Electrooxidative Metal-Free Cyclization of 4-Arylaminocoumarins with DMF as C1-Source

Article abstract of DOI:10.1002/adsc.202100146

An environmentally-benign electrochemical approach for the construction of quinoline derivatives employing N,N-dimethylformamide (DMF) as the methine source has been devised by cyclization of 4-(phenylamino)-2H-chromen-2-ones. In a user-friendly undivided cell, 6H-chromeno[4,3-b]quinolin-6-ones were obtained under chemical oxidant-free and transition-metal-free conditions in 43–92% yields with high functional tolerance. (Figure presented.).

Full text of DOI:10.1002/adsc.202100146

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DOI: 10.1002/adsc.202100146
Electrooxidative Metal-Free Cyclization of 4-Arylaminocoumarins
with DMF as C1-Source
Yiyi Weng,a, b,* Hantao Chen,a Nanhui Li,a Long Yang,b and Lutz Ackermannb,*
a
College of Pharmaceutical Sciences, Zhejiang University of Technology, 310014, Hangzhou, (People’s Republic of China)
E-mail: wengyoyo@163.com
b
Institut fuer Organische und Biomolekulare Chemie, Georg-August-Universitaet Gottingen, Tammannstrasse 2, 37077,
Goettingen, Germany
E-mail: Lutz.Ackermann@chemie.uni-goettingen.de
Manuscript received: February 1, 2021; Revised manuscript received: April 12, 2021;
Version of record online: ■■■, ■■■■
Supporting information for this article is available on the WWW under https://doi.org/10.1002/adsc.202100146
At present, the CÀ C bond formations under tran-
Abstract: An environmentally-benign electrochem-
sition metal-catalyzed[10] and metal-free electrochem-
ical approach for the construction of quinoline
derivatives employing N,N-dimethylformamide
(DMF) as the methine source has been devised by
cyclization of 4-(phenylamino)-2H-chromen-2-ones.
In a user-friendly undivided cell, 6H-chromeno[4,3-
b]quinolin-6-ones were obtained under chemical
ical conditions have been disclosed.[11–14] Especially,
metal-free catalyzed electrochemistry enabled wide
applications for green protocols of alkylations,[12]
arylations,[13] and dehydrogenative couplings[14] (Sche-
me 1a). New methods for CÀ C bond formations by
electrochemistry remain underdeveloped for an effi-
oxidant-free and transition-metal-free conditions in
cient access to valuable organic structural motifs.
43–92% yields with high functional tolerance.
Almost all CÀ C bond coupling reactions using
DMF as a carbon source have the disadvantages of
high temperature and harsh conditions.[15] Therefore,
Keywords: Electrochemistry; Metal-free catalytic
there is a strong demand for methods that use DMF as
strategy; CÀ C bond formation; N,N-dimeth-
a carbon source to construct CÀ C bond under mild
ylformamide; Chromene-fused quinoline
conditions.
We have previously reported a route using copper
as catalyst and inexpensive industrial raw material
Chromene-fused quinoline derivatives are wildly dis- DMF as the carbon source in the presence of tert-butyl
tributed in natural products,[1] and display excellent
performance towards pharmacological and biological
activities, such as anti-cancer activity,[2] selective non-
steroidal progesterone receptor modulators,[3] glucocor-
ticoid modulators,[4] and estrogen receptor β-selective
ligands.[5] Additionally, there are also important appli-
cations in biotechnology, such as dyes for mitochon-
drial imaging[6] and fluorescent pH sensor (Figure 1).[7]
In recent years, electrocatalysis has been recognized
as a versatile and environmentally-friendly tool for
molecular synthesis due to its effective nature, atom-
efficiency, and environmentally-friendly merits.[8]
Thus, electrochemistry has enabled significant devel-
opments of C–H functionalization for the formation of
CÀ C and CÀ Het bonds.[9] Among them, the selective
CÀ C formation is a crucial challenge for the modifica-
tion of natural products, pharmaceutically active
compounds, and ligands for catalytic transformations.
Figure 1. Some examples of chromene-fused quinolines.
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was isolated in 92% yield when substrate 1a was
directly electrolyzed at a 10 mA constant current in a
mixed solution of NaI, NaOAc, and NaHSO3 in
°
anhydrous DMF at 95 C without additional metals
(entry 1). Compared with NaI as the iodide salt, the
yield dropped slightly to 89% when employing KI as
the iodide additive (entry 2). However, only 45% yield
of product 2a was obtained using nBu4NI instead of
NaI (entry 3). As was expected, when switching iodide
salts to other additives, such as NaBr, nBu4NBF4, and
LiClO4, no desired product was detected (entries 4–6).
In addition, the participation of water reduced the
yield, and this proved that anhydrous conditions were
particularly important for this reaction (entries 7–8).
Further control experiments confirmed the essential
role of electricity, as this resulted in lower yields or
even no desired product (entries 9–11). Interestingly, it
Scheme 1. The development of CÀ C bond formations.
perbenzoate (TBPB) as the oxidant to construct 6H- was found that carbon rods or platinum plates were not
chromeno [4,3-b] quinoline-6-ones.[16] Thus, we have advantageous as anodes than glassy carbon (GC)
proposed a strategy for metal-free electrocatalytic anodes (entries 12–13). The yield was decreased
°
cyclizations of 4-arylaminocoumarins using DMF as slightly when the reaction was performed at 100 C
the carbon source in the absence of toxic and (entry 14).
expensive chemical oxidant in a mild, efficient, and
environmentally-friendly manner (Scheme 1b).
With the optimized conditions in hand, the substrate
scope was next investigated to illustrate the robustness
We commenced our studies by using 4-(phenyl- of the electrooxidative catalytic strategy (Scheme 2).
amino)-2H-chromen-2-one (1a) as the model substrate
to identify the optimal reaction conditions (Table 1).
The desired 6H-chromeno[4,3-b]quinolin-6-one 2a
Table 1. Optimization of the reaction conditions.[a]
Entry
Deviation from standard conditions
Yield/%[b]
1
2
3
4
5
6
7
8
None
92
89
45
0
0
0
77
55
78
82
0
0
52
80
KI instead of NaI
nBu4NI instead of NaI
NaBr instead of NaI
nBu4NBF4 instead of NaI
LiClO4 instead of NaI
DMF/H2O (14:1) instead of DMF
DMF/H2O (5:1) instead of DMF
I=8 mA
I=15 mA
No current
C as anode
Pt as anode
9
10
11
12
13
14
°
100 C
Scheme 2. Synthesis of 6H-chromeno [4,3-b] quinoline-6-ones
(2a–2l) a,b. aReaction conditions: Undivided cell, Glassy carbon
anode, Pt plate cathode, 1 (0.20 mmol), NaI (0.20 mmol),
[a] Reaction conditions: Undivided cell, Glassy carbon anode, Pt
plate cathode, 1a (0.20 mmol), NaI (0.20 mmol), NaOAc
(0.20 mmol), NaHSO3 (0.24 mmol), anhydrous DMF
NaOAc (0.20 mmol), NaHSO3 (0.24 mmol), anhydrous DMF
b
°
°
(3.0 mL), constant current=10 mA, 95 C, 12 h.
(3.0 mL), constant current=10 mA, 95 C, 12 h. Isolated yield
[b] Yields of isolated product.
of 2.
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As shown, electro-donating groups on the aniline ring
The scalability of electrooxidative metal-free cycli-
showed moderate to excellent reactivity in the syn- zation with DMF as the methine source was evaluated
thesis of 6H-chromeno[4,3-b]quinolin-6-ones 2a–2e, in 8.0 mmol scale (Scheme 4). The reaction furnished
2j–2l (62–92%). Furthermore, halide substituents (F, the desired product in 51% yield, which shows great
Cl, Br) were well-tolerated, providing the correspond- potential of this electrooxidative metal-free cycliza-
ing products (2f–2h) in 63–81% yields. However, tion.
compared with common electron-donating groups,
phenyl-substituted substrate (2i, 52%) obtained lower electrooxidative intramolecular CÀ C bond formation, a
yield, which may be due to steric hindrance. series of mechanistic studies was carried out
In order to understand the mechanism of the
Finally, the substitution effect on both the coumarin (Scheme 5). Two equivalents of iodine (Scheme 5a) or
and the benzene ring was taken into consideration N-iodosuccinimide (NIS) (Scheme 5b) were tested as
(Scheme 3). The reaction performed well with either the oxidant directly in the reaction of substrate 1a,
the coumarin moiety or the aromatic ring containing since the iodide could be oxidized under electro-
electron-donating or electron-withdrawing substituents chemical conditions. In the absence of electricity,
(4a–4l, 44–80%).
neither iodine nor NIS as the oxidant can be used to
obtain the desired product 2a. Radical inhibition
experiment with butylated hydroxytoluene (BHT) was
carried out to probe whether the reaction occurred by a
radical reaction pathway or not. As was expected, no
desired product could be observed by adding one
equivalent of BHT, which indicates that the radical
involves mechanism (Scheme 5c). We could infer from
the results of parallel experiments that the methine
group of the cyclization product should be from the
methyl group on DMF (Figure S1, see SI).
Furthermore, we performed cyclic voltammetry
(CV) experiments to gain further insights into this
reaction mechanism (Figure S2, See SI). Substrate 1a
was oxidized within several waves at 1.10 VSCE, 1.55
V
SCE, and 1.90 VSCE. When NaOAc, NaHSO3, NaI, and
DMF were added, the oxidation potential of the
mixture was 1.60 VSCE, which reduced the partial
oxidation potential of 1a and promoted the reaction.
Based on our studies, a plausible mechanistic
pathway was proposed in Scheme 6. In the first step,
the anodically generated iodine radicals attack sub-
strate 1a to afford intermediate A, which is converted
to intermediate B after releasing iodine anion. At the
Scheme 3. Synthesis of 6H-chromeno [4,3-b] quinoline-6-ones
(4a–4l) a,b. aReaction conditions: Undivided cell, Glassy carbon
anode, Pt plate cathode, 3 (0.20 mmol), NaI (0.20 mmol),
NaOAc (0.20 mmol), NaHSO3 (0.24 mmol), anhydrous DMF
b
°
(3.0 mL), constant current=10 mA, 95 C, 12 h. Isolated yield
of 4.
Scheme 4. Gram scale experiment.
Scheme 5. Controlling experiments.
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Acknowledgements
Generous support by the NSFC (Grant No. 21506190,
21978273), the CSC (Scholarship to Y. Weng) the DFG
(Gottfried-Wilhelm-Leibniz award to L. A.) are gratefully
acknowledged.
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Electrooxidative Metal-Free Cyclization of 4-Arylaminocou-
marins with DMF as C1-Source
Adv. Synth. Catal. 2021, 363, 1–6
Y. Weng*, H. Chen, N. Li, L. Yang, L. Ackermann*