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 (Ea) and decreases the endothermicity (Eendo), Ea and Eendo are still high in C−H activation by a palladium(0) monodentate phosphine model complex, Pd(PH3)2:  Ea = 37 kcal/mol and Eendo = 34 kcal/mol for CH4, Ea = 32 kcal/mol and Eendo = 23 kcal/mol for CH3CN, and Ea = 25 kcal/mol and Eendo = 11 kcal/mol for CH2(CN)2, where MP4SDQ values are given. However, Ea becomes significantly low and the reaction becomes exothermic in the C−H activation of CH2(CN)2 by a chelate phosphine model complex; Ea = 18 kcal/mol and Eexo = 11 kcal/mol in a simple model Pd(dipe) in which two PH3 ligands are placed to mimic bis(dicyclohexylphosphino)ethane, and Ea = 19 kcal/mol and Eexo = 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.