Electrochemical oxidation of 1-chloro(bromo)-2,2,6,6-tetramethylpiperidines

Article abstract of DOI:10.1134/S1070363211100197

Electrochemical oxidation of 1-haloamines of the 2,2,6,6- tetramethylpiperidine series results in the formation of relatively stable radical-cations, detected by the methods of cyclic voltammetry and ESR spectroscopy. The final products of electrochemical oxidation of these haloamines are stable nitroxyl radicals. Pleiades Publishing, Ltd., 2011.

Full text of DOI:10.1134/S1070363211100197

ISSN 1070-3632, Russian Journal of General Chemistry, 2011, Vol. 81, No. 10, pp. 2151–2156. © Pleiades Publishing, Ltd., 2011.
Original Russian Text © E.S. Kagan, V.V. Yanilkin, N.V. Nastapova, V.I. Morozov, I.Yu. Zhukova, I.I. Kashparov, V.P. Kashparova, 2011, published in
Zhurnal Obshchei Khimii, 2011, Vol. 81, No. 10, pp. 1699–1704.
Electrochemical Oxidation
of 1-Chloro(bromo)-2,2,6,6-tetramethylpiperidines
a
b
b
b
E. S. Kagan , V. V. Yanilkin , N. V. Nastapova , V. I. Morozov ,
a
a
a
I. Yu. Zhukova , I. I. Kashparov , and V. P. Kashparova
a
South-Russian State Technical University, ul. Prosveshcheniya 132, Novocherkassk, 346421 Russia
e-mail: kagan29@mail.ru
b
Arbuzov Institute of Organic and Physical Chemistry, Kazan Scientific Center,
Russian Academy of Sciences, Kazan, Tatarstan, Russia
e-mail: yanilkin@iopc.ru
Received September 27, 2010
Abstract—Electrochemical oxidation of 1-haloamines of the 2,2,6,6-tetramethylpiperidine series results in the
formation of relatively stable radical-cations, detected by the methods of cyclic voltammetry and ESR
spectroscopy. The final products of electrochemical oxidation of these haloamines are stable nitroxyl radicals.
DOI: 10.1134/S1070363211100197
Earlier [1] we have shown that the electrolysis of
,2,6,6-tetramethylpiperidine (I) in the presence of
radical III can be conducted also by oxidation on the
anode in a diaphragm electrolyzer, using amine I as a
substrate, and subsequent chemical reduction of the
formed oxoammonium ion IV (Scheme 1) with sodium
nitrite. On this basis, the method of synthesis of
2,2,6,6-tetramethylpiperidine-1-oxyl by the electrolysis
of N-chloro-2,2,6,6-tetramethylpiperidines or 2,2,6,6-
tetramethyl-piperidine hydrochloride in the two-phase
system water–methylene chloride was elaborated.
2
chloride ions in the diaphragm-free electrolyzer in the
two-phase system methylene chloride–water results in
the corresponding nitroxyl radical, 2,2,6,6-tetra-
methylpiperidine-1-oxyl (III) (Scheme 1). The nitroxyl
radical is obtained in a good yield also by the
electrolysis of 1-chloro-2,2,6,6-tetramethylpiperidines
under similar conditions. The preparative synthesis of
Scheme 1.
electrolysis
NaCl
electrolysis
Na SO
electrolysis
Na SO
2
4
2
4
+
N
N
N
N
H
Cl
O
O
I
II
III
IV
That was the first and the so far the only example of
electrochemical oxidation of 2,2,6,6-tetramethyl-
piperidine to the corresponding nitroxyl radical.
upon oxidation of chloramine II was suggested, which
included the stages of formation of radical cation V,
elimination of the chlorine molecule with subsequent
hydroxylation, elimination of protons and electron
transfer (Scheme 2).
In the previous report [2] we have used the methods
of voltammetry and ESR spectroscopy to investigate
the electrochemical oxidation of 1-chloro-2,2,6,6-
tetramethylpiperidine (II) in acetonitrile. Based on the
data obtained, the mechanism of formation of the
nitroxyl radical III and the oxoammonium ion IV
The present study is a continuation of this work and
was undertaken to determine the generality of the
reaction, the effect of the nature of the halogen atom
and the substituent in position 4 of the piperidine ring
2
151

2
152
KAGAN et al.
Scheme 2.
_e
H O
_H+
2
II
_1
/2 Cl2
+
N
N
N
+
Cl
OH
V
_e
_H+
_e
+
N
N
O
O
III
IV
on the reaction of electrochemical oxidation of 1-halo-
,2,6,6-tetramethylpiperidines. In the present work,
the reaction of electrochemical oxidation of
haloamines VI–X is studied using the methods of
CVA, ESR spectroscopy, and by preparative
electrolysis.
2
OH
NHCOCH3
OH
NHCOCH3
N
N
N
N
N
Br
Br
Br
Cl
Cl
VI
VII
VIII
IХ
Х
Cyclic voltammograms (CVA) of haloamines VI–
X obtained in the MeCN/0.1 M Bu NBF medium on a
potentials and doubled (Table 1). In the anode range,
for all compounds VI–X one one-electron oxidation
peak is detected, the bromamines being oxidized only
slightly easier than chloramines. For chloramine II and
bromamine VI the difference of potentials of the
oxidation peaks is as low as 50 mV (Table 1). For all
compounds regardless of the nature of the halogen
atom the current of the oxidation peak ip,ox regularly
decreases with the rate of the potential sweep υ in the
4
4
glass carbon electrode are principally similar to the
curves of the earlier studied chloramine II [2]. For
chloramines, one extended irreversible reduction peak
is registered in the far cathode range of potentials, for
bromamines this peak is shifted to lower cathode
–
1
Table 1. CVA data for halogenamines II, VI–X on a glass
range from 200 to 20 mV s (Figs. 1, 2), and the plot
–3
1/2
carbon electrode in CH
υ = 100 mVs
3
CN/0.1 M Bu
4
NBF
4
, (c = 2×10 M),
ip,ox–υ is linear, suggesting the diffuse nature of the
–1
peak current. On the reverse branch of CVA for all
rates of potential sweep an intense peak of re-reduction
is registered, shifted by 60–70 mV to lower anode
values relative to the oxidation peak. This is indicative
of reversibility of both the stage of the electron transfer
and the whole electrode process of one-electron
oxidation; in other words, it points to a considerable
stability of the formed radical cations of haloamines
during the time of voltammetry measurements
Comp. no.
E
p,ox, (Ep,rered), V
Ep,red, V
а
II
1.11 (1.05)
1.06 (1.00)
–2.10
–1.36
–1.81
–1.49
–1.69
–1.56
–2.13
YI
(
0.35)
1.08 (1.01)
0.39)
YII
(
VIII
IX
1.10 (1.02)
1.18 (1.11)
(
seconds). Typical CVAs for bromo- and chloramines
are depicted in Figs. 1, 2, and the electrochemical
characteristics from the CVA data for compounds II,
VI–X are given in Table 1.
X
1.20 (1.13)
–2.26
a
Earlier obtained data [2].
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ELECTROCHEMICAL OXIDATION
Current
2153
[
5 μA
Current
[
5 μA
Potential, V
Potential, V
Fig. 1. CVA curves for oxidation of chloramine X (c =
Fig. 2. CVA curves for oxidation of bromamine VII (c =
–3
–
3
2
×10 M) on a glass carbon electrode in CH
Bu NBF at different potential sweep rates, mV s :
1) 200; (2) 100; (3) 50; and (4) 20.
3
CN/0.1 M
2×10 M) on a glass carbon electrode in CH
Bu NBF at different potential sweep rates, mV s : (1)
200; (2) 100; (3) 50; and (4) 20.
3
CN/0.1 M
–
1
–1
4
4
4
4
(
Electrochemical oxidation of unsubstituted steri-
cally hindered amines of the 2,2,6,6-tetramethyl-
piperidine series under similar conditions also pro-
ceeds with the formation of radical cations, which,
however, cannot be experimentally detected (the
cathode peak of re-reduction is absent) due to fast
deprotonation with the formation of the corresponding
aminyl radical detected by the ESR method [3, 4].
Radical cations of haloamines II, VI–X were directly
registered by the ESR spectroscopy method when
performing the electrooxidation of the Pt electrode
with the potential of +1.1 V for bromamines and
The experimental spectra completely coincide with
the theoretical spectra calculated using the WinSIM
EPR DESIGN V. 9.5 program [5]. Oxidation of
bromamine VII leads to a radical cation of the same
+
1.3 V for chloramines in the resonator of the ESR
spectrometer at a low temperature (–10°С). In Figs. 3,
, typical ESR spectra of radical cations obtained by
oxidation of bromamine VI (Fig. 3) and chloramine X
Fig. 4) as well as the spectra of the corresponding
4
(
nitroxyl radicals obtained after completion of the
process of oxidation and subsequent re-reduction at
0
V are shown.
OH
NHCOCH3
Strength of magnetic field, mT
Fig. 3. ESR spectra (1) of the radical cation of bromamine
VI generated electrochemically at Е = +1.10 V in MeCN/
N
N
N
0.1 M Bu
nitroxyl radical III obtained after completion of the process
of oxidation and subsequent re-reduction at 0 V (cVI
4 4
NBF on the Pt electrode at –10°С, and (2) the
O
III
O
VIIa
O
VIIIa
=
–
3
2×10 М).
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 81 No. 10 2011

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154
KAGAN et al.
Strength of magnetic field, mT
Strength of magnetic field, mT
Fig. 4. ESR spectra of (1) the radical cation of chloramine
X generated electrochemically at Е = +1.20 V in MeCN/0.1
M Bu NBF on the Pt electrode at –10°С, and (2) the
4 4
Fig. 5. ESR spectra of the radical cation of bromamine VI
generated electrochemically at Е = +1.10 V in MeCN/0.1 M
nitroxyl radical VIIIaobtained after completion of the
Bu
(
4 4
NBF on the Pt electrode at various temperatures, °С:
process of oxidation and subsequent re-reduction at 0 V
–3
–
3
1) –10, (2) +10, and (3) +30 (cVI = 2×10 М).
X
(c = 2×10 М).
type that is formed from bromamine X although in
much lower yield. The characteristics of the ESR
spectra are determined by the nature of the halogen
intensity of the signal of the corresponding nitroxyl
radical VIIIa remains low. The intensity of this signal
drastically increases if all the processes of oxidation
and re-reduction are carried out at room temperature.
Therefore, in the case of bromamines, the formation of
oxoammonium ion and, afterward, the nitroxyl radical,
takes place to a considerable extent, whereas in the
case of chloramines at least two channels of decay of
the primary radical cations exist, the ratio of their rates
being dependent on the temperature. At room tem-
perature, the contribution of the route via the oxo-
ammonium ion formation increases, while at lower
temperatures another route predominates, whose nature
is not clear yet. The characteristics of the ESR spectra
of the nitroxyl radicals III, VIIа–VIIIа are given in
Table 2.
(
Table 2).
When the temperature increases, the intensity of the
ESR signal of radical cations decreases (Fig. 5), which
is indicative of a decrease of their stability.
The decrease of the intensity of the ESR signal after
the electrolysis is switched off has shown that radical
cations of bromamines are less stable than radical
cations of chloramines. In particular, the decay half-
time of the radical cation of compound X at 243 K is
~
~
8 min, while for radical cation of compound VIII it is
3 min at 243 K and 45 s at 283 K. With this, no other
paramagnetic species, for example, the aminyl radicals,
are detected.
Therefore, the radical cations of 1-halogeno-
2,2,6,6-tetramethylpiperidines formed upon electro-
chemical oxidation in acetonitrile are substantially
more stable than of the corresponding 2,2,6,6-
tetramethylpiperidines, that makes them accessible for
experimental investigation by the methods of CVA and
ESR under conventional conditions in the second time
scale. One of the reasons of a higher stability of the
radical cations formed by oxidation of N-halopyridines
After completion of the process of oxidation of
haloamines II, under the conditions of generation of
radical cations, the subsequent reduction of the
solutions at 0 V results in appearing the signal of the
corresponding nitroxyl radical III, VIIа, VIIIа
(
Figs. 3, 4), which is intense in the case of brom-
amines VI–VII and weak for chloramines II, IX–X.
Even for a protracted electrolysis of chloramine X the
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 81 No. 10 2011

ELECTROCHEMICAL OXIDATION
2155
Table 2. Parameters of the ESR spectra of radical cations formed upon electrochemical oxidation of halogenamines II, VI–X,
radical cation XI obtained by photolysis of chloramine II in СF COOH solution, and nitroxyl radicals III, VIIа–VIIIа
3
3
5
79
a Cl, mT
a Br, mT
N
Н
Comp. no.
Number of lines
12
g-Factor
2.0093
a , mT
3
а СН , mТ
3
7
81
a Cl, mT
a Br, mT
а
II
2.110
0.600
0
.500
VI
6
6
2.0230
2.0230
2.0093
2.10
2.10
2.081
1.85
1
.99
1.85
.99
VII
VIII
1
12
0.585
.486
0
III
3
3
3
2.0057
2.0059
2.0060
1.57
1.59
1.54
VIIа
VIIIа
b
XI
2.00382
1.87
1.23
a
b
From [2]. From [6].
is delocalization of the unshared electron to the N–
halogen bond, as is the case in nitroxyl radicals. In
both cases, no splitting of the signals on the methyl
protons is observed in the ESR spectra. Another reason
is that the halogen atoms, as distinct from hydrogen,
are incapable of elimination as cations. When the
process of elimination of proton is suppressed by
carrying out the photolysis of 1-chloro-2,2,6,6-
converted into more stable products. When the
processes of oxidation and re-reduction of the products
of oxidation are carried out at 0 V at room tempera-
ture, an intense signals of the corresponding nitroxyl
radicals appear for all the studied compounds. No
other radical species could be detected by the ESR
method.
The results obtained for haloamines are in complete
agreement with those for chloramine II [2] and are
indicative of the same mechanism of the process of
their oxidation as shown in Scheme 2. The one-
electron oxidation affords radical cations. Two radical
cations slowly react with each other to form the
molecular halogen and two iminium ions. The
reactions of iminium ions with water molecules from
the solution lead to oxoammonium ions via the step of
formation of hydroxylamines and nitroxyl radicals.
These particles in the present case are not the final
products but only intermediates since their oxidation
potential is substantially lower than the oxidation
potential of haloamines [8, 9]. That is why the process
of re-reduction of the oxoammonium ion is an
obligatory step after the oxidation process to obtain
and detect the nitroxyl radical. We have performed
such a reduction at 0 V; with this, apparently, the
halogen molecules are also reduced to the halide ions.
tetramethylpiperidine II in the СF COOH solution
3
(
Scheme 3), it is also possible to register the radical
cation of unsubstituted 2,2,6,6-tetramethylpiperidine
(XI) whose lifetime under these conditions is 100 ms
[6]. The methods of synthesis of ammonium radical
cations and the pathways of their decay were
considered earlier [6, 7]. In the ESR spectrum of the
obtained radical cation XI the splitting of the signal on
the hydrogen atom and on the methyl protons is
observed (Table 2).
Scheme 3.
CF COOH
+
N
3
N
Cl
H
II
ХI
During protracted electrolysis (5–7 min) the radical
cations of haloamines VI–X both at room and lower
temperature undergo chemical transformations being
Preparative electrosynthesis of the nitroxyl radicals
was successful only when using haloamines II and VI
as substrates. In the latter case, the yield of 2,2,6,6-
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2
156
KAGAN et al.
tetramethylpiperidine 1-oxyl (III) was 92% to the
converted bromamine VI. In other cases, unidentified
products of more deep oxidation were formed.
crystallization from hexane 1.34 g (32%) of the
unreacted 1-bromo-2,2,6,6-tetramethylpiperidine VI
was isolated. To the water layer 2 g of sodium nitrite
was added, keeping pH in the range of 4–5 by addition
of 3 М hydrochloric acid, and extracted with
methylene chloride (3×30 ml). Methylene chloride was
distilled off, the residue distilled in vacuum, collecting
the fraction with bp 78–80°С (5 mm Hg). The yield of
2,2,6,6-tetramethylpiperidine-1-oxyl (III) 1.92 g (62%
to the starting compound or 90% to the reacted
1-bromo-2,2,6,6-tetramethylpiperidine), mp 36–37°С
(vacuum sublimation). A mixed probe with the
authentic sample [12] does not give depression of the
melting point.
EXPERIMENTAL
Cyclic voltammograms were registered using a PI-
5
3
0-1 potentiostat on a two coordinate recorder Н
07/2. The working electrode was glass carbon disc
electrode (d = 2 mm), pressed into fluoroplast. Prior to
each measurement the electrode was mechanically
–
1
polished. Potential sweep rate υ was 100 mV s . The
potentials were measured relative to the silver
reference electrode Ag/0.01 М AgNO in MeCN. The
3
dissolved oxygen was removed by bubbling argon into
the solution, temperature 295 K.
REFERENCES
Investigation by the method of the ESR spectro-
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combining the ESR spectrometer “Radiopan SE/X-
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544,” potentiostat PI-50-1 and electrochemical cell.
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known procedures [10].
5
6
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2
,2,6,6-Tetramethylpiperidine-1-oxyl (III) from
1
-bromo-2,2,6,6-tetramethylpiperidine (VI). The
synthesis is carried out in a standard diaphragm-free
electrolyzer of 150 ml capacity equipped with water
jacket, thermometer and mechanical stirrer [11].
Anode and cathode were platinum plates of 10 cm
surface area. The electrolyzer was charged with 80 ml
of water, 5.6 g (0.04 mol) of sodium sulfate, 4.4 g
7
8
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9
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(
0.02 mol) of 1-bromo-2,2,6,6-tetramethylpiperidine
VI and 30 ml of methylene chloride. The conditions of
–
2
electrolysis: current density 0.05 А cm (current
.5 А), temperature 20–25°С, vigorous stirring. The
electrolysis was stopped after passing 1.2 А h
1
1
0. Kashparova, V.P., Zhukova I.Yu., and Kagan, E.Sh.,
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0
–
1
(
2.2 F mol ) amount of electricity. The water layer
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(Electrochemical synthesis of Organic Preparates),
Rostov: Rostov. Gos. Univ., 1981.
was separated from organic and extracted with
methylene chloride (2×20 ml). Organic extracts were
combined and analyzed by TLC and GC. From the
analysis data, the organic layer contains a mixture of
compounds, from which after removal of solvent and
12. Rozantsev, E.G., Svobodnye iminoksil’nye radikaly,
Moscow: Khimiya, 1970.
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