An ionic liquid-supported ruthenium carbene complex: A robust and recyclable catalyst for ring-closing olefin metathesis in ionic liquids

Article abstract of DOI:10.1021/ja021484x

The synthesis of an ionic liquid-supported olefin metathesis catalyst derived from Grubb's ruthenium carbene complex is described. This new supported catalyst has been used in BMI·PF₆ solvent, and this allowed success in solving the challenging problem of catalyst recycling. The IL catalyst in BMI·PF₆ can be recovered and reused up to 10 consecutive cycles in RCM reactions of several dienes with excellent conversions. Moreover, the IL catalyst shows a remarkable stability in BMI·PF₆ and can be stored several months without loss of activity. These results clearly demonstrate the importance of anchoring an imidazolium ionic liquid pattern to the catalyst to avoid its leaching from the BMI·PF₆ phase. Copyright

Full text of DOI:10.1021/ja021484x

Published on Web 07/15/2003
An Ionic Liquid-Supported Ruthenium Carbene Complex: A Robust and
Recyclable Catalyst for Ring-Closing Olefin Metathesis in Ionic Liquids
Nicolas Audic, Herve´ Clavier, Marc Mauduit,* and Jean-Claude Guillemin*
Laboratoire de Synthe`ses et ActiVations de Biomole´cules, UMR CNRS 6052, Ecole Nationale Supe´rieure de Chimie,
Institut de Chimie de Rennes, 35700 Rennes, France
Received December 30, 2002; E-mail: jean-claude.guillemin@ensc-rennes.fr; marc.mauduit@ensc-rennes.fr
Scheme 1. Synthesis of IL Catalyst 10a
Ring-closing metathesis (RCM)1 is a powerful method to generate
heterocyclic and macrocyclic motifs present in many natural
products. Several well-defined homogeneous ruthenium carbene
complexes such as 1-52 are efficient catalyst precursors, are air
and moisture stable, and are extremely tolerant toward different
organic functional groups.
a Reaction conditions: (a) 2.2 equiv NaH, 2.2 equiv i-PrI, DMF, THF,
rt, 90%. (b) 1.05 equiv Br2, 0.04 equiv HOAc, CH2Cl2, rt, 98%. (c) 1 equiv
LiAlH4, THF, 0 °C, 95%. (d) 1.5 equiv Bu3SnCHCH2, 3 mol % Pd(PPh3)4,
toluene, 110 °C, 75%. (e) 1.5 equiv Et3N, CH2Cl2, 0 °C to rt. (f) 2 equiv
LiBr, THF, DMF, rt, 74% overall for two steps. (g) 2 equiv 1-methylimi-
dazole, toluene 110 °C. (h) HPF6, H2O, 0 °C, 87% overall for two steps.
(i) 1.5 equiv 1, 1.25 equiv CuCl, CH2Cl2, rt, 78%.
The concept of recoverable and recyclable catalysts has become
extremely important from both the environmental and economical
points of view,3 and various strategies were employed to recover
and reuse the catalyst for the RCM reaction.4
methyl-3(4-hydrophenyl)propionate 6, etherification of the phenol
group with isopropyliodide, followed by bromination of the aromatic
ring and reduction of the ester group afforded 7 in 84% yield.
Introduction of the vinyl group was accomplished by a Pd-catalyzed
Stille coupling in 75% yield. Alkylation of 1-methylimidazole with
the bromine 8, followed by anion exchange in water with HPF6,
afforded the desired hexafluorophosphate imidazolium salt 9, which
was purified by silica gel chromatography. Following Hoveyda’s
method4e for the incorporation of the Ru center, the imidazolium
styrenyl ether ligand 9 was treated with a slight excess of Grubbs’s
catalyst 1 in the presence of CuCl. Removal of excess of 1 by a
simple filtration in acetone followed by a crystallization in a 1/1
CH2Cl2/pentane mixture afforded pure IL catalyst 10 (checked by
elementary analysis) in 78% yield as an air-stable brown powder.
The N,N-diallyltosylamide 11 was directly added into a pre-
formed solution of 2.5 mol % IL catalyst 10 in BMI‚PF6. Complete
formation of the cyclic olefin 12 was obtained after heating at 60
°C for 45 min, and no cross-metathesis product has been observed
(Table 1). Importantly, isolation of the pure product was easily
accomplished by extraction with toluene, and the IL solution
containing IL-cat 10 was reused for the next cycle of metathesis.
Excellent conversions were obtained for up to nine consecutive
cycles of recycling and reuse. We have also tested the Grubbs’s
catalyst 1 and Hoveyda’s catalyst 4 diluted in BMI‚PF6 under the
same conditions of recycling and reuse (Table 1). After the second
cycle, low conversion was obtained caused by extraction of the
catalyst to the organic phase. These results show the importance
of attaching an imidazolium ionic liquid pattern to the catalyst to
avoid its leaching from the IL phase.
Ionic liquids (IL) are a new class of solvents5 which present
interesting properties such as nonvolatility, high stability, and easy
recyclability. Because these solvents are immiscible with many
organic solvents, it was very attractive to perform RCM and attempt
to recover and recycle the ionic liquid layer containing the Ru
catalyst after a simple extraction of the product. Pioneers of using
this strategy, Buijsman and co-workers,6a have reported that the
Grubbs’s Ru catalyst precursor 1 dissolved in 1-butyl-3-methylimi-
dazolium hexafluorophosphate (BMI‚PF6)7 promoted the RCM of
several dienes for at least three cycles. However low conversion
was obtained at the last cycle caused by extraction of the catalyst
to the organic phase. More recently, Dixneuf and co-workers6b have
described RCM performed by ruthenium allenylidene salts in BMI‚
PF6, but the catalyst proved to be efficient only for the first two
cycles due to the slow decomposition of the catalytic system.
During the development of the biphasic Rh-catalyzed hydro-
formylation in IL, several research groups have clearly demonstrated
that cationic ligands were especially suitable to avoid catalyst
leaching from the ionic liquid layer.8 In light of these results, we
thought that application of this strategy for RCM in IL by
introduction of an ionic liquid pattern (as an alkyl imidazolium
salt) directly bound to the ligand should avoid the problem of
catalyst leaching. Moreover, the resulting ionic catalyst should be
completely soluble in IL and would allow the RCM reaction to be
carried out under standard homogeneous conditions.
Herein, we report the synthesis of an alkyl imidazolium salt-
supported ruthenium catalyst 10 (i.e. IL catalyst 10) and its use
and recycling in BMI‚PF6.
The route for the synthesis of the ruthenium IL catalyst 10 is
illustrated in Scheme 1. Starting from the commercially available
We have investigated the chemical stability of the IL catalyst
10 in BMI‚PF6 solution and accomplished the ninth run after 3
9
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J. AM. CHEM. SOC. 2003, 125, 9248-9249
10.1021/ja021484x CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
Table 1. Comparative Recycling and Reuse in BMI‚PF6 of IL
Catalyst 10 and Catalysts 1 and 4 in the RCM of Diene 11
demanding olefin 17, we used more drastic conditions (5 mol %
of IL catalyst 10, at 60°C over 4 h). Although conversions of 92%
were obtained for the first two cycles, they decreased dramatically
to reach 73% after the third cycle due to the decomposition of the
catalyst after repeated heating. Concerning the oxygen-containing
dienes such as 19, 21, and 23, the IL catalyst proved to be efficient
only for the first two cycles. We attribute this to the slow but
competing decomposition of the catalytic system where a stable
oxygen-ligated ruthenium carbene complex can be formed due to
the presence of oxygen’s substrate. These limitations of IL catalyst
10 led us to develop a more efficient IL catalyst, based on complex
5, to promote efficient RCM with trisubstituted or oxygen-
containing olefins. Its preparation is underway in our laboratory.
In summary, we have developed an ionic liquid-supported catalyst
for olefin metathesis carried out in IL. The IL catalyst has shown
high activity with a remarkable recyclability and can be stored in
BMI‚PF6 several months without loss of activity.
cycle (% conv.a)
catalyst
1
2
3
4
5
6
7
8
9
10b
10
1
4
>98 >98 >98 >98 >98 96 92 92 92 95
>98
>98
20
40
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
20
a Determined by 1H NMR spectroscopy analysis. b 13 as starting material.
months. A similarly high conversion was obtained, showing that
IL catalyst remains active. Moreover, this recycled catalyst solution
was used for the metathesis of a second substrate 13 (cycle 10) to
lead to the cyclized product 14 with 95% conversion, but the crude
NMR spectrum showed a contamination by small amounts (<5%)
of compound 12, product of the previous reaction. This contamina-
tion can be explained by the partial miscibility of toluene in BMI‚
PF6 (23 wt % at 26°C)9 which keeps small amounts of product in
the ionic liquid phase. At last, importantly, when the IL catalyst
10 lost its activity after repeated use, the BMI‚PF6 solvent could
be recovered by treatment with black carbon6b and reused with a
new loading of catalyst.
Acknowledgment. We thank G. Quillien for the preparation
of BMI‚PF6, Dr. M. Garayt and Dr. R. Gre´e for helpful discussion,
and the Ministe`re de la Recherche et de la Technologie for financial
support (Grant to N.A. and H.C.).
Supporting Information Available: Experimental details and
analytical data for the work described (PDF). This material is available
free of charge via the Internet at http://pubs.acs.org.
Having established the recyclability and reuse of IL catalyst 10
in BMI‚PF6, we tried several RCM reactions with other substrates
(Table 2). Ring-closing metathesis of 13 has afforded the six-
membered ring 14 in quantitative conversion for each of six
consecutive cycles.
References
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conversion after the seventh cycle. However, with the sterically
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