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Table 1. Cu-bpy HP-MB as oxidation catalyst.
course of our recent efforts to apply organic–inorganic
hybrid polymers in organic synthesis, we succeeded in the
catalytic synthesis of a-hydroxy ketones by using {[Cu(bpy)-
G
A
G
N
dine).[4–6] Thus we attempted to construct the Cu-bpy
hybrid-polymer-encapsulated magnetic beads (HP-MB) in
this study.
[b]
Entry
Cu(BF4)2 [equiv]bpy [equiv]Yield [%]
G
1
2
3
4
5
10
89
92
74
36
Amino-functionalized magnetic beads (ꢀ200 nm diame-
ter, ꢀ130 mmolgÀ1 NH2 content, ꢀ40 emugÀ1 magnetic sus-
ceptibility, and 15 m2 gÀ1 specific surface area) containing ap-
10
20
100
20
40
200
proximately 70% iron oxide[7] were mixed with Cu
(BF4)2
N
[a]Catalyst amount as the monomeric structure based on the functionali-
ty on the parent magnetic beads. [b]Yield of isolated product after treat-
(10 equivalents to the amine content) and bpy (20 equiva-
lents to the amine content) in EtOH. The resulting mixture
was gently agitated at ambient temperature for eight hours.
The excess of added reagents was readily washed out with
EtOH after making the beads cohere under an external
magnetic field. The extreme simplicity of the preparation of
these HP-MB self-assembled magnetic beads was remark-
able. Although superparamagnetic nanoparticle-supported
catalysts have been studied recently, the synthesis of the ma-
terials described in those reports involved several complicat-
ed transformations.[8]
ment with P(OEt)3.
U
thin. It was also noteworthy that the catalytic activity of Cu-
bpy HP-MB was higher than that of free {[Cu(bpy)(BF4)2-
(H2O)2](bpy)}n (48 h, 53%) under the current conditions,
suggesting that the catalytic oxidation took place on the sur-
face of the hybrid polymer. Importantly, magnetic beads
R
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G
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treated with Cu(BF4)2 without the bpy ligand gave a signifi-
G
cant amount of the protonated product of 1. Moreover, in-
stead of the use of amino-functional magnetic beads, when a
commercially available amino-methylated polystyrene was
applied as the support, the catalytic activity of the resulting
“polymer-supported HP” in the oxidation of 1 decreased to
provide a-hydroxy ketone 2 with a yield of only 20%. This
fact could be reasonably explained by considering the incor-
poration of the HP into the polymer matrix in the conven-
tional support. Based on these results, we can conclude that
the organic–inorganic hybrid phase formed on the surface of
the magnetic beads was crucial for catalytic activity, and this
is an advantage of the use of the magnetic beads.
Although the magnetic beads thus obtained did not show
any obvious change in color, characteristic changes to the
surfaces of the magnetic beads were observed in SEM
images. As shown in Figure 2, small projections were ob-
The scope and generality of the new direct method is
summarized in Table 2. Various substrates were converted
to a-hydroxy ketones in good yields under mild conditions.
In particular, not only cyclic ketones, but acyclic ketones
were also transformed to the corresponding a-hydroxy ke-
tones, with chemical yields higher than those obtained by
using free Cu-bpy (Table 2, entries 7 and 9).[4a]
Figure 2. Scanning electron microscope (SEM) images of: a) original
amino-functionalized magnetic beads, and; b) Cu-bpy organic–inorganic
hybrid-polymer-encapsulated magnetic beads.
served to stick out from the spherical surface of the magnet-
ic beads. These projections were probably the result of ani-
sotropic growth of the crystalline Cu-bpy hybrid polymer.
To our delight, the newly prepared Cu-bpy HP-MB
showed high catalytic activity for the aerobic oxidation of
silyl enolates. Thus, based on the NH2 functionality of the
parent magnetic beads, only 2 mol% of the Cu-bpy HP-MB
was enough to catalyze the reaction of 1 to provide a-hy-
droxy ketone 2 in 92% yield (Table 1,entry 2). The effects
The Cu-bpy HP-MB was readily recovered by using an
external magnetic field in air, and the recovered catalyst
could be recycled. For example, after the reaction of Cu-
A
in Table 1), the magnetic beads were quantitatively recov-
ered by magnetic separation, and the catalyst was repeatedly
reused to give the a-hydroxy ketone 2 (2nd use: 92%
(48 h), 3rd use: 89% (48 h), 4th use: 85% (48 h), 5th use:
84% (48 h)).
of amounts of Cu
MB on the catalytic activity of the oxidation were examined.
N
The extremely simple preparation procedure for the HP-
MB compounds allows us to use high throughput screening
to determine the most efficient catalysts. The catalytic activi-
ties of a series of metal salts-bpy HP-MBs are correlated in
The HP-MB prepared by using ten equivalents of Cu
showed the highest catalytic activity (Table 1, entry 2).
ICP analysis of the HP-MB used in entry 2 in Table 1
showed that the 2 mol% NH2 functionality on the parent
magnetic beads resulted in 6 mol% Cu atoms and that the
organic–inorganic phase formed on the beads was quite
Figure 3. The cationic CuII-bpy HP-MB catalysts (Cu
and Cu
estingly, the readily prepared Co
provided the adduct 2 with a yield of 65%, although the
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Chem. Eur. J. 2008, 14, 882 – 885
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
883