Mendeleev Commun., 2011, 21, 26–28
NHOH
NHOH
NO2
NH
–
–
0.10
0.05
e–
–
OH–
NO2
NO2
NHOH
0
0
.00
.05
NH
NH2
2
e–
NO2
1
NOH
NO2
NO2
–
100
–300
–500
–700
–900 –1100
E/mV vs. SCE
NO2
Figure 2 (1) CV curve of a 10 mm 4-NPHA solution at a disk carbositall
electrode (d = 3 mm) in 0.1 m Bu NClO /DMF 3 h after 0.1 F of electricity
Scheme 2
4
4
was passed through the solution at the potential of the first step, and (2) CV
curve of a 4 mm solution of 4,4’-dinitroazobenzene. The scan rate was
under the conditions where the time of the simultaneous presence of
4-NPHA and its anion in the reaction mixture is minimum.4
The combination of 4-NPHA and its anion with the formation
of 4,4'-dinitroazobenzene should simultaneously result in the
formation of the hydroxide anion (Scheme 2). The essence of the
chain reaction is as follows: the hydroxide anion undergoes
protonation by 4-NPHA to produce the 4-NPHA anion, which
reacts with 4-NPHA affording dinitroazobenzene and the hydroxide
anion (Scheme 3). This scheme assumes that the result of the
chain reaction should be independent of the method of generation
of the NPHA anion. Therefore, we generated the 4-NPHA anion
by the interaction of 4-NPHA with a strong base. The addition
–
1
0
.1 V s , and the cell temperature was 298 K.
that the changes observed are related to the occurrence of the
chain (cyclic) reaction initiated by the electrolysis product.
A comparison of the CV curve of the product of this reaction
(
Figure 2, curve 1) with the voltammogram of 4,4'-dinitroazo-
§
benzene (Figure 2, curve 2) suggests that they are identical. The
HPLC data indicate that 4,4'-dinitroazobenzene is formed in
¶
7
4% yield under the described conditions (Scheme 1). In addi-
tion, the test solution contains small amounts of the 4-NPHA
†
†
anion (7%) and 4-nitroaniline (3%) formed in reaction (1).
NHOH
of 0.1 equiv. of Et NOH to a 0.01 m solution of 4-NPHA in
4
DMF followed by stirring for 3 h results in the formation of
4
,4'-dinitroazobenzene in 78% yield (according to HPLC data).
0
.1 F, 3 h
O2N
N
N
NO2
‡‡
0
.1 M Bu4NClO4
DMF
At a preparative scale, the product was isolated in 74% yield.
Thus, the experimental data indicate that the mechanism of
the electrochemically initiated transformation of 4-NPHA into
4,4'-dinitroazobenzene can be described by Scheme 3.
Note that, as a rule, the pre-electrode electroinitiated chain
reactions occur with a high rate accompanied by the fast (within
several minutes) drop of the current of the electroreduction of
7
4%
NO2
NOH
NO2
NH2
+
+
1
0,11
the substrate.
No similar effect is observed in this case. The
NO2
3%
reason is not clear, but one may suppose that the nucleophilic
replacement of a hydroxyl group in 4-NPHA by 4-NPHA anion
gives rise to Ar–NH–N(OH)–Ar, which is relatively stable and
dehydrates slowly giving the final product (4,4'-dinitroazobenzene).
This intermediate product may be responsible for the cathodic
peak different from those of starting material and final products
observed at CV curves during the reactions, but it disappears
when the reaction completes.
7
%
Scheme 1
The formation of 4,4'-dinitroazobenzene as the major product
of the chain reaction suggests that this reaction is initiated by the
interaction of the 4-NPHA anion formed by reaction (1) with the
starting compound.
This is also indicated by the fact that reaction (1) becomes
predominant and the overall yield of its products reaches 69%
in the case of exhaustive electrolysis at potentials of the limiting
current of the first step of the electroreduction of NPHA, i.e.,
In contrast to 4-NPHA, unsubstituted N-phenylhydroxylamine
has a much lower electron affinity, and it cannot be reduced
cathodically at the available range of the potentials. The result
of the experiment with Et NOH addition in case of N-phenyl-
4
hydroxylamine is different from that for 4-NPHA. No forma-
tion of azobenzene is observed, most probably, because of the
much lower acidity of N-phenylhydroxylamine, as compared with
§
4
,4'-Dinitroazobenzenewassynthesizedfrom4,4'-dinitroazoxybenzene
by the electrochemical reduction to the corresponding hydrazo compound
followed by its oxidation similarly to a procedure described in ref. 7.
4
§
§
4-NPHA. In favour of the suggestion says the fact that the con-
,4'-Dinitroazoxybenzene was synthesized from 4-nitroaniline by the
version of N-phenylhydroxylamine to azobenzene in alkaline media
8
reported method.
13
in an inert atmosphere takes an extremely long time (10 days).
¶
HPLC was carried out on the Diaspher-110-C16 column (5 mm, 2.0×80 mm)
‡‡
using a mixture MeCN/0.1 m phosphate buffer with pH 3 in ratios of 25:75
In the synthesis, 0.1 equiv. Et NOH was added to 0.1 m deaerated with
4
and 50:50 as the mobile phase and the UV detector at l = 350 nm.
The yield of the 4-NPHA anion was determined by the absence of the
oxidation peak of 4-NPHA in the CV curve of the obtained solution and
by the appearance of 4-NPHA under the chromatographic conditions
argon solution of 4-NPHA in DMF. In the course of reaction, 4,4'-dinitro-
††
9
1
azobenzene was precipitated, mp 230°C (lit., mp 227–229°C). H NMR
1
3
(300 MHz, DMSO-d ) d: 8.12 (d, 4H), 8.43 (d, 4H). C NMR (75 MHz,
6
2
6
2'
6'
3
5
DMSO-d ) d: 123.88, 123.97 (C , C , C , C ), 125.00, 125.14 (C , C ,
C , C ). MS, m/z (%): 272 (M , 3), 150 (35), 122 (100), 92 (40), 76 (98),
6
3'
5'
+
(
under these conditions, the 4-NPHA anion is protonated by the acidic
mobile phase).
64 (26), 50 (64).
–
27 –