Electrochemical Oxidative C?H Cyanation of Quinoxalin-2(1H)-ones with TMSCN

Article abstract of DOI:10.1002/ejoc.202100348

Both quinoxalin-2(1H)-ones and nitriles are valuable organic compounds, and it is an interesting task to introduce cyano into quinoxalin-2(1H)-ones. Herein a regioselective C?H cyanation of quinoxalin-2(1H)-ones was developed with a nucleophilic cyano source TMSCN under electrochemical oxidative conditions. This process allowed the synthesis of C3 cyanated quinoxalin-2(1H)-ones in moderate to excellent yields in the absence of transition-metal catalysts and organic hydroperoxides.

Full text of DOI:10.1002/ejoc.202100348

Communications
doi.org/10.1002/ejoc.202100348
Electrochemical Oxidative CÀ H Cyanation of Quinoxalin-
2(1H)-ones with TMSCN
Yanling Zhan,[a] Yifan Li,[a] Jinwen Tong,[a] Ping Liu,*[a] and Peipei Sun*[a]
group to organic compounds is synthetically attractive for
organic chemists. Chelation-assisted transition-metal-catalyzed
CÀ H activation has been well developed for the CÀ H cyanation
of arenes[16] or heteroarenes[17] in the past decade (Scheme 1a).
However, this strategy required the introduction of directing
groups to the substrates and the utilization of expensive Rh, Ru
or Pd, etc. transition-metal catalysts.[18] Direct CÀ H cyanation of
arenes[19] or heteroarenes[20] is an alternative and efficient
protocol because it enables the formation of cyanated products
without the prefunctionalization of substrates. In these reac-
tions, the choice of cyano sources was significant for obtaining
high efficiency. Electrophilic cyanating reagents, such as
BrCN,[19a] N-cyano-N-phenyl-p-toluenesulfonamide (NCTS),[19e,20b]
could smoothly react with electron-rich arenes. However, these
reagents had some disadvantages such as high toxicity of BrCN
Both quinoxalin-2(1H)-ones and nitriles are valuable organic
compounds, and it is an interesting task to introduce cyano into
quinoxalin-2(1H)-ones. Herein a regioselective CÀ H cyanation of
quinoxalin-2(1H)-ones was developed with a nucleophilic cyano
source TMSCN under electrochemical oxidative conditions. This
process allowed the synthesis of C3 cyanated quinoxalin-2(1H)-
ones in moderate to excellent yields in the absence of
transition-metal catalysts and organic hydroperoxides.
Quinoxalin-2(1H)-ones are present in a variety of natural
products and can be widely found in pharmaceutical molecules
with antimicrobial, antifungal, antithrombotic, and antitumor
activities, etc.[1] Quinoxalin-2(1H)-one skeleton can also be
utilized as a building block in the design of fluorescent dye and
fluorogenic probe.[2] A series of active quinoxalinone derivatives
bearing a C3-substituted group has been extensively studied,
due to their diverse biological activities.[3] Therefore, the
synthesis of C3-substituted quinoxalin-2(1H)-ones has gained
much attention in recent years. One of the most efficient
methods to access C3-substituted quinoxalin-2(1H)-ones is the
direct transformation of quinoxalin-2(1H)-ones without
prefunctionalization.[4] The past years have witnessed extensive
efforts on the direct C3 arylation,[5] heteroarylation,[6]
alkylation,[7]
trifluoromethylation,[8a]
trifluoroalkylation,[8b]
difluoromethylation,[8c] acylation,[9] amination,[10] alkoxylation,[11a]
fluoroalkoxylation,[11b] phosphonation[12] or sulfenylation[13] of
quinoxalin-2(1H)-ones. Nevertheless, the introduction of a CN
group to the C3-position of quinoxalin-2(1H)-ones is still highly
desirable.
Compounds containing cyano functional group, namely
nitriles, are a valuable class of organic compounds due to their
wide application in pharmaceuticals, agrochemicals, and materi-
als, etc.[14] Moreover, nitriles are easily converted to other
important organic compounds such as amides, carboxylic acids,
esters, amines, ketones, aldehydes, etc. via the cyano
transformation.[15] Therefore, the facile introduction of a cyano
[a] Y. Zhan, Y. Li, J. Tong, Dr. P. Liu, Prof. P. Sun
College of Chemistry and Materials Science, Jiangsu Provincial Key
Laboratory of Material Cycle Processes and Pollution Control,
Jiangsu Collaborative Innovation Center of Biomedical Functional Materials,
Nanjing Normal University
Nanjing 210023, People’s Republic of China
E-mail: pingliu@njnu.edu.cn
sunpeipei@njnu.edu.cn
http://schools.njnu.edu.cn/chem/person/ping-liu
http://schools.njnu.edu.cn/chem/person/peipei-sun
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/ejoc.202100348
Scheme 1. Direct CÀ H cyanation with nucleophilic cyano sources.
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or high cost of NCTS. A series of metal cyanides were used as
nucleophilic cyano sources. Some of them, like NaCN, KCN,
CuCN,[16d] AgCN,[19f] are highly toxic to the environment. Non-
toxic K3[Fe(CN)6][19d] and K4[Fe(CN)6],[20a] were considered as the
ideal cyanide sources, but a trans-cyanation process with a
palladium catalyst was needed (Scheme 1b). Some cheap
organic solvents, including CH3CN,[19c] CH3NO2,[16a] and DMF,[16b]
were also employed as the cyanating reagents. Due to their low
reactivity, equimolar quantities of metal salts or additives were
required for the cyanation. Thus, the development of more
environmentally friendly and efficient CÀ H cyanation methods
is an area worth exploring.
Electrochemical oxidative reactions are one of the most eco-
friendly tools for the direct CÀ H functionalization of arenes and
heteroarenes since the usage of stoichiometric amounts of
oxidants and additives can be avoided in this process.[21] In
2019, Zeng and co-workers demonstrated the successful
application of electrochemical oxidation to achieve the direct
C3 phosphonation,[22a] trifluoromethylation[22b] and amination[22c]
of quinoxalin-2(1H)-ones. In 2020, Wang, Pan et al. demon-
strated that the electrochemical deoxygenative functionaliza-
tion with alkyl carbazates allowed the C3 alkylation of
quinoxalin-2(1H)-ones.[22d] Recently, electrochemical oxidative
C3 thiolation,[22e] alkoxylation,[22f] decarboxylative alkylation,[22g]
alkylation[22h] and phosphorylation[22i] of quinoxalin-2(1H)-ones
were reported by several groups. In 2018, Gooßen revealed that
the direct electrochemical CÀ H cyanation of electron-rich
(hetero)arenes could be achieved by using NaCN as the
nucleophilic cyano source.[23] In 2020, the group of Jin described
a direct CÀ H cyanidation of quinoxalin-2(1H)-ones by employing
NH4SCN as the cyanide source in the presence of 4 equiv. of
tert-butyl hydroperoxide (TBHP).[24] Herein, we report an electro-
chemical oxidative CÀ H cyanation of electron-deficient quinox-
alin-2(1H)-ones with a nucleophilic cyano source TMSCN in the
absence of hydroperoxides under transition-metal-free condi-
tions (Scheme 1c).
Cyclic voltammetry (CV) experiments of methylquinoxalin-
2(1H)-one (1a) and TMSCN were performed to determine the
oxidation potential of the two substrates. As shown in Figure 1,
the oxidation of 1a occurred at a lower potential (2.16 V vs SCE,
blue curve) than that of TMSCN (no distinct oxidation peak) in
the 0–2.5 V region. This indicated that the electrocatalytic
oxidation of 1a could occur first to initiate the reaction.
The electrochemical reaction was performed by employing
1-methylquinoxalin-2(1H)-one (1a, 0.5 mmol, 1.0 equiv.) and
TMSCN (1.5 mmol, 3.0 equiv.) as the model substrates to
research the reaction conditions. The reaction was carried out
in an undivided cell equipped with a graphite felt anode and a
platinum plate cathode (Table 1). When acetonitrile was used as
a solvent and nBu4NBF4 as the supporting electrolyte, the
reaction did not occur at all (entry 1). Surprisingly, when the
reaction was conducted in a mixed solvent of CH3CN (8 mL) and
KH2PO4/K2HPO4 pH 9 buffer solution (2 mL) in the presence of
the electrolyte nBu4NBF4 under 5 mA constant current, the
desired product 4-methyl-3-oxo-3,4-dihydroquinoxaline-2-car-
bonitrile (3aa) was obtained in 50% yield after 12 h (entry 2).
Different CH3CN/phosphate buffer (pH 9) volume ratios were
Figure 1. CV scans (scan rate 100 mv·sÀ 1) of substrates: (a) Blank (nBu4NPF6
(0.02 M) in MeCN); (b) TMSCN (0.02 M) in blank. (c) 1-Methylquinoxalin-2(1H)-
one (1a, 0.02 M) in blank.
n
investigated with the electrolyte Bu4NBF4 (entries 2–4). A 1:1
ratio gave the best result (entry 4). Replacing the electrolyte
nBu4NBF4 by nBu4NPF6 led to a lower yield (entry 5, 71%). A high
n
yield of 3aa (93%) was obtained in the absence of Bu4NBF4,
the phosphate buffer (pH 9) being the sole electrolyte (entry 6).
The use of a pH 8 phosphate buffer solution gave a much lower
yield of 3aa (25%, entry 7). Moreover, inferior results were
obtained with NH3/NH4Cl (pH 9) or Na2CO3/NaHCO3 buffer
(pH 9) (entries 8 and 9), maybe due to their lower conductiv-
ities. When platinum plates or graphite felts were used as both
anode and cathode, lower yields of 3aa were obtained, 30%
and 37% respectively (entries 10 and 11). Reducing the reaction
time to 6 h resulted in a lower yield of 3aa (55%, entry 12).
Increasing the reaction time to 18 h did not bring an increase in
the reaction yield (92%, entry 13). Reducing the amounts of
TMSCN to 2.0 equiv. resulted in a lower reaction yield (75%,
entry 14) and increasing it to 4.0 equiv. brought no improve-
ment of the yield (86%, entry 15). Unfortunately, when non-
toxic cyano source K4[Fe(CN)6] was used instead of TMSCN, the
reaction did not occur (entry 16). Finally, a control experiment
without electricity was conducted, and no desired product was
detected (entry 17).
With the optimized reaction conditions in hand, we ex-
plored the scope of 1-methylquinoxalin-2(1H)-ones. Initially, we
investigated the effect of different substituents on the aromatic
ring of quinoxalin-2(1H)-ones 1, and the results are summarized
in Scheme 2. Both electron-withdrawing (F, Cl, Br) and an
electron-donating methyl group on the benzene ring of
substrates 1 had little influence on the yields of cyanated
products 3ab–3ag (55–87%). Gratifyingly, 1-methylbenzo[g]
quinoxalin-2(1H)-one (1h) also worked well to give the desired
product 3ah in 70% yield. We wondered whether our method
could be applied to electron-rich arenes. When 1,3,5-trimeth-
oxybenzene (1i) or 1,3-dimethoxybenzene (1j) was treated with
TMSCN under the standard conditions, the desired products 3ai
and 3aj were obtained in 78% and 77% yields, respectively.
Next, we examined the influence of the N-substituents of
quinoxaline-2(1H)-ones on this reaction (Scheme 3). The sub-
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Table 1. Optimization of the reaction conditions.[a]
Entry Electrode Electrolyte Solvent/buffer[c]
[mL:mL]
Time
[h]
Yield
[%][b]
1
C(+)/
Pt(À )
C(+)/
Pt(À )
C(+)/
Pt(À )
C(+)/
Pt(À )
C(+)/
Pt(À )
C(+)/
Pt(À )
C(+)/
Pt(À )
C(+)/
Pt(À )
C(+)/
Pt(À )
Pt(+)/
Pt(À )
C(+)/
C(À )
nBu4NBF4
CH3CN
12
12
12
12
12
12
12
12
12
12
12
6
0
2
nBu4NBF4
CH3CN/pH 9 (8:2)
CH3CN/pH 9 (2:8)
CH3CN/pH 9 (6:6)
CH3CN/pH 9 (6:6)
CH3CN/pH 9 (6:6)
CH3CN/pH 8 (6:6)
50
43
85
71
93
25
35
30
30
37
55
92
75
86
0
3
nBu4NBF4
4
nBu4NBF4
5
nBu4NPF6
6
–
–
–
–
–
–
–
–
–
–
–
–
7
Scheme 2. Scope of various substituents on the aromatic ring of quinoxalin-
2(1H)-ones. Reaction conditions: 1 (0.5 mmol, 1.0 equiv.), TMSCN (1.5 mmol,
3.0 equiv.), graphite felt anode, platinum plate cathode (10 mm×10 mm),
CH3CN/KH2PO4/K2HPO4 pH 9 buffer solution (1:1, (v/v), 12 mL), constant
current (5 mA), 2.2 F/mol, undivided cell, at room temperature in the
presence of open air. aIsolated yield based on 1.
8[d]
9[e]
10
11
12[f]
13[g]
14[h]
15[i]
16[j]
17[k]
CH3CN/buffer B
(6:6)
CH3CN/buffer C
(6:6)
CH3CN/pH 9 (6:6)
CH3CN/pH 9 (6:6)
CH3CN/pH 9 (6:6)
CH3CN/pH 9 (6:6)
CH3CN/pH 9 (6:6)
CH3CN/pH 9 (6:6)
CH3CN/pH 9 (6:6)
CH3CN/pH 9 (6:6)
C(+)/
Pt(À )
C(+)/
Pt(À )
C(+)/
Pt(À )
C(+)/
Pt(À )
C(+)/
Pt(À )
C(+)/
Pt(À )
respectively. In addition, different N-benzyl substituted quinoxa-
line-2(1H)-ones 2m–2s, possessing electron-donating (OMe and
Me) or electron-withdrawing (CN, CF3, F, Cl) groups, were
successfully converted into the desired product 4am–4as in
moderate to excellent yields (58–92%). Similarly, different N-
phenyl of quinoxaline-2(1H)-ones did not have an obvious
influence on the reaction efficiency and provided the corre-
sponding products 4at–4av in high yields (86–91%). To our
delight, the unprotected quinoxaline-2(1H)-one (2w) was also a
viable reaction partner for this transformation and gave the
desired product 4aw in 70% yield.
In order to demonstrate the practicality of this electro-
chemical CÀ H cyanation protocol, a scale-up reaction of 1-
methylquinoxalin-2(1H)-one (5.0 mmol) with TMSCN was con-
ducted (Scheme 4a). Delightfully, the efficiency of this trans-
formation was not affected and 75% yield of the product 3aa
was obtained. Then 3aa was used for a further transformation
(Scheme 4b). When 3aa was treated with La(OTf)3 (10 mol%)
18
12
12
12
12
0
[a] Reaction conditions: 1a (0.5 mmol, 1.0 equiv.), TMSCN (1.5 mmol,
3.0 equiv.), graphite felt anode, platinum plate cathode (10 mm×10 mm),
electrolyte (0.1 M), constant current (5 mA), 2.2 F/mol, solvent (12 mL),
undivided cell, at room temperature in the presence of open air. [b]
Isolated yield based on 1a. [c] Buffer solution prepared from KH2PO4 and
K2HPO4 at 4 M with respect to phosphate ion. [d] Buffer B (NH3/NH4Cl aq.,
pH=9). [e] Buffer C (Na2CO3/NaHCO3 aq., pH=9). [f] 6 h, 1.1 F/mol. [g] 18 h,
3.3 F/mol. [h] TMSCN (2.0 equiv.). [i] TMSCN (4.0 equiv.). [j] K4[Fe(CN)6]
instead of TMSCN. [k] Without electricity.
°
strates 2 bearing N-alkyl groups, both chain alkyl (ethyl, n-butyl)
and cycloalkyl (cyclopropylmethyl, cyclohexylmethyl) groups,
were well compatible for this reaction and generated the target
products 4aa–4ad in 78–84% yields. Moreover, quinoxaline-
2(1H)-ones 2 substituted with reactive functional groups, such
as cyanomethyl, ester group, phenacyl, 3-bromopropyl, etc.
underwent the electrochemical CÀ H cyanation with high
efficiency to give the products 4ae–4ai in 75–87% yields.
Quinoxaline-2(1H)-one 2j bearing hydroxyethyl was smoothly
tolerated in the reaction, affording the C3 cyanated product 4aj
in 84% yield. It was worth noting that unsaturated allyl and
propargyl groups were also tolerated, and corresponding
products 4ak and 4al were obtained in 86% and 90% yields,
and Pd(OAc)2 (5 mol%) in CH3COOH/H2O (2:1) at 60 C for 12 h,
the hydrolysis of the cyano group resulted in the desired
product 4-methyl-3-oxo-3,4-dihydroquinoxaline-2-carboxamide
(5) in 75% yield.
To understand the mechanism of this CÀ H cyanation
reaction, a control experiment was carried out. When TMSCN
was not added to the reaction mixture under the standard
conditions, no desired product 3aa was observed and most of
the starting material 1a was recovered, indicating that CH3CN
was not the cyano source (Scheme 5).
Based on the above experimental results and previous
reports, a plausible mechanism for this electrochemical CÀ H
cyanation is proposed in Scheme 6. Firstly, quinoxalin-2(1H)-one
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Scheme 5. Control experiment.
Scheme 6. Proposed mechanism.
Scheme 3. Scope of N-substituted quinoxaline-2(1H)-ones. Reaction condi-
tions: 2 (0.5 mmol, 1.0 equiv.), TMSCN (1.5 mmol, 3.0 equiv.), graphite felt
anode, platinum plate cathode (10 mm×10 mm), CH3CN/KH2PO4/K2HPO4
pH 9 buffer solution (1:1, (v/v), 12 mL), constant current (5 mA), 2.2 F/mol,
undivided cell, at room temperature in the presence of open air. aIsolated
yield based on 2.
nucleophilic cyano source TMSCN to deliver radical B.[25] Then,
intermediate B would be oxidized further to the intermediate
cation C. Deprotonation of the cation intermediate would lead
to the formation of the final product 3aa. The cathodic reaction
is the reduction of water to dihydrogen gas, which is favored
on a low-hydrogen overvoltage Pt cathode.
In summary, we have established an electrochemical
oxidative protocol for the direct CÀ H cyanation of quinoxalin-
2(1H)-ones or of electron-rich arenes with TMSCN. The reaction
furnished a variety of C3 cyanated quinoxalin-2(1H)-ones in
satisfactory yields without using toxic transition metals. The
nature of substituents on the benzene ring, electron-donating
or electron-withdrawing, had little effect on the yields. Oxidiz-
able functional groups such as a primary alcohol, a terminal
olefin, and terminal acetylene, and a furan ring were left
untouched.
Scheme 4. Gram-scale reaction and further transformation of the product
3aa.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (Project 21672104, 21502097) and the
Priority Academic Program Development of Jiangsu Higher
Education Institutions.
1a would undergo a single-electron transfer (SET) oxidation at
the anode to generate the radical cation intermediate A.[22a,f]
Subsequently, the intermediate A would be captured by the
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Manuscript received: March 20, 2021
Revised manuscript received: March 26, 2021
Accepted manuscript online: March 30, 2021
Eur. J. Org. Chem. 2021, 2193–2197
www.eurjoc.org
2197
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