Synthesis Of Carbazole Via Biaryl Carbon-Hydrogen Bond Activation Reaction

Heterocyclic compounds have important practical applications in dyes,functional materials,and household chemicals.They are also widely present in natural products,medicines,and pesticides,and have outstanding chemical and biological activities.As an important heterocyclic structural unit,the construction of carbazole molecular structure is very important.People initially obtained carbazole,which was mainly extracted from coal tar.For the synthesis of carbazole,in the past few decades,chemical workers at home and abroad have carried out in-depth research,giving a variety of cut off carbazole molecules.Most of these synthesis methods form carbazole structures through the construction of carbon hetero bonds.Unfortunately,these methods have drawbacks such as narrow range of substrates,harsh reaction conditions,poor atomic economy,and strong harm to the environment.Therefore,it is imperative to find new ways to construct this structure.The transition metal-catalyzed C-H bond activation has become one of the research focuses in the field of organic chemistry in recent years and is known as the holy grail of metal organic chemistry.The strategy of combining transition metal catalyzed C-H bond activation with dehydrogenation coupling is characterized by its high efficiency,high selectivity and atomic economy and green chemistry requirements,and is an important means of constructing carbon-carbon bonds.Based on this background,we chose aniline and cyclohexanone derivatives to directly construct unsymmetric carbazole rings through the dehydroaromatization/C(sp2)-C(sp2)series activation process.The method is simple and efficient,which not only saves the tedious steps of preparing the aryl halide of the raw material,but also obtains good yield and regioselectivity,and at the same time makes the reaction meet the requirements of green chemistry.The investigation of the mechanism revealed that the reaction is a two-step tandem reaction.First,the dehydroaromatization of the imine formed by aromatic amines and cyclohexanone produces the diphenylamine intermediate,which is then intramolecularly generated by the action of palladium.The dehydrogenative coupling between the biaryl groups thus forms carbazole.This article is divided into three parts.In the first part,the palladium-catalyzed carbon-hydrogen bond activation reaction and the synthesis of carbazole ring were reviewed.In the second part,palladium-catalyzed dehydroaromatization-aryl coupling cascade reactions were studied.The third part is the experimental part.