Ultrasound and ionic liquid: An efficient combination to tune the mechanism of alkenes epoxidation

Article abstract of DOI:10.1016/j.ultsonch.2011.10.007

In this proof of concept study, the advantageous properties of both H ₂O₂/NaHCO₃/imidazole/Mn(TPP)OAc oxidation system and MOPyrroNTf₂ ionic liquid have been combined under ultrasonic irradiation to give an exceptionally favorable environment for Mn(TPP)OAc catalyzed olefin oxidations. The results reveal the crucial role played by the ultrasonic irradiations that influence drastically the oxidation process. In MOPyrroNTf₂ and under ultrasonic irradiation, the mechanism probably involves an oxo-manganyl intermediate at the expense of the classical bicarbonate-activated peroxide route.

Full text of DOI:10.1016/j.ultsonch.2011.10.007

Ultrasonics Sonochemistry 19 (2012) 390–394
Contents lists available at SciVerse ScienceDirect
Ultrasonics Sonochemistry
journal homepage: www.elsevier.com/locate/ultsonch
Short Communication
Ultrasound and ionic liquid: An efficient combination to tune the mechanism
of alkenes epoxidation
Gregory Chatel a,b, Catherine Goux-Henry , Nathalie Kardos , Joel Suptil , Bruno Andrioletti
⇑
,
Micheline Draye
a
b
a
a
b,
a,
⇑
Laboratoire de Chimie Moléculaire et Environnement, Université de Savoie, CISM, Campus scientifique, Le Bourget du Lac Cedex 73376, France
Institut de Chimie et Biochimie Moléculaire et Supramoléculaire (Equipe CASYEN), UMR-CNRS 5246, Université Claude Bernard Lyon 1, Bâtiment Curien (CPE), 43, Boulevard du 11
b
Novembre 1918, Villeurbanne Cedex 69622, France
a r t i c l e i n f o
a b s t r a c t
Article history:
In this proof of concept study, the advantageous properties of both H O /NaHCO /imidazole/Mn(TPP)OAc
2
2
3
Received 5 September 2011
Received in revised form 7 October 2011
Accepted 9 October 2011
2
oxidation system and MOPyrroNTf ionic liquid have been combined under ultrasonic irradiation to give
an exceptionally favorable environment for Mn(TPP)OAc catalyzed olefin oxidations. The results reveal
the crucial role played by the ultrasonic irradiations that influence drastically the oxidation process. In
Available online 20 October 2011
MOPyrroNTf
mediate at the expense of the classical bicarbonate-activated peroxide route.
Ó 2011 Elsevier B.V. All rights reserved.
2
and under ultrasonic irradiation, the mechanism probably involves an oxo-manganyl inter-
Keywords:
Epoxidation
Alkenes
Ionic liquids
Mn porphyrin
Ultrasound
1
. Introduction
2 2
that a combination of hydrogencarbonate and H O (10 equiv.) al-
lows the epoxidation of variously substituted alkenes with conver-
sions ranging from 20% to 90% [1a]. In addition, most reactions
were carried out in a volatile, often toxic organic solvent and
bleaching of the catalyst was observed.
In the recent years, the use of ionic liquids has become a topic of
much interest. Their application as reaction media for a wide vari-
ety of synthetic processes is an area of intense research and new
approaches involving ionic liquids are proposed for catalyst sepa-
ration and recycling [8]. Ionic liquids display many interesting
properties which make them very attractive for catalysis. In these
almost vapourless, air and moisture stable solvents, polar or ionic
catalysts can be immobilized without any post-functionalization,
allowing easy recycling.
2 2
Utilization of hydrogen peroxide, H O , as an oxidant has re-
ceived much attention in the recent years because of its low envi-
ronmental impact. In addition, it is reasonably cheap [1], safely
storable [2] and generates water as only theoretical co-product
[
2 2
1]. Moreover, H O is more accessible than other oxidizing agents
such as organic peracids and organic hydroperoxides for liquid
phase reactions and it advantageously exhibits a high active oxy-
gen content [3]. However, it is a rather slow oxidizing agent in
the absence of activators such as bicarbonate ions [4]. Accordingly,
the use of bicarbonate-activated hydrogen peroxide system (BAP)
was proven to be very versatile, allowing the epoxidation of both
lipophilic and hydrophilic alkenes [1a].
Olefin epoxidation, catalyzed by iron or manganese metallopor-
phyrin complexes [5], constitutes a milestone in the rapid evolu-
tion of this research area including in a chiral form [6]. Recently,
Ultrasound-promoted synthesis has attracted much attention
during the past few decades. The most successful applications of
ultrasound were found in the field of heterogeneous chemistry
involving solids and metals. In fact, ultrasound is known to en-
hance some processes [9] through a physical phenomenon called
cavitation, which is the formation, growth and collapse of bubbles
in an elastic liquid. By imploding, these bubbles create locally high
pressure (up to 1000 bar) and temperature (up to 5000 K) that lead
to high-energy radical mechanisms but also generate some inter-
esting physical effects [10]. Thus, the enhancement of catalyst
activity by low frequency ultrasonic irradiation (e.g. f = 20 kHz) is
due to (1) the improvement of mass transfer between the liquid
2 2 3
the combination of H O /NaHCO /imidazole/Mn(TPP)OAc has been
proposed in the literature for the selective oxidation of olefins [7].
However, in the latter report, one cannot exclude an important
contribution of the metal-free BAP process, as it has been shown
⇑
Corresponding authors. Tel.: +33 (0)472 446 264; fax: +33 (0)472 448 160 (B.
Andrioletti), tel.: +33 (0)479 758 859; fax: +33 (0)479 758 674 (M. Draye).
E-mail addresses: bruno.andrioletti@univ-lyon1.fr (B. Andrioletti), micheline.-
draye@univ-savoie.fr (M. Draye).
1
350-4177/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.ultsonch.2011.10.007

G. Chatel et al. / Ultrasonics Sonochemistry 19 (2012) 390–394
391
and the catalyst surface, (2) reduction of particle size, increase of
surface area and (3) acceleration of suspended particle motion in-
duced by shock waves and microstreaming.
argon bubbling. Then, 1-octylbromide (333 mmol, 58.0 mL,
1 equiv.) was added and the resulting mixture was refluxed for
17 h under argon. Finally, the solvent was evapored under reduced
pressure (rotary evaporator). The residue was washed thoroughly
with ethyl acetate (3 ꢀ 100 mL) and dried under reduced pressure
to afford the MOPyrroBr first generation ionic liquid as an hygro-
scopic white solid in 92% yield.
Recent reports in the literature describe the use of manganese
[
11] or molybdenum [12] salts for promoting the epoxidation of ole-
fins in an ionic liquid medium. While environmentally friendly and
very efficient, this strategy is limited to achiral transformations.
Herein, we demonstrate that under ultrasound activation in an
appropriate ionic liquid, it is possible to completely by-pass the li-
gand-free mechanism and induce a metalloporphyrin based route.
MOPyrroBr: mp 98 °C. H NMR (d ppm, CDCl
3.85 (m, 4H), 3.62–3.68 (m, 2H), 3.30 (s, 3H), 2.30–2.31 (m, 4H),
1.73–1.83 (m, 2H), 1.27–1.36 (m, 10H), 0.88 (t, 3H, J = 6.6 Hz). 13
DEPT NMR (d ppm, CDCl , 75.5 MHz): CH d 64.8 (2C), 64.5 (2C),
2.0, 29.6, 29.4, 26.8, 24.5, 22.9, 22.1. CH d 49.1, 14.4.
3
, 300 MHz): d 3.84–
C
3
2
2
. Experimental
3
3
2.1. Reagent, apparatus and analysis
2.4. Synthesis of MOPyrroNTf
2
1H NMR and 13C DEPT NMR spectra were recorded in CDCl
(Eur-
3
MOPyrroBr (307 mmol, 85.3 g, 1 equiv.) was added to a solution
of LiNTf (245 mmol, 70.4 g, 0.8 equiv.) in distilled water (200 mL).
The mixture was stirred at room temperature under argon for 5 h
and extracted with CH Cl
(3 ꢀ 70 mL). The combined organic
phases were then washed with water (3 ꢀ 20 mL), brine
2 ꢀ 15 mL) and dried over MgSO . After filtration and evaporation
under reduced pressure (rotary evaporator, 2 ꢀ 10 mbar), the de-
sired MOPyrroNTf second generation ionic liquid was obtained as
a pale yellow liquid in 93% yield.
iso-Top, Saint Aubin, France) at 23 °C using a Bruker DRX300 spec-
trometer, at 300 MHz and 75.5 MHz for 1H and 13C, respectively.
2
Chemical shifts (d) are reported in ppm relative to tetra-methylsi-
lane (TMS). Gas chromatography was performed on a GC9000series
gas chromatograph from Fisons Instruments using flame-ionization
detector and equipped with an UB1P capillary column (dim-
2
2
(
4
ꢁ3
ethylpolysiloxane 30 m ꢀ 0.32 mm ꢀ 0.25
lm) from Interchim.
Mass spectra were taken on a HP 5973 MSD coupled to a HP 6890
2
GC and equipped with an Optima 5 capillary column (dim-
1
MOPyrroNTf
.37 (m, 2H), 3.06 (s, 3H), 2.21 (m, 4H), 1.65–1.76 (m, 2H),
1.20–1.33 (m, 10H), 0.83 (t, 3H).
2 3
: H NMR (d ppm, CDCl , 300 MHz): d 3.57 (m, 4H),
ethylpolysiloxane 30 m ꢀ 0.32 mm ꢀ 0.25
lm) from Macherey-Na-
gel. FTIR spectra were recorded on a Nicolet 380 spectrometer and
3
ꢁ1
Ò
reported in cm . Ultrasound was generated by a Digital Sonifier
1
3
C DEPT NMR (d ppm, CDCl
2C), 31.0, 28.4, 28.3, 25.6, 23.3, 21.9, 20.8. CH
ZnSe crystal, neat): max 2958, 2930, 2861, 1487, 1349, 1180, 1134,
3
, 75.5 MHz): CH
2
d 64.0 (2C), 63.9
S-250D from Branson (Pelec = 11 W). A 3 mm diameter tapered
(
3
d 13.4, 47.8. IR (ATR,
microtip probe operating at a frequency of 20 kHz was used and its
ꢁ1
m
acoustic power in water (Pacous.vol = 0.667 W mL ) was determined
by calorimetry using a procedure described in the literature [13].
Viscosity measurements were recorded on a Anton Paar AMVn Vis-
cosimeter at 20 °C, associated with water content measurements
on Metrohm 831KF coulometer (Karl Fisher method).
ꢁ1
1
054 cm
.
2.5. General procedure for the epoxidation of alkenes in MOPyrroNTf
2
Mn(TPP)OAc (1.5
3 mL) under sonication for 2 min. Imidazole (15
dium bicarbonate (0.25 mmol, 21 mg, 0.25 equiv.) and alkene
1 mmol, 0.13 mL, 1 equiv.) were added to the solution thermo-
stated at 25 °C (minichiller cooler). Hydrogen peroxide (2.5 mmol,
.25 mL, 2.5 equiv., 30% solution in water) was added under ultra-
l
mol, 1.1 mg) was dissolved in MOPyrroNTf
2
Except pyrrolidine that was distilled under reduced pressure,
chemicals used during these investigations were obtained and
used without further purification. 1-methylpyrrolidine, 1-octylbro-
(
lmol, 1.0 mg), so-
(
mide, ethyl acetate, cyclooctene, styrene, a-pinene, hydrogen per-
oxide (30%: wt.% solution in water), imidazole and sodium
bicarbonate were purchased from Acros. Pyrrole was purchased
from Alfa Aesar, benzaldehyde and cyclohexene from Aldrich,
0
sonic irradiation (3 mm Ø tapered microtip probe, Pelec = 11 W).
Additional amounts of sodium bicarbonate (0.25 mmol, 21 mg,
cyclohexane from Chimie-Plus Laboratoires and LiNTf
2
was ob-
0
.25 equiv.) and hydrogen peroxide (2.5 mmol, 0.25 mL, 2.5 equiv.)
tained from Solvionic.
were added portion-wise every 15 min over 1 h. After 1 h of soni-
cation (Fig. 1), the reaction mixture was throughly washed with an
ether/cyclohexane (3:2 v/v) mixture (5 ꢀ 2 mL) to extract the
remaining alkene and the newly formed products. The ether/cyclo-
hexane phase was analyzed by gas chromatography. The chemical
2
.2. Synthesis of Mn(TPP)OAc
Tetraphenylporphyrin (TPP) was prepared in propionic acid
according to the literature method [14].
3
TPP (732 mol, 0.450 g, 1 equiv.), CH COONa (4.4 mmol,
0
1
1
yields were determined by H NMR after purification by column
chromatography on silica gel with cyclohexane as eluent.
l
.360 g, 6 equiv.) and manganese acetate (11.0 mmol, 1.9 g,
5 equiv.) were refluxed in glacial acetic acid under an argon atmo-
sphere for 3 h. Complexation was monitored by TLC. After evapora-
tion of the solvent under reduced pressure (rotary evaporator), the
3. Results and discussion
residue was solubilized in CH
2
Cl
2
and filtered through a 10 cm high
The hydrophilicity of the aqueous bicarbonate-activated H O2
2
alumina pad to remove the remaining TPP free base in the first
fraction, before increasing the polarity of the eluent with EtOH
and collect the porphyrin manganese complex as a greenish band.
Then, the solvents were evaporated and the solid was dried under
system and the lipophilicity of alkenes urged us to develop a favor-
able ionic liquid system which can provide a suitable reaction envi-
ronment for both hydrophilic and hydrophobic molecules. For
these reasons, we decided to carry out the catalytic reaction in
MOPyrroNTf2 (Fig. 2) [15], an air, moisture and electrochemically
stable hydrophobic ionic liquid. Indeed, MOPyrroNTf2 displays a
water content of 1165.2 ± 13.9 ppm and exhibits a low viscosity
ꢁ
3
reduced pressure (2 ꢀ 10 bar) to afford 517 mg of Mn(TPP)OAc
as green crystals in 97% yield.
2
ꢁ1
2.3. Synthesis of MOPyrroBr
of (g
= 237, 15 mm s at 20 °C) allowing a perfect solubilization
affords an ideal
of the porphyrin catalyst. Hence, MOPyrroNTf
2
A
solution of 1-methylpyrrolidine (400 mmol, 41.6 mL,
interfacial contact area for mass transfer between the water and
the organic phases.
1
.2 equiv.) in ethyl acetate (250 mL) was degassed for 15 min by

3
92
G. Chatel et al. / Ultrasonics Sonochemistry 19 (2012) 390–394
ꢁ
ꢁ
reaction H
2
O
2
þ HCO $ HCO [1a] (the desired epoxidation reac-
3
4
tion does not occur in the absence of sodium hydrogencarbonate).
However, as mentioned above, under ultrasonic irradiation, the
high cavitational intensity favors the H
2 2
O dissociation resulting
in an immediate decrease in H concentration [16]. The net re-
2 2
O
sult is an instantaneous decrease in formation of the active oxi-
dant-(peroxomonocarbonate ion) and a subsequent decrease in
olefin oxidation. When MOPyrroNTf was used as solvent (Table 1,
2
entries 4 and 5) almost no reaction occured, both in silent condi-
tions and under ultrasonic activation, showing a crucial solvent ef-
fect in the extent of reaction completion.
2
As MOPyrroNTf partly dissolves cyclooctene, it forms at least a
biphasic medium with the oxidant slowly transferring from the
aqueous phase to the organic one, hence limiting the conversion.
An attempt was made to increase the rate of mass transfer [16]
using ultrasonic irradiation. However, this had no significant effect
in the absence of catalyst. Keeping in mind that the development of
an environmentally benign and versatile methodology for enantio-
selective epoxidation of olefins constitutes the ultimate goal in
catalysis, we then explored the use of the model Mn(TPP)OAc to
2 2 3
perform the oxygen transfer from H O /NaHCO to the olefin sub-
strates. The reaction was first carried out with cyclooctene as sub-
strate according to the conditions previously described [7].
Catalytic tests in acetonitrile resulted in all cases in catalyst
bleaching. In addition, as shown in Table 2 (entries 1 and 2), when
acetonitrile was used as reaction medium, conversions of cyclooc-
tene to cyclooctene oxide were comparable to those obtained
without catalyst whatever the reaction time (Table 1, entries 1
and 2). This suggests that the catalyst does not play any key role
in the oxidation process under those experimental conditions,
and that the oxidation proceeds according to the classical BAP
mechanism. Conversely, when the reaction was carried out under
ultrasonic irradiation (Table 2, entry 3) the yield of epoxide re-
mained in the 30% range whereas it was limited to 8% in the ab-
sence of Mn(TPP)OAc (Table 1, entry 3). This observation
suggests that under ultrasound activation the mechanism is differ-
ent, and probably involves the manganese porphyrin possibly
through the already known oxo-manganyl intermediate [17]. This
hypothesis was further verified when the reaction was carried out
Fig. 1. Ultrasonic experimental equipment.
O
O
N
N
S
S
F3C
CF3
O O
2
Fig. 2. Structure of the MOPyrroNTf ionic liquid.
As detailed in Table 1, the BAP system affords the selective for-
mation of cyclooctene oxide in modest yields at 25 °C in silent con-
ditions when acetonitrile is used as solvent (even if one equivalent
of NaHCO
3
is added portionwise). But, as the reaction time in-
2
in MOPyrroNTf . In this reaction medium and in silent conditions,
creases, the yield of cyclooctene oxide increases too. These results
suggest that, in agreement with literature data [3a], the catalyst-
free bicarbonate-activated H O constitutes an efficient, but slow
2 2
oxidizing system for the selective epoxidation of cyclooctene in
acetonitrile. Under ultrasonic activation (Table 1, entry 3) the reac-
tion is still selective but the yield in cyclooctene oxide is reduced
by a factor of three suggesting a deactivation of the hydrogencar-
the catalyst does not bleach; but, probably due to the limited mass
transfer afforded by mechanical stirring, a low 12% conversion was
obtained after 5 h of reaction at room temperature (Table 2, entry
4). Interestingly, upon ultrasound activation the rate transfer of
oxidizing species is enhanced, resulting in 72% conversion in only
1 h (Table 2, entry 5). This result undoubtedly confirms that under
ultrasonic irradiation, the BAP-epoxidation does not occur. Even
more, it highlights the crucial role played by the porphyrin catalyst
that is prevented from bleaching by the ionic liquid.
bonate-activated H
the active peroxymonocarbonate ion, HCO4 according to the
2
O
2
. The BAP system relies on the formation of
ꢁ
Table 1
BAP-mediated epoxidation of cyclooctene.
H
2
O
2
, NaHCO
3
O
solvent, 25°C
a
Entry
Solvent
Activation method
Time (h)
Epoxide yield (%)
Selectivity (%)
1
2
3
4
5
CH
CH
CH
3
3
3
CN
CN
CN
–
–
))))
–
))))
1
5
1
1
1
26b (4d)
100
100
100
100
>99
b
54
c
8
1
3
b
(<1d)
MOPyrroNTf
MOPyrroNTf
2
c
2
a
b
c
GC-yield (based on starting cyclooctene) measurements.
Optimized reaction conditions: 1 equiv. substrate, 10 equiv. H
Optimized reaction conditions: 1 equiv. substrate, 10 equiv. H
Portionwise addition of the oxidant (4 ꢀ 0.25 equiv.).
2
O
O
2
, 0.53 equiv. NaHCO
3
, 3 mL solvent, T = 25 °C.
3
, 3 mL solvent,)))) ultrasonic irradiation: f = 20 kHz, P = 11 W (electric).
2
2
, 4 ꢀ 0.25 equiv. NaHCO
d

G. Chatel et al. / Ultrasonics Sonochemistry 19 (2012) 390–394
393
Table 2
Effect of catalysis.
Entry
Solvent
Activation method
–d
Time (h)
Epoxide yielda (%)
34b (5e)
Selectivity (%)
1
2
3
4
5
CH
3
CH
3
CH
3
CN
CN
CN
1
5
1
5
1
100
100
100
100
>99
d
b
–
))))
54
d
c
33
b
MOPyrroNTf
MOPyrroNTf
2
–
))))
12
72 (66f)
c
2
a
GC-yield (based on starting cyclooctene).
Optimized reaction conditions: 1 equiv. substrate, 10 equiv. H
Optimized reaction conditions: 1 equiv. substrate, 10 equiv. H
b
c
2
O
2
, 0.53 equiv. NaHCO
3
, 1.5 mol% imidazole, 0.15 mol% Mn(TPP)OAc, 3 mL solvent, T = 25 °C.
, 1.5 mol% imidazole, 0.15 mol% Mn(TPP)OAc, 3 mL solvent,)))) ultrasonic
2
O
2
, 4 ꢀ 0.25 equiv. NaHCO
3
irradiation: f = 20 kHz, P = 11 W(electric).
d
Bleaching of catalyst.
e
Portionwise addition of the oxidant (4 ꢀ 0.25 equiv.).
f
Isolated yield.
Table 3
Epoxidation of various alkenes.
Entry
Substrate
Solvent
Catalyst
Activation method
Epoxide yielda (%)
99b (99e)
Selectivity (%)
1
2
3
4
5
6
7
8
9
Cyclohexene
Cyclohexene
Cyclohexene
Cyclohexene
Styrene
Styrene
Styrene
Styrene
CH
CH
3
CN
CN
None
–
–
))))
))))
–
100
>99
>99
>99
97
>99
96
91
100
>99
>99
82
Mn(TPP)OAcd
None
98
b
3
c
MOPyrroNTf
MOPyrroNTf
2
1
c
f
2
Mn(TPP)OAc
None
95 (91 )
b
e
CH
CH
3
CN
CN
61 (60 )
Mn(TPP)OAcd
None
–
73
b
3
c
MOPyrroNTf
MOPyrroNTf
2
))))
))))
–
–
))))
))))
7
c
f
2
Mn(TPP)OAc
None
86 (74 )
b
e
a-Pinene
a-Pinene
a-Pinene
a-Pinene
CH
CH
3
CN
CN
10 (2 )
Mn(TPP)OAcd
None
11
b
10
11
12
3
c
MOPyrroNTf
MOPyrroNTf
2
2
c
49 (39f)
2
Mn(TPP)OAc
a
b
c
GC-yield (based on starting cyclooctene).
Optimized reaction conditions: 1 equiv. substrate, 10 equiv. H
Optimized reaction conditions: 1 equiv. substrate, 10 equiv. H
2
O
2
, 0.53 equiv. NaHCO
3
, 1.5 mol% imidazole, 0.15 mol% Mn(TPP)OAc, 3 mL solvent, T = 25 °C, 1 h.
, 1.5 mol% imidazole, 0.15 mol% Mn(TPP)OAc, 3 mL solvent,)))) ultrasonic
2
O
2
, 4 ꢀ 0.25 equiv. NaHCO
3
irradiation: f = 20 kHz, P = 11 W (electric), 1 h.
d
Bleaching of catalyst.
e
Portionwise addition of the oxidant (4 ꢀ 0.25 equiv.).
f
Isolated yield.
Based on literature about Mn-porphyrin catalytic epoxidation
with H , imidazole was added in reaction medium (Tables 2
and 3). Indeed, epoxide yields obtained without imidazole are very
low and axial ligand role of imidazole (proximal effect) was shown
by Mansuy and co-workers [17,18].
The combination ionic liquid/ultrasound leads to better conver-
sions with, in addition, a decreased reaction time. These optimized
conditions were applied to a series of substrates displaying differ-
ent electronic and/or steric properties and the results were gath-
ered in Table 3.
49% yield (Table 3, entry 12), whereas the silent conditions affor-
ded the corresponding epoxide in a limited 10% conversion (Ta-
ble 3, entry 9). The isolated yields confirmed that almost no
degradation occured using our optimized conditions. Indeed, in
all cases, the yields are good to excellent – 66%, 91%, 74% and
39% – for cyclooctene oxide, cyclohexene oxide, styrene oxide
2 2
O
and
a-pinene oxide, respectively, and the selectivity is close to
100% despite a limited decrease under ultrasound activation.
Analysis of Table 3 (entries 1, 2 and 4) reveals that cyclohexene
oxidation readily affords the corresponding epoxide, whatever the
experimental conditions. However, entry 3 (Table 3) confirms that
in the absence of catalyst, the BAP system is deactivated under
ultrasound irradiation. A similar trend is observed when styrene
4. Conclusion
In our work the use of ultrasonic irradiation in MOPyrroNTf
2
prevents the olefin epoxidation via the bicarbonate-activated
hydrogen peroxide system in the presence of a manganese porphy-
rin. In the afore-mentioned conditions, epoxidation most likely
takes place via a classical high-valent oxo-managanese porphyrin
complex. Indeed, our study demonstrates that ultrasound favor
the fast degradation of the transient oxidant active peroxymono-
carbonate intermediate (half life ꢂ5 min, pH 7.4 under silent
and
For all investigated reactions, the best conditions involve the use
of MOPyrroNTf as solvent, Mn(TPP)OAc as catalyst and ultrasonic
irradiation. Under these conditions, the challenging trisubstituted
-pinene double bond is selectively epoxidised in an encouraging
a-pinene are used as substrates (Table 3, entries 7 and 11).
2
a

3
94
G. Chatel et al. / Ultrasonics Sonochemistry 19 (2012) 390–394
conditions) allowing a fast epoxidation via the porphyrin complex
17]. Thus, the unique combination of ultrasound activation and
ionic liquid should allow the use of a chiral complex for the
enantioselective epoxidation of olefins in the presence of the
[8] (a) R. Rogers, R. Sheldon, Ionic Liquids as Green Solvents: Progress and
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[
(
2
b) K. Seddon, Catalytic reactions in ionic liquids, Chem. Commun. 23 (2001)
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2 2 3
environmentally benign H O /NaHCO combination. Further work
is currently in progress in our laboratories to develop enantioselec-
tive epoxidation of olefins under this smart ultrasound/ionic liquid
system.
[
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