Electrochemical Cross-Coupling of C(sp2)?H with Aryldiazonium Salts via a Paired Electrolysis: an Alternative to Visible Light Photoredox-Based Approach

Article abstract of DOI:10.1002/adsc.201901011

Photoredox-based C?H bond functionalization constitutes one of the most powerful and atom-economical approaches to organic syntheses. During this type of reaction, single electron transfer takes place between the photocatalyst (PC) and redox- active substrates. Electrosynthesis also involves electron transfer between substrates and electrodes. In this paper, we focus upon electrochemical cross-coupling of C(sp²)?H with aryldiazonium salts and have developed an efficient electrochemical approach to the Minisci-type arylation reaction. The constant current paired electrosynthesis proceeds in a simple undivided cell without external supporting electrolyte, features a wide range of substrates and is easy to scale-up. These results demonstrate that photoredox-based cross-coupling of C(sp²)?H with aryldiazonium salts can also proceed successfully under paired electrolysis conditions, thereby contributing to understanding of the parallels between photosynthesis and electrosynthesis. (Figure presented.).

Full text of DOI:10.1002/adsc.201901011

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DOI: 10.1002/adsc.201901011
Electrochemical Cross-Coupling of C(sp2)À H with Aryldiazonium
Salts via a Paired Electrolysis: an Alternative to Visible Light
Photoredox-Based Approach
Yang-ye Jiang,a Gui-yuan Dou,a Luo-sha Zhang,a Kun Xu,a R. Daniel Little,b and
Cheng-chu Zenga,*
a
Beijing Key Laboratory of Environmental and Viral Oncology, College of Life Science & Bioengineering, Beijing University of
Technology, Beijing 100124, People’s Republic of China
E-mail: zengcc@bjut.edu.cn
Department of Chemistry & Biochemistry, University of California, Santa Barbara, Santa Barbara, CA 93106–9510, USA
b
Manuscript received: August 13, 2019; Revised manuscript received: September 24, 2019;
Version of record online: ■■■, ■■■■
Supporting information for this article is available on the WWW under https://doi.org/10.1002/adsc.201901011
Abstract: Photoredox-based CÀ H bond functionalization constitutes one of the most powerful and atom-
economical approaches to organic syntheses. During this type of reaction, single electron transfer takes place
between the photocatalyst (PC) and redox- active substrates. Electrosynthesis also involves electron transfer
between substrates and electrodes. In this paper, we focus upon electrochemical cross-coupling of C(sp2)À H
with aryldiazonium salts and have developed an efficient electrochemical approach to the Minisci-type
arylation reaction. The constant current paired electrosynthesis proceeds in a simple undivided cell without
external supporting electrolyte, features a wide range of substrates and is easy to scale-up. These results
demonstrate that photoredox-based cross-coupling of C(sp2)À H with aryldiazonium salts can also proceed
successfully under paired electrolysis conditions, thereby contributing to understanding of the parallels
between photosynthesis and electrosynthesis.
Keywords: paired electrolysis; photoredox-based reactions; cross-coupling; quinoxalin-2(1H)-ones; aryldiazo-
nium salts
Introduction
a single electron transfer (SET) with the substrate to
give its cation radical, PC+.. Simultaneously, the
The functionalization of CÀ H bonds has attracted substrate is reduced to the corresponding anion radical,
increasing attention in organic chemistry, medicinal SubÀ .. Thereafter, a series of conversions (e.g., bond
chemistry and material sciences due to their atom cleavage, protonation or coupling) converts SubÀ . to an
economy and cost efficiency characteristics. Moreover, intermediate, Int1. When the redox potential of PC+.
this approach is particularly important in the late-stage is higher than that of Int1, a second SET may take
functionalization of molecules of medicinal interest.[1]
Although metal-catalyzed methodology has been well
developed for the CÀ H bonds functionalization, stoi-
chiometric amounts of oxidant are frequently required
to regenerate the active catalyst.[2]
Photoredox-based visible light initiated reactions,
catalyzed either by a transition metal complex,[3] or by
organic dyes[4] have emerged as a powerful alternative
strategy. Mechanically, a photoexcited state, PC*, is
produced when the PC is irradiated using visible light
(see Figure 1a). In turn, PC* behaves as a strong
oxidant or reductant toward the substrate, Sub. Using
oxidative quenching as an example, the PC* undergoes
Figure 1. Comparison of photoredox-based process and paired
electrolysis.
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place to afford the product and regenerate the PC
(Figure 1a). Overall, this type of reaction only con-
Results and Discussion
sumes light energy (photons). Nevertheless, the photo- We chose quinoxalin-2(1H)-one (1a) and phenyldiazo-
catalyst-based sequence is photocatalyst-dependent and nium salt 2a as the model substrates to demonstrate
its success depends upon the compatibilities of the our concepts. Initially, the reaction was carried out
potentials of the substrate with PC (here, E(PC*/PC+.) under photochemical conditions using eosin Y as
compared with E (SubÀ ./Sub), and E(PC+./PC) vs E photocatalyst. As expected, the desired coupling
(Int2/Int1) ).
Organic electrochemistry has emerged as an effi-
product 3aa was isolated in a 65% yield (Table 1,
cient, green and environmentally friendly method, by
which to achieve the CÀ H bond functionalization.[5]
Given the inherent parallels that both electrosynthesis
and photoredox chemistry utilize electrons as reagents
to activate the substrate molecule, many organic trans-
formation have been translated from photoredox-
catalysed methodology to electrochemical processes
and vice versa.[6] This concept is illustrated in Fig-
ure 1b. The electrochemical reduction of a Sub
generates the corresponding SubÀ ., which converts to
an intermediate, Int1. If the electrochemical reaction is
carried out in an undivided cell, the Int1 may be
oxidized at the surface of the anode to form a second
intermediate, Int2, which then goes on to produce the
product. In this electrochemical process, only electric-
ity energy is consumed. One main advantage of the
electrochemistry strategy is that the potential of the
electrodes (anode and cathode) can be controlled by
the operator, rather than depending upon the potentials
accessible to the photocatalyst; the electrodes function
as the PC. In addition, workup, separation and
isolation is required to recycle PC, whereas the
electrode can simply be removed from solution.
Table 1. Optimization of the reaction conditions.[a]
entry deviation from standard conditions
yield
(%)[b]
1
2
3
4
eosin Y, DMSO, blue LEDs, N2
none
CH3CN, MeOH, DMF or DMA as solvent
CH3CN:DMF (5:1), MeOH:DMF (5:1) or
MeOH:DMSO (5:1) as solvent
no electricity
65[c]
84
17–44[d]
23–57[d]
5
6
7
8
0[e]
2 mA
6 mA
82[e]
76[e]
76
No external electrolyte
[a] Reaction conditions: 1a (0.2 mmol) and 2a (0.4 mmol) in
°
DMSO (3 mL), undivided cell, 25 C, current of 4 mA,
graphite felt and Ni plate cathode (working area: 1 cm2) and
constant current electrolysis (CCE) for 2.5 h, 1.88 F/mol.
[b] Yield determined by HPLC.
[c] Isolated yield.
[d] n-Bu4NClO4 (1.5 equiv.) was used as electrolyte.
[e] Et4NBF4 (1.5 equiv.) was used as electrolyte.
Arylation reactions are widely used in organic
synthesis to construct arylated compounds.[7] Photo-
catalyzed arylation of C(sp2)À H bonds has emerged as
a promising and useful strategy. It has been reported
that, by employing Ru,[8] and Ir[9]-based transition entry 1). With this result in mind, we chose to explore
metal photocatalysts, or an organic dye such as eosin the transformation electrochemically. Notably, without
Y,[10] under visible light irradiation, aryl diazonium current, the desired product was not detected (Table 1,
salts undergo a series of transformations with (hetero) entry 5). It was observed that the best results could be
arenes or alkenes to give arylated products (Figure 2a). obtained by conducting a constant current electrolysis
Based on these understanding, we envisioned that the at 4 mA/cm2 in an undivided cell equipped with a
photocatalyzed arylation of C(sp2)À H bonds might also graphite felt anode and a nickel plate cathode without
proceed electrochemically. This communication de- supporting electrolyte. Under these conditions, the
scribes our efforts and demonstrates for the first time desired product 3aa was obtained in an 84% yield
that electrochemical paired electrolysis is an alternative (Table 1, entry 2). Although the 2.0 equiv. phenyl-
to the visible light photoredox-based oxidation quench- diazonium salt was used, the biphenyl was not
ing approach (Figure 2b).
detected. Use of other solvents such as CH3CN,
MeOH, DMF and DMA provided 17%–44% yields of
3aa (Table 1, entry 3). The use of mixed solvent
systems did not improve the reaction efficiency
(Table 1, entry 4). The electrochemical arylation
proved insensitive to the choice of electrolyte, although
the use of Et4NBF4 provided a slightly improved yield
(72% vs 64%–71%; see SI Table S2 for details). It was
observed that the anode and cathode materials play an
important role (see SI Table S3 for details). In addition,
Figure 2. Arylation of C(sp2)À H bonds.
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an almost identical yield (82%) of 3aa was obtained electrochemical conditions. Next, quinoxalinones with
when the current was reduced from 4 to 2 mA/cm2 substituents on the aryl ring were investigated under
(Table 1, entry 6), but the reaction time increased to the standard condition. When the substituents on the
5 h. Moreover, increasing the current to 6 mA/cm2 aryl ring corresponded to electron-donating group, 6,7-
resulted in lower yield of 3aa (Table 1, entry 7). To di-CH3, or halogen, 6-Cl, 6,7-di-Cl and 6,7-di-F, the
our delight, the electrochemical reaction also worked corresponding products 3oa–3ra were obtained in
well without Et4NBF4 as the external supporting moderate to good yields (68%–83%). Notably, when
electrolyte and a comparable yield of 3aa was obtained pyrazin-2-ol was used as the substrate, product 3sa, a
(Table 1, entry 8), which highlights the advantages of natural product and antibacterial agent of TN82, was
this chemistry as a “green and sustainable” process.
obtained in a 76% yield. When benzo[g]quinoxalin-2
Having the optimal reaction conditions in hand, we (1H)-one was electrolyzed under the standard con-
turned to investigate the generality of the protocol. As ditions, the corresponding product 3ta was isolated in
shown in Table 2, initially, when N-substituted quinox- a 49% yield. Besides, electron-deficient heterocycle
quinoxaline,1u, also proceeded well to give the desired
3ua in a 56% yield.
Table 2. Substrate scope of heterocycles.[a,b]
To further explore the practicality of the electro-
chemical arylation reaction, reactions of quinoxalin-2
(1H)-one 1a with a variety of aryldiazonium salts 2
were investigated and the results are presented in
Table 3. The electrochemical reactions exhibited an
Table 3. Substrate Scope of Aryldiazonium Couplings.[a,b]
[a] Reaction conditions: 1 (0.2 mmol) and 2a (0.4 mmol) in
°
DMSO (3 mL), undivided cell, 25 C, current of 4 mA,
graphite felt and Ni plate cathode (working area: 1 cm2) and
electrolysis for 2–4 h, 1.5–3.0 F/mol.
[b] Isolated yield.
[a] Reaction conditions: 1a (0.2 mmol) and 2 (0.4 mmol) in
alinones were allowed to react with 2a under the
°
DMSO (3 mL), undivided cell, 25 C, current of 4 mA,
graphite felt and Ni plate cathode (working area: 1 cm2) and
electrolysis for 2–4 h, 1.5–3.0 F/mol.
[b] Isolated yield.
standard conditions, the corresponding arylated prod-
ucts were afforded in moderate to good yields. For
example, N-methyl and N-allyl quinoxalinones gave
the corresponding products 3ba and 3ca in 61% and
45% yields, respectively. Notably, in the case of 1c,
the arylation reaction occurred selectively on the excellent tolerance for both electron-donating and
heterocyclic ring, instead of the allylic subunit. N- electron-withdrawing groups on the aryl ring. For
benzyl substituted quinoxalinones, bearing an electron- instance, with 4-CH3 and 4-OCH3 on the aryl diazo-
donating group (R=4-CH3, 4-OCH3, 4-t-Bu, 3,5-di- nium salts, the desired products 3ab and 3ac were
CH3) or a halogen atom (R=F, Cl, Br) also worked obtained in 64% and 67% yields, respectively. In
well and gave the corresponding products 3da–3la in addition, halogen atom (4-F, 4-Cl and 4-Br) substituted
49%–70% yields. For the N-acetyl substituted quinox- aryl diazoniums can also work well and the corre-
alinone 1m (R2 =CH2CO2Et), the arylated adduct 3ma sponding products 3ad–3af were obtained in moderate
was obtained in a 54% yield. In the case of N-amidyl yields (54%–57%). However, when the strong elec-
substituted quinoxalinone 1n, the adduct 3na, whose tron-withdrawing nitro group was appended to C4 of
framework is common to antitumor and antimicrobial the aryl ring, the product 3ag was not detected and the
agents, was obtained in a 39% yield under the standard starting material 1a was recovered. In contrast, when
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electron-withdrawing groups, 4-CF3, 4-CN and 4-
SO2CH3 were substituted on the aryl ring, the desired
products 3ah–3aj were obtained in 32%–62% yields.
In addition, when the substituents were 2-CH3 and 3-
CH3, the aryl diazonium salts also worked satisfac-
torily and gave the corresponding products 3ak and
3al in 46% and 20% yields. What’s more, the product
3am was obtained in a 41% yield when the α-
naphthalene diazonium salt was used as the source of
aryl radical. Notably, the products 3an and 3ao were
not detected when the 2-Br or 2-I-substituted phenyl-
diazonium salts were employed, due to the possible
reduction reaction of aryl bromides or aryl iodides in
the electrochemical condition.[11,12] In addition, the
Scheme 2. Control experiments.
heterocyclic aromatic diazonium salt 2p could also nitrogen atmosphere (run 4). These results clearly
give the corresponding product 3ap in 54% yield. show that cathodic reduction leads to the formation of
However, when the heterocyclic aryl diazonium salt, the aryl radical. It is noteworthy that 3aa could be
2q, was used as the aryl radical precursor, the desired isolated in a 36% yield in a divided cell (run 5) and
product 3aq was not detected and the starting material improved to a 54% yield in an undivided cell (run 6)
was recovered.
when only catalytic amount of charge was applied.
The practicality of the protocol was demonstrated This information indicates that a radical chain reaction
by using the aqueous solution and a scale-up reaction pathway may also be involved, in addition to the
(see SI Table S4 for details). As shown in Scheme 1, anodic oxidation.
Cyclic voltammograms (CV) of diazonium salt 2h
in the absence and presence of 1a were also recorded
in an effort to provide additional mechanistic insights.
As shown in Figure 3, diazonium salt 2h exhibited an
irreversible reduction wave at À 0.85 V vs Ag/AgNO3
(0.1 M in CH3CN), corresponding to the reduction of
2h to form the phenyl diazo radical, 5. In the presence
of 1a, the reduction current of 2h decreased from
Scheme 1. Gram-Scale Experiment.
33 μA to 19 μA. The result discloses that, in addition
to the heterogenerous electron transfer of 2h with the
when 20 mmol of 1s (1.92 g) was treated with cathode, an intermediate stemmed from 1a may under-
40 mmol of aryl diazonium salt 2a by using a
circulation pump in conjunction with an undivided cell
that was equipped with graphite as anode and cathode,
the product 3sa was obtained in 53% yield (1.82 g
(10.6 mmol), see the SI Figures S5–S7 for the details),
the gram scale reaction time is similar to the small
scale, suggesting that the electrochemical protocol
might be useful for small scale industrial settings.
To examine the reaction mechanism, a series of
control experiments were first performed. As shown in
Scheme 2, when the mixture of 1a and 2a was
electrolyzed in the presence of 2.0 equiv. of TEMPO
under the standard reaction conditions, only a trace of
the desired product 3aa along with a radical trapping
product 4 was detected by GC-MS (run 1). The results
implicate that a radical process involves a phenyl
radical. In addition, it was observed that 3aa was not
detected when the electrolysis was carried out in the
anodic compartment of a divided cell under the
standard reaction conditions (run 2), whereas, 53%
yield of 3aa was obtained in the cathodic compartment
(run 3). Moreover, nearly the same yield of 3aa was
obtained when the process was conducted under a
Figure 3. Cyclic voltammograms of 2h and related compound
in 0.1 M n-Bu4NBF4/DMSO using platinum disk as the working
electrode, Pt wire, and Ag/AgNO3 (0.1 M in CH3CN) as the
counter and reference electrode, respectively, at a scan rate of
0.1 VsÀ 1: Background (black line), 5 mM of 1a (red line),
5 mM of 2h (blue line) and 5 mM of 2h in the presence of
15 mM of 1a (green line).
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Experimental Section
General Procedure for the Synthesis of 3-aryl-
substituted Quinoxalinones
An undivided cell was equipped with a graphite felt anode
(1.0×4 cm2) and a Ni plate cathode (1.0×4 cm2) and connected
to a direct (DC) regulated power supply. To the cell was added
quinoxalinone (1, 0.2 mmol), aryl diazonium salt (2, 0.4 mmol)
and 3.0 mL of DMSO. The mixture waÀs2 electrolyzed using
°
constant current conditions (~4 mAcm ) at 25 C under
magnetic stirring. When TLC analysis indicated that the
electrolysis was complete (witnessed by the disappearance of
the quinoxalinone, 1.5–3.0 F/mol), the reaction mixture was
extracted with DCM for 4 times. And the organic solvent was
washed by water for 5–8 times, dried over Na2SO4, and
concentrated in vacuo. The residue was purified by column
chromatography on silica gel to afford the desired pure product.
Scheme 3. Proposed mechanism for the electrochemical aryla-
tion of 1a with aryldiazonium salts.
go homogeneous electron transfer with 2h in solution,
thereby decrease the concentration of 2h.
Acknowledgements
Based on the control experiments, CV analysis and
literature reports,[13] two pathways for the electro-
chemical arylation of C(sp2)À H with aryldiazonium
salts are suggested and illustrated in Scheme 3. Thus,
the aryl diazonium salts are initially reduced at the
surface of the cathode to generate corresponding
phenyl diazo radicals 5. Following loss of molecular
N2, the resulting aryl radical 6 adds to quinoxalin-2
(1H)-one 1a generating radical intermediate 7. Sub-
sequently, intermediate 7 is oxidized at the anode to
afford the corresponding product 3 (pathway 1). In
addition to the paired electrolysis pathway, intermedi-
ate 7 also undergoes homogeneous electron transfer in
solution with the aryldiazonium salts followed by loss
of a proton to give the product 3, along with the
generation of 5 (pathway 2). Since the radical chain
reaction also induce the formation of phenyl diazo
radicals 5, only catalytic amount of charge is required.
This work was supported by grants from the National Key
Technology R&D Program of China (2017YFB0307502) and
the National Natural Science Foundation of China (No.
21871019).
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FULL PAPER
Electrochemical Cross-Coupling of C(sp2)À H with Aryldiazo-
nium Salts via a Paired Electrolysis: an Alternative to Visible
Light Photoredox-Based Approach
Adv. Synth. Catal. 2019, 361, 1–7
Y.-y. Jiang, G.-y. Dou, L.-s. Zhang, K. Xu, R. D. Little, C.-c.
Zeng*