Asymmetric Transfer Hydrogenation of Aromatic Ketones
reaction with the [Rh(cod)Cl]2/NHC catalyst system
(Table 4, Entries 3 and 4). Introduction of an electron-with-
drawing group, however, led to much lower enantio-
selectivity, but higher yield (Table 4, Entry 6). On the other
hand, a slight negative effect on yield and enantioselectivity
was observed when performing the ATH with acetonaph-
thone in lieu of acetophenone, presumably due to increased
substrate bulk (Table 4, Entries 7 and 1). This was con-
firmed when increasing the bulkiness of the R group, next
to the active center, where very low activity and enantio-
selectivity were recorded (Table 4, Entries 2 and 8).
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Conclusions
In summary, we have developed a new and flexible pro-
cedure for the synthesis of chiral azolium salts, precursors
to NHCs, derived from (S)-pyroglutamic. We have also
demonstrated the successful application of rhodium cata-
lysts containing chiral NHC ligands in the ATH of aro-
matic ketones. Good yield and enantioselectivity were ob-
served for acetophenone as the substrate. Although the re-
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hydrogenation catalysts incorporating a chiral NHC ligand.
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Experimental Section
General Procedure for Catalytic Transfer Hydrogenation of Ketones:
In a 20-mL dry Schlenk tube under an argon atmosphere, the in
situ catalyst was prepared by stirring a solution of imidazolium salt
7 (0.06 mmol) and tBuOK (0.066 mmol) in degassed and dried
THF (5 mL) for 30 min at room temperature. [Rh(cod)Cl]2
(0.01 mmol) was added, and the resulting mixture was heated at
60 °C for 1.5 h. After evaporation of THF, iPrOH (5 mL), KOH
(0.02 mmol), and the corresponding ketone (1.0 mmol) were suc-
cessively added. The reaction mixture was heated at 80 °C for 20 h.
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raphy column on silica gel (pentane/diethyl ether, 95:5). Enantio-
meric excess values were determined by chiral HPLC or GC analy-
sis.
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Acknowledgments
We are grateful to the Interchim Company for a doctoral fellowship
to A.A., the Centre National de la Recherche Scientifique (CNRS-
UMR 8182), and the Université Paris-Sud 11 for financial support.
We also thank Emilie Kolodziej for technical assistance with the
HPLC analysis.
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