Mechanism of disproportionation of 2,2,6,6-tetramethyl-1,4- dioxopiperidinium perchlorate in acidic aqueous medium

Article abstract of DOI:10.1007/s11172-009-0248-3

The cation of 2,2,6,6-tetramethyl-1,4-dioxopiperidinium perchlorate disproportionates in acidic aqueous medium to give piperidinoxyl and/or 1-hydroxypiperidine, nitrone, and acetone. A mechanism of this reaction was proposed and the rate constants for its particular steps were calculated.

Full text of DOI:10.1007/s11172-009-0248-3

Russian Chemical Bulletin, International Edition, Vol. 58, No. 9, pp. 1824—1827, September, 2009
1824
Mechanism of disproportionation of 2,2,6,6ꢀtetramethylꢀ
1,4ꢀdioxopiperidinium perchlorate in acidic aqueous medium
V. A. Golubev and V. D. Sen´
Institute of Problems of Chemical Physics, Russian Academy of Sciences,
1 prosp. Akad. Semenova, 142432 Chernogolovka, Moscow Region, Russian Federation.
Fax : +7 (496) 515 5420. Eꢀmail: senvd@icp.ac.ru
The cation of 2,2,6,6ꢀtetramethylꢀ1,4ꢀdioxopiperidinium perchlorate disproportionates
in acidic aqueous medium to give piperidinoxyl and/or 1ꢀhydroxypiperidine, nitrone, and
acetone. A mechanism of this reaction was proposed and the rate constants for its particular
steps were calculated.
Key words: oxoammonium salts, nitroxide radicals, nitrones, autoreduction, kinetics,
mechanism, 2,2,6,6ꢀtetramethylꢀ4ꢀoxopiperidineꢀ1ꢀoxyl.
Table 1. Yields of the key products of the reaction of salt 1 with
Nitroxide radicals and oxoammonium salts of the piꢀ
water at different HClO4 concentrations ([1]0 = 0.05 mol L–1
25 °C, reaction time 4 h)
,
peridine series are widely used as catalysts of oxidation of
alcohols to carbonyl compounds.1—5 The efficiency
of these catalysts is largely determined by the stability of
Nꢀoxopiperidinium cations, which are direct oxidants
for the alcohol group. In aqueous solution, Nꢀoxoꢀ
piperidinium cations are converted to the corresponding
nitroxides6 and other products, which were partially charꢀ
acterized.7,8 The mechanism of these transformations
is unknown.
[HClO4]0
/mol L–1
Product yield (mol.%)*
2
3
4
Acetone
HClO4
98
—
—
—
0
63
2
1
2
—
—
44
32
1
1
3
4
33
—
42
47
1
2
0.1
1.0
2.0
26
11
6
39
43
42
4
3
1
1
—
Previously,6 it was assumed and later,9 evidence was
obtained for the reduction of oxopiperidinium salts to
nitroxide radicals by the hydroxide anion of water. The
authors of a review10 believe that of all known cases,
autoreduction of the 4ꢀmethoxyꢀ2,2,6,6ꢀtetramethylꢀ
1ꢀoxopiperidinium cation giving H2O2 is the most conꢀ
vincing proof for oxidation of the OH– ion by an organic
oxidant.9 It was found,11,12 however, that H2O2 quickly
reacts with oxoammonium salts by reducing them to
nitroxides and, hence, the formation of H2O2 upon
autoreduction of oxoammonium salts is impossible. The
autoreduction induced by OH– anions is known and has
been studied in detail for other strong organic oxidants,
for example, quinones13 or perchlorotriphenylmethyl
radicals and perchloroquinone derivatives.14 In none of
the cases, unambiguous proof was obtained for the oxidaꢀ
tion of OH– anions.
* Per mole of initial salt 1.
due to the fact that reduction can be studied over a broad
pH range owing to the activating effect of the 4ꢀoxo group.
In aqueous solution, salt 1 is partly reduced to nitroxide
2 and to 1ꢀhydroxypiperidinium salt 3 and is partly conꢀ
verted to acetone and 5,5ꢀdimethylꢀ3ꢀoxoꢀ1ꢀpyrroline
Preliminary experiments with 4ꢀsubstituted oxopiperiꢀ
dinium salts have shown that the yields of transformation
products of the oxopiperidinium cation and the reaction
rates depend on the pH and the substituent nature.15 In
this work we studied in detail the transformations of
2,2,6,6ꢀtetramethylꢀ1,4ꢀdioxopiperidinium perchlorate
(1) in aqueous acidic medium. The choice of this salt is
(1)
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1767—1770, September, 2009.
1066ꢀ5285/09/5809ꢀ1824 © 2009 Springer Science+Business Media, Inc.

Disproportionation of oxoammonium cations
Russ.Chem.Bull., Int.Ed., Vol. 58, No. 9, September, 2009 1825
Scheme 1
1ꢀoxide (4). The ratio of the reaction products depends
on the acidity of the medium (Table 1).
The transformation of dioxopiperidinium salt 1 in
water follows the stoichiometric equation (1).
As a result of complex disprportionation, two molꢀ
ecules of cation 1 undergo singleꢀelectron reduction to
radical 2, while one molecule of cation 1 undergoes twoꢀ
electron oxidation to acetone and oxopyrroline oxide 4.
Water is involved in this reaction as a nucleophile rather
than a reducing agent, as no water oxidation products
(H2O2 or O2) are formed. The ClO4– anion is not changed
during the reaction.
;
(4)
On acidification of an aqueous solution of salt 1 with
perchloric acid, the yield of oxopyrroline oxide 4 increases,
and instead of radical 2, the product of twoꢀelectron reꢀ
duction of cation 1, 1ꢀhydroxypiperidinium cation 3, is mainly
;
(5)
formed. When the concentration [HClO4] > 1 mol L–1
,
the reaction stoichiometry roughly corresponds to the
equation (see Table 1):
(6)
(2)
The kinetics of reaction of salt 1 with water was studꢀ
ied in a water—acetonitrile solution of HClO4. The
dioxopiperidinium cation is consumed according to a firstꢀ
order kinetic equation the rate constants of which (k´) are
summarized in Table 2. As water concentration increases,
the k´ values linearly increase. Thus, cation 1 reacts with
water according to a secondꢀorder kinetic equation. The
bimolecular rate constants k″ = k´/[H2O] are also given
in Table 2. As the HClO4 concentration increases, the
reaction rate decreases and the 1/k″ value (see Table 2)
linearly increases according to the experimental equation
(7)
1/k″ = 1.19•104 + 2.83•104[HClO4].
(3)
The results of studying the kinetics and stoichiometry
of the reaction of dioxopiperidinium cation 1 with water
are consistent with Scheme 1.
The first step (reaction (4)) is reversible addition of
water to cation 1 to give Nꢀhydroxypiperidine Nꢀoxide 5
(see Refs 16, 17). Compound 5 is unstable and undergoes
fragmentation along reaction (5) yielding acetone and
Nꢀhydroxypyrrolidinone 6. This product is oxidized with
cation 1 to final pyrroline oxide 4 (reaction (6)). Cation 1
undergoes twoꢀelectron reduction to hydroxypiperidinium
cation 3. The redox reaction of cations 1 and 3 (reacꢀ
tion (7)) results in radical 2 by a known mechanism.18,19

1826
Russ.Chem.Bull., Int.Ed., Vol. 58, No. 9, September, 2009
Golubev and Sen´
Table 2. Rate constants for the decomposition of salt 1 in water—acetonitrile solutions of HClO4 at 25 °C
[1]0•102
[H2O]0,
mol L–1
[HClO4]0•102
k´•104/s–1
k″•105
10–4/k″
/L mol–1 s–1
/mol s L–1
3.50
3.50
0.72
3.30
3.30
3.10
3.50
6.94
6.94
1.25
12.50
12.50
12.50
22.20
44.40
50.00
6.3
4.2
8.8
11.1
11.1
8.3
9.1
6.1
6.3
6.0
6.0
4.5
3.6
1.1
1.6
1.6
1.7
1.7
2.2
2.8
13.90
18.50
18.50
18.50
13.90
5.0
The equilibrium constant for the lastꢀmentioned reacꢀ
The disproportionarion rates of radical 2, k–4 =
tion K4 = [2]2[H+]2/([1][3]) = (8.3 0.2)•10–2 mol2 L–2
= (2.7 0.2)•10–2 L2 mol–2 s–1 and the rate constant for
the reverse reaction k4 = 2.3•10–3 s–1 at 25 °C. As a result
of the inductive effect of the carbonyl group for radical 2,
the constant k–4 is 8 times lower, k4 is 360 times higher,
and K4 is 2800 times higher than for 2,2,6,6ꢀtetramethylꢀ
piperidinꢀ1ꢀoxyl.18,19 The k–4[H+] and k4/[H+] values are
considerably higher than the observed rate constant k″ for
decomposition of cation 1 (see Table 2). Therefore, equiꢀ
librium between radical 2 and the products of its disproꢀ
portionation 1 and 3 during decomposition of cation 1 is
established rather fast.
at 25 °C. According to this K4 value, at [H+] = K4
=
1/2
= 0.29 mol L–1, equal amounts of compounds 1, 2, and 3
occur in equilibrium. At [H+] < 0.1 mol L–1, this equilibꢀ
rium is shifted toward radical 2 and at [H+] > 1 mol L–1
it is shifted toward its disproportionation products 1 and
3. Therefore, in weakly acidic medium, radical 2 is formed
as the final product, and cation 1 decomposes according
to Eq. (1). In highly acidic medium, the resulting cation 3
weakly reacts with cation 1, and the latter decomposes by
reaction (2).
According to Scheme 1, the rate of consumption of
the dioxopiperidinium cation 1 and accumulation of the
final oxopyrroline oxide 4 at quasiꢀsteadyꢀstate concenꢀ
trations of intermediates 5 and 6 is given by the equation
Thus, the transformation of dioxopiperidinium cation
1 to radical 2 in aqueous medium is not caused by water
oxidation. NꢀHydroxypyrrolidine 6, resulting from fragꢀ
mentation of Nꢀhydroxypiperidine Nꢀoxide 5, is direct
reducing agent of cation 1. Like alcohols20 and dihydroꢀ
benzopyridines,21 Nꢀhydroxypyrrolidine 6 performs twoꢀ
electron reduction of cation 1 to Nꢀhydroxypiperidine.
Nitroxide 2 is formed upon the subsequent redox reaction
between cation 1 and Nꢀhydroxypiperidine.
–0.5d[1]/dt = d[4]/dt =
= k1k2 [1][H2O]/(k2 + k–1[H+]).
(8)
According to this equation, the experimental rate conꢀ
stant k″ can be written as
k″ = k1k2/(k2 + k–1[H+]).
Experimental
According to experimental equation (3), the reciproꢀ
IR spectra were recorded in the range of 400—4000 cm–1
on a Specord 75ꢀIR spectrometer in mineral oil. UV spectra
were recorded on a Specord UVꢀVIS spectrometer in ethanol.
ESR spectra were measured at room temperature on an
EPAꢀ2М instrument. 1H NMR were run in CDCl3 on a
Bruker DXP instrument (200 MHz).
2,2,6,6ꢀTetramethylꢀ1,4ꢀdioxopiperidinium perchlorate (1)
was obtained from 2,2,6,6ꢀtetramethylꢀ4ꢀoxopiperidineꢀ1ꢀoxyl
(2) by a previously described22 procedure and reprecipitated
from acetonitrile with ether.
cal k″ value increases linearly with increase in [H+]:
1/k″ = 1/k1 + k–1[H+]/(k1k2).
(9)
From Eqs (3) and (9), it is possible to determine the
value k1 = 8.4•10–5 L mol–1 s–1 and the ratio of rate
constants k–1/(k1k2) = 2.83•104 s, k1k2/k–1 = k2K1 =
3.5•10–5 s–1, and k–1/k2 = 2.4 L mol–1
.
According to Scheme 1, the quasiꢀsteadyꢀstate concenꢀ
tration of radical 2 is as follows: [2] = (K4[1][3])1/2/[H+].
This depends on the [H+] concentration and is expected
to change little during decomposition of dioxopiperidiꢀ
nium salt 1, because the sum [1] + [3] varies from [1]0 to
[1]0/2. Thus during decomposition of salt 1 in aqueous soluꢀ
tion acidified with perchloric acid ([HClO4]0 = 1 mol L–1),
the concentration of radical 2 rapidly reaches 12% of the
initial concentration [1]. After 3 h, when the salt has
mainly decomposed, it decreases to 10%.
Preparative decomposition of salt 1 was carried out in aqueous
solution of HClO4 ([HClO4] = 1 mol L–1) at 25 °C for 4 h. The
oxopyrroline oxide 4 thus formed was extracted with chloroform
and purified by chromatography on a column with silica gel
using ether—hexane (1 : 1) as the eluent. Oxopyrroline oxide 4
crystallized as colorless plateꢀlike crystals with m.p. 41—42 °C
(hexane). Lit. data:23 m.p. 43.0—43.5 °C. Compound 4, which
1
we obtained from salt 1, was identical as regards H NMR, IR,
and UV spectra to oxopyrroline oxide synthesized by a previously
described procedure.23 The amount of pyrroline oxide 4 and

Disproportionation of oxoammonium cations
Russ.Chem.Bull., Int.Ed., Vol. 58, No. 9, September, 2009 1827
radical 2 in the reaction solution was determined by HPLC on a
Milichrom chromatograph (a 2×64 mm column, Separon C18,
5 μm, 50% aqueous methanol as the eluent). In this system, the
retention volumes for products 4 and 2 are 240 and 320 μL,
respectively.
6. V. A. Golubev, E. G. Rozantsev, M. B. Neiman, Izv. Akad.
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The amount of 1ꢀhydroxypiperidinium salt 3 in the reaction
solution was determined after alkalification of the solution to
рH 13 and oxidation of salt 3 with atmospheric oxygen to radical
2 the formation of which was monitored by HPLC. The amount
of acetone was determined by GLC on an LKhMꢀ80 chromꢀ
atograph (3×1500 mm column, Separon VD1, 100 μm, 159/175°,
nitrogen as a carrier gas, 30 mL min–1). The retention time of
acetone was 240 s. The amount of HClO4 formed upon decomꢀ
position of salt 1 in water was determined by potentiometric
titration.
8. G. Moad, E. Rizzardo, D. H. Solomon, Tetrahedron Lett.,
1981, 22, 1165.
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Decomposition kinetics of dioxopiperidinium salt 1 was studied
in an aqueousꢀacetonitrile solution of HClO4. The consumption
rate of compound 1 was determined by monitoring the change in
the optical density (D) of the solution at ν = 26000 cm–1. In the
coordinates lnD = k´t, the curves of variation D are linear up to
about 50% consumption of cation 1.
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7764.
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cals (Novosibirsk, 2005, 20—24 September), Novosibirsk,
2005, p. 94.
The equilibrium constant K4 = [2]2[H+]2/([1][3]) was
determined by two methods: from disproportionation of radical
2 and from reaction of salts 1 and 3 in aqueous HClO4 ([1] = [3] =
= 1 mol L–1) at 25 °C. In both cases, the equilibrium conꢀ
centrations of radical 2 were determined by ESR. In the former
case, the equilibrium concentrations of cations 1 and 3 were
calculated by the relation [1] = [3] = ([2]0 – [2])/2, and in the
latter case, the relations [1] = [1]0 – [2] and [3] = [3]0 – [2]
were used. The equilibrium concentrations of [H+] was taken to
be [HClO4]0.
The rate constant for disproportionation of 2,2,6,6ꢀtetraꢀ
methylꢀ4ꢀoxopiperidineꢀ1ꢀoxyl (2) was determined by ESR from
the initial segments of the consumption curve of radical 2 in
aqueous HClO4 ([HClO4] = 0.3 and 1.0 mol L–1) at 25 °C.
Radical 2 is consumed by a secondꢀorder kinetic equation.
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Received April 13, 2009;
in revised form June 8, 2009