Study On The Mechanism Of Pd(Ⅱ)/Rh(Ⅰ) Complex Catalyzing The Activation Of Aliphatic C(sp~3)-H

The direct functionalization of non-activated C–H bonds has realized the maximization of atom-and step-economy in the process of chemical synthesis.But due to the high stability of C–H bonds,the process often requires harsh reaction conditions,which has drastically limited their applications in the synthesis of complex organic molecules.Transition-metal-catalyzed C–H activation can transform C–H to carbon-heteroatom（C–C、C–O、C–N）bonds under mild reaction conditions and has high chemoselectivity.The advantages of catalysis as simpleness,high efficiency and according with the concept of green chemistry have been reflected in the development of organic chemistry.In this paper,the progress of related research in palladium and rhodium catalysis is briefly reviewed.We have performed density functional theory（DFT）calculations on palladium-and rhodium-catalyzed reactions,using two different reaction systems.Transient directing group（DG）has been successfully applied to assist the activation of C–H bonds.In this paper,we have performed density functional theory（DFT）calculations to investigate the mechanism for ligand-assisted Pd（II）-catalyzed C（sp³）–H arylation.In the presence of quinoline-8-carbaldehyde（Ar^QCHO）ligand,the reaction starts with the nucleophilic addition of 2-butylamine with the ligand to generate the transient imine DG,which binds to the Pd（II）center via bidentate coordination.The sequential C（sp³）–H activation,oxidative addition of[Ph₂I]⁺with the palladacycle,and C–C reductive elimination yield the final product of arylation.Instead of the traditional inner-sphere concerted metalation deprotonation（CMD）mechanism,a novel deprotonation mechanism for the rate-determining C（sp³）–H activation is discovered;that is,the methyl group is deprotonated by an out-of-sphere pivalate.A comparison with the results of reaction without ligand indicates that the square planar geometry formed by the transient DG with Pd（II）significantly reduces the distortion energies,which ultimately makes the C（sp³）–H activation kinetically favorable.In addition,we have used density functional theory（DFT）calculations to explore the mechanism of Rh（I）-catalyzed 2-butanone?-C（sp³）–H bonds coupling with 3-hexyne.Attempts to capture the Rh–H intermediate via experiments were unfruitful,but they speculate that the C–H activation occurs through a metal–hydride pathway rather than a CMD pathway.The key steps in a matal–hydride pathway including in situ-generated hydrazone DG derived from coupling between 2-butanone and 3-hexyne,Rh（I）coordination with the DG facilitates oxidative addition to generate Rh（III）–H intermediates,alkyne insertion,reductive elimination and the corresponding CMD pathway involving CMD-type C–H activation transition states,alkyne migratory insertion and protodemetallation have been calculated.The alkyne migrates insertion into the Rh–H selectively ruling out alkyne insertion into Rh–C and Rh–N as we can see from the energy barrier.The results show that the CMD pathway by acac^-and TsO^-makes this reaction kinetically unfavorable due to the higher energy barrier.The calculated results of the replacement（p-MeOC₆H₄）₃P with acac^-elucidate that the true reactant is phosphine ligand.The related studies above have strengthened the understanding of the mechanism of Pd（II）/Rh（I）catalysis,our calculations will be helpful in the rational design of effective catalysts,ligands and reaction condition.