Structure of the intermediates in the industrial separation of perinone isomers

The industrial synthesis of perinone results in a mixed crystal containing an isomer mixture of trans- and cis-perinone. The isomers are separated by treatment with alcoholic KOH, which leads to a pale yellow precipitate and a yellow solution. The precipitate is subsequently hydrolyzed to produce pure trans-perinone (Pigment Orange 43). The yellow solution is treated with dilute acids to give cis-perinone (Pigment Red 194). The chemical structure and the stoichiometry of the intermediate pale yellow precipitate has never been determined, although this intermediate has been produced on a multi-ton scale for more than 80 years. The intermediate was assumed to be a “potassium hydroxide addition compound”. X-ray single-crystal analyses reveal that this assumption is wrong. The KOH/ethanol treatment actually causes a double ring-opening of the perinone. This results in the tetra-potassium salt of 4,8-bis(benzimidazolato)-naphthalene-1,5-dicarboxylate. Two phases were isolated: The α-phase, K4[C26H12N4O4]·3C2H5OH·6H2O, from the industrial synthesis and the β-phase, 1.5(K4[C26H12N4O4])·5C2H5OH·4H2O, from a recrystallization in KOH/ethanol. The molecular and crystal structures are confirmed by solid-state NMR spectroscopy through combination of 13C CPMAS and MAS experiments, which revealed that the solvent layers in the crystals show a highly dynamic solvent disorder in the solid state. The intermediate of cis-perinone, which is formed in the isomer separation step, has been isolated as well. Also this intermediate has a ring-opened molecular structure, as revealed by single-crystal X-ray diffraction.