On the formation of mauvein: Mechanistic considerations and preparative results

Article abstract of DOI:10.1515/znb-2009-0622

The reaction of aniline (AH₂) with an oxidizing agent in acidic solution gives rise to the formation of a mixture of products containing, besides a variety of oligoanilines named as Aniline Black or Polyaniline, Mauvein as a deeply purple phena

Full text of DOI:10.1515/znb-2009-0622

On the Formation of Mauvein: Mechanistic Considerations
and Preparative Results
Christoph Heichert and Horst Hartmann
Fakulta¨t Mathematik und Naturwissenschaften, Technische Universita¨t Dresden,
Mommsenstr. 13, 01062 Dresden, Germany
Reprint requests to Prof. Dr. Horst Hartmann. E-mail: Hartmann@iapp.de
Z. Naturforsch. 2009, 64b, 747 – 755; received March 29, 2009
Dedicated to Professor Gerhard Maass on the occasion of his 60th birthday
The reaction of aniline (AH2) with an oxidizing agent in acidic solution gives rise to the forma-
tion of a mixture of products containing, besides a variety of oligoanilines named as Aniline Black
or Polyaniline, Mauvein as a deeply purple phenazine derivative. Although this Mauvein synthe-
sis was developed by W. H. Perkin more than 150 years ago and has opened the era of industrial
dyestuff chemistry, the detailed mechanism of this reaction has remained rather unclear until today.
The elucidation of the mechanism of the Mauvein formation as an oxidative coupling process of
AH2 is hindered by the fact that several different types of coupling reactions occur simultaneously.
Among them the C,N-coupling reaction is the most important one and responsible for the formation
of Polyaniline and Mauvein. It gives rise to the formation of, e. g. 2 different aniline dimers, 7 trimers
and more than 20 tetramers including Mauvein as one representative of these tetramers.
In the present study, the oxidative coupling of a mixture of AH2 and 4-amino-4ꢀ-(N-anilino)-di-
phenylamine (TaH2) as one of the aniline trimers was studied in more detail. The reaction leads to
the formation of Mauvein in satisfactory yields and, after some manipulations, widely free of by-
products. By using simple aniline derivatives as co-reagents in the oxidative coupling with TaH , the
reaction can be extended to the synthesis of different Mauvein derivatives most of which have2been
unknown to date.
Key words: Mauvein, Phenazine Dyes, Synthesis, Mechanism
Introduction
tor of a large chemical industry in Europe and overseas
was opened [4].
Stimulated by an idea of August Wilhelm von
Hofmann, anno 1856, his eighteen years old student
Remarkably, Perkin was not the first scientist who
studied the oxidation of aniline and its derivatives, but
William Henry Perkin tried to synthesize quinine by he was the first one who produced a useful dyestuff
oxidation of crude aniline. Quinine, its chemical struc- in practicable amounts and brought it to the market.
ture known only roughly as a nitrogen-containingcom- Before him the German chemist Friedlieb Ferdinand
Runge, who isolated aniline for the time first from tar
pound and widely used at that time as an anti-malaria
drug, was available only via an elaborated way from in 1838 and provided the basis for its inexpensive avail-
the Cinchona bark [1]. Other than expected, instead of ability in larger amounts, had studied the oxidation of
aniline also and thereby developed the synthesis of an
the desired quinine, Perkin obtained in the course of
his experiments with aniline and some of its derivatives intensively colored product named Aniline Black [5].
a deeply colored reaction mixture which contained a This product was able to dye textile fibres also, but it
purple compound able to dye wool and silk in brilliant was not used practically later on due to its low dyeing
quality. Therefore, Perkin entered the modern history
ploratory study and started to produce the novel dye on as the pioneer of the dyestuff industry.
shadows [2]. Therefore Perkin decided to finish his ex-
a technical scale. Under the name Aniline Purple, Mau-
Although the procedure of Mauvein production was
vein or Mauve this product was sold immediately [3]. rather simple – a sulfuric acid solution of crude aniline
With this discovery the era of tar industry as the initia- is to treat with potassium bichromate [6] – it has sev-
c
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C. Heichert – H. Hartmann · On the Formation of Mauvein
Scheme 1.
eral malices making it difficult to handle and demands a precondition for the formation of this oxidation prod-
of the producer a great deal of skillness and engage- uct indicating that this position is essential for the re-
ment. Especially the low yields of the pure dye were actions involved.
the most critical points indicating that the essential
dye-forming process is rather complicated. It depends
to a large extent on the quality of the starting aniline
and gives better yields of product if some toluidines,
especially ortho- and para-toluidine, are admixed to
aniline.
More detailed studies on the structure of Mauvein
have demonstrated that an original technical sample
of Perkins Violet consists of a mixture of seven dif-
ferent compounds containing methyl groups at dif-
ferent positions in the parent phenazine core [11].
The methyl groups originate from the starting mate-
Although Perkin soon recognized that his Mauvein rial used, an aniline/toluidine mixture. The different
is not an uniform product but consists of two differ- Mauvein derivatives have been named, as depicted in
ent components at least, the structures of these com- Scheme 2, as Pseudomauvein, Mauvein A, Mauvein B,
pounds could not be elucidated by him correctly at Mauvein B2, Mauvein C, Mauvein C25, and Mauvein
this time. Sometimes later, however, Otto Fischer and C25b [12]. In accordance with the Chemical Abstract
Eduard Hepp as well as Rudolf Nietzki found that a Service the enumeration of the ring positions is per-
phenazine moiety is the basic structure of Mauvein [7]. formed as depicted in Scheme 2.
These chemists and few others found in the course
of subsequent studies of the aniline oxidation process
several further products derived from aniline, such as
Azophenines, Indulines, and Nigranilines [8]. More-
over, different kinds of Aniline Blacks could be pre-
pared in the meantime, and different suggestions for
their structures have been made (Scheme 1) [9]. More
recently Aniline Black received much attention as an
organic electrically conducting material [10].
Despite of a variety of different studies on the mech-
anism of the Mauvein formation, the process is not
clear in all details as yet. As indicated by ESR and
Raman spectroscopic measurements, the process sta•rt+s
with the formation of an aniline cation radical AH2
from aniline AH2 initiated by the oxidative agents ap-
•+
plied [13]. This highly reactive cation radical AH2
attacks an unchanged aniline AH2, which under the
experimental conditions is in an equilibrium with the
In respect to the Mauvein formation it is worth men- corresponding anilinium ion AH3+, giving rise to the
tioning that the use of anilines with a free 4-position is formation of different types of products. As primary
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C. Heichert – H. Hartmann · On the Formation of Mauvein
749
Scheme 2.
•+
products of the reaction of the cation radical AH2
the compounds TaH2 and QaH4 can be generated in
with the parent aniline AH2, benzidine BH2, o-amino- the course of an electrophilic attack of the imino group
diphenylamine CH2 and p-amino-diphenylamineDH2 of quinone diimine D at the para-position of AH2 or
as well as hydrazobenzene HAz could be detected, DH2 (Route a), the other compounds can be formed in
which are transformed immediately into the corre- the course of a nucleophilic attack of the amino groups
sponding oxidation products diphenquinone diimine of AH2 or DH2 at the position b or c of the quinone
B, N-phenyl-1,2-quinone diimine C, N-phenyl-1,4- diimine double bound of D (Route b or Route c, re-
quinone diimine D, and azobenzene Az, respectively spectively) [16]. The electrophilic attack of the aniline
•+
at the 4ꢀ-position of 4-amino-
(Scheme 3) [14].
cation radical AH2
diphenylamineDH2, analogous to its reaction with ani-
line AH2, has been discussed by several authors, but
can be ruled out as an alternative route for generating
TaH2 because this reaction is overruled by the redox
reaction mentioned bef•o+re. It is worth mentioning that
the radical cation DH2 formed in the course of the
synproportionation of DH2 with D was made responsi-
ble for the formation of QaH4 by dimerization during
the aniline oligomerization [17].
Although no detailed information on the relative
amounts of the formed products exists at yet, N-
phenyl-1,4-quinone diimine D seems to be the main
reaction product occurring in the course of this ox-
idative dimerization process and can be considered
as the essential precursor in the Mauvein forma-
tion [15]. It is generated by the oxidation of para-
amino-diphenylamine DH2 – probably by the attack of
the aniline cation radical AH2•+ on DH2 – and can re-
act either with the starting aniline AH2 or with its own
Unfortunately, no information on the relative
4-amino-diphenylamine precursor DH2 [16]. For these amounts of the individual compounds formed in the
reactions three different pathways may be considered reaction of D with AH2 or DH2 is available at yet.
which give rise to the formation of either the trimers Nevertheless, in respect to their structure only the men-
TaH2, TbH2, and TcH2 or of the tetramers QaH4, tioned trimers TaH3, TbH3, and TcH3 (R = H) are
QbH4, and QcH4, respectively (Scheme 4). Whereas to be considered as Mauvein precursors. By contrast,
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750
C. Heichert – H. Hartmann · On the Formation of Mauvein
Scheme 3.
the corresponding tetramers QaH4, QbH4, and QcH4 ever, the yields are rather low in this case, and the prod-
(R = C6H5NH) can be considered solely as polyaniline uct is accompanied by a variety of by-products very
precursors, of which QaH4 is the most favored prod- difficult to separate from the target compound. This
uct. In analogy to DH2, this aniline tetramer can re- is in contrast to the oxidative coupling of DH2 with
act, after its oxidation, to the corresponding quinone 3-amino-diphenylamine studied by Nietzki in 1896
diimine derivatives QaH2 or Qa, either with DH2 or which gives rise to the formation of Mauvein in sat-
with QaH4, forming oligomers with six (SH6) or eight isfactory yield [7]. It has to be pointed out, however,
(OH8) aniline moieties (Scheme 4).
that 3-amino-diphenylamine is no a direct aniline cou-
pling product and therefore has to be synthesized by a
special synthetic route.
Results and Discussion
The oxidative coupling of TaH3 (or of DH2) with
Because the ability of the aniline trimer TaH3 to aniline and several of its derivatives AiH2 was per-
function as an educt for Mauvein has not yet been stud- formed in analogy to a procedure used for the synthesis
ied, we have checked this possibility and found that of Safranine [18] by adding an eqimolar mixture of the
this trimer is a versatile starting material for preparing corresponding compounds dissolved in methanol into
the title compound in satisfactory yields. Simultane- a solution of K3Fe(CN)6 in aqueous sodium acetate
ously, we also checked the performance of the aniline (0.5 M) and refluxing the mixture until no starting ma-
dimer DH2 in the same reaction, and found that this terials could be detected by TLC. In all cases this cou-
compound can also serve as a Mauvein educt. How- pling gives rise to the formation of the corresponding
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C. Heichert – H. Hartmann · On the Formation of Mauvein
751
Scheme 4.
Mauvein derivatives Mi. Whereas in the case of TaH3 line tetramers QdH4 – QhH4 are formed among which
the corresponding Mauveins Mi were obtained in satis- only the compounds QfH4 and QgH4 are the essen-
factory yields, mostly and only accompanied with few tial educts for the Mauvein formation. Whereas these
other by-products, by starting with DH2 mainly a va- aniline tetramers are subsequently transformed into
riety of other products, such as Polyaniline as well as the corresponding quinone diimine derivatives QiH2
different types of phenazines, were formed.
(not depicted in Scheme 5), from which the Mauvein
derivatives Mi can be generated via the intermediate
To separate these by-products from the target Mau-
veins the crude product was first extracted with boil- formation of the dihydrophenazine derivatives MiH,
ing ethyl acetate and then with hot methanol. The the other aniline tetramers QdH4, QeH4 and QhH4
methanolic solution was filtered after cooling to re-
move sparingly soluble Polyaniline and treated with
are transformed into higher aniline oligomers which
contaminate the target Mauveins and make expensive
an aqueous solution of Na2ZnCl4. This procedure re- purification procedures necessary for obtaining neat
moves the paramagnetic Polyaniline impurities and products.
transforms the less soluble Mauvein acetates obtained
as primary products into better soluble tetrachlorozin-
cates which can finally be isolated as lustrous green,
bronze-colored crystals.
The structure of the Mauvein derivatives Mi pre-
pared as described above follows unambiguously from
their analytical data. Thus, all Mauveins Mi exhibit in
their mass spectra characteristic molecular ion peaks
1
It should be mentioned that the complete separation and give rise in their H NMR spectra, in accordance
of paramagnetic Polyaniline is essential to obtain sat- with other studies [11, 12], to characteristic signals at
isfactory 1H NMR spectra of the Mauvein products.
5.9, 6.3, 7.5, 8.0, 8.1, and 8.2 ppm, stemming from
From these preparative results it follows that the the protons attached to their phenazinium moieties.
dye-forming process very probably starts, as depicted Whereas most of these signals are split into doublets,
in Scheme 5, with the quinone diimine intermediates the signals at 7.5 ppm appear as double doublets. The
+
ꢀ
TaH or Ta H (or with their protonated species TaH2
doublets at 5.9 and 6.3 ppm stem from the protons at-
and Taꢀ H2+) which are generated from the TaH3 tached to the 6- and 4-position of the phenazine moi-
educt by oxidation. Immediately after their forma- ety. These protons are significantly shielded by the ring
tion the appropriate aniline derivatives AiH2 are added current of the N-phenyl moieties at N(5), as can be
1
at their quinone diimine moieties. Thereby the ani- seen, e. g. by comparing the H NMR spectral data
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752
C. Heichert – H. Hartmann · On the Formation of Mauvein
Scheme 5.
of the Mauveins Mi with the corresponding data of 9- These absorptions are not much influenced by the sub-
methyl-2,7-bis(methylamino)phenazinium perchlorate stituents R at the N-phenyl moiety of the phenazine
which exhibits a low-field singlet at 6.70 ppm for the core and are responsible for the purple color of all
protons at C(1) and C(9) [19]. According to X-ray Mauvein derivatives.
diffraction [20] and gas phase electron diffraction stud-
ies [21] or DFT calculations [22] these phenyl rings
are twisted from the phenazine plane with an angle of
about 45◦. Finally, the signals of the protons at the N-
linked phenyl groups are found as multiplets between
7.1 and 8.1 ppm.
In summary, the previous considerations as well
as the experimental data suggest that the ani-
line trimer N-(4-aminophenyl)-Nꢀ-phenyl-benzene-
1,4-diamine (TaH2) is an essential intermediate in the
Mauvein formation process. This compound is able, af-
ter transformation into the corresponding quinone di-
All Mauveins Mi exhibit, as can be seen in Fig. 1 imine derivative Ta, to react with aniline at one of
for the parent Mauvein Ma as an example, deep colors its C-C double bonds to yield four different aniline
originating from intense absorptions at about 550 nm. tetramers QeH4, QfH4, QgH4, and QhH4, of which
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C. Heichert – H. Hartmann · On the Formation of Mauvein
753
the literature by reaction of N-phenyl-1,4-phenylene diamine
with 4-fluoronitrobenzene in DMSO [23]. Yield 65 %; m. p.
139 – 140 ◦C (ref. [23a]: 132 – 134 ◦C; ref. [23b]: 139 ◦C).
b) Preparation of N-(4-aminophenyl)-Nꢀ-phenyl-benz-
ene-1,4-diamine (TaH2): Into a solution of N-(4-nitro-
phenyl)-Nꢀ-phenyl-benzene-1,4-diamine (1.53 g, 5,0 mmol)
in acetic acid (15 mL) Zn dust (2.61 g, 40 mmol) was added
in small portions with stirring at r. t. After the color of the
reaction mixture had turned to yellow-brown (after about
3 min) methanol (40 mL) was added. Then the resulting
slurry was filtered by suction through a pad of celite, the filter
cake was washed with methanol (10 mL), and the combined
filtrates were neutralized with methanolic KOH (0.25 M).
The resulting filtrate was used directly for the Mauvein syn-
thesis given below.
Fig. 1. Absorption spectrum of Pseudomauvein (Ma), mea-
sured in methanol (c = 10−5 mol L−1).
Preparation of Mauveins (Mi). General procedure
only the second and third act as Mauvein educts,
whereas the other ones and the trimeric precursors
TbH2 and TcH2 are responsible for the other prod-
a) Oxidative coupling: To a boiling solution of K3Fe-
(CN)6 (10.86 g, 33 mmol) in 0.5 M aqueous sodium ac-
ucts formed. Alternatively, the quinone diimine deriva- etate (200 mL) a solution of N-(4-aminophenyl)-Nꢀ-phenyl-
benzene-1,4-diamine (TaH2, 5 mmol) in methanol, followed
by the appropriate aniline derivative AiH2 (7.5 mmol) dis-
solved in methanol (5 mL), was added. The resulting mix-
ture which turned immediately to deep purple was refluxed
for 3 h. After addition of methanol (150 mL), refluxing was
continued for further 30 min. The hot mixture was filtered
by suction and the filter cake washed with methanol (50 mL)
until the filtrate was pale purple. The filtrate was evaporated
in vacuo and the residual aqueous phase left standing over
tive Ta can react with its precursor TaH2 analogously
to yield different types of aniline tetramers, such as
QaH4, QbH4, QcH4 and QdH4, which are obviously
unable to act as Mauvein precursors.
To get a more deeper insight in the Mauvein for-
mation process and also, alternatively, into the pro-
cess of the Aniline Black formation, it seems neces-
sary to estimate implicitly the actual concentrations of
all the intermediates formed as well as their reactivi-
ties and different reaction pathways under the experi-
mental conditions applied. Although for such investi-
gations the required physicochemical methods, espe-
cially spectro-electrochemical methods, are generally
◦
night at 20 C. The raw Mauvein precipitated was collected
by filtration, washed with water (50 mL) and ethyl acetate
(50 mL), and dried at 120 ◦C.
b) Purification of the raw material: The solid residue of
step a) was first triturated and digested with boiling ethyl
well-established, they unfortunately have not been em- acetate (2 × 75 mL) to remove impurities and then refluxed
◦
with methanol (200 mL) for 2 h. After cooling to 20 C, the
mixture was filtered through a fine pore paper filter and the
residual filter cake washed with methanol until the filtrate
was pale purple. The combined filtrates were added to a re-
fluxing solution of Na2[ZnCl4] (12.63 g, 50 mmol) dissolved
in 0.02 M HCl (100 mL). The mixture obtained was con-
centrated in vacuo at r. t. and the residual aqueous phase left
standing over night at 20 ◦C. The solid product was filtered,
washed with 0.02 M HCl (10 mL), water (25 mL) and ethyl
acetate (50 mL), and dried at 120 ◦C.
ployed at yet.
Experimental Section
General information
The following instruments and analytical techniques were
used: Melting points: Kofler hot-stage microscope, cor-
rected; NMR: Bruker 500 MHz spectrometer DRX 500P;
[D6]DMSO as solvent; δ in ppm (assignment); UV/Vis:
Perkin Elmer spectrometer Lambda 900; methanol as sol-
vent. MS: Bruker Esquire MS with Ion Trap detector and
ESI-Source, measurement parameters: fragmentation volt-
age: 1 – 200 V, corona voltage 140 – 1800 V.
Thus, the following Mauvein derivatives were obtained in
black-green, bronze micro-crystals.
Preparation of N-(4-aminophenyl)-Nꢀ-phenyl-benzene-1,4-
diamine
Bis-(3-amino-5-phenyl-7-phenylamino-5-phenazinium)
tetrachlorozincate (Ma)
◦
1
a) The preparation of the N-(4-nitrophenyl)-Nꢀ-phenyl-
Yield 40 %; m. p. 288 – 290 C. – H NMR: δ = 5.92 (d,
benzene-1,4-diamine precursor was performed according to J = 2.0 Hz, 1H), 6.25 (s, J = 2.0 Hz, 1H), 7.13 (m, 3H), 7.32
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C. Heichert – H. Hartmann · On the Formation of Mauvein
(m, 3H), 7.50 (dd, J = 2.0 Hz, J = 9.2 Hz, 1H), 7.64 (m, 2H), 1H), 7.56 (m, 2H), 8.04 (d, 1H), 8.08 (m, 2H), 10.12 (d, J =
7.75 (m, 1H), 7.81 (m, 2H), 8.00 (d, J = 9.2 Hz, 1H), 8.08 3.8 Hz, 1H, NH). – UV/Vis: λmax (logε) = 548 nm (5.01). –
(m, 3H), 10.16 (s, 1H, NH). – UV/Vis: λmax (logε) = 549 nm MS: m/z = 393.2 (calcd. 393.46 for C25H21N4O).
(5.01). – MS: m/z = 363.2 (calcd. 363.43 for C24H19N4).
Bis-[3-amino-5-(3-methoxypheny)l-7-phenylamino-5-
phenazinium] tetrachlorozincate (Me)
Bis-[3-amino-5-(4-chlorophenyl)-7-phenylamino-5-
phenazinium] tetrachlorozincate (Mb)
◦
Yield 26 %; m. p. 248 – 250 C (dec.). – 1H NMR: δ =
◦
Yield 16 %; m. p. 226 – 230 C (dec.). – 1H NMR: δ =
3.83 (s, 3H, CH3O), 5.99 (d, J = 2.2 Hz, 1H), 6.32 (d, J =
2.2 Hz, 1H), 7.17 (m, 4H), 7.31 – 7.36 (m, 5H), 7.48 (dd, J =
2.2 Hz, J = 9.3 Hz, 1H), 7.73 (m, 1H), 8.00 (d, J = 9.3 Hz,
1H), 8.09 (d, J = 9.3 Hz, 1H), 8.11 (s 1H, NH), 10.16 (s, 1H,
NH). – UV/Vis: λmax (logε) = 549 nm (5.04). – MS: m/z =
393.2 (calcd. 393.46 for C25H21N4O).
5.93 (d, J = 1.9 Hz, 1H), 6.23 (s, J = 1.9 Hz, 1H), 7.16 (m,
3H), 7.34 (m, 3H), 7.52 (dd, J = 1.9 Hz, J = 9.2 Hz, 1H),
7.72 (d, J = 8.6 Hz, 2H), 7.94 (d, J = 8.6 Hz, 2H), 7.99
(d, 5H), 8.00 (d, 1H, J = 9.3 Hz, 1H), 8.08 (d, J = 9.3 Hz,
1H) 8.12 (s, 2H, NH), 10.17 (s, 1H, NH). – UV/Vis: λmax
(logε) = 550 nm (5.06). – MS: m/z = 397.2 (calcd. 397.88
for C24H18N4).
Bis-[2-amino-5-(2-methoxyphenyl)-7-phenylamino-5-
phenazinium] tetrachlorozincate (Me)
Bis-[3-amino-5-(4-methylpheny)l-7-phenylamino-5-
phenazinium] tetrachlorozincate (Mc)
◦
1
Yield 41 %; m. p. 223 – 225 C. – H NMR: δ = 3.73 (s,
3H, CH3O), 5.97 (d, J = 2.3 Hz, 1H) 6.28 (d, J = 2.3 Hz,
1H), 7.08 – 7.16 (m, 3H), 7.26 – 7.36 (m, 4H), 7.47 (dd, J =
2.2 Hz, J = 9.3 Hz, 1H), 7.50 (dd, J = 2.2 Hz, J = 9.3 Hz,
1H), 7.73 (t, 1H), 7.97 (d, J = 9.3 Hz, 1H), 8.9 (d, J = 9.3 Hz,
1H) 8.16 (s, 2H, NH), 10.19 (s, 1H, NH). – UV/Vis: λmax
(logε) = 550 nm (5.02). – MS: m/z = 393.2 (calcd. 393.46
for C25H21N4O).
Yield 31 %; m. p. 273 – 275 ◦C (dec.). – 1H NMR (in
CD3NO2): δ = 2.56 (s, 3H, CH3, 6.21 (s, 1H, NH), 6.23 (d,
J = 2.2 Hz, 1H) 6.50 (d, J = 2.2 Hz, 1H), 7.21 (t, 1H), 7.26
(d, J = 7.5 Hz, 2H), 7.36 – 7.42 (m, 5H), 7.58 (dd, J = 2.2 Hz,
J = 9.3 Hz, 1H), 7.65 (d, J = 8.2 Hz, 2H), 8.02 (d, J = 9.3 Hz,
1H), 8.07 (d, J = 9.3 Hz, 1H), 8.15 (2H, NH). – UV/Vis: λmax
(logε) = 548 nm (5.05). – MS: m/z = 377.2 (calcd. 377.46 for
C25H21N4).
Bis-[3-amino-5-(4-nitrophenyl)-7-phenylamino-5-
phenazinium] tetrachlorozincate (Mf )
Bis-[3-amino-5-(4-methoxyphenyl)-7-phenylamino-5-
phenazinium] tetrachlorozincate (Md)
◦
Yield 2 %; m. p. 177 – 180 C. – 1H NMR: δ = 5.92 (s,
1H), 6.26 (s, 1H), 7.15 (m, 3H), 7.30 – 7.36 (m, 3H), 7.49 (d,
1H), 7.64 (d, 1H), 7.81 (m, 2H), 7.99 – 8.10 (m, 3H), 10.14
(s, 1H, NH). – UV/Vis: λmax (logε) = 549 nm (5.03). – MS:
m/z = 409.2 (calcd. 408.43 for C24H18N5O2).
◦
Yield 19 %; m. p. 263 C. – 1H NMR: δ = 3.89 (s, 3H,
CH3O), 5.99 (d, J = 2.2 Hz, 1H), 6.34 (s, J = 2.2 Hz, 1H),
7.17 (m, 3H), 7.35 (m, 4H), 7.50 (dd, J = 2.2 Hz, J = 9.3 Hz,
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