Theoretical Study On Ligand Electronic Effects In Pd(Ⅱ)/Rh(Ⅰ)-Catalyzed Conjugate Addition Reactions

Transition metal-catalyzed conjugate addition reaction is the efficient catalytic reaction for the construction of C-C bonds.Palladium and rhodium are the most used transition metal catalysts in conjugate addition reactions.The mechanism of the conjugate addition reaction catalyzed by Pd（Ⅱ）and Rh（Ⅰ）has been revealed,and the insertion step is confirmed to be the rate-determining step of the reaction.The electronic effect of the ligand coordinated with the transition metal plays a key role in the conjugate addition reaction,but quantifying the ligand electronic effect of pyridine-type ligands is rare.Ⅰn this thesis,ligands with different electronic effects on metal centers are constructed.By using density functional theory（DFT）,Pd（Ⅱ）or Rh（Ⅰ）catalyzed conjugate addition reactions are calculated for the quantitative study on electronic ligand effects.The main contents and results are as follows:（1）Pd（Ⅱ）/Rh（Ⅰ）catalyzed conjugate addition reactions with 26 different bidentate bipyridine（bpy）ligands have been investigated.The activation energy barrier of the insertion step（ΔG_a）,the binding energy（ΔG_B）of the bpy ligand to the metal center M（Pd（Ⅱ）or Rh（Ⅰ））,the Mayer bond order（MBO）of N1 or N2（N1/N1’position on 2,2’-bpy）atom on the bpy ligand-M bond,the average occupied orbital energy of Pd（Ⅱ）/Rh（Ⅰ）4d orbital and the Hirshfeld charges of N¹and N²are calculated.The results show that when both the C4and C4’positions of the bpy ligand are linked to strong electron-withdrawing groups（EWG）,the insertion step of the conjugate addition reaction is favorable either in the Pd（ΙⅠ）-or Rh（Ι）-catalyzed systems.On each side of the bpy ligand,the effect of MBO onΔG_ais greater for N¹-M than for N²-M.Ⅰn the Pd（ΙⅠ）-catalyzed system,the MBO of N¹-M and N²-M is positively correlated withΔG_a,while it is negatively correlated in the Rh（Ι）-catalyzed system.This is due to the dominant"π-back donation"effect of Rh（Ⅰ）on the bpy ligand and theσ-donation of the ligand on Pd（ΙⅠ）in the relevant system.（2）The activation energy barriers of insertion steps（ΔG_a）catalyzed by Pd（ΙΙ）/Rh（Ι）with 10 bidentate P,N-type cis-and trans-ligands,monodentate pyridine-and N-heterocyclic carbine-type ligands are calculated,and fitting with the MBOs of N1/N2（nitrogen for olefin-adjacent/para pyridine ligands）-metal（for P,N-type ligands and monodentate pyridine-type ligands）or C1/C2（carbon for olefin adjacent/para N-heterocyclic carbine-like ligands）-metal（for N-heterocyclic carbine-type ligands）.The results indicate that when the bidentate P,N ligands coordinated,either Pd（ΙΙ）-or Rh（Ι）-catalyzed system favors the insertion step when a strong EWG is bonded to the C4 position of the pyridine ring.And the trans ligand leads to a lowerΔG_athan that of the cis one.Similar to the bpy ligand,the effect of MBO onΔG_ais greater for N1-M than for N2-M on each side of the ligand.For Pd（ΙΙ）-catalyzed system,the fitted equations betweenΔG_aand MBOs of N1-Pd and N2-Pd are similar to those of bpy ligand,indicating theσ-donation is still dominant for Pd（ΙΙ）.While the fitted equations for Rh（Ι）-catalyzed system change a lot from those of bpy ligand,suggesting that the"π-back donation"effect is competitive with theσ-donation effect in Rh（Ι）-catalyzed system.When the ligand is a monodentate N-heterocyclic carbene,theΔG_adecreases compared to that of bpy ligand,and the MBO of C1-Pd is positively correlated withΔG_ain Pd（ΙΙ）-catalysis,but the opposite is true for C2-Pd.The effect of hydrogen bond leads to a significant increase inΔG_awhen the substituents at the C3 position are-SO₃H and-NO₂.The MBO of both C1 and C2-Rh are negatively correlated withΔG_afor Rh catalysis.For all ligands,the"π-back donation"effect of the d orbital of Rh（Ι）is obvious.This thesis screens a parameter MBO which can quantitatively describe the electronic effects of ligands for conjugate addition reactions,and explores the difference between the electronic effects of Pd（Ι）and Rh（Ι）.This not only provides a method for quantitative analyzing the electronic effects of ligand,but also provides sufficient theoretical guidance for the rational design of efficient transition metal-based catalysts by adjusting the ligands to achieve conjugate addition reaction.