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Paired electrosynthesis using
different mediators regenerated at
the anode and cathode:
Dioxygen (O
2
) is
cathodically reduced in aqueous and
non-aqueous solutions to reactive oxygen
species such as H
2
O
2
and O
2
−
√,
respectively, which are oxidizing reagents
for organic compounds. Hence, cathodic
oxidation of organic substrates is realized
using such cathodically generated oxygen
species. However, the concentration of
cathodically generated H
2
O
2
is so low that
the oxidation rate of organic compounds
with H
2
O
2
is not enough for a practical
electrolytic process. Catalysts are therefore
necessary for the practical cathodic
oxidation to be paired with the anodic
oxidation. Nonaka and his co-workers
developed a unique cathodic oxidation
system using a tungstate/pertungstate redox
mediator and applied this to the paired
electrosynthesis, as shown in
Figure 5.14
[58].
N
-Hydroxyamine derivative as a single
starting substrate is indirectly oxidized to
the nitrone as a single product with HWO
5
−
derived from HWO
4
−
and H
2
O
2
in a
cathodic chamber, and with halogen (X
2
)
anodically formed from X
−
in an anodic
chamber. The total cathodic and anodic
current efficiencies for the nitrone were very
high (190%).
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