Communications
Table 1. Optimization of the reaction conditions.[a]
have been exploited extensively as efficient catalysts in various
oxidation reactions owing to both their resistance towards oxi-
dation and compatibility with various oxygen sources.[8] Specif-
ically, POMs can be used as ideal inorganic ligands for the
design and construction of various inorganic-ligand-coordinat-
ed metal catalysts that are more thermally stable and have
better redox properties than common organometallic com-
plexes. In this context, Anderson-structured POMs[9,10] are very
important, as they are single-metal anions that are supported
by ring-like polyoxometalate clusters, which are composed of
six edge-sharing MO6 (M=W or Mo) octahedra surrounding a
central, edge-sharing metal heteroatom to form an octahedron
(XO6) with six protons. Such inorganic-ligand-coordinated
structures with redox and charge-transfer properties provide a
quick and stable state variation of the atomic valence and can
be used to solve the instability and activity problems of organ-
ometallic complex catalysts, thus providing a new potential
pathway for the aerobic oxidation routine. Very recently, we re-
ported the oxidation of aldehydes in an organic–inorganic
hybrid ligand supported iron-catalyst system and provided the
first examples of the aerobic oxidation of aldehydes to carbox-
ylic acids in water under mild conditions by using a hybrid An-
derson-structured iron(III)/POM catalyst;[11] we demonstrated
the excellent catalytic activity of POM-supported metal cata-
lysts in oxidation reactions.
Entry
1 [mol%]
Additive
Yield[b] [%]
1
2
3
4
5
6
7
8
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.5
1.0
0.1
0.1
0.1
0.1
–
NaCl
NaBr
NaF
56
68
70
65
63
99
75
35
10
80
82
85
17
12
7
KCl
Na2CO3
NaHCO3
Na2SO4
Na2SO3
CH3CO2Na
HCO2Na
Et3N
NH4Cl
NaOH
HCl
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
9
10
11
12
13
14
15
16
17
18[c]
19[d]
20[e]
21[f]
99
99
90
92
<15
<5
[a] Reaction conditions: Catalyst 1 (0.1 mol%), piperonal (1.5 mmol), O2
(balloon), additive (0.1 equiv.), and H2O (2 mL). [b] Yields were calculated
by 1H NMR spectroscopy. [c] Reaction was performed at 258C. [d] Reac-
tion was performed at 708C. [e] Reaction was performed under atmos-
pheric pressure performed in air. [f] Reaction was performed under a ni-
trogen atmosphere.
Inspired by these results, herein we report a simple, mild,
and efficient procedure for the aerobic oxidation of aldehydes
in water by using an inorganic-ligand-supported copper cata-
lyst,[12] (NH4)4[CuMo6O18(OH)6] (catalyst 1, Figure 2). Compared
with our previous organic hybrid iron catalyst, the present
pure inorganic copper catalyst is simpler with a defined struc-
ture, which is helpful to clarify the structure–activity relation-
ship of the catalytic reactions and also shows more potential
for future industrial productions. Copper catalyst 1 was easily
prepared in high yield through a simple one-step route in
water at 1008C; it has a CuII core and a planar arrangement of
six MoVI centers. The MoVIO6 inorganic scaffold, supported by
the central heterometal, greatly enhances the Lewis acidity of
the catalytically active sites and also enables the edge-sharing
MO6 unit to act as a ligand analogous to those used in tradi-
tional organometallic complexes.
ingly, oxidation of piperonal in the absence of catalyst 1 did
not result in the formation of any of the desired product. We
investigated the influence of additives on the reaction in the
presence of catalyst 1. The efficiency of the transformation was
strongly affected by additives (Table 1, entries 1 and 2). Addi-
tives with a neutral salt, such as NaCl, NaBr, NaF, and KCl, gave
only moderate yields of the oxidized product (Table 1, en-
tries 2–5). Upon adding Na2CO3 to the reaction mixture, benzo-
ic acid was obtained in 99% yield (Table 1, entry 6), whereas
the addition of NaHCO3 reduced the yield of benzoic acid to
75% (Table 1, entry 7). The use of Na2SO4 further decreased
the yield to 35% (Table 1, entry 8), whereas the presence of
Na2SO3 decreased the yield to 10% (Table 1, entry 9). Other
basic additives, such as CH3COONa, HCOONa, and Et3N, gave
benzoic acid in yields of 80, 82, and 85%, respectively (Table 1,
entries 10–12). The acidic additive NH4Cl severely inhibited the
oxidation reaction (Table 1, entry 13). With either a strong acid
or base as the additive, low yields were observed (Table 1, en-
tries 14 and 15), possibly because 1 becomes less stable and
decomposes under these conditions. These results show that
additives clearly influence the activity of catalyst 1, probably
because the acid–base properties of POMs are highly tunable.
The influences of the catalyst loading and temperature on
the product yields were also investigated. Increasing the cata-
lyst loading did not change the yield (Table 1, entries 6, 16 and
17), which demonstrated that altering the catalyst loading had
The presented method is operationally simple and avoids
the use of expensive, toxic, air/moisture-sensitive, and com-
mercially unavailable organic ligands.
In the initial catalytic tests to explore the activity of inorgan-
ic-ligand-supported copper-catalyst 1 for the oxidation of alde-
hydes, piperonal was chosen as the model substrate, with O2
(in a balloon) as the sole oxidant at 508C (Table 1). Not surpris-
Figure 2. Synthesis of the inorganic-ligand-supported copper catalyst.
&
ChemCatChem 2018, 10, 1 – 6
2
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