Pyridinium bromide as a new mediator for electrochemical transformations involving CH-acids

Article abstract of DOI:10.1016/j.mencom.2019.07.010

Pyridinium bromide has been tested as a new mediator for electrochemical synthesis of functionalized cyclopropane from alkylidenemalononitriles and CH-acids. The process was performed in an undivided cell with the use of substoichiometric amount of PyHBr

Full text of DOI:10.1016/j.mencom.2019.07.010

Mendeleev
Communications
Mendeleev Commun., 2019, 29, 391–392
Pyridinium bromide as a new mediator for
electrochemical transformations involving CH-acids
Anatoly N. Vereshchagin,* Evgeniya O. Dorofeeva,
Michail N. Elinson, Victor A. Korolev and Mikhail P. Egorov
N. D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, 119991 Moscow,
Russian Federation. E-mail: vereshchagin@ioc.ac.ru
DOI: 10.1016/j.mencom.2019.07.010
R2
R1
R2
Pyridinium bromide has been tested as a new mediator for
electrochemical synthesis of functionalized cyclopropane
from alkylidenemalononitriles and CH-acids. The process
was performed in an undivided cell with the use of substoichio-
metric amount of PyHBr that serves both as a mediator and
a supporting electrolyte, and MeOH or MeCN as the solvents.
X
X
PyHBr (0.75 equiv.) / MeCN
CN
R1
X
+
NC
NC
graphite anode
Fe cathode
constant current electrolysis
undivided cell
CN
X
X = CN, 6 examples,
48–72%
X = COOMe, 6 examples,
38–67%
Electrosynthesis is a competitive method of modern organic
chemistry.1–3 The importance of electrochemical synthesis can
hardly be overestimated because of its great and, in some cases,
unique possibilities for performing various transformations of
organic compounds.4,5 In recent decades, indirect electrooxidation
of organic compounds involving mediators has been the subject
of intensive studies.6
Cheap and available halides are well-known and popular
inorganic mediators7 that provide processing in an undivided
cell. On the other hand, CH-acids are convenient objects for the
preparation of various compounds by indirect electrooxidation
in alcohols in the presence of sodium halides as mediators.8,9
Despite the advantages and significant achievements described,
the use of alkali metal halides as mediators in alcohol solutions for
carrying out organic electrosyntheses has several disadvantages.
The use of methanol or ethanol as solvents is not good for poorly
soluble substrates. Significant limitations can be caused by certain
side processes.10 Quaternary ammonium salts are widely used in
electrochemistry as supporting electrolytes. Recently, the electro-
initiated oxidative amination of benzoxazoles11 and sulfonamides12
using tetraalkylammonium halides and ammonium iodide as
redox catalysts, respectively, was reported.
Here, we present pyridinium bromide as a new mediator for
electrochemical transformations. We started our studies employing
benzylidenemalononitrile 1a and malononitrile 2a as the model
reactants for constant current electrolysis (100 mA cm–2) in an
undivided cell equipped with graphite rod as the anode and iron
plate as the cathode (Scheme 1, Table 1). Initially, we optimized
the catalytic efficacy of PyHBr in methanol (entries 1–10). The
current density of 50 mA cm–2, 0.75 equiv. of PyHBr, 20°C,
2.2 F mol–1 were found to be the optimum conditions for obtaining
product 3a (entry 5). The use of smaller amount of PyHBr
leads to drop in the yields (entries 1, 2), apparently, due to
incomplete regeneration of the mediator. Raising the current
density up to 100 mA cm–2 (entry 4) or temperature up to 30°C
(entry 8) also results in decrease in the yield, which may be caused
R2
R1
R2
Table 1 Electrocatalytic transformation of benzylidenemalononitrile 1a and
malononitrile 2a into tetracyanocyclopropane 3a with PyHBr as mediator.a
X
X
i
CN
CN
1a–h
+
R1
X
NC
NC
Current
density/
Eletricity Isolated
Equiv.
of PyHBr
X
Entry Solvent
T/°C
passed/
yield of
2a X = CN
2b X = COOMe
mA cm–2 F mol–1
3a (%)
3a–l
1
2
3
4
5
6
7
8
9
10
11
12
13
14
MeOH 0.30
MeOH 0.50
20
20
20
20
20
20
10
30
20
20
20
20
20
20
100
100
100
100
100
100
100
100
50
25
50
50
50
2.0
2.0
2.0
2.0
2.2
2.5
2.2
2.2
2.2
2.2
2.2
2.2
2.2
2.2
25b
42
60
60
68
61
44
32b
72
68
62
72
35b
25b
Yield
(%)
3
R1
R2
X
1
R1
Ph
R2
MeOH 0.75
MeOH 1.00
MeOH 0.75
MeOH 0.75
MeOH 0.75
MeOH 0.75
MeOH 0.75
MeOH 0.75
a
b
c
d
e
f
H
H
H
H
H
a
b
c
d
e
f
g
h
i
Ph
H
H
H
H
H
CN
CN
CN
CN
CN
CN
COOMe
COOMe
COOMe
COOMe
COOMe
COOMe
72
66
64
58
48
60
67
63
67
64
44
38
4-MeC6H4
4-ClC6H4
3-BrC6H4
4-NO2C6H4
4-MeC6H4
4-ClC6H4
3-BrC6H4
4-NO2C6H4
(CH2)5
(CH2)5
H
H
3-MeOC6H4
3-NO2C6H4
Ph
H
H
H
H
H
H
g
h
4-MeC6H4
3-MeOC6H4
3-BrC6H4
3-NO2C6H4
4-NO2C6H4
EtOH
MeCN
DMF
0.75
0.75
0.75
j
k
l
DMSO 0.75
50
Scheme 1 Reagents and conditions: i, 1 (10 mmol), 2 (10 mmol), PyHBr
a Benzylidenemalononitrile 1a (10 mmol), malononitrile 2a (10 mmol),
(7.5 mmol), MeCN (20 ml), undivided cell, 20°C, 2.2 F mol–1, current
solvent (20 ml), undivided cell. b NMR data.
density of 50 mA cm–2
.
© 2019 Mendeleev Communications. Published by ELSEVIER B.V.
on behalf of the N. D. Zelinsky Institute of Organic Chemistry of the
Russian Academy of Sciences.
– 391 –

Mendeleev Commun., 2019, 29, 391–392
by acceleration of side electrochemical oligomerization of the
acid A. Next, ylide B can be formed from intermediate A and
pyridine. Finally, addition of ylide B at alkylidenemalononitrile
gives rise to cyclopropane 3. The last stage was postulated previously
in the chemical synthesis of cyclopropanes from pyridinium
ylides and benzylidenemalononitriles.13
To summarize, we have employed pyridinium bromide as a
new mediator for electrosynthesis of functionalized cyclopropanes
from CH-acids and benzylidenemalononitriles. The procedures
for electrolysis and isolation of products are simple and can be
used both under laboratory conditions and in larger reactors.
reactants. The low yield and insufficient conversion of starting
compounds were observed when electrolysis was carried out at
10°C (entry 7). Ethanol is slightly less effective medium than
methanol (entry 11). Since PyHBr is soluble in aprotic solvents,
electrolysis was performed in MeCN leading to product 3a in
good yield (entry 12).
The reaction scope with regard to dimethyl malonate 2b was
also explored. Under the optimal conditions, tetracyanocyclo-
propanes 3a–f or dimethyl 2,2-dicyano-3-arylcyclopropane-
1,1-dicarboxylates 3g–l were obtained from alkylidenemalono-
nitriles 1a–h (both with electron-withdrawing and electron-
donating substituents in aromatic ring) and CH-acids 2a,b (see
Scheme 1). †
This study was supported by the Russian Foundation for
Basic Research (project no. 18-33-00301).
Possible mechanism for the current transformation is outlined
in Scheme 2. Bromine is formed at the anode, which can be
observed by some colouration. The cathodic deprotonation of
pyridinium cation leads to formation of free pyridine. The
evolution of hydrogen at the cathode is observed, especially
when electrolysis is conducted without stirring of the reaction
mixture. Deprotonation of CH-acid with pyridine in solution and
further bromination of CH-acid anion leads to intermediate CHBr
Online Supplementary Materials
Supplementary data associated with this article can be found
in the online version at doi: 10.1016/j.mencom.2019.07.010.
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Anode:
– 2e
2PyH+
+ 2e
Br2
H2
Py
2Py
+
Cathode:
X
X
X
Py
Py, Br2
Br
Py
2
– PyHBr
X
– PyHBr
X
Br–
A
R1 R2
1
Py
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– Py
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Scheme 2
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†
General procedure. An undivided cell was equipped with a graphite
anode and iron cathode (5 cm2 area each) and connected to a DC regulated
power supply. The cell was charged with the corresponding alkylidene-
malononitrile 1 (10 mmol), CH-acid 2 (10 mmol), PyHBr (7.5 mmol),
and MeCN (20 ml). The mixture was electrolyzed using constant current
conditions (50 mA cm–2) at room temperature under magnetic stirring
(TLC control, full consumption of olefin 1). The reaction solution was then
concentrated under reduced pressure, the residue was treated with water,
and the product was extracted with dichloromethane (3 × 20 ml), the
extract was dried over Na2SO4 and concentrated. The residue was purified
by crystallization from methanol to afford the desired cyclopropane 3.
For characteristics of products 3, see Online Supplementary Materials.
Received: 17th January 2019; Com. 19/5806
– 392 –