Electrochemical amination. Selective introduction of two amino groups into an aromatic ring

Article abstract of DOI:10.1134/S1070363217010042

Indirect cathodic amination of anisole via a Ti(IV)–NH₂OH system in aqueous solutions of sulfuric acid is studied. The major products of the radical cation substitution in these media are para- and ortho-anisidines and 4-methoxy-1,3-phenylenediamine. The most efficient electrochemical process takes place in 10–12 М H₂SO₄. Under these conditions, complete conversion of the source of amino radicals is observed, and the total current yields, which correspond to the yields per hydroxylamine, reach 60%.

Full text of DOI:10.1134/S1070363217010042

ISSN 1070-3632, Russian Journal of General Chemistry, 2017, Vol. 87, No. 1, pp. 16–21. © Pleiades Publishing, Ltd., 2017.
Original Russian Text © Yu.A. Lisitsyn, A.V. Sukhov, 2017, published in Zhurnal Obshchei Khimii, 2017, Vol. 87, No. 1, pp. 20–25.
Electrochemical Amination. Selective Introduction
of Two Amino Groups into an Aromatic Ring
Yu. A. Lisitsyn* and A. V. Sukhov
Butlerov Chemical Institute, Kazan (Volga) Federal University,
ul. Kremlevskaya 18, Kazan, Tatarstan, 420008 Russia
*e-mail: Yuri.Lisitsyn@kpfu.ru
Received June 16, 2016
Abstract—Indirect cathodic amination of anisole via a Ti(IV)–NH2OH system in aqueous solutions of sulfuric
acid is studied. The major products of the radical cation substitution in these media are para- and ortho-anisidines
and 4-methoxy-1,3-phenylenediamine. The most efficient electrochemical process takes place in 10–12 М
H2SO4. Under these conditions, complete conversion of the source of amino radicals is observed, and the total
current yields, which correspond to the yields per hydroxylamine, reach 60%.
Keywords: cathode, Ti(IV)/Ti(III) mediator system, hydroxylamine, anisole, radical cation aromatic substitution,
4-methoxy-1,3-phenylenediamine
DOI: 10.1134/S1070363217010042
The chemical amination of aromatic substrates with
hydroxylamine and transition metal compounds in
acidic aqueous and aqueous-organic media most
commonly form isomeric monoamino derivatives as
major products [1–9]. Electrochemical functionalization
in dilute sulfuric acid solutions gives the same results
[10–13]. However, in catholytes with H2SO4 con-
centrations higher than 7 M, a deeper substitution takes
place, specifically, diamino derivatives are formed in
significant amounts along with monosubstitution products
[12, 13].
substitution efficiency criterion we used the total
current yield of amino compounds. Unlike the
conditions used in [12, 13] for benzene amination, the
quantity of electricity passed through the catholyte
(482.4 C) was enough for not only partial, but also for
complete theoretical conversion of hydroxylamine,
based on the consumption of one electron per hydroxyl-
amine molecule. To prevent electrochemical release of
hydrogen in 1.5–10 М H2SO4 solutions we used a
mercury cathode, and, in more acidic media, when
mercury can be oxidized by Ti(IV), a platinum elec-
tron was used.
Until now consecutive introduction of two amino
groups to the aromatic ring has been studied with only
one substrate, namely benzene [12, 13]. The goal of
the present work was to find out how the acidity of the
sulfuric acid electrolyte affects the results of amination
of anisole, a compound containing a substituent that
makes the aromatic ring more reactive toward amino
radical cations.
The amination of anisole in 1.5 М H2SO4 gives not
only isomeric anisidines, but also 4-methoxy-1,3-
phenylenediamine 1. The current yields of anisidines
and diamine 1 at 20°С are 5.9 and 1.6%, respectively
(Fig. 1a). As the H2SO4 concentration is increased up
to 4.5 М, the total substitution efficiency decreases
(Fig. 1b). The decrease of the amination yield is
associated with fact that reaction (1) involves
exclusively the unprotonated form of hydroxylamine
[12, 13], and the concentration of this form decreases
upon the increase of the acidity of the electrolyte.
Increasing concentration of Ti(III) generated both on
the cathode and by methoxyaminocyclohexadienyl
radical cations with Ti(IV) increases the fraction of
The electrolysis of the Ti(IV)–NH2OH–C6H5OMe
system was performed in 1.5–15 М aqueous H2SO4
under conditions favoring the synthesis of isomeric
anisidines in aqueous-organic electrolytes containing
sulfuric acid [14, 15]. Anisole was taken in a large
excess with respect to hydroxylamine (0.046 and
0.005 mol, respectively), and, therefore, as the main
16

ELECTROCHEMICAL AMINATION. SELECTIVE INTRODUCTION
(a) (b)
17
1'
2
3'
2'
3
2
[H2SO4], M
[H2SO4], M
Fig. 1. (а) Effect of H2SO4 concentration on the (1, 1') current yield of methoxyphenylynydiamine 1 and (2, 2') total yield of
isomeric anisidines and (b) (3, 3') total current yield of all indirect anisole amination products on a (1–3) mercury cathode (11.0 cm2)
at 20°С and (1'–3') platinum cathode (8.2 cm2) at 40°С. (×) Data obtained in 4.5 and 10 М H2SO4 on a Hg cathode at 40°С and on a
Pt cathode at 20°С, respectively ([Ti(IV)] = 0.1 М, [NH2OH] = 0.2 М, i = –2 mA/cm2).
aminyl radicals consumed in reaction (2) that competes
with substitution.
aromatic ring toward amino radical cations and the
substitution regioselectivity are mainly associated with
the presence of the methoxy group.
Ti(III) + NH2OH → [Ti(IV)NH2]· + OH–,
(1)
H+
[Ti(IV)NH2]· + Ti(III)
2Ti(IV) + NH3.
(2)
In the catholytes containing H2SO4 concentra-tions
higher than 5 М, because of the increased solubility of
anisole, increased concentration of ion pairs, and
decreased rate of reaction (2) explained by the shift of
equilibrium (3) to the right, an increase of the total
yield of mono- and diamino derivatives is observed
(Fig. 1b). Therewith, the current yield of methoxy-
phenylenediamine increases to the greatest extent
(Fig. 1a). The only product whose yield in these media
further decreases is meta-anisidine (Fig. 2).
The slight increase in the current yield of diamine
1 and even stronger decrease in the total yield of
anisidines under these conditions (Fig. 1a) point to the
fact that a certain fractionof anisidinium ions are
activated as the acid concentration increases.
The isomer distribution of phenylenediamines
observed during the electrochemical amination of
benzene and aniline [12, 13], as well as the ion
molecular composition of aqueous solutions of H2SO4,
reported in [16], suggest that aniline and its
monosubstituted derivatives in sulfuric acid media can
exist as ArNH3+ cations, ArNH3+, HSO4¯ ion pairs, and
ArNH2, HSO4¯ associates formed by the reaction of
aminium and sulfate ions. Compound 1 is the only
anisole diamination product. Consequently, taking into
account the isomer com-position of anisidines (Fig. 2),
we can suppose that the increase of its current yield
(Fig. 1a) is associated with increasing concentrations
of р-МеОC6H4NH3+, HSO4¯ and о-МеОC6H4NH3+,
HSO4¯ ion pairs in the catholyte. The charge of the
ammonium ion in the ion pairs is compensated by the
charge of the counterion (the effect of the positive
charge on the aromatic ring is exclusively through the
field effect), and, therefore, the reactivity of the
[Ti(IV)NH2]· + H+ ⇄ Ti(IV) + NH3+·.
(3)
The total efficiency of the indirect amination of
anisole in 10 М H2SO4 on a mercury electrode is as
high as 59.9% (Fig. 1b). The replacement of the
mercury electrode by a platinum electrode does not
increase the yield of amino compounds under these
conditions, though the smaller area of the latter
electrode leads to a lower rate of reaction (2) because
of the lower intensity of the cathodic generation of
Ti(III). This result is likely to be associated with the
electro-chemical release of hydrogen (Fig. 3), which
becomes appreciable, when the potential of the
platinum electrode is below –0.18 to –0.20 V.
Raising the experimental temperature to 40°С
eliminates the problems associated with the use of a
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LISITSYN, SUKHOV
Current yield, %
[H2SO4], M
[H2SO4], M
Fig. 2. Dependence of the current yield of (1, 1') meta-, (2, 2') para-, and (3, 3') ortho-anisidines on H2SO4 concentration during the
indirect anisole amination on a (1–3) mercury cathode at 20°С and (1'–3') platinum cathode at 40°С. (×) Data obtained in 4.5 and
10 М H2SO4 on a Hg cathode at 40°С and on a Pt cathode at 20°С, respectively.
platinum cathode (Fig. 3) and makes it possible to
aminate anisole not only in 10 М H2SO4, but also in
less acidic solutions, in particular, in 8 М H2SO4 (Figs. 1
and 3). The nature of the cathode material has almost
no effect on the composition of the aromatic amination
products, because the role of the electrode in the
electrochemical process is to reduce Ti(IV) exclusively
[12, 13].
The amination efficiency remains almost invariable
in 10–12 М acid solutions and then sharply decreases
(Fig. 1b). The dependences of the current yield of
phenylenediamine 1 and the total current yield of
anisidines on H2SO4 concentration peak at 10 and 13.5 М
acid concentrations (Fig. 1a) respectively. The
decrease of the current yield of diamine 1 is accom-
panied by the sharpening of the slope of the depen-
dence of the total current yield of anisidines (Fig. 1a).
This points to a decrease in the rate of the reaction of
monoamines with amino radical cations in media with
a high protogenic activity, and, apparently, to a
decrease in the concentration of the р-МеОC6H4NH3+,
HSO4¯ and о-МеОC6H4NH3+, HSO4¯ ion pairs.
E, mV
4-Methoxy-1,2-phenylenediamine 2 along with
diamine 1 are produced in the catholytes containing 9–
14 M H2SO4. The highest current yields of about 0.2%
for compound 2 were observed in 11–13 М solutions.
Such a low yield of the product suggests that these
media contain very little of the associate of para-
anisidine molecule and hydrosulfate ion. The low
concentration of the associate is obviously explained
the electron-donor effect of the methoxy group, which
enhances the strength of bonds in the +NH3 group. As a
result, anisidines, unlike aniline [12, 13], are present in
sulfuric acid media almost exclusively as anisidium
cations, both free and ion-paired.
Q, C
Fig. 3. Change in the potential of the (1–3) platinum and
(4) mercury cathode at (1, 4) at 20°С and (2, 3) 40°С in
aqueous solutions of (2) 8, (1, 4) 10, and (3) 15 М H2SO4.
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ELECTROCHEMICAL AMINATION. SELECTIVE INTRODUCTION
19
m, g
Р, %
[H2SO4], M
[H2SO4], M
Fig. 5. Effect of H2SO4 concentration on the weight of
anisole remaining after the electrolysis of the Ti(IV)–
NH2OH–C6H5OMe system on a (1) mercury cathode at 20°С
and (2) platinum cathode at 40°С. (×) Data obtained in 4.5
and 10 М H2SO4 on a Hg cathode at 40°С and on a Pt
cathode at 20°С, respectively.
Fig. 4. Distribution of (1) para-, (2) ortho-, and (3) meta-
anisidines in H2SO4 solutions at 40°С.
The decrease in the total yield of isomeric anisi-
dines at H2SO4 concentrations higher than 13.5 М
(Fig. 1a) is associated with the protonation and sulfona-
tion of anisole, as well as by the formation of dipro-
tonated аnisidines (anisidine dications, similarly to
isomeric phenylenediamine dications [12, 13], do not
methoxyphenylenediamines in 10 М H2SO4 requires
about 0.19 g of anisole, which fairly agrees with the
data in Fig. 5.
After completion of amination in 8–13 М acid, the
catholytes had colors characteristic of Ti(III) solutions,
from light to black violet. The catholytes containing
13.5–15 М H2SO4 were dark cherry-colored and
changed color to violet on dilution with water. Adding
hydroxyl-amine sulfate to dilute catholyte samples led
to a disappearance of the violet color. This fact
suggests that hydroxylamine is completely consumed
on electrolysis in 8–15 М H2SO4 solutions and that the
current yields of anisole amination products cor-
respond to the yields for the amino radical source.
·+
react with the electrophilic radical cations NH3). The
protonation of the anisole methoxy group is evidenced
by an increase in the current yield of meta-anisidine in
10–13.5 М H2SO4 solutions (Fig. 2) and a sharp
decrease of the fraction of ortho-anisidine among the
resulting anisidines in solutions with H2SO4 concentra-
tions higher than 13 М (Fig. 4). The sulfonation of the
aromatic substrate is evidenced by the lower currents
yield of all the three isomeric anisidines at acid
concentrations higher than 13.5 М (Fig. 2) and the
dependence of the mass of anisole remaining after
electrolysis on H2SO4 concentration (Fig. 5). At 40°С
appreciable anisole sulfonation begins to occur at
H2SO4 concentrations higher than 11–12 М (Fig. 5).
Note that since the aromatic substrate was taken in a
large excess with respect to the source of amino
radicals, we did not give considerable attention to the
completeness of the substrate transfer from the
electrochemical cell to the extract; the cell was only
five times washed with water. Inspite of this the
efficiency of the transfer was quite high. For example,
formation the obtained amounts of anisidines and
At the end of this research, to assess the relative
contribution of para- and ortho-anisidines to phenyl-
enediamine 1 formation, we performed amination of
these substrates in 10 М H2SO4 at 40°С. The electrolysis
of the Ti(IV)–NH2OH–р-МеОC6H4NH2 gave diamines
1 and 2 with the yields of 64.3 and 0.46% per current
and hydroxylamine respectively. The amination of
ortho-anisidine resulted in formation of compound 1
exclusively (current yield 58.8%). The yields of diamino
compounds per anisidine in both cases were quanti-
tative. These results, as well as the data in Fig. 2, allow
us to state that para-anisidine contributes more to the
formation of phenylenediamine 1 that the ortho isomer.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 87 No. 1 2017

20
LISITSYN, SUKHOV
Thus, we presented here a fairly rare precedent of a
was then diluted with cold water to obtain the final
H2SO4 concentration of about 1.5 М, then rendered
weakly acidic by adding saturated aqueous NaOH under
cooling, and finally neutralized with NaHCO3. The
electrolysis products were extracted with chloroform.
highly selective radical cation process, where the conse-
cutive introduction of two groups into an aromatic ring
leads to an almost exclusive formation of one disub-
stitution product, namely 4-methoxy-1,3-phenylenediamine.
Analysis of the resulting amines was performed on
a Khromatek Kristall 5000.2 with flame ionization
detector, the temperature of a CP-Sil 8 CB capillary
column (60 m × 0.25 mm × 0.25 µm) and the helium
pressure were 160°C and 200 kPa, respectively. The
qualitative composition of the amination products was
also determined on a Chrom-5 chromatograph (flame
ionization detector, glass columns 3.5 m × 3 mm,
liquid phases XE-60 and SE-30).
EXPERIMENTAL
The amination of anisol and para- and ortho-
anisidines was performed in a three-electrode glass
electrochemical cell equipped with a reflux condenser,
a thermostatic jacket, and a ceramic diaphragm between
the cathode and anode compartments. The design of
this cell allows use of both a platinum (8.2 cm2) and a
mercury (11.0 cm2) cathodes with the constant surface
area of the latter under vigorous stirring of the catholyte.
The following chemicals were used: 15% solution
of titanium(IV) sulfate in aqueous 4 М H2SO4 (analy-
tical grade), H2SO4 (chemical grade), hydroxylamine
sulfate (Acros, 99%) recrystallized from water, NaOH
(analytical grade), NaHCO3 (chemical grade), distilled
anisole (Acros, 99%), chloroform (analytical grade),
ortho- and meta-anisidines (Lancaster, 98 and 99%)
distilled in a vacuum over KOH (Acros, 85%),
sublimed para-anisidine (Lancaster, 99%), as well as
4-methoxy-1,3-phylenediamine and 4-methoxy-1,2-
phylenediamine prepared by neutralization of their
sulfate (Aldrich, 98%) and hydrochloride (Lancaster,
98%), commercial mercury (Р0) purified by
simultaneous treatment with air oxygen and 10%
aqueous solution of distilled HNO3 (chemical grade),
and twice distilled water.
The catholyte (25 mL) contained sulfuric acid of
required concentration, 0.1 M Ti(IV), and 0.2 M
NH2OH. The highly dispersed emulsion of 5 mL of the
aromatic substrate in the electrolyte was obtained by
means of magnetic stirring. Before electrolysis the
oxygen dissolved in the emulsion was removed by
bubbling argon passed through a Drexel bottle filled
with an aqueous solution of H2SO4 and C6H5OCH3. As
the inert gas was bubbled through the catholytes with
14 and 15 М H2SO4, the anisole phase disappeared. In
the course of electrolysis argon was passed over the
emulsion or solution (in 13.5 М H2SO4 the two-phase
system transformed into one-phase).
The concentrations of anisidines in 10 М H2SO4
were 0.2 М, the cathode was a platinum electrode.
The amination of the aromatic substrates was
performed using Autolab PGSTAT 302N or IPC-Pro
MF potentiostats galvanostats at a current density of
–2 mA/cm2. The computer control of cathode current
and potential (measured against a silver chloride
electrode) was duplicated by means of GDM 8145 and
Keithley 2000/E multimeters, respectively. For repro-
ducible potentials, before electrolysis the platinum
electrode was washed with acetone and water and then
subjected to anodic‒cathodic polarization in aqueous
sulfuric acid of required concentration at a current
density of ±0.5 А. The anode was a platinized pla-
tinum, and the anolyte was aqueous Н2SO4 with the
same concentration as in the catholyte.
ACKNOWLEDGMENTS
The work was financially supported by the Russian
Scientific Fund (project no. 16-03-01061).
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