Mechanism of electrochemical oxidation of 1-chloro-2,2,6,6- tetramethylpiperidine

Article abstract of DOI:10.1134/S1070363209050235

In contrast to 2,2,6,6-tetramethylpiperidine and other aliphatic amines, at the electrochemical oxidation of 1-chloro-2,2,6,6-tetramethylpiperidine a sufficiently stable cation-radical is formed. Its formation is confirmed by the data of cyclic voltammetry and electron paramagnetic resonance. Further transformation of the cation-radical leads to the formation of 2,2,6,6-tetramethylpiperidin-1-oxyl.

Full text of DOI:10.1134/S1070363209050235

ISSN 1070-3632, Russian Journal of General Chemistry, 2009, Vol. 79, No. 5, pp. 1001–1003. © Pleiades Publishing, Ltd., 2009.
Original Russian Text © E.Sh. Kagan, V.V. Yanilkin, V.I. Morozov, N.V. Nastapova, I.Yu. Zhukova, I.I. Kashparov, V.P. Kashparova, 2009, published in
Zhurnal Obshchei Khimii, 2009, Vol. 79, No. 5, pp. 829–831.
Mechanism of Electrochemical Oxidation
of 1-Chloro-2,2,6,6-tetramethylpiperidine
a
b
b
b
E. Sh. Kagan , V. V. Yanilkin , V. I. Morozov , N. V. Nastapova ,
a
a
a
I. Yu. Zhukova , I. I. Kashparov , and V. P. Kashparova
a
South-Russian State Technical University (Novocherkassk Polytechnical Institute),
ul. Prosveshcheniya 132, Novocherkassk, 346421 Russia
e-mail: kagan29@mail.ru
b
Arbuzov Institute of Organic and Physical Chemistry, Kazan Research Center,
Russian Academy of Sciences, ul. Arbuzova 8, Kazan, 420088 Tatarstan, Russia
e-mail: yan@iopc.knc.ru
Received December 4, 2009
Abstract―In contrast to 2,2,6,6-tetramethylpiperidine and other aliphatic amines, at the electrochemical
oxidation of 1-chloro-2,2,6,6-tetramethylpiperidine a sufficiently stable cation-radical is formed. Its formation
is confirmed by the data of cyclic voltammetry and electron paramagnetic resonance. Further transformation of
the cation-radical leads to the formation of 2,2,6,6-tetramethylpiperidin-1-oxyl.
DOI: 10.1134/S1070363209050235
Earlier [1] we showed that electrolysis of 2,2,6,6-
tetramethylpiperidine (I) in the presence of chloride
ions in a diaphragmless elecrolyzer in a two-phase
system methylene chloride–water afforded the respec-
tive nitroxyl radical, 2,2,6,6-tetramethylpiperidin-1-
oxyl (III) (Scheme 1). The same nitroxyl radical is
formed also at the electrolysis under similar conditions
of 1-chloro-2,2,6,6-tetramethylpiperidine (II, Scheme 1).
However, mechanism of this reaction remained
unclear. Scheme 1 shows an oxidation of 2,2,6,6-tetra-
methylpiperidine I into nitroxyl radical III and
oxoammonium salt IV via 1-chloro-2,2,6,6-tetrame-
thylpiperidine II.
with the diaphragm-type electrolyzer, in the organic
phase remains unreacted II, while water phase contains
oxoammonium salt IV. No other product was detected
in the water phase. For the isolation of the reaction
products the oxoammonium salt IV was reduced with
sodium nitrite or sodium hydroxide into the respective
nitroxyl radical III which was readily soluble in
organic solvents and could be easily recovered from
the water phase.
The result obtained probably means that in a
diaphragmless electrolyzer the nitroxyl radical III is
formed in the process of chloroamine II oxidation.
It is suggested [2–4] that in the first step of
oxidation of amines a single electron transfer occurs
and the respective cation-radical is formed, despite the
fact that in the majority of cases the cation-radicals
have not been identified owing to their short life-time,
Preparative synthesis of radical III can also be
performed by oxidation on anode in an electrolyzer
with a diaphragm-separated cells using compound I as
an initial substance (Scheme 1). At the electrolyzis
Scheme 1.
electrolysys
Na SO
electrolysys
Na SO
electrolysys
NaCl
+
N
2
4
N
2
4
N
N
H
Cl
O
O
III
IV
I
II
1001

1
002
KAGAN et al.
Scheme 2.
_e
H O
2
+
N
N
1/2 Cl2
N
_H+
N
+
Cl
Cl
OH
II
V
_e
_H+
_e
+
N
N
O
O
III
IV
and their formation has been presumed from the
character of the formed final products.
figure). The difference in the peaks potential ∆Е =
60 mV is typical of the reversible one-electron process
р
[
5] and evidences the formation of the primary cation-
In contrast to 2,2,6,6-tetramethylpiperidine and to
the majority of the other aliphatic amines, the
electrochemical oxidation of II (Scheme 2) in
acetonitrile afforded a rather stable cation-radical V
radical V. In the time scale of registration of CVA
curves (seconds) the cation-radicals enter chemical
reactions that is indicated by the decrease in the
relative height of the re-reduction peak at the
diminished potential sweep rate. One of the reaction
products is molecular chlorine whose shallow
reduction peak is registered at the potential +0.10 V.
(
the half-life time at 243 K is ~10 min), whose
existence was evidenced by the cyclic voltammetry
CVA) and ESR spectroscopy. Scheme 2 shows the
(
assumed mechanism of formation of nitroxyl radical
III and oxoammonium salt IV.
We also succeeded to register the cation-radicals V
and identify them by ESR spectroscopy by carrying
out electrooxidation of II on a Pt electrode in an
ampoule directly in the resonator of the ESR
spectrometer at low temperature (243 K). The
spectrum of cation-radical V consists of 12 peaks with
On the CVA curves of chloroamine II at the
temperature 22°С an oxidation peak is fixed at +1.10 V
(
relatively to Ag/0.01 M AgNO in MeCN) and its
3
conjugated peak of re-reduction at +1.04 V (see
the following characteristics: g = 2.0093, a = 2.11 mT,
N
3
5
3 7
a = 0.6 mT, a = 0.5 mT.
Cl
Cl
Proceeding from the data obtained, we describe the
process of formation of nitroxyl radical III and oxo-
ammonium ion IV at the oxidation of chloroamin II by
Scheme 2 that includes the steps of formation of
cation-radical V, elimination of chlorine molecule
followed by hydroxylation, elimination of protons and
electron transfer.
–5
–2
1
2
3
4
[
1×10 A cm
0
0.2
0.4
0.6
0.8
1.0
1.2 E, V
EXPERIMENTAL
2
,2,6,6-Tetramethylpiperidin-1-oxyl from 1-chlo-
ro-2,2,6,6-tetramethylpiperidine. The synthesis was
carried out in a diaphragm type electrolyzer of 150 ml
capacity. For the anode and cathode were used
platinum plates of the area 20 cm and 10 cm res-
pectively. To the electrolyzer anode cell was placed
7.1 g (0.05 mol) of sodium sulfate dissolved in 80 ml
Cyclic voltammogram curves of 1-chloro-2,2,6,6-tetra-
-3
methylpiperidine (I) (c 3×10 M) on a glass carbon
electrode in the CH CN/0.1 M Bu NBF medium at varied
potential sweep rates, V s : (1) 0.2 ; (2) 0.1; (3) 0.05; and
4) 0.02.
2
2
3
4
4
–
1
(
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 79 No. 5 2009

MECHANISM OF ELECTROCHEMICAL OXIDATION
1003
of a buffer solution (0.05 M Na HPO and 0.05 M
was collected and 1.74 g of 1-chloro-2,2,6,6-tetra-
methylpiperidine was recovered.
2
4
KH PO ; рН 6.86), and 3.5 g (0.02 mol) of 1-chloro-
2
4
2
,2,6,6-tetramethylpiperidine in 30 ml of methylene
chloride. To the catode cell was loaded 40 ml of 10 %
sodium sulfate solution.
REFERENCES
1
. Kashparova, V.P., Vlasova, E.V., Zhukova, I.Yu.,
Kagan, E.Sh., and Il’chibaeva, I.B., Elektrokhimiya,
The electrolysis was carried out at the current
–
2
power 1.5 А (current density 0.075 А cm ). The syn-
thesis was stopped after passing 4 F of electricity. The
organic layer was separated from the water phase. To
the water layer was added 10% sodium hydroxide
solution to рН 10–11 and the product was extracted
with methylene chloride (3×30 ml ). Organic extracts
were combined, the solvent was distilled off, and
residue was purified by sublimation in a vacuum (10–
2
007, vol. 43, no. 10, p. 1.
2
3
. Chow, Y.L., Danen, W.C., Nelsen, S.F., and Rosen-
blatt, D.N., Chem. Rev., 1978, vol. 78, no. 3, p. 243.
. Randall, S., Deinhammer, Ho, M., Anderegg, J.W., and
Porter, M.D., Langmuir, 1994, no. 10, p. 1306.
4. Yip, R.W., Vidoczy, T., Snyder, R.W., and Chow, Y.L.,
J. Phys. Chem., 1978, vol. 82, no. 10, p. 1194.
1
5 mm Hg). Finally 0.65 g of compound III was
5
. Esida, K., Elektrookislenie v organicheskoi khimii. Rol’
kation-radikalov kak itermediatov v sinteze (Elec-
trooxidation in Organic Chemistry. Cation-Radicals as
Intermediates in the Synthesis), Beletskaya, I.P., Ed.,
Moscow: Mir, 1987.
obtained, mp 35–36°С, yield 82% on the reacted
compound II. From organic layer the solvent was
removed in a rotory evaporator and the residue was
distilled in a vacuum, fraction 61–63°С (7 mm Hg)
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 79 No. 5 2009