Thermally Induced Degradation of 2,3,5,6-Tetrachloroterephthalylidenebis(o-aminoaniline)

Article abstract of DOI:10.1021/jo00153a022

The bis(Schiff base) 1, 2,3,5,6-tetrachloroterephthalylidenebis(o-aminoaniline), obtained by low-temperature solution condensation of o-phenylenediamine with 2,3,5,6-tetrachloroterephthalaldehyde, suffers fragmentation at 100 degC in dipolar aprotic solvent media under strictly anaerobic conditions, giving benzimidazole (3) and 1,2,4,5-tetrachlorobenzene (4).The same fragmentation of 1, although accompanied by side reactions, is observed in refluxing ethanol or toluene.The reaction involves fission of the C-C bonds connecting the central tetrachlorophenylene segment with the outer two substituent groups and probably proceeds via the ring-tautomeric bis(imidazoline) form of 1.Neither the bis(benzimidazole) 2, 1,4-bis(benzimidazol-2-yl)-2,3,5,6-tetrachlorobenzene, in which the outer two substituent groups already exist in the aromatized state, nor the bis(Schiff base) 6, terephthalylidenebis(o-aminoaniline), lacking the four bulky chloro groups at the center unit, undergo such C-C bond cleavage under the conditions indicated.These findings show that it is the combined effect of steric buttressing at the perchlorophenylene segment and imidazolation, i. e., aromatization, of the outer substituent groups which provides the driving force for the fragmentation observed.In a suitable oxidative environment, cyclodehydrogenation of the bis(Schiff base) 1 competes efficaciously enough with the degradative process to suppress the generation of the fragmentation products, and only the bis(benzimidazole) 2 is isolated.