Application Of Iminyl Radicals In Visible-Light Photoredox Catalysis

The reaction of nitrogen radicals promoted by visible light is an important method to construct nitrogenous compounds.In particular,as a kind of nitrogen radical,iminyl radical has good reactivity and rich functional group transformation in the reaction,which has attracted the attention of many chemists.Due to the C(sp²)-N bond in its molecule,iminyl radical belongs to?-type radical,which has both electrophilic and nucleophilic properties.The reaction with iminyl radical as the intermediate can undergo four main classes of reactions:1)addition to arenes,2)intramolecular hydrogen atom transfer and subsequent reactions,3)addition to alkenes,4)Norrish type-I fragmentation(?cleavage of-carbon-carbon bonds)and subsequent reactions.According to the properties of the above iminyl radicals,we carried out our research work,including the following four parts:The first part:A series of keto-?-alkenylation products with high trans/cis ratio were synthesized by the remote C(sp³)-H bond alkenylation of oxime esters and phenylvinyl boric acids via visible light photoredox catalysis.In this strategy,an excited photosensitizer and an oxime ester undergo a single electron transfer via oxidative quenching,and the resultant iminyl radical undergoes 1,5-HAT to generate an alkyl radical.The alkyl radical add to a phenylvinyl boric acid,and the final product is obtained by oxidative,deboronation and hydrolysis.The strategy has good substrate adaptability and functional group tolerance,which complements to the remote C(sp³)-H functionalization reaction.The second part:Remote alkenylation of nitriles was achieved by the visible light catalytic C-C bond alkenylation of cyclobutanone oxime esters with phenylvinyl boric acids.26 alkenyl products with high trans/cis ratio were obtained,with good substrate adaptability and functional group tolerance.The third part:The alkene-substituted 2H-pyrrole compounds were constructed with high diastereoselectivity by visible light catalyzed intermolecular iminylalkenylation of?,?-unsaturated oxime esters and phenylvinyl boric acids.Under mild reaction conditions,E-or Z-products could be achieved by changing the solvent:E-and Z-products can be obtained with dichloromethane(CH₂Cl₂)and tetrahydrofuran(THF)as solvents,respectively,with excellent diastereoselectivity(dr>20:1).Through theoretical calculation and mechanism verification,we propose a possible mechanism to account for stereoselectivity.The fourth Part:A photoredox/palladium-cocatalyzed enantioselective alkylation of racemic secondary carbonates with 4-alkyl-1,4-dihydropyridines under visible light irradiation.The present study provides a method for the preparation of optically active diarylalkanes from racemic diarylmethyl carbonates by a dynamic kinetic asymmetric transformation(DYKAT).This photoredox/palladium dual catalysis strategy expands the scope of the asymmetric Pd-catalyzed benzylic substitution reaction and serves as its potential alternative and complement.