Efficient Electrocatalysis for the Preparation of (Hetero)aryl Chlorides and Vinyl Chloride with 1,2-Dichloroethane

Article abstract of DOI:10.1002/anie.201814570

Although the application of 1,2-dichloroethane (DCE) as a chlorinating reagent in organic synthesis with the concomitant release of vinyl chloride as a useful byproduct is a fantastic idea, it still presents a tremendous challenge and has not yet been achieved because of the harsh dehydrochlorination conditions and the sluggish C?H chlorination process. Here we report a bifunctional electrocatalysis strategy for the catalytic dehydrochlorination of DCE at the cathode simultaneously with anodic oxidative aromatic chlorination using the released HCl as the chloride source for the efficient synthesis of value-added (hetero)aryl chlorides. The mildness and practicality of the protocol was further demonstrated by the efficient late-stage chlorination of bioactive molecules.

Full text of DOI:10.1002/anie.201814570

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
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Accepted Article
Title: Efficient electrocatalysis for the preparation of (hetero)aryl
chlorides and vinyl chloride with 1,2-dichloroethane
Authors: Ning Jiao, Yujie Liang, Fengguirong Lin, Yeerlan Adeli, and
Rui Jin
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201814570
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10.1002/anie.201814570
Angewandte Chemie International Edition
DOI: 10.1002/anie.2018(will be filled in by the editorial staff)
Organic Electrochemistry
Efficient electrocatalysis for the preparation of (hetero)aryl chlorides and
vinyl chloride with 1,2-dichloroethane
Yujie Liang, Fengguirong Lin, Yeerlan Adeli, Rui Jin, and Ning Jiao*
The incorporation of a chlorine atom has been demonstrated to
have a profound effect on enhancing biological properties of small
molecules.[5] In addition, chlorinated arenes and heterocycles are
frequently utilized as valuable intermediates in cross-coupling
reactions.[6] Therefore, the development of highly efficient and
atom-economic chlorination under mild conditions has been a long-
term goal of synthetic scientists. The traditional electrophilic
aromatic chlorination with Cl2, SO2Cl2, and t-BuOCl as chlorinating
reagents is a straightforward method for aryl chlorination. However,
Abstract: Although the application of DCE as a chlorinating
reagent in organic synthesis concomitant with release of
vinyl chloride as a useful byproduct is a fantastic idea, it is
still a big challenge and is hardly achieved because of the
harsh dehydrochlorination conditions and the inert C-H
chlorination process. Here we report a bifunctional
electrocatalysis strategy for catalytic dehydrochlorination of
DCE at cathode simultaneously with the anodic oxidative
aromatic chlorination using the released HCl as chloro- their toxic and aggressive nature has plagued their wide applications
in organic synthesis.[7] In the past decades, some classical solid
source for the efficient synthesis of value-added (hetero)aryl
chlorides. The mildness and practicality of the protocol was
further demonstrated by the efficient late stage chlorination
of bioactive molecules.
chlorinating reagents (e.g. NCS, DCDMH, TCCA, CBMG) are
developed.[8] Despite their high efficiency and wide applications, the
atom-economy is low by using these chlorinating reagents leaving
the extra part as the wastes.
Alternatively, the in situ generation of electrophilic Cl+ species by
the oxidation of chloride ion has been widely utilized as an
oxidative strategy normally using stoichiometric amounts of K2S2O8,
PhI(OAc)2, DDQ or some metal salts as oxidants, which would
generate unwanted wastes.[9] In contrast, electrochemical oxidation
has been recognized as an environmentally friendly strategy in
organic synthesis, since generally no sacrificial oxidants are
required only with the electricity consumption.[10] However, the
electrochemical chlorination was particularly more difficult than the
iodination and bromination due to the difficulty to oxidize Cl-
through anodic oxidation. The rarely reported electrochemical
chlorination required the assistance of palladium catalysis with a
directing group in an divided reaction cell.[10t] Inspired by these
fantastic work[10] and our oxidative halogenation protocols,[11] we
envisage that, if the dehydrochlorination of DCE could occur under
the electrochemical conditions by controlling the current intensity
producing vinyl chloride with the incidental formation of HCl,
which provides a chance for anodic oxidative C-H chlorination
(Scheme 1b). The in situ HCl generation and consumption would
avoid the usage of strong acidic condition and therefore enable the
selective chlorination with broad substrate scope including inactive
electron-deficient (hetero)arenes.
The challenging issue of this hypothesis is how the
dehydrochlorination process of DCE could be executed by
electrochemistry. The thermal cracking dehydrochlorination of DCE
for vinyl chloride manufacture is known via chloroethyl radical
intermediate, which is produced from the pyrolysis of 1,2-DCE
itself.[12] Gas phase DCE, like many other halocarbons, has been
reported to have an extensive electron attachment chemistry.[13] It
has a large electron capture cross section for electron hunting to
form an excited negative ion DCE-*, which can decompose promptly
on metal surface to lead to the chloroethyl radical and chemisorbed
chlorine.[14] The generation of chloroethyl radical supports our
hypothesis that an electrochemical strategy could enable the
dehydrochlorination process of DCE, where the DCE would be
absorbed and dissociated on the surface of the electrode. The
electrons may be released from the surface of electrode and enter
into the antibonding C-C1 σ* orbital of DCE, thus triggering the
cleavage of C-Cl bond to produce vinyl chloride and HCl at mild
conditions. Furthermore, it may also be possible to enable the anodic
oxidative C-H chlorination with the in situ generated HCl,[10t] and
therefore complete the electrochemical circle (Scheme 1b). Herein,
1,2-Dichloroethane (DCE), an inexpensive colorless liquid, has
been widely used as a common solvent in various organic
transformations. In industry, DCE was used predominantly for vinyl
chloride monomer manufacture via pyrolysis, which is typically run
o
at 500-550 C and 25-35 bar.[1] Alternative process such as using
heterogeneous catalysts to catalyzed dehydrochlorination of DCE
o
has also been explored (usually performed at > 240 C) with the
formation of HCl byproduct which could be reused in the
oxychlorination process, but fast deactivation is observed on all the
catalysts due to the coke formation that blocks the active sites of the
catalysts (Scheme 1a).[2]
Inspired by the tremendous chlorinated natural products and
approved drugs,[3] the C-H chlorination of (hetero)arenes and
bioactive molecules has been an attractive procedure for drug
discovery and development. Recently, although a special 7-
azaindole group directed C-H chlorination was developed using
DCE as the chlorinating reagent,[4] that protocol is restricted by the
expensive Rhodium-catalyst as well as 2.0 equivalent copper-
oxidants, and the very limited substrate scope. Ideally, if the C-H
chlorination reaction could be achieved by using DCE as a
chlorinating reagent, it may produce vinyl chloride and H2 as useful
byproducts. This transformation looks wonderful with high atom
efficiency. However, this one-stone-two-birds approach to
(hetero)aryl chlorides and vinyl chloride from DCE is generally too
difficult to be practical because of the harsh dehydrochlorination
conditions and the tandem inert C-H chlorination process.
[]
Y. Liang, F. Lin, A. Yeerlan, R. Jin, and Prof. N. Jiao
State Key Laboratory of Natural and Biomimetic Drugs,
School of Pharmaceutical Sciences, Peking University,
Xue Yuan Rd. 38, Beijing 100191, China
Fax: (+) 86-10-82805297
E-mail: jiaoning@pku.edu.cn
Homepage: http://sklnbd.bjmu.edu.cn/nj/
Prof. N. Jiao
State Key Laboratory of Organometallic Chemistry,
Chinese Academy of Sciences,
Shanghai 200032, China
Supporting information for this article is available on the
WWW under http://www.angewandte.org.
we report
a
bifunctional electrocatalysis for catalytic
1
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10.1002/anie.201814570
Angewandte Chemie International Edition
dehydrochlorination of DCE and direct C-H chlorination of
(hetero)arenes at mild conditions (Scheme 1c). This chemistry
provides a practical and high atom-efficient protocol for the
preparation of various high-valued (hetero)aryl chlorides with the
formation of vinyl chlorides and H2 as the byproducts. To the best of
our knowledge, this represents the first example that DCE is
activated and utilized as an efficient chlorinating reagent for
aromatic chlorination by electrolysis.
which holds great potential for the further application in large-scale
vinyl chloride manufacture. The generation of ethylene also
supports the formation of a DCE- ion transition state by electron
capture from the Pt surface, which follows a similar process
depicted in Scheme 1b to furnish the chloroethyl radical. This
radical would finally lead to the ethylene formation by stripping of a
Cl atom, or like in the thermal cracking dehydrochlorination process
of DCE to produce vinyl chloride. The results are consistent with
our mechanistic hypothesis.
Scheme 2. Electrolysis enabled dehydrochlorination of DCE.
Encouraged by these promising results, we further tested the
possibility of direct aromatic chlorination in one pot. Initially, we
investigated the chlorination of N-(4-chlorophenyl)acetamide 1a
with DCE under electrochemical conditions (Table S1). After
extensive attempts, we found the chlorination of 1a proceeded
efficiently under constant-current electrolysis at 10 mA for 10 h,
using graphite anode, Pt-plate cathode, and n-Bu4NOH as electrolyte
in an undivided cell (Table S1, entry 1). Replacing the Pt-plate
cathode with Ni-plate would lead to the efficiency decrease (Table
S1, entry 2). Surprisingly, when replaced the Pt-plate cathode with
Cu-plate, no desired chlorination product was formed (Table S1,
entry 3). This is probably due to the instability of Cu-plate under the
electrochemical condition, since the destruction of Cu-plate was
observed after electrolysis 10 h. Other electrolytes were also tested,
but lower yields were obtained (Table S1, entries 4-8). The reaction
became sluggish when performed at room temperature (Table S1,
entry 9). Decreasing the electric current to 5 mA proved also viable,
albeit prolonged reaction time is required for full consumption of
starting material (Table S1, entry 10). Control experiment showed
that the reaction did not occur without electricity (Table S1, entry
11).
With the optimized conditions in hand, the substrate scope was
then investigated (Table 1). Notably, various para-substituted
anilides even with strong electron-withdrawing groups, could all be
smoothly chlorinated with good to excellent yields (2a-2i). When
the para position of acetanilides was not occupied, the bis
chlorination products were produced (2a, 2j-2k). Additionally,
chlorination of N-(naphthalen-2-yl)acetamide and N-(naphthalen-1-
yl)pivalamide were also facile (2m, 2n). In addition, anisole was
chlorinated in excellent yield with 16:1 para/ortho selectivity (2o).
In some cases, the loading of Cu(OAc)2 (5 mol%) could increase the
yields.
Significantly, the protocol can also be extended to the selective
chlorination of some druglike molecules. The sensitive benzylic C-
H of naproxen methyl ester (3a), the reactive imide N-H bond of
aminoglutethimide derivative (3b), the fragile isoxazole group of
leflunomide (3c), and even the subtle hydroxyl group of paracetamol
(3d), were also compatible in this protocol. These results
demonstrate the mildness and practicality of the chlorination
protocol.
Encouraged by the efficient chlorination of arenes, the substrate
scope with regard to heterocycles were then examined (Table 1). To
our delight, reactions of thiophenes (4a, 4b), pyrazole (4c),
imidazole (4d), indoles (4e-4g), and some pyridine and quinolone
anilides (4h-4l) could all lead to clean chlorination with high
regioselectivity, affording mono-chlorination products in most cases
even for substrates with multiple potential chlorination sites. It is
Scheme 1. Bifunctional electrocatalysis for dehydrochlorination of
DCE and the anodic oxidative C-H chlorination.
Based on our mechanistic design (Scheme 1b), we first tested
the viability of dehydrochlorination of DCE under electrochemical
conditions. Intriguingly, when the reaction was conducted in an
undivided cell filled with liquid DCE (15 mL, ~192 mmol) and n-
Bu4NBF4 (1 mmol) as electrolyte using graphite anode and Pt-plate
o
cathode at 60 C under constant-current electrolysis I = 100 mA,
notable gas bubble was observed in the reaction vessel. After
electrolysis overnight, a gas chromatographical examination of the
outlet volatile organic compounds indicated that vinyl chloride was
indeed produced as the major component with 45% selectivity, but a
notable amount of ethylene (53%) and small amount of other
components (2%) were also observed in this case using this
electrolyte. Furthermore, when the readily available and inexpensive
n-Bu4NOH (1 mmol, 37% MeOH solution) was used as electrolyte
instead of n-Bu4NBF4, the desired vinyl chloride was produced in
48% selectivity along with generation of ethylene (46%), a small
amount of 1,1-dichloroethylene (5%) and other components (1%)
(Scheme 2). A notable amount of HCl was also produced, which
was observed in both the outlet flow as well as the reaction solution,
and was detected by GC-MS (Scheme 2, and see Supporting
Information). It should be noted that, the ethylene and HCl could be
efficiently reused to generate DCE via ethylene oxychlorination for
15]
further vinyl chloride preparation.[2,
Therefore, this protocol
should be very useful, since after reusing the ethylene and HCl to
produce DCE, the vinyl chloride would almost be the sole product.
Thus, the atom efficiency of this protocol should be very high,
2
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10.1002/anie.201814570
Angewandte Chemie International Edition
noteworthy that some inactivated chemicals which are not accepted
by NCS, reacted smoothly under the current electrochemical
conditions (2h, 2i, 3c, 4i, 4j, 4l) (see Figure S8, Supporting
Information).
the stoichiometric amount of base and the HCl that produced from
DCE dehydrochlorination would serve as chloride source.[16] In
addition, when performed the reaction in the absence of substrate
under constant current electrolysis overnight, the reaction mixture
clearly became acidic by pH measurement. Moreover, the outlet gas
was also collected in a balloon among them the HCl was proved by
GC-MS (Scheme 4b, and see in the Supporting Information). It is
noteworthy that, without electricity, the reaction mixture remains
basic after stirring overnight. Furthermore, the employment of
stoichiometric amount of TEMPO significantly inhibited the
chlorination reaction, probably due to the suppression of the radical
process (see Figure S6, Supporting Information).
Table 1. Scope of the electrochemical aromatic C-H chlorination with DCE.
Scheme 4. Mechanistic studies.
Furthermore, we conducted cyclic voltammetry experiment
(Figure 1). Based on preliminary measurements, we chose the 0-1.5
V potential window for cyclovoltammetry in DCE. As it can be seen
in this potential window, one oxidation wave appeared on the
forward potential sweep (1.06 V) and one reduction wave (0.94 V)
exhibited on the backward potential sweep at a Pt disc electrode at
100 mV s-1 in 0.1 M n-Bu4NOH in DCE (Figure 1a). This redox
couple is probably due to the oxidation of Cl- by the forward sweep
and the reduction of DCE to DCE- by the backward sweep. This
assumption is supported by the cyclic voltammetry of the n-Bu4NCl
(3 mM) with n-Bu4NBF4 (0.1 M) in DCE, which exhibited also one
oxidation wave (1.11 V) and one reduction wave (0.70 V) (Figure
1b). Furthermore, with substrate 1a (3 mM) and n-Bu4NBF4 (0.1
M ) in DCE, an oxidation occurred at 1.03 V was observed (Figure
1c). This is probably due to the oxidation of 1a or the oxidation of
Cl- (Figure 1c). Moreover, the oxidative potential of 1a is further
decreased to 0.81 V under basic condition and the oxidation of Cl-
was also observed at 1.10 V at this condition (Figure 1d). This is
consistent with the cyclic voltammetry of 0.1 M n-Bu4NOH in DCE
(Figure 1a) where the oxidation of Cl- occurred at 1.06 V. Based on
these results, we assume that anodic oxidation of 1a would form a
cationic intermediate,[17] which then interact with the chloride anion
to generate the chlorinated product. It should be noted that the exact
reaction pathway depends highly on the oxidative potential of the
substrate.[18] If the oxidative potential of a substrate is higher than
the Cl-, the anodic oxidation of Cl- is preferential, thus, an
electrophilic aromatic chlorination mechanism would be favored.
Based on the above results, a plausible mechanism for the
chlorination of arenes and the formation of vinyl chloride and
ethylene was proposed (see Figure S7, Supporting Information).
[a] Reaction conditions: 1 (0.5 mmol), n-Bu4NOH (1 mmol, used as
37% MeOH solution), graphite anode, Pt-plate cathode, I = 10 mA
o
(janode ≅ 4.4 mA cm−2), DCE, 60 C, undivided cell. Isolated yields.
[b] Constant current = 20 mA (janode ≅ 8.9 mA cm−2). [c] Reaction
conditions: 1 (0.5 mmol), 2,4,6-trimethylaniline (10 mol%), NCS
(0.6 mmol), DCM (0.25 M) at r.t. for 12 h (see ref 8c). [d] Reaction
conditions: 1 (0.5 mmol), n-Bu4NBF4 (1 mmol), Cu(OAc)2 (0.05
o
equiv), DCE, 60 C, I = 10 mA (janode ≅ 4.4 mA cm−2), graphite
anode, Pt-plate cathode, undivided cell. [e] n-Bu4NOH (1 mmol)
without MeOH was used. [f] Trace amount of bis chlorination
product was observed.
The scalability of this electrochemical aromatic C-H chlorination
was further evaluated. To our delight, this reaction can be easily
scaled up to 15 mmol scale using a simple and readily made device,
which shows great potential of this method for industrial synthesis
(Scheme 3).
Scheme 3. Gram-scale synthesis
To get more insights into the mechanism, some experiments were
performed. Firstly, we test the constancy of the assembled undivided
cell for aromatic chlorination. Delightedly, after the first round
chlorination of 1a (0.5 mmol), another 1 mmol of 1a was simply
added into the same vessel, which sequentially proceed smoothly
without extra addition of electrolyte or using new electrode. We
repeated this same process 2 times, the efficiency was not affected
by the continual substrate addition producing the desired chlorinated
product 2a in 85% yield based on the total 3 mmol consumption of
1a (Scheme 4a). This result suggests that the reaction did not rely on
3
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10.1002/anie.201814570
Angewandte Chemie International Edition
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-0.2 0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
E vs (Ag/AgNO3)/V
Figure 1. Cyclic voltammetry experiment. Recorded on a Pt disc
electrode at 100 mVs-1 in (a) 0.1 M n-Bu4NOH in DCE; (b) 3 mM
n-Bu4NCl and 0.1 M n-Bu4NBF4 in DCE; (c) 3 mM 1a and 0.1 M n-
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n-Bu4NOH in DCE.
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In conclusion, we have developed an ideal bifunctional
electrocatalysis strategy for catalytic dehydrochlorination of DCE
simultaneously with the aromatic chlorination using the released
HCl. The common solvent DCE is ideally employed as a reactant
producing vinyl chloride and as a chlorinating reagent for efficient
aromatic chlorination. Notably, this external oxidant free
electrocatalysis can also be easily scaled up and is demonstrated
very green and sustainable with the tolerance of inactive chemicals.
Considering the benign process for DCE dehydrochlorination, and
the broad substrate scope of aromatic chlorination, this method
would open new avenue for vinyl chloride and (hetero)aryl chlorides
preparation in an efficient and environmentally benign manner. The
present bifunctional electrocatalysis strategy is expected to be of
high interest to scientists from both academia and industry.
Acknowledgements
Financial support from the National Basic Research Program of
China (973 Program) (No. 2015CB856600), and the National
Natural Science Foundation of China (21632001, 21772002,
81821004) are greatly appreciated. We thank Prof. Chengchu Zeng
and Yangye Jiang at Beijing University of Technology for helpful
discussion on the cyclic voltammetry experiment, and thank Prof.
Wei Li and Wei Qin at Nankai University for assistance with the
analysis of the composition of gas mixture. We also thank Weijin
Wang in this group for reproducing the results of 3a and 4i.
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Received: (will be filled in by the editorial staff)
Published online on (will be filled in by the editorial staff)
Keywords: electrochemistry, chlorination, radicals,
dehydrochlorination, C-H functionalization
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10.1002/anie.201814570
Angewandte Chemie International Edition
Entry for the Table of Contents
C-H Functionalization
Y. Liang, F. Lin, A. Yeerlan, R. Jin, and N. Jiao *
__________ Page – Page
Efficient electrocatalysis for the preparation of
(hetero)aryl chlorides and vinyl chloride with 1,2-
dichloroethane
A bifunctional electrocatalysis strategy for catalytic dehydrochlorination of DCE at
cathode simultaneously with the anodic oxidative aromatic chlorination using the
released HCl as chloro-source for the efficient synthesis of value-added (hetero)aryl
chlorides is demonstrated.
5
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