Highly Monoselective Arylation Of Benzamide Via Visible Light Induced Pd-catalyzed C-H Activation

Transition metal catalyzed C-H bond activation is one of the most effective means of constructing C-C bonds and C-X bonds.After decades of development,the cross-coupling reaction catalyzed by the transition metal including Rh,Ru and Pd has made great progress,especially in the direction of bidentate ligands,it has become a vital organic synthesis methodology because of the good effect in the catalytic reaction of cheap mental.However,this type of reaction often requires higher temperature or Grignard reagent,and the removal of the directing group is also under extremely harsh conditions.In order to solve the above problems,the new C-H activated functionalization combined with free radical chemistry has gradually moved on to the centre of modern organic synthesis because of its moderate characteristics.Herein,we discuss the study on the highly monoselective arylation of benzamide via visible light induced Pd-catalyzed C(sp~2)-H activation at room temperature,which can be divided into three parts as follow:(1)The Pd-catalyzed C-H functionalization of benzamide derivatives and the transition metal-catalyzed C-H bond activation reaction involving aryldiazonium salts and the project background,significance and research contents were introduced,respectively.(2)A serious of experiments on the Pd-catalyzed C-H bond activated arylation of benzamide and aryldiazonium salts at room temperature has been carried out to explore the effects of photocatalysts,palladium salts,solvents and visible light.And other investigations were used to explore the scope and limitations of this reaction.Then,an approach for the preparation of[1,1’-biphenyl]-2-carboxamides from benzamides was described.(3)Based on the previous literature reports and free radical capture experimental results,a plausible mechanism of the synthesis of[1,1’-biphenyl]-2-carboxamides was proposed.Upon irradiation with visible light,aryldiazonium salt first reacts with photocatalyst to generate an aryl radical and the ortho-hydrocarbon of benzamide undergoes C-H bond activation with the Pd-catalyst to form a 5-membered palladacycle.Then,this palladacycle is combined with the free radical to afford a Pd(Ⅲ)intermediate via a radical addition process.After a single-electron-transfer steps,a Pd(IV)intermediate can be formed.And the desired product is obtained via reductive elimination of the intermediate.