Enzymatic coupling of chloroanilines with syringic acid, vanillic acid and protocatechuic acid

Abstract

To clarify the reaction mechanism and the reactivity of phenolic acids with chloroanilines, the reactions of 4-chloroaniline, 3,4-dichloroaniline and 2,4,5-trichloroaniline with vanillic acid, syringic acid and protocatechuic acid were investigated. Anilines alone were not transformed by a laccase isolated from the fungus Rhizoctonia praticola, but numerous cross-coupling products were formed in the presence of a phenolic acid. Protocatechuic and syringic acid readily reacted with anilines, but the reaction with vanillic acid was very weak. From the structures of the generated reaction products, it could be concluded that there was no difference in the cross-coupling reaction between a chloroaniline with a phenolic acid. The nucleophilic attack of the investigated chloroaniline resulted in the formation of an imine linkage at the 6-position of a quinone formed from protocatechuic acid and at the 1-position (through a quinone group) of syringic acid. Dimer formation between vanillic acid and chloroaniline could not be detected, but vanillic acid formed a quinone dimer which then reacted with chloroaniline, generating a hybrid trimer.