Linear Paired Electrolysis—Realising 200 % Current Efficiency for Stoichiometric Transformations—The Electrochemical Bromination of Alkenes

Article abstract of DOI:10.1002/anie.202016413

The generation of bromine by oxidation of bromide anions at the anode and reduction of molecular oxygen at the cathode to hydrogen peroxide resulted in the overall formation of two molecules of Br₂ (=four electron oxidation) by passing just two electrons through the solution. The bromine was used for the bromination of alkenes and thereby a linear paired electrolysis was attained which resulted in current efficencies of up to 200 %. Also, the diiodination of cyclohexene as well as the electrophilic aromatic bromination of an electron-rich arene were realised both in 168 % current efficiencies.

Full text of DOI:10.1002/anie.202016413

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How to cite: Angew. Chem. Int. Ed. 2021, 60, 9996–10000
International Edition:
German Edition:
doi.org/10.1002/anie.202016413
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Electrosynthesis
Linear Paired Electrolysis—Realising 200% Current Efficiency for
Stoichiometric Transformations—The Electrochemical Bromination of
Alkenes
Julia Strehl, Marvin L. Abraham, and Gerhard Hilt*
In memory of Prof. Eberhard Steckhan
Abstract: The generation of bromine by oxidation of bromide
anions at the anode and reduction of molecular oxygen at the
cathode to hydrogen peroxide resulted in the overall formation
of two molecules of Br2 (= four electron oxidation) by passing
just two electrons through the solution. The bromine was used
for the bromination of alkenes and thereby a linear paired
Scheme 1. Linear paired electrolysis.
electrolysis was attained which resulted in current efficencies of
up to 200%. Also, the diiodination of cyclohexene as well as
the electrophilic aromatic bromination of an electron-rich
arene were realised both in 168% current efficiencies.
A seminal example for such a linear paired electrolysis
with a theoretical 200% current efficiency was published by
Nonaka who investigated the formation of nitrones from
hydroxylamines (Scheme 2).[3]
E
lectrochemical transformations are coupled processes
where either an oxidation or a reduction represents the
centre of interest. In most cases the electrochemical reaction
at the counter electrode is of little concern (e.g. generation of
hydrogen from protons).[1] In contrast, in a paired electrolysis
both electrochemical reactions are producing valuable prod-
ucts and various types of such transformations have been
realised.[2] For example, in a convergent paired electrolysis
À
a single product (Nu E) can be formed when a nucleophile
(NuÀ) is formed at the cathode and an electrophile (E+) at the
anode and the reaction of these species in the bulk solution
leads to the product. In this case, 100% of the electrons
supplied by the cathode and 100% of the electrons absorbed
at the anode are responsible for the production of the product
and an overall current efficiency of 200% is possible—at least
in principle.
A more complicated case to realise is an oxidation, such as
the conversion of a single substrate to a single product with
200% current efficiency. In this case the anodic reaction as
well as the cathodic reaction should generate the same
product (Scheme 1). This paradoxon can only be solved by
a sacrificial substance.
Scheme 2. Linear paired electrolysis for the oxidation of hydroxyl-
amines.
The oxidation of bromide ions to bromine at the anode
accomplished the oxidation of the hydroxylamine 1 at the
anode. Using molecular oxygen as sacrificial substance, the
reduction at the cathode led to the formation of hydrogen
peroxide. In combination with tungstenate, a peroxo-tung-
stenate was proposed to accomplish the oxidation of the
substrate 1 “via a reductive process”. The authors reported
a current efficiency (ce) of up to 185% for the formation of
the desired nitrones 2.[4]
Another example of a 200%-cell linear paired electrol-
ysis, that can be found in the literature concerning organic
transformations, was reported in a diploma thesis by Arns
from the Steckhan group in 1998 (Scheme 3).[5] The results for
[*] M. Sc. J. Strehl, M. L. Abraham, Prof. Dr. G. Hilt
Institut fꢀr Chemie, Universitꢁt Oldenburg
Carl-von-Ossietzky-Strasse 9–11, 26111 Oldenburg (Germany)
E-mail: gerhard.hilt@uni-oldenburg.de
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.202016413.
ꢂ 2021 The Authors. Angewandte Chemie International Edition
published by Wiley-VCH GmbH. This is an open access article under
the terms of the Creative Commons Attribution License, which
permits use, distribution and reproduction in any medium, provided
the original work is properly cited.
Scheme 3. Linear paired electrolysis for the oxidation of furane.
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Table 1: Results of the electrochemical bromination of cyclohexene in
a linear paired electrolysis.
the oxidation of furane in up to 195% current efficiency were
never published elsewhere and represent the basis for this
investigation (Scheme 3).
As a catalyst for the reduction of molecular oxygen a N-
methylated 1,10-phenanthroline-5,6-dione catalyst (3;
PDMe) was utilised and in methanol solution furane was
converted to the dimethoxy derivative 4 in good overall yield.
However, when we adopted this reaction for the bromination
of cyclohexene[6,7] we recognised some inconsistencies. The
outline of the desired reaction is shown in Scheme 4.
Entry
Variations from initial conditions
Yield[a]
1
2
3
4
5
6
7
8
none
75% (ce: 150%)
79% (ce: 158%)
66% (ce: 131%)
80% (ce: 160%)
81% (ce: 162%)
24% (ce: 48%)
35% (ce: 70%)
71% (ce: 142%)
81% (ce: 162%)
65% (ce: 130%)
67% (ce: 134%)
81% (ce: 162%)
95%[d] (ce: 190%)
À58C
158C
electrode distance: 9 mm[b]
glassy carbon cathode (1.50 cm2)[c]
Pt anode (1.50 cm2)
both electrodes (1.50 cm2)
7.5 mA
9
12 mA
0.1 m H2SO4
0.3 m H2SO4
150 rotations/min
250 rotations/min
10
11
12
13
Scheme 4. Quinone-mediated linear paired electrolysis for the bromi-
nation of cyclohexene.
Unless otherwise stated, the reactions were performed in an undivided
cell with a platinum anode (active surface area: 2.55 cm2) and a glassy
carbon electrode (active surface area: 2.55 cm2) with an electrode
distance of 11 mm on a 0.5 mmol scale. The reaction mixture was
saturated with oxygen for 10 minutes at 08C prior to electrolysis. [a] The
yield was determined by GC analysis of the crude reaction mixture using
mesitylene as internal standard. [b] This change was kept for entries 5–
13. [c] This change was kept for entries 6–13. [d] Isolated yield.
The current demand for high efficiencies in chemistry and
the preservation of energy in general are of high interest.
Therefore, the perspective of doubling the efficiency of
a chemical reaction by an innovative technique, such as
a linear paired electrolysis, was investigated.
A linear paired electrolysis is principally performed in an
undivided cell design and passing 1.0 F through the solution
would result in complete conversion of the cyclohexene giving
a 200% current efficiency. However, when the reaction is
performed in a divided cell, the efficiency of the anodic as
well as the cathodic reaction can be determined independ-
ently. Also, in a divided cell 2.0 F have to be passed through
the solution to obtain theoretical 100% conversion. Accord-
ingly, the optimisation of the linear paired electrolysis was
first conducted in a divided cell and later in an undivided cell
to determine the efficiency of the coupled processes. In fact,
the presence of quinones, such as 3 or anthraquinone, which
was reported to facilitate the formation of H2O2,[8] proved to
be effective for the bromination of cyclohexene in the cathode
compartment but the effect was marginal, so that the further
reactions were conducted in the absence of these quinones
(for details, see SI).
(À12 to + 208C). Afterwards, the influence of the electrode
distance (1–3 cm), the electrode surface (3.5–6.0 cm2), the
current (5–15 mA), and the stirring rate (150–750 rpm) were
examined. The best results for the temperature dependence of
the reaction were obtained at À58C, but for general
convenience, we decided to perform the electrolysis at 08C
with an ice bath and not at higher or lower temperatures
(Table 1, entries 1–3). The product generation was mostly
influenced by the surface areas of the electrodes (Table 1,
entries 4–7), less by the electrode distance or the electrode
material itself. Reducing the electrode surface of the platinum
anode by about 50% resulted in almost doubling of the
current density and led to low overall performance while
small variations of the current (Table 1, entries 8/9) had
a comparable little effect. With respect to yield and current
efficiency the amount of sulfuric acid was found to be optimal
at 0.2 m. A factor that is quite relevant and is overlooked (or
irrelevant) in many organic reactions is the stirring rate (the
size of the stir bar[10] was kept constant), which intensively
affects the mass transport. An electrochemical short-cut
(oxidising bromide to bromine at the anode, transport to
the cathode and reduction of bromine to bromide—or the
oxidation of H2O2 at the anode) had to be avoided. Therefore,
we checked the stirring rate as well and the best results were
obtained when 250 rpm were applied. Slower stirring might
cause a too slow mass transport of the cathodically generated
H2O2 into the bulk solution where Br2 is formed. If this
process happens near the cathode, reduction of Br2 to
bromide can occur and a reduced current efficiency is
The oxidation of bromide to bromine in the anode
compartment of a divided cell, where bromine equilibrates
with the tribromide anion (Br3 ),[9] proved to be uncritical and
À
therefore, we turned our attention to the cathode compart-
ment to optimise the reduction of oxygen to H2O2 to produce
bromine as well. The preliminary optimisation of the cathodic
oxidation reaction was performed in a divided cell under
oxygen atmosphere to verify good starting conditions for this
part of the electrolysis. Subsequently, the extensive optimisa-
tion was realised in an undivided cell design and is summar-
ised in Table 1 (for further results see SI). First, we tested
several bromide sources (NaBr, KBr, and nBu4NBr) and
solvents (DMF, TFE, HFIP, and CH3CN) and their water
content (0–20 equiv), as well as the reaction temperature
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observed. Faster stirring rates led to a diminished overall
performance as well, probably by a faster mass transport of
the electrogenerated species to the respective counter elec-
trode (see SI).
The scope of the linear paired electrolysis for the
bromination of alkenes was investigated following the
reaction outlined in Scheme 5.
yield of 92% (ce: 184%) on a 2.5 mmol scale. While the
bromination of styrene is performed in student laboratories
worldwide within minutes and in almost quantitative yield,
the linear paired electrochemical version, which takes around
80 minutes, led to unidentified side-products and a low
current efficiency of only 112% for 5d. Terminal alkenes as
well as 1,1-disubstituted alkenes led to good to acceptable
results as well (5 f/5h). However, 5-decene proved to be an
excellent substrate. With 98% isolated yield of 5g after
passing 1.0 F electricity through the solution a current
efficiency of 196% was obtained. A perfect current efficiency
of 200% was obtained for product 5i, indicating that the more
electron-rich the alkene the faster the bromination occurs
thereby avoiding the electrochemical short-cut. Next, we
turned our attention towards non-conjugated dienes, such as
b-citronellene, and experienced that the dibrominated prod-
uct 5j, isolated in 78% yield (ce: 156%) with a high
chemoselectivity (> 95%) was formed as the main product.
The high chemoselectivity for the bromination of a trisubsti-
tuted double bond over a monosubstituted double bond might
be caused by the slow generation of the Br2 over 80 minutes
electrolysis whereas the drop-wise addition of Br2 in a chem-
ical bromination leads to low chemoselectivities.
In comparison with the outstanding work performed by
Schꢀfer more than 40 years ago (bottom of Scheme 6),
electrochemical methods are now able to generate a selective
dibromination of dienes, such as 5j, via the herein provided
methodology or a chemoselective electrochemical debromi-
nation (by an electrochemical E1cB mechanism) to generate
product 7 under potential-controlled conditions.[12] Also,
a linear paired electrolysis for the diiodination of cyclohexene
was investigated and the desired product 5k was isolated in
84% yield after passing 1.0 F of electricity through the
solution, which resulted in a current efficiency of 156%. Not
at all surprisingly, based on the relatively high oxidation
potential of chloride anions, an analogous dichlorination of
cyclohexene failed.
Scheme 5. Linear paired electrolysis for the bromination of cyclohex-
ene.
On a semi-preparative scale of 0.5 mmol the electrolysis
was conducted at 10 mA utilising a platinum anode (active
surface area: 2.55 cm2), and a glassy carbon cathode (active
surface area: 1.50 cm2; electrode distance 9 mm)[11] to pass
À
1.0 F (per carbon carbon double bond) through the solution.
The results of these electochemical bromination reactions
are summarised in Scheme 6.
The linear paired electrolysis of cyclic alkenes (5a–5c)
provided good to excellent results with the exception of
cyclooctene for no obvious reason. Larger-scale bromination
was performed for 5a and resulted in a slightly diminished
Surprisingly, in chemical control reactions, cyclohexene
reacted with H2O2 in the presence of bromide ions (reac-
tion (1) in Scheme 7), but no brominated product 5 was
formed. When sulfuric acid was added to this solution
(reaction (2) in Scheme 7), almost complete conversion
towards 5a was observed in a short period of time. The
hypothesis that persulfate (Caroꢁs acid) was formed in situ
and acts as oxidising agent to generate bromine was disproven
because the same fast reaction towards 5a was observed when
HBr (reaction (3) in Scheme 7) was added instead of sulfuric
Scheme 6. Linear paired electrolysis for the bromination of alkenes.
[a] 2.5 mmol scale.
Scheme 7. Chemical control experiments.
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acid to the cyclohexene/H2O2 solution. Accordingly, under
the reaction conditions of the electrolysis an acidic medium is
useful to facilitate the formation of bromine from the
electrogenerated H2O2 and further catalysts or additives are
not needed.
To identify functional group compatability, we then
performed a tolerance test introduced by Glorius.[13] Selected
results are summarised in Table 2 (for further details see SI).
Several functional groups were compatible with the linear
electrolysis conditions for the bromination reaction of
alkenes, such as alkynes, nitriles, esters, alkyl/aryl halides,
and ethers. Some other functional groups led to moderate
tolerance, such as nitrobenzene derivatives (based on the easy
reduction of the nitro group) and electron-rich arenes (see
below). Epoxides were protonated by sulfuric acid and some
side reactions occurred. For unknown reasons an aldehyde
showed a considerable transformation to unidentified side-
products. Aniline and dodecylamine were protonated by the
sulfuric acid and precipitated from the solution which looks
like an incompatability. Nevertheless, when electron-rich
arenes were applied as additive in the compatability test,
brominated side-products could be identified by GCMS.
Scheme 8. Linear paired electrolysis for the bromination of 8.
Acknowledgements
We thank Prof. J. Dickschat for providing a copy of the
Diploma thesis of T. Arns from the chemistry department in
Bonn, Germany. Open access funding enabled and organized
by Projekt DEAL.
Conflict of interest
The authors declare no conflict of interest.
Table 2: Selected examples of a functional group tolerance test using
cyclohexene as substrate.
Keywords: 200% cell · alkene · arene · bromination ·
linear paired electrolysis
Entry Additive
Yield[a] (additive) Yield[a] (product)
1
1-dodecyne
96%
96%
86%
24%
7%
84%
77%
87%
75%
77%
79%
59%
80%
84%
72%
49%
74%
2
octanenitrile
3
4
methylbenzoate
aniline
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dodecylamine
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91%
54%
66%
79%
83%
67%
69%
8
9
10
11
12
1,2-epoxy-n-octane
valerophenone
dibutyl ether
1-bromo-1-nitrobenzene
N,N-dimethyl aniline
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In conclusion, we have optimised a linear paired electrol-
ysis for the dibromination of alkenes in up to 100% yield and
a perfect current efficiency of 200%. Several important
reaction parameters were identified, optimised, and a func-
tional group tolerance test identified other starting materials
which can be applied in the linear paired bromination, such as
electron-rich arenes. In a preliminary reaction, the diiodina-
tion of cyclohexene was realised while the corresponding
dichlorination was not. Nevertheless, in light of the current
interest in electrochemical transformation, the increase of
current efficiency for stoichiometric processes could be
demonstrated.
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Manuscript received: December 10, 2020
Revised manuscript received: February 15, 2021
Accepted manuscript online: March 3, 2021
Version of record online: March 24, 2021
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