Synthetic studies related to diketopyrrolopyrrole (DPP) pigments. Part 3: Syntheses of tri- and tetra-aryl DPPs

Novel synthetic methodologies leading towards 2,3,5-triaryl- and 2,3,5,6-tetraaryl-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-diones (tri- and tetra-aryl-DPPs) and their derivatives have been investigated. Direct arylation of 3,6-diphenyl-DPP was possible using 1-fluoro-2,4-dinitrobenzene. Acylation of ethyl 2-aryl-4,5-dihydro-5-oxopyrrole-3-carboxylates with N-arylbenzimidoyl chlorides in the presence of a strong base gives the novel 2,3,6-triaryl-DPPs together with the corresponding uncyclised enamines. A new and simple method for the synthesis of ethyl 1,2-diaryl-4,5-dihydro-5-oxopyrrole-3-carboxylates has led to an alternative route to triaryl-DPPs via reaction with benzonitrile under basic conditions, and combination of this with the benzimidoyl chloride methodology has enabled the synthesis of variously substituted 2,3,5,6-tetraphenyl-DPPs.