Theoretical Investigations On Transition Metal Palladium And Visible Light-Catalyzed C-N And C-C Bond Formation Reactions

Constructing a catalytic cycle process to achieve the formations of carbon-carbon bonds and carbon-heteroatom bonds in efficient and inexpensive ways is an important topic in organic synthetic chemistry.Nowadays,the catalytic reactions of transition metal complex have overcome the defects of traditional synthesis methods and occupied the dominant position of the cross-coupling reactions.At the same time,the catalytic reactions driven by visible light have made great progress with advantages of environmental friendliness.Through the density functional theory(DFT)calculations,we systematically studied the transition metal palladium-catalyzed C-N coupling reaction pathways and the visible light-catalyzed carbon radical addition reaction process.It was revealed that the catalytic systems of two commonly used palladium catalyst ligands have different rate-limiting steps,mainly because of their differences in spatial structures and electrical properties.Dithiocarbonyl anions play an important role in the formation of carbon radicals,and the addition of carbon radicals is greatly affected by the natural charge of their terminal carbons.For the transition metal palladium-catalyzed C-N coupling reactions,our calculation results demonstrated that the Buchwald-Hartwig amination reaction consists of three steps:oxidative addition,deprotonation,and reductive elimination,where oxidative addition and reductive elimination are endothermic steps.The rate-limiting step of Brett Phos catalytic system is oxidative addition,and that of Ru Phos catalytic system is reductive elimination,which is rationalized by the fact that Brett Phos has relatively larger steric hindrance and electron-richer substituents than Ru Phos.Then we introduced different reaction systems serving as control groups,which indicated that Brett Phos is suitable for primary amines or secondary amines with smaller steric hindrance,while Ru Phos is suitable for secondary amines with greater steric hindrance.In addition,we found that amines with large size substituents or halogens with electron-withdrawing groups would make the activation energy barriers of reactions reduced.For the visible light-catalyzed carbon radical addition reactions,our calculation results displayed that the entire photocatalytic cycle is divided into two reaction parts.In the first half,the dithiocarbonyl anion catalyzes the formation of carbon radicals through the S_N2 replacement and the C-S bond cleavage.Among them,the iodine atom is easier to leave;electron-withdrawing groups near the C-S bond would make it difficult to be cleavaged.In the second half,there are three destinations for carbon radicals:addition to olefins and abstraction of hydrogen from hydrogen donors;addition to olefins and cyclization;attack of?-carbon and deprotonation.The less negative charge the terminal carbon carries,the more favorable for carbon radical to attack it.We also studied the possible side reactions and by-products,which are useful to understand the observed experimental results.