Flexible on-site halogenation paired with hydrogenation using halide electrolysis

Article abstract of DOI:10.1039/d0gc04362a

Direct electrochemical halogenation has appeared as an appealing approach in synthesizing organic halides in which inexpensive inorganic halide sources are employed and electrical power is the sole driving force. However, the intrinsic characteristics of direct electrochemical halogenation limit its reaction scope. Herein, we report an on-site halogenation strategy utilizing halogen gas produced from halide electrolysis while the halogenation reaction takes place in a reactor spatially isolated from the electrochemical cell. Such a flexible approach is able to successfully halogenate substrates bearing oxidatively labile functionalities, which are challenging for direct electrochemical halogenation. In addition, low-polar organic solvents, redox-active metal catalysts, and variable temperature conditions, inconvenient for direct electrochemical reactions, could be readily employed for our on-site halogenation. Hence, a wide range of substrates including arenes, heteroarenes, alkenes, alkynes, and ketones all exhibit excellent halogenation yields. Moreover, the simultaneously generated H₂at the cathode during halide electrolysis can also be utilized for on-site hydrogenation. Such a strategy of paired halogenation/hydrogenation maximizes the atom economy and energy efficiency of halide electrolysis. Taking advantage of the on-site production of halogen and H₂gases using portable halide electrolysis but not being suffered from electrolyte separation and restricted reaction conditions, our approach of flexible halogenation coupled with hydrogenation enables green and scalable synthesis of organic halides and value-added products.

Full text of DOI:10.1039/d0gc04362a

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Flexible on-site halogenation paired with
hydrogenation using halide electrolysis†
Cite this: Green Chem., 2021, 23,
2037
Xiao Shang, Xuan Liu and Yujie Sun
*
Direct electrochemical halogenation has appeared as an appealing approach in synthesizing organic
halides in which inexpensive inorganic halide sources are employed and electrical power is the sole
driving force. However, the intrinsic characteristics of direct electrochemical halogenation limit its reac-
tion scope. Herein, we report an on-site halogenation strategy utilizing halogen gas produced from halide
electrolysis while the halogenation reaction takes place in a reactor spatially isolated from the electro-
chemical cell. Such a flexible approach is able to successfully halogenate substrates bearing oxidatively
labile functionalities, which are challenging for direct electrochemical halogenation. In addition, low-
polar organic solvents, redox-active metal catalysts, and variable temperature conditions, inconvenient for
direct electrochemical reactions, could be readily employed for our on-site halogenation. Hence, a wide
range of substrates including arenes, heteroarenes, alkenes, alkynes, and ketones all exhibit excellent
halogenation yields. Moreover, the simultaneously generated H2 at the cathode during halide electrolysis
can also be utilized for on-site hydrogenation. Such a strategy of paired halogenation/hydrogenation
maximizes the atom economy and energy efficiency of halide electrolysis. Taking advantage of the on-
site production of halogen and H2 gases using portable halide electrolysis but not being suffered from
electrolyte separation and restricted reaction conditions, our approach of flexible halogenation coupled
with hydrogenation enables green and scalable synthesis of organic halides and value-added products.
Received 25th December 2020,
Accepted 17th February 2021
DOI: 10.1039/d0gc04362a
rsc.li/greenchem
chlorination) holds a unique position because of the preva-
lence of organic halides in the synthesis of natural products,
Introduction
Since sustainability has become a prime direction for organic pharmaceuticals, and industrially important chemicals.25–28
synthesis, there is no doubt that the renaissance of organic By virtue of facile electrochemical oxidation of halide anions,
electrosynthesis will continuously attract increasing attention, non-toxic and inexpensive inorganic halides can be utilized as
in that electricity can be generated from sustainable resources, halogen sources, representing greener alternatives to expensive
while stoichiometric oxidants/reductants could be avoided in and/or toxic counterparts used in conventional halogenation
organic redox reactions.1,2 Organic electrosynthesis also reactions.29 For instance, Raju et al. reported two-phase
enables precise control of conversion and selectivity by adjust- (chloroform/water) electrochemical bromination of aromatic
ing multiple electrochemical parameters including electrodes, compounds using NaBr as the bromine source30 and Lei’s
electrolytes, applied potential, current, electrocatalysts, etc.1–3 group studied the electrochemical oxidative halogenation of
These years have witnessed impressive achievements in heteroarenes by employing HX/NaX (X = Br, Cl) in N,N-di-
organic electrosynthesis from both anodic4–6 and cathodic7–9 methylformamide/water (Fig. 1).31 Cu(OAc)2-catalyzed electro-
perspectives, such as alcohol oxidation,10–12 allylic C–H oxi- chemical bromination of amides using NH4Br as the bromine
dation,13 diazidation,14 dichlorination,15 heterodifunctionali- source has also been reported.32
zation of alkenes,16 anodic C–C bond cleavage,17 hydro-
dimerization of aldehdyes,18 redox mediator-assisted trans- electrosynthesis, several limitations exist in these aforemen-
formations,13,19–21 and paired electrolysis.22–24
tioned direct electrochemical halogenation processes. For
Nevertheless, due to the inherent requirements of organic
Among many important organic electrochemical reactions, instance, the oxidation potential of the bromide anion dictates
direct electrochemical halogenation (e.g., bromination and a small potential window for direct electrochemical bromina-
tion, which bears limited tolerance to oxidatively labile func-
tional groups (e.g., amines). Secondly, transition metal cata-
lysts are frequently employed in various halogenation reac-
Department of Chemistry, University of Cincinnati, Cincinnati, OH 45221, USA.
E-mail: yujie.sun@uc.edu
tions; however, the presence of redox-active metal species may
severely interfere with direct electrochemical halogenation. In
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
d0gc04362a
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Fig. 1 Reported direct electrochemical bromination and a schematic of paired on-site halogenation/hydrogenation described in this work.
addition, aqueous or polar organic solvents are typically tively labile functional groups, and can also be performed
required in electrolyte solutions, which are not always compati- under conditions incompatible with direct electrochemical
ble with certain halogenation reactions conducted in nonpolar halogenation, such as the use of redox-active metal catalysts
organic media.33 Furthermore, the separation of supporting and nonpolar organic solvents. Further taking advantage of H2
electrolytes from halogenated products results in additional produced on the cathode, our on-site halogenation can be
cost. Finally, the reduction reaction on the cathode, usually H2 seamlessly coupled with on-site hydrogenation, maximizing
evolution, has not been well utilized in reported direct electro- the energy return of electricity input for the halide electrolysis.
chemical halogenation. Overall, these collective limitations of
direct electrochemical halogenation call upon a more flexible
and versatile strategy for the synthesis of organic halides,
Results and discussion
Feasibility of on-site bromination using electrochemically
which can utilize both electricity and low-cost halide sources.
Inspired by decoupled water splitting,34 paired electro-
lysis,22–24 and hydrogenation in an isolated chamber utilizing
produced Br2
34–36
electrochemically
produced
H2
or
Pd-adsorbed In order to prove the feasibility of our on-site halogenation
hydrogen,37–39 we envision that it is feasible to conduct on-site strategy, an undivided cell in the two-electrode configuration
halogenation employing electrochemically generated Br2 (or employing Pt as both the anode and cathode was first adopted
Cl2), taking advantage of the phase separation of Br2 (or Cl2) to perform the electrolysis of saturated NaBr in H2SO4 at a con-
from the electrolyte solution. As shown in Fig. 1, the one-com- stant current of 1 A. The produced Br2 gas in the headspace of
partment electrochemical cell in direct electrochemical halo- the electrochemical cell was transported to another chamber
genation is replaced by an H-type cell for the electrolysis of in- for bromination reactions. Because of the spatial separation
organic halides in aqueous electrolytes. The generated volatile between organic bromination and bromide electrolysis, the
halogen gas (Br2 or Cl2) readily migrates through a Teflon bromination condition is completely independent of the elec-
tubing to an isolated chamber for on-site halogenation. In the trolysis condition. As shown in Scheme 1, six representative
meantime, H2 produced on the cathode is transported to organic substrates, including arenes, heteroarenes, aromatic
another chamber for on-site hydrogenation. Consequently, ketones, alkenes, and alkynes, could be successfully bromi-
these two spatially separated chemical reactions from halide
nated using on-site produced Br2. It should be noted that the
electrolysis effectively bypass restrictions in conventional direct electrochemical bromination of these substrates has
electrochemical halogenation. Herein, we demonstrate that been reported with reasonable to excellent yields
(50–98%).32,40,41 Specifically, following our on-site bromination
such a flexible on-site halogenation strategy is applicable to a
wide variety of organic substrates, including those with oxida- strategy, anisole (1a) exhibited high reactivity and the corres-
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its applicability towards those substrates which are challen-
ging for direct electrochemical bromination. An apparent
limitation of direct electrochemical bromination is the low tol-
erance of substrates bearing oxidatively labile functionalities,
such as amine groups. For instance, the oxidation potential of
organic amines usually ranges from 0.3 to 1.3 V vs. the satu-
rated calomel electrode (SCE);44 however, the redox potential
of Br2/Br− is ∼0.8 V vs. SCE.45 Consequently, the applied poten-
tial necessary for bromide oxidation in direct electrochemical
bromination of organic amines will inevitably result in the oxi-
dation of many amine groups, leading to side-products and
low faradaic efficiency. Fig. 2a shows the cyclic voltammo-
grams of aniline (3a), 4-chloro-benzenamine (3b), 5-chloro-2-
pyridinamine (3c), N,N-dimethyl-benzenamine (3d), and tetra-
n-butylammonium bromide in CH2Cl2. The onset potential of
bromide oxidation is ∼0.75 V vs. SCE, while the onset oxi-
dation potentials of these four amines are observed at ∼0.63,
0.69, 0.90, and 0.58 V vs. SCE, respectively, which are appar-
ently less positive or close to the former. Any potential
required for bromide oxidation (>0.75 V vs. SCE) will definitely
result in undesirable amine oxidation, as reflected in the poor
yields (33–59%) from previous reports for the electrochemical
bromination of these amines.31,46 In striking contrast, our on-
site bromination completely avoids the direct interaction of
amine substrates with any electrode; therefore, higher bromi-
Scheme 1 On-site bromination of representative aromatics, alkyls,
alkenes, and alkynes.
ponding 4-bromoanisole (2a) was produced in 90% yield. In
addition, selective bromination of aromatic ketones, such as
on-site bromination on the side alkyl chain in 4-acetylbenzo-
nitrile (1b) to yield 4-(2-bromoacetyl)benzonitrile (2b), also
proceeded smoothly with a decent yield of 81%. Our on-site
bromination approach was also amenable to heteroarenes. For
instance, benzo[b]thiophene (1c) could be brominated to
produce 3-bromobenzo[b]thiophene (2c) with an excellent
yield of 85%. It’s worth noting that all these three bromination
reactions were conducted at room temperature in CHCl3,
different from the strongly acidic condition of bromide
electrolysis.
Notably, our on-site bromination was able to demonstrate
exceptionally high efficiency in converting alkenes to vicinal
dibromides, such as from styrene (1d) to (1,2-dibromoethyl)
benzene (2d) and from 2-vinylnaphthalene (1e) to 2-(1,2-
dibromoethyl)naphthalene (2e) with yields of 98% and 94%,
respectively. Besides alkenes, dibromination of alkynes is fun-
damental to provide versatile intermediates for synthetic
applications.42,43 A recognized challenge for the successful
dibromination of alkynes is the high reactivity of the resulting
dibromides bearing a CvC bond that may undergo further
bromination. To our delight, on-site bromination was able to
produce
(1,2-dibromoprop-1-en-1-yl)benzene
(2f)
from
1-phenyl-1-propyne (1f) with a high yield of 82%. Such a high
conversion efficiency was probably attributed to a more precise
control of the Br2 feed rate in our on-site bromination. It is
necessary to mention that these dibromination reactions took
place in CHCl3 at 0 °C, a temperature condition inconvenient
for direct electrochemical bromination.
Fig. 2 (a) Cyclic voltammograms of aniline (3a), 4-chloro-benzenamine
(3b), 5-chloro-2-pyridinamine (3c), and N,N-dimethyl-benzenamine (3d)
compared to that of tetra-n-butylammonium bromide (TBAB) on a
glassy carbon electrode (3 mm in diameter) in CH2Cl2. The concen-
tration of the substrate was 10 mM. Scan rate: 50 mV s−1. (b) On-site
On-site bromination of organic substrates with oxidatively
labile functional groups
After proving the effectiveness of on-site bromination for the
aforementioned organic substrates, we next sought to explore bromination yields of the above four amine substrates.
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nation yields would be anticipated. Indeed, as shown in (6) in 80% yield, comparable to the reported results using con-
Fig. 2b, on-site bromination of 3a and 3b both afforded decent ventional brominating reagents.55 For comparison, the on-site
yields of 4-bromoaniline (4a, 70%) and 2-bromo-4-chloroani- bromination without AlCl3 only produced 2-bromoacetophe-
line (4b, 84%), respectively. The less reactive 3c could also be none and 2,2-dibromo-1-phenylethanone (Fig. S3†), highlight-
on-site brominated to form 3-bromo-5-chloro-2-pyridinamine ing the important role played by the Lewis acid catalyst AlCl3.
(4c) with a yield of 70%. Compared to aniline, 3d achieved a
higher bromination yield (80%) at the para-position, resulting On-site bromination of substrates in solvents challenging for
in 4-bromo-N,N-dimethylaniline (4d), probably due to the
steric hindrance of the dimethylamine group in preventing
bromination at its ortho-position. Overall, no amine group oxi-
dation was detected for these four amine substrates, superior
to direct electrochemical bromination.
electrochemistry
Aqueous or polar organic solvents with a supporting electrolyte
are mandatory for direct electrochemical halogenation. Such a
requirement of reaction medium is incompatible with certain
halogenation reactions, which are preferred to be conducted in
nonpolar organic solvents (e.g., Et2O and CCl4). As an example,
α-bromination of cyclic ketones exhibits a faster reaction rate
and higher yield in nonpolar solvents like Et2O or CCl4 than in
polar solvents like CHCl3 or MeOH.56–58 In this regard, our on-
site bromination strategy could be conveniently adopted to
perform the α-bromination of cyclohexanone (7) in Et2O at
0 °C as shown in Scheme 3. The desirable product of 2-bromo-
cyclohexanone (8) was obtained with a high yield of 90%.
Moreover, the absence of a supporting electrolyte in the bromi-
nation chamber renders product separation/purification extre-
mely facile.
On-site bromination of organic substrates with metal catalysts
For substrates with multiple functionalities, there may exist
several potential bromination sites. In order to form the desir-
able bromination product selectively, redox-active metal cata-
lysts have been frequently employed.47–50 For instance, bromi-
nation of acetophenone (5) may take place on the benzene ring
or the side acetyl group (Scheme 2a). The deactivation effect of
the acetyl group in 5 renders aromatic bromination thermo-
dynamically challenging. Actually, aromatic bromination using
the direct electrochemical method is always limited to those
activated aromatics.29,30 Instead, direct electrochemical bromi-
nation of 5 produced the acetyl brominated product exclu-
sively.33 Nevertheless, Lewis acid catalysts such as FeBr3 and
AlCl3 have been shown to interact with Br2 to form strongly
electrophilic brominating reagents, which are able to accom-
plish aromatic bromination of deactivated aromatics.51–54
However, the redox activity of these metal catalysts may
severely interfere with direct electrochemical bromination and
lead to complicated product mixtures. As shown in Scheme 2b,
following our on-site bromination strategy, AlCl3 was intro-
duced to the bromination reactor isolated from bromide elec-
trolysis. Consequently, aromatic bromination of acetophenone
(5) proceeded smoothly to deliver 1-(3-bromophenyl)ethanone
Flexibility in the bromide source for on-site bromination
In addition to great suitability for challenging substrates and
reaction conditions, our on-site bromination strategy also exhi-
bits excellent flexibility in the choice of the bromide source for
the on-site production of Br2. As shown in Fig. 3, bromide elec-
trolysis utilizing KBr, LiBr, and HBr all produced superior
results for the on-site bromination of styrene, with yields
Scheme 3 On-site bromination of cyclohexanone in diethyl ether at
0 °C.
Scheme 2 (a) Different bromination pathways of acetophenone with or
without a Lewis acid catalyst. (b) On-site bromination of acetophenone
(5) using AlCl3 as a Lewis catalyst.
Fig. 3 Dibromination yield of styrene to produce (1,2-dibromoethyl)
benzene (2d) using various bromide sources and in the presence of
other common anions (50 mM) for the on-site production of Br2.
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>90%. It should be noted that HBr is actually a byproduct of
many bromination reactions; hence, our on-site bromination
strategy realizes the recycling of HBr, which is otherwise a
waste, further improving atom economy and minimizing the
environmental impact. Due to the relatively low oxidation
potential of Br2/Br−, the presence of common inorganic
anions like CO32−, NO3−, and PO43− exerts negligible influence
on the bromination performance when NaBr was used as the
bromine source (Fig. 3). Therefore, our on-site bromination
does not require high purity of inorganic bromides, another
advantage compared to those utilizing conventional brominat-
ing reagents such as N-bromosuccinimide.
Scheme 5 On-site hydrogenation of acetophenone (5) to produce
α-methyl-benzenemethanol (10) using H2 input produced from water
electrolysis.
hydrogenation catalyst. A high yield of 87% was achieved for
the production of α-methyl-benzenemethanol (10), demon-
strating the success of on-site hydrogenation with electro-
chemically produced H2 gas and paving the way to paired halo-
genation/hydrogenation.
Applicability of on-site chlorination
Encouraged by the above success of on-site bromination, we
envisioned that such a strategy could be extended to facile
chlorination taking analogous advantage of the phase separ-
ation of Cl2 from on-site electrolysis of inexpensive inorganic
chlorides. As a proof of concept, NaCl electrolysis was
employed to produce Cl2 and dichlorination of styrene (1d)
was selected as a representative reaction (Scheme 4). To our
delight, in diethyl ether at 0 °C, our on-site produced Cl2 was
able to transform styrene into (1,2-dichloroethyl)benzene (9)
with a decent yield of 80%. Such a performance is apparently
greener than those conventional chlorination approaches
using expensive and/or environmentally deleterious chlorinat-
ing reagents such as KMnO4/HCl59 and Ph2SO/(COCl)2.60 Our
(1,2-dichloroethyl)benzene yield (80%) is also comparable to
that (85%) of a recently reported electrosynthetic strategy
using a Mn(III) complex as the electrocatalyst,15 while much
simpler product isolation and purification were the inherent
advantages of our approach.
Paired on-site bromination and hydrogenation using
electrochemically produced Br2 and H2
The most appealing scenario of our on-site halogenation strat-
egy is to couple it with on-site hydrogenation from both
atomic and economic perspectives. Eventually, we assembled a
divided electrochemical cell for bromide electrolysis which
could produce Br2 on the anode and H2 on the cathode simul-
taneously, which were transported to separated bromination
and hydrogenation chambers, respectively. As illustrated in
Scheme 6, styrene (1d) bromination and acetophenone (5)
hydrogenation were selected as sample reactions. The resulting
yields of (1,2-dibromoethyl)benzene (2d) and α-methyl-benze-
nemethanol (10) were 98% and 85%, respectively, highlighting
the great efficiency of on-site bromination coupled with on-site
On-site hydrogenation using electrochemically produced H2
With the aim of maximizing atom economy and energy return
for halide electrolysis, the byproduct H2 formed on the
cathode is better utilized for valuable synthetic applications,
such as hydrogenation. It is known that ketone hydrogenation
presents a straightforward access to secondary alcohols, which
are useful synthons for the synthesis of pharmaceuticals, agro-
chemicals, and liquid crystals.61,62 In order to demonstrate the
feasibility of on-site hydrogenation, we decided to carry out the
hydrogenation of acetophenone (5) in an isolated chamber
using H2 stream produced from water electrolysis in a divided
cell (Scheme 5). Commercially purchased Pt/C was used as the
Scheme 6 Schematic of on-site bromination paired with on-site
hydrogenation with a representative example of styrene (1d) bromination
coupled with acetophenone (5) hydrogenation.
Scheme 4 On-site chlorination of styrene in diethyl ether at 0 °C.
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hydrogenation using electrochemically co-produced Br2 and
H2 from bromide electrolysis. Based on the aforementioned
results, it is reasonable to envision that on-site chlorination
paired with hydrogenation would be equally effective.
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Conclusions
In summary, our reported on-site halogenation approach ame-
liorates several inherent limitations that plague direct electro-
chemical halogenation and hence represents a convenient and
alternative strategy for electricity-driven halogenation with
greater flexibility, exhibiting superior tolerance to oxidatively
labile functionalities, low-polar solvents, redox-active metal
catalysts, and challenging temperature conditions for electro-
chemistry. We further demonstrate that coupled with on-site
hydrogenation, our strategy maximizes the energy return of
halide electrolysis and presents exceptional greenness for
organic halogenation and hydrogenation reactions. Such a
strategy of chemical reactions using electrochemically gener-
ated reagents but performed in an isolated space is potentially
applicable to many other organic reactions.
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Conflicts of interest
There are no conflicts to declare.
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Acknowledgements
Y. S. acknowledges the financial support of the National
Science Foundation (CHE-1914546), the Herman Frasch
Foundation (820-HF17), and the University of Cincinnati.
NMR experiments were performed on a Bruker AVANCE
NEO 400 MHz NMR spectrometer (funded by NSF-MRI grant
CHE-1726092).
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