Paired Electrosynthesis of Aromatic Azo Compounds from Aryl Diazonium Salts with Pyrroles or Indoles

Article abstract of DOI:10.1002/adsc.202001457

An efficient paired electrosynthesis of functionalized aromatic azo compounds through a diazenyl radical pathway has been developed. The paired electrolysis simultaneously uses the anode and cathode reactions to avoid the electricity expense of oxidation/reduction of sacrificial substances, thereby maximizing energy efficiency and achieving high atom economy. The in vitro cytotoxicity of all compounds was evaluated against four cancer cell lines by MTT assay. Results showed that the aromatic azo compounds exhibited good antitumor activity in vitro, and one of the compounds induced tumor cell apoptosis. (Figure presented.).

Full text of DOI:10.1002/adsc.202001457

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DOI: 10.1002/adsc.202001457
Paired Electrosynthesis of Aromatic Azo Compounds from
Aryl Diazonium Salts with Pyrroles or Indoles
Mu-Xue He,+a Yu-Zheng Wu,+a Yan Yao,a Zu-Yu Mo,b Ying-Ming Pan,a and
Hai-Tao Tanga,*
a
State Key Laboratory for Chemistry and Molecular Engineering of Medicinal Resources, School of Chemistry and
Pharmaceutical Sciences of Guangxi Normal University, Guilin 541004, People’s Republic of China
E-mail: httang@gxnu.edu.cn
Pharmacy School of Guilin Medical University, Guilin, 541004 People’s Republic of China
b
+
These authors contributed equally to this work.
Manuscript received: November 23, 2020; Revised manuscript received: March 9, 2021;
Version of record online: ■■■, ■■■■
Supporting information for this article is available on the WWW under https://doi.org/10.1002/adsc.202001457
iminyl radicals for the preparation of polycyclic
imidazole derivatives and assorted pyridine-fused
Abstract: An efficient paired electrosynthesis of
functionalized aromatic azo compounds through a
polycyclic N-heteroaromatic compounds under electro-
diazenyl radical pathway has been developed. The
chemical conditions, respectively.[5,6] As an important
paired electrolysis simultaneously uses the anode
structural motif of nitrogen-containing compounds, azo
and cathode reactions to avoid the electricity
compounds are not only important synthetic colorants
expense of oxidation/reduction of sacrificial sub-
in the dye industry, but also used as indicators or food
stances, thereby maximizing energy efficiency and
additives.[7] The electrophilic diazonium function al-
achieving high atom economy. The in vitro cytotox-
lows the addition of nucleophiles to provide azo
icity of all compounds was evaluated against four
compounds, and the diazonium salts substituted with
cancer cell lines by MTT assay. Results showed that
electron-withdrawing substituents show high chemical
the aromatic azo compounds exhibited good anti-
selectivity.[8] When the aryl diazonium salt undergoes a
tumor activity in vitro, and one of the compounds
free radical mechanism reaction, the electron donating
induced tumor cell apoptosis.
substitution generally obtains
a
higher yield.[9]
Although many methods can be used to generate
diazenyl radicals,[10] the synthesis of azo compounds
by electrolysis-generating diazenyl radical has not
been reported yet.
Keywords: Paired electrolysis; N-centered radical;
Diazenyl radical; Antitumor activity
Organic electrosynthesis is a controllable and green
synthesis tool.[11] In most electrochemical conversions,
Nitrogen-centered radicals are a class of universal either anodic oxidation or cathodic reduction can
intermediates that construct nitrogen-containing com- obtain the desired products. However, only one
pounds in a wide range of chemical transformations.[1] electrode is used to convert the reaction substrate, and
Organic electrochemistry promotes chemical trans- some sacrificial reagents (protic solvents, metal elec-
formations through electron transfer, which has be- trodes, and electrolytes) are usually required to
come an efficient synthetic technique to generate promote the reaction on the counter electrode.[12] By
nitrogen-centered radicals in recent years.[2] At present, contrast, paired electrolysis simultaneously uses both
various N-centered radicals have been produced using anode and cathode reactions to synthesize the target
electrochemical techniques, such as amidyl, amidinyl, compound, which avoids the electricity expense of
iminyl and iminoxyl radicals, and many important oxidation/reduction of sacrificial substances, thereby
skeletons have been synthesized by them maximizing energy efficiency and achieving high atom
(Scheme 1a).[2,3] For instance, Waldvogel and Moeller’s economy.[13] Therefore, we report herein that the
group reported the electrochemical synthesis of amidyl reduction reaction of aryl diazonium salts at the
radical from anilides to directly construct cathode produces diazenyl radical, which then disso-
benzoxazoles.[4] Xu’s group exploited amidinyl and ciate to the anode and react with functionalized
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nBu4NBF4 was the electrolyte in solvent DMSO at
room temperature and under argon protection (entry 1).
The control experiment verified the necessity of each
reaction condition. In the absence of electricity, the
desired product 3aa cannot be obtained (entry 2).
Under air conditions, the yield of 3aa dropped to 73%
°
(entry 3). The reaction at 60 C resulted in a sharp
decline in yield (entry 4). Other solvents such as EtOH
(entry 5), CH3CN (entry 6), DMF (entry 7) and CH2Cl2
(entry 8) had lower reaction yields than DMSO. The
changes of electrolyte had a negative effect on the
yield of 3aa (entries 9–10). Under different constant
currents, the results showed that increasing (Table S1,
entry 1) and decreasing (Table S1, entry 2) the current
both lead to a decrease in the yield. We also tested the
yields change under different electrode conditions, and
the yield decreased to different degrees (Table S1,
entries 3–5).
With the clarification of the optimal reaction
conditions, we first conducted a substrate range
analysis by changing the substituents of the aryl
diazonium salt (Scheme 2). In the absence of substitu-
ents, aryl diazonium salt 1c reacts with pyrrole 2a to
Scheme 1. Generation of various N-centered radicals via elec-
trolysis.
pyrroles or indoles to construct pharmacologically yield product 3ca at 69%. The para position of aryl
active aromatic azo compounds (Scheme 1b). diazonium salt can replace electron-donating OMe
We first chose aryl diazonium salt 1a and pyrrole (3aa, 83%), Me (3ba, 78%), and electron-withdrawing
2a as model substrates to optimize the electrolysis halogens (e.g., F [3fa, 63%], Cl [3ga, 64%], and Br
reaction conditions (Tables 1 and S1). The results of [3ha, 60%]), which have good yields. Moreover,
electrolysis were optimal when the reaction was compound 3ga confirmed the crystal structure.[14]
carried out at a constant current of 5 mA in an Given that the former has a strong electron-donating
undivided cell equipped with reticulated vitreous ability, the yield of electron-donating groups is higher
carbon (RVC) anode and Pt plate cathode and when than that of electron-withdrawing groups. Because
Table 1. Optimization of the reaction conditions.[a]
Entry
Variation from standard conditions
Yield[b] (%)
1
2
3
none
no electricity
under air
83
NR
73
°
4
5
6
7
reaction at 60 C
EtOH as solvent
CH3CN as solvent
DMF as solvent
42
27
trace
64
8
9
10
CH2Cl2 as solvent
nBuNPF6 as electrolyte
Et4NBF4 as electrolyte
46
77
79
[a] Reaction conditions: Reticulated vitreous carbon (RVC)
anode (100 PPI, 1 cm×1 cm×1.2 cm), Pt plate cathode
(1 cm×1 cm), undivided cell, 1a (0.3 mmol, 1.0 equiv.), 2a
(0.9 mmol, 3.0 equiv.), electrolyte (1.0 equiv.), DMSO
(6 mL), room temperature, argon, 5 mA, 2 h (1.2 FmolÀ 1).
[b] Isolated yields. NR=no reaction.
a
Scheme 2. Substrate scope of aryl diazonium salts. Reaction
conditions: reticulated vitreous carbon (RVC) anode (100 PPI,
1 cm×1 cm×1.2 cm), Pt plate cathode (1 cm×1 cm), undivided
cell, constant current=5 mA, 1a–1k (0.3 mmol, 1.0 equiv.), 2a
(0.9 mmol, 3.0 equiv.), nBu4NBF4 (1.0 equiv.), DMSO (6 mL),
room temperature, argon, 5 mA, 2 h. Isolated yields.
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diazenyl radicals are considered to be electrophilic The reaction of substrates with substituents on the
species due to the negatively polarized N=N group, indolebenzene ring with aryl diazonium salt 1a
which require electron-donating substrate 1 to stabilize resulted in a series of moderate yields (3am–3ap,
and then tend to react with electron-rich substrate 2. 62%–91%). Among them, the electron-donating group-
The methoxy disubstituted product can be obtained substituted substrates have a higher yield than the
with a yield of 84% (3da). Moderate yields were also electron-withdrawing group substrates.
obtained in the meta- and ortho-substitutions of aryl
We conducted some control experiments to study
diazonium salts (3ia, 59%; 3ja, 57%). Both biphenyl- the mechanism of this reaction (Scheme 4). The azo
diazonium salt (3ea, 72%) and naphthalene diazonium compound 3aa was not detected by adding 4 equiv.
salt (3ka, 49%) reacted with pyrrole to obtain TEMPO (Scheme 4a) or 3 equiv. BHT (Scheme 4b) to
moderate yields.
the optimal reaction system. Product 4 can be detected
A series of aryl azo compounds was synthesized by by HRMS when adding 4 equiv. TEMPO (Scheme 4a).
the reaction of aryl diazonium salt 1a with pyrrole The reaction of aryl diazonium salt 1a with styrene
derivatives (Scheme 3). First, monomethyl and was carried out under standard conditions, and the
polymethyl substituted pyrroles can be involved in the desired products 5 and 6 were detected by HRMS
reaction to obtain a good yield (3ab–3ad, 77%–80%). (Scheme 4c). According to the above experimental
As a substrate, 1-methylpyrrole can also be used to results, the radical intermediates are involved in the
obtain a good yield (3ae, 73%). Electron-withdrawing electrocatalytic reaction. In the absence of other
group-substituted pyrroles, such as 2-acetylpyrrole (2f) reactants, pyrrole 2a did not dimerize under standard
and pyrrole-2-carbonitrile (2g), react with 1a to obtain conditions (Scheme 4d). This result indicated that
lower yields. The aryl diazonium salt 1a reacts with 2- pyrrole is not oxidized to the pyrrole radical cation on
methylindole (2h) and 3-methylindole (2i) to get a the anode.[15] In the absence of current and electrolyte,
large difference, and the results showed that it is easier the reaction of 1a and 2a did not produce the target
to attach to the third position of indoles. The product 3aa (Scheme 4e). We operated the reaction in
participation of 2-phenylindole in the reaction also the divided cell, and the results showed that the target
resulted in good yield (3aj, 76%). 1-Methylindole, product 3aa was not produced at either anode or
which has a substituent at the 2-position can also give cathode (Scheme 4f). The above reaction results
the products with a good yield (3ak, 70%; 3al, 86%). indicated that the aryl diazonium salt did not react with
pyrrole in the form of a radical cation, but was reduced
to a radical intermediate to participate in the reaction
process.
Based on the above experiments and cyclic
voltammetry experiments (Figure S2), we proposed a
possible reaction mechanism with 1b and 2a as
substrates
under
electrochemical
conditions
Scheme 3. Substrate scope of pyrroles. aReaction conditions:
reticulated vitreous carbon (RVC) anode (100 PPI, 1 cm×
1 cm×1.2 cm), Pt plate cathode (1 cm×1 cm), undivided cell,
constant current=5 mA, 1a (0.3 mmol, 1.0 equiv.), 2b–2p
(0.9 mmol, 3.0 equiv.), nBu4NBF4 (1.0 equiv.), DMSO (6 mL),
room temperature, argon, 5 mA, 2 h. Isolated yields.
Scheme 4. Control Experiments.
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(Scheme 5). First, the aryl diazonium salt 1b was
reduced to aryldiazenyl radical intermediate A at the
cathode,[16] and the tetrafluoroborate anion was formed.
Aryldiazenyl radical intermediate A reacted with
pyrrole 2a to generate radical intermediate B. Radical
intermediate B was oxidized to cation intermediate C
at the anode. Subsequently, intermediate C was
deprotonated by tetrafluoroborate and finally formed
the target azo product 3ba.
The proposed reaction mechanism can be demon-
strated by the cyclic voltammetry experiment (Fig-
ure S2). According to the cyclic voltammograms, there
is a reduction peak (À 1.201 V vs. Fc/Fc+) when only
Table 2. IC50 (μM) values for compounds 3ea, 3ga and 3ha.
Compounds HeLa
T-24
SKOV3
MGC-803
3ea
3ga
3ha
3.2�1.2 1.5�1.3 10.5�0.8 5.4�1.3
4.5�1.4 3.3�0.9 10.4�1.5 6.2�2.0
7.9�1.3 5.8�0.4 12.6�0.7 9.5�1.9
the aryl diazonium salt 1b is present (Figure S2-1, 3ea on the T-24 cell line was 1.5�1.3 μM, which
curve b), indicating that it has undergone a cathodic indicated a significant inhibitory effect on tumor cells.
reduction process without an anodic oxidation process. On the basis of these results, the antitumor mechanism
There is no redox peak when only pyrrole 2a exists of 3ea on the T-24 cell line was further studied.
(Figure S2-2, curve c), showing that there is no redox Detailed experimental results are described in the
process. In the cyclic voltammograms of 1b in the Supporting Information.
presence 2a (Figure S2-3, curve d), there is a reduction
In summary, we have developed a method to
peak (À 1.201 V vs. Fc/Fc+) and an oxidation peak synthesize pharmacologically active aromatic azo
(1.294 V vs. Fc/Fc+), implying that the aryl diazonium compounds through the reaction of functionalized
salt 1b was reduced to intermediate A first, and pyrroles or indoles with aryl diazonium salts under
intermediate A reacted with 2a to form intermediate B, paired electrolysis conditions. Under our paired elec-
then intermediate B was anodized to intermediate C. trosynthesis conditions, the diazonium salt was reduced
Finally, the intermediate C was deprotonated to to the diazenyl intermediate at the cathode to promote
generate the target compound 3ba. The curve of the the reaction process, which uses both anode and
synthesized compound 3ba shows no redox peak cathode reactions to achieve high atom economy and
(Figure S2-4, curve e), indicating that the synthesized maximize energy efficiency. The in vitro cytotoxicity
compound has no further redox process.
of all compounds against four cancer cell lines was
As mentioned above, aromatic azo compounds can screened by using MTT assay. Among them, com-
not only be used as dyes and photoswitches, but also pounds 3ea, 3ga, and 3ha all showed good antitumor
have good biological activity. Here, compounds 3aa– activity against the T-24 tumor cell line. Preliminary
3ka and 3ab–3ap were used to conduct in vitro analysis on the mechanism of action showed that
cytotoxicity study of four cancer cell lines, Hela, T-24, compound 3ea inhibited T-24 cell apoptosis.
SKOV3, and MGC-803, via MTT assay, which were
screened using 5-FU as the positive control. As shown
Experimental Section
in Table 2, the IC50 value of compound 3ga was 3.3�
0.9 μM, showing good antitumor activity against the T- Synthesis of Aromatic Azo Compounds
24 cell lines. In addition, the IC50 value of compound
A 10 mL three-necked round-bottomed flask was charged with
aryl diazonium salts (0.3 mmol, 1.0 equiv.), pyrrole derivatives
(0.9 mmol, 3.0 equiv.), and nBu4NBF4 (0.3 mmol, 1 equiv.).
The flask was equipped with a reticulated vitreous carbon RVC
(100 PPI, 1 cm×1 cm×1.2 cm) anode and a platinum plate
(1 cm×1 cm) cathode. DMSO (6 mL) was added. Electrolysis
was carried out at room temperature using a constant current of
5 mA until the substrate was completely consumed (monitored
by TLC, about 2 h). After the reaction was completed, the
solvent was extracted with ethyl acetate. The aqueous phase
was extracted with ethyl acetate (3×30 mL). The combined
organic solution was washed with brine, dried over MgSO4 and
concentrated under reduced pressure. The resulting solution was
purified by silica gel column chromatography using ethyl
acetate/petroleum ether to afford the desired products.
Scheme 5. Proposed mechanism.
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Acknowledgements
We thank the National Natural Science Foundation of China
(22061003), Guangxi Natural Science Foundation of China
(2019GXNSFAA245027), Guangxi Key R&D Program (No.
AB18221005) and Improvement of teachers’ scientific research
ability of Guangxi Province of China (RZ1900005748) for
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Paired Electrosynthesis of Aromatic Azo Compounds from
Aryl Diazonium Salts with Pyrroles or Indoles
Adv. Synth. Catal. 2021, 363, 1–6
M.-X. He, Y.-Z. Wu, Y. Yao, Z.-Y. Mo, Y.-M. Pan, H.-T.
Tang*