Low-Temperature Route to 1,2-Benzoylenebenzimidazole Ladder Structure

Abstract

Low-temperature catalytic conversion of N-(o-aminophenyl)phthalamic acid (I) was studied as a model reaction for the synthesis of ladder poly(aroylenebenzimidazole)s. The treatment of I with acetic anhydride/pyridine or trifluoroacetic anhydride resulted in the selective and quantitative cyclodehydration to yield either imide or isoimide structures respectively. The imidization was accompanied by acylation of the ortho amino group to form lateral acetamide or trifluoroacetamide substitutes. Thermodynamically unstable N-(o-trifluoroacetamidophenyl)phthalisoimide (III) underwent secondary cyclization to yield ladder 1, 2-benzoylenebenzimidazole (V) between 130 and 150 °C. The conversion of this reaction did not exceed 35% because it competed with the thermal isomerization of III to the stable N-(o-trifluoroacetamido-phenyl)phthalimide (IV). The cyclization of N-(o-trifluoroacetamidophenyl)phthalisoimide (III) was found to be possible even at room temperature. The formation of 30–35% of the ladder 1, 2-benzoylene-benzimidazole (V) was observed after III was dissolved in DMF and stored at room temperature for 4–10 h. This also was accompanied by the catalytic isoimide-to-imide isomerization. The obtained data may be useful for the further development of novel low-temperature approaches to the synthesis of ladder poly(aroylenebenzimidazole)s. Synthesis of aromatic polyimides and polyisoimides with lateral alkylamide or trifluoroalkylamide from available aromatic dianhydrides and tetraamines may also be of practical interest.