could not react. The heterogeneous nature of the catalysis was verified by a
Hg-poisoning experiment and a hot filtration experiment. And the catalyst could
be reused for five cycles without significant loss of activity and aggregation of
nanoparticles.
2.2 Chiral Diamine-Modified Nanoparticle Catalysts
Asymmetric hydrogenation of nonactivated aromatic ketones is an atom economical
process to prepare chiral alcohols. However, early attempts using chiral ligandmodified heterogeneous catalysts, such as proline-modified Pd/C, cinchonamodified Pt/C, and chiral organotin-modified Pt or Rh, could not achieve high
enantioselectivity (~20% ee) [37–40].
In 2007, Zhao et al. reported R,R-1,2-diphenylethylene diamine ((R,R)-DPEN)modified Ru on γ-Al 2 O 3 -catalyzed asymmetric hydrogenation of acetophenone
[41]. A phosphine ligand was additionally employed to effect on the conversion
and enantioselectivity, and a relatively high ee (60.5%) was obtained. In the same
year, Chen et al. reported asymmetric hydrogenation of aromatic ketones in an ionic
liquid using a monophosphine TPPTS [P(m-C 6 H 4 SO 3 Na)]-stabilized Ru catalyst
and chiral diamine 3 as a modifier [42]. RuCl 3 ‧3H 2 O was reduced by H 2 in the
presence of TPPTS as a stabilizer to generate Ru nanoparticles whose size was about
5 nm. The reactions were performed in a co-solvent system with ionic liquid
[RMIM]Ts (1-alkyl-3-methylimidazolium p-methylphenylsulfonates, R ¼ ethyl,
butyl, octyl, dodecyl, hexadecyl), and various aromatic ketones were hydrogenated
with high conversions and moderate to high enantioselectivities (Scheme 7). The
ionic liquid substituted with the longer alkyl chain decreased enantioselectivity
because the Ru catalyst modified with hydrophilic chiral diamine 3 was reduced
due to its high solubility in water. This Ru nanoparticle catalyst system was more
active than the corresponding metal complex (RuCl 2 (TPPTS) 2 ) under the same
conditions, although similar high enantioselectivities were obtained by the
metal complex. The chiral alcohol products could be separated by extraction with
Scheme 7 TPPTS-stabilized Ru in ionic liquids catalyzed asymmetric hydrogenation of aromatic
ketones
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T. Yasukawa and S. Kobayashi
Hg-poisoning experiment and a hot filtration experiment. And the catalyst could
be reused for five cycles without significant loss of activity and aggregation of
nanoparticles.
2.2 Chiral Diamine-Modified Nanoparticle Catalysts
Asymmetric hydrogenation of nonactivated aromatic ketones is an atom economical
process to prepare chiral alcohols. However, early attempts using chiral ligandmodified heterogeneous catalysts, such as proline-modified Pd/C, cinchonamodified Pt/C, and chiral organotin-modified Pt or Rh, could not achieve high
enantioselectivity (~20% ee) [37–40].
In 2007, Zhao et al. reported R,R-1,2-diphenylethylene diamine ((R,R)-DPEN)modified Ru on γ-Al 2 O 3 -catalyzed asymmetric hydrogenation of acetophenone
[41]. A phosphine ligand was additionally employed to effect on the conversion
and enantioselectivity, and a relatively high ee (60.5%) was obtained. In the same
year, Chen et al. reported asymmetric hydrogenation of aromatic ketones in an ionic
liquid using a monophosphine TPPTS [P(m-C 6 H 4 SO 3 Na)]-stabilized Ru catalyst
and chiral diamine 3 as a modifier [42]. RuCl 3 ‧3H 2 O was reduced by H 2 in the
presence of TPPTS as a stabilizer to generate Ru nanoparticles whose size was about
5 nm. The reactions were performed in a co-solvent system with ionic liquid
[RMIM]Ts (1-alkyl-3-methylimidazolium p-methylphenylsulfonates, R ¼ ethyl,
butyl, octyl, dodecyl, hexadecyl), and various aromatic ketones were hydrogenated
with high conversions and moderate to high enantioselectivities (Scheme 7). The
ionic liquid substituted with the longer alkyl chain decreased enantioselectivity
because the Ru catalyst modified with hydrophilic chiral diamine 3 was reduced
due to its high solubility in water. This Ru nanoparticle catalyst system was more
active than the corresponding metal complex (RuCl 2 (TPPTS) 2 ) under the same
conditions, although similar high enantioselectivities were obtained by the
metal complex. The chiral alcohol products could be separated by extraction with
Scheme 7 TPPTS-stabilized Ru in ionic liquids catalyzed asymmetric hydrogenation of aromatic
ketones
284
T. Yasukawa and S. Kobayashi