Merging Photoredox Catalysis With Transition Metal Catalysis:Direct C4-H Amination Of 8-hydroxyquinoline Derivatives

A protocol for Ag/Ru-cocatalyzed regioselective C-H amination of8-hydroxyquinoline esters with pyrazoles was explored in this paper,The model reaction proceeded smoothly selecting pyrazole as amination reagent,Ru?bpy?₃?PF₆?₂ and AgNTf₂ as the cocatalyst,K₂S₂O₈ as oxidant,CH₃CN as the solvent in argon by the blue light irradition at room temperature,obtaining the target product 3aa successfully?Scheme 1?.The remarkable features of this reaction as follows:high regioselectivity,simple system,mild reaction conditions,and moreover,it is not necessary to add additives such as an acid and a base,and the functional group is compatible.The 8-hydroxyquinoline ester bearing an electron withdrawing group,an electron withdrawing group or an electrically neutral group all could be reacted under standard conditions,affording the corresponding product in moderate yields.It is noted that the electronic effect has little effect on the yield.When 3.0equiv 2,6-ditert-butyl-4-methylphenol?BHT?or 2,2,6,6-tetramethylpiperidin-1-oxyl?TEMPO?was added to this amination reaction,the reaction could not occur at all.Under optimized conditions,The pyrazole radical was detected by HR-MS when 3.0equiv 2,6-ditert-butyl-4-methylphenol?BHT?was added to the reaction.These consequences imply that this amination reaction might proceed via a radical process?Scheme 2?.???Scheme 1???Scheme 2...