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Heterocyclic Azo Dyes by Oxidative Coupling.

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Heterocyclic Azo Dyes by Oxidative Coupling [l]
Dedicated to ProJ Dr. 0. Bayer on the occasion of his 60th birthday
New findings in the field of oxidative. coupling reactions are reported. These involve: extension of this type of reaction to amidrazone systems displaying (vinylogous) tautomerism
and to unsymmetrical diarylhydrazines; elucidation of the coupling mechanism for these
arylhydrazines and for the amidrazone systems previously described; the properties of some
dyes, complexed with heavy metals, and the synthesis of tetraazapentamethine and pentaazapentamethine dyes.
A. Introduction
Four years ago, a novel synthesis of azo dyes, some
of which were unobtainable and others only indirectly
obtainable via conventional azo coupling, was reported
under the same title as the present article [3].
The principle upon which this reaction is based represents an extension of the-familiar oxidative coupling of
p-phenylenediaminewith aromatic amines, phenols, and
reactive methylene compounds, whereby, in a completely
general fashion, the p-phenylenediamine is replaced by
compounds possessing the amidrazone system (I) or its
vinylogue (2). These compounds capable of undergoing
coupling reactions will be designated herein after as
Both the hydrazone and the coupling component can be
varied within wide ranges, as demonstrated in the
following examples [*I :
543 mp [41
[I] XXIII. Communication in this series. XII. Communication,
S. Hiinig and F. MiiNer, Liebigs Ann. Chem. 651, 89 (1962).
[2] Address: Chemisches Institut der Universitat Wiirzburg,
Rontgenring 1 1 , Wiirzburg, Germany.
[3] S. Hiinig et al., Angew. Chem. 70, 215 (1958).
[4] S. Hiinig and G. Kobrich, Liebigs Ann. Chem. 617, 216 (1958).
[*] All optical data reported in this paper refer to solutions in
methanol, unless stated otherwise.
- 4 €I@’
= 500 mp [61
Since the appearance of the paper mentioned above, the
number of hydrazones suitable for coupling and the
number of dyes synthetized with their aid have greatly
increased. In the present paper, only those results will
be discussed which have led to extensions of the oxidative coupling principle and to an understanding of the
course of reaction.
B. Oxidative Coupling with Amidrazones
Capable of Tautomerism
As seen in the examples above, the heterocyclic nitrogen
atom of the “hydrazones” always carries an alkyl or
aryl group to stabilize the hydrazone form. As was
stressed previously [3], hydrogen in the same position
leads to more extensive isomerization, and hence to
ready elimination of nitrogen upon oxidation [7].
[5] S . Hunig and K. H. Fritsch, Liebigs Ann. Chem. 609, 143
[6] S. Hiinig and F. Miiller, Liebigs Ann. Chem. 651, 73 (1962).
[7] E. Thielepape and 0 . Spreckelsen, Ber. dtsch. chem. Ges. 55,
2929 (1922).
Angew. Chem. internnt. Edit. 1 Vol. I (1962) ] No. 12
It was to be expected that, on incorporating an amidrazone system into a heterocycle with a lower tendency
to aromatize, oxidative coupling would still occur,
provided tautomerization is not blocked. This proves
true with 2-hydrazinobenzothiazole,1 1 % of which is
converted into the expected azo dye (3) with u-naphthol
using potassium ferricyanide at pH 9 181 [*I.
+ H*c-N--N-
\A -s
~ / I L ~ N -CH-N.=N-s--l
N ~
(5) R
= H,
= 515, 615
(6) R = N-O~,amax= 605,640 my
Yields (determined optically)
R = H, 11 %; R = NOz, 13 %; R = NHCOCH3,O %
Substituents on C-6 of the benzothiazole structure that
lower the basicity of the heterocyclic nitrogen atom
should stabilize the hydrazone form and thereby
incre-se the coupling yield; the reverse should be true
of substituents that increase its basicity. Actually,
under identical reaction conditions, with 6-nitrohydrazinobenzothiazole the yield of dye increases to
73 %, whereas the 6-acetamino derivative forms no dye
at all [S]. With amidrazone compounds without aromatizable rings, coupling of the types capable of tautomerizing was also to be expected. Nevertheless, it was
The structure of (7) has been proved independently.
2-Hydrazinobenzothiazole condenses with triethyl orthoformate to give the hydrazine (9) [*], which can
be dehydrogenated easily to yield the dye (7) [lo]. The
42 %
surprising that the strongly basic compound aminoguanidine, underwent a smooth reaction, to give the
dye (4), which has a long-wave absorption, in striking
contrast to the N-alkyl and N-aryl derivatives [6].
reaction of 2-hydrazinobenzothiazoles is thus closely
parallel to the formation of cationic tetraazapentamethinecyanines, such as (IO) [3,11], which were obtained
by Besthorn as early as 1910, although he was unable to
establish their structures [12].
C. Tetraampentamethine Dyes
On alkaline, oxidation of 2 -hydrazinobenzothiazole in
the presence of traces of formaldehyde, the solution
becomes intensely purple; with 6-nitro-2-hydrazinobenzothiazole, a deep blue solution is formed, from
which acid precipitates a red dye [8]. This reaction,
which has since been discovered independently by Sawicki,who developed it into a sensitive test for aliphatic
aldehydes [9], leads to the anions (5) and (6) of the
tetraazapentamethine dyes (7) and (8).
[8] E. &either, Diploma Thesis, Universitlt Marburg 1957.
[*] The tautomerism of the dye will not be discussed.
[9] E. Sawicki and Th. Hauser, Analytic. Chem. 32, 1434 (1960).
Angew. Chem. internnt. Edit. J VoI. I (1962) 1 No. I2
(10) R = H,
= 670.630 mp (acetone)
[*I The position of the H-atoms in the chain has not been proved.
[ 101 F. Briihne. Ph:D. Thesis, Universitlt Marburg 1960.
[Ill S. Hiinig and K . H . Fritsch, Liebigs Ann. Chem. 609, 172
[I21 E. Besthorn, Ber. dtsch. chem. Ges. 43, 1519 (1910).
64 1
This reaction, too, has recently been expanded by Sawicki
into an extremely sensitive means of detecting aliphatic
aldehydes [13] and even aldehyde-2,4-dinitrophenylhydrazones [14].
It seemed desirable to consider the partially alkylated
dyestuff (12) for comparison as a connecting link
between (5) and (10). In this case also, the oxidative
coupling follows the scheme:
tetraazapentamethinecyanines is excluded, this phenomenon may be attributed to the formation of pentaazapentamethinecyanines,which were first isolated as
purple dye salts after oxidation of N-methylbenzothiazol-Zone hydrazone hydrofluoroborate with lead
tetraacetate in weakly acidic methanolic solution [ 151.
On chromatographic purification, however, both the
symmetrical dyes (5) and (10) appear in addition,
hence, a transfer of the formaldehyde residue must have
taken place in the solution:
(IS) R = OCH3, A,,
= 596 mp. o = 59 200(acetoNtrile)
It is thus conceivable that (12) is (also) formed via the latter
two components. The ease of exchange of the aldehyde
group raises the question whether the formation of tetraazapentamethinecyanines should be classified as an oxidative
coupling at all. For example, N-methylbenzothiazol-2-one
hydrazone could add onto the formaldehyde derivative ( ] I ) ,
to form (13), which then undergoes dehydrogenation.
(14) R=H,AmaX= 552rnw,~=55500(acetonitrile)
This path is improbable, since the long-wave extinction of a
solution of (11) in acetic acid or ethanol is not depressed on
addition of N-methylbenzothiazol-2-one
hydrazone, but, on
the contrary, is slightly enhanced [lo].
Both the anionic dye (5) and the cationic dye (10)
belong to the fully symmetrical cyanine type. Their
longest absorption bands lie in the same region and
display high molar extinctions. The absorption maxima
[9,11,13], which are dependent on the solvent and the
temperature, are probably due to geometric isomerism
of the azamethine chain. As is to be expected the absorption maximum undergoes a hypsochromic shift in
the transition to the uncharged, unsymmetricil ,,cyanine
base” (12), with simultaneous decrease of the molar
The structure of these alkali-sensitive dyes, which contain the
longest chain of nitrogen atoms with conjugated double
bonds so far known, can be derived from their smooth
reductive cleavage with Tic13 in acetonitrile. Two equivalents
of reducing agent are consumed, and the dye is decolorized.
The two cleavage products, viz. the original hydrazone and
the azidinium salt (16) - a representative of a class of
substances discovered by Balfi [16] - can both be dearly
identified. In contrast, the pentaazamethinecyanines are
formed by nucleophilic addition of hydrazones onto azidiniun
salts, whereby the excess azidinium salt, which is known to
be reduced with great ease [16], acts as dehydrogenating
agent. This synthesis can be generalized to produce (unsymmetrical) dyes by using other heterocycles [15].A further
proof for the constitution of the new dyes is provided by their
thermal decomposition : triazatrimethine- and monoazamonomethinecyanines (17) and (18) are formed with elimination of nitrogen.
D. Pentaazapentamethinecyanines
A fugitive purple to blue coloration is observed on
oxidation of many hydrazones in the absence of a
coupling component: Provided that the formation of
It is surprising that the oxidative linking of two heterocyclic hydrazone mclecules gives a compound containing an azamethine chain with an odd number of nitrogen
atoms. Perhaps this circumstance is due to the reaction
1131 E. Sawicki, Th. R. Hauser. Th. W. Stanley, and W. Elbert,
Analytic. Chem. 33, 93 (1961).
[14]E. Sawicki, Th. R . Hauser, and F. T. Fox, Analytica chim.
Acta 26, 229 (1962).
[15] S. Hlinig, H. Balli, and H. Quast, Angew. Chem. 74, 28
[16]H. BaZIi, Angew. Chem. 70,442 (1958);Liebigs Ann. Chem.
647,11 (1961).
Angew. Chem. internat. Edit.
Vol. 1 (1962) 1 No. 12
of the active entitj of the oxidative coupling (see Section
F), e.g. (19), with excess hydrazone as coupling
= ,S;
(25): X = -CH-CH--;
(26): X
Table I. Optical data for the dyes j24)-(26), and their 1: 1
complexes with heavy metals [I91
Bathochromic effect due
Free dye
1'1 Amax
to complex formation
The investigation was extended to the quinoline derivative (25) and the benzimidazole derivative (26), to
vary the heterocycle's basic ity [191. The bathochromic
effect increases in all cases in the sequence Zn2+<Ni2+
<Cu2+, which at the same time also corresponds to the
general series of increasing complex stability [20]. Ag+
and Hgz+, as expected, came only slight color shifts,
since these ions necessitate linearly arranged ligands
[21], and hence chelation does not result in an energy
increase. The Cu2+complexes have been more thoroughly investigated and, according to results obtained by
the method of Job [22], exist in the case of the weakly
basic benzothiazole derivative as the 1:I complex (27),
The adduct (20) initially formed is again substituted by
the electrophile (19) in weakly acidic medium to give
(21), which, after dehydrogenation and cleavage of Nmethylbenzothiazol-2-oneimine (22), yields the pentaazamethine dye (14) [17]. The ruptule of an N-N bond
in the fashion described is reasonable: at pH 6-9, a
solution of N-methylbenzothiazol-2-onehydrazone
with potassium ferricyanide, evolves only 45-50 %
nitrogen [18]. The main product is not the azine (23),
expected to be formed according to the equation:
but surprisingly, is the imine (22), along with 10 % of
the azine (23) [17]. As a result, only 1.2 equivalents of
oxidizing agent are consumed.
The formation of the imine might be due to an intramolecular redox reaction of the dimer (20) - an assumption
which remains to be investigated with isotopic nitrogen.
E. Metal Complexes of Diazamerocyanines
As described earlier, assisted by the quinoline nitrogen
atom, the diazamerocyanine (24) forms deeply colored
complex salts with heavy metals in acid solutions [3,5].
[17] H . Quast, Diploma Thesis, Universitat Marburg 1961.
(181 H. BaZfi, Ph.D. Thesis, Universitat Marburg 1956.
Angew. Chem. internat. Edit. 1 Vol. 1 (1962)
1 No. 12
and in the cme of the stroligly basic quinoline and
benzimidazole derivitives as the 1:2 complex (28) [*I.
The 1:2 complex formerly assumed [5] in the benzothiazole dye (24) can be produced only under spccial
coliditions (Im,,
= 630 mp) [23]. These complex salts
belong to the few types which exist only in the acid
range (PH 1 to 7).
[19] E. Grigat, Ph.D. Thesis, Universitgt Marburg 1959.
[20] H . Irving and R. J. P. Williams, Nature (London) 162, 746
[21] G. Schwarzenbach, Experientia Suppl. V (1956).
(221 P. fob, Ann. Chimie (10) 9, 118 (1928); (11) 6,97 (1936); cf.
W . C. Vasburgh, A. R . Cooper, and R . K. Gould, J. Amer. chem.
SOC.63, 437 (1961).
[*I The number of solvent ligands is unknown. The formulae
show only one resonance structure.
[23] H. Klamberg, Personal Communication.
Table 1 shows that, even in the basic state, the length of the
C=O bond, and thereby also the negative character of the
carbonyl oxygen atom both increase in the series (24)
through (26). This explains the transition to 1 :2 complexes
with (25) and (26). All three dyes display positive solvatochromism, which means that the nonpolar azino skeleton
structure becomes increasingly important [24]. The cyaninelike bond distribution increases with increasing C=O bond
length, i. e. the excitation energy decreases in the series (24)
to (26) in the color-bound system, which latter can be
treated like a disturbed electron gas [25].
A comparison of the disturbance of the x-electron system
caused by complex formation is made possible by using
solutions in methanol of the three dyes, which contain metal
ions in vast excess. Under these conditions, (24), (25), and
(26) all form 1:1 complexes, the spectra of which display the
bathochromic shifts shown inTable 1. The magnitude of the
effect is thus determined by the polarizability of the dye, and
not by its polarity. This means that the effect decreases in
the sequence (24)>(25)>(26) for all three cations of Table
1. For the same reason, the difference between the bathochromic effects of the strongly active Cu2+ ion and the weakly
active Zn2+ ion decreases in the same sequence.
this ratio becomes 1 :2. The dehydrogenation step thus
seems to be sufficiently well ratified.
However, the polarogram does not exclude the possibility
that the ratio could be 2 : 4 instead of 1:2, i.e. at least a
small portion of the hydrazone could be raised to the very
high-energy oxidation stage (32), which is doubtlessly an
extremely active diazonium coupling ion and which could
hence effect the coupling in its entirety. The choice of an
appropriate coupling component permits a decision to be
made. Oxidation stage (31) can also couple with phenols
bearing a para-substituent X, which is able to undergo
anionic cleavage, whereby the dye (35) is formed directly,
according to the reaction scheme above. Oxidation stage
1I + Y - 0 - 5 0
F. Mechanism of the Reaction
The arguments presented earlier [3] already indicated
that the oxidative coupling reaction of the “hydrazones”
proceeds analogously to the same reaction with p phenylenediamines [26]. Thereby, two equivalents of
oxidizing agent are consumed, and the mesomerismstabilized oxidation stage (31)
(32) of the diazonium ion cannot react in this manner. In its
case, coupling can only take place by elimination of a psubstituent (Y)as a cation. In accordance with this, the 4fluoro-2,6-diethylphenol(36) tested did not couple with p nitrobenzene diazonium salt. In contrast, oxidative coupling
yields even more dye than with 2,6-diethyIphenol (37) [28].
is reached, which then reacts by electrophilicsubstitution
with the coupling components to give the leuco dye,
which can be dehydrogeilated easily:
Thus, a total of four equivalents of oxidizing agent are
consumed. The pH dependence of the polarographic
hah wave potential of several hydrazones confirms this
concept [27]. The ratio of transferred protons to transferred electrons is 1 :1 within the pH range in which the
hydrazone is present in the cationic form (29) ;however,
at higher pH, i. e. starting from the free hydrazone (30),
[24] Cf., for example, S . Hiinig and 0 . Rosenthal, Liebigs Ann.
Chem. 592, 161 (1955); K. Dimrutit, S.-B. Ges. Beford. ges. Naturwiss. Marburg 76, issue no. 3, 3 (1953).
1251 H . Kuhn, 2. Elektrochem. angew. physik. Chem. 53, 165
[26] J. Eggers and H . Frieser, Z . Elektrochem. Ber. Bunsenges.
physik. Chem. 60, 372 (1956); I . Eggers, ibid. 60, 987 (1956);
L. K. J. Tong and M . C. Glesmann, J. Amer. chem. SOC.78, 5827
(1956); 79, 583, 592 (1957).
[27] S. Hiinig and H . Balli, Liebigs Ann. Chem. 628, 56 (1959).
The reaction course by way of the diazonium stage (32) is
thus eliminated, and the formulation via the “diazeniumstage” (31) gains in importance [281.
G. Oxidative Coupling of N,N-Diarylhydrazines
a) The Resulting Dyes
The converse applicptionof the phenylogue principle to
the p-phenylenediamines and of the vinylogue principle
to amidrazones ( I ) leads to hydrazines, which have
1281 S. Hiinig, H . Balii, H . Nother, and H. Geiger, Liebigs Ann.
Chem. 628, 75 (1959).
Angew. Chem. internat. Edit. J Vol. I (1962) J No. 12
been subjected to oxidative coupling as dialkylhydrazine
(38), alkylarylhydrazines (39), and diarylhydrazines
The reaction, which in most features closely follows
that of the classic model (see below), requires discussion
at tnis point, although the coupling products shall no
longer be considered as azo dyes ill the strict sense.
Oxidative coupling of diarylhydrazines gives dyes in
yields of I0 to SO%, since the concurrent coupling
between two identical molecules to give tetrazenes [31]
sometimes becomes the main reaction.
Although no coupling products can be isolated with
(38) and (39), unsymmetrical diarylhydrazines, such as
(41) and (43), react with both phenols and aniines,
yielding the expected dyes after the oxidative coupling:
b) Mechanism of Dyestuff Formation
The identification of the dialkyldiazenium ion (46),
which originates from oxidation of unsymmetxical dialkqlhydrazines in acid solution [32], and the analogy
of the oxidative coupling of unsymmetrical diarylhydrazines to that between p-phenylenediamines and
hydrazones of type (I) and (2) would also seem to
The phenol dyes, such as (42), are quinone monohydrazones, while the cationic amine dyes are derived
from quinone diimmoniumions [29], and are therefore
particularly sensitive to hq drolysis
Even 1-phenyl-A2-pqrazoline, which can be considered
as an intramolecular aldehyde hydrazone and which is
known to exert coupling activity toward diazonium
salts [30], can be linked oxidatively to unsymmetric
diarylhydraziaes. As can be recognized from the resonance structures (45a) and (45b), the dye salt corresponds to an unsymmetrical 1,3-diazatrirnethinecyanine
dye with terminal heteroatoms which are not part of a
(45b). =,A,
558 m p
The oxidative coupling principle haq thus beeit extended
to cover a class of compounds that only remotely
resemble their classic model, the p-phenylenediamines.
[*I Only one resonance structure is drawn in each case.
[29] S. Hiinig and P. Richters, Chem. 91, 442 (1958).
[30] G. F. Duffin and J. D. Kendall, J. chem. SOC.(London) 1954,
Angew. Chem. internat. Edit. 1 Vol. I (1962)
1No. 12
indicate the existence of similar intermediate stages
here, For example, 1,l-diphenylhydrazine (47) should
first be dehydrogenated to the mesomeric diazenium ion
(48), which wodd then substitute the coupling component (e.g. diphenylamine) electrophilically, and then
the leuco dye (49) would immediately be dehydrogenated to give the dye cation (50).
\e -
The dye (50) was isolated by Wieland as early as 1910,
after oxidation of (47) in acid solution, without an
addition of diphenylamine [33]. The reaction course
proposed by him after a careful study [34] is, however,
different. According to Wieland, diphenylhydroxylamine (51), obtained from diphenylhydrazine via an
unknown path, reacts with diphenylhydrazine to give
[31] E. Fiscker, Liebigs Ann. Chem. 190, 167 (1878).
[32] W. R. McBrideand H. W. Kruse, J. Amer. chem. SOC.79,
572(1957); W. H. Urry, H. W.Kruse, and W. R . McBride, ibid.
79, 6568 (1957); W. R . McBride and E. M. Bens, ibid. 81, 5546
[33] ff. Wieland and E. Wecker, Ber. dtsch. chern. Ges. 43, 3260
[34] H. Wieland and A. Roseen, Ber. dtsch. chem. Ges. 45, 494
the hypothetical triazane (52), which is rearranged by
acid to the leuco dye (49), and is finally dehydrogenated
to yield the red dye salt (50).
ing between paths A and B is furnished by the reaction
of tetra-p-tolylhydrazine(55) and diphenylhydrazine in
acid solution [lo]:
Actually the dye is formed from diphen>lhydrazineeven
in the absence of an oxidizing agent, when diphenylhydroxylamine or tetraphenylhydrazine (53) are added
[34]. The cleavage by acid of (53) into diphenylamine
and diphenylhydroxylamine was already known [35].
The "enormous reactivity" of diphenylhydroxylaminein
acid solution is unquestionably based on the formation
of a diarylazenium cation (54), which is familiar from
diverse examples [36} and which must also appear as a
primary product in the cleavage of tetraarylhydrazines.
The question is therefore to decide whether (54) reacts
(through A) with diphenylhydrazine to give the symmetrical triazane (52) first, or whether it acts (trough B) as
a strong dehydrogenating agent and gives rise to the
coupling diphenyldiazenium ion (48) directly.
+ Ph/N--NHz
\- - -/
According to H. Wieland the unsymmetrical triazane
(56) is tc be expected in this case. Acid cleavage of (56)
would produce both the diphenyldiazeniumcation (48)
and the di-p-tolyldiazenium cation (57). Thus, a path
for the formation of two dye salts would be opened: to
(50) by way of (49), and to (59) by way of (58). However, actually only the diphenyl dye (50) is formed [*].
Oxidative formation of dyes therefore is based on the
reaction principle of oxidative coupling, which encompasses a surpiisingly wide range of applicability. Tbis
principle is a variant of electrophilic substitution of aromatic compounds and reactive methylene compounds ;
the attacking partner is not formed by all acid-base reaction - as for sulfonation or nitration - but instead by
-F Ph/N=NH
Even if the hypothetical triazane (52) were to be
formed, its proton-catalyzed rearrangement would have
to be channelled by way of C to D and lead to the
coupling path proposed above. A means of differentiat1351 H. WieIand and St. Gamberjan, Ber. dtsch. chem. Ges. 39,
1501 (1906).
[36] Cf. K. H. Meyer and W.Reppe, Ber. dtsch. chem. Ges. 54,
327 (1912).
I wish to thank the persons named at the heading of this
article for their individual contributions to the investigations described above as well as W. Brenninger for his
skilled assistance. My special thanks for the generous
promotion of this work program are expressed to the
Fonds der Chemischen Industrie, and the Deutsche Forschungsgemeinschaft; both institutions also supported
some co-workers with stipends. Sincere thanks are also
due to the Badische Anilin und Sodafabrik for additional
Received, July 30th, 1962
[A 233157 IE]
[*] The joint oxidation of diphenylhydrazine and di-p-tolylhydrazine leads, as expected, to a mixture of the dyes (50) and
(59).The rate of coupling of (48) and (57) is therefore of the
same order of magnitude.
Angew. Chem. internat. Edit. f Vol. I (1962) f No. I2
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