Mechanism of autoreduction of bis(4-methoxyphenyl)-oxoammonium perchlorate in aqueous alkali

Article abstract of DOI:10.1134/S1070428011090090

Autoreduction of bis(4-methoxyphenyl)oxoammonium perchlorate in aqueous alkali follows a mechanism different from that generally accepted for diaryloxoammonium salts. Bis(4-methoxyphenyl)-oxoammonium cation undergoes hydrolysis to the corresponding quinone imine oxide and methanol, the latter gives rise to methoxide ion which reduces the oxoammonium cation to intermediate bis(4-methoxyphenyl)-hydroxylamine. The reaction of bis(4-methoxyphenyl) hydroxylamine with the initial cation yields bis-(4-methoxyphenyl)nitroxyl, and the quinone imine oxide undergoes disproportionation to N-(4-methoxyphenyl)-1,4- benzoquinone imine and oxidation products. Pleiades Publishing, Ltd., 2011.

Full text of DOI:10.1134/S1070428011090090

ISSN 1070-4280, Russian Journal of Organic Chemistry, 2011, Vol. 47, No. 9, pp. 1313–1317. © Pleiades Publishing, Ltd., 2011.
Original Russian Text © V.A. Golubev, V.D. Sen’, 2011, published in Zhurnal Organicheskoi Khimii, 2011, Vol. 47, No. 9, pp. 1296–1300.
Mechanism of Autoreduction of Bis(4-methoxyphenyl)-
oxoammonium Perchlorate in Aqueous Alkali
V. A. Golubev and V. D. Sen’
Institute of Chemical Physics Problems, Russian Academy of Sciences,
pr. Akad. Semenova 1, Chernogolovka, Moscow oblast, 142432 Russia
e-mail: senvd@icp.ac.ru
Received December 9, 2010
Abstract—Autoreduction of bis(4-methoxyphenyl)oxoammonium perchlorate in aqueous alkali follows
a mechanism different from that generally accepted for diaryloxoammonium salts. Bis(4-methoxyphenyl)-
oxoammonium cation undergoes hydrolysis to the corresponding quinone imine oxide and methanol, the latter
gives rise to methoxide ion which reduces the oxoammonium cation to intermediate bis(4-methoxyphenyl)-
hydroxylamine. The reaction of bis(4-methoxyphenyl)hydroxylamine with the initial cation yields bis-
(4-methoxyphenyl)nitroxyl, and the quinone imine oxide undergoes disproportionation to N-(4-methoxy-
phenyl)-1,4-benzoquinone imine and oxidation products.
DOI: 10.1134/S1070428011090090
Oxoammonium salts are used as selective stoichio-
metric oxidants or catalysts in oxidation of alcohols
[1–5]. Their catalytic efficiency in aqueous medium is
limited by the stability of oxoammonium cations
which are converted into nitroxyl radicals and other
by-products. The rate of this transformation strongly
depends on the structure of oxoammonium ion and pH.
For example, autoreduction of 2,2,6,6-tetramethyl-1,4-
dioxopiperidinium perchlorate in alkaline medium is
determined by the transformation of dioxopiperidinium
ion into cyclic hydroxylamines as shown in Scheme 1
[6]. Cyclic hydroxylamines reduce dioxopiperidinium
ion to nitroxyl radicals, thus being oxidized to the
corresponding cyclic nitrones.
Scheme 1.
O
O
O
OH–
OH–
Me
Me
Me
Me
OH
Me
Me
Me
Me
Me
Me
Me
Me
N
–H2O
N
O
N
O
HO
OH
O
O
CH2
Me
Me
Me
Me
Me
Me
Me
N
O
N
OH
O
O
O
OH–
OH–
Me
Me
Me
Me
Me
Me
Me
Me
Me
N
–H2O
–Me2CO
N
O
N
CH2
HO O–
OH
Scheme 2.
·
HO O–
N
2OH–
O
N
O
N
2
2
+ Oxidation products
R
R
R
R
R
R
1313

GOLUBEV, SEN’
1314
Oxoammonium salts of the aromatic series also
The primary products of the reaction of oxoammo-
nium ion VI with hydroxide ion were found to be
quinone imine oxide II and methanol (yield 70–99%,
Scheme 4) but not N-oxide R2N(O)OH. Facile hydrol-
ysis of the ether moiety in cation VI is likely to be
favored by considerable contribution of quinoid struc-
ture VIb. Increase of the concentration of alkali accel-
erates hydrolysis of VI; however, the yield of quinone
imine oxide II simultaneously decreases (see table).
The yield of II also decreases as the initial concentra-
tion of salt I rises.
undergo one-electron reduction by the action of bases
[7, 8]. Meyer and Reppe [8] proposed a hypothetical
mechanism for the autoreduction of such salts in al-
kaline medium, which included addition of hydroxide
ion to diaryloxoammonium cation R2N+=O with
formation of N-hydroxy N-oxide R2N(O)OH. The sub-
sequent disproportionation of the latter yields nitroxyl
radical R2N–O· and unidentified oxidation products
(Scheme 2).
In the present work we studied in detail the mech-
anism of autoreduction in aqueous alkali of a typical
diaryloxoammonium salt, bis(4-methoxyphenyl)oxo-
ammonium perchlorate (I). It was found that the rate of
decomposition of salt I in alkaline medium strongly
increases as the concentration of hydroxide ion rises; at
[OH–] > 0.1 M the reaction rate is determined by the
rate of salt dissolution. Decomposition of the cation in
salt I leads to the formation of quinone imine oxide II,
nitroxyl III, quinone imine IV, amine V, and methanol
(Scheme 3, see table).
Methanol liberated by hydrolysis of cation VI is
partly consumed for the reduction of VI to radical III.
This follows from the lower yield of methanol as
compared to the yield of quinone imine oxide II, as
well as from increase in the yield of III upon addition
of methanol to the reaction mixture (see table).
Methanol is a weak reducing agent in neutral and acid
media, and it almost does not reduce cation VI during
the hydrolysis of salt I. Therefore, the hydrolysis of VI
in pure water yields 99±1% of quinone imine oxide II
and methanol, whereas the yield of radical III is less
than 0.1% (see table).
Compounds II–IV were isolated as individual sub-
stances and identified on the basis of spectral data and
by comparing with authentic samples (see Experimen-
tal). Amine V was detected by chromatography and
identified by retention volume and UV spectrum. Apart
from the above substances, formaldehyde, formic acid,
and carbon dioxide were formed as a result of oxida-
tion of methanol. Perchlorate ion does not participate
in the process, and it can be recovered quantitatively as
potassium perchlorate.
Oxoammonium ion VI is reduced to radical III
mainly with methoxide ion according to Scheme 5
[9, 10]. An intermediate product in this reaction is
N-oxide A which undergoes decomposition into form-
aldehyde and diarylhydroxylamine B. Comproportion-
ation of the latter with cation VI yields two molecules
of III. Disproportionation of a small amount of hy-
droxylamine B gives amine V and radical III [8].
Scheme 3.
MeO
OMe
O
OMe
MeO
OMe
KOH
+
–KClO4
N
O
I
N
O
N
O
ClO4
·
II
MeO
III
O
OMe
OMe
+
+
N
N
H
IV
V
Scheme 4.
MeO
OMe
MeO
OMe
OH–
II
+ MeOH
N
O
N
O–
VIa
VIb
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 47 No. 9 2011

MECHANISM OF AUTOREDUCTION OF BIS(4-METHOXYPHENYL)OXOAMMONIUM
1315
Scheme 5.
VI
MeO
OMe
MeO
OMe
2III
MeO–
VI
N
N
–MeOH
O–
H
2B
O
OH
2III + V
H2C
A
B
Scheme 6.
OH
H
OH
O
OMe
O
OMe
OH–
II
II
Tars
–IV, –OH–
N
N
O–
O–
C
D
Appreciable accumulation of these species (V and III)
is observed after complete consumption of cation VI.
table). The same series corresponds to increase in the
rate of reduction of 2,2,6,6-tetramethyl-1-oxopiperi-
dinium ions [11].
Taking into account that, unlike II, the yield of III
increases with rise in the concentration of alkali, the
rate of reduction of VI with methoxide ion increases
more rapidly than does the rate of hydrolysis of VI
with hydroxide ion as the concentration of the latter
increases. In the reaction with VI, methoxide ion is
partly oxidized to carbon dioxide through intermediate
formation of formaldehyde and formate ion. The yield
of III increases upon addition of a mixture of CH2O
and HCO2Na. Judging by the gain in the yield of III at
[OH–] = 0.05 M, the rate of reduction of VI increases
in the series formate ion < MeOH < formaldehyde (see
The fourth decomposition product obtained from
cation VI is quinone imine IV which is formed by
reduction of quinone imine oxide II. This follows from
reduction of the concentration of quinone imine oxide
II and accumulation of quinone imine IV in the reac-
tion mixture after complete consumption of cation VI.
The stoichiometry and kinetics of transformation of
quinone imine oxide II in alkaline medium were
studied by separate experiments. As a result of dispro-
portionation, ~50% of II is reduced to quinone imine
IV, and its other part is converted into oxidation prod-
Reaction of bis(4-methoxyphenyl)oxoammonium perchlorate (I) with KOH at ~20°C
Yield (HPLC), mol %
[I]0 ×102, [OH–]0 ×102, Additive
Solvent
Water
M
M
(0.1 M)
II
III
<0.1
IV
–
V
MeOH
1.0
1.0
0.0
5.0
50
–
99±1
92±2
85±2
84±1
69c
–
98±2
a
–
05.3±0.3
10±1
11±1
25c
1.2±0.1
2.0±0.2
1.6±0.1
1.2c
0.3±0.1
0.3±0.1
25% MeCN
25% MeCN
Water
1.0
–
–
79±3
1.0
50
00.2±0.05
78±3
b50b.00
55
–
Water
a
a
a
a
a
1.0
5.0
5.0
5.0
MeOH
CH2O
HCO2Na
87±2
77±2
85±2
10±1
19±1
08±2
0.8±0.1
00.2±0.05
00.3±0.05
<0.1
<0.1
<0.1
25% MeCN
25% MeCN
25% MeCN
1.0
1.0
a
b
c
The yield was not determined.
Sodium hydroxide was used instead of potassium hydroxide.
Yield of the isolated product.
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 47 No. 9 2011

GOLUBEV, SEN’
1316
ucts. The reaction rate is proportional to the concentra-
tions of II and alkali and is given by the equation
∂[II]/∂τ = –k[II][OH–]. The bimolecular rate constant
in 25% aqueous acetonitrile at 20°C and [KOH] =
(5–50)×10–2 M is k = 2.3×10–4 l mol–1 s–1. A probable
mechanism involves addition of hydroxide ion to
nitrone II with formation of intermediate product C
and its isomers with alternative position of hydroxy
group in the quinoid fragment. Intermediate C reduces
quinone imine oxide II to quinone imine IV and is
thus oxidized assumingly to quinone imine oxide D
according to Scheme 6. The transformation C→D is
accompanied by regeneration of hydroxide ion which
acts as catalyst. We failed to isolate imine oxide D or
its isomers because of tarring.
described in [12], Vret = 440 μl, Rf 0.79. The kinetics of
the reaction of quinone imine oxide II with potassium
hydroxide were studied in 25% aqueous acetonitrile.
The current concentration of II was determined by its
peak area (HPLC) compared to acetanilide (internal
standard).
Bis(4-methoxyphenyl)oxoammonium perchlo-
rate (I) was prepared as described in [7] with account
taken of the recommendations given in [13]. The prod-
uct was purified by double reprecipitation from aceto-
nitrile (spectral grade) with diethyl ether. Decomposi-
tion point 166–168°C; published data [7]: decomposi-
tion point 155°C. UV spectrum (MeCN), λmax, nm (ε,
l mol–1 cm–1): 578 (38500), 390 (9670), 259 (7400),
228 (7600), 205 (19800). 1H NMR spectrum (CD3CN),
δ, ppm: 4.14 s (6H, CH3), 7.32 d (4H, m-H, J =
9.3 Hz), 8.07 br.d (4H, o-H, J = 8.6 Hz). 13C NMR
spectrum (CD3CN), δC, ppm: 59.08 (CH3), 135.43
(Cm), 146.51 (Co), 162.5 (CN), 173.69 (Cp).
Thus autoreduction of cation VI to nitroxyl radical
III in alkaline medium is not the result of its dispro-
portionation, as was presumed in [8], but the reducing
agent is methoxide ion generated by hydrolysis of VI.
Insofar as the rate of hydrolysis of cation VI is higher
than the rate of its reduction with methoxide ion, the
maximal yield of radical III is ~25%. Otherwise, up to
86% of VI would be reduced to III. The proposed
mechanism of autoreduction of salt I is likely to be
general for all diaryloxoammonium salts having
alkoxy groups in the cation.
Reaction of bis(4-methoxyphenyl)oxoammonium
perchlorate (I) with sodium hydroxide. Perchlorate
I, 1.72 g (5 mmol), was added to 11 ml of an aqueous
solution of NaOH (c = 0.5 M), and the resulting sus-
pension was stirred for ~20 min at 20°C. A brown
material was formed; it was filtered off, washed with
water, and dried under reduced pressure. Yield 1.18 g.
The filtrate was treated with ethyl acetate (2×5 ml).
The extracts were combined and evaporated under
reduced pressure to obtain 31 mg of a brown sub-
stance. Both products were subjected separately to
chromatography in a 28×220-mm column charged
with KSK silica gel (40–50 μm). The column was
eluted with chloroform–methanol (100:3), and the
eluate composition was monitored by TLC. From the
brown precipitate we isolated 675 mg (59 mol %) of
quinone imine oxide II and 305 mg (25 mol %) of
stable radical III. From the extract we isolated 20 mg
(1.9 mol %) of quinone imine IV.
EXPERIMENTAL
The IR spectra were recorded on a Specord 75IR
spectrometer. The UV spectra were measured on
a Specord UV-VIS spectrophotometer. The NMR spec-
tra were obtained on a Bruker AIII instrument; signals
1
were assigned using two-dimensional HSQC H–13C
technique. The melting points were determined on
a PHMK hot stage. HPLC analysis was performed on
a Milikhrom chromatograph [2×64-mm column packed
with Nucleosil C18 (5 μm); detection at λ 260 nm;
eluent 50% aqueous acetonitrile]; acetanilide (Vret
=
4-Methoxy-N-(4-oxocyclohexa-2,5-dienylidene)-
benzenamine oxide (II). Orange needles, mp 126°C
(from benzene–hexane); published data [13]: mp 125–
126°C; Vret = 155 μl, Rf 0.4. UV spectrum (EtOH),
λmax, nm (ε, l mol–1 cm–1): 385 (27100), 266 (5400),
228 (10500), 202 (22200). IR spectrum (KBr), ν,
cm–1: 3108, 3065, 3055, 3032 and 3005 (C–Harom),
2973, 2945, 2908, 2850 (CH3), 1622, 1608 (quinone),
110 μl) was used as internal standard for quantitative
measurements. Silufol UV-254 plates were used for
TLC, eluent chloroform–acetonitrile (10 :1). The
amount of methanol was determined by GLC on
an LKhM-80 chromatograph equipped with a flame
ionization detector and a 4×1650-mm column packed
with Poropak Q (80–100 mesh); oven temperature
130/170°C; carrier gas nitrogen, flow rate 32 ml/min;
ethanol was used as internal standard; retention time, s:
MeOH, 200; EtOH, 450 s. Bis(4-methoxyphenyl)-
amine (V) was synthesized according to the procedure
1
1598, 1510, 1472, 1448 (arom.). H NMR spectrum
(CDCl3), δ, ppm: 3.88 s (3H, CH3), 6.24 d.d (1H, m′-H,
J = 1.9, 10.25 Hz), 6.62 d.d (1H, m′-H, J = 1.9,
10.2 Hz), 7.00 m (2H, m-H), 7.26 d.d (1H, o′-H, J =
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 47 No. 9 2011

MECHANISM OF AUTOREDUCTION OF BIS(4-METHOXYPHENYL)OXOAMMONIUM
1317
2.9, 10.25 Hz), 7.44 m (2H, o-H), 8.00 d.d (1H, o′-H,
J = 2.9, 10.3 Hz). 13C NMR spectrum (CD3Cl), δC,
ppm: 55.84 (CH3), 114.46 (Cm), 126.29 (Co), 126.13,
128.89, 129.89, 131.61 (Co′, Cm′), 138.85 (NC), 143.10
(Cp), 161.88 (N=C), 186.52 (Cp′).
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