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J. Pernak et al. / Tetrahedron Letters 52 (2011) 4342–4345
CH3
H3C Si
CH3
CH3
H3C Si
CH3
CH3
Si
CH3
CH3
Si
CH3
O
O
Si CH3
CH3
O
O
Si
CH3+ CH2=CH(CH2)5CH3
CH2
CH3
H
CH2(CH2)5CH3
Scheme 2. Hydrosilylation of 1-octene.
Figure 3. The yield of product over 10 runs with the recovered catalytic system for the hydrosilylation of 1-octene catalyzed by Pt or Rh complexes immobilized in pyrylium
IL 6.
ILs are recognized as potential media for the immobilisation of
catalysts and were used with considerable success in a wide range
of laboratory scale reactions.19 Having experience with catalytic
systems for hydrosilylation processes, based mainly on rhodium
siloxide complexes immobilized in imidazolium and phosphonium
ILs,20–22 we decided to test pyrylium ILs. In the present study we
employed a rhodium complex [Rh(PPh3)3Cl] and complexes of
platinum at different oxidation states, namely [Pt(PPh3)4],
[Pt(PPh3)2Cl2], and [PtCl4], immobilized in liquid pyrylium sulfo-
nate 6. The obtained systems were used as catalysts for the model
reaction of octene hydrosilylation (Scheme 2).
Our studies showed that immobilisation of the complexes in
sulfonate 6 gave biphasic systems which enabled easy separation
of the products from the catalytic system and its reuse in subse-
quent catalytic runs. In all cases only one product was formed
(the b addition product). Figure 3 shows the yields of product over
10 reaction cycles for the reaction catalyzed by a particular metal
complex immobilized in pyrylium IL 6.
The catalytic activity of most of the systems studied was high
and enabled product yields above 50% to be obtained in subse-
quent runs, except for the catalytic system based on [PtCl4]. How-
ever, the highest catalytic activity as well as high stability and
reproducibility were shown by the systems based on [Rh(PPh3)3Cl]
and [Pt(PPh3)2Cl2], which made it possible to obtain the product in
yields over 90% in 10 subsequent runs.
Pyrylium ILs are characterized by thermal stability and have
very low, but not null vapor pressure. The hydrosilylation process,
employed in our study as a test reaction, is usually carried out
without a solvent. ILs chosen for such an application have to be
of high purity (no contaminants able to poison the catalyst) and
should dissolve and immobilize the metal complex, while main-
taining its high catalytic activity.
interact with the products. The ILs applied in this study have
met all these requirements which resulted in a highly effective
catalytic system for the reaction studied (particularly in the case
of the rhodium siloxide complex). Another advantage of the IL
was its immiscibility with the reactants and ability to form well-
separated phases, which made isolation of the post-reaction mix-
ture from the catalytic system very easy. The hydrosilylation in
the presence of four different complexes of metals indicated differ-
ences in their catalytic performance, particularly when they were
reused. In the case of [Rh(PPh3)3Cl] and [Pt(PPh3)2Cl2] complexes,
immobilization in the IL was excellent and they were well-pro-
tected against leaching and showed high catalytic activity. The
two other complexes were not so efficiently immobilized in the
IL, which resulted in their gradual leaching from the catalytic sys-
tem as reflected by the decreasing product yield. This fact was con-
firmed by results of post-reaction mixture analysis performed by
ICP-OES which showed no presence of Rh or Pt in the case of the
former two complexes, whereas in that of the remaining two com-
plexes the post-reaction mixture obtained after subsequent reac-
tion runs contained platinum.
4-Pyrones, obtained from petroleum feedstock and renewable
resources (maltol), are reactive cyclic compounds, capable of form-
ing both oxonium cation and cocrystals with organic acids. Very
strong sulfonic acids lead to pyrylium ionic liquids due to a proton-
ation reaction. On the other hand, carboxylic acids are too weak to
create ionic bonds, but are strong enough to yield hydrogen bond
based cocrystals. Both kinds of product could be obtained in one
step with high efficiency, and purity. These thermally-stable pyry-
lium ionic liquids are useful as immobilizers and dissolving agents
in hydrosilylation reactions.
Acknowledgments
The pyrylium IL used by us did not play the role of solvent, but
that of a metal complex immobilizing agent. Moreover, the IL
should form a bi- or multiphase system with substrates and prod-
ucts, be stable in the presence of reactive substrates and should not
We are grateful to The Ministry of Science and Higher Education
(Poland) for financial support project No. N 204 165 436 and
Poznan University of Technology – grant 32-172/2011.