Flavanthrone, production

The syntheses of three polycyclic anthraquinones, indanthrone (53), pyranthrone (55a) and flavanthrone (55b), are illustrated in Scheme 4.7. In spite of the structural complexity of the products, the syntheses of these types of compound are often quite straightforward, involving, for... 

The times of half-reduction of five typical vat dyes are listed in Table 3.3. Most of the commercially important products give values within the range 25-500 seconds. Few dyes are as slow to reduce as Cl Vat Red 1 and even fewer are reduced as quickly as flavanthrone. In fact, when these times were measured the average particle size of the sample of flavanthrone under test was greater than that of the Cl Vat Green 1 sample, which was reduced at least ten times more slowly [26]. 

The formation of indanthrone and flavanthrone, as well as alizarin, during the alkali fusion of 2-aminoanthraquinone can be explained mechanistically on the basis of the initial loss of a proton. The resulting anionic species can be represented as a resonance hybrid and is also tautomeric (Scheme 6.12). Primary 1-hydroxylation of 2-aminoanthraquinone is probably the first step in the formation of the alizarin by-product (compare Scheme 6.8). Such an attack may initiate the formation of flavanthrone [31 ]. It is also possible to envisage the formation of all three species by a radical mechanism [32]. 

P.Y.179, an isoindolinone/cobalt complex pigment, was introduced to the market only few years ago but the production has recently been discontinued. It was recommended for use in paints, especially in automotive finishes. The pigment produces a reddish yellow shade. High lightfastness and excellent weatherfastness are an asset in pastel colors. Besides, good transparency made P.Y.179 a suitable product for metallic finishes. Yet, it is not quite as weatherfast as the equally reddish yellow P.Y.24, a flavanthrone pigment. 

Flavanthrone, like indanthrone, must be extremely pure in order to develop useful pigment properties. Subsequent finishing converts the thus prepared material into an appropriate product for use in paints or plastics. 

Converting the crude product into its leuco form, separating the intermediate, and reoxidizing the compound to form pure flavanthrone. 

Flavanthrone Yellow, together with its chemical structure, is listed in the Colour Index under Constitution No. 70600. It was temporarily known as Pigment Yellow 112, but now it is exclusively referred to as Pigment Yellow 24. Since some time sales products of P.Y.24 are not listed anymore in the catalogues of the manufacturers, but the grades are still available on the market. 

A typical use of 61 is, by fusion with potassium hydroxide at high temperature, in the production of flavanthrone (67) (Scheme 15). Fusion of 61 with potassium hydroxide under milder conditions affords the important anthraquinone azines known as indanthrones (Travis Chapter 1). The main vat-dye reactions proceed via anthrimides, which are both dyes and intermediates, since they are readily converted into cyclic carbazole derivatives. For example, benzanthrones, such as 68 and 69, dibenzanthrones, isodibenzanthrones, and their derivatives and substitution products, are important in vat-dye manufacture. An example of their use is in the synthesis of Cl Vat Olive T (70) (Scheme 16). Benzanthrone acridones made from 1-aminoanthraquinones are important green dyes. Certain vat-dye reactions require high-boiling organic solvents. 

Chloroanthraquinone is especially useful as a base material for the production of 2-aminoanthraquinone, from which the vat dye indanthrene (Vat Blue 4) is produced by a method discovered by Rene Bohn in 1901, involving a reaction in alkaline solution with atmospheric oxygen in the presence of potassium nitrate at 150 to 200 °C. At higher temperatures and using antimony-(V)-chloride or aluminum chloride, flavanthrone (Vat Yellow 1) is obtained this is nowadays produced from l-chloro-2-aminoanthraquinone and PA. 

---