Cyclization Reaction of 2, 5-Dianilino-3, 6-dihydroterephthalic Acid Diethyl Ester

Abstract

When 2, 5-dianitino-3, 6-dihydroterephthalic acid diethyl ester (1) was heated in toluene, reaction proceeded according to eq. [1]._??_-N-H-_??_-O=C-EtO-N-H-_??_-C=O-OEt→step1 _??_-O=C-N-H-_??_-C=O-OEt→step2 _??_-O=C-N-H _??_-N-H-O=C-_??_… [1] (1) (2) (3) (1)(2)(3)The activation energy of step-1 was 42.1 kcal/mol and that of step-2 was 86.0 kcal/mol. The half cyclized product (2) can be synthesized with high purity when the reaction temperature is controlled at around 230°C.The reaction occurs only in nonpolar solvents. The presence of alcohol in the solvent strongly inhibited the reaction and gave ester-exchanged products. Since the reaction is not catalized by acids or bases and is not affected by the substituents on the aromatic ring of (1), ionic transition state is probably not involved in the rate determining step.Large difference in activation energy between step-1 and step-2 can be explained by the transition state of highly strained bond angle in step-2; the increase of rigidity of cyclohexadiene ring in (2) as a result of half cyclization requires high energy to form the transition state.