Table 2 Scope of 3-catalyzed asymmetric transfer hydrogenationa
and enantioselectivity than its homogeneous counterpart. In
particular, this imidazolium-based hybrid silica can not only
substitute Bu4NBr to function as a phase-transfer catalyst but
also boosts both catalytic and enantioselective performances in
a cooperative manner in aqueous medium. More importantly,
the heterogeneous catalyst can be recovered conveniently and
subsequently reused at least 10 times without affecting the
catalytic efficiency, showing a promising potential for practical
application in asymmetric synthesis.
Entry
Substrate
Product
Conv.b (%)
Eeb (%)
1
499
97 (95)c
2
3
499
499
98 (97)c
97 (96)c
We are grateful to China NSF (20673072), Shanghai STDF
(10DJ1400103, 10JC1412300 and 12NM0500500) and Shanghai
MEC (12ZZ135 and S30406) for financial support.
4
5
499
499
93
Notes and references
92 (67)c
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92
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Reaction conditions: catalyst 3 (26.7 mg, 4.0 mmol of Ir based on ICP
analysis), HCO2Na (0.27 g, 10.0 mmol), ketone (0.40 mmol) and
b
2.0 mL of water, at 40 1C for 1.0–18.0 h. Determined by chiral GC
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An important feature of design of any heterogeneous catalyst
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was recovered easily and reused repeatedly when acetophenone
was chosen as a substrate. In ten consecutive reactions, recycled
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Table S1 and Fig. S9w). The high recyclability should be due to
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non-covalent physical adsorption.9 Evidence to support this
view comes from ICP analysis, in which the amount of Ir after
the tenth recycle was 26.89 mg per gram catalyst, with only
6.8% of Ir being leached out.
In addition to acetophenones (Table 1), catalyst 3 could also
be applied to the asymmetric transfer hydrogenation of other
cyclic and acyclic aryl ketones in aqueous medium (Table 2). It
is noteworthy that the ester moiety can be tolerated in a
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homogeneous counterpart were also observed in all cases.7a,14
In conclusion, we have developed an imidazolium-based
hybrid silica as a solid support and prepared a bifunctional
heterogeneous catalyst that displays higher catalytic activity
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This journal is The Royal Society of Chemistry 2012