Methine Type Isoindoline Pigments

More recent publications describe a variety of isoindoline pigments with different chemical structures. Such species comprise an isoindoline ring attached to two methine bridges. 

These pigments have the following general chemical structure  

R1 through R4 represent CN, CONH-alkyl, or CONH-aryl. R1 and R2 on the one hand and R3 and R4 on the other hand can be also members of a heterocyclic ring system. 

Examples, such as 47 type pigments, have been described by BASF and CibaGeigy [15,16]  

Likewise, suitable starting materials are iminoisoindolines, especially diimino-isoindoline (l-amino-3-iminoisoindolenine), which reacts with a cyanoacetamide NCCH2CONHR5 to afford a mono-condensation product 49. Further reaction with a compound containing an activated methylene group (such as cyanoacetamide derivatives, barbituric acid) yields the desired pigment 47  

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Pigment Yellow 139 (24) is a methine-type isoindoline that has a red shade. 

The methine type comprises isoindoline derivatives which, like tetrachloro-isoindolinone pigments, have only recently been described. One or usually two equivalents of a compound containing an activated methylene moiety are attached to one equivalent of isoindoline. The list of compounds featuring activated methylene groups includes cyanacetamide or heterocycles such as barbituric acid or tetrahydroquinolinedione. 

In the pigment molecules, the substituents are attached to the isoindoline ring either through a carbon bond (methine type) or through a nitrogen bond (azo-methine type). 

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