Study On The Free Radical Self-coupling Reaction Of Anilines And Its Substituent Effect

The development of efficient synthetic methodologies to prepare structuraldiversity of benzidine derivatives has received increasing attention in recent years dueto their applicability in a wide variety of domains such as azodyes, staining reagent,molecular machines and organic functional materials. Synthetic methodologies based oniron have undergone an explosive growth due to the readily accessibility, favorablesafety profile and low cost of iron derivatives. We have investigated the iron-promotedfree radical oxidative self-coupling of anilines, under which various structurally diverseanilines were transformed to corresponding benzidines efficiently.The FeCl36H2O-promoted oxidative self-coupling of anilines has been developed,and the high selective migration of N-methyl of amine to construct diarylmethanes wasalso explored through adjusting the reaction conditions. The reaction conditions such assolvent, additive, reaction temperature and oxidant were screened. Moreover, electroniceffects, substituents effects and steric effects of the substrates on the reaction are testedin our research. Prolonging the reaction time to24h, trimer product of3-ethoxyl-N,N-dibenzylaniline was obtained in52%yield. Utilizing our developedmethod, a set of photoactive materials involving benzidine unit were synthesized.Meanwhile, LC-MS and controlling experiments were conducted to give insight into themechanism, and a free radical mechanism was proposed.During the investigation of substituent effect of the self-coupling reaction, we findthat unprotected anilines were coordinated with FeCl3·6H2O and only recovered startingmaterials after treatment. To reduce the basicity of substrates, dibenzyl substitutedanilines were applied and showed good reactivity. While the ortho-substituents stericrepulsions of dibenzyl substituted anilines inhibited the self-coupling reactions. In orderto decrease the steric shielding around the nitrogen, N-benzylanilines were introducedand the reaction proceeded smoothly. However, some strong electronic-withdrawinggroups are still big setback to undergo the self-coupling reaction to providecorresponding benzidines. To our delight, N,N-dimethyl substituted anilines haveexcellent functional group tolerance to give corresponding self-coupling products.Our developed iron-promoted oxidative self-coupling of anilines has advantages ofmild conditions, simple operation and good tolerance of functional group and has thepotential for industrial application.