Encapsulation of Pd(OAc)2 catalyst in an ionic liquid phase confined in silica gels. Application to Heck-Mizoroki reaction

Article abstract of DOI:10.1039/b902909e

Ionogels encapsulating Pd(OAc)₂ were successfully used as catalysts in Heck-Mizoroki coupling reactions. Reaction rates were compared to homogeneous systems showing no significant difference. Leaching tests showed that catalysis actually took p

Full text of DOI:10.1039/b902909e

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New Journal of Chemistry
An international journal of the chemical sciences
Volume 33 | Number 10 | October 2009 | Pages 1981–2184
www.rsc.org/njc
ISSN 1144-0546
PAPER
André Vioux et al.
Encapsulation of Pd(OAc)2 catalyst in
an ionic liquid phase confined in silica
gels. Application to Heck–Mizoroki
1144-0546(2009)33:10;1-0
reaction

PAPER
www.rsc.org/njc | New Journal of Chemistry
Encapsulation of Pd(OAc)2 catalyst in an ionic liquid phase confined
in silica gels. Application to Heck–Mizoroki reaction
a
b
b
a
´
Sabrina Volland, Marina Gruit, Thomas Regnier, Lydie Viau,
b
a
´
Olivier Lavastre* and Andre Vioux*
Received (in Montpellier, France) 11th February 2009, Accepted 17th April 2009
First published as an Advance Article on the web 26th May 2009
DOI: 10.1039/b902909e
Ionogels encapsulating Pd(OAc)2 were successfully used as catalysts in Heck–Mizoroki coupling
reactions. Reaction rates were compared to homogeneous systems showing no significant
difference. Leaching tests showed that catalysis actually took place in the ionic liquid phase
confined within the silica matrix. The absence of leaching of palladium, the trapping of salt
by-product in ionogels and the ability to produce convenient ready-to-use catalyst doses open
interesting potential applications.
Another approach is the immobilization of ionic liquids in
Introduction
silica matrices by sol–gel processing, which has been recently
reported.12–17 It results in a more intimate biphasic system
than the simple impregnation of oxide particles. The confined
ionic liquid was shown to fill 3D-interconnected mesopores
(mean pore diameters in the 7–20 nm range, depending on the
ratio of ionic liquid to silica), which gives rise to percolating
ionic conduction paths.13 However, despite confinement
the ionic liquid nanophase was shown to keep liquid-like
dynamics.12 Accordingly these ionogels gave ionic conductivity
performances close to those of pure ionic liquids.13 The
interconnected pore structure of ionogels should facilitate
the internal diffusion of reactants and products in catalyzed
reactions. Nevertheless, whereas numerous applications
of SILF have been reported, particularly in transition
metal catalysis, such as Rh-catalyzed hydroformylations,16
hydrogenations,18 Pd-catalyzed Heck–Mizoroki14,15 and
Suzuki–Miyaura19 reactions, and Rh-, Pd-, and Zn-catalyzed
hydroaminations,20–22 there are to date only a few papers on
the use of ionogels in the same area. Actually Deng et al.
reported the enhanced catalytic performance of the carbonyl-
ation of aniline into ureas and carbamates in the presence of
ionogels loaded with Rh(PPh3)3Cl and Pd(PPh3)2Cl2.23,24
Reactions were carried out without additional solvent in an
autoclave at 135–180 1C under 5 MPa CO pressure. These
promising results prompted us to evaluate the efficiency of this
method of catalyst encapsulation when operating in the
presence of an organic solvent, in the synthesis of metal-free
organic compounds, especially in Heck–Mizoroki coupling
reactions.
The concept of confining catalysts on porous supports is far
from new, but it is still the subject of numerous studies
involving organic chemistry, materials chemistry, physico-
chemistry, calculations etc.1,2
A supported liquid-phase
catalyst (SLPC), which consists of a catalyst component
dissolved in a liquid (PEG,3 water,4 ionic liquid5 etc.) coated
on the surface of a high-surface porous material, belongs to
this common concept.
Ionic liquids have been claimed as green solvents, especially
for sustainable metal catalyzed reactions, with high efficiency
and recyclability.6,7 However ionic liquids still are expensive,
which is very restrictive for large scale uses. A second limitation
arises from the high viscosity of ionic liquids, which implies
highly energy-consuming stirring, not suitable for industrial
applications. Moreover, in biphasic systems, mass transfer
limitations mean that the sole catalyst involved in the process
is that contained in the narrow diffusion layer at the liquid–
liquid interface.
The immobilization of the ionic liquid phase containing the
metal catalyst as a thin film adsorbed on an inorganic
support has been recently developed to circumvent these
limitations.5,8–10 Typically silica support beads with 25 wt%
of ionic liquid loading are easily obtained as free flowing
powders. Actually, the supported ionic liquid film (SILF)
approach provides major advantages: (i) a small amount of
ionic liquid, (ii) an inexpensive immobilization of the metal
catalyst without using coupling agents, (iii) a possible confine-
ment effect in relation to the concentration of both metal
catalyst and substrate in the ionic liquid film, (iv) a possible
effect of the nature of the surface support on the selectivity of
reactions.11 Moreover it could be particularly suitable for
continuous fixed-bed reactors. However, the main drawback
is the risk of leaching of ionic liquid after a few cycles.
Actually, the palladium-catalyzed Heck–Mizoroki reaction
is one of the most powerful tools for the coupling of olefins
with aryl and vinyl halides. Usually it is performed in polar
solvents (typically dimethylformamide and acetonitrile), in the
presence of phosphine ligands, which are required to stabilize
the palladium catalyst species, especially with less reactive
bromide and chloride substrates. It has been shown that the
reaction could be performed in room-temperature ionic
liquids,25–29 even without additive phosphine ligands.25,30
The mechanism by which the phosphine stabilizes the
a Institut Charles Gerhardt de Montpellier, UMR 5253
´
CNRS-UM2-ENSCM-UM1, Universite Montpellier 2, cc 1701,
F34095, Montpellier, France. E-mail: vioux@univ-montp2.fr
´
Sciences Chimiques de Rennes, UMR 6226 CNRS Universite Rennes 1,
Campus de Beaulieu, F35042, Rennes, France
b
ꢀc
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2009 New J. Chem., 2009, 33, 2015–2021 | 2015

Scheme 1 Heck coupling of 4-iodobenzene and ethyl acrylate.
palladium is not completely clear to date.31–34 However, the
[BMI][TFSI], and 1-butyl-3-methylimidazolium hexafluoro-
phosphate, [BMI][PF6], were synthesized by metathesis
reaction using lithium bis(trifluoromethylsulfonylimide) and
potassium hexafluorophosphate, respectively.42
effect of the ionic liquid solvent on the activity of palladium
catalysts has been reported.28,35,36 For instance higher
conversion and better selectivity to the trans isomer were
found in 1-butyl-3-methylimidazolium bromide ([BMI][Br])
than in the analogous tetrafluoroborate salt ([BMI][BF4]).37
In the first part, we tested the ability of Pd(OAc)2 encapsulated
in ionogels to act as a catalyst without leaching out into the
bulk solution. The coupling of ethyl acrylate with iodobenzene
was chosen as a model reaction (Scheme 1), as this typical
arylation reaction has been widely studied in bulk ionic
liquids25,29,38 and supported ionic liquid systems as well.15,39
In the second part, we applied the catalyst ionogels to the
coupling of 4-vinyl pyridine with 4-iodo-N,N-dibutylaniline
(Scheme 2). The coupling product is of interest as a labeling
molecule in biology and it is important to be able to synthesize
it without any metal traces.40
After the metathesis reaction, the ionic liquids were washed
several times with aliquots of water until no more halogen
residues could be detected by the AgNO3 test. The 4-iodo-
N,N-dibutylaniline was synthesized according to the literature.40
Synthesis
Synthesis of catalyst ionogels for the coupling reaction of
ethyl acrylate with iodobenzene
Ionogel without phosphine. For the reaction without
phosphine, a stock dichloromethane solution was prepared
by dissolving Pd(OAc)2 (22.4 mg; 0.1 mmol) into 2 ml of
CH2Cl2. 0.2 mL of this solution was added to a mixture of
[BMI][Br] (0.74 g, 0.5 eq.) and formic acid (2 mL, 2 eq.); the
resulting solution was stirred at room temperature for 10 min.
Finally, tetramethoxysilane (TMOS) (1 mL, 1 eq.) was added
and the mixture was stirred for another 10 min. A yellow
ionogel pellet (Fig. 1) was obtained after 1 h and was aged
until no more volatiles evolved (constant weight was observed
after about two weeks). The final concentration of Pd in ionic
Experimental
Instrumentation
The Heck couplings of 4-vinyl pyridine were performed in
parallel with a 24-tube semi-automated synthesis robot
liquid was 1 Â 10À2 mol LÀ1
.
(Syncore from Buchi). Gas chromatography analyses were
¨
performed on a HP 6890 series gas chromatograph using a
SE30 column, dihexyl ether as internal reference. NMR
spectra were recorded on a Bruker DPX 300 (1H 300 MHz)
Ionogel with phosphine. In this case the stock solution was
obtained by dissolving Pd(OAc)2 (11.2 mg, 0.5 Â 10À4 mol)
and triphenylphosphine (26.3 mg, 1 Â 10À4 mol) into 10 mL of
dichloromethane. 0.2 mL of this solution was added to the
mixture of [BMI][Br] and formic acid as described above.
The final concentration of palladium in ionic liquid was
1
spectrometer at room temperature. H chemical shifts (d) are
given in ppm and are referenced to residual solvent peaks
1
(CDCl3: d 7.26 ppm H; d 77.0 ppm 13C). Coupling constants
(J) are reported in Hz. The elemental analysis of palladium
was determined by ICP-MS and carried out by the Centre of
Chemical Analyses of the Centre National de la Recherche
Scientifique (Vernaison, France).
1 Â 10À3 mol LÀ1
.
Synthesis of catalyst ionogels for the coupling reaction of
4-vinyl pyridine with 4-iodo-N,N-dibutylaniline. The palladium(II)
complex was prepared by mixing Pd(OAc)2 (1 eq.) and PPh3
(2 eq.) in dichloromethane (concentration in Pd(II) 1.2 Â
10À2 mol LÀ1) and stirring at room temperature for 2 h.
250 mL of tetramethoxysilane (TMOS) and 250 mL of methyl-
trimethoxysilane (MTMOS) were mixed with 0.37 g of
[BMI][Br] and 1 mL of formic acid. After few minutes, the
ionic liquid was totally dissolved. 150 mL of the palladium
complex solution in CH2Cl2 were added so that the palladium
Chemicals
The following chemicals were purchased and used as received:
iodobenzene 98%, tetramethoxysilane 98% and methyl-
trimethoxysilane 97% from Alfa Aesar; 1-bromobutane,
Pd(OAc)2 and formic acid 99% from Acros Organics; ethyl
acrylate from Merck; PPh3 99% from Lancaster. Toluene
was distilled over sodium before use; triethylamine, dichloro-
methane and 1-methylimidazole 99% were distilled over
CaH2; 4-vinylpyridine was distilled before use.
concentration in the ionic liquid was 3.6 Â 10À3 mol LÀ1
.
(Additional experiments indicated that, if the concentration of
Pd(II) per litre was higher than 3.6 Â 10À3 mol LÀ1 in the ionic
liquid, the resulting ionogels were not transparent and the
results were not reproducible). After a short stirring to obtain
1-Butyl-3-methylimidazolium bromide, referred to as
[BMI][Br], was prepared according to the literature.41 1-Butyl-
3-methylimidazolium
bis(trifluoromethylsulfonylimide),
Scheme 2 Heck coupling of 4-vinylpyridine and 4-iodo-N,N-dibutylaniline.
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This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2009
2016 | New J. Chem., 2009, 33, 2015–2021

After cooling to room temperature, the solution in tube B
was separated from the ionic liquid by means of a liquid–
liquid extraction. The ionogel was filtrated and washed
with toluene. In all cases, the solvent was removed under
reduced pressure affording the target compound. The
conversion and selectivity E/Z of crude product were
1
determined by H-NMR.
Fig. 1 Pd-doped ionogel pellet.
1H-NMR (CDCl3) E isomer: d 8.54 (2H, d, J = 6 Hz), 7.38
(2H, d, J = 9 Hz), 7.26 (2H, d, J = 6 Hz), 7.15 (1H, d, J =
16 Hz), 6.73 (1H, d, J = 16 Hz), 6.62 (2H, d, J = 9 Hz), 3.32
(4H, q, J = 7 Hz), 1.57 (4H, m), 1.37 (4H, m), 0.97
(6H, t, J = 7 Hz).
13C-NMR (CDCl3) E isomer: d 149.65 (2 CH Ar), 128.98
(1 CHQ), 128.56 (2 CH Ar), 120.34 (2 CH Ar, 1 CHQ),
111.11 (2 CH Ar), 50.75 (2 CH2), 29.48 (2 CH2), 20.35 (2 CH2),
14.02 (2 CH3).
1H-NMR (CDCl3) Z isomer: d 8.44 (2H, d, J = 6 Hz), 7.22
(2H, d, J = 6 Hz), 7.09 (2H, d, J = 9 Hz), 6.59 (1H, d, J =
12 Hz), 6.20 (1H, d, J = 12 Hz), 6.47 (2H, d, J = 9Hz), 3.26
(4H, q, J = 7 Hz), 1.57 (4H, m), 1.37 (4H, m), 0.96 (6H, t,
J = 7 Hz).
13C-NMR (CDCl3) Z isomer: d 149.73 (2 CH Ar), 134.02
(1 CHQ), 130.33 (2 CH Ar), 123.51 (2 CH Ar), 122.98
(1 CHQ), 110.96 (2 CH Ar), 50.68 (2 CH2), 29.44 (2 CH2),
20.35 (2 CH2), 14.02 (2 CH3).
an orange homogeneous mixture the solution can be cast in
one pellet or distributed into 4 Eppendorf tubes to generate
several duplicates of ionogels showing the same shape, the
same active surface and the same amount of Pd(II).
This step must be done before the gelation which occurred
within 2 h. After aging for two weeks at room temperature the
ionogels were obtained as orange transparent pellets or small
cones (each containing 4.5 Â 10À7 mol of Pd) (Fig. 2).
Coupling reaction of ethyl acrylate with iodobenzene. The
ionogel encapsulating the Pd catalyst was put in the glass
‘‘basket’’ of the reactor. Toluene (17 mL), iodobenzene
(0.1 mL, 0.90 mmol), ethyl acrylate (0.2 mL, 1.8 mmol) and
triethylamine (0.2 mL, 1.4 mmol) were added under nitrogen
atmosphere. The palladium loading for the phosphine free
reaction was 1.1% and in the case of the phosphine reaction,
0.1%. The solution was stirred at 100 1C and the reaction
was monitored by GC (dihexyl ether as internal reference).
When the reaction was complete, the reactor was left to cool to
room temperature. Excess toluene and triethylamine were
evaporated affording ethyl cinnamate.
Results and discussion
Choice of the ionic liquid
Preliminary tests were carried out to check the solubility and
stability of Pd(OAc)2 in different imidazolium salts: [BMI][Br],
[BMI][PF6] and [BMI][TFSI]. Clear yellow solutions were
obtained in the cases of [BMI][PF6] and [BMI][TFSI]; never-
theless these solutions turned into black colloids within a few
hours under ambient conditions. This phenomenon was even
worse in the presence of formic acid (Fig. 3). However, the
behavior of [BMI][Br] was different, affording a yellow
solution when mixed with Pd(OAc)2 in diluted form, or red
solution when concentrated, and remaining stable over weeks
even in the presence of formic acid (Fig. 3). The formation of
Coupling reaction of 4-vinyl pyridine with 4-iodo-N,N-dibutyl-
aniline. 150 ml of Pd(OAc)2–PPh3 solution in CH2Cl2
(1.2 10À2 mol LÀ1) were introduced, after 2 h of mixing, into
two tubes A and B. After evaporation of CH2Cl2 the tubes A
and B contained 1.8 Â 10À6 mol of Pd. 0.37 g of ionic liquid
[BMI][Br] were added to the tube B. 4 small ionocats were
added into a third tube C (entry 3) in order to have the same
amount of Pd in tubes A, B and C. 1.8 ml of toluene–NEt3
(4/1, v/v) containing 1.8 Â 10À4 mol of 4-vinylpyridine
and 4-iodo-N,N-dibutylaniline were added under nitrogen
atmosphere into tubes A, B and C and stirred with an orbital
shaker at 80 1C for 72 h.
Fig. 3 Pd(OAc)2 in [BMI][PF6] (left) before and after addition of
HCOOH; Pd(OAc)2 in [BMI][Br] (right) before and after addition
of HCOOH.
Fig. 2 Pd-doped cone ionocats.
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This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2009 New J. Chem., 2009, 33, 2015–2021 | 2017

carbene ligand, which would be able to stabilize palladium
species,37 was first assumed, but no carbene was detected at
room temperature by 1H NMR. The red color is more likely to
be related to the formation of [PdBr4]2À species.43
Accordingly, the ionogels, the synthesis of which spreads
over weeks, were prepared from 1-butyl-3-methylimidazolium
bromide. The ionogels were synthesized using a sol–gel route
first described by Sharp44 and already applied to the synthesis
of monolith ionogels.13 Typically, silica precursors (1 eq.) and
formic acid (7.8 eq.) were dissolved in [BMI][Br] (0.5 eq.) and
stirred at room temperature. After gelation, the gels were left
for aging until a constant weight (no more volatiles evolved)
was obtained affording yellow homogeneous pellets or cones
(Fig. 1 and 2).
Model reaction: coupling reaction of ethyl acrylate with
iodobenzene
The activity of catalyst ionogels was first evaluated in the
coupling reaction of ethyl acrylate with iodobenzene
(Scheme 1) using a glass ‘‘basket’’ reactor (Fig. 4), so that
the Pd-doped ionogel soaked in toluene solvent (17 mL)
whereas it was not ground by the stirring magnet (the glass
featured small holes allowing free exchange from the solvent to
the ionogel).
For comparison, the coupling reaction was also carried out
in toluene under classical homogeneous catalysis conditions,
in the presence and absence of triphenylphosphine. The
loading of palladium was 1.1% in the case of phosphine free
systems, but due to solubility restrictions the loading with
Pd(OAc)2–PPh3 was 0.11%. Long reaction times were
observed, in relation to the very low substrate concentrations
used. The progress of coupling reactions under homogeneous
conditions and that of reactions in the presence of catalyst
ionogels are compared in Fig. 5. A marked initiation period is
Fig. 5 Heck reaction of 4-iodobenzene and ethyl acrylate as a
function of time (A) without phosphine (B) with phosphine;
K ionogel, & homogeneous.
visible in ionogel systems in the absence of phosphine, whereas
in the presence of triphenylphosphine the initial rate is
higher than under homogeneous conditions. Comparing the
two ionogel systems, the reaction rate was higher in the
presence of phosphine even though the catalyst concentration
was 10 times lower. Note that an increase in maximal
conversion was noticeable in ionogel systems, particularly in
the presence of phosphine. At the end of the reaction, the
ionogel which was fragmented, black and embedded in the salt
products was removed with the basket. No leaching of the
ionic liquid to the solvent mixture was observed based on
the 1H NMR spectrum of the filtrate after evaporation of
volatiles.
A key point for heterogeneous catalysis is the leaching of
metal species. Here the absence of palladium leaching was
checked by the ‘‘hot filtration technique’’.45 The hot reaction
mixture was filtered off from the reactor at a low conversion
ratio while the temperature was held at 100 1C. As displayed in
Fig. 6, the coupling reaction stopped at this moment, thus
evidencing that the palladium catalyst species were kept
encapsulated within the ionogel while the reaction ran. This
supported the catalyzed reaction actually taking place in the
ionic liquid phase confined in the silica network. This point
was confirmed by trace analysis of Pd measured by ICP-MS:
0.9 r x r 1 ppm (r0.4 wt%) was detected in the filtrate.
Note that ionogel pellets underwent significant fragmentation
during the reaction course (Fig. 4), which could be related to
Fig. 4 Basket reactor before (top) and after reaction (bottom).
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This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2009
2018 | New J. Chem., 2009, 33, 2015–2021

hereafter referred to as ionocats, were prepared (Fig. 2), each
containing 4.5 Â 10À7 mol of Pd and 0.092 g of [BMI][Br].
Three parallel experiments were performed (Table 1) to
compare the homogeneous catalyst, the homogeneous catalyst
in the presence of ionic liquid and the ionocats. The
palladium–substrate ratio was kept constant at 1% and the
dilution was the same. After 72 hours, similar conversions
were obtained as shown in Table 1. However, a second use of
ionogel catalysts was found to result in a considerable
lowering of the yield (o5%). Moreover an important
fragmentation of the pellets was observed, as in the ethyl
cinnamate synthesis.
Discussion
Similar reaction rates were found under classical homo-
geneous conditions and in the presence of Pd catalyst
encapsulated in ionogels, without apparent limitations related
to the diffusion of reactants and products between the internal
ionic liquid and the external solvent, whereas leaching tests
showed that catalysis actually took place in the ionic liquid
phase confined within the silica matrix. It is a major advantage
to implement coupling reactions without any contamination of
products with metal traces, for such application as drugs and
molecular probes in life science.
Another feature was that the triethylammonium salt
by-product was not dispatched in the toluene phase, but was
kept in the ionic liquid. The extensive formation of salt in the
course of the reaction resulted in dramatic fragmentation and
embedding of catalyst ionogels. The question arises, to what
extent the continuous in situ formation of triethylammonium
iodide modifies the nature of the ionic liquid medium and
accordingly the course of the reaction. Unfortunately washing
the catalyst ionogels for reusing is not possible since [BMI][Br]
is water soluble. However, this efficient salt trapping by
ionogels represents a practical advantage. Usually, homo-
geneous Heck–Mizoroki coupling reaction involves a purification
step to separate metal residues, salts and target products.
This purification step, generally column chromatography or
liquid–liquid extraction with water, is time-consuming and
needs significant amounts of solvent. Using catalyst ionogels,
simply decanting could be enough to eliminate both metal
and salt residues when coupling reactions are quantitative.
Alternatively a tea-bag-like reaction could be carried out,
using a basket, as in the coupling of ethyl acrylate with
iodobenzene.
Fig. 6 Hot filtration analysis using ionogels (A) without phosphine
(B) with phosphine; K continuous reaction, m reaction before hot
filtration, D reaction after hot filtration.
the action of triethylamine base on the silica network or/and
the extensive formation of triethylammonium bromide within
the mesopores.
Coupling of 4-iodo-N,N-dibutylaniline and 4-vinyl pyridine
The preparation of ionogels was slightly modified by using a
50 : 50 TMOS–MTMOS mixture as starting silicon alkoxides.
The idea was to decrease the hydrophilicity of pore walls,
which could favor the diffusion of organic reactants. The
absence of leaching in toluene was checked again, using a
somewhat different protocol. Toluene was added to the
Pd(II)–PPh3 containing ionogel. After heating for 2 hours at
80 1C, the solution was removed and engaged in the Heck
coupling reaction under the usual conditions; once again no
catalysis was detected. On the other hand, the ionogel pellet
heated for 2 h at 80 1C in toluene was shown to be still able to
induce the catalytic coupling reaction.
The last interest for the encapsulation of Pd catalyst into
ionogels is the opportunity to generate duplicates of catalysts
from the same mother solution by dispatching the sol–gel
solution into small tubes before gelation. One can easily use
one, two or more of these ionocats containing the same
amount of Pd catalyst, depending on the amount required
for the reaction.
The catalytic properties of the Pd(II) catalyst-containing
ionogel for the coupling of p-vinylpyridine and 4-iodo-N,N-
dibutylaniline were checked in toluene–NEt3 (4/1 v/v) at 80 1C
for 72 h. An orbital shaker (Syncore system from Buchi) was
used, rather than a classical magnetic stirrer, to prevent
grinding the pellet to dust. Drawing advantage from the
opportunity to easily generate duplicates of catalysts from
the same mother solution by dispatching the sol–gel solution
into small tubes before gelation, a series of cone ionogels,
As supported ionic liquid films (SILF), ionocats involve low
amounts of ionic liquids and combine the advantages of
homogeneous catalysis and of heterogeneous catalysis.
However, the processing of metal catalyzed reactions is quite
different in ionogels and SILF. In the latter, reactants diffuse
through the residual voids in pores, the walls of which are only
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This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2009 New J. Chem., 2009, 33, 2015–2021 | 2019

Table 1 Conversion and selectivity %
Entry
Conditionsa
Conversionb (%)
Selectivityb E/Z (%)
1
2
3
Toluene–NEt3–Pd(OAc)2–2PPh3
Toluene–NEt3–[BMI][Br]–Pd(OAc)2–2PPh3
Toluene–NEt3–ionocats
25
20
20
95/5
97/3
98/2
a
b
Ratio of palladium to substrate: 1%. Measured by H NMR.
1
coated with the liquid catalyst film. On the other hand, in
ionogels, reactants diffuse in the 3D continuous liquid phase
which fills all the pores. Similarly products diffuse back out of
the material according to the two different routes. Thus,
diffusion should be by far easier in SILF. However, in the
liquid phase SILF are subjected to the risk of extensive
leaching, in relation to the minute amount of ionic liquid on
the support. Even in the case of complete immiscibility in the
feedstock–product mixture, the ionic liquid film may be
mechanically removed from the support by the liquid flow.
As a consequence, SILF look like the most convenient process
for continuous gas-phase reactions, whereas it is demonstrated
here that ionogels offer an interesting alternative for liquid-
phase reactions.
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Conclusion
In conclusion, Pd containing ionogels prepared by the
incorporation of a Pd complex into the ionic liquid solution
before gelation were shown to be active in the Heck–Mizoroki
coupling reaction. These ionogels could be cast as small
cones containing identical Pd contents, thus providing
convenient ready-to-use catalyst doses for organic synthesis.
No significant difference was noticeable by comparison
with corresponding homogeneous catalysis. No leaching of
catalytic species took place during the reaction. Moreover,
ammonium salts were shown to be trapped in the ionogels.
This provides an efficient method for isolating the products,
free of metal and salt residues, and offers some advantages
over SILF when operating in the presence of an organic
solvent.
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Acknowledgements
This study was performed with the supports of the
Regional Councils of Languedoc-Roussillon (grant SV)
and of Britanny (grant MG) and was carried out with
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