A Theoretical Study of the C−H Activation of Methane Derivatives. Significant Effects of Electron-Withdrawing Substituents

The sp³ C−H activation of CH₃CN and CH₂(CN)₂ by palladium(0) complexes was theoretically investigated with the ab initio MO/MP4 method. Although introduction of an electron-withdrawing CN group lowers the activation energy (E_a) and decreases the endothermicity (E_endo), E_a and E_endo are still high in C−H activation by a palladium(0) monodentate phosphine model complex, Pd(PH₃)₂:  E_a = 37 kcal/mol and E_endo = 34 kcal/mol for CH₄, E_a = 32 kcal/mol and E_endo = 23 kcal/mol for CH₃CN, and E_a = 25 kcal/mol and E_endo = 11 kcal/mol for CH₂(CN)₂, where MP4SDQ values are given. However, E_a becomes significantly low and the reaction becomes exothermic in the C−H activation of CH₂(CN)₂ by a chelate phosphine model complex; E_a = 18 kcal/mol and E_exo = 11 kcal/mol in a simple model Pd(dipe) in which two PH₃ ligands are placed to mimic bis(dicyclohexylphosphino)ethane, and E_a = 19 kcal/mol and E_exo = 6 kcal/mol in a more realistic model Pd(H₂PCH₂CH₂PH₂). The acceleration by the CN group is interpreted in terms of the charge-transfer interaction from Pd to the π* orbital of CH₂(CN)₂ into which the C−H σ* orbital mixes. These computational results suggest that the C−H activation by a palladium(0) complex easily occurs when electron-withdrawing groups are introduced on the sp³ carbon atom and a chelate phosphine is used as a ligand.