Co-immobilization of transition-metal complexes and ionic liquids in a polymeric support for liquid-phase hydrogenations

Article abstract of DOI:10.1016/S0040-4039(02)02843-5

A new recyclable heterogeneous system is reported for reactions with a transition-metal catalyst in ionic liquid medium. It consists of a polymeric phase in which both the ionic liquid and the transition-metal catalyst are incorporated. The system is readily prepared by simple mixing of the components. In hydrogenations, the polymeric system always outperformed the 'classical' biphasic systems with ionic liquids and it could be re-used successfully without loss of activity.

Full text of DOI:10.1016/S0040-4039(02)02843-5

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 44 (2003) 1195–1198
Co-immobilization of transition-metal complexes and ionic
liquids in a polymeric support for liquid-phase hydrogenations
Adi Wolfson, Ivo F. J. Vankelecom* and Pierre A. Jacobs
Centre for Surface Chemistry and Catalysis, Faculty of Agricultural and Applied Biological Sciences, Katholieke Universiteit
Leuven, Kasteelpark Arenberg 23, 3001 Leuven, Belgium
Received 31 October 2002; accepted 11 December 2002
Abstract—A new recyclable heterogeneous system is reported for reactions with a transition-metal catalyst in ionic liquid medium.
It consists of a polymeric phase in which both the ionic liquid and the transition-metal catalyst are incorporated. The system is
readily prepared by simple mixing of the components. In hydrogenations, the polymeric system always outperformed the ‘classical’
biphasic systems with ionic liquids and it could be re-used successfully without loss of activity. © 2003 Elsevier Science Ltd. All
rights reserved.
1. Introduction
used in gas-phase hydrogenations only, where their
stability towards leaching could not be evaluated. Other
types of ILs, functioning as Lewis acid catalysts, were
immobilized by impregnation on inorganic supports10
or charcoal11 and applied in Friedel–Crafts liquid-phase
alkylations and acylations, respectively.
Transition-metal complexes (TMCs) are widely known
to be active and selective catalysts in various homoge-
neous reactions1,2. However, their recycling and separa-
tion from the formed products often remains
problematic. Numerous methods have been developed
already to heterogenize the complexes for this pur-
pose, but leaching of the complexes during reaction
and lowered activities or selectivities are often
troublesome.2–4
We report here the preparation of a new type of
supported IL-phase (SILP) for TMC-catalyzed reac-
tions, made by simple mixing of the TMC and the IL
with poly(diallyldimethylammonium chloride) (Fig. 1).
The thus formed polymeric phase simultaneously
heterogenizes the TMC and the IL. For two classical
catalysts, this will be proven to form an active and
selective leach-free system in liquid-phase hydrogena-
tions.
An increasingly popular and attractive way to achieve
heterogenization is by dissolving the TMCs in an ionic
liquid (IL) phase, which is then applied in biphasic
catalysis5–8. In addition to the heterogenization, the IL
sometimes even stabilizes the complex or acts as a
modifier that accelerates the reaction and enhances the
selectivity. On the other hand, IL-biphasic reactions
can not simply be run continuously and activities are
often lowered due to mass transfer limitations. Further-
more, in spite of the biphasic nature of the system, the
product work-up might be complicated when numerous
extractions steps are required to obtain reasonable
product yields.
2. Results and discussion
First, the leaching of both the IL (1-butyl-3-methylimi-
dazolium hexafluorophosphate, bmimPF6) and the
Supporting the IL on a solid-phase could solve these
problems. Pd/C and Rh-complexes were thus already
mixed with ILs and subsequently supported on
poly(vinylidene fluoride)-based membranes9. They were
Figure 1. The 1-butyl-3-methylimidazolium hexafluorophos-
phate (bmimPF6) IL (a) and the poly(diallyldimethylammo-
nium chloride) support (b).
* Corresponding author. Tel.: +32-16-321594; fax: +32-16-321998;
e-mail: ivo.vankelecom@agr.kuleuven.ac.be
0040-4039/03/$ - see front matter © 2003 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(02)02843-5

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A. Wolfson et al. / Tetrahedron Letters 44 (2003) 1195–1198
Figure 2. Representative hydrogenation reactions.
TMC from the SILP was screened. It was found that
the strong interactions between the IL-containing
TMC-phase and the support prevented their leaching to
the contacting solvent. Indeed, while solvents that dis-
solve the IL induced leaching from the SILP, leaching
was absent in solvents that form a biphasic system with
the IL. In general, every solvent that can be used in a
biphasic IL-system can thus also be used with the SILP.
The only exception is water, a solvent that would
dissolve the polymer. In protic solvents that form two-
phase systems with IL, the SILP was found to swell,
but no IL-leaching was observed.
neous reactions. Homogeneous reference reactions
could not be done in diethyl ether, toluene and hexane
due to the poor solubility of the complex in these
solvents. In addition, as mentioned before, the solubil-
ity of the polymer in water prevented the use of water
in the SILP-system. Moreover, attempts to prepare a
leach-free heterogeneous polymeric system without the
addition of IL failed.
Table 1. Homogeneous, biphasic and heterogeneous
hydrogenations of 2-cyclohexen-1-one with Wilkinson’s
catalyst in different solventsa
The hydrogenations of 2-cyclohexen-1-one and 1,3-
cyclooctadiene (Fig. 2a and 2b, Tables 1 and 2) with
Wilkinson’s catalyst were tested under three different
conditions: homogeneous, biphasic IL and SILP. The
homogeneous hydrogenation of 2-cyclohexen-1-one
(Table 1, entries 1 and 2) was successfully performed in
dichloromethane and tetrahydrofuran (THF). Joo et
al.12 found that water could enhance the activity of
Wilkinson’s catalyst. Indeed, the reaction rate of the
THF-system was doubled when water was added (entry
3). The presence of the polymer in solution did not
influence catalytic performances (entry 4). As expected,
performing reference reaction in biphasic IL-systems
yielded substantially lower activities than the homoge-
neous reactions, but the TMC could now be re-used
successfully (entries 5 and 6). The heterogeneous SILPs
were tested in several solvents (entries 7–9). All activi-
ties were clearly higher than for the biphasic IL-systems
and about as high as for the best water-free homoge-
Entry
Reaction
Solvent
TOFb (h−1
(re-use)
)
1
2
3
4
Homogeneous
Dichloromethane
THF
8
3
15
15
THF+waterc
THF+water
+polymerc,d
Diethyl ether
Toluene
5
6
7
8
9
Biphasic IL
SILP
2.7 (3.1)
1.1 (0.9)
9.0 (9.3)
7.0 (7.1)
7.7 (7.9)
Diethyl ether
Toluene
Hexane
a Reaction conditions: 1 mmol complex, S/C=140, 5 bar H2, 30°C, 3
h. Homogeneous: 2 mL solvent. Biphasic: 1 g bmimPF6, 4 mL
solvent. Heterogeneous: 0.2 g polymer, 1 g bmimPF6, 4 mL solvent.
b TOF=turn-over frequency.
c 1 mL of each solvent.
d Addition of 0.2 g polymer.

A. Wolfson et al. / Tetrahedron Letters 44 (2003) 1195–1198
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Table 2. Homogeneous, biphasic and heterogeneous hydrogenations of 1,3-cyclooctadiene with Wilkinson’s catalyst in differ-
ent solventsa
Entry
Reaction
Solvent
TOF (h−1) (re-use)
Selectivityb
1
2
3
4
Homogeneous
Biphasic
SILP
Dichloromethane
Diethyl ether
Diethyl ether
Hexane
13
66
65
67
64
5.9 (6.2)
14.1 (14.6)
15.0 (15.2)
a Reaction conditions: 1 mmol complex, S/C=100, 5 bar H2, 30°C, 35% conversion. Homogeneous: 2 mL solvent. Biphasic: 1 g bmimPF6, 4 mL
solvent. Heterogeneous: 0.2 g polymer, 1 g bmimPF6, 4 mL solvent.
b Selectivity to cyclooctene.
The enhanced activity of the SILP-system compared to
the biphasic IL-system, is believed to be due to
improved mass transfer between the supported IL-
phase and the substrate containing solvent phase. Upon
re-use, activities of both the biphasic IL and the SILP-
systems surprisingly increased slightly. The homoge-
neous reaction also showed such a somewhat increasing
activity in time, but this phenomenon was not further
studied. Atomic absorption spectroscopy was done on
the extractant phases, but metal traces were never
detected.
functions as proton donor in this type of reaction,14
protic solvents are usually required. In the alcoholic
solvents, formation of the corresponding acetals was
observed throughout, explaining the lower selectivities
in Table 3. Changing from methanol to isopropanol
induced a lower activity, but a higher selectivity to
methyl hydroxybutyrate at constant enantioselectivity
(entries 1 and 2). The biphasic IL-system in isopropanol
yielded lower activity and selectivity when compared to
the isopropanol homogeneous reaction (entry 3).
Again, the SILP-reaction in isopropanol showed higher
activity (entry 4) than the biphasic IL-reaction, but the
performances of the homogeneous reactions this time
could not be reached. It is evident that mass transfer in
the polymeric system becomes more critical for this fast
reaction. A more profound characterization and opti-
mization of the supporting polymeric phase could pos-
sibly solve this problem. Here as well, the SILP-system
could be re-used without significant loss of activity,
selectivity or enantioselectivity and no traces of com-
plex were detected in the liquid phases.
The reduction of 1,3-cyclooctadiene to cyclooctene and
cyclooctane with Wilkinson’s catalyst (Fig. 2b) under
homogeneous, biphasic IL and SILP-conditions was
also investigated (Table 2). At similar conversion levels,
the selectivities were comparable for all systems. Here
again, the traditional biphasic IL-system showed a
lower activity than both the homogeneous and the
SILP-catalyzed reaction, due to the quality of the mix-
ing. The SILP-system was now even slightly faster than
the homogenous reference reaction. Re-use of both
recyclable systems showed again a slightly increased
activity with unchanged selectivity.
3. Conclusion
In order to expand the application of the new heteroge-
neous system to other catalytic systems, the asymmetric
reduction of methyl acetoacetate with Ru-BINAP was
selected (Fig. 2c). It was previously found that the
presence of high amounts of water decreased the enan-
tioselectivity of homogeneous hydrogenations with Ru-
BINAP, probably due to dissociation of the chloro
ligand13. Hence, in the preparation of the Ru-BINAP/
SILP-system, the aqueous polymer solution was first
evaporated, followed by a re-dissolution of the crys-
talline polymer in methanol. Since the solvent also
The poly(diallyldimethylammonium chloride) based
SILP provides an easy way to heterogenize both the IL
and the TMC. This new, leach-free system was success-
fully tested and re-used in a selection of liquid-phase
hydrogenations. It thus offers the potential to use ILs
under continuous reaction conditions and to facilitate
the separation of the products from the reaction liquid.
For all reactions studied, the activity of the SILP was
higher than for the ‘classical’ biphasic IL-system, as
ascribed to improved mass transfer between the IL-
Table 3. Homogeneous, biphasic and heterogeneous enantioselective hydrogenations of methyl acetoacetate with Ru-BINAPa
Entry
Reaction
Solvent
TOF (h−1) (re-use)
Ee (%)
Selectivityb
1
2
3
4
5
Homogeneous
Methanol
103
74
88
16 (15)
29 (29)
99
99
99
97
97
85
93
91
83
87
Isopropanol
Methanolc
Isopropanol
Isopropanol
Biphasic
SILP
a Reaction conditions: 2 mmol complex, S/C=140, 40 bar H2, 60°C, 1 h. Homogeneous: 4 mL solvent. Biphasic: 1 g bmimPF6, 4 mL solvent.
Heterogeneous: 0.2 g polymer, 1 g bmimPF6, 4 mL solvent.
b Selectivity to methyl hydroxybutyrate due to the formation of the corresponding acetal.
c Addition of 0.2 g polymer.

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A. Wolfson et al. / Tetrahedron Letters 44 (2003) 1195–1198
phase and the solvent. The selectivities equaled those of
the homogeneous reference reactions. Whereas the recy-
clable system based on Ru-BINAP was slower than the
homogeneous reference system, the Wilkinson-SILP
showed activities that were comparable to those of the
homogeneous reaction.
product from the IL-phase. Atomic absorption spec-
troscopy was done on the collected extractant phases,
but metal traces were never detected.
4.4. Reactions with Ru-BINAP
For the enantioselective hydrogenation of methyl aceto-
acetate with Ru-BINAP, 2 mmol of complex was used
with 32.5 mg of substrate and 4 mL of solvent at 40 bar
hydrogen pressure at a temperature of 60°C.
4. Experimental
4.1. Materials
Conversion, selectivity and enantioselectivity were
determined via GC-analysis with a chiral column, Chi-
raldex G-TA, after extraction of substrate and product
from the IL-phase. Atomic absorption spectroscopy
was done on the collected extractant phases, but metal
traces were never detected.
Complexes, substrates and ionic liquid were purchased
from Fluka. Solvents were obtained from Across. An
aqueous 20 wt% solution of poly(diallyldimethylammo-
nium chloride) (average molecular weight 200,000–
350,000 Da) was purchased from Aldrich.
4.2. General procedures
Acknowledgements
The homogeneous, the biphasic and the SILP-reactions
were done in 10 mL stainless steel pressure reactors
with magnetic stirring.
A.W. acknowledges a postdoctoral fellowship in the
frame of the IAP-network in supramolecular chemistry
and catalysis, sponsored by the Belgian Federal Gov-
ernment. The research is done in the same framework.
A grant from the Concerted Research Action of the
Flemisch Government (GOA) is gratefully acknowl-
edged.
In the IL-biphasic reactions, the TMC was added to the
bmimPF6-phase before addition of the reactant
mixture.
The SILP-systems were prepared as follows. The TMC
was dissolved in 1 g of bmimPF6 and then mixed with
1 g of a 20 wt% aqueous or methanolic polymer
solution. This solution was dried by means of a nitro-
gen flow at 80°C. The air sensitive Ru-BINAP complex
was handled under nitrogen atmosphere. After the first
run, all reactants were extracted from the IL-phase by
adding 3×4 mL of the organic solvent. The catalyst was
re-used by adding a fresh substrate solution to the
SILP.
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The reduction of 1,3-cyclooctadiene with Wilkinson’s
catalyst was performed under the same conditions.
Samples were taken after 3 h for the reduction of
2-cyclohexenone, while samples were withdrawn as a
function of time for the reduction of 1,3-cyclooctadiene
in order to compare the performances at similar conver-
sion levels. Both, conversion and selectivity were deter-
mined via GC-analysis with a capillary column, type
CP-Wax-58 CB, after extraction of substrate and