1610
S. C. Zinner et al. · Enantioselective Hydrosilylation
tion temperature did not significantly improve stereos- Experimental Section
electivity in any case, but always resulted in very low
yields.
Catalyst synthesis
Compound 1 was synthesized according to a literature
procedure [6]. GC spectra for determination of the catalytic
result were measured on a Varian gas chromatograph CP-
3800 (column: FactorFour VF-5 ms) equipped with an FID
detector.
Conclusion
The catalytic activity and enantioselectivity of the
complexes [(4X,5X)-1,3-bis[2,6-diisopropylphenyl]-
4,5-ditert-butylimidazolin-2-ylidene][1,5-cyclooctadi-
ene]-iodorhodium(I) with X = R or S for the hydrosi-
lylation of prochiral ketones was investigated. Activity
and enantiomeric excess were found to depend on
solvent, substrate, silane, and catalyst enantiomer
General procedure for catalytic reactions
Compound 1 (0.6 mmol) was dissolved in the appropri-
ate solvent (2 mL). The appropriate ketone (10 mmol) and
diethylenglycoldibutylether (375 µL) were added as inter-
employed. Among the solvents tested, tetrahydrofuran nal GC standards. Then the silane (60 mmol) was injected.
After a certain time, a sample of 1 mL was taken from the
reaction mixture. Methanol (1 mL) and sodium hydroxide
(1 mL, 0.5 M) were added to deprotect the silylated products
for analysis. After ten minutes of stirring, the product was
extracted using diethyl ether and filtered through glass wool.
The ratio of products and starting materials were determined
by GC analysis.
– as the strongest donor – led to the highest conversion.
Interestingly, the (−)-enantiomer of 1 displayed a
much higher catalytic activity as the corresponding
(+)-enantiomer. By comparing the hydrosilylation of
different substrates it was shown that the catalytic
activity of the (–)-enantiomer of 1 decreases in
the following order: activated substrates (B and C)
> acetophenone (A) > deactivated substrates (D and
E). Significant enantioselectivity was only achieved
for 4-trifluormethylacetophenone (C). Further studies,
particularly including a more detailed study of the
initiation step will be performed in our group.
Acknowledgement
This work was supported by the Elitenetzwerk Bayern
(scholarship for S. C. Z.).
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