Metal-Free Electrochemical Oxidative Dihalogenation of Quinolines on the C5 and C7 Positions Using N-Halosuccinimides

Article abstract of DOI:10.1002/ejoc.202001066

An efficient and convenient method for electrochemically oxidative dichlorination or dibromination of 8-aminoquinolines on C5 and C7 positions using N-halosuccinimides (NCS and NBS) as the halogen source was described. Substrates with various functional g

Full text of DOI:10.1002/ejoc.202001066

Communication
doi.org/10.1002/ejoc.202001066
EurJOC
European Journal of Organic Chemistry
Electrochemical Synthesis
Metal-Free Electrochemical Oxidative Dihalogenation of
Quinolines on the C5 and C7 Positions Using
N-Halosuccinimides
Jiahao Hou,[a] Kai Wang,[b] Changjun Zhang,[b] Tingting Wei,[a] Renren Bai,[a] and
Yuanyuan Xie*[a,b]
Abstract: An efficient and convenient method for electro- Substrates with various functional groups were transformed
chemically oxidative dichlorination or dibromination of smoothly and the halogenated products were obtained in good
8-aminoquinolines on C5 and C7 positions using N-halosuccin- to excellent yields. These transformations feature transition-
imides (NCS and NBS) as the halogen source was described. metal-free, oxidant-free, and short reaction time.
Introduction
traditional methods.[6] Inspired by Cu-catalyzed C5 chlorination
of quinolinamides by Stahl and co-workers in 2013,[7] the func-
tionalization of halogenation of quinolinamides has received in-
creasing attention of researchers. After then, a number of transi-
tion-metal-catalyzed[8] and metal-free[9] halogenation reactions
of quinolinamides employing various halogen sources have
been reported successfully by Xie's, Huang's, Zhang's and other
groups. Nevertheless, most halogenation reactions of quinolin-
amides were concentrated on the C5 position, while the C5 and
C7 positions dihalogenation reactions have been rarely re-
ported. In 2015, Xie's group has reported a convenient method
for Pd-catalyzed synthesis of 5,7-dichlorinated acetylquinolin-
amide using N-chlorosuccinimide (Scheme 1, a).[8a] In 2016,
Zhang's group developed a Cu-catalyzed 5,7-dibromination of
quinolinamides using NaBr (Scheme 1b).[8d] After then, Zhang's
Quinoline derivatives are very important heterocyclic com-
pounds, which have wide applications in pharmaceutical chem-
istry,[1] materials chemistry,[2] and organic synthesis.[3] Espe-
cially, many halogen-containing quinolines have been found in
natural products and bioactive molecules.[4] (Figure 1). For ex-
ample, chloroquine compounds are believed to have a curative
effect in the treatment of novel coronavirus pneumonia (COVID-
19) patients.[4d,4e] Considering the prominent pharmacological
activities of quinoline derivatives, the development of efficient
synthesis for halogenated quinoline derivatives has got consid-
erable attention than ever before.
Figure 1. Drugs and biologically active compounds containing halogenated
quinolines scaffolds.
Over the past few decades, the conversion of C–H bonds to
C–X (X = halogen) bonds via direct C–H functionalization[5] has
become an excellent strategy in organic synthesis instead of
[a] J. Hou, T. Wei, R. Bai, Prof. Dr. Y. Xie
College of Pharmaceutical Sciences, Zhejiang University of Technology,
Hangzhou, P.R. China
E-mail: xyycz@zjut.edu.cn
http://www.zjutyxy.zjut.edu.cn/frontend/showNews.html?hashcode=
397542200
[b] K. Wang, C. Zhang, Prof. Dr. Y. Xie
Collaborative Innovation Center of Yangtze River Delta Region Green
Pharmaceuticals, Zhejiang University of Technology,
Hangzhou, P.R. China
Supporting information and ORCID(s) from the author(s) for this article are
available on the WWW under https://doi.org/10.1002/ejoc.202001066.
Scheme 1. Dihalogenation of quinolinamides on the C-5 and C-7 positions.
6382 © 2020 Wiley-VCH GmbH
Eur. J. Org. Chem. 2020, 6382–6386

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group achieved 5,7-dichlorination of quinolinamide with Oxone as H2O and methol were used and the yields were significantly
and HCl in 2018 (Scheme 1c).[9g] Although advances have been lower than acetonitrile (Table 1, Entries 3–4). Screening of halo-
made in the dihalogenation of quinolinamides, transition-metal genation reagents revealed that no desired product was ob-
catalysts, and toxic oxidants/additives were still used, which tained when NaBr was used and 1,3-dibromo-5,5-dimethyl-
made these reactions not very uneconomical and ecofriendly. hydantoin (DBDMH) was less effective than NBS (Table 1, Entries
So, it is meaningful to explore a method that is more efficient 5–6), due to the different activities of bromine in this transfor-
and green.
During the past years, electrochemical organic synthesis,
mation. Meanwhile, the 3a yield was reduced to 69 % when
electrolyte was replaced to nBu4NBr (Table 1, Entry 7). Subse-
quently, the effects of the different electrode materials and sur-
face area in this reaction were also investigated. The yields of
the target product have been reduced to varying degrees, as
replacing C anode with RVC or Pt plate(Table 1, Entries 8–9),
and replacing Pt cathode with C rod (Table 1, Entry 10). As
predicted, the yield of the desired product was decreased when
constant current declined to 10 mA, so high current density
showed a favorable effect on the formation of 3a (Table 1, Entry
11), and no product was obtained without electricity (Table 1,
Entry 12).
using safe and green electrons as oxidants or reductants to
replace the conventional dangerous and toxic oxidants, has de-
veloped rapidly and become a utility tool in organic synthe-
sis.[10] Most recently, Fang and co-workers demonstrated an
electrochemical Cu-catalyzed C5 bromination of quinolinam-
ides using NH4Br as halogenation reagent.[8n] However, the
electrochemical dihalogenation of quinolinamides on C5 and
C7 positions has not been reported. Herein, we reported an
unprecedented, high-efficiency, electrochemical oxidate C5 and
C7 dihalogenation of 8-aminoquinoline amides with N-halosuc-
cinimides (Cl, Br) (Scheme 1). This work was performed under
With the optimized conditions in hand (Table 1, Entry 2), a
air for 15 minutes at 50 °C and the desired products were ob- range of quinolinamides was applied to obtain the correspond-
tained in good to excellent yields.
ing dibromide products (Table 2). The results showed excellent
functional group tolerance, and the substrates containing elec-
tron-donating groups and electron-withdrawing groups could
obtain the desired products in good to excellent yields (3a–3q).
For example, the substrates bearing methyl, methoxyl, or tert-
Results and Discussion
Initially, N-(quinolin-8-yl)benzamide 1a and N-bromosuccin-
imide (NBS) 2 were chosen as model substrates in acetonitrile
under constant current electrolysis to search for optimal condi-
tions (Table 1). The electrochemical reactions were carried out
in an undivided cell which was equipped with a carbon rod as
the anode and Pt plate as the cathode. Pleasantly, the target
product 3a was obtained in 95 % isolated yield with 15 mA
constant current using nBu4NBF4 as electrolyte at 50 °C (Table 1,
Entry 1). Only 51 % yield of 3a was obtained when the tempera-
ture dropped to R.T. (Table 1, Entry 2). Reaction solvents such
Table 2. Dibromination of quinoline amides.[a,b]
Table 1. The effect of electrolysis conditions on the yield of the target product
3a in the bromination reaction.[a,b]
Entry
Variation from standard
Yield [%][b]
1
2
None
R.T. instead of 50 °C
95
51
3
4
5
6
7
8
9
10
11
12
H2O instead of CH3CN
CH3OH instead of CH3CN
NaBr instead of NBS
DBDMH instead of NBS
nBu4NBr instead of nBu4NBF4
RVC anode instead of C anode
Pt anode instead of C anode
C cathode instead of Pt cathode
10 mA instead of 15 mA
no electricity
18
43
trace
60
69
32
80
15
70
N.R
[a] Reaction condition: 1a (0.3 mmol), 2 (4.0 equiv.), electrolyte (0.3 equiv.),
CH3CN (3.0 mL), C anode (d = 6 mm), Pt plate cathode (0.5 cm × 1 cm), I =
15 mA, 50 °C, 15 min. [b] Isolated yields. RVC: Reticulated vitreous carbon
anode. N.R = no reaction.
[a] Reaction condition: 1 (0.3 mmol), 2 (4.0 equiv.), nBu4NBF4 (0.3 equiv.),
CH3CN (3.0 mL), C anode (d = 6 mm), Pt plate cathode (0.5 cm × 1 cm), I =
15 mA, 50 °C, 15 min. [b] Isolated yields.
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butyl on the benzene ring of benzamides reacted smoothly to substrate was replaced by thiophene, and given corresponding
afford the target products in 76 %-92 % yields (3b–3f), and hal- product in 53 % yield (5m). A substrate containing a C-2 methyl
ogen-substituted substrates also gave desired products in 78 %- substituent in the quinoline portion also provided the desired
87 % (3g–3m). In addition, substrates containing a strong elec- product in moderate yield (5n). Unfortunately, C-6 methoxyl
tron-withdrawing group, such as trifluoromethyl, can also be substituent on the quinoline ring did not get the desired prod-
transformed to the corresponding product in good yield (3n). uct under optimal conditions (5o). Alkylamides, such as trimeth-
Moreover, the substituent at different positions (ortho-, meta- ylacetamide, also did not obtain the target product (5p).
or para-) of the benzene ring has no significant effect on the
reaction (e.g., 3b and 3c; 3h, 3i, and 3l; etc.). To our delight,
heterocycle amide with thienyl performed well in the reaction
and provided the desired product with satisfactory yield (3o).
Furthermore, substrates amides containing a C-2 methyl or C-6
methoxy substituent on the quinoline ring also gave the di-
bromination products in good yields (85 % and 76 %, 3p and
3q). Sadly, trimethylacetamide did not get the desired product
under optimal conditions (3r).
In further studies, we found that the reaction conditions of
bromination could be applied to chlorination, and dichloride
products were obtained in moderate to good yields (Table 3).
Owing to the activity of N-Chlorosuccinimide (NCS) is lower
than NBS,[9c] the yields were lower than the dibromination ones.
Similar to dibromination, the substrates with electron-donating
and electron-withdrawing groups could be given the desired
products in moderate to good yields (5b–5l). Moreover, the
chlorination could be realized when the benzene ring of the
Then, the iodization was further investigated using N-iodo-
succinimide (NIS) as a halogenation reagent. Unfortunately, the
reaction cannot take place because of its poor reactivity.
Next, the scalability of this electrochemical bromination was
explored to demonstrate the synthetic value (Scheme 2). The
desired product was isolated in 83 % yield when 5 mmol
(1.24 g) 1a was involved, whereas, the reaction time should be
extended to 30 minutes. Next, the herbicide derivative 3aa
could be synthesized from dibromide product 3a according to
the method previously reported in the literature.[11]
Table 3. Dichlorination of quinoline amides.[a,b]
Scheme 2. A gram scale experiment.
In order to further understand the reaction mechanism,
some quinolinamide analogs 6, 7, and naphthylamide 8 were
investigated under the optimal reaction conditions (Scheme 3).
None of these substrates were successful in obtaining the aim
products, even if the reaction time is prolonged. These phe-
nomena proved that the free NH of amides and the nitrogen
atom on quinoline scaffold are necessary.
Scheme 3. Nonreactive amide compounds.
Radical inhibition experiments were conducted to investi-
gate the radical nature of the reaction. When radical scavengers
such as TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxy) or BHT
(butylated hydroxytoluene) were added 5.0 equiv. in the reac-
tion system, none desired products were detected (Scheme 4a).
This result implied that the reaction might be involved in radical
steps.
Further studies were conducted to prove whether the di-
bromide products were directly formed without monobromide
products. The formation of monobromoquinolinamide was de-
tected by ESI-HRMS when the reaction underwent the optimal
conditions for 3 minutes, which was isolated and verified by
NMR spectra, revealed that halogenation happened on C5 posi-
tion of quinoline (Scheme 4b, and see the Supporting Informa-
tion for 10 NMR spectra details). Then, N-(5-bromoquinolin-8-
yl)benzamide (10) was exposed to the dibromination condi-
[a] Reaction condition: 1 (0.3 mmol), 4 (4.0 equiv.), nBu4NBF4 (0.3 equiv.),
CH3CN (3.0 mL), C anode (d = 6 mm), Pt plate cathode (0.5 cm × 1 cm), I =
15 mA, 50 °C, 15 min. [b] Isolated yields.
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Conclusion
In conclusion, we have reported an environmental-friendly elec-
trochemical reaction protocol, metal-free, oxidant-free, and re-
mote C–H halogenation on the C5 and C7 positions of 8-amino-
quinoline amides using N-halosuccinimides as a halogen source
in a short reaction time. This transformation showed high effi-
ciency, and well-tolerated functionalization, providing the de-
sired products in good to excellent yields. And some controlled
experiments confirmed that free-radical reaction mechanism
was involved in the reaction.
Acknowledgments
This work was supported by the National Natural Science
Foundation of China, NSFC (No. 21978273 and 21576239) and
National Key R&D Program of China (No. 2018YFC0214100).
Scheme 4. Mechanistic studies.
tions as starting material, and the desired compound 3a was
isolated with 97 % yield. These results proved that the dibrom-
ide products did generate from C5 monobromide compounds
step by step (Scheme 4c).
Keywords: Amides · Dihalogenation · Electrochemical
synthesis · Nitrogen heterocycles · Synthetic methods
Next, cyclic voltammetry (CV) experiments were carried out,
and this exploration showed that the oxidation potential of 1a
is lower than others′ (see the Supporting Information for de-
tails).
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