
reaction time (Table 1, entry 8) and an increase in temperature up
to 80 °C (Table 1, entry 9) were useful, but yields became quantita-
tive in 2 h and 60 °C using DES formed by a more hydrophobic
quaternary ammonium salt, showing how useful is the tunability
of the DES. The use of TOAMsO is shown in entries 6 and 10 of Table
1.
The method was extended to the use of different alcohols. Pri-
mary alcohols gave good yields: as already observed for the acids,
also for alcohols an increase in the chain length, from methanol to
ethanol, n-butanol, and n-octanol, leads to more difficult reactions.

5154
V. De Santi et al. / Tetrahedron Letters 53 (2012) 5151–5155
Table 1
Fischer esterification of aliphatic acids with alcohols in DES formed by PTSA and various quaternary ammonium saltsa
Entry
Acid
Alcohol
Quaternary Ammonium salt in the DES
Time (h)
Temperature (°C)
Yieldb (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
Lauric
Lauric
Lauric
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
TCyAMsO
2
2
2
2
2
2
2
4
2
2
2
2
2
2
2
6
2
2
6
2
2
2
60
60
60
60
60
60
60
60
80
60
60
60
60
60
80
80
60
80
80
60
80
80
97 (94)c
—
ChCl ZnCl2
d
—
16
Octanoic
Palmitic
Palmitic
Stearic
Stearic
Stearic
Stearic
Lauric
Lauric
Lauric
Lauric
Lauric
Lauric
Lauric
Lauric
Lauric
Acetic
Isobutyric
Trimethylacetic
TCyAMsO
TCyAMsO
TOAMsO
TCyAMsO
TCyAMsO
TCyAMsO
TOAMsO
TCyAMsO
TCyAMsO
TCyAMsO
TCyAMsO
TCyAMsO
TCyAMsO
TCyAMsO
TCyAMsO
TCyAMsO
TCyAMsO
TCyAMsO
TCyAMsO
90 (85)c
77
93
55
54
71
97
81
78
74
42
59
85
50
66
72
96
85
MeOH
MeOH
EtOH
n-BuOH
n-OctOH
Cyclohexanol
Cyclohexanol
Cyclohexanol
Isopropanol
Isopropanol
Isopropanol
n-OctOH
n-OctOH
n-OctOH
73
a
b
c
4.5 mmol of acid; carboxylic acid: alcohol: DES = 1:1:0.3.
Yield in mol % by gc, i.s. tButylbenzene.
Yield by weight in parentheses.
d
The reaction medium is a solution of PTSA (1.5 mmol) in water (0.70 ml).
Table 2
Fischer esterification of acids bearing aromatic moiety with alcohols in DES formed by PTSA and various quaternary ammonium saltsa
Entry
Acid
Alcohol
Quaternary ammonium salt in the DES
Time (h)
Temperature (°C)
Yield b(%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
Benzoic
Benzoic
Benzoic
Benzoic
Benzoic
Benzoic
Benzoic
Benzoic
Benzoic
Benzoic
Benzoic
Benzoic
Benzoic
Benzoic
Benzoic
Benzoic
Phenylacetic
Phenylacetic
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
n-OctOH
n-OctOH
n-OctOH
n-OctOH
n-OctOH
n-OctOH
n-OctOH
2-(4-Hydroxyphenyl)-ethanol
2-(4-Hydroxyphenyl)-ethanol
MeOH
TCyAMsO
TCyAMsO
—
2
2
2
4
6
17
4
2
2
4
60
80
80
80
80
80
80
60
80
80
80
80
80
80
80
80
80
80
40
53
22
69
58
59
65
19
41
55
64
86
72
88
60c
49c
65
82
d
TCyAMsO
TCyAMsO
TCyAMsO
TCyATos
TCyAMsO
TCyAMsO
TCyAMsO
TCyAMsO
TCyAMsO
TBnAMsO
TCyATos
TCyAMsO
TCyATos
TCyAMsO
TCyATos
6
17
4
4
17
17
2
MeOH
4
a
b
c
4.5 mmol of acid; carboxylic acid:alcohol:DES = 1:1:0.3.
Yield by gc, i.s. tButylbenzene.
Yield by weight; identification of ester produced by comparison with 1H NMR in the literature.24
The reaction medium is a solution of PTSA (1.5 mmol) in water (0.70 ml).
d
Last but not least, we used our model reaction for reusability
tests. It was possible to conduct the esterification of lauric acid
with methanol in TCyAMsO-PTSA for more consecutive cycles.–
Results reported in Table 3 show no loss of the dual catalyst-solvent
Table 3
Recycling of TCyAMsO-PTSA system in the
preparation of methyl lauratea
Cycle
Yieldb (%)
1
2
3
4
8
8
95
97
94
98
92c
36
–
For the test of reusability, as the reaction time was reached, the mixture was
decanted while hot. As two clearly separated phases are formed, the upper liquid
layer was spilled off with a Pasteur pipette and weighted, then analysed by gc, after
addition of the internal standard. The lower layer formed by DES and water was used
for 4 consecutive cycles without any treatment. A drying in vacuo at 60 °C overnight
was carried out on DES for further cycles. Tests for reagent/product entrapment in
DES were carried out: after the ester had spilled off, DES was extracted with
petroleum ether and thereafter with diethyl ether and extracts analysed by gc after
addition of the internal standard. Analysis evidenced no presence of the reagent lauric
acid, and small amounts (2–5%) of the ester methyl laurate produced.
a
4.5 mmol of acid; lauric acid: alcohol:
DES = 1:1:0.3.
Yield by gc, i.s. tButylbenzene.
b
c
After water removal from DES in vacuum
at 60 °C.

V. De Santi et al. / Tetrahedron Letters 53 (2012) 5151–5155
5155
7. Deetlefs, M.; Seddon, K. R. Green Chem. 2010, 12, 17–30.
functions of the DES for 4 consecutive cycles, with no treatment on
DES.
8. Gorke, J.; Scrienc, F.; Kazlauskas, R. Biotechnol. Bioprocess Eng. 2010, 15, 40–53.
9. Abbott, A. P.; Capper, G.; Davies, D. L.; Rasheed, R. K.; Tambyrajah, V. Chem.
Commun. 2003, 70–71.
10. Abbott, A. P.; Boothby, D.; Capper, G.; Davies, D. L.; Rasheed, R. K. J. Am. Chem.
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11. Abbott, A. P.; Harris, R. C.; Ryder, K. S.; D’Agostino, C.; Gladden, L. F.; Mantle, M.
D. Green Chem. 2011, 13, 82–90.
12. Dominguez de Maria, P.; Maugeri, Z. Current Opinion Chem. Biol. 2011, 15, 220–
225.
For further cycles the yield remained high only with a simple
treatment on DES consisting in water elimination from DES in
vacuo at 60 °C. In fact after 8 cycles the yield was still good, 92%,
but a test showed that the yield was quite lower (36%) if DES
had no treatment.
13. Chen, Z.; Zhu, W.; Zheng, Z.; Zou, X. J. Fluorine Chem. 2010, 131, 340–344.
14. Abbott, A. P.; Capper, G.; McKenzie, K. J.; Ryder, K. S. Electrochim. Acta 2006, 51,
4420–4425.
Acknowledgments
15. Abbott, A. P.; Capper, G.; Davies, D. L.; Rasheed, R.; Shikotra, P. Inorg. Chem.
2005, 44, 6497–6499.
16. Abbott, A. P.; Cullis, P. M.; Gibson, M. J.; Harris, R. C.; Raven, E. Green Chem.
2007, 9, 868–872.
17. Sunitha, S.; Kanjial, S.; Redyy, P. S.; Prasad, R. B. N. Tetrahedron Lett. 2007, 48,
6962–6965.
Support of this work by MIUR (Ministero dell’Istruzione,
dell’Università e della Ricerca Scientifica), Italy (PRIN 2008, Grant
No. 2008AZT7RK_002) is gratefully acknowledged. Professor N.
Spreti is gratefully acknowledged for measures of viscosity.
18. Brinchi, L.; Germani, R.; Savelli, G. Tetrahedron Lett. 2003, 44, 2027–2029.
19. Brinchi, L.; Germani, R.; Savelli, G. Tetrahedron Lett. 2003, 44, 6583–6585.
20. The viscosity of DES was measured with a Brookfield Dial Reading viscosimeter
RVT model. For each analysis, the sample was kept in a silicone oil bath at
constant temperature.
References and notes
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