DPP pigments

The formation of a DPP molecule was first reported in 1974 as a minor product in low yield from the reaction of benzonitrile with ethyl bro-moacetate and zinc. A fascinating study by research chemists at Ciba Geigy into the mechanistic pathways involved in the formation of the molecules led to the development of an efficient one-pot synthetic procedure to yield DPP pigments from readily available starting materials, as illustrated in Scheme 4.10. The reaction involves the treatment of diethyl succinate (1 mol) with an aromatic cyanide (2 mol) in the presence of a strong base. The reaction proceeds through the intermediate 88, which may be isolated and used to synthesise unsymmetrical derivatives. 

Furo[3,4-c]pyrrolediones are important intermediates in the synthesis of diketo-pyrrolopyrrole (DPP) pigments. Smith and coworkers have described the preparation of several different 3,6-diaryl-substituted furo[3,4-c]pyrrole-l,4-diones by microwave-assisted cyclization of readily available 4-aroyl-4,5-dihydro-5-oxo-2-arylpyrrole-3-carboxylates (Scheme 6.192) [353]. While conventional heating in Dowtherm A at 230-240 °C for 64 h provided only moderate product yields, microwave irradiation of the neat starting material at 250 °C for 10 min provided significantly increased yields. 

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. 

These studies were followed up by Ciba-Geigy and systematically developed into a synthetic route to a group of very fast red pigments, known as diketo-pyrrolopyrrole pigments (DPP pigments) [3, 4], since the brilliant red crystals of 141 turned out to be extremely insoluble. 

DPP pigments are synthesized by reacting succinic ester with benzonitriles in the presence of alcoholate in the corresponding alcohol for base catalysis. Originally starting from sodium methylate/methanol an important step toward a significantly improved yield was achieved by reaction of succinic tert.-alkylester in sodium tert.-alkylalcoholate/tert.-alkylalcohol. 

Another path affording novel pigments is the mixed synthesis yielding solid solutions. The mixed synthesis of two DPP pigments can result in solid solutions, which consist of the two symmetrically and the unsymmetrically substituted DPP pigment. Solid solutions can also be obtained from DPP pigments and quin-acridone pigments. 

Within the range of DPP pigments presently offered to the market, P.R.254 plays the most important role. The pigments differ in their substituents and position at the phenyl ring. The p-position is the preferred one, at present only one pigment with a substituent in the m-position is commercially available. 

Some years ago the DPP pigment has been introduced to the market. It produces medium orange shades and is recommended mainly for the use in plastics and printing inks. Special grades for these applications are on the market. 

P.R.264 provides bluish red shades, which are similar to those of P.R.177, an aminoanthraquinone pigment. The fastness properties are similar to the other types of the DPP pigment class. The commercially available grade is distinguished... 

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