A Combined Experimental and Computational Study of Ligand Effects on C-H Bond Activation by Palladium and Platinum Complexes

Abnormally large kinetic hydrogen/deuterium isotope effects (KIEs, ~ 20) are measured for the protonolysis of several dimethylpalladium(II) complexes with various bidentate ligands by trifluoroethanol (TFE) at room temperature. Analyses of semiclassical KIE theory suggest that the occurrence of hydrogen tunneling needs to be invoked in order to explain these KIE values, which is further supported by the KIE-temperature-dependence study for the protonolysis of (dppe)Pd(CH₃)₂ by CF₃CD₂OD/CF₃CH₂OH. Density functional theory (DFT) computation suggests that protonation at the MII-C bond is kinetically preferred over protonation at the metal center for the protonolysis of (COD)Pt(CH₃)₂ by TFA and the dimethylpalladium(II) complexes by TFE in dichloroethane. The computation further indicates the significant contribution of hydrogen tunneling in the abnormally large KIEs observed experimentally. The monomethylpalladium(II) complex, (COD)Pd(CH₃)Cl (COD = 1,5-cyclooctadiene), undergoes both benzene C-H activation and migratory insertion of olefin, with the former faster than the latter, at room temperature under the assistance of an anionic β-diketiminate ligand, to yield η³-(2-R-cyclooctenyl)palladium(II) β-diketiminate (R = methyl or phenyl). DFT computation result suggests that bisindolide-type ligands and carbenearyl-type ligands are likely to lead to faster benzene C-H bond activation as well as lower relative VIII barrier heights of the C-H bond activation versus the insertion of olefins than those in monomethyl palladium(II) with β-diketiminate. Several pyridine-like ligands were found to improve Pd(OAc)₂-catalyzed allylic oxidation of allylbenzene to cinnamyl acetate by p-benzoquinone in acetic acid. The best ligand examined, bipyrimidine, was used to identify the catalyst precursor for this system, (bipyrimidine)Pd(OAc)₂, which was fully characterized. Mechanistic studies suggest the reaction takes place through disproportionation of (bipyrimidine)Pd(OAc)₂ to form a bipyrimidine-bridged dimer, which reacts with olefin to form a PdII-olefin adduct, followed by allylic C-H activation to produce (η³-allyl)PdII species. The (η³-allyl)PdII intermediate undergoes a reversible acetate attack to generate a Pd⁰-(allyl acetate) adduct, which subsequently reacts with p-benzoquinone to release allyl acetate and regenerate (bipyrimidine)Pd(OAc)₂. No KIE is observed for the competition experiment between allylbenzene-d₀ and allylbenzene-d₅(CD₂=CD-CD₂-C₆H₅), suggesting that allylic C-H activation is not rate determining. Catalytic allylic acetoxylations of other terminal olefins as well as cyclohexene were also effected by (bipyrimidine)Pd(OAc)₂.