B. Zhang et al. / Journal of Organometallic Chemistry 744 (2013) 108e112
109
by FT-IR, H-, 13C NMR spectroscopy and elemental analysis (see the
1
perrhenate ionic liquids (IPILs) of the general type [C
n
mim][ReO
2nþ1-3-methylimidazolium; n ¼ 2 (1), 4 (2), 5 (3), 6
4), 8 (5), 10 (6), 12 (7), see Fig. 1), which are very efficient reaction
4
]
(
C
n
mim ¼ 1-C
n
H
Supporting information for details).
(
media for the epoxidation of simple olefins (cyclooctene, 1-octene,
styrene) using aqueous hydrogen peroxide and hydrogen peroxide
urea adduct (UHP) as oxidants [40]. Due to negligible solvente
2.3. Catalytic oxidation of sulfides
To a stirred solution of sulfide (10 mmol) and [C mim][ReO ]
4
4
perrhenate interactions in these ILs, H
with the Re-oxo ligands, which activates H
ceptible for epoxidation. Further research, summarized in this pa-
per, shows that IPILs can be used in catalytic amounts for the
oxidation of sulfides to sulfones with hydrogen peroxide as oxidant
2
O
2
can form hydrogen bonds
and makes it sus-
4 4
(0.1955 g, 5 mol%) in [C mim][BF ] (2 mL), an aqueous solution of
2
O
2
hydrogen peroxide (35% in water) (3.5 mL, 40 mmol) is added in 2e3
ꢀ
portions at 60 C. The progress of the reaction is followed by TLC. The
reaction mixture is extracted with diethyl ether (5 ꢂ 10 mL) and the
extract is dried over anhydrous MgSO . The yield and selectivity of
4
and conventional [C
tetrafluoroborate) ionic liquid as solvent under mild conditions.
4
mim][BF
4
]
(1-butyl-3-methylimidazolium
methyl phenyl sulfone are calculated from calibration curves
(r > 0.999) recorded using 3-methylanisole and 1,4-diacetylbenzene
2
as internal standard. The crude product is obtained by rolling evap-
oration and purified by column chromatography separation (silica gel
using hexane/ethyl acetate 90:10 v/v). The RTIL phase is diluted with
2
. Experimental section
CH
2 2 2
Cl and then treated with MnO to destroy the excess peroxide.
2
.1. General remarks
The obtained liquid is first dried over anhydrous MgSO
4
and then for
. Fresh substrate and hydrogen
ꢀ
4
h in vacuo at 50 C to remove CH
Cl
2 2
All preparations and manipulation involving air sensitive ma-
peroxide are then added for a new reaction cycle. All products are
terials were performed using standard Schlenk techniques under
argon atmosphere. Solvents were dried by standard procedures
1
13
characterized by melting point, H NMR, C NMR and IR spectroscopy
see Supporting information).
(
(
2 2 2 2
Et O over Na/benzophenone; CH Cl over CaH ), distilled under
ꢀ
argon and kept over 4 A molecular sieves. All chemicals (purchased
from Acros Organics or Aldrich) were of analytical grade and used
as received. H NMR, C NMR spectra were recorded on a Bruker
Avance DPX-400 spectrometer and chemical shifts are reported
relative to the residual signal deuterated solvent. IR spectra were
recorded on Varian FTIR-670 spectrometer, using a GladiATR
3. Results and discussion
1
13
3.1. Characterization of the IPIL compounds 1e7
Physical data such as density, melting point and decomposition
temperature were determined (see Table 1). Compounds 1e5 are
liquids at room temperature and a glass transition process can be
detected below room temperature according to differential scan-
ning calorimetry (DSC) data. Compounds 6 and 7 show melting
points of 38.3 and 48.3 C, respectively. The thermogravimetric
analysis (TGA) data indicate that all the compounds show negligible
volatility and high thermal stability with a decomposition onset
accessory with
a diamond ATR element. Thermogravimetry
coupled with mass spectroscopy (TG-MS) was conducted utilizing a
Netzsch TG209 system; typically about 10 mg of each sample were
heated from 25 C to 1000 C at 10 K min . Differential Scanning
Calorimetry (DSC) was performed on a Q2000 series DSC instru-
ꢀ
ꢀ
ꢁ1
ꢀ
ment, typically about
2
mg of each sample were heated
ꢀ
ꢀ
ꢁ1
from ꢁ100 C to 150 C at 10 K min . Density was measured by an
Anton Paar DMA4500 densimeter. Catalytic runs were monitored
by GC methods on a HewlettePackard instrument HP 5890 Series II
equipped with an FID, a Supelco column Alphadex 120 and a
HewlettePackard integration unit HP 3396 Series II. Melting points
were determined by MPM-H2 melting point meters. TLC was per-
formed on silica gel 60F254 plates procured form E. Merck. Silica
gel (0.06e0.2 mm 60A) was used for column chromatography.
ꢀ
temperature near 400 C (see Table 1).
3.2. Influence of solvent, oxidant and catalyst
We studied the catalytic properties of IPILs 2, 5 and 7 for the
2 2
oxidation of thioanisole using aqueous H O and UHP as oxidants.
For comparison, various oxidants and catalysts in different reaction
media were also tested. The results are given in Table 2. Several
[
[
C
C
4
mim][PF
mim][NTf
6
]
(1-butyl-3-methylimidazolium tetrafluoroborate),
] (1-butyl-3-methylimidazolium bis(trifluoromethy-
organic solvents and ILs as solvents, such as [C
-methylimidazolium bis(trifluoromethylsulfonyl)imide), [C
HSO (1-butyl-3-methylimidazolium hydrogen sulfate) and
mim][PF ] (1-butyl-3-methylimidazolium hexafluorophosphate)
as oxidant
4 2
mim][NTf ] (1-butyl-
4
2
3
4
mim]
lsulfonyl)imide), [C
hydrosulfate) and [C
hexafluorophosphate) were synthesized according to literature
procedures [41e43].
4
mim][HSO
mim]PF
4
]
(1-butyl-3-methylimidazolium
(1-butyl-3-methylimidazolium
[
[
4
]
4
6
C
4
6
2 2
were applied. Oxidation of thioanisole with aqueous H O
was found to be strongly solvent dependent. When using conven-
tional solvents, such as n-hexane and toluene (Entries 1e2) the yield
of sulfone is below 60%, and the selectivity toward sulfone is less
2.2. Preparation of IPILs 1e7
The imidazolium perrhenates 1e7 are prepared according to a
published procedure [40]. The ionic liquids 1e7 are characterized
Table 1
Physical data of ionic liquids 1e7.
Compounds
Melting point/Glass transition
temperature
Density
[g/cm3]
Decomposition
temp. onset
ꢀ
ꢀ
[
C]
[ C]
1
2
3
4
5
6
7
ꢁ15.2
ꢁ76.5
ꢁ73.9
ꢁ61.1
9.2
2.157
1.963
1.851
1.794
1.658
1.527
1.433
395.2
399.3
396.7
393.9
380.2
381.3
383.5
38.3
48.3
Fig. 1. Imidazolium perrhenate ILs 1e7 (IPILs).