J. Horn, W. Bannwarth
SHORT COMMUNICATION
To gain more insight into this unexpected accelerating over several reaction cycles. Furthermore, the ruthenium
effect, a series of experiments with unmodified (S)-BINAP content in the product was very low, which confirms the
and unmodified silica gels with different pore sizes was con- strong retention of the ruthenium catalyst on the surface
ducted. To visualise possible accelerating effects more of the fluorous silica. Ru impurities of the product ranged
clearly, the reactions were stopped after one hour at a point between 1.6 and 4.5 ppm, which is in significant contrast to
of moderate conversion.
the reactions with the pertinent untagged complex where
As can be seen in Table 4, the presence of silica gel with more than 300 ppm of ruthenium was observed. This corre-
a pore size of 500 Å during the hydrogenation of 5 led to a sponds to a maximum leaching of 1.5% of the total amount
distinct acceleration of 40% without loss of enantio- of ruthenium per run. As a result of these advantages, the
selectivity, whereas other silica gels with smaller pore sizes described strategy is recommended for the application of
gave less conversion. Even more significant accelerations oxidation-sensitive perfluorinated catalysts.
were observed with substrates 7 and 8 in combination with
000-Å pore-size silica gel (Table 5).
Finally, an accelerating effect of unmodified porous silica
(500 and 1000 Å pore size) was observed which was in one
case responsible for an up to sevenfold increase in the reac-
tion rate without concomitant loss of enantioselectivity.
1
Table 5.Hydrogenations of 7 and 8.[a]
Experimental Section
Supporting Information (see footnote on the first page of this arti-
cle): Procedures and analytical data for the asymmetric hydrogena-
tion reactions.
Acknowledgments
We thank S. Hirth-Walther for ruthenium elemental analyses and
G. Fehrenbach for HPLC and GC analyses. We are grateful to
Grace GmbH & Co. KG, Worms, for the donation of silica gel with
defined pore sizes.
[
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a] Reaction conditions: 2.6 µmol [RuCl (benzene)] , 5.2 µmol (S)-
1
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Cinnamic acid derivative 8 showed a conversion of 88%
after 42 h, which corresponds to a sixfold increase in the
reaction rate, also in the presence of 1000-Å silica gel. The
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BINAP ligand 1/Ru complex in the asymmetric hydrogena-
tions of olefins. By means of noncovalent immobilisation
on FSG, which was suspended in the reaction mixture, the
catalyst could be easily isolated and reused several times.
This methodology avoided the use of perfluorinated sol-
vents. However, the addition of BTF was essential for opti-
mal stereoselectivity. This is the first example of a perfluo-
rous BINAP catalyst that can retain its catalytic activity
[
[
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