Synthesis and characterization of p-phenylenediamine derivatives bearing an electron-acceptor unit

Article abstract of DOI:10.1021/jo048324j

(Chemical Equation Presented) Two series of aniline oligomers bearing fused heterocycles as an electron-acceptor unit were synthesized. They consist of aniline or its derivatives as an electron donor and benzothiadiazole (BT) or quinoxaline (QX) as an ele

Full text of DOI:10.1021/jo048324j

Synthesis and Characterization of p-Phenylenediamine
Derivatives Bearing an Electron-Acceptor Unit
Hidehiro Sakurai, Mendra T. S. Ritonga, Harumi Shibatani, and Toshikazu Hirao*
Department of Applied Chemistry, Graduate School of Engineering, Osaka University,
Yamada-oka, Suita, Osaka 567-0871, Japan
hirao@chem.eng.osaka-u.ac.jp
Received September 22, 2004
Two series of aniline oligomers bearing fused heterocycles as an electron-acceptor unit were
synthesized. They consist of aniline or its derivatives as an electron donor and benzothiadiazole
(BT) or quinoxaline (QX) as an electron-acceptor unit. Benzothiadiazoles 1-3 were synthesized by
palladium-catalyzed amination. Quinoxalines 4-6 were prepared by palladium-catalyzed amination
or transformation from the benzothiadiazoles. These compounds showed a HOMO-LUMO gap
smaller than those of their analogues such as thiophene-substituted BT/QXs. Cyclic voltammetry
revealed that the electrochemical behavior is dependent on the position of the acceptor heterocycle.
Chemical oxidation with Ag2O afforded the corresponding 1,4-quinonediimine derivatives as an
E,E-isomer, stereoselectively. As for the BT pentamer analogues 2 and 3, the first oxidation selec-
tively occurred at the amino group adjacent to the benzothiadiazole unit, giving the regiospecific
half-oxidized derivatives. Furthermore, the fully oxidized derivative 24 was isolated and character-
ized.
Introduction
protect semiconductors or metals.7 Polyanilines exist in
three different discrete redox forms, which include the
fully reduced leucoemeraldine base (LE), the semioxi-
dized emeraldine base (EB), and the fully oxidized
pernigraniline base (PE) as shown in Scheme 1.8 The
oxidation states of the π-conjugated polymers and oligo-
mers interconvert each other,9 permitting the construc-
tion of a catalytic system for oxidation reactions. Poly-
anilines serve as synthetic metal catalysts under an
oxygen atmosphere in the dehydrogenative oxidation of
benzylamines, 2-phenylglycine, and 2,6-di-t-butylphe-
nol.10 Similar catalysis is also performed with the qui-
nonediimine derivatives.11 The quinonediimine moiety of
The π-conjugated polymers have attracted much at-
tention in the application to electrical materials depend-
ing on their electrical properties. Among π-conjugated
polymers, polyanilines are particularly attractive as a
result of their excellent electrical properties and stabi-
lity.1 Extensive investigation has provided useful materi-
als such as batteries,2 light-emitting diodes,3 antistatic
packaging and coatings,4 microelectronic devices,5 elec-
trochemical chromatography,6 and corrosion inhibitors to
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10.1021/jo048324j CCC: $30.25 © 2005 American Chemical Society
Published on Web 03/09/2005
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J. Org. Chem. 2005, 70, 2754-2762

p-Phenylenediamines Bearing an Electron-Acceptor Unit
SCHEME 1. Redox Process of Polyaniline
mers, solution-processible materials are scarce and only
a precursor polymer based on pyrrole and 2,1,3-ben-
zothiadiazole has been reported.18 In our design of low-
band-gap conjugated systems for application to material
devices and complexation with transition metals, the use
of a moderately strong acceptor that bears coordination
sites is required. 2,1,3-Benzothiadiazole18,19 and quinoxa-
line,20 which bear two electron-withdrawing imine (CdN)
nitrogens, are known as typical electron-accepting units.
When these units are combined with electron-donating
units, the donor-acceptor polymers or oligomers may
exhibit low band gap. However, most of these donor-
acceptor polymers comprise pyrrole, thiophene, or phen-
ylene as a donor unit. Introduction of acceptor units such
as benzothiadiazole or quinoxaline into an aniline oligo-
mer chain is expected to give novel π-conjugated com-
pounds that exhibit different redox properties as com-
pared with those of aniline oligomers.5 In this paper, we
report the synthesis and characterization of p-phenylene-
diamine derivatives bearing a benzothiadiazole or qui-
noxaline unit. Not only 1 or 4 but also the longer
derivatives 2 and 3 bearing the benzothiadiazole unit or
5 and 6 bearing the quinoxaline unit in a different
position (Figure 1) were synthesized and characterized
in order to study the position effect of the acceptor unit
in the aniline oligomer chain.21
the emeraldine base has been revealed to be capable of
participating in the complexation with transition met-
als.12
Band gap control of π-conjugated polymers is a re-
search issue of ongoing interest. Controlling the band gap
may give a polymer with the desired electrical and optical
properties, and reduction of the band gap to approxi-
mately zero is expected to afford a conducting polymer.13
The synthetic principles for lowering the band gap of
linear conjugated polymers have been reviewed by Ron-
cali.14 One successful and potentially versatile strategy
to achieve low-band-gap conjugated polymers follows the
pioneering work of Havinga15 and involves the alterna-
tion of electron-rich (donor, D) and electron-deficient
(acceptor, A) units in the π-conjugated polymer chain. In
recent years, electrochemical polymerization has been
employed to prepare polymers consisting of an electron-
withdrawing moiety such as 2,1,3-benzothiadiazole, thieno-
[3,4-b]pyrazine, or [1,2,5]thiadiazolo[3,4-g]quinoxaline,
and an electron-releasing bithiophene or N,N′-dimethyl-
bipyrrole. The poly(D-A) systems exhibit optical band
gaps as low as 0.5 eV.16 An extremely narrow optical band
gap of 0.3 eV was recently reported for an electrogener-
ated polymer derived from a bithiophenic precursor
involving thieno[3,4-b]pyrazine and 3,4-ethylenedioxy-
thiophene.17 The application of π-conjugated systems to
the fields of electronic and photonic devices, however,
requires solution-processible materials, and hence the use
of electrochemical polymerization is not straightforward.
Within the class of low-band-gap alternating D-A poly-
Results and Discussion
1. Synthesis of 1-6. p-Phenylenediamine derivatives
1, 2, and 3 bearing a benzothiadiazole unit were prepared
as shown in Scheme 2. Amination of 4,7-dibromo-2,1,3-
benzothiadiazole (7)22 was carried out by modifying a
method reported for the preparation of aniline oligomers
by Buchwald et al.23 Palladium-catalyzed amination of
7 with aniline (8) using bis(2-(diphenylphosphino)phenyl)
ether (DPEphos)24 as a ligand in refluxing toluene
afforded the bis-coupled product 1 in 91% yield. The
reaction of 7 with N-phenyl-1,4-phenylenediamine (9)
also gave 2 in 55% yield. For the preparation of unsym-
(18) van Mullekom, H. A. M.; Vekemans, J. A. J. M.; Meijer, E. W.
Chem. Commun. 1996, 2163.
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1997, 53, 10169. (b) van Mullekom, H. A. M.; Vekemans, J. A. J. M.;
Meijer, E. W. Chem. Eur. J. 1998, 4, 1235. (c) Raimundo, J. M.;
Blanchard, P.; Brisset, H.; Akoudad, S.; Roncali, J. Chem. Commun.
2000, 939. (d) Morikita, T.; Yamaguchi, I.; Yamamoto, T. Adv. Mater.
2001, 13, 1862. (e) Jayakannan, M.; van Hal, P. A.; Janssen, R. A. J.
J. Polym. Sci. A 2002, 40, 2360. (f) Kitamura, C.; Saito, K.; Nakagawa,
M.; Ouchi, M.; Yoneda, A.; Yamashita, Y. Tetrahedron Lett. 2002, 43,
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Chem. Commun. 1993, 194. (b) Hirao, T.; Higuchi, M.; Oshiro, Y.;
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J. Org. Chem. 1997, 62, 1072.
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A.; Kubota, K. J. Am. Chem. Soc. 1994, 116, 4832. (b) Yamamoto, T.;
Kanbara, T.; Ooba, N.; Tomaru, S. Chem. Lett. 1994, 1709. (c)
Yamamoto, T.; Zhou, Z.-H.; Kanbara, T.; Shimura, M.; Kizu, K.;
Maruyama, T.; Nakamura, Y.; Fukuda, T.; Lee, B.-L.; Ooba, N.;
Tomaru, S.; Kurihara, T.; Kaino, T.; Kubota, K.; Sasaki, S. J. Am.
Chem. Soc. 1996, 118, 10389.
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(b) Higuchi, M.; Imoda, D.; Hirao, T. Macromolecules 1996, 29, 8277.
(c) Hirao, T.; Yamaguchi, S.; Fukuhara, S. Tetrahedron Lett. 1999, 40,
3009. (d) Hirao, T.; Yamaguchi, S.; Fukuhara, S. Synth. Met. 1999,
106, 67. (e) Moriuchi, T.; Bandoh, S.; Miyaishi, M.; Hirao, T. Eur. J.
Inorg. Chem. 2001, 651. (f) Hirao, T.; Fukuhara, S.; Otomaru, Y.;
Moriuchi, T. Synth. Met. 2001, 123, 373.
(21) Preliminary results have been reported. See: (a) Ritonga, M.
T. S.; Sakurai, H.; Hirao, T. Tetrahedron Lett. 2002, 43, 9009. (b) Hirao,
T.; Sakurai, H.; Ritonga, M. T. S. Proc. Electrochem. Soc. 2003, 12,
189.
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85, 4673. (b) Bredas, J. L. Synth. Met. 1987, 17, 115.
(14) Roncali, J. Chem. Rev. 1997, 97, 173.
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14, 3081.
(17) Akoudad, S.; Roncali, J. Chem. Commun. 1998, 2081.
J. Org. Chem, Vol. 70, No. 7, 2005 2755

Sakurai et al.
FIGURE 1. p-Phenylenediamine derivatives bearing a benzothiadiazole (1-3) or quinoxaline (4-6) unit.
SCHEME 2. Synthesis of Benzothiadiazoles 1-3a
SCHEME 3. Synthesis of Quinoxalines 4-6 from
5,8-Dibromoquinoxaline (Method A)a
a Reagents and conditions: (a) 10 mol % Pd(OAc)2, 11 mol %
BINAP, 280 mol % t-BuONa, 200 mol % 8, toluene, reflux, 72 h.
(b) 10 mol % Pd(OAc)2, 11 mol % BINAP, 280 mol % t-BuONa,
200 mol % 9, toluene, reflux, 72 h. (c) 5 mol % Pd(OAc)2, 5.5 mol
% BINAP, 140 mol % t-BuONa, 100 mol % 8, toluene, reflux, 16
h. (d) 5 mol % Pd(OAc)2, 5.5 mol % DPEphos, 140 mol % t-BuONa,
100 mol % 10, toluene, reflux, 72 h.
a Reagents and conditions: (a) 10 mol % Pd(OAc)2, 11 mol %
DPEphos, 280 mol % t-BuONa, 200 mol % 8, toluene, reflux, 27
h. (b) 10 mol % Pd(OAc)2, 11 mol % DPEphos, 280 mol % t-BuONa,
200 mol % 9, toluene, reflux, 72 h. (c) 5 mol % Pd(OAc)2, 5.5 mol
% DPEphos, 140 mol % t-BuONa, 100 mol % 8, toluene, reflux, 20
h. (d) 5 mol % Pd(OAc)2, 5.5 mol % DPEphos, 140 mol % t-BuONa,
100 mol % 10, toluene, reflux, 72 h.
SCHEME 4. Transformation from
Benzothiadiazoles to Quinoxalines (Method B)a
metrical derivative 3, the amination of 7 with an equimo-
lar amount of 8 was performed at 80 °C to give the
monocoupled product 11 selectively, followed by the
coupling with the aniline trimer 10.
a Reagents and conditions: (a) 900 mol % zinc powder, AcOH,
70 °C, 3 h. (b) 120 mol % glyoxal, EtOH-H2O, rt, 6 h.
mately 1 ppm lower-field shift was observed with the
N-H attached to the BT/QX ring (Figure 2).
Quinoxaline derivatives 4-6 were synthesized via two
routes (methods A and B). Method A is based on the
palladium-catalyzed amination of 5,8-dibromoquinoxaline
(12)25 in the presence of BINAP or DPEphos as a ligand
(Scheme 3). Benzothiadiazole derivatives 1-3 could be
transformed to the corresponding quinoxalines 4-6,
respectively. As shown in Scheme 4 (method B), reduction
of 1 with zinc powder gave the corresponding o-phen-
ylenediamine derivative, followed by the condensation
with glyoxal to afford 4 in 39% yield. The preparation of
5 and 6 was also carried out under similar conditions.
1H NMR of 1-6 indicates that hydrogen bonding exists
between the imine nitrogen on the benzothiadiazole or
quinoxaline ring and the amine adjacent to the hetero-
cycle. Chemical shifts of the N-H apart from the BT/QX
ring ranges from 5.6 to 5.8 ppm, whereas an approxi-
2. X-ray Crystal Structure. Further structural in-
formation of 2 and 5 was obtained by X-ray crystal-
lography, as shown in Figures 3 and 4, respectively. As
for the structure of 2‚H2O, the sums of angles at the
amine nitrogen atoms are 359.5° (N2), 359.6° (N3), 359.1°
(N12), and 359.9° (N13), respectively, supporting that
they possess sp3 hybrid structures. The dihedral angles
of N1-C1-C2-N2 and N11-C101-C102-N12 are 5.4°
and -1.8°, respectively. H1 and H3 are faced on the BT
nitrogens. The distances of N1-H1, N2-H1, N11-H3,
and N12-H3 are 2.42, 0.95, 2.67, and 0.92 Å, respec-
tively. The angles of N2-C2-C1 and N12-C102-C101
are 115.9° and 117.4°, respectively, smaller than those
of N2-C2-C3 and N12-C102-C103 (128.0° and 126.6°).
These data indicate that hydrogen bonding exists be-
tween the imine nitrogen of the BT ring and the amine
adjacent to the heterocycle. Aromatic rings are twisted
against each other. The steric interaction between the
hydrogen atom at C3 and the hydrogen atom at C5,
(25) (a) Tsubata, Y.; Suzuki, T.; Miyashi, T.; Yamashita, Y. J. Org.
Chem. 1992, 57, 6749. (b) Yamamoto, T.; Sugiyama, K.; Kushida, T.;
Inoue, T.; Kanbara, T. J. Am. Chem. Soc. 1996, 118, 3930.
2756 J. Org. Chem., Vol. 70, No. 7, 2005

p-Phenylenediamines Bearing an Electron-Acceptor Unit
A similar structural feature was observed with 5. The
sums of angles around the amine nitrogen atoms are
358.4° (N2), 359.0° (N3), 359.9° (N12), and 353.5° (N13),
respectively, showing their sp3 hybrid structures. The
dihedral angles of N1-C1-C2-N2, C1-C2-N2-H1,
N11-C101-C102-N12, and C101-C102-N12-H3 are
1.3°, 9.9°, -1.3°, and -2.3°, respectively. H1 and H3 are
faced on the QX nitrogens. The distances of N1-H1, N2-
H1, N11-H3, and N12-H3 are 2.24, 0.91, 2.18, and 0.93
Å, respectively. The angles of N2-C2-C1 and N12-
C102-C101 are 116.1° and 115.3°, respectively, smaller
than those of N2-C2-C3 and N12-C102-C103 (127.1°
and 128.1°, respectively). The dihedral angle between the
QX ring and the adjacent phenylene is 29.9°. The
dihedral angle between the phenylene and the terminal
phenyl ring is 47.8°.
The hydrogen bondings of the BT and QX systems are
significantly different. The length of the intramolecular
hydrogen bond (N2-H1‚‚‚N1) in the QX system is 2.23
Å, which is longer than 2.42 Å in the BT system. These
data indicate that the strength of hydrogen bonding in
the QX system is greater than that in the BT system.
These data are in good agreement with the results
observed in the solution state; in the NMR spectra, the
chemical shift of N-H attached to the QX ring was
shifted to lower field than that adjacent to the BT ring
(Figure 2). The difference in the hydrogen bonding is
considered to be partly due to the difference of the N lone
pairs between BT/QX.
3. UV-vis Spectra. The absorption maxima of 1-6
in chloroform are shown in Table 1. The analogous
compounds bearing pyrrolyl, thiophenyl, or phenyl moi-
eties as a donor unit are also shown for comparison. The
absorption maximum of 1 was 522 nm, exhibiting a
HOMO-LUMO gap considerably smaller than that of
thiophenyl 15 and phenyl analogues 16 and almost the
same as that of the pyrrolyl analogue 14, which is known
to give the smallest HOMO-LUMO gap among these
donor-acceptor compounds. Furthermore, longer wave-
length absorption maximum was observed with 2 and 3
at 550 and 546 nm, respectively. Like in the BT systems,
QX derivatives 4-6 also exhibited a smaller HOMO-
FIGURE 2. 1H NMR chemical shifts of the N-H’s.
however, causes the phenylene ring to rotate away from
this orientation, resulting in a conformation with a
dihedral angle of 50.7° between the BT ring and the
adjacent phenylene. The dihedral angle between the
phenylene and the terminal phenyl ring is 47.5°. Two
molecules are present in a face-to-face manner with an
interplanar distance of ca. 3.6 Å between the benzothia-
diazole moieties, which form a π-stacked dimer in the
crystal packing (see Supporting Information).
FIGURE 3. Molecular structure of 2‚H2O.
J. Org. Chem, Vol. 70, No. 7, 2005 2757

Sakurai et al.
FIGURE 4. Molecular structure of 5.
TABLE 1. Absorption Maxima of the Compounds 1-6
and 14-19
FIGURE 5. Cyclic voltammogram of 1 (1.0 × 10-3 M) in
MeCN (0.10 M Bu4NClO4) at a platinum working electrode
with scan rate ) 100 mV s-1 under an argon atmosphere.
SCHEME 5. Plausible Redox Processes of 1
FIGURE 6. Cyclic voltammograms of 2 and 3 (1.0 × 10-3 M)
in MeCN (0.10 M Bu4NClO4) at a platinum working electrode
with scan rate ) 100 mV s-1 under an argon atmosphere.
cesses at +0.10 and +0.44 V vs Fc/Fc+ (Figure 5). The
reduction wave indicates one-electron reduction process
at -1.91 V. The QX derivative 4 showed similar electro-
chemical behavior with the redox potentials at -2.02,
+0.06, and +0.45 V, respectively.
The results of cyclic voltammography of 2, 3, 5, and 6
are shown in Scheme 6 and Figure 6. A scan from -2.2
to +1.4 V at 100 mV s-1 revealed three reversible one-
electron redox processes and one two-electron process in
every case. The first redox potential (E1) is dependent
on both the heterocyclic unit and its position. Unsym-
metrical 3 is reduced into the radical anion at -1.95 V,
which is 100 mV lower than for symmetrical 2 (-1.85
V), and 6 (-2.05 V) is also 110 mV more readily reduced
than 5 (-1.94 V). An approximately 100 mV difference
was observed between BT and QX derivatives. In con-
trast, the next two redox processes, B f C and C f D,
are relatively independent of both the heterocyclic unit
and its position. The BT/QX series 2/3 and 5/6 possess a
similar second redox potential (E2) and the difference
LUMO gap as compared with those of 18 and 19 and
nearly the same as that of 17. The absorption maxima
of 5 and 6 were 540 and 538 nm, respectively. The
intramolecular hydrogen bond might also play an im-
portant role to lower the HOMO-LUMO gap.
4. Electrochemical Properties. The redox behavior
of 1-6 was investigated by cyclic voltammetry. The
electrochemical behavior of 1 in acetonitrile may be
explained by a redox process as shown in Scheme 5. A
scan of 1 in acetonitrile from -2.2 to +1.2 V at 100 mV
s-1 revealed two reversible one-electron oxidation pro-
2758 J. Org. Chem., Vol. 70, No. 7, 2005

p-Phenylenediamines Bearing an Electron-Acceptor Unit
SCHEME 6. Plausible Redox Processes of the Pentamers
SCHEME 7. Chemical Oxidation of 1 and 4
originated from the heterocycle BT/QX is only 30-40 mV
(2: -0.09 V, 3: -0.08 V) (5: -0.12 V, 6: -0.12 V). The
third redox potential (E3) showed a slightly similar
tendency to E1, but the positional and heterocyclic
difference is about 50 mV, which is apparently smaller
than the difference in E1. On the other hand, a distinct
positional difference was observed in the two-electron
redox process (D f E). The wave of 3 appeared at +0.35
V, which is much lower than that of 2 (+0.62 V). This
oxidation corresponds to the formation of the fully
oxidized form E. Such a lower E4 may be due to the
structure of the half-oxidized form D bearing the phen-
ylenediamine moiety, where the next-HOMO lies.
5. Chemical Oxidation. Next, the chemical oxidation
was investigated. When 1 was treated with 1.2 molar
amounts of Ag2O in THF, the (E,E)-quinonediimine
derivative 20 was obtained quantitatively and no other
isomers were detected (Scheme 7). The E,E-geometry was
preliminarily determined by NOE, which was observed
between Ha and Hb, and further confirmed by the
coupling constant 2J (13C, 15N).26 The compound possess-
ing 15N-labeled quinonediimine nitrogens was prepared
from Ph15NH2. The coupling constants of 15N with Ca and
Cb were measured as 7.46 and 2.71 Hz, respectively. This
assignment is in good agreement with the NOE results.
The same stereoisomer was selectively obtained in the
oxidation of 4 and the geometry of 21 was also deter-
mined by NOE and 2J (13C, 15N) (2JCaN ) 7,22 Hz,
2JCbN ) 1.20 Hz). Such stereoselectivity might be induced
by both kinetic and thermodynamic control. As stated in
section 2, the starting reduced forms 1 and 4 favor the
geometry in which H is faced on BT/QX and Ph locates
away from BT/QX. Therefore, the oxidation may lead to
the E,E-isomer under kinetic control. In addition, the
E-geometry may also be favored thermodynamically due
to the steric repulsion between the phenyl group and
fused heterocycle.
Judging from their oxidation potentials, the behaviors
of 2 and 3 in chemical oxidation are expected to be
different. Oxidation of 2 with an equimolar amount of
Ag2O afforded a half-oxidized form 22, which corresponds
to D in the redox cycle shown in Scheme 6. Oxidation
proceeded selectively at the BT position and 22 was
isolated as one isomer, indicating that 22 also possesses
the (E,E)-quinonediimine unit. Because of the high
oxidation potential of 22, no further oxidation proceeded
even though an excess amount of Ag2O was added under
similar conditions. On the other hand, the fully oxidized
form of 3 (24) is considered to be obtained with ease due
to the low oxidation potential. Actually, 3 was oxidized
with an equimolar amount of Ag2O to give 23, which was
further converted to 24 quantitatively by treatment with
another equimolar amount of Ag2O (Scheme 8). It is
noteworthy that 23 was also isolated as one stereoisomer,
(26) Kalinowski, H.-O.; Berger, S.; Braun, S. In Carbon-13 NMR
Spectroscopy; Wiley: Chichester, 1998; p 572.
J. Org. Chem, Vol. 70, No. 7, 2005 2759

Sakurai et al.
SCHEME 8. Chemical Oxidation of 2, 3, 5, and 6
SCHEME 9. Chemical Oxidation of B3N2 and B5N4
while 24 was obtained as a mixture of the stereoisomers
and was very susceptible to moisture. Similar behavior
should be expected in the oxidation of QX derivatives 5
and 6. Unfortunately, control of mono- or dioxidation was
difficult using Ag2O as an oxidant and isolation of pure
25, 26, or 27 failed. This is partly because the third
oxidation potentials of 5 and 6 are not so different as
compared with those of BT derivatives. Further optimi-
zation of oxidation conditions should be required to
isolate them.
There are two distinctive features in the chemical
oxidation of the benzothiadiazole and quinoxaline deriva-
tives as compared with the unsubstituted aniline oligo-
mers (B3N2 and B5N4). One is that the initial oxidation
selectively occurs at the heterocyclic position and the
other is that the configuration of the quinonediimine on
the BT/QX is E,E-geometry. This selectivity provides a
uniform oxidized product (EB stage) and makes their
chemical/physical properties tunable. In general, B3N2
is oxidized to yield a 1:1 isomeric mixture, and the
oxidation of B5N4 affords a mixture of EB1 and EB2
(Scheme 9).27 On the contrary, positional isomers 22 and
23 are independently prepared in the BT system and the
comparison suggests important information on the prop-
erties of such heterocycle-hybridized polyaniline systems.
In addition, it should be pointed out that E,E-configura-
tion on BT/QX provides the bidentate coordination sites
for transition metals. It has been reported by us12 and
other groups28,29 that the hybrid systems of synthetic
metals and transition metals are of potential as novel
catalysts and materials. The BT/QX system is expected
to develop a rational design for hybrid systems with
transition metals. Their complexation behaviors will be
described elsewhere.
The UV-vis spectra of 23 and 24 in acetonitrile were
investigated (Figure 7). The absorption maximum of 23
is 560 nm, exhibiting a HOMO-LUMO gap smaller than
that of 3 (546 nm). The absorption maximum of the fully
oxidized form 24 (500 nm) was observed at a shorter
wavelength compared to that of 3. This behavior is
similar to the feature as observed with ordinary polya-
niline, i.e., the half-oxidized form (EB in Scheme 1) shows
red-shifted maximum and the fully oxidized form (PE)
shows blue-shifted maximum.30
The electrochemical properties of the half-oxidized
forms 22/23 were also investigated. The cyclic voltam-
mograms of 22 and 23 are shown in Figure 8. The redox
potentials and calculated energy data are listed in Table
2. The first reversible redox waves at -2.04 V for 22 and
-2.08 V for 23 are assigned to the one-electron reduction
(28) (a) Pickup, P. G. J. Mater. Chem. 1999, 9, 1641. (b) Kokil, A.;
Shiyanovskaya, I.; Singer, K. D.; Weder, C. J. Am. Chem. Soc. 2002,
124, 9978. (c) Kokil, A.; Huber, C.; Caseri, W. R.; Weder, C. Macromol.
Chem. Phys. 2003, 204, 40. (d) Wang, F.; Lai, Y.-H.; Han, M. Y. Org.
Lett. 2003, 5, 4791 and references therein.
(29) (a) Kowalski, G.; Pielichowski, J. Synlett 2002, 2107. (b)
Kowalski, G.; Pielichowski, J.; Jasieniak, M. Appl. Catal., A 2003, 247,
295.
(30) Albuquerque, J. E.; Mattoso, L. H. C.; Balogh, D. T.; Faria, R.
M.; Masters, J. G.; MacDiarmid, A. G. Synth. Met. 2000, 113, 19.
(27) The related paper has been reported. See: MacDiarmid, A. G.;
Zhou, Y.; Feng, J. Synth. Met. 1999, 100, 131.
2760 J. Org. Chem., Vol. 70, No. 7, 2005

p-Phenylenediamines Bearing an Electron-Acceptor Unit
TABLE 2. UV-vis Spectra and Electrochemical Data in MeCN for 1-6, 22, and 23
compound
1
2
3
4
5
6
22
548
23
560
λmax (nm)
(eV)
522
550
546
492
540
540
2.37
-1.91
+0.10
+0.44
2.25
-1.85
-0.09
+0.18
+0.62
2.27
-1.95
-0.08
+0.13
+0.35
2.52
-2.02
+0.06
+0.45
2.30
-1.94
-0.12
+0.13
+0.57
2.30
-2.05
-0.12
+0.07
+0.44
2.26
-2.04
-1.58
-1.38
+0.16
+0.32
2.21
-2.08
-1.67
-1.38
+0.03
+0.31
E1 (V)
E2 (V)
E3 (V)
E4 (V)
E5 (V)
cated that the electrochemical properties depend on the
chain length and the position of the heterocyclic unit.
Chemical oxidation afforded only (E,E)-quinonediimine
derivatives. The UV-vis spectra showed a similar feature
as observed with ordinary polyaniline, i.e., the half-
oxidized form showed a red-shifted maximum and the
fully oxidized form showed a blue-shifted maximum.
Large positional effect on the spectroscopic and electro-
chemical properties was observed in both reduced forms
2/3 and half-oxidized forms 22/23. The present informa-
tion may permit the design of novel aniline-heterocycle
conjugate polymers.
FIGURE 7. UV-vis spectra in MeCN (0.05 mM) of 3, 23, and
24 under an argon atmosphere.
Experimental Section
Compound 1. Compound 7 (294 mg, 1.0 mmol), Pd(OAc)2
(23 mg, 0.1 mmol), DPEPhos (54 mg, 0.1 mmol), and t-BuONa
(269 mg, 2.8 mmol) were mixed in toluene (5 mL). Aniline (218
µL, 2.4 mmol) was added dropwise to the solution, which was
refluxed for 27 h. The reaction mixture was cooled and filtered
through Celite. The Celite was rinsed with toluene and the
mixture solution was concentrated. Purification by column
chromatography (10% of CH2Cl2/hexane) gave 1 as a wine-red
solid (289 mg, 91%).
Compound 2. Compound 7 (294 mg, 1,0 mmol), N-phenyl-
1,4-phenylenediamine (405 mg, 2.2 mmol), Pd(OAc)2 (23 mg,
0.1 mmol), DPEPhos (54 mg, 0.1 mmol), and t-BuONa (269
mg, 2.8 mmol) were mixed in toluene (5 mL). The reaction
mixture was refluxed for 72 h. The mixture was cooled and
filtered through Celite. The Celite was rinsed with toluene and
the mixture solution was concentrated. Purification by column
chromatography (50% of CH2Cl2/hexane) gave 2 as a dark blue
solid (275 mg, 55%).
Compound 11. Compound 7 (294 mg, 1.0 mmol), Pd(OAc)2
(11.5 mg, 0.05 mmol), DPEPhos (27 mg, 0.05 mmol), and
t-BuONa (135 mg, 1.4 mmol) were mixed in toluene (5 mL).
Aniline (109 µL, 1.2 mmol) was added dropwise to the solution,
and the reaction mixture was stirred at 80 °C for 20 h. The
reaction mixture was cooled and filtered through Celite. The
Celite was rinsed with toluene and the mixture solution was
concentrated. Purification by column chromatography (hexane)
gave 11 as a yellow solid (236 mg, 77%).
Compound 3. 4-(4′-Aminophenyl)aminodiphenylamine (138
mg, 0.5 mmol), 11 (184 mg, 0.6 mmol), Pd(OAc)2 (5.6 mg, 0.025
mmol), DPEPhos (15.3 mg, 0.028 mmol), and t-BuONa (67.3
mg, 0.7 mmol) were mixed in toluene (5 mL). The reaction
mixture was refluxed for 72 h. The mixture was cooled and
filtered through Celite. The Celite was rinsed with toluene and
the mixture solution was concentrated. Purification by column
chromatography (CH2Cl2) gave 3 as a violet solid (137 mg,
54%).
FIGURE 8. Cyclic voltammograms of 22 and 23 (1.0 × 10-3
M) in MeCN (0.1 M Bu4NClO4) at a platinum working
electrode with scan rate ) 100 mV s-1 under an argon
atmosphere.
of the thiadiazole moiety as observed with 2 and 3. The
pseudoreversible redox waves at -1.58 and -1.38 V for
22 are assignable to the reduction of the quinonediimine
moiety, while the waves for 23 were observed at -1.67
and -1.38 V. The fourth redox potential (E4) is dependent
on the position of the BT unit. Unsymmetrical 23 was
130 mV more readily oxidized than the symmetrical 22.
This behavior might be consistent with the difference in
the fourth redox potential (E4) of 2/3. Interestingly, the
redox waves corresponding to the formation of the fully
oxidized form were observed as two waves of the one-
electron process on 22/23 (E4 and E5), despite one wave
of two-electrons process for 2/3 (E4). The HOMO-LUMO
gap of 23 is smaller than that of 22, which is in good
agreement with the results of their absorption maxima
in the UV-vis spectra.
Compound 4 (by Method A). Compound 12 (288 mg, 1.0
mmol), Pd(OAc)2 (23 mg, 0.1 mmol), BINAP (62 mg, 0.1 mmol),
and t-BuONa (269 mg, 2.8 mmol) were mixed in toluene (10
mL). Aniline (218 µL, 2.4 mmol) was added dropwise to the
solution, which was refluxed for 72 h. The reaction mixture
was cooled and filtered through Celite. The Celite was rinsed
with toluene and the mixture solution was concentrated.
Conclusion
In summary, we have established the synthesis of
p-phenylenediamine derivatives bearing a benzothiadia-
zole or quinoxaline unit, in which a small HOMO-LUMO
gap system was attained. Cyclic voltammography indi-
J. Org. Chem, Vol. 70, No. 7, 2005 2761

Sakurai et al.
Purification by column chromatography (30% of CH2Cl2/
hexane) gave 4 as a wine-red solid (150 mg, 48%).
mixture was cooled and filtered through Celite. The Celite was
rinsed with toluene and the mixture solution was concentrated.
Purification by column chromatography (hexane) gave 13 as
an orange solid (174 mg, 63%).
Compound 6 (by Method A). 4-(4′-Aminophenyl)amino-
diphenylamine (138 mg, 0.5 mmol), 13 (165 mg, 0.6 mmol),
Pd(OAc)2 (5.6 mg, 0.025 mmol), BINAP (17.4 mg, 0.028 mmol),
and t-BuONa (67.3 mg, 0.7 mmol) were mixed in toluene (5
mL). The reaction mixture was refluxed for 72 h. The mixture
was cooled and filtered through Celite. The Celite was rinsed
with toluene and the mixture solution was concentrated.
Purification by column chromatography (CH2Cl2) gave 6 as a
violet solid (84 mg, 34%).
Compound 6 (by Method B). To a suspension of 3 (250
mg, 0.5 mmol) in a mixture solution of acetic acid (4.0 mL)
and deionized water (1.5 mL) was added zinc powder (485 mg,
7.5 mmol) at 60 °C. The mixture was stirred for 12 h at 70 °C,
filtered while hot, and trapped into 50% w/w of sodium
hydroxide aqueous. After being cooled in the ice bath, the
mixture was extracted with ether. The organic phase was
washed with brine and dried over Na2SO4. Evaporation of the
solvent gave the corresponding o-phenylenediamine derivative
as a pale violet solid. Without other purification, ethanol (30
mL) and then an aqueous solution of glyoxal (40%, 73.5 mg,
0.5 mmol) were added dropwise. The mixture was heated to
reflux for 16 h and cooled. The mixture solution was concen-
trated. Purification by column chromatography (CH2Cl2) gave
6 as a violet solid (27 mg, 11%).
Compound 4 (by Method B). To a suspension of 1 (318
mg, 1.0 mmol) in a mixture solution of acetic acid (4.0 mL)
and deionized water (1.5 mL) was added zinc powder (970 mg,
15 mmol) at 60 °C. The mixture was stirred for 12 h at 70 °C,
filtered while hot, and trapped into 50% w/w of sodium
hydroxide aqueous. After being cooled in the ice bath, the
mixture was extracted with ether. The organic phase was
washed with brine and dried over Na2SO4. Evaporation of the
solvent gave the corresponding o-phenylenediamine derivative
as a pale red solid. Without other purification, ethanol (30 mL)
and then an aqueous solution of glyoxal (40%, 147 mg, 1.0
mmol) were added dropwise. The mixture was heated to reflux
for 16 h and cooled. The mixture solution was concentrated.
Purification by column chromatography (30% of CH2Cl2/
hexane) gave 4 as a red-wine solid (112 mg, 39%).
Compound 5 (by Method A). Compound 12 (288 mg, 1,0
mmol), N-phenyl-1,4-phenylenediamine (405 mg, 2.2 mmol),
Pd(OAc)2 (23 mg, 0.1 mmol), BINAP (62 mg, 0.1 mmol), and
t-BuONa (269 mg, 2.8 mmol) were mixed in toluene (10 mL).
The reaction mixture was refluxed for 72 h. The mixture was
cooled and filtered through Celite. The Celite was rinsed with
toluene and the mixture solution was concentrated. Purifica-
tion by column chromatography (50% of CH2Cl2/Hex) gave 5
as a dark blue solid (114 mg, 23%).
Compound 5 (by Method B). To a suspension of 2 (250
mg, 0.5 mmol) in a mixture solution of acetic acid (4.0 mL)
and deionized water (1.5 mL) was added zinc powder (485 mg,
7.5 mmol) at 60 °C. The mixture was stirred for 12 h at 70 °C,
filtered while hot, and trapped into 50% w/w of sodium
hydroxide aqueous. After being cooled in the ice bath, the
mixture was extracted with ether. The organic phase was
washed with brine and dried over Na2SO4. Evaporation of the
solvent gave the corresponding o-phenylenediamine derivative
as a pale blue solid. Without other purification, ethanol (30
mL) and then an aqueous solution of glyoxal (40%, 73.5 mg,
0.5 mmol) were added dropwise. The mixture was heated to
reflux for 16 h and cooled. The mixture solution was concen-
trated. Purification by column chromatography (50% of CH2-
Cl2/hexane) gave 5 as a dark blue solid (42 mg, 17%).
Acknowledgment. The use of the facilities of the
Analytical Center, Graduate School of Engineering,
Osaka University is acknowledged. We thank Dr.
Toshiyuki Moriuchi for the X-ray crystallography. We
also thank Ms. Toshiko Muneishi for the help of NMR
measurement. This work was partially supported by a
Grant-in-Aid for Scientific Research on Priority Areas
(no.13128206) from the Ministry of Education, Sports,
Science and Technology, Japan.
Supporting Information Available: X-ray crystallo-
graphic data of 2 and 5 in CIF format and analytical and
spectroscopic data of all compounds described in Experimental
Section. This material is available free of charge via the
Internet at http://pubs.acs.org.
Compound 13. Compound 12 (288 mg, 1.0 mmol), Pd(OAc)2
(23 mg, 0.1 mmol), BINAP (62 mg, 0.1 mmol), and t-BuONa
(269 mg, 2.8 mmol) were mixed in toluene (5 mL). Aniline (218
µL, 2.4 mmol) was added dropwise to the solution, and the
reaction mixture was stirred for 16 h at 80 °C. The reaction
JO048324J
2762 J. Org. Chem., Vol. 70, No. 7, 2005