Photoredox Catalytic Cascade Radical Addition Reactions To Construct 1,4-diketone-functionalized Quinoxalin-2(1H)-one Derivatives

Quinoxalin-2（1H）-one as a representative of non-aromatic nitrogen heterocycles,which has attracted wide attention because of its remarkable physiological and pharmacological activities.With the deepening of research,it is found that most of the drugs containing quinoxalinone structures are generally modified at the C-3 position.Therefore,its C3-H functionalization has become a research hotspot in recent years.Meanwhile,visible light-catalyzed redox reactions have developed rapidly due to the characteristics of green,environmental protection and relatively mild reaction conditions.Therefore,the construction of quinoxalinone C3-H functionalization using a sustainable photocatalytic platform has been widely reported.Besides the direct addition of photocatalysis-yielding radicals to 3-position of quinoxalin-2（1H）-ones,a handful of three-component cascade reactions have been described,wherein olefins were used to preferentially accept radicals to generate new radical intermediates to experience the second addition process.Given the robust ability of cascade strategy to enrich the varieties of quinoxalin-2（1H）-ones,the exploration of new protocols remains a highly attractive task.Herein,we report photoredox catalytic reactions of quinoxalin-2（1H）-ones with 2-bromo-1-arylethan-1-ones as the partners,leading to a series of valuable 1,4-diketone-functionalized quinoxalin-2（1H）-one derivatives with satisfactory results.This is an unprecedented cascade reaction strategy,involving radical addition to quinoxalin-2（1H）-ones and the resultant exocyclic olefin intermediates to accept a second addition of radicals generated from bromide substrates or to experience SN₂ reaction with bromides.In this special tandem addition method,we constructed asymmetric 1,4-diketone-functionalized quinoxalin-2（1H）-one derivatives by pre-forming exocyclic olefin intermediates,which react with one molecule ofα-bromoaryl ethanone subsequently.Based on the experimental results,we provide two pathways,photocatalytic radical addition and equivalent inorganic base-promoted SN₂ reaction.