S. Hiinig' W'irzburg University 87 WErzburg, West Germany
Oxidative Coupling: A New Approach to Azo Chemistry
In 1954 part of our group started to investigate the oxidative coupling reactions of certain hydrazine derivatives. Since then the field has been widely explored, the general character of the reaction has been well established, and a great variety of unusual azo compounds have become available (1). These facts will justify the attempt to review our research. All the results I am going to discuss are due to the excellent co-operation of my students, who are mentioned in the footnote. Their endurance, skill, and imagination enables me to present this material. Development of the Principal Idea
dyes (3). The unique character of the Nietzki-Fischer reaction is demonstrated by the fact that even today the most important processes of color photography are based on it. , No doubt the ability to couple in such a manner is due to a special structural feature of the p-phenylenediamine, I. This feature might be the arrangement of two amino groups (one of which is primary) attached to the ends of two (formal) double bonds. If this is correct, the insertion of one nitrogen atom into the ring together with the removal of one methine group from the ring, according to VI, should lead to a molecule of similar activity.
At the very beginning we became fascinated by the oxidative coupling of p-phenylenediamines, I, with naphthols, 11, and anilines, 111,reactions discovered by Nietzki in 1877 (2).
Since pyridone methides are sensitive to acid, the methine group is replaced by nitrogen thereby forming Nmethylpyridone-4-hydrazone, VII. Because of the known qualitative equivalence of 2- and Psuhstituents in pyridine derivatives, compound VIII should exhibit properties similar to those of I X in which heteroatoms represent one double bond. Actually all of these snbstances undergo smooth oxidative coupling in complete analogy to the p-phenylenediamines (la). For example, the hydrazone VII combines with a-naphthol oxidatively to form the violet dye XI, which can he
These reactions are remarkable in different respects: Since four electrons and either four or three protons must he abstracted, the reaction must involve more than one step. Nevertheless, both the indaniline, IV, and the indamine, V, dyes are formed smoothly and very rapidly at room temperature. This behavior enabled Fischer in 1911 to combine photographic development with the oxidative formation of indaniline 'Based on eontrihutions of H. Balli, E. Breither, W. Brenninger, F. Briihne, K. H. Fritsch, H. Geiger, E. Grigat, M. Herbert, H. Hermann, G. Kaupp, W. Kniese, G. Kbbrieh, W. Lampe F. Miiller. H. Nether. K. H. Oette. H. Quast, R. D. Rausohenh a c h , ~ : ~ c h i i t zand , H. Werner.
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classified as a diazamerocyanine. N-methyl-benzthiazolone-hydrazone, X, first described by Besthorn (4) in 1910, is extremely suitable for these oxidative coupling reactions. With dimethylaniline, 111, it. yields the intensely colored blue-violet azo quaternary ion XII. Ions of this type cannot be obtained directly by normal azo coupling. Variation of the Hydrazone Structure
From the latter, diaza-trimethine cyanines (e.g., XXI) are produced. Asymmetric azines, such as XX, can also serve as coupling components when oxidized together with the hydrazone X X in acidic solution. The intensely colored blue-green product XXIII belongs to the hitherto unknown class of tetraaza-pentamethinecyanines ( I b),
The large number of variations that are possible with the hydrazone structure, already typified by VII-X is further illustrated by examples XIII-XVIII. According to XIII-XV, the five-membered heterocycle may contain from two to four nitrogen atoms, XVI shows that sulfur instead of nitrogen may serve as a heteroatom, while XVII demonstrates that even a lack of aromaticity in the heterocycle is tolerated. Furthermore, the hydrazone group can even be attached to an open chain carbon atom XVIII. ,CH"
H&V-~~ N-N
N-i%
By this reaction 0.01 fig of formaldehyde can be detected on the spot plate if X and an oxidizing agent are added (5). Already in 1910 Besthorn had obtained a crystalline dye by heating the azine XXII with ferric chloride in a strong solution of hydrogen chloride, thereby discovering the oxidative coupling of heterocyclic hydrazones unintentionally. However, he made no suggestions concerning either the nature of the reaction or the structure of the product (4). Mechanism of Oxidafive Coupling of the Hydrozones ( I )
XVII n=4,6,6
Consequently, the necessary strnctural requirements for. this coupling can be summarized as the amidrazone systems represented by XIX.
\
I
l
l
N+C=C,+C=N-NHI / XIX
n = 0,l
Variation of the Coupling Component (lo, b)
-
Besides the above mentioned phenols and aromatic amines, reactive methylene compounds (e.g., malononitrile XX) or certain types of enamines (e.g., 1,3,3trimethyl-2-methylene-indolenine) can also be used.
Polarographic methods disclose that the hydrazones in question, XXIV, lose two electrons and one proton on oxidation thereby forming the electrophilic intermediate XXV which could be the active coupling species (la). However, polarography would not detect even a few percent of XXVI, the higher oxidized form. The hypothetical quaternary diazonium ion, XXVI, would he very much more electrophilic than XXV and thus would dominate the reaction. The two possible reaction paths can be distinguished by introducing either of the two phenols XXVII or XXX, both of which have the ortho positions blocked. XXVII and XXX differ in the replacement of the p-hydrogen atom by fluorine. Since fluorine can only leave the molecule as an anion, formation of dye XXIX cannot result from the reaction between XXVI and XXX even if the intermediate XXXII would develop. Actually, both phenols XXVII -and X X X couple oxidatively with XXIV, yielding the same amount of dye XXIX under identical conditions. Depending upon the nature of the different para-substituents, four oxidation equivalents will be consumed in the sequence XXIV 4 XXVII -+ XXIX, as opposed to XXIV + XXXI XXIX in which only
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I
R XXIV -2F
xxvm 3
xxv XXM
2c -Hf
XXXI
R XXX
I
R
R
R
XXVI
XXXII
two are required. This experiment rules out the intermediate XXVI and establishes XXV as the coupling species ( I b ) , in full accord with the oxidative coupling of thc p-phenylendiamines, where XXXIII has been shown to be the active species (6).
X X X ~ Self-coupling of the Hydrozones ( l b )
l e d tetraacetate or If an oxidizing agent, such iodine, is added in one portion to a fairly concentrated acidic solution of the hydrazone X, a violet color develops. From the rapidly fading solution a black-violet perchlorate could be isolated which, by analysis, spectroscopic comparison, and degradation experiments, @ 1 "N-BH
-
+
~+N
