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catalyst. In contrast to BMImBF4 and BMImPF6,
DMPIIm lacks acidic proton and BFꢀ4 and PF6ꢀ anions.
To elucidate which mechanism is at work, we have car-
ried out hydrogenation of aromatic ketones in a mixture
of n-butyl-2,3-dimethylimidazolium tetrafluoroborate
(BDMImBF4) and isopropanol. Interestingly, complete
conversions were obtained for all the substrates tested
under the same conditions (although with slightly lower
eeÕs) (see the last column in Table 1). This control experi-
ment supported the argument that the generation of
Arduango carbenes under catalytic conditions is respon-
sible for the low catalytic activity observed for the reac-
tions carried out in BMImBF4 and BMImPF6. This
work thus shows that the nature of RTILs can have a
significant influence on their performance in catalytic
reactions.
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7. A typical procedure for the synthesis of Ru precatalysts: a
mixture of [Ru(benzene)Cl2]2 (23 mg, 0.046 mmol) and L1
(78 mg, 0.1 mmol) in anhydrous DMF (4 mL) was heated
at 100 °C under nitrogen for 1/2 h and was then cooled to rt
for the addition of (R,R)-DPEN (21.5 mg, 0.1 mmol). With
stirring, DPEN dissolved in ꢁ15 min and the color of
solution slowly changed from orange-red to yellow. After
stirring at 80 °C for 2 h, solvent was removed under
vacuum to give desired yellow solid of precatalyst, which
was used for asymmetric hydrogenation reactions without
further purification.
8. A typical procedure for asymmetric hydrogenation of aro-
matic ketones: 3.3 lmol of precatalyst was transferred into a
vial inside a glove box. Under N2 gas flow, 2 mol% KOtBu
in 0.5 mL of isopropanol was syringed into the vial,
followed by the addition of 1-acetonaphthone (0.5 mL,
3.3 mmol) and BMImBF4 (0.5 mL). The reaction mixture
was quickly transferred into a stainless steel autoclave and
sealed. After purging with H2 for six times, final H2
pressure was adjusted to 700 psi. After 24 h, the autoclave
was depressurized and the reaction mixture was washed
with 3.1 mL of hexane–IPA (30/1; v/v) three times. The
organic solvent was removed under reduced pressure and
the residue was passed through a mini silica gel column
using diethyl ether as the eluent. An aliquot was analyzed
on GC to give conversion and ee values.
We have also demonstrated that both DMPIIm and cata-
lyst 1 can be recycled and reused several times for
asymmetric hydrogenation of 1-acetonaphthone in a
mixture of DMPIIm/isopropanol.9 With 0.1 mol% cata-
lyst loading, 1-acetonaphthone was hydrogenated at
98.5% ee and 100% conversion, 98.1% ee and 100% con-
version, 95.9% ee and 92% conversion, 94.5% ee and
40% conversion, 93.1% ee and 32% conversion, and
92.9% ee and 32% conversion, for six consecutive runs.
Direct current plasma (DCP) spectroscopy further
showed that no appreciable leaching of Ru occurred
during the extraction of organic products. We estimated
from DCP experiments that less than 0.01% of the Ru
catalyst has leached into the chiral alcohol phase from
the DMPIIm phase.
In summary, we have successfully applied polar phos-
phonic acid-derived Ru(BINAP)(DPEN)Cl2 systems
for asymmetric hydrogenation of aromatic ketones in
RTILs with high eeÕs. Both RTIL and RTIL-immobi-
lized catalysts were recycled and reused with no appre-
ciable leaching of Ru into the chiral alcohol products.
We believe that immobilization of homogeneous asym-
metric catalysts in RTILs represents an interesting
approach toward catalyst recycle and reuse and
preventing the leaching of toxic metals into the organic
products.
Acknowledgements
We acknowledge financial support from NSF (CHE-
0208930). We thank Dr. Allen F. Glazner for helpwith
DCP experiments and Covalent Associates for supply-
ing DMPIIm. W.L. is an Alfred P. Sloan Fellow, an
Arnold and Mabel Beckman Young Investigator, a
Cottrell Scholar of Research Corp, and a Camille Drey-
fus Teacher–Scholar.
9. For IL and catalyst recycle and reuse experiments, the
reaction mixture was extracted with degassed distilled
hexane–IPA three times under N2 gas flow. The organic
layer was siphoned from the RTIL and the IL phase was
dried under vacuum for 12 h before the addition of new
ketone substrate, KOtBu, and IPA for another round of
hydrogenation reaction.
References and notes
1. (a) Knowles, W. S. Adv. Synth. Catal. 2003, 345, 3–13; (b)
Noyori, R. Adv. Synth. Catal. 2003, 345, 15–32; (c) Zhou,