Triethylsulfonium bistriflimide as the reaction medium in catalyzed and uncatalyzed cycloaddition [4 + 2]

Article abstract of DOI:10.1007/s10562-009-0211-9

Triethylsulfonium bistriflimide, [S_2.2.2][NTf₂], has been tested and compared with other ionic liquids and molecular solvents as a medium in Diels-Alder reaction between cyclopentadiene and dimethyl maleate. Triflates and chlorides of different metals have been combined with [S _2.2.2][NTf₂] and the catalytic activity of the systems formed have been determined. The effect of concentration of the catalysts in sulfonium ionic liquid and reactants on the yield and endo:exo ratio has been established. The representative catalyst-Yb(OTf)₃?xH ₂O in [S_2.2.2][NTf₂] has been examined in the reaction of cyclopentadiene with various dienophiles. The use of sulfonium ionic liquids permitted recycling the catalysts. For the best four catalytic systems, the products have been isolated.

Full text of DOI:10.1007/s10562-009-0211-9

Catal Lett (2010) 134:147–154
DOI 10.1007/s10562-009-0211-9
Triethylsulfonium Bistriflimide as the Reaction Medium
in Catalyzed and Uncatalyzed Cycloaddition [4 + 2]
Ewa Janus
Bo z_ ena Bittner
•
Received: 30 April 2009 / Accepted: 1 November 2009 / Published online: 14 November 2009
Ó Springer Science+Business Media, LLC 2009
Abstract Triethylsulfonium bistriflimide, [S2.2.2][NTf2],
has been tested and compared with other ionic liquids and
molecular solvents as a medium in Diels–Alder reaction
between cyclopentadiene and dimethyl maleate. Triflates
and chlorides of different metals have been combined with
Diels–Alder reaction was also one of the first and most
important reactions in which ionic liquids were used [7–
11]. Ionic liquids have extremely attractive properties.
They are characterized by a very low vapour pressure even
in high temperatures and high thermal stability [12] in
comparison to molecular solvents. Recently, some attempts
have been made to explain the efficiency of ionic liquids as
solvents in the Diels–Alder reaction. Solvent parameters of
various ionic liquids and molecular solvents have been
analysed. Attempts of understanding both, cation and anion
effect on the reaction rate and stereoselectivity have been
undertaken [13, 14]. Those studies have demonstrated that
both solvent and solute are important for the solvent effect.
The hydrogen bond donating character of the cation sub-
stituents or bulky cations of ionic liquid can play a role in
change of the stereoselectivity, however, each group of
dienophiles has to be considered individually. It has been
found that, higher stereoselectivities and rates of the Diels–
Alder reaction are obtained when Lewis acids added.
In recent years a merge of catalysts with ionic liquids has
become a very promising direction in catalysis. The first
examples of catalyst systems comprising of Lewis acids and
ionic liquids were chloroaluminates [15, 16]. However, they
were moisture sensitive and difficult to recover.
[
S2.2.2][NTf ] and the catalytic activity of the systems
2
formed have been determined. The effect of concentration
of the catalysts in sulfonium ionic liquid and reactants on
the yield and endo:exo ratio has been established. The
representative catalyst—Yb(OTf) ÁxH O in [S2.2.2][NTf2]
3
2
has been examined in the reaction of cyclopentadiene with
various dienophiles. The use of sulfonium ionic liquids
permitted recycling the catalysts. For the best four catalytic
systems, the products have been isolated.
Keywords Sulfonium ionic liquid Á Lewis acids Á
Cycloaddition Á Diels–Alder reaction
1
Introduction
The Diels–Alder reaction is one of the most powerful
methods for the synthesis of carbo- or heterocyclic com-
pounds. Water [1–3], superheated or supercritical water
[
4], supercritical carbon dioxide [5] and also macrocyclic
Abbott and co-workers prepared a range of Lewis-acidic
ionic liquids based on choline chloride and ZnCl or SnCl
hosts [6] have been tested as media of this reaction. The
2
2
[17]. Some of them were employed in the Diels–Alder reac-
tion resulting in high yields and stereoselectivities [18].
However, the undesired reaction between the alcohol side
chain and the carbonyl starting materials occurred in these
ionic liquids. This feature was mostly observed after the cat-
alyst was recycled. Moreover, the viscosity of Abbott’s sys-
tems was high and a co-solvent would have had to be used.
Mixtures of Lewis acid catalysts with neutral ionic liq-
uids containing [PF ], [BF ] or [SbF ] anions have been
Electronic supplementary material The online version of this
article (doi:10.1007/s10562-009-0211-9) contains supplementary
material, which is available to authorized users.
E. Janus (&) Á B. Bittner
Department of Organic Chemical Technology, West Pomeranian
University of Technology, Pulaski Str. 10, 70-322 Szczecin,
Poland
e-mail: ejanus@ps.pl
6
4
6
123

1
48
E. Janus, B. Bittner
studied. A common limitation for their use was the pres-
ence of water or strong Lewis acids, which caused
decomposition of the anions.
(trifluoromethylsulfonyl) imide were bought from Solvent
Innovation (Cologne, Germany), 1-butyl-3-methylpyr-
idinium bis (trifluoromethylsulfonyl) imide and 1-butyl-1-
methylpiperidinium bis (trifluoromethylsulfonyl) imide
were purchased from IoLiTec (Denzlingen, Germany). The
purity of ionic liquids was 98% and higher, as stated by the
specifications of the manufacturers. The rest of halides
anions and water were present as contaminations and could
have an effect on the reaction. Therefore, the ionic liquids
were washed with deionized water until halides were not
detectable in aqueous residues as indicated by potentio-
Ionic liquids proved to be good solvents in the catalyzed
reactions combined with bistriflimide anion, because of
their hydrophobic, noncoordinating property [19]. Imi-
dazolium salts have become most widely used in recent
years [20–28]. In our earlier studies [29], we showed that
the catalytic systems consisting of trihexyltetradecylpho-
sphonium bistriflimide and Lewis acids have been highly
effective and reusable.
As a continuation of our investigation of ionic liquids with
different cations, our attention was drawn to triethylsulfo-
nium bis (trifluoromethylsulfonyl) imide. Sulfonium ionic
liquids with bistriflimide anion [30, 31] exhibit low viscos-
ity, lower than ammonium, phosphonium and piperidinium
and comparable to imidazolium ionic liquids with the same
anion. A more recent report provided the information that the
metric titration with AgNO . Then the ionic liquids were
3
dried at 45 °C, under the pressure of 5 mbar for 24 h until
the water content was below 50 ppm. The water content in
ionic liquids and organic solvents was determined by
coulometric KF titration, using Metrohm 831 KF Coulo-
meter. The density of [S2.2.2][NTf ] was determined by
2
measuring the weight of ionic liquid in a pycnometer
(capacity of 5 ml). The dynamic viscosity was calculated
based on measurements of the kinematic viscosity with
micro-Ubbelohde viscometer (capillary diameter 0.7 mm
and constant 0.1). Thermogravimetric analysis was per-
formed on a TA Instruments thermoanalyzer (Model SDT
2960) at a heating rate of 10 °C/min to a maximum tem-
perature 320 °C and in an air atmosphere (gas flow
110 mL/h).
viscosity of triethylsulfonium bistriflimide [S2.2.2][NTf ] is
2
the lowest (33 mPa s [32] or 30 mPa s [33] at 25 °C), among
the sulfonium bistriflimides based on alkyldimethylsulfoni-
um, [S1.1.R], alkyldiethylsulfonium, [S2.2.R] and alky-
lethylmethylsulfonium, [S1.2.R] cations, where the alkyl
chain was changed from methyl to pentyl. Only the viscos-
ities of [S2.2.3][NTf ] and [S
2
][NTf ] were close to the
2
2.2.1
viscosity of [S2.2.2][NTf ] and were 33 mPa s and 36 mPa s,
2
respectively [34]. Low viscosity of the ionic liquid can
facilitate its recovery and reuse and also may enhance cata-
lyst solubility. Sulfonium ionic liquids with bistriflimide
anion are also thermally stable up to 300 °C under nitrogen
atmosphere [34].
The physical and thermal properties of [S .2.2][NTf2]
2
-
3
were as follows: the density was 1.468 g cm (at 25 °C),
the dynamic viscosity 30 mPa s (at 25 °C) and the
decomposition temperature (onset of decomposition peak)
was set to 225.0 °C.
To date, sulfonium ionic liquids have been recognized
as good electrolytes for lithium batteries [34]. In the pat-
ented literature [35] we found information about acidic
ionic liquids prepared from aluminium, alkylaluminium,
gallium or alkylgallium halides and sulfonium halide and
their application in alkylation of aromatics with alkenes.
On the other hand no data have been reported regarding the
use of sulfonium bistriflimide as the medium in the cata-
lyzed reactions.
Extra dry toluene (99.85%, water \50 ppm), and
anhydrous ethanol (300 ppm of water) were purchased
from Acros Organics. Dimethyl maleate (96%) was bought
from Aldrich. Cyclopentadiene was obtained as a result of
thermal cracking of dicyclopentadiene (C95% pure) which
was purchased from Fluka. Metal chlorides and tri-
fluoromethanesulfonates used as the catalysts were com-
mercial products, purchased from Aldrich: yttrium triflate
(98%), ytterbium triflate (99.9%), ytterbium triflate hydrate
(degree of hydration 1-2, Yb, 25–28% approx.), yttrium
chloride (anhydrous, 99.9%), ytterbium chloride (anhy-
drous, 99.99%), magnesium triflate (97%), calcium triflate
(99.9%) lithium triflate (99.995%), zinc triflate (98%),
sodium triflate (98%), potassium triflate (98%), silver tri-
flate (C99%). Cyclohexanone (min 99.8%), used as an
internal standard for GC analysis, was bought from Merck.
In the present study, the application of triethylsulfonium
bistriflimide as a medium in the Diels–Alder reaction in
combination with various Lewis acid catalysts has been
studied.
2
Experimental
2
.1 Materials
2.2 Procedure for the Diels–Alder Reaction
Ionic liquids, triethylsulfonium bis (trifluoromethylsulfo-
nyl) imide, 1-butyl-3-methylimidazolium bis (trifluoro-
methylsulfonyl) imide, 1-butyl-1-methylpyrrolidinium bis
The reaction was carried out in a 4 mL screwed vials,
equipped with a stirring bar. Dimethyl maleate (4 mmol)
was added to 1 mL of solvent. The following ionic liquids
1
23

Triethylsulfonium Bistriflimide as the Reaction Medium
149
have been used as solvents: triethylsulfonium bis (trifluo-
and cyclopentadiene. The second and the following runs
were conducted in the recycled triethylsulfonium bis (tri-
fluoromethylsulfonyl) imide with no futher addition of the
catalyst.
romethylsulfonyl) imide [S2.2.2][NTf ], 1-butyl-3-methyl-
2
pyridinium bis (trifluoromethylsulfonyl) imide [(Py1,3)4.1
]
[
NTf ], 1-butyl-1-methylpiperidinium bis (trifluorometh-
2
ylsulfonyl) imide [Piper1.4][NTf ], 1-butyl-3-methylimida-
2
zolium bis (trifluoromethylsulfonyl) imide [IM1.4][NTf2],
1
-butyl-1-methylpyrrolidinium bis (trifluoromethylsulfo-
3 Results and Discussion
nyl) imide [Pyrr1.4][NTf ]. Toluene or ethanol were also
2
used as solvents. The reaction started when cyclopentadi-
ene (6 mmol) was added. When the reaction was carried
out in the presence of a catalyst, the catalyst was dissolved
in the solvent before adding the reactants. The resulting
mixture was stirred at a temperature ranging from 10° to
The aim of the study was to investigate the use of sulfo-
nium ionic liquid as recyclable medium for the Diels–Alder
reaction in the presence of Lewis acid catalysts. The
reaction between cyclopentadiene and dimethyl maleate
has been chosen as model reaction (Scheme 1). It has been
performed in triethylsulfonium bis (trifluoromethylsulfo-
4
5 °C. The reaction progress was monitored by GC anal-
ysis until total conversion of dimethyl maleate or cyclo-
pentadiene. When complete conversion was not achieved
the reaction was carried out for 24 h. Reaction yield and
endo:exo ratio were determined by GC analysis, the latter
nyl) imide, [S2.2.2][NTf ] in a temperature range of
2
10°–45 °C. For comparison, several other ionic liquids
consisting of bistriflimide anion and various cations (imi-
dazolium, pyrrolidinium, pyridinium, piperidinium) and
molecular solvents were studied as media for the model
reaction at 25 °C.
1
13
was confirmed by the H NMR and C NMR analysis. GC
analysis was carried out using Carlo Erba GC 8000 TOP
instrument, equipped with FID, Restek capillary column,
As shown in Table 1, the highest reaction rate constant
TM
RXI -17 (30 m long with diameter of 0.53 mm and
at 25 °C was obtained for sulfonium ionic liquid,
1
2
.5 lm film layer).
[S2.2.2][NTf ]. The endo:exo ratio in this ionic liquid was
2
lower than in the polar solvent-ethanol, but was slightly
higher than in toluene. Endo selectivities were comparable
in various ionic liquids, but they were enhanced when the
reaction temperature was decreased (Table 1, entries 1–4).
.3 Recycling Experiments
A portion of a catalyst (24.8 mg of Yb(OTf) ÁxH O;
3
2
1
2.9 mg of Mg(OTf) ; or 11.2 mg of YbCl or 7.8 mg of
2
The second order rate constant, k increased with the
2
3
YCl ) was weighed and introduced into a 5 mL round-
3
reaction temperature, according to the Arrhenius equation.
The acceleration of the reaction through catalysis is
attractive as it permits avoiding the use of high temperature
and pressure. As mentioned before, the most effective
catalysts for Diels–Alder reaction are Lewis acids (AlCl3,
SnCl , TiCl , BF ). Since the 1990s much interest has been
bottomed flask, equipped with a magnetic stirrer. The
catalyst was dissolved in 1 mL of triethylsulfonium bis
(
trifluoromethylsulfonyl) imide. Next 0.580 g (4 mmol) of
dimethyl maleate and 0.410 g (6 mmol) of cyclopentadiene
were added. The flask was closed with a rubber septum and
placed in a thermostated bath at 25 °C. The course of the
reaction was controlled by the GC analysis. When the
reaction went to completion diethyl ether was added in
order to extract the product. The reaction mixture with
ether was allowed to stir at room temperature. Next the
stirring was turned off and phases were separated. The
ionic liquid phase was then 3 times washed with ether
4
4
3
focused on the use of scandium, yttrium, lanthanum and
other metal compounds. Due to their chemical stability
trifluoromethanesulfonates and bis (trifluoromethylsulfo-
nyl) imides have been successfully used as Lewis acid
catalysts in many chemical reactions [36–39]. To date no
data has been published for the use of sulfonium ionic
liquid together with the Lewis acids catalysts.
(
3 9 5 mL) to provide the complete recovery of the
The activity of metals triflates and chlorides has been
studied in triethylsulfonium bis (trifluoromethylsulfonyl)
imide. All catalysts were used in the concentration of
product. The lower layer containing ionic liquid and the
catalyst was dried under vacuum and recharged with
dimethyl maleate and cyclopentadiene. The next run was
then conducted in the recycled mixture.
The product was also removed from the reaction mixture
by distillation. When the reaction was completed, the
reaction flask was connected to the distillation unit and
H
R
H
R
R
R
R
R
5
-norbornene-2, 3-dicarboxylic acid dimethyl ester was
cyclopentadiene
diemthyl maleate
R=COOCH3
endo
exo
distilled off directly under vacuum (5 mbar) at 127 °C. The
residual ionic liquid-catalyst mixture was then cooled to
room temperature and recharged with dimethyl maleate
2,3-bis(methoxycarbonyl)-5-norbornene
Scheme 1 The reaction of cyclopentadiene and dimethyl maleate
123

1
50
E. Janus, B. Bittner
Table 1 Comparison of the reaction rate constants and endo:exo
ratios for Diels–Alder reaction between cyclopentadiene and dimethyl
maleate in various solvents
endo isomer. Yttrium and ytterbium triflates are the most
active onces, although the main drawback was the vigorous
course of the reaction. The exothermic effect was observed
and side polymerization reaction of cyclopentadiene took
place (reaction mixture became dark). The polymerization
of the cyclopentadiene was also detected when only this
reactant was mixed with ionic liquid and yttrium or ytter-
bium triflate. The vigorous course of the reaction was still
observed, even if the concentration of the anhydrous
a
6
Entry Solvent
Reaction
temperature (dm mol
°C)
k
2
Á10
endo:exo
) ratio
3
-1 -1
s
(
1
2
3
4
5
6
7
8
9
1
a
[S2.2.2][NTf
[S2.2.2][NTf
[S2.2.2][NTf
[S2.2.2][NTf
2
2
2
2
]
]
]
]
10
25
3.6
12.5
18.3
28.0
11.7
10.8
10.7
8.3
3.5
3.2
3.1
2.9
3.1
3.15
2.8
2.9
2.7
3.8
35
Y(OTf) or Yb(OTf) was decreased to 10% of the original
3
45
3
level. The replacement of the anhydrous ytterbium triflate
by the hydrate of this salt, resulted in the suppression of the
competitive reaction. This indicates that traces of water
inhibited the side reaction. With YCl3 and YbCl3 the
reaction proceeded slower than with triflates of these
metals, although the use of chlorides increased endo:exo
ratios. These changes could be attributed to the less bulky
chloride anion (than triflate anion) and thus dienophile can
more easily access the respective metal salts.
[IM1.4][NTf
[Pyrr1.4][NTf
[(Py1,3)4.1][NTf
2
]
25
2
]
25
2
] 25
[Piper1.4][NTf
Toluene
2
]
25
25
5.0
0
Ethanol
25
7.5
Solvents’ abbreviations: [S2.2.2][NTf
fluoromethylsulfonyl) imide, [(Py1,3)4.1][NTf
pyridinium bis (trifluoromethylsulfonyl) imide, [Piper1.4][NTf
butyl-1-methylpiperidinium bis (trifluoromethylsulfonyl) imide, [IM1.4
NTf ]—1-butyl-3-methylimidazolium bis (trifluoromethylsulfonyl)
imide, [Pyrr1.4][NTf ]—1-butyl-1-methylpyrrolidinium bis (trifluoro-
methylsulfonyl) imide
2
]—triethylsulfonium bis (tri-
]—1-butyl-3-methyl-
]—1-
2
2
]
Generally catalysts like Mg(OTf) , Ca(OTf) , Zn(OTf) ,
2
2
2
[
2
LiOTf show relatively medium activity. However, the studied
reaction took place smoothly with excellent yield in the
presenceofthese catalysts. The endoselectivity wasincreased
in comparison to that in the reaction without catalyst. Sodium,
potassium and silver trifluoromethanesulfonate were not that
active as catalysts in the Diels–Alder reaction.
2
Table 2 The yields and stereoselectivities of the Diels–Alder
cycloaddition between cyclopentadiene and dimethyl maleate in
[
metal catalysts
S
2.2.2][NTf
2
] as the reaction medium and in the presence of various
Some of the Lewis acids i.e. Al(OTf) , Bi(OTf) ,
3
3
Cu(OTf) , In(OTf) , InCl , Sc(OTf) or Sn(OTf)2 not
3
2
3
3
a
Catalyst
Reaction time (min) Yield (%) endo:exo ratio
mentioned in Table 2, were also screened as catalysts.
However, a huge exothermic effect, an increase of vis-
cosity or even the sample solidification was observed in
their presence. The desired product of the reaction between
dimethyl maleate and cyclopentadiene was not found, but
cyclopentadiene polymerization occurred instead.
b
3
Y(OTf)
5
95
96
95
90
92
92
92
66
85
30
31
24
25
10.2
10.2
9.7
b
Yb(OTf)
3
5
Yb(OTf)
3
ÁxH
2
O
15
60
60
30
30
60
60
60
60
60
YCl
YbCl
Mg(OTf)
3
14.8
13.6
9.4
3
Comparison was also made for the reaction progress
2
(Fig. 1) and stereoselectivities in the presence of the rep-
Ca(OTf)
Zn(OTf)
LiOTf
2
7.1
resentative catalyst, Yb(OTf) ÁxH O, in the sulfonium
3
2
2
9.1
ionic liquid, [S
][NTf ] and the presence of two
2
2
.2.2
8.1
molecular solvents as media. The highest reaction rate was
NaOTf
KOTf
3.5
found for [S2.2.2][NTf ] (Fig. 1). Also in this medium the
2
3.3
highest endo:exo ratio was achieved (9.7). The catalyst was
also active in toluene but the reaction proceeded slower.
Only in ethanol the catalyst was not active and minor
differences were observed between the reaction carried out
with and without Yb(OTf) ÁxH O. The endo:exo ratio in
AgOTf
3.4
Without catalyst 60
3.2
a
Concentration of the catalyst was 0.04 mmol per 1 mL of
b
[S2.2.2][NTf ] and per 4 mmol of dimethyl maleate; Exothermic
2
3
2
reaction
toluene with the ytterbium catalyst was 8.7. It was lower
than for sulfonium ionic liquid. In ethanol, the corre-
sponding value was only 4.5.
0
.04 mol/L of ionic liquid and 1 mol% vs. dimethyl
The next step in our study was to examine the influence
of the concentration of selected catalysts in sulfonium ionic
liquid on the yield and stereoselectivity of the reaction. The
following catalysts: Yb(OTf) ÁxH O, Mg(OTf) , YbCl ,
maleate. The results are shown in Table 2. Yttrium, ytter-
bium, magnesium, calcium, lithium and zinc compounds
have appeared as the active catalysts significantly
improving the reaction rate and promoting formation of
3
2
2
3
LiOTf were chosen and used in concentrations of 0.04,
1
23

Triethylsulfonium Bistriflimide as the Reaction Medium
151
0
.02 and 0.013 mol per litre of sulfonium ionic liquid, but
catalyst concentration to 0.013 mol/L caused a decrease in
the endo stereoselectivity.
in the constant concentration of 1 mol%, versus dimethyl
maleate (Table 3). These concentrations of the catalysts in
ionic liquids guaranteed the homogeneity of the reaction
system. The reaction slightly slowed down when the con-
centration of the catalyst in ionic liquid was decreased and
longer time was necessary to obtain a quantitative yield.
Only in the presence of Yb(OTf) ÁxH O, regardless of its
When the concentrations of catalysts versus dimethyl
maleate decreased to 0.5 mol% or less, in the presence of
each of the three chosen catalysts (Figs. 2, 3, 4), the
reaction had slowed down significantly. In the presence of
Yb(OTf) ÁxH O or Mg(OTf)2, the stereoselectivity
3
2
remained almost unchanged in the range of catalyst con-
3
2
concentration in sulfonium ionic liquid, over 90% yield
was achieved within 15 min. The use of each catalyst in the
concentration of 0.02 mol/L, gave the highest stereoselec-
tivity towards endo isomer. The subsequent decrease in the
centration studied. In the presence of YbCl a significant
3
decrease in the endo:exo ratio, from above 14.0 to 8.4
(Table 4) was noted when the catalyst concentration was
decreased to 0.5 mol%.
100
1
00
9
0
0
0
0
0
0
0
0
0
9
0
0
0
YbCl
.50 mol%
1.00 mol%
0.75 mol%
0.50 mol%
0.25 mol%
3
1
8
7
6
5
4
3
2
1
8
7
2
[S2.2.2][NTf ]
Toluene
Ethanol
60
5
4
3
2
1
0
0
0
0
0
0
0
20
40
60
80
100
120
140
160
0
0
20
40
60
80
100
120
140
160
Reaction time [min]
Reaction time [min]
Fig. 1 The effect of the reaction medium on the dependence of yield
of the product on the reaction time in the presence of Yb(OTf)
ÁxH
24.8 mg of Yb(OTf) O was dissolved in 1 mL of solvent)
ÁxH
Fig. 2 The dependence of yield on the reaction time for different
YbCl concentrations versus dimethyl maleate. Conditions of the
reaction: the concentration of YbCl in [S2.2.2][NTf ] was 0.02 mol/L,
cyclopentadiene:dimethyl maleate molar ratio 1.5; reaction temper-
ature: 25 °C
3
2
O
3
(
3
2
3
2
2
Table 3 The effect of the catalyst concentration in [S2.2.2][NTf ] on
the yield and stereoselectivity of the Diels–Alder reaction
1
00
Catalyst
Concentration of the Reaction Yield endo:exo
catalyst in
time
(min)
(%)
ratio
9
0
0
0
0
[S
2.2.2][NTf
2
]
8
7
6
(mol/L)
YbCl
3
0.04
60
60
60
60
60
60
60
60
60
15
15
15
92
90
61
92
90
83
67
58
56
95
90
93
13.6
14.2
8.8
9.3
9.6
8.3
8.1
8.4
8.4
9.7
9.5
9.2
0
0
.02
.013
50
3 2
Yb(OTf) •xH O
1
1
0
.50 mol%
.00 mol%
.75 mol%
Mg(OTf)
2
0.04
4
3
2
1
0
0
0
0
0
0
.02
0.50 mol%
.25 mol%
0
.013
LiOTf
0.04
0
0
.02
.013
0
0
10
20
30
40
50
60
70
80
Yb(OTf)
3
ÁxH
2
O 0.04
Reaction time [min]
0
0
.02
Fig. 3 The dependence of yield on the reaction time for different
Yb(OTf) O concentrations versus dimethyl maleate. Conditions
.013
3
ÁxH
2
Conditions of the reaction: 0.04 mmol of catalyst, 4 mmol of dime-
thyl maleate; 6 mmol of cyclopentadiene; reaction temperature 25 °C
of the reaction: the concentration of Yb(OTf) ÁxH O in [S2.2.2][NTf2]
3
2
was 0.02 mol/L; the rest parameters as in Fig. 1
123

1
52
E. Janus, B. Bittner
1
00
The reusability of ionic liquids is perhaps one of their
greatest potentials, in particular where this can be linked
with recycling of a catalytic active mixture of ionic liquid
with metal catalyst. In the present study the recovery and
reuse of the catalytic systems consisting of the tri-
ethylsulfonium bis (trifluoromethylsulfonyl) imide and
Yb(OTf) ÁxH O, Mg(OTf) , YbCl or YCl3 were per-
9
0
0
0
0
0
0
0
0
0
8
7
6
5
4
3
2
1
3
2
2
3
formed. Two methods of the product removal from the
catalytic active mixture were used. The first one was the
extraction with organic solvent and the other was direct
distillation from the reaction mixture. In the extraction
method, diethyl ether and toluene were applied as solvents.
Diethyl ether proved to be more appropriate because it
generated less ionic liquid loss. Recovery of the product by
the extraction was only possible for metal triflates—
Yb(OTf) ÁxH O, Mg(OTf) but impossible for YbCl or
Mg(OTf)
2
1
1
0
0
.50 mol%
.00 mol%
.75 mol%
.50 mol%
0.25 mol%
0
0
10
20
30
40
50
60
Reaction time [min]
Fig. 4 The dependence of yield on the reaction time for different
Mg(OTf) concentrations versus dimethyl maleate. Conditions of the
reaction: the concentration of Mg(OTf) in [S2.2.2][NTf ] was
.02 mol/L; the rest parameters as in Fig. 1
3
2
2
3
2
YCl . For catalytic systems consisting of Yb(OTf) ÁxH O
3
3
2
2
2
or Mg(OTf) and [S
2
][NTf ] over five runs were per-
2
2.2.2
0
formed (Table 5). Metal chlorides were not active in the
second run though. The reaction proceeded as slowly as
without catalyst. The reason for this is the moisture sen-
sitivity of metal chlorides and their deactivation under
extraction conditions. Better recycling effects for metal
Table 4 The effect of the concentration of the catalyst versus
dimethyl maleate on the stereoselectivity of the Diels–Alder product
2
in [S2.2.2][NTf ] as the reaction medium
Concentration of
the catalyst
(mol% vs.
dienophile)
endo:exo ratio
chlorides, dissolved in [S2.2.2][NTf ], were accomplished
2
Yb(OTf)
3
2
ÁxH O
YbCl
3
Mg(OTf)
2
by direct distillation of the product (Table 6). This method
is more environmentally friendly because it eliminates the
use of volatile organic solvents and a pure product is
separated at once. Catalyst-ionic liquid mixture became
darker following the distillation, which is an important
drawback, but five runs were performed with high
endo:exo ratio and only a little decrease in TOF. Tri-
ethylsulfonium bistriflimide and the catalytic system con-
sisting of this ionic liquid and Yb(OTf) ÁxH O were also
1
1
0
0
0
.5
9.7
9.7
9.5
9.7
9.6
15.0
14.2
14.0
8.4
9.8
9.6
8.4
8.1
8.2
.75
.5
.25
4.8
Conditions of the reaction: the concentration of catalyst in
] was 0.02 mol/L, cyclopentadiene:dimethyl maleate
3
2
[S2.2.2][NTf
2
tested in the reaction of cyclopentadiene with other di-
enophiles. The results are presented in Table 7. The
molar ratio 1.5; reaction temperature: 25 °C
2
Table 5 Recycle studies of the catalytic systems: [S2.2.2][NTf ]/
Yb(OTf)
3
2
ÁxH O and [S2.2.2][NTf
2
]/Mg(OTf)
2
2 3
Table 6 Recycle studies of the catalytic systems [S2.2.2][NTf ]/YCl
and [S2.2.2][NTf
2 3
]/YbCl
Run Catalyst
Yb(OTf)
Run Catalyst
3
ÁxH
2
O
Mg(OTf)
2
YCl
3
YbCl
3
a
b
endo: exo TOF
ratio
endo: TOF
exo ratio (mol mol
-
1
-1
)
-1 -1
h )
a
a
(mol mol
h
endo: exo TOF
ratio
endo: exo TOF
ratio
-1
-1
)
-1 -1
h )
(mol mol
h
(mol mol
1
2
3
4
5
9.1
9.0
8.7
8.6
7.8
364
372
364
335
283
9.1
8.9
8.8
8.0
7.7
99
94
93
45
45
1
2
3
4
5
14.8
14.7
14.5
13.6
13.4
97
95
92
70
56
13.6
13.7
13.1
12.8
12.3
95
92
93
66
53
The effect of the number of recycle runs on the yields and endo/exo
ratios; Removal of the product from catalytic systems by the
extraction with diethyl ether
The effect of the number of recycle runs on the yields and endo:exo
ratios; Removal of the product from the catalytic systems by direct
distillation
a
b
Determined for the reaction time 15 min; determined for the
reaction time 60 min
a
Determined for the reaction time 60 min
1
23

Triethylsulfonium Bistriflimide as the Reaction Medium
153
Table 7 The yield and endo:exo ratio in the Diels–Alder reaction between cyclopentadiene and various dienophiles in [S2.2.2][NTf
2
] and in the
catalytic system Yb(OTf)
3
2 2
ÁxH O-[S2.2.2][NTf ]
R
3
R
2
R
3
R2
R
4
R1
R
2
R3
R
4
R
1
endo
exo
R
1
R4
Dienophile
R
1
R
2
R
3
R
4
Concentration of the catalyst
a
Reaction time
Yield (%)
endo:exo ratio
(
mol% vs. dienophile)
Methyl-vinyl ketone
Ethyl-vinyl ketone
C(O)CH
3
H
H
H
H
H
H
H
H
H
H
H
H
H
H
–
0
–
0
–
0
–
5
–
5
–
5
–
1
15 min
15 min
15 min
15 min
24 h
1 h
87
95
42
90
9
8.0
10.5
3.4
.1
.25
.5
.0
.0
.0
.0
C(O)C
C(O)C
2
H
H
5
5
H
10.0
2.5
4
-hexen-3-one
2
CH
H
3
72
44
70
22
65
38
58
4
11.3
4.0
Methyl acrylate
Ethyl acrylate
Butyl acrylate
Methacrolein
C(O)OCH
3
2 h
2 h
5.2
C(O)OC
C(O)OC
2
H
5
9
H
2 h
3.6
2 h
6.4
4
H
H
2 h
4.7
2 h
5.6
CH
3
H
CHO
1 h
4.2
30 min
82
3.1
The concentration of Yb(OTf)
5 °C
3
ÁxH
2 2
O in [S2.2.2][NTf ] was 0.02 mol/L, cyclopentadiene:dienophile molar ratio was 1.5; reaction temperature
2
a
The optimal concentration vs. dienophile was given
concentration of the catalyst in [S2.2.2][NTf ] was constant.
2
The catalytic system of [S2.2.2][NTf ] and Yb(OTf) Á
2
3
The optimized endo selectivity and yield for different di-
enophiles are outlined in Table 7.
xH O proved to be active in the reaction of cyclopentadi-
2
ene with various dienophiles. The optimal catalyst con-
centration in [S2.2.2][NTf ] was 0.02 mol/L, but that versus
2
dienophile was between 0 and 5 mol% depending on the
dienophile.
4
Conclusions
Acknowledgments This work was supported by the Polish Ministry
of Science and Higher Education, Grant No. N205 055 31/2491.
Triethylsulfonium bis (trifluoromethylsulfonyl) imide
proved to be a good solvent for the Diels–Alder reaction
between cyclopentadiene and dimethyl maleate, in the
presence and absence of Lewis acids. The reaction rate in
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