Oxidative Coupling of N,N- and 2,6-Disubstituted Aniline Derivatives Mediated by Cerium(IV) Ions in Aqueous Perchloric Acid

Article abstract of DOI:10.1007/PL00010205

N,N- and 2,6-disubstituted anilines were oxidized by cerium(IV) perchlorate in aqueous solutions of perchloric acid to the corresponding derivatives of 4,4′-diphenoquinonediimine and N-phenyl-p-phenylenediimine in high yields. The type of the products obtained was determined by the properties of the substituents, by steric hindrance, and by the concentration of protons in the reaction mixtures. On the basis of experimental data and quantum mechanical calculations, which were performed using the semi-empirical AM1 method, a reaction mechanism was suggested.

Full text of DOI:10.1007/PL00010205

Monatshefte fuÈr Chemie 130, 249±256 (1999)
Oxidative Coupling of N,N- and
2,6-Disubstituted Aniline Derivatives
Mediated by Cerium(IV) Ions in
Aqueous Perchloric Acid
Ã
ꢀ
Piotr Seliger, Magdalena Wodzinska, and Jozef Dziegiec
Â
ꢀ
 Â
ꢀ
Department of General and Inorganic Chemistry, University of èodz, PL-90136 èodz, Poland
ꢀ
Summary. N,N- and 2,6-disubstituted anilines were oxidized by cerium(IV) perchlorate in aqueous
solutions of perchloric acid to the corresponding derivatives of 4,40-diphenoquinonediimine and N-
phenyl-p-phenylenediimine in high yields. The type of the products obtained was determined by the
properties of the substituents, by steric hindrance, and by the concentration of protons in the reaction
mixtures. On the basis of experimental data and quantum mechanical calculations, which were
performed using the semi-empirical AM1 method, a reaction mechanism was suggested.
Keywords. N,N- and 2,6-Disubstituted anilines; Cerium(IV) perchlorate; Products of oxidation;
Mechanism; Quantum mechanical AM1 calculations.
Cer(IV)-Ionen-vermittelte oxidative Kupplung von N,N- und 2,6-disubstituierten
È È
Anilinderivaten in waûriger Perchlorsaure
Zusammenfassung. N,N- und 2,6-disubstituierte Aniline wurden durch Cer(IV)perchlorat in
È
È
È
waûrigen Losungen von Perchlorsaure in hohen Ausbeuten zu den ensprechenden Diphenodiiminen
und N-Phenyl-p-phenylendiiminen oxidiert. Die Art der Produkte wird dabei durch die
Eigenschaften des Substituenten, die sterische Hinderung und die Protonenkonzentration im
Reaktionsmedium bestimmt. Auf der Basis von experimentellen Daten und quantenchemischen
Rechnungen mit Hilfe der semiempirischen AM1-Methode wurde ein Reaktionsmechanismus
vorgeschlagen.
Introduction
Investigations on the chemical and electrochemical oxidation of 2,6-disubstituted
anilines have shown that the products of these one-electron reactions are deri-
vatives of benzidine (I), 4-aminodiphenyleneamine (II), and hydrazobenzene (III)
[1±16].
Experimental data show that type and constitution of the products depend
strongly on the concentration of protons in the reaction medium and on the basic
Ã
Corresponding author

250
P. Seliger et al.
Scheme 1
properties of the oxidized aniline derivatives. Accordingly, benzidine derivatives
are obtained from aniline precursors with relatively strong basic properties in
media with high proton concentrations. Hydrazobenzene derivatives are obtained
from aniline derivatives with less basic properties in basic or less acidic media
[3, 4, 6, 11]. The mechanisms suggested in previous papers do not explain such
dependencies. In this paper, using experimental results regarding the oxidation of
2,6-disubstituted anilines by cerium(IV) perchlorate and quantum mechanical
calculations performed using the AM1 method, we attempt to explain the in¯uence
of the concentration of the protons in the reaction medium and of the basic
properties of anilines on the type and constitution of the products obtained. On the
other hand, the experimental results regarding the oxidation of N,N- and 2,6-
disubstituted anilines by cerium(IV) ions indicate that it is possible to develop
simple and fast methods for the preparation of the corresponding derivatives of
4,40-diphenoquinonediimine and/or N-phenyl-p-phenylenediimine in high yields
[13, 14].
Results and Discussion
When oxidizing N,N-dimethyl- (1a) and N,N-diethylaniline (1b) by cerium(IV)
ions in aqueous solutions of perchloric acid, we found that the only products of
these processes (which involve reduction of 2.0 mmol of oxidizing agent per
1.0 mmol of aniline derivative) were the corresponding N,N,N0,N0-tetraalkyldiphe-
noquinone-4,40-diammonium perchlorates (2a, b). Moreover, we observed that the
reaction velocity decreased considerably with increasing proton concentration.
However, in media with a perchloric acid concentration of 4.0 M, in which the
reactions proceeded very slowly, we observed neither a lowering of the formal
potential of the Ce(IV)/Ce(III) system nor a colour change of the solution during
the addition of N,N-dialkylanilines. This indicates that under such conditions the
formation of a donor-acceptor complex of the oxidizer-reducer takes place only to
a very small degree. What plays a crucial role in the formation of these complexes
is the electron pair of the amino nitrogen atom. Such an effect was not observed
during the reaction of cerium(IV) ions with 2,6-dimethyl- (1c), 2,6-diethyl- (1d),

Oxidative Coupling of Disubstituted Anilines
251
Scheme 2
and 2-isopropyl-6-methyl-aniline (1e) with protonation constants three orders of
magnitude lower than those of N,N-dialkylanilines. The oxidation of these anilines
occurred almost immediately. In 0.4 M aqueous solutions of perchloric acid, 1c and
1d were oxidized almost quantitatively to 3c and 3d, and with higher
concentrations of perchloric acid, the additional formation of 2c and 2d was
observed. In the case of 1e, a mixture of 2e and 3e is obtained irrespective of the
reaction conditions. When the 2,6-disubstituted aniline derivatives with protonation
constants logK < 2.0, e.g. as 2-chloro-6-methyl- (1f ), 2,6-dichloro- (1g), or 2,6-
di¯uoro-aniline (1h), are oxidized by cerium(IV) ions, the corresponding N-
phenyl-p-phenylenediimine derivatives (3f±h) are obtained irrespective of the
proton concentration in the reaction media. Products 3f±h partially dissolved in the
reaction medium and underwent hydrolysis to the corresponding p-benzoquinone
derivatives (4f±h) and substrates (1f±h). This caused an increase in the amount of
reduced cerium(IV) ions by 1.0 mmol of 1f±h, and in the case of 1h the amount
even equalled 2.8 mmol. The results are summarized in Scheme 2 and Table 1.
The results shown in Table 1 unequivocally indicate that the type and
constitution of the products of the oxidation of anilines by one-electron oxidizing
agents strongly depend on the proton concentration in the reaction medium, on the
steric hindrance, and on the basic properties of the anilines ± but in fact, the most
crucial factors are the properties of the ®rst products of their oxidation, i.e. cation
radicals CR.
The cation radicals (CR) of 2,6-disubstituted anilines can undergo the reactions
shown in Scheme 4.
According to the experimental results, the process CR!CROH does not take
place. This is in agreement with the results obtained for the oxidation of 2,6-
disubstituted phenols [17±19] which show that the hydroxylation process of the
carbon atoms C4 of the cation radicals occurs only if charges on the C4 atoms are
positive (> 0.03). The process R!C is unlikely to occur either, because with the

252
P. Seliger et al.
Table 1. Oxidation of substituted anilines 1a±h with Ce(IV) in H2O or H2O-acetonitrile (Ã) solutions
in room temperature; conversion rate: 100%
Aniline
1a
Molar ratio
ox:red
Time
(min)
[HClO4]
(mol Á dmꢀ3
0.4
4.0
Product
(%)a
)
1
2
2.2Æ0.1
2.1Æ0.1
40
2a (>95)#
2a (>95)b
140
3
4
1b
2.1Æ0.1
20
50
0.4
4.0
2b (>95)#
2b (>95)b
2.2Æ0.1
5
6
1c
2.2Æ0.1
2.1Æ0.1
0
0
0.4
4.0
3c (>95)#
2c (65)#, 3c (25)#
7
8
1d
2.3Æ0.2
0
0
0.4
4.0
3d (>95)#
2.1Æ0.1
2d (55)#, 3d (35)#
9
1e
2.3Æ0.2
2.1Æ0.1
0
0
0.4
4.0
2e (40)#, 3e (50)#
10
2e (60)#, 3e (30)#
11
12
1f
2.3Æ0.1
0
0
0.4
4.0
3f (>95)#
2.5Æ0.1
3f (70)#, 4f (20)
13*
14
1g
2.3Æ0.1
2.2Æ0.1
0
0
0.4
4.0
3g (80)#, 4g (10)
3g (85)#, 4g (5)
15
16
a
1h
2.4Æ0.2
0
0
b
0.4
4.0
3h (80)#, 4h (10)
3h (60)#, 4h (30)
2.8Æ0.2
Yields based on the amount of aniline added; from spectroscopic measurements; #: product
precipitates during oxidation
Scheme 3
charges on the carbon atoms C4, which are 0.14±0.16 for the corresponding cations
C, it is dif®cult to explain the lack of the corresponding p-iminobenzoquinones
among the reaction products.
The quantum mechanical calculations show that the process CR!DiC is not
probable either. Some authors try to connect this process with the second peak
observed only on CVA curves (at potentials about 0.2 V higher than the formation
potential of CR) of N,N-dialkylanilines [11]. It seems that this peak is associated
with one of the stages of the oxidation of one of the intermediate dimers, e.g. a
cation radical dimer, to 2a, b. This leads to the conclusion (Scheme 3, Table 3) that
in the case of N,N-dialkylanilines one of the possible processes which the cation

Oxidative Coupling of Disubstituted Anilines
253
Table 2. Calculated data for reaction presented in Scheme 3; Hf : heat of formation (kJ Á molꢀ1), ÁH: heat of reaction
(kJ Á molꢀ1), ꢀyx: partial charge of atom x in form y
ꢀN1
ꢀ1C4
ÁH1ꢀ2 ꢀ2N
logK
ꢀ2C4
ÁH2ꢀ3 ꢀ3N
ꢀ3C4
ÁH3ꢀ4 ꢀ4N
ꢀ4C4
Substituents
Hf1
Hf2
Hf3
Hf4
R1 ˆ Me,
0.06
0.05
0.00
0.00
ꢀ0.07 212.9
ꢀ0.26 ꢀ0.17 732.2
ꢀ0.08 0.00
357.9
363.9
285.7
289.5
290.3
264.0
247.8
235.2
ꢀ0.25 ꢀ0.13
R2 ˆ R3 ˆ H
764.4
5.03
130.0
862.2
372.8
R1 ˆ Et,
ꢀ0.07 209.8
ꢀ0.26 ꢀ0.17 726.3
ꢀ0.08 0.00
ꢀ0.25 ꢀ0.13
R2 ˆ R3 ˆ H
719.9
664.9
6.53
82.4
808.7
325.3
R1 ˆ H,
ꢀ0.06 207.5
ꢀ0.32 ꢀ0.17 737.2
ꢀ0.21 0.01
ꢀ0.23 ꢀ0.11
R2 ˆ R3 ˆ Me
3.70
25.1
762.3
200.7
R1 ˆ H,
ꢀ0.06 211.1
ꢀ0.33 ꢀ0.17 731.1
ꢀ14.9
ꢀ0.22 0.01
ꢀ0.23 ꢀ0.11
R2 ˆ R3 ˆ Et
621.3
3.80
716.2
158.4
R1 ˆ H,
ꢀ0.01 ꢀ0.06 214.0
625.6 3.75
ꢀ0.33 ꢀ0.17 731.9
ꢀ0.21 0.01
ꢀ0.23 ꢀ0.11
R2 ˆ Me, R3 ˆ iPr
ꢀ7.7
724.2
167.2
R1 ˆ H,
0.00
0.00
0.01
ꢀ0.05 192.5
679.3 1.97
ꢀ0.33 ꢀ0.17 759.9
ꢀ0.20 0.01
ꢀ0.21 ꢀ0.10
R2 ˆ Cl, R3 ˆ Me
24.5
784.4
201.1
R1 ˆ H,
ꢀ0.05 177.6
696.1 <2
ꢀ0.34 ꢀ0.17 781.8
ꢀ0.20 0.01
ꢀ0.19 ꢀ0.10
R2 ˆ R3 ˆ Cl
26.4
808.2
208.7
R1 ˆ H,
ꢀ0.02 173.1
383.4 1.13
ꢀ0.32 ꢀ0.13 787.3
ꢀ290.8
ꢀ0.18 0.03
ꢀ0.18 ꢀ0.08
ꢀ115.6
R2 ˆ R3 ˆ F
496.5
Scheme 4
radicals CR can undergo is their recombination along with the formation of a C±C
bond between the aromatic rings.
This process is determined by the steric effects. In some media, especially at
low concentration of perchloric acid, the process with de®nitely the lowest
enthalpy is possible: CR‡1a, b ! cation radical of the dimer with a C±C bond
between the aromatic rings. The oxidation reaction enthalpies of the cation radicals

254
P. Seliger et al.

Oxidative Coupling of Disubstituted Anilines
255
of the dimers are higher by about 81.0Æ5.0 kJ/mol than the enthalpies of the
reactions 1a, b!CR, and they can explain the presence of the second peaks on
the voltammograms of the N,N-dialkylanilines. In the case when there is no
steric hindrance (1c±h), the processes CR‡1 lead to the formation of the cation
radical of the dimers with C±N±C bonding between the rings (Table 3). The
reaction enthalpies of their oxidation to dications are higher by about 220.0 (1c)
to 245.0 kJ/mol (1h) than the oxidation reaction enthalpies of the aniline
molecules 1c±h.
When 2,6-disubstituted anilines (1f±h), whose cation radicals should most
readily undergo the process of deprotonation, are oxidized by cerium(IV) ions, the
‡
‡
stages CR$R, 1H $1, and R‡CR, R‡1H , R‡1 or R‡R lead to the formation of
dimers with C±N±C bonds between the aromatic rings. The same situation is
observed for anilines 1c±e whose cation radicals have stronger basic properties
(Scheme 3, Table 2) in media with lower proton concentrations. Such conclusions
result from the analysis of the quantum mechanical calculations (Table 2 and 3). It
follows that the enthalpies of reactions with participation of the radical, which lead
to the formation of dimers with a C±N±C bond between aromatic rings, are lower
than those of the C±C dimer formation; this is in agreement with the experiment.
Although the quantum mechanical calculations were performed for the gas
phase, they are in very good agreement with the experimental data, suggesting that
they can be used in this kind of experimental modelling.
The presented experimental results show that the use of cerium(IV) ions as a
mediator of the oxidative coupling of aniline derivatives seems to provide a simple
and economical method for the synthesis of 2a±e or/and 3c±h; methods for the
electrochemical regeneration of the oxidizing agent in puri®ed post-reaction
solutions (®ltration through active carbon) have been described [20].
Experimental
Melting points were determined with a melting points apparatus (Veb Nagema). UV/Vis spectra of
the reaction mixtures with various molar concentration ratios of the reagents and the compounds
obtained were recorded on a Philips PU9630 spectrophotometer. IR spectra were recorded on a
Specord IR M-80, and NMR spectra on a Tesla BS 487 NMR spectrometer (60 MHz). The quantum
mechanical calculations (AM1 [21], MOPAC 6 package) were carried out on a 486DX/50 computer
with 16 MB RAM. The geometries of all analyzed structures were calculated using the PRECISE
option. All internal coordinates were taken into account, and no assumptions as to the symmetry
were made during the geometry optimization.
General procedure for the oxidation of N,N- and 2,6-disubstituted anilines 1a±h on the example
of 2,6-dimethylaniline (1c)
In 25 cm3 of an aqueous solution of 0.8 M HClO4 containing 0.3 M cerium(III) perchlorate, 2.5 mmol
1c are dissolved. An aqueous solution of 0.8 M HClO4 containing 0.24 M cerium(IV) perchlorate and
0.06 M cerium(III) perchlorate is added dropwise within 5 min at room temperature. The oxidation
process occurs immediately. The moment when an excess of cerium(IV) ions appears (which
indicates completeness of the reaction) is detected potentiometrically. The precipitate (almost
ꢁ 100% yield) is ®ltrated, washed with aqueous 0.1 M HClO4 and H2O, and dried at 80ꢂC to a solid
mass.

256
P. Seliger et al.: Oxidative Coupling of Disubstituted Anilines
Yield: 0.378 g N-(2,6-dimethylphenyl)-20,60-dimethyl-p-phenyleneimineamonium perchlorate
(95%); m.p.: >600 K, UV (acetonitrile): ꢁmax ˆ 292 nm (" ˆ 1.7Â104); IR (KBr): ꢂ ˆ 2900, 2880,
1660, 1600, 1590, 1210, 880, 785 cmꢀ1
.
In the case of oxidation of 1a,b which occurs more slowly, the amount of cerium(IV) ions
necessary for a 100% conversion of the reducing agent was assessed experimentally, and those
processes were followed until the cerium(IV) ions disappeared in the reaction mixture. In the case of
1c, the mixture of the products was separated chromatographically on an Al2O3 column with CH2Cl2
as eluent. The remaining quinones in the post-reaction solutions (4g±h) were extracted with CCl4.
The general procedure for the conversion of the anilines 1a±h to 4,40-diiminediphenoquinone
and/or to N-phenyl-p-phenylenediimine derivatives in 4.0 M perchloric acid solution is similar; 2a,b
are dissolved in the reaction medium, and in the case of 1c±e, the product mixtures are separated
chromatographically on an Al2O3 column using CH2Cl2 as eluent. Quinones 4f±h remaining in the
solution are extracted with CCl4.
Acknowledgements
The presented investigations were carried out within Project NO. 3 T09B 139 08 and supported from
the KBN grant for 1995±1997.
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Received March 26, 1998. Accepted (revised) June 2, 1998