A theoretical study of the C-H activation of methane derivatives. Significant effects of electron-withdrawing substituents

The sp(3) 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 (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(H2PCH2-CH2PH2). The acceleration by the CN group is interpreted in terms of the charge-transfer interaction from Pd to the pi* orbital of CH2(CN)(2) into which the C-H sigma* 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 sp(3) carbon atom and a chelate phosphine is used as a ligand.