SYNTHESIS OF 3,5-DIBROMO- AND 2,4,5-TRIBROMO-1,8-NAPHTHALIMIDES

3,5-Dibromo-1,8-naphthalic acid and 2,4,5-tribromo-1,8-naphthalic acid were synthesized by oxidation of 1,5,7-tribromoacenaphthylene and 3,5,6-tribromoacenaphthylene by sodium dichromate in anhydrous acetic acid. The obtained naphthalic acids were heated for 2 hours at 110 °C, at which the acids were completely converted to corresponding naphthalic anhydrides with the overall yields of 53% and 45% respectively. Anhydrides were crystallized from acetic anhydride to form white needle crystals with melting points 207.5-208 °C and 222-223 °C for 3,5-dibromo-1,8-naphthalic anhydride and 2,4,5-tribromo-1,8-naphthalic anhydride respectively. 3,5-Dibromo-1,8-naphthalimide and 2,4,5-tribromo-1,8-naphthalimide were obtained by ammonolysis of corresponding anhydrides with aqueous ammonia solution. Formation of naphthalimide cycle takes place in several stages. At the first step ammonia molecule attacks the electrophilic carbon atom of carbonyl group with opening of the anhydride cycle following with proton migration from nitrogen to oxygen atom to form the monoamide of 1,8-naphthalenedicarboxylic acid. Further, the nitrogen atom of the amide group attacks the carbon atom of the carboxylic group to form a six-membered imide ring with cleaving the water molecule. Thus, the first stage of ammonolysis was carried out at room temperature to avoid the ammonia evaporation, the further cyclization reaction was carried out at 80 оC. 3,5-Dibromo-1,8-naphthalimide has been obtained by this procedure with the yield of 90 % as light yellow crystals with melting point 292-293 °C. In case of 2,4,5-tribromo-1,8-naphthalimide the product contained some unreacted 2,4,5-tribromo-1,8-naphthalic anhydride as impurity. The reaction mixture was treated with a 2% aqueous solution of sodium hydroxide to remove anhydride impurity. Target 2,4,5-tribromo-1,8-naphthalimide was obtained with the yield of 67 % as light yellow crystals with melting point > 300 °C. The structure of the synthesized naphthalimide derivatives has been confirmed by IR and NMR 1Н spectroscopy