Reactions of aromatic nitro compounds: LXXIV.* azo coupling of anionic nitroarene σ-complexes with aromatic diazo compounds. Effect of reaction conditions on the yield of nitroazobenzenes

Article abstract of DOI:10.1023/A:1012440710073

Effect of a number of factors on the yield of nitroazobenzenes in the reaction of 4-nitro-and 4-dimethylaminobenzenediazonium tetrafluoroborates with anionic σ adducts derived from 1,3-dinitrobenzene was studied. Conditions were found which allow nitroazobenzenes to be prepared in 80-90% yield.

Full text of DOI:10.1023/A:1012440710073

Russian Journal of Organic Chemistry, Vol. 37, No. 3, 2001, pp. 401 403. Translated from Zhurnal Organicheskoi Khimii, Vol. 37, No. 3, 2001,
pp. 426 428.
Original Russian Text Copyright
2001 by Blokhina, Shakhkel’dyan, Atroshchenko, Alifanova, Gitis, Kaminskii, Moiseev.
Reactions of Aromatic Nitro Compounds:
LXXIV.* Azo Coupling of Anionic Nitroarene -Complexes
with Aromatic Diazo Compounds. Effect of Reaction Conditions
on the Yield of Nitroazobenzenes
N. I. Blokhina, I. V. Shakhkel’dyan, Yu. M. Atroshchenko, E. N. Alifanova,
S. S. Gitis, A. Ya. Kaminskii, and D. N. Moiseev
Tolstoy Tula State Pedagogical University, pr. Lenina 125, Tula, 300026 Russia
e-mail: reaktiv@tspu.tula.ru
Received December 2, 1999
Abstract Effect of a number of factors on the yield of nitroazobenzenes in the reaction of 4-nitro- and
4-dimethylaminobenzenediazonium tetrafluoroborates with anionic adducts derived from 1,3-dinitrobenzene
was studied. Conditions were found which allow nitroazobenzenes to be prepared in 80 90% yield.
Azo coupling reaction remains so far the main
method for preparation of aromatic azo compounds
[2]. Taking into account the low electrophilicity of
diazo compounds, aromatic amines, phenols, and
naphthols are generally used as azo component. Direct
introduction of the azo group into electron-deficient
systems such as nitroarenes is difficult. This obstacle
can be circumvented by preliminarily increasing the
nucleophilicity of nitroaromatic substrate. We showed
in [3 6] that anionic -complexes formed by nitro-
benzenes, nitronaphthalenes, and nitroanthracenes
react with arenediazonium salts through replacement
of one nitro group by phenylazo group. As a result,
a number of new nitroazo compounds were syn-
thesized. However, the yield of azo compounds in the
above reactions was insufficiently high because of
concurrent oxidation [3, 4] of the -complex to the
initial nitro compound with diazonium salt. In the
present work we examined the effect of reaction
conditions (reactant ratio, temperature, and solvent
nature) on the yield of azo compounds. As model
reaction we chose the reaction of 1,3-dinitrobenzene
-complex I with 4-nitro- and 4-dimethylaminoben-
zenediazonium tetrafluoroborates, studied previously
in [6] (Scheme 1). These diazonium salts exhibit
appreciably different reactivities in both azo coupling
and redox processes [7]. The data in Table 1 show
Scheme 1.
Table 1. Effect of the ratio -complex diazonium salt
on the yield of 3-nitroazobenzenes IIa and IIb in a THF
DMF mixture (5: 2, by volume)
Yield, %
Molar ratio
-complex diazonium salt
IIa
IIb
1: 1
1: 2
1: 3
1: 4
69
81
67
59
72
47
44
44
R = NO2 (a), N(CH3)2 (b).
____________
*
For communication LXXIII, see [1].
1070-4280/01/3703-0401$25.00 2001 MAIK Nauka/Interperiodica

402
BLOKHINA et al.
Table 2. Effect of the ratio 1,3-dinitrobenzene KBH4 on
the yield of 3-nitroazobenzenes IIa and IIb in a THF
DMF mixture (5: 2, by volume)
that the yield of azo compounds IIa and IIb can be
increased to 70 80% by variation of the reactant ratio.
At an equimolar reactant ratio the yield of 3,4 -dinitro-
azobenzene (IIa) was slightly lower than the yield of
4-dimethylamino-3 -nitroazobenzene (IIb). With
2 equiv of p-nitrobenzenediazonium tetrafluoroborate,
the yield of IIa increases to 81%. Further raising the
amount of the diazo component leads to reduced yield
of IIa, presumably because of increasing contribution
of the oxidation pathway (pathway b in Scheme 1).
4-Dimethylaminobenzenediazonium tetrafluoroborate
shows a different relation. In the presence of 2 equiv
of the diazo compound the yield of IIb falls almost
twofold, and further increase in the amount of the
diazonium salt does not affect the yield of IIb.
As follows from Scheme 1, the process consists of
two stages. The first stage, formation of anionic
-complex, is reversible. It was interesting to estimate
the effect of excess reducing agent on the yield of the
final product. Experiments were performed without
isolation of adduct I from the solution to which
appropriate diazonium salt was added. By using
4 equiv of KBH4 with respect to initial 1,3-dinitro-
benzene we succeeded in obtaining 89% of azo com-
pound IIb. In the case of more active 4-nitrobenzene-
diazonium tetrafluoroborate the use of excess reducing
agent decreased the yield of product IIa (Table 2).
Obviously, this is the result of side processes, e.g.,
reduction of the diazo compound with excess potas-
sium tetrahydridoborate.
Molar ratio -complex diazonium salt
Molar ratio
1: 1
1: 2
1,3-DNB KBH4
IIa
IIb
IIa
IIb
1: 1
1: 2
1: 3
1: 4
69
58
43
38
72
72
82
89
81
52
37
37
71
73
85
88
Table 3. Effect of the temperature on the yield of 3-nitro-
azobenzenes IIa and IIb in a THF DMF mixture (5: 2,
by volume)
Yield, %
Temperature,
C
IIa
IIb
0
10
20
30
40
81
68
64
42
24
89
87
87
83
78
Table 4. Effect of the solvent on the yield of 3-nitroazo-
benzenes IIa and IIb (equimolar reactant ratio, 0 C)
Insofar as diazonium salts are unstable compounds,
the azo coupling is usually carried at low temperature.
We have found that mixing of the reactants is accom-
panied by considerable increase in temperature, i.e.,
the reaction is exothermic. As might be expected, the
optimal reaction temperature is 0 C (Table 3).
Yield, %
Solvent
IIa
IIb
Dioxane
THF
0
0
0
0
Acetonitrile
DMF
DMSO
Ethanol
Water
28
53
21
28
0
46
66
24
45
0
The yield of compounds IIa and IIb strongly
depends on the polarity and acid base properties of
the solvent (Table 4). On the whole, more polar
solvents (with larger dielectric constant ) favor
formation of the nitroazo compounds. In weakly polar
solvents, such as tetrahydrofuran and dioxane, the
yield of IIa and IIb was very poor since salt-like
reagents are almost insoluble in these solvents. Water
promotes fast protolytic decomposition of the anionic
-complex to initial 1,3-dinitrobenzene.
Table 5. Effect of nonpolar solvents (30 vol %) on the
yield of 4-nitroazobenzenes IIa and IIb in DMF
Yield, %
Nonpolar
We also examined the reactions in binary mixtures
of THF with polar solvents (see figure). Initially, the
yield of nitroazobenzenes increases as the fraction of
polar component rises; the maximal yield is attained
at a polar component fraction of 30%. Further rise in
the latter results in a slight decrease of the yield, and
then it no longer changes up to 100% fraction of the
component
IIa
IIb
Benzene
Dioxane
Pentane
THF
52
34
27
69
61
49
38
72
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 37 No. 3 2001

REACTIONS OF AROMATIC NITRO COMPOUNDS: LXXIV.
(a)
403
(b)
Effect of polar solvents on the yields of (a) 3,4 -dinitroazobenzene (IIa) and (b) 4-dimethylamino-3 -nitroazobenzene (IIb)
in THF: (1) water, (2) ethanol, (3) acetonitrile, (4) DMSO, and (5) DMF.
polar component. This pattern can be explained in
terms of formation of contact ion pairs by the reagents
in weakly ionizing solvents, which decompose in
going to more polar medium. Comparison of the data
given in Tables 4 and 5 shows that addition of a non-
polar solvent to DMF increases the yield of nitroazo
compound only when the nonpolar component is THF.
The result of the present study is that we succeeded
in raising the yield of nitroazobenzenes in the reac-
tions of anionic -complexes with benzenediazonium
tetrafluoroborates to 80 90%. The optimal conditions
are as follows: solvent THF DMF (5:2, by volume),
temperature 0 C, and molar reactant ratio 1,3-DNB
KBH4 4-X-C6H4N+2 BF4 1:1:2 (X = NO2) or 1:4:1
(X = NMe2). The developed procedure extends syn-
thetic potential of the azo coupling reaction, for it
provides the possibility of using accessible nitro-
arenes as azo component and obtaining new nitroazo
compounds.
Shishkin, O.V., and Andrianov, V.F., Russ. J. Org.
Chem., 2000, vol. 36, no. 5, pp. 684 692.
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The azo coupling of anionic -complexes with
benzenediazonium tetrafluoroborates was performed
by the procedure described in [6]. The solvents were
purified by standard methods [8].
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RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 37 No. 3 2001