ROMPgel-supported tris(triphenylphosphine)rhodium(I) chloride: A selective hydrogenation catalyst for parallel synthesis

Article abstract of DOI:10.1016/S0040-4039(03)00351-4

ROMPgel-supported tris(triphenylphosphine)rhodium(I) chloride has been prepared and the immobilised catalyst has been effectively employed in selective hydrogenations of a variety of alkenes and terminal alkynes.

Full text of DOI:10.1016/S0040-4039(03)00351-4

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 44 (2003) 2703–2707
ROMPgel-supported tris(triphenylphosphine)rhodium(I) chloride:
a selective hydrogenation catalyst for parallel synthesis
,
Erik Arstad, Anthony G. M. Barrett* and Livio Tedeschi
Department of Chemistry, Imperial College of Science, Technology and Medicine, Exhibition Road, London SW7 2AY, UK
Received 19 November 2002; accepted 6 February 2003
Abstract—ROMPgel-supported tris(triphenylphosphine)rhodium(I) chloride has been prepared and the immobilised catalyst has
been effectively employed in selective hydrogenations of a variety of alkenes and terminal alkynes. © 2003 Elsevier Science Ltd.
All rights reserved.
In parallel synthesis immobilized reagents1 combine the
advantages of simple purification, in that only filtration
and washing are required to remove large excesses of
reagents, with the ease of reaction monitoring associ-
ated with classical solution phase methodologies. As a
result, immobilized reagents are ideal in the synthesis of
compound libraries. In the last few years a range of
solid-supported reagents has become available and
there has been an ongoing effort to improve their
loading and physical properties. Among the different
solid supports, to which reagents can be attached,
polymers derived from ring-opening metathesis (ROM)
of norbornene and 7-oxanorbornene derivatives are
particularly attractive. They can be prepared from read-
ily available and highly functionalised monomers upon
treatment with ruthenium carbenes.2 The resulting
polymers (ROMPgels) have high loadings and undergo
significant swelling in various solvents.3
problem has been addressed by the attachment of the
catalyst to polystyrene,5 aluminosilicate,6 alumina7 and
modified silica gel.8 However, it has yet to be demon-
strated that the resulting supported catalysts are gener-
ally applicable in parallel synthesis. An alternative to
polymer supports has been the use of ionic liquids in
biphasic systems.9 Ionic liquids are immiscible with
most organic solvents but show high affinity for metal
complexes. Therefore, they can be efficiently employed
in catalytic hydrogenations using the Osborn10 complex
or the Wilkinson catalyst.11 In a biphasic ionic liquid/
organic solvent system the metal complex is completely
retained by the ionic liquid, while the reduction product
can be recovered from the organic phase. Rhodium
contamination in the products was thereby minimized.
In addition, reaction rates and selectivities for hydro-
genations performed in ionic liquids are often improved
compared with conventional solvents.
Catalytic hydrogenation is one the most important
functional group interconversions and tris(triphenyl-
phosphine)rhodium(I) chloride, Wilkinson’s catalyst, is
widely used for the reduction of unconjugated alkenes.4
It is selective for sterically unhindered substrates, but
the activity diminishes rapidly for more hindered com-
pounds with tetrasubstituted alkenes being inert, a fea-
ture widely explored in selective hydrogenations.
Despite its advantages, Wilkinson’s catalyst suffers
from one serious inherent disadvantage, namely the
need to separate the catalyst from the product. This
We envisioned that ROMP methodology could be
employed to prepare a novel supported Wilkinson cata-
lyst, allowing fine-tuning of the polymer support in
order to provide the best chemical environment for
catalyst activity. We investigated the use of different
ROM polymers, containing triarylphosphine 1, imida-
zolium units 2 or a mixture of the two. The synthesis of
ionic ROMPgels allowed us to explore the possibility of
incorporating the advantages of ionic liquids into a
solid supported reagent (Scheme 1).
ROMPgel-supported triphenylphosphine 5 was pre-
pared as previously reported,12 while the imidazolium
hexafluorophosphate monomer 2 was obtained in three
steps from commercially available starting materials
(Scheme 2). N-Alkylimidazole 8 was prepared accord-
Keywords: ROMPgels; catalytic hydrogenation; Wilkinson’s catalyst;
imidazolium salts; supported reagents.
* Corresponding author. Tel.: +44-20-759-45766; fax: +44-20-759-
45805; e-mail: agmb@ic.ac.uk
0040-4039/03/$ - see front matter © 2003 Elsevier Science Ltd. All rights reserved.
doi:10.1016/S0040-4039(03)00351-4

,
E. Arstad et al. / Tetrahedron Letters 44 (2003) 2703–2707
2704
Scheme 1.
Scheme 2. Reagents and conditions: (a) BnCl, PhMe, reflux, 24 h; (b) KPF6, CH2Cl2, rt, 12 h.
ing to a reported procedure13 and allowed to react with
benzyl chloride in toluene to give the imidazolium
chloride 9 in 78% yield. Ion exchange using potassium
hexafluorophosphate in dichloromethane gave the salt 2
in quantitative yield.
triphenylphosphine groups, attachment of the catalyst
was achieved by ligand displacement, giving rise to the
proposed structures 12 and 13. In the case of polymer
7, we investigated the possibility of supporting the
Wilkinson catalyst through non-bonding interactions
with the ionic ROMPgel (Scheme 3).
Addition of the second-generation Grubbs catalyst 3
(1 mol%) to the imidazolium monomer 2 afforded the
ionic ROMPgel 7 in 94% yield. The triphenylphos-
phine/imidazolium co-polymer 6 was obtained simply
by adding the above-mentioned catalyst to a 2:3 mix-
ture of the corresponding monomers. For simplicity
ROMPgel 6 is depicted as a block co-polymer, however
the structure is probably random (Scheme 1). The cross
linker 4 (10 mol%) was used in the synthesis of both
ROMPgels 6 and 7.
The resulting polymeric rhodium complex catalysts
were screened for their ability to catalyse the hydro-
genation of terminal alkenes (Table 1). When the cata-
lysts 12 and 13 were compared, using the same weight
percentage of polymer to substrate, the catalyst 12
turned out to be significantly more active than 13. This
clearly shows that, unfortunately, the ionic moiety does
not have the desired rate-enhancing effect on the cata-
lytic reduction. The polymer 14, obtained by heating
the ionic ROMPgel 7 to reflux in the presence of
Wilkinson catalyst 11, showed no activity.
Tris(triphenylphosphine)rhodium(I) chloride (11) was
anchored to the ROMPgels 5, 6 and 7 by heating a
suspension of the polymer and either of the rhodium
complexes 10 or 11 in dichloromethane for 18 h
(Scheme 3). In the case of polymers containing
The conditions for catalytic hydrogenation using the
best-performing polymer 12 were further optimised.14
To prevent the backbone double bonds interfering with

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E. Arstad et al. / Tetrahedron Letters 44 (2003) 2703–2707
2705
Scheme 3.
the catalyst, the polymer 12 was exposed to hydrogen
(50 psi, 1:1 THF:ethanol, 48 h, 20°C). This procedure is
likely to reduce backbone double bonds, which are
accessible to the rhodium complexes, thereby prevent-
ing competitive reduction in the presence of a substrate.
The reductive step also proved efficient in washing out
loosely bound rhodium complexes. As determined by
ICP–AES the resulting polymer contained 6.86%
rhodium, which corresponds to a loading of 0.66 mmol/
g. The phosphorous to rhodium ratio was found to be
2.78.
seen from Table 2, the ROMPgel-supported catalyst 13
is selective and allows for reduction of terminal alkenes
and alkynes in the presence of more hindered alkenes.
Most noteworthy is the selective reduction of a ring
alkene in the presence of a trans-disubstituted alkene.
Also, alkenes can be reduced in the presence of other
functionalities such as ketones, esters, amides and nitro
groups. In all cases the products are obtained in good
to excellent yields and purities.
In conclusion, ROMPgel-supported tris(triphenylphos-
phine)rhodium(I) chloride has been prepared by load-
ing the complex 10 onto ROMP-supported
triphenylphosphine, and the immobilised catalyst has
been utilised for selective hydrogenation of alkenes.
To compensate for the reduced diffusion rate of large
substrates, i.e. steroids, generally encountered in trans-
formations with supported reagents/catalysts a fixed
mass percentage of 100% of polymer 13 to substrate
was used. In this way a number of alkenes/alkynes were
selectively reduced and obtained in high yield and
purity (Table 2).
Acknowledgements
In a typical experiment, a degassed (N2) mixture of
THF and ethanol (1:1, 2 ml) was added to the substrate
(0.040 g) and the polymer 13 (0.040 g) under nitrogen.
The nitrogen was replaced with hydrogen, the reaction
mixture connected to a hydrogen source and shaken
vigorously. When the reaction had gone to completion
the resulting mixture was filtered, the polymer washed
repeatedly with dichloromethane and diethyl ether, and
the combined filtrates concentrated in vacuo. As can be
We thank Merck KGaA (to L.T.), the Norwegian
Research Council (to E.A.), the EPSRC for their sup-
,
port of this research, Glaxo SmithKline for the gener-
ous endowment (to A.G.M.B), the Royal Society and
Wolfson Foundation for a Royal Society–Wolfson
Research Merit Award (to A.G.M.B), and the Wolfson
Foundation for establishing the Wolfson Centre for
Organic Chemistry in Medical Science at Imperial
College.
Table 1.

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E. Arstad et al. / Tetrahedron Letters 44 (2003) 2703–2707
2706
Table 2. Selective hydrogenation of alkenes/alkynes using ROMPgel-supported (PPh3)3RhCl 13
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