Catalytic activity of heteroaromatic N-oxides in the hydrolysis of 2,3,5,6-tetrachloro-p-benzoquinone

Article abstract of DOI:10.1134/S1070363206010233

Pyridine and quinoline N-oxides catalyze hydrolysis of 2,3,5,6-tetrachloro- p-benzoquinone to 2,5-dichloro-3,6-dihydroxy-p-benzoquinone in dilute aqueous acetonitrile. Their catalytic activity is much higher than that of the corresponding azines. Quinoline N-oxides react with 2,3,5,6-tetrachloro-p- benzoquinone in a concentrated dioxane solution in the presence of water to give 2,5-dichloro-3,6-dihydroxy-p-benzoquinone salts. Pleiades Publishing, Inc. 2006.

Full text of DOI:10.1134/S1070363206010233

ISSN 1070-3632, Russian Journal of General Chemistry, 2006, Vol. 76, No. 1, pp. 126 128.
Pleiades Publishing, Inc., 2006.
Original Russian Text A.V. Ryzhakov, L.L. Rodina, 2006, published in Zhurnal Obshchei Khimii, 2006, Vol. 76, No. 1, pp. 129 131.
Catalytic Activity of Heteroaromatic N-Oxides
in the Hydrolysis of 2,3,5,6-Tetrachloro-p-benzoquinone
A. V. Ryzhakov* and L. L. Rodina**
* Karelian Research Center, Russian Academy of Sciences, pr. A. Nevskogo 50, Petrozavodsk, 185003 Russia
e-mail: aryzhakov@nwpi.krc.karelia.ru
** St. Petersburg State University, pr. Universitetskii 26, St. Petersburg, 198504 Russia
Received May 30, 2005
Abstract Pyridine and quinoline N-oxides catalyze hydrolysis of 2,3,5,6-tetrachloro-p-benzoquinone to
2,5-dichloro-3,6-dihydroxy-p-benzoquinone in dilute aqueous acetonitrile. Their catalytic activity is much
higher than that of the corresponding azines. Quinoline N-oxides react with 2,3,5,6-tetrachloro-p-benzoqui-
none in a concentrated dioxane solution in the presence of water to give 2,5-dichloro-3,6-dihydroxy-p-benzo-
quinone salts.
DOI: 10.1134/S1070363206010233
Heterocyclic N-oxides are well known as effective
catalysts in nucleophilic substitution and nucleophilic
addition reactions. Their catalytic activity is much
higher than the activity of the parent heterocycles
which are more basic than N-oxides [1]. The observed
supernucleophilicity of N-oxides is sometimes ratio-
nalized in terms of spatial accessibility of the oxygen
atom, the ability to form H-complexes, high polariza-
bility of the N O bond, interactions with the solvent,
etc. These factors usually favor stabilization by
N-oxides of transition states in the rate-determining
stage. However, in many cases the mechanism of their
catalytic action remains unclear.
In a strongly alkaline medium, compound I is con-
verted into chloranilic acid (II) almost instantaneously.
Acid II anion thus formed has a violet color and
absorbs at
= 530 nm ( = 980, aqueous aceto-
nitrile, 1:1)m. axThe reaction in weakly alkaline or
neutral media occurs at a much lower rate. The
process includes two concurrent reactions, noncata-
lytic and catalytic hydrolysis. The apparent rate
constants (which characterizes the overall process) at
various concentrations of N-oxide III were determined
by Eq. (1):
k = 2.3t 1log(D /D
Dt).
(1)
We previously showed that pyridine and quinoline
N-oxides promote the catalytic transformation of tetra-
cyanoethylene into pentacyanopropene salts in the
presence of water [2]. The reaction begins with forma-
tion of a 1:1 N-oxide tetracyanoethylene charge-trans-
fer complex which is quite sensitive to traces of water.
In the present work we studied a catalytic hydrolysis
of another electron-deficient compound, 2,3,5,6-tetra-
chloro-p-benzoquinone (chloranil, I), to 2,5-dichloro-
3,6-dihydroxy-p-benzoquinone (chloranilic acid, II)
in the presence of 4-methoxypyridine N-oxide (III),
pyridine N-oxide (IV), and quinoline N-oxides Va
Vd (for substitution pattern, see table) and sodium
hydrogen carbonate in aqueous acetonitrile (1:1,
by volume).
1
Here, k is the first-order rate constant (min ); t, reac-
tion time (min); Dt, optical density of the reaction
solution at = 530 nm at a moment t; and D , optical
density by the end of the process.
Figure shows the dependence of k on the concentra-
tion of 4-methoxypyridine N-oxide. The dependence
is linear and is described by Eq. (2):
Crystalline complexes formed by quinoline N-oxides and
chloranil
N-Oxide
Yield, IR spectrum,
,
O
N
X
1
no.
%
cm
O
O
Va
Vb
Vc
Vd
H
73
70
75
55
1265
1317
1285
1300
OH
Cl
Cl
Cl
Cl
Cl
Cl
4-CH3
4-OCH3
4-Cl
HO
O
II
O
I
126

CATALYTIC ACTIVITY OF HETEROAROMATIC N-OXIDES
127
k = 0.8133c 0.0057; r = 0.995.
(2)
1.6
1.4
1.2
1.0
0.8
0.6
0.4
Here, c is the concentration of 4-methoxypyridine
N-oxide (M). The intercept on the y axis (5.71
10 min ) corresponds to the first-order rate constant
3
1
(k0) of the noncatalytic reaction. The experimental
3
1
value of k0 is (5.84 0.51) 10 min .
0
2
4
6
8
10
For comparison, pyridine N-oxide (IV) and pyri-
dine were used as catalysts in some experiments. As
might be expected, N-oxide IV showed a slightly
lower catalytic activity than that of N-oxide III having
a strong electron-donor methoxy group. The rate
constant k of the reaction in the presence of N-oxide
c
103, M
Plot of the apparent rate constant for hydrolysis of
2,3,5,6-tetrachloro-p-benzoquinone (I) vs. concentra-
tion of 4-methoxypyridine N-oxide (III).
3
IV (concentration 5 10 M) was (7.14 0.57)
3
1
10 min . The catalytic activity of pyridine at the
transfer complexes of heteroaromatic N-oxides with
2,3,5,6-tetrachloro-p-benzoquinone (I). Our attempts
were successful in the case of quinoline N-oxides
Va Vd. The corresponding 1:1 complexes were
formed on mixing saturated solutions of the initial
components in anhydrous dioxane; they were isolated
as bright red crystalline substances which decomposed
at about 140 C (see table). The IR spectra revealed
only a slight shift of the N O stretching vibration
same concentration was much lower, k = (2.26
2
1
0.18) 10 min . The pKa values of the conjugate
acids of N-oxide IV and pyridine in water are 0.79
and 5.25, respectively [3], i.e., the latter is more basic
by a factor of 104.46. However, its catalytic activity is
greater only by a factor of 3.16. Therefore, the basi-
city factor is not crucial in the hydrolysis catalyzed
by N-oxides. A similar catalytic activity ratio of
N-oxides and their unoxidized analogs was observed
previously in other nucleophilic reactions [4, 5].
1
frequency (no more than 5 cm ) in the adducts re-
lative to the initial N-oxides. This means that the
N-oxide group is not involved directly in complex
formation and that the complexes are formed via ,
Activation of the hydrolysis of chloranil (I) by
heteroaromatic N-oxides can be rationalized assuming
interactions. The formation of
,
charge-transfer
initial formation of
, charge-transfer complexes in
complexes by N-oxides and chloranil conforms to the
principle of hard and soft acids and basis: -acceptors
react more efficiently with orbitals of donors (soft
center). Crystalline complexes of Va Vc with I in the
presence of water undergo fast hydrolysis to the cor-
responding salts VIa VIc of quinoline N-oxides with
chloranilic acid, which were isolated as solvates with
a water molecule.
which the N-oxide acts as -donor, and compound I,
as -acceptor. Elimination of chloride ion from the
charge-transfer complex is more facile than from free
chloranil molecule, and transition state in the nucleo-
philic substitution reaction is also stabilized.
In order to obtain proofs for the above assumption,
we made an attempt to isolate individual charge-
O
X
OH
Cl
Cl
H2O
Va Vc I
H2O + 2HCl
HO
N
O
O
VIa VIc
Presumably, like pyridine N-oxides, quinoline
N-oxides catalyze hydrolysis of chloranil, but the
reaction in a concentrated aqueous dioxane solution
leads to formation of salts VIa VIc which separate
from the solution.
EXPERIMENTAL
The IR spectra were recorded on a UR-20 spec-
trometer from samples dispersed in mineral oil.
The rate of hydrolysis of 2,3,5,6-tetrachloro-p-
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 76 No. 1 2006

128
RYZHAKOV, RODINA
benzoquinone in the presence of N-oxides was deter-
mined by the accumulation of chloranilic acid using a
KFK-3 photometer ( = 530 nm, d = 10 mm). In all
Va Vc in 1 ml of dioxane and of 1 mmol of chloranil
in 4 5 ml of dioxane were mixed, 0.05 ml of water
was added, and the mixture was left overnight at room
experiments, the initial concentrations were as follows: temperature. The precipitate was filtered off, washed
3
1
chloranil (I), 1 10 M; NaHCO3, 5 10 3 M;
aqueous acetonitrile (1:1), 4 ml; temperature 19 C.
The concentration of 4-methoxypyridine N-oxide was
with dioxane (2 2 ml) and diethyl ether (3 2 ml),
and dried in air.
4
2
REFERENCES
varied from 5 10 to 1 10 M. The apparent rate
constants (k) was calculated using first-order equa-
tion (1).
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Heterocyclic N-Oxides, London: Academic, 1971.
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and purified by the procedure described in [6]. Pyri-
dine was distilled before use. Statistical treatment of
the rate constant values was performed by calculating
standard deviation of the arithmetic mean [7].
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Molecular complexes of quinoline N-oxides
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mmol of N-oxide Va Vd in 2 ml of anhydrous di-
oxane and of 2 mmol of chloranil in 10 ml of dioxane
were mixed, and the mixture was left overnight at
room temperature. The precipitate was filtered off,
washed with dioxane (2 2 ml) and diethyl ether
(3 2 ml), and dried in air.
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Salts VIa VIc of quinoline N-oxides and chlor-
anilic acid. Hot solutions of 1 mmol of N-oxide
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 76 No. 1 2006