Indanthrone and Flavanthrone Pigments

In 1901, R. Bohn synthesized indanthrone and flavanthrone. Both compounds are thus among the oldest synthetic vat dyes known. 

Vat dyes of the highest quality have derived their name from indanthrone, a blue compound, which was originally known as Indanthrene Blue. The compound has been used for a long time as a pigment and is registered in the Colour Index as Pigment Blue 60, 69800 (91). 

The primary synthetic route proceeds via oxidative dimerization of 2-aminoan-thraquinone in the presence of an alkali hydroxide. 2-aminoanthraquinone, for instance, is fused with potassium hydroxide/sodium hydroxide at 220 to 225°C in the presence of sodium nitrate as an oxidant. New techniques involve air oxidation of 1-aminoanthraquinone at 210 to 220°C in a potassium phenolate/sodium acetate melt or in the presence of small amounts of dimethylsulfoxide. A certain amount of water which is formed during the reaction may be removed by distillation in order to improve both efficiency and yield. 

One out of several possible options for the synthesis of indanthrone from 2-aminoanthraquinone is illustrated in the following scheme  

The reactions involve a variety of preliminary equilibria, which open up a number of different pathways. Therefore, the reaction conditions during indanthrone synthesis must be followed exactly [14,15]. 

---