Angewandte
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Chemie
case, the reaction tolerated the presence of methanol solvent
reached. In this case, the presence of the methanol solvent led
even when methanol was added directly to the cathodic
chamber. As in the earlier reactions, a Zn-cathode was
employed to minimize the competing hydrogen evolution
reaction, and a divided cell was used to avoid re-oxidation of
to a shift in the reduction potential for the Ph As=O of
3
approximately 350 mV to an Ep/2 value of À2.10 V vs. Ag/
AgCl. This potential is significantly more positive than the
onset potential needed for the reduction of methanol,
a scenario that enabled the paired electrolysis reaction shown
in Scheme 10b.
the Ph As product generated at the cathode. The reaction did
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benefit from the oxidation reaction being conducted on
a scale about three times that of the reduction, presumably
because of a background hydrogen evolution reaction con-
suming some of the current at the cathode. With this caveat,
both reactions proceeded in good yield. With the observation
The result obtained for the reduction of Ph As=O
3
indicated that a successful pairing of an anodic cyclization
in methanol solvent with the reduction of Ph P=O will require
3
an electrocatalyst that operates at a potential that is only
250 mV more positive than the reduction potential of
triphenylphosphine oxide (in methanol), a difference in
potential that is consistent with the energy difference
commonly seen between an electrocatalyst and its targeted
that the reduction of Ph As=O could tolerate the presence of
3
methanol, the reaction highlighted in Scheme 10b was con-
ducted. Both reactions proceeded well.
Insight into the success of this reaction was gained by
cyclic voltammetry (see the Supporting Information for the
[45]
substrate.
CV data). As a baseline, the reduction potential of Ph P=O
3
was measured in acetonitrile at a glassy carbon electrode and
found to be Ep/2 = À2.50 V vs. Ag/AgCl. The reduction wave
was found in the same place as the reduction wave for
methanol which has an onset potential of around À2.2 V vs.
Ag/AgCl, an observation that showed why the reduction of
Recycling Sacrificial Anodes
While the chemistry discussed above targeted reduction
reactions that do not require the use of a sacrificial anode,
a large number of reduction reactions (the generation of an
anionic nucleophiles, the use of electrogenerated bases, etc.)
do require such an electrode. In those cases, the direct pairing
of the cathodic reaction with the oxidation of an organic
molecule can be ruled out. However, an oxidation can still be
utilized to make the corresponding reduction more sustain-
able. Reduction reactions paired with a sacrificial anode still
produce a stoichiometric waste product that can be recycled
with the energy used at the auxiliary electrode of an anodic
reaction.
Ph P=O fails in the presence of methanol. The addition of
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TMSCl to the reduction of Ph P=O in acetonitrile led to
3
a shift in the reduction potential for Ph P=O of approximately
3
8
00 mV (Ep/2 = À1.70 V vs. Ag/AgCl), presumably due to
either silylation of the Ph P=O prior to the reduction or rapid
3
trapping of the radical anion derived from Ph P=O reduction.
3
This shift made the reduction of Ph P=O significantly more
3
favorable than the hydrogen evolution reaction, a change that
enabled the success of the reactions shown in Scheme 8. Of
course, the presence of the methanol solvent in the reaction
shown in Scheme 9 destroyed the TMSCl and stopped the
desired reduction.
Consider the reactions shown in Scheme 11. For each
oxidation reaction, a Zn-electrode was employed as the
2
+
When the cyclic voltammetry experiment was repeated
cathode and a Zn -salt was added to cathodic chamber. The
cathode was weighed before and after the reaction to
with the use of Ph As=O in acetonitrile, the potential
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0
measured for the Ph As=O was E = À2.45 V vs. Ag/AgCl.
determine the weight of Zn that was deposited on the
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p/2
This result was a surprise given the success of the reaction
shown in Scheme 10b relative to the same reaction using
Ph P=O (Scheme 9). Since the reduction of Ph As=O did not
electrode. Both reactions proceeded smoothly leading to
good yields of the anodic products and over 80% recovery of
[
46]
Zn on the cathode. The reactions were run using equal
molar amounts of substrate for both the anodic and cathodic
reactions indicating that the hydrogen evolution reaction was
not competitive with the desired reduction.
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3
require the presence of the TMSCl, we had assumed it was
easier to reduce than Ph P=O. However, the 50 mV differ-
3
ence in reduction potential for the two substrates was far too
small for such an explanation, especially with a methanol
onset potential of Ep/2 = À2.2 V vs. Ag/AgCl.
The key issue turned out to be a difference between the
CVand preparative electrolyses. The CV data reported above
was obtained in the absence of methanol solvent while the
preparative reactions shown in Scheme 9 and Scheme 10b
Pairing Reactions on Different Scales
In the introduction, we mentioned that since current can
be split, paired electrochemical reactions do not have to be
required methanol. When the CV experiment utilizing Ph P= conducted using reactions that are required on the same scale.
3
O was repeated in the presence of methanol, a small shift in
the reduction potential to Ep/2 = À2.35 V vs. Ag/AgCl was
observed along with a significant increase in current. It
appeared that the reduction of the triphenylphosphine might
help catalyze a reaction with the methanol, an observation
that was consistent with the preparative reaction leading to
One can engineer an electrolysis cell to demonstrate this
capability, but there are simpler ways to achieve that
demonstration. Take for example the chemistry illustrated
in Scheme 12. The reactions shown were run in electro-
chemical cells linked together in a series format. In this
format, the cathode of the first cell was wired to the anode of
the second cell, the cathode of the second cell was wired to the
anode of the third cell, etc. A constant current was then
passed through each of the cells and all of the electrodes
a complete recovery of the Ph P=O starting material.
3
When the CVexperiment utilizing Ph As=O was repeated
3
in the presence of methanol, a different conclusion was
Angew. Chem. Int. Ed. 2021, 60, 2 – 10
ꢀ 2021 Wiley-VCH GmbH
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