Pyranthrone

In 1901, mercury cataly2ed a-sulfonation of anthraquinone was discovered, and this led to the development of the chemistry of a-substituted anthraquinone derivatives (a-amino, a-chloro, a-hydroxy, and a,a -dihydroxyanthraquinones). In the same year R. Bohn discovered indanthrone. Afterward flavanthrone, pyranthrone, and ben2anthrone, etc, were synthesi2ed, and anthraquinone vat dyes such as ben2oylaniinoanthraquinone, anthrimides, and anthrimidocarba2oles were also invented. These anthraquinone derivatives were widely used to dye cotton with excellent fastness, and formed the basis of the anthraquinone vat dye industry. 

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... 

Scheme 4.7 Syntheses of the polycyclic anthraquinones indanthrone (53), pyranthrone (55a) and flavanthrone f55b)...

An alternative route to pyranthrone, involving baking 1,6-dibenzoylpyrene (6.96) with aluminium chloride, was also devised by Scholl (Scheme 6.18). The Scholl reaction is a key step in the synthesis of several polycyclic quinones the cyclisation of 1-benzoylnaphthalene to give benzanthrone (6.73) has already been mentioned. The mechanism of this cyclodehydrogenation reaction may involve an initial protonation step, if traces of water are present, or complexation with aluminium chloride. Electrophilic substitution is thereby... 

Commercial attention focuses on the derivatives of the pyranthrone molecule at a varying level of halogenation. Most are orange but others exhibit a dull medium to bluish red shade. Due to their good weatherfastness pyranthrone pigments are used for high grade industrial finishes. 

All polycyclic pigments, with the exception of triphenylmethyl derivatives, comprise anellated aromatic and/or heteroaromatic moieties. In commercial pigments, these may range from systems such as diketopyrrolo-pyrrol derivatives, which feature two five-membered heteroaromatic fused rings (DPP pigments) to such eight-membered ring systems as flavanthrone or pyranthrone. The phthalo-cyanine skeleton with its polycylic metal complex is somewhat unique in this respect. 

Most pigments derived from vat dyes are structurally based on anthraquinone derivatives such as indanthrone, flavanthrone, pyranthrone, or dibromoan-thanthrone. There are other polycyclic pigments which may be used directly in the form in which they are manufactured. This includes derivatives of naphthalene and perylene tetracarboxylic acid, dioxazine (Carbazole Violet), and tetrachloro-thioindigo. Quinacridone pigments, which were first introduced in 1958, and recently DPP pigments have been added to the series. 

This class includes polycarbocyclic compounds which are at least formally derived from the anthraquinone structure. The products are considered members of the higher condensed carbocyclic quinone series, which even in the absence of additional substituents provide yellow to red shades. Halogenation is frequently found to afford cleaner shades and improved fastness properties. Heading the list of such derivatives are pyranthrone, anthanthrone, and isoviolanthrone pigments. 

Pyranthrone pigments are related to the pyranthrone structure (99), which is formally derived from flavanthrone (Sec. 3.7.3.2) in that the nitrogen atoms are replaced by CH groups. 

Manufacture of the unsubstituted compound 99 (Pigment Orange 40, 59700) has recently been discontinued. Other commercially available pyranthrone pigments include bromo, chloro, or bromo/chloro derivatives of the parent structure. 

Pyranthrone is commonly prepared by Ullmann reaction of 1 -chloro-2-methyl-anthraquinone (100), followed by double ring closure. 

Heating 101 with aqueous sodium hydroxide in diethyleneglycol monomethyl-ether or with alkali acetate, a reaction which may also be performed in other polar organic solvents, such as dimethylformamide, N-methylpyrrolidone, or di-methylacetamide at 150°C to 210°C, also provides pyranthrone. As an alternative, 101 may be cyclized to form pyranthrone in a two-phase reaction, carried out in the presence of quarternary ammonium salts in a phase-transfer system comprising an aqueous and an organic phase [20]. 

Halogenated pyranthrones may be obtained by two different routes. One route proceeds via ring closure of halogenated 2,2 -dialkyl-1,1 -dianthraquinonyl, in accordance with the synthesis of pyranthrone. The same end may be accomplished by halogenation of ready-made unsubstituted pyranthrone. 

Pyranthrone may be halogenated, for instance, in chlorosulfonic acid in the presence of small amounts of sulfur, iodine, or antimony as a catalyst. This procedure necessitates intermediate separation and purification of pyranthrone after manufacture, because the products, unless purified, fail to furnish the solvent fastness which is characteristic of a typical pigment. 

While 3,3 -dichloro-2,2 -dimethyl-1,1 -dianthraquinonyl is easily converted into well-defined 6,14-dichloropyranthrone, direct halogenation affords changing amounts of chloro and bromo derivatives with undefined substitution patterns on the pyranthrone ring. Depending on the amount of added halogen, an average of two to four halogen atoms per molecule is expected. 

An opaque form of pyranthrone may also be prepared by treating the corresponding pigment in a polar organic solvent (such as isobutanol) at elevated temperature in the presence of some (0.5 to 10%) halogenated anthraquinone or another anthraquinone derivative. 

In addition to 6,14-dibromopyranthrone, there are some mixed halogenated pyranthrone derivatives which exhibit equally useful pigment properties. 

Commercially Available Pyranthrone Pigments and Their Application... 

Until recently also some of the halogenated pyranthrone types had been offered to the market. Currently the producer has ceased their production and does not list them in his sales catalogues anymore. The pigments, however, are still available on the market, giving reason to furthermore characterize their application and fastness properties in this book. 

P.R.216 lends color to all types of industrial paints, it is heat stable up to 200°C. Like other pyranthrone pigments, it is suitable for use in unsaturated polyester systems, in which it is resistant to peroxides. 

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