Phenazine

Phenazine (C₁₂H₈N₂ or C₆H₄N₂C₆H₄), also called azophenylene, dibenzo-p-diazine, dibenzopyrazine, and acridizine, is a dibenzo annulated pyrazine and the parent substance of many dyestuffs, such as the eurhodines, toluylene red, indulines and safranines.

Synthesis

A classic method for the synthesis of phenazine is the reaction of nitrobenzene and aniline in the Wohl-Aue reaction (1901). Other methods are known, e.g.:

• by distilling the barium salt of azobenzoate
• by passing aniline vapor over lead oxide
• by the oxidation of dihydrophenazine, which is prepared by heating pyrocatechin with o-phenylenediamine.
• It is also formed when ortho-aminodiphenylamine is distilled over lead peroxide.

Phenazine crystallizes in yellow needles which melt at 174-177 °C, and are only sparingly soluble in alcohol. Sulfuric acid dissolves it, forming a deep-red solution.

Derivatives

• The more complex phenazines, such as the naphthophenazines, naphthazines and naphthotolazines, may be prepared by condensing ortho-diamines with ortho-quinones or by the oxidation of an ortho-diamine in the presence of α-naphthol, and by the decomposition of ortho-anilido-(-toluidido- et cetera)- azo compounds with dilute acids.
• If alkyl or aryl-ortho-diamines be used, azonium bases are obtained. The azines are mostly yellow in color, distill unchanged and are stable to oxidants. They add on alkyl iodides readily, forming alkyl azonium salts, anhydride formation also taking place between these hydroxylgroups. It dissolves in concentrated sulfuric acid with a yellowish-green fluorescence.
• The rhodamines, which are closely related to the phthaleins, are formed by the condensation of the alkyl metaaminophenols with phthalic anhydride in the presence of sulfuric acid. Their salts are fine red dyes. By the entrance of amino or hydroxyl groups into the molecule dyestuffs are formed. The mono-amino derivatives or eurhodines are obtained when the arylmonamines are condensed with orthoamino azo compounds; by condensing quinone dichlorimide or para-nitrosodimethyl aniline with monamines containing a free para position, or by oxidizing ortho-hydroxydiaminodipbenylamines. They are yellowish-red solids, which behave as weak bases, their salts undergoing hydrolytic dissociation in aqueous solution. When heated with concentrated hydrochloric acid the amino group is replaced by the hydroxyl group and the phenolic eurhodols are produced.

Diaminophenazine

The symmetrical diaminophenazine is the parent substance of the important dyestuff toluylene red or dimethyldiaminotoluphenazine. It is obtained by the oxidation of orthophenylene diamine with ferric chloride; when a mixture of para-aminodimethylaniline and meta-toluylenediamine is oxidized in the cold, toluylene blue, an indamine, being formed as an intermediate product and passing into the red when boiled; and also by the oxidation of dimethylparaphenylene diatnine with metatoluylene diamine. It crystallizes in orange-red needles and its alcoholic solution fluoresces strongly. It dyes silk and mordanted cotton a fine scarlet. It is known commercially as neutral red. For the phenazonium salts, see safranine. Phenazone is an isomer of phenazine, to which it bears the same relation that phenanthrene bears to anthracene.

Natural products

Many phenazine compounds are found in nature and are produced by bacteria such as Pseudomonas spp., Streptomyces spp., and Pantoea agglomerans. These phenazine natural products have been implicated in the virulence and competitive fitness of producing organisms. For example, the phenazine pyocyanin produced by Pseudomonas aeruginosa contributes to its ability to colonise the lungs of cystic fibrosis patients. Similarly, phenazine-1-carboxylic acid, produced by a number of Pseudomonas, increases survival in soil environments and has been shown to be essential for the biological control activity of certain strains.

Biosynthesis

Phenazine biosynthesis branches off the shikimic acid pathway at a point subsequent to chorismic acid. Two molecules of this chorismate-derived intermediate are then brought together in a diagonally-symmetrical fashion to form the basic phenazine scaffold. Sequential modifications then lead to a variety of phenazine with differing biological activities.

References