Two-Phase Electrochemical Generation of Aryldiazonium Salts: Application in Electrogenerated Copper-Catalyzed Sandmeyer Reactions

Article abstract of DOI:10.1021/acs.orglett.0c02013

The electrochemical generation of aryldiazonium salts from nitroarenes in a two-phase system (ethyl acetate/water) was reported for the first time. Some compounds including azo, azosulfone, and arylazides were prepared in good yields with good purity. Cathodically generated aryldiazoniums and anodically produced copper(Ι) ions were used to perform Sandmeyer reactions. To improve the method, an H-type self-driving cell equipped with a Zn rod as an anode was introduced and used for two-phase aryldiazonium production.

Full text of DOI:10.1021/acs.orglett.0c02013

pubs.acs.org/OrgLett
Letter
Two-Phase Electrochemical Generation of Aryldiazonium Salts:
Application in Electrogenerated Copper-Catalyzed Sandmeyer
Reactions
Hamed Goljani, Zahra Tavakkoli, Ali Sadatnabi, and Davood Nematollahi*
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ABSTRACT: The electrochemical generation of aryldiazonium salts from nitroarenes in a two-
phase system (ethyl acetate/water) was reported for the first time. Some compounds including azo,
azosulfone, and arylazides were prepared in good yields with good purity. Cathodically generated
aryldiazoniums and anodically produced copper(Ι) ions were used to perform Sandmeyer
reactions. To improve the method, an H-type self-driving cell equipped with a Zn rod as an anode
was introduced and used for two-phase aryldiazonium production.
lectrochemical synthesis is a well-established method for
the synthesis of organic compounds.1 The most important
advantages of electrochemical methods over conventional
synthesis methods are the ability to carry out the reaction
under mild conditions, the lack of a need for additional
chemical reagents such as oxidizing or reducing agents, the
easy scalability, and the environmental friendliness.1
been published on the use of electrochemistry in the
Sandmeyer reaction.10 In this paper, the authors report a
general electrochemical strategy for the Sandmeyer reaction
using aryldiazonium tetra-fluoroborate or anilines as the
starting material in the presence of NBS, CBrCl3, CH2I2,
CCl4, LiCl, and NaBr as halogen sources.
However, it is still important to develop a more efficient
strategy for the Sandmeyer reaction without the use of
aromatic amines. Our proposed method to perform the
Sandmeyer reaction provides a facile electrochemical approach
for the halogenation of nitroarenes in a one-pot two-phase
system without any organic or inorganic reducing agent. In
addition, in this method, the anodic oxidation of copper was
employed for the preparation of copper(Ι).
This strategy is also successfully used for the synthesis of
diarylsulfide and isothiocyanate derivatives. Diarylsulfides
possess very important biological activities.11,12 There are
many efficient methods for the synthesis of diarylsulfides using
transition metals.13−15 To extend the usability of the method,
the cathodically produced aryldiazoniums and anodically
generated copper(Ι) ions were successfully used for the
synthesis of diarylsulfides.
E
Aryldiazonium salts have an important role in chemistry
because of their application in a wide range of chemical
reactions.2 The synthesis of aryldiazoniums from anilines is
very valuable, but its disadvantage is the use of aromatic
amines, which are toxic.3 Because of this drawback, we focused
on the development of an efficient procedure for the synthesis
of aryldiazoniums from nitroarenes in a two-phase electro-
chemical cell (ethyl acetate/water). This method provides a
novel strategy without using aromatic amines. As the first part
of the study, the electrogenerated aryldiazoniums were
subjected to react with naphthalene and phenol to synthesize
arylazo compounds. In the next step, we synthesized arylazo
sulfones by the reaction of the produced aryldiazoniums with
benzensulfinic acid; then, we used this reagent for the synthesis
of arylazides. To evaluate the usability of the method, two
examples of arylazides have been synthesized in this work. In
general, electrochemically generated aryldiazoniums have been
successfully used for the synthesis of arylazo, arylazo sulfone,
and arylazide compounds. Aryldiazoniums are also an
important source of aryl radicals.4 The aryl radicals were
then converted to halobenzenes in the presence of copper(I)
ions.5 In this regard, a number of valuable methods have been
reported.6−9 In addition, a comprehensive paper has recently
Received: June 17, 2020
https://dx.doi.org/10.1021/acs.orglett.0c02013
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Cyclic voltammograms (CVs) of p-nitrophenol (PNP) and
p-dinitrobenzene (DNB) in a two-phase ethyl acetate−water
system are shown in Figure 1. It should be noted that in these
Figure 1. (Ι) CVs of 5 mM PNP in two-phase ethyl acetate−water.
(a,c) Water contains HClO4 (0.25 M). (b) Water contains H2SO4 1.0
M. (ΙΙ) CV of 5 mM DNB in a two-phase ethyl acetate−water
(HClO4 0.25 M) system at the glassy carbon (GC) electrode. Scan
rate: 100 mV/s. In all voltammograms, the working electrode is
located in the ethyl acetate phase.
experiments, the working electrode is located in the ethyl
acetate phase. The CV of PNP (Figure 1I,a) was recorded in
such a way that (1) the potential was initially scanned to
negative values and then to positive values, and (2) the water
phase contained perchloric acid. The voltammogram shows a
well-defined irreversible cathodic peak (C0) that corresponds
to the reduction of the nitro group with two reversible couples
(A1/C1 and A2/C2) that correspond to the redox couples of 4-
(hydroxyamino)phenol/4-nitrosophenol and 4-aminophenol/
4-iminocyclohexa-2,5-dienone, respectively.16 The CV of PNP
under the same conditions, when the water phase contains
sulfuric acid, is shown in Figure 1I,b. Under these conditions,
the voltammogram does not show peak C0, which is evidence
that the reduction of the nitro group requires protons, and
sulfuric acid does not provide them due to the impossibility of
entering the ethyl acetate phase.
The CV of PNP under the same conditions of Figure 1I,a,
when the potential was initially scanned to positive values, is
shown in Figure 1I,c. Under these conditions, CV does not
show the redox couples A1/C1 and A2/C2, which confirms that
the reduction of the nitro group is necessary to create A1/C1
and A2/C2 peaks. Figure 1II, shows the CV of DNB under the
same conditions of Figure 1I,a. The most important change
observed in the CV of DNB is the presence of two cathode
peaks, C0 and C1, which are attributed to the reduction of the
two nitro groups of DNB.16
Figure 2. Top: Electrochemical cell for aryldiazonium production.
Bottom: H-type galvanic cell and reactions.
Simultaneously, the LSVs of the produced diazonium17
recorded in the water phase indicate an increase in the
concentration of diazonium with the progress of coulometry
(see the Supporting Information, Figure S1).
It should be noted that in the absence of a two-phase system,
the diazonium salt could be reduced at the cathode to generate
the corresponding radical.
The first reaction that we examined involved the reaction of
aryldiazonium with 2-naphthol to synthesize (1b) (Pigment
Red 1). For this purpose, after the consumption of 300 C, the
aryldiazonium was added to an alkaline solution of 2-naphthol.
Various parameters have been evaluated to optimize the
conditions for the synthesis of 1b (Table S1). It was found that
the use of a carbon cathode, ethyl acetate/water (1/0.7 v/v),
perchloric acid (0.25 M), and sulfuric acid (1.2 M) provides
the best conditions for the synthesis of 1b (entry 1).
The versatility of the method is shown through the reaction
of some compounds with electrochemically generated
aryldiazoniums (Table 1).
To carry out the in situ production of aryldiazoniums, in an
ethyl acetate/water (HClO4 0.25 M, NaNO2 1.5 equiv, and
H2SO4 1.2 M) system, we used carbon and stainless-steel
electrodes as a cathode and anode, respectively, located in the
ethyl acetate phase in a divided cell at −0.45 V (Figure 2).
Controlled potential coulometry was performed in the ethyl
acetate phase containing DNB at −0.45 V. The electrolysis
progress was monitored by linear sweep voltammetry (LSV). It
shows a continuous decrease in cathodic peaks C0 and C1 over
the charge passed. At −0.45 V, one of the nitro groups is
converted to an amine group. The 4-nitroaniline that forms in
the next step by entering the water, in the presence of sodium
nitrite, is converted to the corresponding aryldiazonium.
To develop the method, an H-type galvanic cell was used for
two-phase diazonium production. The anodic compartment
consists of a zinc rod in aqueous solution (1.2 M H2SO4), and
the cathodic compartment consists of a carbon rod in the ethyl
acetate solution. Other conditions are similar to those used in
Table S1, entry 1. In this strategy, the electrons needed to
reduce the nitroarenes are produced by the oxidation of zinc.
The H-type cell and the reaction mechanism are shown in
Figure 2.
The synthesis of 1b under optimized conditions, which is
accompanied by the color change of the solution, is also shown
in Figure 2. The color change is related to the conversion of
dinitrobenzene (yellow) to Pigment Red 1 (red).
B
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Table 1. Scope of the Azo, Azosulfone, and Aryl Azide
Synthesis
Scheme 1. Proposed Mechanism for the Electrochemical
Sandmeyer Reaction
a
To further explore the scope of the reaction, various
nucleophiles were employed to react with aryldiazoniums
under the optimized conditions (Table 2). This method is easy
to carry out and is performed in an undivided cell under
constant current conditions using stainless-steel and copper
electrodes.
a
Electrolysis was performed in a divided cell equipped with four
carbon rods as the cathode and a stainless-steel anode in ethyl acetate
(40 mL) containing arylnitrobenzene (0.5 mM). The water phase (25
mL) contained 1.2 M H2SO4 and 0.25 M HClO4.
Under the optimized conditions, we explored the scope of
the reaction using a range of arylnitro compounds shown in
Table 1. The simplicity of implementation and the lack of a
need for a current/voltage source are the advantages of this
method, but increasing the reaction time along with the need
for a sacrificial zinc electrode are its disadvantages.
a
Table 2. Scope of the Synthesis of 2b−k
The results confirm that we were able to develop an efficient
method for the synthesis of azo, azo sulfone, and arylazide
compounds from nitroarenes in 52−81% overall yields.
In this work, the aryldiazonium was also used in the
Sandmeyer reaction. In this regard, a novel electrochemical
strategy was established by using a copper rod as anode. To
achieve this goal, two methods were developed. In the first
method, the produced diazonium was added to an aqueous
solution containing electrogenerated copper(Ι) ions, HCl,
HBr, or KI (0.1 M) (Scheme 1).
In this method, the halogenation of aryldiazonium was
successfully carried out in aqueous solution under constant
current conditions. This method was also used for isothiocya-
nation by the reaction of aryldiazonium with sodium
thiocyanate.
A systematic study of the reaction was carried out to obtain
the optimal conditions for the synthesis of p-chloronitroben-
zene (2c) (Table S2).It was found that the use of copper and
stainless steel as the anode and cathode, respectively, in 0.1 M
sulfuric acid at current density of 25 mA/cm2 provides the best
conditions for the synthesis of 2c (Table S2, entry 1). These
results confirm the essential role of copper(I) ions as a catalyst.
The yield also depends on the amount of charge passed. The
effect of the electricity passed on the yield of 2c was
investigated under the optimal values (Table S2, entry 1). It
was found that when 0.26 F/mol of electricity was passed, 2c
was obtained in maximum yield.
a
Generation of aryldiazonium salts was performed using data given in
Table S1 (entry 1) and Figure S2. The Sandmeyer reaction was
carried out in an undivided cell containing 0.1 M of H2SO4 and 0.1 M
of HCl equipped with a copper rod as the anode and a stainless-steel
plate as the cathode at 25 mA/cm2. Charge passed 0.26 F·mol−1.
In another method, the halogenation of aryldiazonium in a
one-pot system was successfully carried out by using a copper
rod as an anode in the water phase. In this strategy, in the ethyl
acetate phase, nitroarene is reduced to the corresponding
amine; then, the produced amine is transformed to the water
phase. In water that contains NaNO2 and HCl or HBr, the
protonated amine is converted to the corresponding
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Authors
aryldiazonium. In the next step, the reaction of aryldiazonium
with anodically generated copper(Ι) ions leads to the
formation of the corresponding aryl radical; eventually, in
the presence of HCl or HBr, the aryl radical is converted to the
desired product. The electrochemical system and the reaction
mechanism are shown in the Figure 3, and the product yields
are given in Table 3.
Hamed Goljani − Faculty of Chemistry, Bu-Ali-Sina University,
Hamedan 65174, Iran
Zahra Tavakkoli − Faculty of Chemistry, Bu-Ali-Sina University,
Hamedan 65174, Iran
Ali Sadatnabi − Faculty of Chemistry, Bu-Ali-Sina University,
Hamedan 65174, Iran
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c02013
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We acknowledge Iran National Science Foundation (INSF) for
financial support of this work. We also acknowledge the Bu-Ali
Sina University Research Council.
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ASSOCIATED CONTENT
* Supporting Information
■
sı
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c02013.
Explanations about the optimizations and experimental
and spectral data for all compounds (PDF)
AUTHOR INFORMATION
Corresponding Author
■
Davood Nematollahi − Faculty of Chemistry, Bu-Ali-Sina
University, Hamedan 65174, Iran; orcid.org/0000-0001-
9638-224X; Email: nemat@basu.ac.ir
D
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(18) The synthesis of diazonium salts was performed in a divided
cell containing arylnitrobenzene (0.5 mmol) in 40 mL of ethyl
acetate, equipped with four carbon rods as a cathode and one zinc rod
as an anode. The synthesis of arylhalide was performed at 10 mA/cm2
in an undivided cell equipped with a copper rod as the anode and a
stainless-steel rod as the cathode in water (25 mL, containing 1.2 M
HCl or HBr and 0.25 M HClO4) (Figure 3).
E
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