10.1002/anie.202013940
Angewandte Chemie International Edition
RESEARCH ARTICLE
Polyoxometalates (POMs), a unique class of
(Table 1, entries 5, 6). After extensive optimization
molecular metal-oxide clusters of transition metals,
are widely used in catalytic transformations,
especially for the catalytic oxidation due to their
electron reservoir abilities and the activity of their
diverse reduced forms[6]. Recently, our groups have
demonstrated that POMs can be used as inorganic
ligand-supported metal catalysts in various organic
reactions[7]. In 2017, we reported a single-sided iron
with the other reaction parameters (for the detailed
conditional screening process, see the Supporting
Information, Table S1), we were pleased to find that
1.0 mol% 1, with 0.1 equivalent of Na2S2O3 and 2.0
equivalent of H2O2 as oxidant in methanol at 60 C
under air was the most optimized reaction condition
(Table 1, entry 1).
Meanwhile, we also detected the production of
azoxybenzene 4a in the reaction system. In the same
way, we screened the reaction conditions in order to
obtain azoxybenzenes selectively (Table S3-S4).
Remarkably, the selections of solvents and additives
proved to have a significant influence on the reaction
pathway and yield. We can selectively produce
azobenzenes or azoxybenzenes by changing solvent
and additive, as well as increasing the temperature
and extending the reaction time. The highest yield of
azoxybenzene can reach to 93%.
º
centered
Anderson-type
POM,
[N(C4H9)4]3
[FeMo6O18(OH)3{(OCH2)3CNH2}] ([FeIIIMo6]), for the
aerobic oxidation of aldehydes in water.
Subsequently, other Anderson-type POM with
different metals central was successfully used in the
oxidation of amines to imines, oxidation of aldehydes
to esters or one-step oxidation of alcohols to esters,
and N-formylation of amines[8]. As part of our ongoing
research in developing new applications of POM-
based catalysts in organic reactions, we report herein
a
new preparation protocol of aromatic azo
compounds with a molecular molybdenum oxide
catalyst [N(C4H9)4]2[Mo6O19] (1) for the selective
oxidation of anilines in the presence of H2O2 under
mild conditions (Scheme 1, c).
Table 1. Optimal reaction conditions
The molecular catalyst [N(C4H9)4]2[Mo6O19] (1)
was prepared from (NH4)6Mo7O24•4H2O and
[N(C4H9)4]Br according to the published literature
reports (Figure S1)[9] and its structure of 1 was
confirmed by FT-IR test (Figure S2). We initiated our
studies by probing various reaction conditions for the
POM catalyzed oxidative dimerization of aromatic
Entry
Deviations from standard
conditions Aa and Bb
None
Yield 3a
Yield 4a
(%)c
99
35
39
-
(%)c
93
52
34
<8
-
1
2
3
4
5
6
(NH4)6Mo7O24
MoO3
-
amines
2 in an environmentally-friendly way.
O2 replacement H2O2
N2 replacement H2O2
-
-
-
Phenylamine 2a was selected as a model substrate
for the optimization of reaction conditions. By
employing 1.0 mol% of [N(C4H9)4]2[Mo6O19] (1) in the
presence of 2.0 equivalent of H2O2 (30% in H2O2) in
[a] 1 (1.0 mol%), 2a (1.0 mmol), methanol (2.0 mL), 30% H2O2
(2.0 equiv), Na2S2O3 (0.1 equiv), 60 ºC, 24 h. [b] 1 (0.5 mol%),
2a (1.0 mmol), MTBE (2.0 mL), 30% H2O2 (2.0 equiv), Na2SO3
(0.05 equiv), 50 ºC, 36 h. [c] Yield determined by GC-MS.
º
methanol at 50 C for 24 h, we were delighted to
observe
the
formation
of
homodimerized
With these optimized reaction conditions in hand,
we sought to examine the substrate scope of the Mo-
catalysed oxidative dehydrogenation reaction (Table
2). Anilines with both electron-rich and electron-
deficient substitutions were readily dimerized to
afford the corresponding symmetric azobenzenes in
excellent yields. Various substituents including Me,
Et, iPr, n-Bu, OMe, OEt, F, Cl and Br were well
tolerated (Table 2, 3b-o). It showed that the
substrates with an electron-withdrawing group are
better than those with the electron-donating group,
maybe due to the existence of the electron-
withdrawing group, which reduces the electron cloud
density of the benzene ring and thus promotes the
reaction process. Moreover, we also explored the
effect of steric hindrance on the reaction. The yield of
para-substituted for electron-withdrawing groups
products was significantly higher than that of ortho-
and meta-substituted products (Table 2, 3k-m). The
same experimental results were obtained for the
substrate of the electron donor product (Table 2, 3b-
c, 3g-i). That means that para-substituted products
are easier to convert to corresponded azo
compounds. The conversion to azobenzene can be
azobenzene 3a as a major product with 49% isolated
yield (Table S1, entry 1). Various additives, such as
Na2CO3, NaHCO3, K3PO4, Na2SO4, Na2SO3 and
Na2S2O3 were investigated, and Na2S2O3 was found
to be an ideal additive for this oxidative dimerization
reaction (Table S1, entries 2-9). Remarkably, when
0.1 equivalent of Na2S2O3 was used, the oxidative
homodimerization proceed smoothly to give
azobenzene 3a in 99% yield (Table S1, entry 9). The
catalyst loading could be decreased to 1.0 mol% of
[N(C4H9)4]2[Mo6O19] (1) (Table S1, entries 10-12).
The reaction worked in a broad range of solvents and
wide range of temperatures (Table S2) and with
MeOH at 60 ºC giving the best reproducibility, highest
yield (Table S2, entry 2). Other Mo catalysts, such as
(NH4)6Mo7O24 and MoO3, were also found to promote
the oxidative dimerization (Table S1, entry 16-17),
however with lower efficiency. No desired aromatic
azo products were detected in the absence of
catalyst 1 (Table 1, entry 4). Moreover, the reaction
was completely blocked in an O2 atmosphere or N2
atmosphere in replacement of H2O2, which indicates
the importance of the oxidant in the reaction system.
But O2 is not reactive enough to activate the catalyst
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