“…The orange color of the P-1,2-PDA dispersion in 1 M HCl (Fig. 9a) suggests a well-defined chemical structure of the oxidation products composed of a dimer, 2,3-diaminophenazine (20), and oligomers (21). The color changes to yellow shades after deprotonation to the corresponding base in alkaline medium.…”
Section: Optical Spectramentioning
confidence: 95%
“…Oxidation of 1,2-phenylenediamine leads to oligomeric product at best (8,20,21), although electrochemical oxidation has been reported to produce polymers (22,23). The other two phenylenediamines, 1,3-PDA and 1,4-PDA, yield materials of limited solubility in organic solvents, but viscometry (24) and chromatographic analysis (8) of soluble parts proved that these products are of polymeric character.…”
Section: Oxidation Of Phenylenediaminesmentioning
confidence: 99%
“…The mixture was stirred during oxidation to avoid temperature gradients. The same experimental set-up, comprising a beaker containing 100 mL of the reaction mixture placed on a magnetic stirrer at room temperature (20)(21)(22)(23) • C), was used to ensure the same surface-to-volume ratio and comparable heat loss in all experiments.…”
“…The orange color of the P-1,2-PDA dispersion in 1 M HCl (Fig. 9a) suggests a well-defined chemical structure of the oxidation products composed of a dimer, 2,3-diaminophenazine (20), and oligomers (21). The color changes to yellow shades after deprotonation to the corresponding base in alkaline medium.…”
Section: Optical Spectramentioning
confidence: 95%
“…Oxidation of 1,2-phenylenediamine leads to oligomeric product at best (8,20,21), although electrochemical oxidation has been reported to produce polymers (22,23). The other two phenylenediamines, 1,3-PDA and 1,4-PDA, yield materials of limited solubility in organic solvents, but viscometry (24) and chromatographic analysis (8) of soluble parts proved that these products are of polymeric character.…”
Section: Oxidation Of Phenylenediaminesmentioning
confidence: 99%
“…The mixture was stirred during oxidation to avoid temperature gradients. The same experimental set-up, comprising a beaker containing 100 mL of the reaction mixture placed on a magnetic stirrer at room temperature (20)(21)(22)(23) • C), was used to ensure the same surface-to-volume ratio and comparable heat loss in all experiments.…”
In view of the contradictory results published in the literature concerning the structure of the indoloquinoxalines obtained by condensation of isatin with an unsymmetrical benzene-1,2-diamine, regioselective syntheses of the 1-, 2-, 3-, and 4-aminoindolo[2,3-b]quinoxalines have been developed. With the four derivatives in hand, isomers obtained by direct cyclocondensations starting from amino or nitro substituted benzene-1,2-diamines could be unambiguously identified. The results of these studies show, on the one hand, that some previous findings need revision and, on the other hand, that the 1-amino isomer could be prepared in one step and in 58% yield.The most classical pathway for the synthesis of indolo [2,3-b]quinoxalines consists in the cyclocondensation of an indole-2,3-dione (1) (isatin) with a benzene-1,2-diamine. When unsymmetrical diamines are used (3-or 4-substituted diamines 5 or 2), the condensation may obviously lead to the HETEROCYCLES, Vol. 75, No. 11, 2008 2745 production of structural isomers i.e. a mixture of the 1-and 4-substituted indoloquinoxalines 6-7 in the first case and of the 2-and 3-isomers in the later one 3-4 (cf. Table 1). Some authors claimed that the results can be rationalized according to the mechanism of the cyclocondensation: the more nucleophilic amino group of the phenylenediamine reacts with the more electrophilic 3-carbonyl group of isatin and a subsequent dehydratation between the second amino group and the lactamic carbonyl affords the tetracycle. 2-4 However, several contradictory examples have been published in the literature.Syntheses starting from benzene-1,2-diamines 2 substituted in the 4-position by various substituents, have been claimed to give only the 2-substituted indoloquinoxalines (3, R = NO 2 4 , Cl 5 , F 6 , CF 3 6 , CH 3 7 ), only the 3-substituted isomer (4, R = Cl 8 , COOH 8 ), one isomer whose structure was not specified (R = COOH 9 , NO 2 2 , F 10 , CF 3 11 ) or a mixture of both derivatives (R = OCH 3 12 , CN 13 , OEt 9 ). Drushlyak et al. claimed to have obtained only the 3-nitroindoloquinoxaline (4a) in a three step pathway: condensation of the 4-nitrobenzene-1,2-diamine (2a) with the N-acetylisatin to give the 3-(2-acetylaminophenyl)-7nitroquinoxalin-2-one, deacetylation and subsequent cyclization in boiling acetic acid. 4Under the same manner, from the corresponding 3-substituted benzene-1,2-diamines 5, the preparation of indoloquinoxalines bearing a substituent in the 1-position 6 and in the 4-position 7 has been published (R = halogen 12 , OCH 3 14 , COOH 15 , COOCH 3 13 ).Structural assignments, usually only based on elemental analyses and, in two cases, on melting points 4,9are not convincing. Furthermore, on most cases, when a mixture of isomers was known to be obtained, they were not separable. As an exception, the 1-and 4-methyl esters have been isolated after flash column chromatography on silica gel but have not been unambiguously identified. 13 Regardless, it was not possible to assign structures from 1 H or 13 C NMR spectra to the ...
“…In the Bamberger–Ham reaction (path c) nitrosobenzenes dimerize under acidic conditions to form phenazines [ 7 ]. Other methods are the condensation of ortho -phenylenediamines with ortho -quinones (path d) [ 8 ], reaction of benzofuroxanes and phenols (the Beirut reaction, path e) [ 9 ], and palladium-catalyzed cyclization of 2-amino-2′-bromophenylenediamines (path f) [ 10 ]. …”
Anilines react with 5-nitroindoles in the presence of t-BuOK in N,N-dimethylformamide (DMF) to form 5-nitroso-4-arylaminoindoles that in turn when treated with N,O-bis(trimethylsilyl)acetamide cyclize to pyrrolo[3,2-a]phenazines. In an alternative approach pyrrolo[3,2-a]phenazines are formed from aminoindoles and nitroarenes.Graphical abstract
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